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Crystallization of mechanical pulp mill effluents through hydrate formation for the recovery of water Gaarder, Cathrine 1993-12-31

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CRYSTALLIZATION OF MECHANICAL PULP MILL EFFLUENTSTHROUGH HYDRATE FORMATION FOR THE RECOVERY OF WATERbyCATHRINE GAARDERB.Sc., Montana State University, 1991A THESIS SUBMITTED IN PARTIAL FULFILLMENT OF THEREQUIREMENTS FOR THE DEGREE OF MASTER OF APPLIED SCIENCEinTHE FACULTY OF GRADUATE STUDIES(Department of Chemical Engineering)We accept this thesis as conforming to the required standardTHE UNIVERSITY OF BRITISH COLUMBIANovember 1993© Cathrine Gaarder, 1993In presenting thisthesis in partialfulfilment of the requirementsfor an advanceddegree at the Universityof British Columbia,I agree that the Library shallmake itfreely available for referenceand study. I furtheragree that permission forextensivecopying of this thesisfor scholarly purposes maybe granted by the head ofmydepartment or by hisor her representatives.It is understood that copyingorpublication of thisthesis for financial gainshall not be allowed withoutmy writtenpermission._______________________Department of Ck( LrjThe University ofBritish ColumbiaVancouver, CanadaDate 3/.DE-6 (2/88)11ABSTRACTThis work presents data toward the development of a new zero-liquid discharge(ZLD) system, the aim of which is to concentrate a liquid effluent thereby recoveringprocess water. The crystallization technology used in this work is clathrate hydratecrystal formation. A screening of clathrate hydrate formers was performed and threewere chosen, propane, carbon dioxide, and a 30-70 mol% propane - carbon dioxidemixture. Experiments were carried out with these substances in a vessel that wasimmersed in a temperature controlled bath. The effluent samples used in this studywere generated at four pulp mills, an unbleached and a bleached thermo-mechanicalpulp mill (TMP1, TMP2), a bleached chemi-thermo-mechanical pulp mill (BCTMP),and a combined bleached BCTMP/TMP mill (CTMP).The aim of this work was two-fold. The prime objective was to studyexperimentally the formation of clathrate hydrate crystals in these effluents.Experiments were designed to measure:(1) The temperature and pressure conditionsat which hydrates form in mechanical pulp mill effluent; (2) The induction time andthe growth rate of crystal formation; and(3)The ability of this apparatus toconcentrate the effluents in-situ. In addition, effluent characteristic tests wereperformed, and qualitative process characteristics were noted. A secondary objectivewas to perform a survey on the liquid effluents generated at TMP and CTMPmills.This survey focused on the type, concentration, source, and environmental impactsassociated with the contaminants typically present in these effluents, on the currentdischarge regulations, and on the available treatment options.It was determined that hydrate crystals can form in these effluents attemperatures above the normal freezing point of water. The measurements werecompared with similar hydrate formation data in pure water. It was also found thatthe presence of impurities did not cause any appreciable change in the hydratecrystal pressure and temperature formation conditions with the TMP and CTMPeffluent samples, but that the BCTMP effluent, which had a high electrolyte andorganic content, did affect these conditions. It was found that the induction periodand growth rate of hydrate crystals are dependent on the driving force and the priorhistory of the effluent. And finally, that some level of in-situ concentration wasattainable with our apparatus.111TABLE OF CONTENTSCHAPTER 1: IntroductionCHAPTER 2: Mechanical Pulp Mill Effluent I:Source of Pollutants and Characteristics2.1 Pulp Production 72.1.1 Debarking 72.1.2 TMP and CTMP Processes 82.1.3 Bleaching 102.2 Wood Chemistry 112.3 Effluent Discharge Parameters2.3.1 Solids2.3.2 Oxygen Demand2.3.3 Toxicity2.3.4 Other Parameters2.4 Source of Pollutants2.4.1 Raw Material2.4.2 Point Source2.4.3 Other Factors2.5 Summary of Effluent Characteristics 24CHAPTERS: Mechanical Pulp Mill Effluent II:Impacts of Pollutants, Discharge Regulations,and Treatment Options 263.1 Receiving Environment 263.2 Impacts3.2.1 Potential Impacts3.2.2 Short-Term Impact Studies3.2.3 Long-term Impact StudiesABSTRACTTABLE OF CONTENTSLIST OF TABLESLIST OF FIGURESACKNOWLEDGEMENTS11111viviiix1712121213151616192227283033iv3.3 Federal Regulations. 343.4 Effluent Treatment Technologies 373.4.1 Biological Systems 383.4.2 Zero-Liquid Discharge Systems 393.5 Comparison of Treatment Options 41CHAPTER 4: Freeze Concentration and Clathrate HydrateConcentration 444.1 Freeze Concentration 444.1.1 Basic Process 454.1.2 Unit Operations 454.1.3 Technology Systems 484.1.4 Industrial Applications 494.1.5 Comparison of Separation Technologies 524.2 Clathrate Hydrate Concentration 544.2.1 Crystal Structure 554.2.2 Concentration Process 574.2.3 Industrial Applications 594.3 Clathrate Hydrate Formation 604.3.1 Thermodynamics 604.3.2 Kinetics 63CHAPTER 5: Experimental Section 645.1 Effluent Samples and Other Materials 645.2 Effluent Characteristic Tests 665.3 Experimental Set-up 675.3.1 Apparatus 685.3.2 Hydrate Formation Vessels 685.4 Experimental Procedures 735.4.1 Qualitative Process Characteristics 735.4.2 Pressure-Temperature Hydrate Formation Conditions .. . . 745.4.3 Induction and Growth Rate Experiments 755.4.4 In-Situ Concentration Experiments 77VCHAPTER 6: Results and Discussion.796.1 Effluent Treatment Options 796.2 Mill Effluent Characteristics 816.3 Selection of Hydrate Formers 916.4 Qualitative Process Characteristics 956.5 Pressure-Temperature Hydrate Formation Conditions 996.6 Induction and Growth Rate Experiments 1106.7 In-Situ Concentration Experiments 1126.8 Overall Discussion 120ChAPTER 7: Conclusions and Recommendations123REFERENCES 125APPENDIX: Effluent Characteristic Tests 138viLIST OF TABLESTABLE 2.1: Compounds in Mechanical Pulp Mill Effluents that areToxic to Fish 18TABLE 2.2: Extractive Content of Several Different Wood Species . . . . 19TABLE 2.3: Untreated TMP and CTMP Pulp Mill EffluentCharacteristics 24TABLE 3.1: Sub-Lethal Effects 29TABLE 3.2: Acute and Sub-lethal Toxicity Data for Several DifferentAquatic Species Exposed to TMP Effluent 32TABLE 3.3: EEM Pre-Design Requirements and their Purpose 35TABLE 3.4: EEM Studies 36TABLE 3.5: Pulp and Paper Effluent Regulations 37TABLE 4.1: Removal Efficiency of Contaminants Typically Present inCTMP Effluents by Evaporation and Freeze Concentration54TABLE 5.1: Design Parameters of the Pulp Mills from which theEffluent Samples Used in this Study were Generated 65TABLE 6.1: Hydrate Former Characteristics for Propane and CarbonDioxide 93TABLE 6.2: Moles of Propane Gas Required and Available to Convert100% of an Effluent Sample to Hydrates 96TABLE 6.3: Propane Hydrate Formation Pressure-Temperature Data . 102TABLE 6.4: Carbon Dioxide Hydrate Formation Pressure-TemperatureData 106TABLE 6.5: Propane-Carbon Dioxide Mixture Hydrate FormationPressure-Temperature Data 108TABLE 6.6: Conductivity, TOC and TIC of CTMP Effluents DeterminedDuring In-Situ Concentration Experiments 120viiLIST OF FIGURESFIGURE 1.1: Zero-Liquid Discharge Mill 4FIGURE 2.1: Simplified Flow Diagram for TMP and CTMP MarketPulp 9FIGURE 4.1: Freeze Concentration Process used in the Desalination ofSeawater 46FIGURE 4.2: Cavities and Unit Cells for Clathrate Hydrate Structures . 56FIGURE 4.3: Clathrate Hydrate Concentration Process used in theConcentration of Mechanical Pulp Mill Effluent 58FIGURE 4.4: Partial Phase Equilibrium Diagram 62FIGURE 5.1: Apparatus used for Clathrate Hydrate FormationExperiments 69FIGURE 5.2: Low Pressure Vessel 70FIGURE 5.3: High Pressure Vessel 72FIGURE 5.4: Schematic Identifying the Driving Force of HydrateFormation 76FIGURE 6.1: Conductivity and Carbon Content of the EffluentSamples 83FIGURE 6.2: Solids Content of the Effluent Samples 84FIGURE 6.3: Conductivity and Carbon Content ofthe TMP2 ConcentrateSamples 87FIGURE 6.4: Solids Content of the TMP2 Concentrate Samples 88FIGURE 6.5: Conductivity and Carbon Content of the CTMPConcentrate Samples 89FIGURE 6.6: Solids Content of the CTMP Concentrate Samples 90FIGURE 6.7: Pressure-Temperature Hydrate Formation Data for theEffluent Samples with Propane 100vi”FIGURE 6.8: Pressure-TemperatureHydrate Formation Data for theCTMP Concentrates with Propane101FIGURE 6.9: Pressure-Temperature HydrateFormation Data for theTMP2 and the CTMP Effluents, and DeionizedWater withCarbon Dioxide105FIGURE 6.10: Pressure-Temperature HydrateFormation Data for theCTMP Effluent, theCTMP/80 Concentrate and DeionizedWater with a Propane-Carbon Dioxide Mixture107FIGURE 6.11: Induction Period and theFirst Growth Rate Period for Run1, 2, 3 and 4 with the CTMP Effluent111FIGURE 6.12: Induction Period, and the Firstand Second Growth RatePeriods for Run 1 with the CTMPEffluent 113FIGURE 6.13: Induction Period, and theFirst and Second Growth RatePeriods for Run 2 with the CTMP Effluent114FIGURE 6.14: Induction Period, and theFirst and Second Growth RatePeriods for Run 3 with the CTMPEffluent 115FIGURE 6.15: Induction Period, and theFirst and Second Growth RatePeriods for Run 4 with the CTMP Effluent116FIGURE 6.16: Ideal In-Situ Concentration118ixACKNOWLEDGEMENTSI would like to express my sincere appreciation to the followingpersons andorganizations:Prof. P. Englezos for his patience and all his supportduring the past two years @ !!!Prof. S. Duff for his input on the effluent survey portionof this thesis.The Natural Sciences and Engineering Research Councilof Canada (NSERC) for theirfinancial support.PAPRICAN for their financial support through the 1992PAPRICAN Merit Award.MacMillan Bloedel for their financial support through the 1993MacMillan BloedelFellowship Award.Fletcher Challenge, Howe Sound Pulp and Paper,Quesnel River Pulp, and LouisianaPacific Canada Ltd. for happily donating effluentsamples.The staff in the Chemical Engineering Department and the Pulp andPaper Centrefor all their help.Rob S. and the Civil Engineering Lab technicians for their adviceand help with theanalytical procedures.YeeTak N. for his help with the initial experiments.Prof. C. Oloman’s “bleaching group”, the CFB group, andin particular RobertFreundlich and Assoc. Ltd for allowing me to use their computers/printers.My office mates Ian H., Susan N., Y-S. Perng, and RoryT. for all of the philosophicaldiscussions held in our office, with a special thankyouto Ian H. for proof reading myinitial draft.Mike P., Ian G., and Ian W. for the Christmas partyAnd last but definitely not least, my parents, Eva and Per, for EVERYTHINGI would like to dedicate this effort in memory of Anthony Carter.1CHAPTER 1: IntroductionThe 1982 report, “Our Common Future”, written by the World Commission onEnvironment and Development clearly stated the need for the development of asustainable environment, where this is defined as development that meets the needsof the present without compromising the ability of the future. This concept, accordingto Wrist (1993), has four broad principles in relation to the forest products sector: (1)Forestry practices must impart minimal impact on the ecosystem and assure asustainable forest yield; (2) Within each major forest ecosystem a certain percentageof forest should be set aside as a preserve to protect biodiversity and maintain arepresentative pool of genetic material;(3) Manufacturing operations must beconducted so as to minimize the impact on the surrounding environment, with thelong term objective to close as many of the systems as possible; and (4) The industrymust assume a greater responsibility for the disposition of its products after use, e.g.recycling of waste paper products. The third principle, the integration of closed-cyclesystems into pulp and paper mill operations, is relevant to the focus of this work.Pulp production processes are classified as chemical, mechanical, or hybridaccording to the technology that they utilize and the pulp yield they produce.Chemical processes (kraft and low-yield suiphite) have the lowest yield at 40 to 55%,mechanical (pressurized groundwood, PGW, and thermo-mechanical, TMP) have thehighest at 85 to 95%, and hybrid processes have a 55 to 85% yield (Smook, 1992). Thechemicals that are added to the digesters in the chemical processes aid in thedegradation of the nonfibrous material in wood. The energy that is added to the2refiners in the mechanical processes aid in defibrating the wood resulting in fibresthat are literally torn apart. Hybrid processes utilize both chemicals and mechanicalenergy to produce pulp, e.g. high-yield suiphite (HYS) and chemi-thermo-mechanical(CTMP) mills (CTMP mills may also be classified as mechanical mills because of theirhigh yields, between 80 to 90%). A rapid expansion in the utilization of high-yieldmechanical and chemi-mechanical processes has occurred during the past severalyears (Leask, 1990). This change has partly resulted from the replacement of low-yield sulphite mills by high-yield mills in order to reduce the discharge ofbiochemically oxygen demanding substances into the environment.The pulp and paper industry requires a large amount of water to run its unitoperations and, as a result, generates a substantial volume of dilute aqueous effluent.According to Statistics Canada (1991) the paper and allied products industrydischarges 30% of the total process water (calculated as water intake less waterconsumption) generated by industrial operations in Canada, including both themanufacturing and the mining sector. In modern mechanical mills approximately 90%of the organics and acutely lethal materials are removed during pulp washing.Therefore, the majority of the water pollution generated by the industry results fromthe pulp rather than the paper division(McCubbin, 1992). The liquid effluent streamsthat are typically generated by these mills are dilute aqueous solutions contaminatedwith organic and inorganic matter which is dissolved and/or suspended. The mostcommon pollution parameters used to control the discharge of these effluents includetotal suspended solids (TSS), toxicity, and biochemical oxygen demand (BOD). Ingeneral, the TSS content results from fibrous matter, the toxicity is mainly attributedto naturally occurring extractive compounds that leach out of the wood during the3pulping process, and the BOD load is generated from both fibrous matter andextractives. Until recently, limited information has been available on thecharacteristics and environmental impacts ofmechanical pulp mill effluents (McLeay,1987).The conventional end-of-pipe treatment systems used to decrease the pollutionload rely predominantly on sedimentation and biological degradation of thecontaminants. These systems are capable of complying with the current dischargelimits, but they may prove to be inadequate if future regulations become increasinglystringent, as is expected (Beaudoin et al. 1991; Colodey,1993). Therefore, new andinnovative zero-liquid discharge(ZLD), or closed-cycle systems are emerging. TheseZLD systems are capable of recovering clean process water from the effluent allowingwater to be recycled back into the mill, refer to Figure 1.1. Without a purificationstep, these effluents can not be reused because dissolved solids will accumulatewithin the unit operations and compromise both production and quality. Successfulimplementation of a ZLD system requires a two-fold approach. Firstly, watermanagement strategies that increase the recycle of internal effluent streams, andminimize the volume of effluent generated must be incorporated. And, in parallelwith these internal measures, a separation technology(s) which is capable ofremovingdissolved solids from the effluent is required. The main “environmental” advantageof a ZLD system is the elimination of any potential negative impacts that might occuras a result of continuously discharging an aqueous effluent into the localenvironment.Separation technologies that have the potential to be used in a ZLD systeminclude evaporation, membrane separation, and crystallization. These technologies4FIGURE 1.1: Zero-Liquid Discharge Mill?iisMWEffluentProcessIWaterConcentrate5are currently at different stages of development for implementation in the pulp andpaper industry. There are two industrial scale ZLD systems operating in Canada.Both operate at bleached CTMP (BCTMP) mills and utilize evaporation as theirseparation technology(Fromson and Leslie, 1993; Arac, 1993). At present,evaporation is the only proven technology, however, both membrane separation andcrystallization are considered to be viable options for the future. The work presentedhere will focus on a crystallization technology, in particular the clathrate hydrateconcentration process.The process which concentrates an aqueous solution by generating ice crystalsfollowed by the physical separation and melting of the resulting crystals, is referredto as freeze concentration (Heist,1979). The temperature at which liquid water formsice will depend on the concentration and nature of the other compounds that arepresent in the solution. Clathrate hydrate concentration, a variant of freezeconcentration, generates clathrate hydrates. These hydrates are ice-like crystallinenon-stoichiometric compounds. They are formed from a mixture of water and at leastone low molecular weight substance. There are over one hundred known compoundswhich can form these crystals including argon, dichiorofluoromethane(R-21), carbondioxide, methane, and propane. Hydrate crystals can form at temperatures severaldegrees above the normal freezing point of water, thereby decreasing the energyrequirements compared with the freeze concentration process. Experimental evidenceto date has shown that impurities are not included within ice or hydrate crystalstructures. This is the basis of crystallization as an effluent concentration process.The primary goal of this work is to experimentally investigate the formationof clathrate hydrates in effluents generated at TMP and CTMP mills to provide6technical data for the subsequent pilot plant and commercial development of aclathrate hydrate concentration process. A secondary goal is to perform a survey onthe liquid effluents generated at TMP and CTMP mills.The specific objectives of this work are to: (1) Determine the type,concentration, source, and environmental impacts associated with the contaminantstypically present in TMP and CTMP effluents, present the current dischargeregulations, and compare the available treatment options; (2) Outline an effluenttreatment selection procedure for TMP and CTMP mills based on the informationcollected during the survey; (3) Determine the characteristics of interest for thespecific mill effluent samples used in this work; (4) Select suitable clathrate hydrateforming substances to be used in the experiments; (5) Qualitatively observe theclathrate hydrate crystal formation process; (6) Determine the minimum pressure, ata given temperature, that hydrate crystals will form; (7) Study the induction periodand the growth rate of hydrate formation; and (8) Study the potential for in-situconcentration of the effluents.In Chapters 2 and 3 a survey on the typical characteristics and environmentalimpacts of TMP and CTMP effluents, the current Federal effluent dischargeregulations, and the available treatment options will be presented. Freezeconcentration and clathrate hydrate concentration will be discussed in Chapter 4. Theexperimental apparatus, procedures and materials will be described in Chapter 5.The results and discussion will be presented in Chapter 6, and the conclusions andrecommendations will be summarized in Chapter 7.7CHAPTER 2: Mechanical Pulp Mill Effluent I:Source of Pollutants and CharacteristicsThe first three sections in this chapter present background informationrequired to determine the characteristics of effluents generated at TMP and CTMPmills. The first section will outline the major unit operations and chemicals that areused in the TMP and CTMP processes, and the second will give a generalintroduction into the composition of wood, the raw material. The third section willdefine the common water pollution parameters in which contaminants are groupedbased on their potential environmental impacts (these parameters will be usedthroughout this work). The fourth section will discuss the type of contaminants thatoriginate from the raw material, the point source from which individual effluentstreams originate throughout the processes, and the effect that certain pulpingpractices have on the concentration of the contaminants. And the fifth section willsummarize the typical characteristics of untreated TMP and CTMP effluents.2.1 Pulp ProductionPulp mills receive wood chips from local sawmills and/or whole logs directlyfrom logging companies. This wood may be processed within a few days or stored forseveral months (sometimes longer) prior to use. The whole logs that are receivedmust be debarked and chipped before being pulped in a TMP or CTMP mill.2.1.1 DebarkingThere are two types of debarking equipment those that use water, such as wet8drums and hydraulic jets, and those that do not, for example, dry drums andmechanical debarkers. The wet debarkers produce a “wet” waste which must bedewatered before it can be used as fuel by on-site hog fuel burners. The drydebarkers, on the other hand, produce a dry waste which can be sent directly to thehog fuel burners.2.1.2 TMP and CTMP ProcessesFigure 2.1 shows a simplified flow diagram of the TMP and CTMP processes.The first step in a TMP process is to wash the wood chips in washers which removegrit, sand, and tramp metal based on the differences in their specific gravity and size.The clean chips are placed in a presteaming vessel in which they are softened bypressurized steam. This thermal pretreatment results in a higher strength pulp bysoftening the lignin to reduce the damage of the long fibre component duringmechanical defibration. The chips are defibrated, torn apart to produce a pulp, in one,or more, consecutive disc refiners in which the chips pass between two serratedplates, one, or both, of which are rotating. These refiners operate under pressure attemperatures over 100 °C (McCubbin,1992). The pulp that is generated is screenedto remove the oversized material with coarse screens (which remove knots, slivers,and unground pieces of wood) and fine screens (which remove fibre bundles and othersmall particles that are up to four times larger than the average fibre length). Thepulp which has a consistency around 0.5% is then dewatered to a consistency between3 to 14% depending on its end-use (McCubbin, 1983).The CTMP process is similar to the TMP process, but the former impregnatesthe wood chips, on a dry wood basis, with a 1 to 5% sodium suiphite (Na2SO3)cc CE IIiiiBLEACHINGiiPRESSiiiDRYERSfWastewaterPulpIChips10solution (Smook, 1992). The impregnation occurs in the presteaming vessel, i.e. priorto preheating and refining. The addition of sulphite increases the quality of the pulpas compared to pulp generated by the TMP process, but it also decreases the pulpyield by 5 to 10%. Pulp quality is enhanced because sulphite decreases thetemperature at which lignin softens thereby aiding in fibre separation and increasingthe long fibre fraction. A further removal of extractives in the pulp can beaccomplished by the addition of surface active agents which improve water absorptionproperties and pulp brightness.2.1.3 BleachingThere are two approaches in the chemical bleaching of pulp. The first is toremove the lignin, and the second is to selectively destroy some of the chromophoricgroups in the pulp. The latter approach, referred to as brightening, is used with high-yield mechanical pulps, i.e. TMP and CTMP. The brightening of these puips isperformed by the addition of a reducing agent, an oxidizing agent, or a combinationof the two. The most common commercially used brightening chemicals are sodiumhydrosulphite (Na2SO4)as a reducing agent, and hydrogen peroxide (H20)as anoxidizing agent (Smook, 1992). Alkali and chelating agents are added to peroxidebrightening units (McCubbin, 1992). The alkali controls the pH and the chelatingagent binds to the metal anions present in the pulp that accelerate the catalyticdecomposition of peroxide. The most commonly used chelating agent is diethylenetriamine-pentaacetic acid (DTPA).112.2 Wood ChemistryThe wood species primarily used include the softwoods pine, spruce, fir andhemlock, and the hardwoods birch, aspen, poplar, and maple(McCubbin, 1983).Wood, on a dry weight basis, contains about 60 to 80% polysaccharides (cellulose andhemicellulose), 20 to 30% lignin, and 1 to 5% extractives. Cellulose accounts for about45% of the wood on a dry weight basis and is a strong, alkali-insoluble carbohydrate.Lignin compounds are hydrophobic, acid-insoluble aromatics located mainly betweenthe wood fibres. They give a tree its structural rigidity.The extractives include a large number of neutral solvent-soluble extracelluarchemical compounds. The extractive content, i.e. the composition and concentration,varies from species to species and may therefore be used as a fingerprintfor thedifferent species. Extractive compounds can be divided into four classes (Sjöström,1993). These include: (1) Terpenoids and steroids, e.g. resin acids, juvabionecompounds, sterols and diterpene alcohols;(2) Fats and waxes, e.g. fatty acids; (3)Phenolic constituents, e.g. lignans; and(4) Inorganics, e.g. metal salts. Extractivesare present at specific sites in a trees structure (Sjöström, 1993). For example, resinacids are located in resin canals and fats and waxes are encapsulated in parenchymacells. The extractives that are present in resin canals are more easily dispersedduring pulping than those found in the parenchyma cells. As a result, wood specieslike Scots pine with 70% of its extractive content present in resin canals will, ingeneral, produce a more toxic effluent than Norway spruce which has only 45% of itsextractive content present in resin acid canals (Sjoström, 1993).122.3 Effluent Discharge ParametersBecause there is a large number of different contaminants present in the liquideffluent the contaminants are grouped according to their environmental impacts. Thewater pollution categories that are of prime concern to the pulp and paper industryinclude solids concentration, oxygen demand, and toxicity. The effluent parametersused to regulate these pollution categories include: (1) total suspended solids (TSS);(2) biochemical oxygen demand (BOD);(3) acute and sub-lethal toxicity tests; (4)adsorbable organic halogens (AOX); and (5) dioxin and furan concentrations. The lasttwo parameters, which include the organochioride compounds, will not be addressedhere because, in general, TMP and CTMP mills do not practice chlorine bleaching.2.3.1 SolidsSolids can be categorized as dissolved or suspended particles. The latter canbe further categorized as either settleable or non-settleable. Non-settleable suspendedsolids are usually colloidal or near colloidal in size and can not be removed bysedimentation whereas the settleable portion can (Smook, 1992). The dissolved solidscontent of an effluent is, in general, not regulated whereas the suspended solidscontent is. The concentration of total suspended solids (TSS) in an effluent isdetermined gravimetrically. The TSS load is defined as the total amount of suspendedsolids discharged per amount of pulp produced.2.3.2 Oxygen DemandA large number of organisms that live in aquatic environments depend on acertain level of dissolved oxygen (DO) in the water for their survival. The actual level13required by an organism is very species dependent, e.g. the minimum DO levelrequired by trout is 4 mg!L (Peavy et al. 1985). The DO level in a natural body ofwater depends on the temperature and the salinity of the water, the rate at whichoxygen is replenished (i.e. by surface aeration, aerated tributaries, and photosyntheticoxygen production by aquatic plants and algae), and the demand for oxygen by theoxygen consuming aquatic organisms. The DO level of natural waters at 20 °C is onaverage 9 mg!L (Smook, 1992).Aerobic microorganisms, whose population is controlled by the availability ofnutrients, can be found in almost any natural body of water. These organismsconsume oxygen as they metabolize the available organic matter. An increase in theavailability of organic matter will increase the population of these organisms andresult in a decrease in the DO level. The increased oxygen demand placed on a bodyofwater by the introduction of an external source ofbiodegradable organic matter canbe determined by a BOD test. The BOD of an effluent stream is determined byplacing a sample of the effluent and a culture of microorganisms in a nutrientsolution for a specified amount of time, after which the DO level is determined. Thetime allotted for oxidation to occur is five days in North America and seven days inEurope. The BOD load, like the TSS load, is defined as the total amount of BODdischarged per amount of pulp produced.2.3.3 ToxicityThe toxicity of a liquid effluent stream can be determined by measuring itsability to kill (lethal or acute toxicity) or interfere with the well being (sub-lethal orchronic toxicity) of an organism by some means other than oxygen deprivation. There14are a large number of different bioassays that can be performed to determine thetoxicity of an effluent.The acute toxicity of an effluent can be determined by the median lethalconcentration (LC) test. The LC5O test measures the effluent concentration that isrequired to kill 50% of a specified test species within a given period of time. Twocommonly used test species are rainbow trout and Daphnia magna, the correspondingexposure periods are 96- and 48-hours respectively.Another commonly used acute toxicity bioassay is the Microtox test whichdetermines the inhibition concentration (IC) value(also referred to as the effectiveconcentration (EC) value). This test measures light emitted by the bacteriumPhotobacterium phosphoreum before and after exposure to increasing concentrationsof an effluent sample. The logarithm of the loss of light caused by the toxicity of aneffluent sample is theoretically proportional to the logarithm of the concentration ofthat effluent sample. The 1C50 (or EC5O) test measures the toxicity of an effluentsample by determining the concentration at which half the light, 50%, of a standardamount of glowing reagent is lost.It should be noted that the toxicity of an effluent increases as the LC or the ICvalue decreases, i.e. an effluent with an LC5O of 50% is more toxic than one with anLC5O of 80%. These acute toxicity tests are known to be ecologically unrealisticbecause they use short exposure times and are performed with high effluentconcentrations, up to 100%. The use of high effluent concentrations does not allow forthe mixing and dilution capabilities of a receiving water. Therefore, acute toxicitytests should only be used to rank the relative toxicity of an effluent compared withother effluents.15The results obtained from sub-lethal toxicity tests, which use more realisticeffluent dilutions and longer exposure periods, more closely reflect what is actuallyoccurring in the environment. The sub-lethal toxicity level of an effluent is moredifficult to determine than the acute toxicity level because sub-lethal toxic impactsmay be manifested in several different ways(as will be described in Section 3.2.1).There are a large number of different bioassays that are used to determinespecific sub-lethal toxic impacts. These include: (1) Body burdens resulting from theexposure of fish to compounds such as resin acids, fatty acids, and cholesterol can bedetermined by analysing the concentration of these compounds in fish bile. Theseresults can then be used to measure the bioconcentration factor (BCF), i.e. comparingthe concentration of resin acids in the bile with the concentration of resin acids in theeffluent; (2) The impact that an effluent has on the growth and reproduction of aspecific species can be determined from the inhibition concentration percentage (ICp)bioassay. This test calculates the concentration of an effluent that would cause agiven percent reduction in growth and reproduction for a specified organism relativeto a control group; and(3) The stress associated with exposure of an organism to aneffluent can be determined by studying changes that occur within the biochemistryof an organism (biomarkers), e.g. the ability of an effluent to induce mixed functionoxidase (MFO) enzyme activity in the liver of a fish. A change in MFO activity canbe measured by determining the MFO activity of a group of fish exposed to aneffluent and comparing this value to the MFO activity level of a control group.2.3.4 Other ParametersOther parameters that may be regulated on a mill to mill basis include i) the16pH, colour, temperature, and nutrient content of the discharged effluent; ii) thedischarge of fish tainting compounds such as phenols, and low boiling hydrocarbons,resin acids, reduced sulphur compounds, aldehydes and ketones; iii) reduced nitrogenor sulphur compounds such as mercaptans which may result in an unacceptable tasteand odour of the receiving water; and iv) the discharge of coliforms into waters thatare classified with a bacteriological water quality standard, i.e., public beaches.2.4 Source of PollutantsThe pollutant load generated by a mill will depend on the wood furnish, thespecific process, the operating conditions, and the rate of water consumption. Aqueouseffluent streams are generated throughout the mechanical pulping process (refer toFigure 2.1). The potential sources include wet debarkers, chip and pulp washers,screens, presses (plug screw feeders), brightening units, and hydrocyclone condensate(Suckling et al. 1993). The majority of pollutants in these streams, i.e., compoundscontributing to the suspended solids, organic, and toxicity loads, originate from theraw material, the wood.2.4.1 Raw MaterialThe compounds of most significance from a pollution prevention point of vieware the extractive compounds. The extractive content in an effluent will vary with thewood furnish used, the wood storage conditions, and variations within a particularspecies, i.e. tree-to-tree, seasonal and geographical variations, the age of the tree, andthe part of the tree that is being considered. The highest extractive content of a treeis in the bark. This is one reason that dry debarkers are more popular than wet17debarkers (Swan, 1973).The extractive compounds of most concern include resin acids, unsaturatedfatty acids, sterols, diterpene alcohols, lignans and juvabione compounds (Carlberget al. 1993; Walden and Howard, 1981; Wong et al. 1978; 1980; Swan,1973). Up to70% of the toxicity in mechanical pulp mill effluents have been attributed to resinacids (Environment Ontario, 1988). Juvabione compounds which have only beendetected in fir species contain a juvenile hormone analog and are therefore similarin nature to insecticides (O’Connor et al. 1992). Another juvenile hormone analog ofinterest is lignan which has been isolated in hemlock (O’Connor et al.1992).Wong et al. (1978) determined that effluent produced from softwoods was moretoxic than that produced from hardwoods, and that pine produces a more toxiceffluent than a spruce/balsam furnish. The reasons being that hardwoods do notcontain resin acids, and pine has a higher resin acid concentration than spruce(Swan, 1973). Walden and Howard (1981) reviewed a large number of studies anddetermined that resin acids accounted for the majority of the toxicity in mechanicalpulp mill effluents, diterpene alcohols were intermediate contributors and theunsaturated fatty acids and juvabione compounds were minor contributors (refer toTable 2.1). Suckling et al. (1993) also found that resin acids appeared to be asignificant contributor to the acute toxicity of simulated BCTMP effluent streamswith the softwood radiata pine as the wood furnish.O’Connor et al. (1992) determined that the acute lethal toxicity of simulatedmechanical pulp mill effluents to both fathead minnows and Ceriodaphnia follow theorder: white pine > balsam fir > hemlock> black spruce> aspen. The order for thechronic toxicity of these effluents to Ceriodaphnia was as follows: balsam fir>18TABLE 2.1: Compounds in Mechanical Pulp Mill Effluents that areToxic to Fish (Walden and Howard, 1981; Thakore andCollins, 1989)Chemical Compound ContributionResin Acids Major: M (SW)Abietic, dehydroabietic, isopimaric, laevopimaric, Major: Dpalustric, sandaracopimaric, neoabietic, 7, 15-isopimaric,_8, 15-isopimaricUnsaturated fatty acids Minor: M (SW,HW)Oleic, linoleic, linolenic, palmitoleic Minor: DDiterpene alcohols Intermediate:Pimarol, isopimarol, dehydroabietal, abietal M (SW)Minor: DJuvabiones Minor: M (HW)Juvabione, juvabiol,A1’-dehydrojuvabione,s”dehydrojuvabiol, dihydrojuvabione todomatuicacid,_3’-deoxy,3’ -hydro-todomatuicOther neutrals Minor: DAbienol, 12 E-abienol, 13-epimanool, 3’,4-divanillyltetrahydrofuranM: refers to mechanical pulpingD: refers to debarkingSW: softwoodHW: hardwoodhemlock> white pine> — black spruce — aspen. The compound primarily responsiblefor the low threshold of Ceriodaphnia, to balsam fir, was dehydrajuvabione. Abreakdown of the important extractives found in the wood species used in thesesimulated effluents can be found in Table 2.2. The age and storage conditions of thewood furnish will affect the toxicity of an effluent, but not the TSS or BOD load(Wong et al. 1978; 1980). It is known that if wood is stored for longer periods of timethe resin and fatty acid compounds will degrade. The extent of degradation will also19TABLE 2.2: Extractive Content of Several Different Wood Species(O’Connor, 1992)Wood Species Total Resin Acids Free FattyAcids*Neutral(mg/L) (mg!L)Compounds**(mg/L)White Pine (SW) 18.2 - 22.5 2.6 - 6.7 NDBlack Spruce 4.4 - 6.8 0.4 - 0.7 ND(SW)Balsam Fir (SW) 0.1 - 0.85 ND 15.5Aspen (11W) 0.7 0.7 NDHemlock (SW) --- --- ND*Subtotal including linoleic and oleic acids.**Subtotal including juvabione and dehydrajuvabione.SW: softwood11W: hardwoodND: Not Detecteddepend on the storage temperature, and whether the wood is stored as chips or logs(Swan, 1973; Leach and Thakore, 1976; Wong et al. 1978; 1980). Resin and fatty acidscan undergo isomerization, oxidation,and/or polymerization reactions if stored for along enough time period. These changes in chemical make-up may result in adecrease in the toxicity of effluents generated with these denatured wood sources(Swan, 1973).2.4.2 Point SourceThe origin of individual effluent streams in the TMP and CTMP processes isshown schematically in Figure 2.1.TSS. Suspended solids can originate in several of the TMP and CTMP unit20operations. Bark, trace metals and sand particles are found in the effluent streamsgenerated by chip washers and wet debarkers. Fibrous matter, residual barkparticles, and grit (produced by the rotatingdisks) are introduced as part of theeffluent produced by pulp washers and dewatering units. Wong et al. (1978;1980)found that the majority of the TSS originating in the wood during the TMP processwas released in pulp washers situated after the first of two consecutive refiners. Thelower pulp yield, i.e. fibre loss, that results from the addition of sodium sulphite ina CTMP process increases the TSS load of these effluents as compared to TMPeffluents. Suckling et al. (1993) found that in a simulated BCTMP process the TSSloads of the effluent generated by the brightening units was higher than thatgenerated in the refiners and released in the pulp washers.BOD. The effluent streams with the highest BOD loads in both the TMP andthe CTMP processes are generated in the wet debarkers, the refiners, and thebrightening units (Wong et al. 1978; 1980; Suckling et al.1993). The effluentproduced by a wet debarker will increase the BOD load due to the high extractivecontent associated with tree bark (McCubbin,1983), as will any residual bark thatmay be present after dry debarking. Stenberg and Norberg (1977) found that woodchips with a bark content of a few percent could increase the BOD load by 3 to 4kgftonne of pulp. The major source of BOD generated at an unbleached TMP mill willresult from the soluble wood extractives. The other wood components, i.e. cellulose,hemicellulose and lignin, do not add to the BOD of TMP effluents because in theirnatural form they can not be readily utilized by microorganisms within the 5-day testperiod. However, the addition of sodium suiphite in the CTMP process aids in thesolubilization of these components resulting in oligosaccharides, simple sugars, low21molecular weight lignin derivatives, and hemi-cellulosic sub stituent groups, e.g. aceticacid. These derivative compounds, which can be readily metabolized bymicroorganisms, result in an increased BOD load in CTMP effluent as compared withTMP effluent. The BOD load generated by a mill is also affected by the use ofoxidizing agents. Malinen et al. (1985) found that the addition of peroxide increasedthe BOD load of CTMP effluents by up to 40%.TOXICITY. The toxicity of an effluent depends on the amount of solubilizedextractive compounds, the metal content, and the presence of residual chemicals. Theextractive compounds can be released in the effluent generated by wet debarkers orin the form of residual bark after dry debarking(if present), in the pulp washerssituated after the refining operation, and in the effluent generated during brightening(if present). The addition of sodium sulphite and/or oxidizing agents will increase thetoxicity of an effluent because these compounds increase the solubility of theextractives in water. As a result, mills that brighten their pulp with peroxides willgenerate more toxic effluents than those that do not, and CTMP effluent will have ahigher toxicity value than TMP effluent.Metals are known to be toxic. The metals that may found in mechanical pulpmill effluents can originate as trace elements introduced with the wood, e.g. copper,zinc, mercury and cadmium, or associated dirt, as impurities in the pulpingchemicals, or from pipe and vessel walls. The presence of metals in these effluentsdoes not result in a significant increase in the overall toxicity because metals requirean acidic environment to be readily solubilized and both the TMP and CTMPprocesses operate under neutral to alkaline conditions.The presence of residual amounts of process chemicals, i.e. brightening agents,22chelating agents, surface agents, and cooking chemicals, may also contribute to theoverall toxicity of an effluent. The actual fate of these chemicals in the environmenthas not been well documented.Wong et al. (1978; 1980) found that the majority of the toxic compoundsoriginating from the wood during the TMP process were released in pulp washerssituated after the first of two consecutive refiners.Suckling et al. (1993) studied individual effluent streams from a simulatedBCTMP process with the softwood radiata pine as the wood furnish. The streamsincluded chip washer effluent, plug screw feeder pressate (psfp)prior to chipimpregnation, psfp prior to primary refining, pulp psfp prior to secondary refining,pulp washer effluent, filtrate following DTPA treatment, and filtrate followingperoxide brightening and washing. They found that effluent stream toxicitydecreasedas follows: psfp prior to chip impregnation> psfp prior to primary refining> filtratefollowing peroxide brightening and washing> pulp psfp prior to secondary refining> pulp washer effluent> chip washer effluent> filtrate following DTPA treatment.A relationship was found between the toxicity of these effluent streams and theirresin acid concentration, with the exception of the brightening plant filtrate whichhad a higher toxicity than predicted. This increased toxicity could be a result of theresidual peroxide remaining in the effluent. It should be noted that the presence ofDTPA did not seem to have any significant affect on effluent toxicity.2.4.3 Other FactorsThe process operating conditions and the rate of water consumption will bothaffect the pollutant load of an effluent generated at a TMP and a CTMP mill. Wong23et al. (1978; 1980) found that the BUD load increased slightly with higher steamingpressures as a result of increased yield loss, but that this did not have an appreciableeffect on toxicity, and that neither toxicity nor BUD were affected by small changesin pretreatment times. However, they postulated that with significant increases inpretreatment times the BUD would increase. Several researchers have found that theBUD ofTMP effluents increased significantly with operating temperatures above 120°C (Idner and Norberg, 1976; Stenberg and Norberg, 1977; Järvinen et al. 1980). Asimilar result was found for CTMP effluents by Malinen et al. (1985) who determinedthat BUD increased with higher pretreatment temperatures but was not significantlyaffected by an increase in pretreatment time. Increasing the energy input during therefining stage of a TMP process has had varying results. Idner and Norberg (1976)found that the BUD increased, whereas Stenberg and Norberg (1977) could not findany correlation, and Wong et al. (1978) determined that neither an increase in energyinput or a redistribution of this energy between the first and second stage refinersaffected the toxicity.The toxicity of an effluent is also dependent on the pH of the effluent becausepH can influence the extent of ionization of toxic compounds, e.g. resin and fattyacids. The pH range that an effluent containing resin and fatty acids will be the leasttoxic is between 8 and 9.5, with the toxicity increasing with decreasing pH (Waldenand Howard, 1981).The typical water consumption rate for TMP and CTMP processes ranges from16 to 150 m3/UDt of pulp (Novatec, 1987). However some of the newer mills are nowapproaching a rate of 10 m3/Adt (Cornacchio and Hall, 1988). The volume of waterused by a mill will affect the toxicity but not the TSS or BUD load of an effluent. The24reason being that toxicity is regulated based on concentration.2.5 Summary of Effluent CharacteristicsThe effluent characteristics of interest include the BOD, TSS, toxicity, pH,temperature, resin acid concentration, fatty acid concentration and flowrate. Typicalvalues of these characteristics for untreated TMP and CTMP mills are given in Table2.3.TABLE 2.3: Untreated TMP and CTMP Pulp Mill EffluentCharacteristics (Cornacchio and Hall, 1988; Habets et al.1991; Kovacs et al. 1992)Characteristics TMP CTMPBOD5 (kg/ADt) 12 to 44 21 to 59TSS (kg/ADt) 20 to 40 3 to 1796 hr-LC50 Acute 1.3 to 35.3% 0.8 to 1.8%Toxicity (v/v)pH 5to9 7to9Temperature (C) 35 to 40 35 to 70Total Resin Acids 2 to 21 26 to 559(mg/L)Total Fatty Acids 0.3 to 6.4 55 to 69.5(mg/L)Flow Rates (m3/ADt) 10 to 150 20 to 30Summary. The majority of the effluents generated by TMP and CTMP millswill contain suspended solids (colloidal and particulate matter), extractivecompounds, lignin, cellulose and hemicellulose fragments, some low molecular weight25compounds, metals and some residual chemicals. Of these compounds the extractivesare of most concern from a pollution prevention point of view.The type and concentration of contaminants that will be present in a particulareffluent will primarily depend on the following:(1) The wood furnish, i.e. softwoodsvs. hardwoods; and (2) The process, i.e. the presence of a debarker (wet vs. dry), theaddition of pulping chemicals(TMP vs. CTMP), and the use of brightening agents(brightened vs. unbrightened pulp). The TSS, BOD and toxicity of an effluent dependon the following: (1) The type and concentration of contaminants that are present;(2)The operating conditions, i.e. temperature, residence time, steaming pressure, energyinput, and pH; and (3) rate of water consumption. In general, the compounds that areprimarily responsible for the TSS of an effluent result from the fibrous matter, thecompounds responsible for an effluents toxicity result from the extractives, and thecompounds responsible for the BOD are generated from both the fibrous matter andthe extractive compounds.26CHAPTER 3: Mechanical Pulp Mill Effluent II:Impacts of Pollutants, Discharge Regulations,and Treatment OptionsThe first section in this chapter will outline the important site-specificreceiving environment data that are required to be able to determine the potentialfor specific impacts to occur at a particular site. The second section will describe thepotential impacts associated with discharging effluents that have high TSS and BODloads and/or are toxic, summarize the results of several short-term studies that havebeen performed with untreated TMP and CTMP mill effluents, and discuss the needfor long-term, in-depth studies. The third section will present the current Federaleffluent regulations and monitoring program requirements. And the fourth and fifthsections will describe and compare the conventional and zero-liquid dischargetreatment technologies.3.1 Receiving EnvironmentTo determine the impacts that result from discharging an effluent into aparticular ecosystem a thorough understanding of the following is required (Owens,1991): (1) The chemistry and variability of the process so as to determine the effluentcharacteristics (refer to Chapter 2); (2) The history of the site and the upstreamsources of pollutants so as to determine all the types of contaminants that arepresent; (4) The sensitivity of the local ecosystem to enrichment and toxicants byperforming lab and field studies; and (5) The chemistry, available dilution, andturnover of the receiving waters.The location of a pulp mill will determine the water quality of the receiving27waters, i.e. river, tributary, lake, or ocean, into which it discharges its effluent. Areceiving water can be characterized by its pH, buffering capacity, temperature,colour, dissolved oxygen level, toxicity, taste, odour, nutrient concentration,productivity (population of microorganisms), and dissolved, suspended and floatablesolids concentration (Smook, 1992; Beak, 1987). The characteristic of primary concernto the pulp and paper industry is pH because it can influence the extent of ionizationof toxic compounds commonly discharged in TMP and CTMP mill effluents (refer toSection 2.4.3). The pH of the dilution water that is specified in the protocol of atoxicity bioassay may not be the same as the pH of the receiving water. Therefore todetermine the “true” toxicity of an effluent the dilution water pH should be adjustedto that of the receiving water. The other receiving water characteristics that werementioned above may also affect the overall impact that an effluenthas on theenvironment. For example, a body of water with an unusually low dissolved oxygenlevel, e.g. 5 mg/b, will not be capable of assimilating the biologically degradablematter present in an effluent without having detrimental effectson the aquaticenvironment.3.2 ImpactsThere are a large number of potential impacts that may result if an effluentis discharged into the environment. The actual impacts will depend on thecharacteristics of the effluent, the extent of effluent treatment prior to discharge, andthe receiving ecosystem at a particular site.283.2.1 Potential ImpactsTSS. Suspended solids can interfere with the transmission of lightin areceiving water. This may decrease the photosynthetic activity ofalgae, and disturbthe predator-prey interactions of aquatic organisms that dependon vision. Thedischarge of suspended solids can also result in stress, secondaryinfections, and/orsuffocation in fish (if enough material gets trapped within theirgills). The settleableportion can cause problems if it forms a sludge blanket on the river,lake, or oceanbottom because this can cause the suffocation of bottom dwellingplants andcreatures, as well as interfere with the growth of aquatic plants(if the blanket is notconducive to good root attachment). In the case wherethe blanket becomes thickenough anaerobic conditions may develop andundesirable organisms such as sludgeworms may become prevalent. These anaerobic conditions could leadto the death ormigration of desirable life forms such as shellfish due to a lackof oxygen, and theproduction of toxic and/or foul smelling gases, e.g. reduced sulphurcompounds. Theproduction of a gas may cause the sludge mats to floatto the surface resulting in anunpleasant sight. The dilution of the non-settleableportion of the suspended solidsby the receiving water is, in general, sufficient to ensure that they willhave aminimal impact on the environment.BOD. The major impact resulting from the discharge of BOD into theenvironment is a decrease in the dissolved oxygen levelof a receiving water. In theextreme case, where the BOD load is very high, the level of dissolvedoxygen can bedepleted to such low levels that both motile fauna whichdo not leave the area andnonmotile fauna die. At lower BOD loads some aquatic organismsmay suffer stress29as a result of a lower than normal dissolved oxygen level. For example, a lowdissolved oxygen level may decrease an organisms growth rate.TOXICITY. The potential impact of discharging an acutely toxic effluent isdeath to those species for which the effluent is acutely toxic. The discharge of a sub-lethally toxic effluent may lead to a large number of potential impacts, refer to Table3.1. The extent to which a sub-lethal impact is manifested will depend on the typeand the concentration of the contaminants that are present. Sub-lethal toxic impactscan be revealed in one or more ways, within several different species, and at varyingorganizational levels. The organizational levels include communities, populations,whole organisms, and levels within an organism, i.e. the organ, tissue, cell,subcellular, and molecular levels.TABLE 3.1: Sub-Lethal Effects (Owens, 1991)Sub-lethal Effects1. Body burdens: resulting from exposure to toxiccompounds, these estimate the toxicant exposurewhich arises from direct bioaccumulation, food chainbiomagnification, environmental transformation (orinternal metabolism).2. Whole organism effects: including reproductive andearly life stage responses.3. Within organism effects on histopathology: includingpathological effects in either gross morphology(deformities) or histology (tissuestructure).4. Within organism effects on physiology andbiochemistry: due to stress, often referred to asbiomarkers.303.2.2 Short-Term Impact StudiesThe studies presented here have examined impacts associated with the toxiccompounds present in untreated effluent generated at TMP and CTMP mills. Specificstudies on the impacts resulting from the discharge of TSSand/or BOD loads will notbe discussed here because the conventional treatment systems, as described inSection 3.4.1, are capable of adequately decreasing both TSS and BOD.The majority of studies performed on the toxic impacts associated with pulpmill effluent have concentrated on the effluent that is generated at kraft mills(McLeay, 1987). However, in the past three years several short-term laboratorystudies have been performed with TMP and CTMP mill effluents. The bioassays thatwere used in the studies that are presented here were briefly described in Section2.3.3.Johnsen et al. (1993) exposed rainbow trout to several effluent dilutions (1:200,1:400, and 1:1000) to determine the extent of bioconcentration of extractivecompounds in the bile of these fish. It was found that the 1:200 effluent dilution wasacutely toxic to trout, and as a result could not be used to determine sub-lethal toxicimpacts. The fish exposed to the 1:400 dilution were placed in a reference water fora four-week recovery period after being exposed to the effluent for eight-weeks. Thebile of the fish exposed to the 1:400 and the 1:1000 dilution had high extractiveconcentrations, in particular resin acids. Based on these findings it was expected thatsignificant physiological responses would occur. However, this was not the case, asonly slight differences could be noted between the exposed fish after a 4-weekrecovery period, and the control group. It was found that the fish were capable ofdecreasing the concentration of resin acids in their bile during the recovery period31without any physiological impacts. These results imply that rainbow trout are ableto adapt to exposure conditions and subsequently recover.Martel et al. (1993) found that mechanical pulp mill effluent did not increasethe mixed function oxidase (MFO) activity in the liver of rainbow trout based on a 96-hour laboratory experiment. Suckling et al. (1993) determined from the Microtox testthat all the individual effluent streams studied during a simulated BCTMPexperiment were toxic. The ICSO values ranging from 0.012 to 21.5% v/v.Carlberg et al. (1993) determined the toxicity of effluents from three differentsources using the Microtox test(with the bacterium Photobacterium phosphoreum)and the LC5O test (with the water flea Daphnia magna). The mills, which all usedNorway spruce (Picea abies) as their wood source, included:(1) A combinedgroundwood/TMP mill in which the pulp was bleached with either hydrosulphite (Al)or peroxide (A2, A3); (2) A TMP mill with hydrosulphite bleaching (B); and (3) acombined high-yield sulphite/groundwood/TMP mill with hydrosuiphite bleaching(C).All three effluents were found to be toxic to the bacterium. Their 1C50 values rangedfrom 0.7 to 2.2%. The LC5O for the water flea was 3.5% for mill B effluent, 22.5% formill C effluent, and 100% for mill A effluent, i.e. the effluent generated at mill A wasnot acutely toxic to this water flea species. In addition to performing the LC5O of millB effluent with a bacterium and a water flea it was also performed with salmon andan algae (Selenastrum). The sensitivity of these species to this TMP effluent indecreasing order was: bacteria> salmon> algae> water flea.Kovacs et al. (1992) performed several toxicity tests on fifteen different high-yield mechanical pulp mill effluents that used various combinations of spruce, fir, andpine as their wood furnish. Results from the three TMP mills will be presented here.32The test species used for the LC5O test were rainbow trout, fatheadminnows, andCerioclaphnia affinis. The 1C50 test was performed with Photobacteriumphosphoreum, and the 1C25 was determined for fathead minnowsand Ceriodaphniaaffinis. The results from these tests can be found in Table 3.2.TABLE 3.2: Acute and Sub-lethal Toxicity Data for SeveralDifferentAquatic Species Exposed to TMP Effluent (Kovacs etal.1992)Species Toxicity TestsLC5O 1C501C25*Rainbow Trout 6 - 28% (96-h)Fathead 2 - 42%(48-h) ---- 1.6 - 3.5%MinnowsWater flea 1 - 28%(48-h) ---- 0.01 - 0.3%Bacterium ---- 1.4 - 17%*These researchers considered a 25% reduction in the growth inhibitionconcentration to be an estimate of the threshold level causing sub-lethaltoxicity.The results in Table 3.1 show the sensitivity of the species used in this study to theseTMP effluents. The acute toxicity, i.e. LC5O and 1C50,decreases in the followingorder: Cerioclaphnia> minnows/trout> bacteria> algae.And the sub-lethal toxicity,manifested as growth rate, i.e. 1C25, shows that Ceriodaphnia was more sensitive tothese TMP effluents than fathead minnows.Buckney (1978) studied a TMP effluent and found that it was highly toxic to33rainbow trout, golden perch, murray cod, mussels, daphnia, and shrimp. The 96-hLC5O values ranged from 7.2 to 48%. This study also determined several sub-lethalresponses which included mussels closing their valves so as to avoid the intake of theeffluent, a darkening in the colour of the fish (which is a common response fordistressed fish), and a decrease in the moult rate of shrimp with an increaseineffluent concentration. Nestmann et al. (1979; 1983) found that the resinacidsneoabietic (tested with yeast andbacteria) and 7-oxodehydroabietic (tested withyeast) exhibit mutagenic activity.Summary. The results found in this section present evidence thatuntreatedTMP and CTMP effluents can be acutely toxic to organisms that aretypically foundin receiving waters, and that they can result in sub-lethal toxic impacts within theseorganisms. Therefore, based on these short-term studies, it can be concluded thatthere is a necessity to implement effluent treatment systems at TMPand CTMP millsthat generate toxic effluents so as to preserve the integrityof the environment. Theselection of a treatment system that can preventall of the potentially negativeimpacts from occurring within a particular ecosystem will depend on theresults oflong-term impact studies.3.2.3 Long-term Impact StudiesSeveral researchers have noted that a general effects modeland/or specificlong-term impacts can not be inferred from short-term impact studies (Martel et al.1993; Johnsen et al. 1993; Kovacs et al. 1992). The major limitation of short-termstudies is their inability to consider the cumulative sub-lethal effects that may resultfrom the continuous discharge of an effluent. Long-term, in-depth assessments that34integrate chemical, toxicological, and biological data at all of the organizational levelsare required to determine the actual impacts that will occur withina specificecosystem. These studies should be performed in both the field andin simulatedstream channels or pools, include organisms from different levels ofthe food-web, andbe performed for a long enough period of time so as to include severallife cycles ofthe test species being considered. In addition, the sub-lethal effects thata specificcontaminant has on an organism, its food source and its predators,and the season,sensitivity, sex and stage of life cycle of the test organism shouldbe determined(Lehtinen, 1991; Hall, 1992; Owens, 1991; Swanson,1992). At present no in-depth,long-term ecosystem studies have been performed withTMP or CTMP mill effluent.However, this will soon change as a result of the newdischarge regulations that havebeen set forth by the Federal government (referto Section 3.3).3.3 Federal RegulationsThe trend of the federal government, spurred on by public opinion, has beento enforce more stringent discharge limits on pulpand paper mill effluents(Environment Canada and Department of Fisheriesand Oceans, 1992a; 1992b).Aqueous effluent generated by the pulp and paper industry is regulated by thefederal government, however, some provinces have additional requirements. TheFisheries Act which included the first Pulp and Paper EffluentRegulations (PPERs)was legislated in 1971, and amended in May of 1992.The two major changes that were legislated in the May 1992 amendment of theFisheries Act required that all of the pulp and paper mills in Canada meet the mostcurrent PPERs, and that they all implement an Environmental Effects Monitoring35(EEM) program (Department of Fisheries and Oceans, 1992). The EEM programistwo-fold. The first part is a pre-design requirement which includes a description ofthe receiving environment and mill data (refer to Table 3.3.TABLE 3.3: EEM Pre-Design Requirements and their Purpose(Colodey, 1993)Pre-designRequirements*PurposeDelineation of effluent mixing zone Integrity of effluent-exposed sites andreference sitesHabitat inventory Consistency between reference andexposed sites, identify confoundinginputs (rivers, other sources), providemapsResource inventory Identification of sentinel and resourcespeciesHistorical receiving environment Documentation of previousimpacts,information assist in sampling designEffluent quality Evaluation of effluentquality inrelation to PPERMill History Description of site history and currentoperations*Pre-design information is required once unless mill operations orenvironmental conditions change substantially.The second part requires a mill to perform in-depth studies every three yearsto examine the effects that discharging its effluent is having on fish, fish habitat, andfisheries resources. The required studies and toxicological tests are outlined in Table3.4.36TABLE 3.4: EEM Studies (Environment Canada and Department ofFisheries and Oceans, 1992)Effects on Fish: Adult fish survey (on selected species)to include: Length, fresh weight, age,gonad weight, egg size, weight of liverand hepatopancreas, externalcondition (obvious abnormalities,prevalence of lesions, tumours,parasites, etc.)Effects on Fish Habitat: Invertebrate community monitoring todelineate the extent of habitatdegradation due to organicenrichment or other forms ofcontamination, and to provide anevaluation of aquatic food resourcesfor selected fish species.Effects on Use of Fisheries Resources: a) Tissue analysis (forthepresence of chlorinated dioxinand furan congeners)b) Tainting evaluationToxicological Tests:a) Fish early life stagedevelopment testb) Invertebrate reproduction testc) Plant toxicity testd) In situ fish lethalitye) Amphipod survival (sedimenttest)f) Invertebrate survival andgrowth (sediment test)The major advantages of implementing the EEM program are its cyclical andsite-specific components. By monitoring the environment every three years the longterm effects, if any, that result from the discharge of mill effluents can bedetermined. This knowledge can then be used to update the regulations. And, becausethe program is site-specific the government will be able to implement specific37regulations, as required, for mills located in sensitive ecosystems.The current Canadian PPERs are given in Table 3.5. The test protocol used bythe Canadian pulp and paper industry for the determination of TSS and BOD can befound in the CPPA Standard Test Methods H.1 and H.2, respectively (CPPA, 1991a;1991b). The current protocol used for the acute toxicity test, the 96-hr LC5O testperformed with rainbow trout is specified by the Fisheries Act (Department of theEnvironment, 1990).TABLE 3.5: Pulp and Paper Effluent Regulations (Department ofFisheries and Oceans,1992)Parameter LimitsDaily MonthlyBOD5 (kg!ADt) 12.5 7.5TSS (kg/ADt) 18.75 11.2596 hr-LC5O Acute 100% 100%Toxicity (v/v)3.4 Effluent Treatment TechnologiesConventional treatment systems can employ primary, secondary and tertiarytechnologies. Primary technologies include clarifiers, flotation cells andsedimentationlagoons which can physically remove 85 to 98% of the settleable portion of thesuspended solids present in a pulp and paper mill effluent (Beaudoin et al. 1991).These units also reduce some of the BOD load. Secondary technologies utilizebiological systems so as to remove dissolved organic material, resulting in a decrease38in the BOD and toxicity of an effluent. The objective of tertiary technologies is tofurther decrease the TSS, BOD, and toxicity of an effluent. Examples of tertiarytechnologies include chemically assisted coagulation and granular filtration.An alternative to conventional treatment systems are zero-liquid discharge(ZLD), or closed-cycle, systems which do not discharge any liquid effluent. These ZLDsystems recover clean water from the effluent and recycle it back into the mill. Itshould be noted that the ZLD systems that will be discussed here requireprimarytreatment of the effluent so as to decrease the TSS load.3.4.1 Biological SystemsBiological systems, which can be either aerobic or anaerobic, decrease BOD andtoxicity by utilizing microbes that metabolize the organic matter present intheeffluent. These systems add nutrients, such as phosphorous and nitrogen,during theprocess to aid in this metabolism. Aerated lagoons (aerated stabilization basins),activated sludge, and biofilters are examples of aerobic technologies, whereasanaerobic contact reactors, upflow anaerobic sludge basins (UASB), fixed filmreactors, combined reactors(fixed film/UASB), fluidized bed reactors, and anaerobiclagoons are examples of anaerobic technologies. The anaerobic technologies discussedhere all require an aerobic polishing step to further reduce the BOD and toxicitylevels. The two most commonly used technologies in North America are aeratedlagoons and activated sludge, with either pure air or oxygen (Landry, 1993).Several TMP and CTMP mills in Canada operate full scale aerobic andanaerobic treatment systems. In general, the mills utilizing aerobic systems arecapable of consistently reducing the TSS, BOD, and toxicity loads to below the39acceptable discharge limits (Black, 1990; Easton et al. 1992; Servizi et al. 1986;Strang, 1992), as are the mills which utilize anaerobic systems (Andersson et al.1987; MacLean et al. 1990).In the past three years several lab and pilot scale studies, some of which arepresented here, have compared the ability of aerobic and anaerobic systems to treatTMP and CTMP effluents. The results from these studies conflict with one another,some advocate aerobic while others advocate anaerobic systems. Schnell et al. (1990)found that both aerobic and anaerobic systems were capable of reducing both thetoxicity and BOD of a TMP effluent, but that the anaerobic systems were notable todecrease the toxicity to an acceptable level on a consistent basis. Lo et al. (1991)found that aerobic systems were more favourable than anaerobic systems in thetreatment of CTMP effluent. Habets et al. (1990) found that an aerobic system wasnot able to reduce the pollutant load of a mixed TMP/CTMP effluent, whereas ananaerobic system was.3.4.2 Zero-Liquid Discharge SystemsSeveral separation technologies, such as evaporation, freeze concentration,andmembrane separation can be utilized in a ZLD system(Beaudoin et al. 1991; Gerbasiet al. 1993). The ZLD systems that utilize membrane separationand/or freezeconcentration require an evaporation step to further concentrate the effluent. This isdue to technical limitations with respect to the final water content that membraneseparation and freeze concentration can attain.The developmental stage at which these three technologies are at differswidely. Evaporation is a mature technology, and has been implemented on an40industrial scale in a vast number of different industries. As a result, it is theseparation technology that is utilized by both of the ZLD mills that are currentlyoperating in Canada.These two mills have implemented ZLD systems for very different reasons. Thefirst ZLD mill was built by Louisiana-Pacific in Chetwynd, BC. This mill required aZLD system because the company decided to site the mill as close as possible to thewood supply (thereby decreasing transport costs of the raw material) even though thissite did not have a receiving water source. It should be pointed out thatthis millinitially used a freeze concentration process, however, due to operational problemsthese units were latter retrofitted to be used as evaporators(refer to Section 4.1.4).The other ZLD mill was built in Meadow Lake, Saskatchewan by Millar WesternPulp Ltd. This mill was first designed to use a conventional biological treatmentsystem. However, this was not considered acceptable to the Saskatchewangovernment or the public. The reason being that effluent generated by the systemwould be “cleaner” than that of the receiving water and as a result would change theoverall ecosystem (albeit in a “positive” way) (Sagert, 1993). A third mill, planned forKitimat, BC, also proposes to operate a ZLD system that will utilize evaporation. Theproposal is currently being assessed by the BC government (Orenda,1991).The use of membrane separation to produce potable water from sea water hasbeen very successful, however, for the treatment of pulp mill effluent it is still at thelab-scale stage of development (Beaudoin et al. 1992; Zaidi et al. 1991). Freezeconcentration has been successfully implemented in both the organics and the foodindustry. The capacity of the freeze units used by these industries are at least 20times smaller than those required by the pulp and paper industry. The specific41separation process that is being considered in thiswork is clathrate hydrateconcentration. This process is a variant of freeze concentration andwill be discussedin the next Chapter.3.5 Comparison of Treatment OptionsThere are advantages and disadvantages associated with the implementationand operation of both conventional (biological) and ZLD systems.The majoradvantages associated with biological systems are:(1) They include the most costeffective treatment methods currently available; (2)They are capable of attaining thecurrent discharge limits; and (3) They do not affect productquality. Thedisadvantages associated with the implementation and operationof biologicaltreatment systems include:(1) The increasing concern about the long-term effectsassociated with discharging high nutrient loads into the environment,especiallynitrogen and phosphorous;(2) The high sludge production, the associatedremoval/disposal costs, and the leachate problems that may occur if this sludge island filled (these sludges concentrate the metals present in the effluent); and (3) Thelarge land requirements.The advantages of implementing a ZLD technology include:(1) There is nopotential for negative impacts to occur in the aquatic environmentbecause thesesystems do not discharge any liquid effluent; (2) The high costs associated withextensive and time consuming long-term ecosystem tests will not be incurred,e.g. theEEM program recently introduced by the Federal government; (3) ZLD mills will notbe affected by future effluent discharge regulations, which may require theimplementation of tertiary treatment technologiesand/or more adequate disposal42treatments for sludge (both of these will increase the cost of biologicalsystems); (4)These mills are easier to site because they do not depend ona receiving water source,and they are more readily accepted by both the public and the regulatory bodies;and(5) They implement water conservation principles (this is of interest to mills locatedin areas where water is a commodity and has an associated cost).The maindisadvantages of ZLD systems include:(1) Their high capital and operating costs(Gorgol, 1991); (2) The large energy requirements associated with these systemswhich pose an environmental disadvantage(which results from the environmentalimpacts associated with generating and transporting the energy required by thesesystems) (Galloway et al. 1991); (3) Their potential to decrease product quality as aresult of water recirculation; (4) The implementation of a ZLD system may resultinan increase in the air pollution load (due primarily to the incinerationof theconcentrate that is produced); and (5) There is a lack oflong-term operatingknowledge about these systems as they have been on-line forless than two years.Wearing et al. (1985) found that the energy andassociated equipment sizerequirements could be reduced substantially if mills were capable of decreasing theirwater-use to flow rates as low as 1.7m3/tonne pulp. These low flow rates could alsomake it feasible to physically treat(recover the chemicals) and combust theconcentrates that are generated by a ZLD system. It is, however, not likely thattheeffluent flow will get much below 10m3/tonne pulp unless new more innovativeinternal recirculation technologies are developed (Cornacchio etal. 1988).Several simulations have recentlybeen performed in which the capital andoperating costs of different ZLD systems have been determined. Paleologou (1993)found that if the effluent flow rate was reduced to less than 5m3/tonne pulp the cost43of a ZLD system utilizing a membrane separation technology wouldbe comparableto the costs associated with a conventional biological system.Jantunen (1993)simulated a TMP mill utilizing an evaporationJmembraneZLD system at an effluentflow rate of 6.9 m3/tonne pulp and compared the capitaland operating costs with asecondary biological treatment system. The results showed that thecapital cost of theZLD system was greater than for the biological system, whereasthe operating costsfor the ZLD system were less than those required by the biologicalsystem (the authornotes that these numbers are very dependent on the volume of waterthat is to betreated). Gerbasi et al.(1993) also compared the capital and operating costs of asimulated TMP mill with the costs for a conventional biologicalsystem. Threedifferent ZLD systems were simulated, a biological/membranesystem, a freezeconcentration system, and an evaporation system, withthe effluent flow rate varyingfrom 5 to 20m3/tonne pulp. In contradiction to Jantunen and Paleologou,Gerbasi etal. found that conventional biological treatment wasthe most economical treatmentoption in each case.Summary. The main advantage of a ZLDsystem is the removal of allpotentially negative impacts that may occur as a result of continuallydischarging aconventionally treated effluent. At present the major drawback ofZLD systems istheir high capital and operating costs. However, as effluentflowrates decrease,discharge regulations become more stringent, and the ZLD technologiesare furtherdeveloped specifically for the recovery of water from TMP and CTMPeffluents thesecosts will decrease relative to conventional systems. Therefore, it can beinferred thatZLD systems will become a viable effluent treatment option in the near future.44CHAPTER 4: Freeze Concentration and Clathrate HydrateConcentrationThe first section in this chapter describes the unit operations used in the freezeconcentration process, reviews the literature on industrial scale applications,anddiscusses the differences between freeze concentration, evaporation,and membraneseparation. The second section introduces clathrate hydrate crystal structure, andaliterature review on the clathrate hydrate process and its applications. And thelastsection discusses the thermodynamics and kinetics of clathrate hydrate formation.4.1 Freeze ConcentrationFreeze concentration refers to the concentration of a solution by crystallization.This process concentrates a solution by generating ice crystals within the aqueousstream followed by the physical removal of these crystals from the solution andsubsequent crystal melting(Heist, 1979). The terms freeze concentration and freezecrystallization are often used interchangeably in industry. However,freezeconcentration is actually a type of freeze crystallization becausethe latter includesall the processes in which heat is removed from a mixture resulting in thecrystallization of a component. Crystallization processes, including freezeconcentration, are based on the formation of pure crystals when a substance changesfrom the liquid to the solid phase. During crystal formation the impurities presentin the original solution will not be contained within the crystal structure. Thetemperature at which ice crystals form can be determined fromfreezing pointdepression data.454.1.1 Basic ProcessThere are several process configurations which can be formulated to recoverwater by freeze concentration. A general process basedon desalination is shownschematically in Figure 4.1. The first step in the process isto cool the originalsolution to a temperature close to its freezing point via a feed-heatexchanger. Thesolution is then fed into a crystallizer in which ice crystals are formed.These crystals,together with the concentrate, are pumped into a separator.A portion of the coldconcentrate is removed and recycled back into the crystallizer to increasethe overallconversion. The crystals are removed from the separatorand fed into a melter inwhich they are melted upon contact with a hot refrigerant stream. Heatis removedfrom the crystallizer and transferred to the melter so as toincrease the energyefficiency of this process. The resultingliquid refrigerant is recycled back into thecrystallizer, and the product water is removedafter exchanging heat with the feedin the feed-heat exchanger. The most important unit operations,when considering theeconomics of the overall separation process, arethe crystallization stage (theformation of the crystals), and the separation andpurification stage (the recovery ofpure crystals) (Hahn, 1986; Heist, 1979).4.1.2 Unit OperationsThe formation of ice crystals occurs in the crystallizer, and proceedsin twosteps: nucleation and growth. The crystals that are formedin the crystallizer may becoated by the impurities that are present in the effluent. Some of theconcentrate mayalso be occluded (entrapped within the interstices) between individual crystals (Bozzaet al. 1965; Maguire, 1987; Gorgol,1991). Both these phenomena are important46FIGURE 4.1: Freeze Concentration Process used in the Desalination ofSeawater (Englezos, 1993a)COMPRESSORFeedConcentrate4I4Product waterSeawaterCooling water47because the degree to which they occur will determine the ease and extentofcomplete separation. The reason that the crystals are coated withthese impuritiesis probably due to the low interfacial tension between thecrystals and theconcentrate (Tleimat, 1980). This surface attraction can be minimizedif the surfacearea to volume ratio of each crystal is small, i.e. crystals are shapedlike spheresrather than snowflakes, and both the surface tension and viscosityof theconcentrated solution are low (Gorgol, 1991; Douglas, 1989; Barduhn,1975). It isthought that the extent of occlusion can be minimized if largecrystals are grown ata slow rate, thereby minimizing the driving force (Sloan, 1990a; Barduhn,1975).Significant progress in the understanding of ice crystal nucleation and growthhas resulted from studies on the application of freeze concentrationin both sea waterdesalination (Harriott, 1967; Viahakis et al.1972; Udall et al. 1965; Delyannis, 1967;Delyannis and Delyannis, 1970; 1973; 1976;1978) and the food-industry (Stockingand King, 1976; Shirai et al. 1985; 1987). It has been postulated that the growth ofthese ice crystals is controlled predominantly by mass and heat transferprocesses(Margolis et al. 1971; Shirai et al. 1985; 1987; Stocking, 1976) and to a lesser extentby the residence time in the crystallizer (Shirai, 1987; Smith et al.1985).The separation and purification of the crystals requires a separation unit, e.g.a wash column, belt press or centrifuge. The most commonly used separationunitsin desalination are wash columns. In these units, which operateby gravity or underpressure, clean water is sprayed onto the top of the ice crystal bed, the washedcrystals are recovered by scraping them off the top, and the concentrate is collectedfrom the bottom of the column.The concentration of impurities by the removal of water will eventually result48in the concentrate interfering with the crystallizationprocess by preventing effectiveseparation or exhibiting an excessive freezingpoint depression. Therefore, it isnecessary to select the extent of concentrationrequired by the process at a level lowenough such that these maximum limitswill not be attained. To concentrate theeffluent beyond this limit evaporation mustbe used.4.1.3 Technology SystemsFreeze concentration systems can be divided into two groups,indirect (Heist,1979; 1981; Hahn,1986) and direct (TechCommentary 1988; Hahn, 1986;Emmermann et al. 1973; Ziering et al.1974; Heist, 1979; 1981). The system that ischosen to concentrate a particular solutionwill depend on the characteristics of thesolution and the end-use of the product water.Indirect systems use a refrigerant to lowerthe solution temperature througha heat exchanger surface in a closed refrigerationcycle. The hot side contains the feedsolution and the cold side contains therefrigerant. These systems, which areexpensive when compared with the otherfreeze options, are commonly used by thefood industry so as to avoid contamination of theproduct solution with therefrigerant.In direct systems a heat exchange surface is not required becausetherefrigerant acts as the heat-transfer medium. Thereare two types of direct systems:(a) triple point (vacuum freezing), and(b) secondary refrigerant. The triple pointsystems use water as the refrigerant by operating theprocess at the triple point ofwater. As water evaporates it removes heat from the system allowingthe solvent tocrystallize. In secondary refrigerant systemsthe refrigerant is mixed with the49solution and acts as the heat transfer mediumby evaporation. The refrigerant thatis used must be nearly insoluble in water such thatit can be removed from both theproduct water and the concentrate, and be recycled back into theprocess. Thesesystems are considered for applications where large volumes ofwater are to betreated and a slight degree of contaminationis acceptable. The clathrate hydrateconcentration process resembles a secondary refrigerant system.4.1.4 Industrial ApplicationsFreeze concentration is currently used in the food industryand in theproduction of high purity organics. It is well suited to the food industrybecause it canseparate water from a solution without the loss of flavour andaroma often associatedwith evaporation and distillation processes(Douglas, 1989; Chowdhury, 1988).However, commercial freeze units in the food industry processless than 190 L!minof solution. The freeze process is also used to purify chemicals, e.g.separate p-xylenefrom a mixture of petrochemicals (Douglas, 1989; Chowdhury, 1988; Heist,1979). Itsprimary advantage to the organics industry is its ability to producehigh purityproducts, in some cases, over 99% (Basta and Fouhy, 1993; Chowdhury,1988).Barduhn (1968; 1975) has reviewed the work on the application of the freezeprocess to the desalination of sea water.A commercial desalination unit designed andbuilt by CBI industries in Yanbu, Saudi Arabia is capable of producing 110L/min ofpotable water from a freeze concentration process (Douglas, 1989;HPD, 1990).Several researchers have performed small scale testing programsto study thetechnical and economical feasibility of using the freeze process to concentrate kraftpulping liquor and bleach plant effluent (Rousseau and Sharpe, 1980; Rousseau, 1981;50Coleman, 1986). The concentration of black liquor was found to be technically viablebut not economical because of the low cost of energy(Coleman, 1986).A pilot plant freeze concentration unit was designed by HPD Inc. and builtinOntario at Tembec’s BCTMP mill during 1991. Kenny et al. (1992) determinedthatthe indirect freeze process used at this pilot plant could reduce theconcentration oflow molecular weight organics (Cl throughC3), resin and fatty acids, the BOD andCOD loads, and the electrolyte content in the recovered process water. However, thenumber of reported results were limited due to operational problems during the testperiod. The results from this pilot project and the lessons learned from CBI’sdesalination plant in Yanbu were used to design an industrial scale freezeconcentration system. This system was designed by HPD Inc. and built inthe fall of1991 at Louisiana-Pacific Canada Ltd.’s (L-P) Bleached CTMP mill near Chetwynd,BC. Details of the effluent treatment process at the L-P mill are proprietary.However, a general outline of the original process is summarized here (Kelsey, 1990;HPD, 1990). The effluent is clarified, the primary sludge is collected, and used as fuelin the hog-fuel boilers. The clarified effluent enters one of four refrigeration units inwhich ice is formed. These units utilize an indirect freezing process with ammoniaas the refrigerant. The slurry that is produced is pumped into a separator in whichthe ice is washed, scraped off, and melted before being pumped back into the mill.This freeze process was designed to treat 4,000 L/min and to concentrate theeffluent from 2% solids up to 10% which would approximately result in an 80%recovery of the water present in the effluent stream. The effluent was to be furtherconcentrated up to 50% solids in an evaporation unit that was designed to use wasteheat from the refrigeration cycle. The mill was designed to reuse 75 to 80% of its51process water, the remaining 20% of this water was expected tobe lost throughevaporation during the drying of the pulp. The make-up water was tobe supplied byon-site wells, as there are no other available water sources at themill site. It wasproposed that the concentrate be sold to kraft mills and used for energyand/orchemical make-up in their black liquor recovery boilers,or be incinerated at the millsite (Kelsey, 1990). It should be noted that Basta and Fouhy(1993) determined thatthe concentrate which would typically be generated by a ZLD system ata mechanicalmill would be similar in elemental composition to kraft black liquor,and have a heatof approximately 9000 kJ/kg.However, several problems with the freeze system built at theL-P mill limitedthe overall recovery of an adequate volume of clean process water. It was foundthatonce ice started to form in the refrigeration units it was difficult to control the growthsuch that the whole system would not freeze up. Scale formationin the refrigerationunit was also noted. It was found that the wash columnscould not adequatelyseparate the ice crystals from the concentrate. This may havebeen partly due to thepresence of colloidally suspended material which enhanced surface attractionand thedegree of occlusion. As a result, the wash columns were replaced with belt-pressunits(Gorgol, 1992). However, the freeze system was still not capable of recovering morethan 40 to 50% of the water present in theeffluent (Arac, 1993). Due to all of theseproblems the mill converted to an evaporation system similar to that used by MillarWestern’s BCTMP-ZLD mill. However, based on the experience attained at the L-Pmill it is believed that with more research freeze concentration has the potential tobecome a viable option for the future(Arac, 1993).Some potential future applications of this process have been suggested by52Douglas (1989). These include the remediation of hazardous waste lagoons,theconcentration of deep mine reject water, the recovery of process materials fromammunition plants, byproduct recovery from organic chemical andpharmaceuticalwaste streams, desalination, and the treatment of metal-plating facility wastewaterin which the recovery ofboth valuable plating metals and water can be accomplished.4.1.5 Comparison of Separation TechnologiesCompared with other separation technologies the major advantageof acrystallization process, e.g. freeze concentration, is that at a given temperatureandpressure only one component will crystallize and it will do so asa pure crystal. Inaddition, freeze concentration has several other specific advantages over bothevaporation and membrane separation.The freeze process has a potential energy savings compared with evaporation.Seven times more energy is required to evaporate water than to freeze it (6.008J/molvs 40.66 J/mol). However, this energy advantage is substantially reducedwhen amultiple effect evaporator is considered. Two other important advantages result fromthe low operating temperatures used in the freeze process. At these lowtemperatures, i.e. below room-temperature, a large portion of the volatile organicsthat are known to be present in TMP and CTMP mill effluents will be removed withthe concentrate. In comparison, the high temperatures required for evaporation willvolatilize these compounds such that they will be present in the recovered water, andas a result ZLD systems which utilize evaporation may require extra separationunits, e.g. stripping columns (Burke, 1990). The low operating temperatures used bythe freeze process will also decrease the potential for scaling and corrosion problems53that can result from the presence of organic and inorganic salts in TMP andCTMPmill effluents. These problems become more prominent at higher temperatures, andas a result more costly materials will be required for the constructionof the unitsused in evaporation.Freeze concentration has two main advantages over membraneseparation.These include: (1) The fouling problems associated with membranes which areexposed to solutions having high organic contents(Zaidi et al. 1991); and (2) Theshortened life expectancy of a membrane exposed to phenols, bacteria,fungi, hightemperatures, high or low pH values, and scale-forming compounds,e.g. calciumcarbonate and calcium sulphate (McCubbin,1992). In a recent study it has beendetermined that a ZLD system which incorporates reverse osmosis and ultrafiltrationis technically feasible. The development of these technologies is ongoing (Paleologouet al. 1993).Gerbasi et al.(1993) did a cost comparison between three closed cycletechnologies and an aerobic treatment system. It was found that the capital andoperating costs increased as follows: biological treatment < membrane separation <evaporation < freeze concentration. However, preliminary results fromthe freezecrystallization desalination plant in Saudi Arabia have shown that the freeze processcan compete economically with membrane separation (Douglas, 1989). HPD Inc.which designs both freeze concentration and evaporation units has determined whichof these two systems deals best with the various contaminants present in CTMPeffluent (refer to Table 4.1). They found that the freeze process is suitablefor mostof the contaminants present in CTMP effluents with the exception of solids, and thatevaporation does not deal well with the volatile compounds. In conjunction with this54they also determined that biological systems are less expensive than evaporationwhich in turn is more economical than freeze concentration.However, if biologicaltreatment is not acceptable and evaporation can not produce water ofan acceptablequality then freeze concentration is a viable option (Gorgol,1991). HPD Inc. havereported four potential applications: (a) CTMP, BCTMP, and Groundwood effluent;(b) Bleach plant effluent; (c) De-inking waste; and (d) Acid recovery.TABLE 4.1: Removal Efficiency of Contaminants TypicallyPresent inCTMP Effluents by Evaporation and Freeze Concentration(Gorgol, 1991)Contaminant Evaporation FreezeConcentrationInorganics High HighLight Organics LowHighVolatile Acids Low HighFatty Acids Medium MediumResin Acids High MediumOrganic Sulphur Low HighFibre/Suspended High LowSolids4.2 Clathrate Hydrate ConcentrationClathrates, a type of inclusion compound, were first discovered by SirHumphrey Davy in 1810, but theirtpractical’ importance did not become evidentuntil 1934 when it was discovered that clathrate hydrates were responsible for theplugging of natural gas pipelines. As a result, the majority of research has focused55on limiting their formation. In the mid 1960’s vast quantities of methane hydrateswere found in the earth’s crust. These natural hydrate formations have the potentialto become very important energy sources in the future if an economical method toextract them can be found (Holder, 1988). On the other hand, these formations mayresult in a further warming of the atmosphere if the current atmospherictemperature increases sufficiently such that these hydrates decompose and releasemethane, a “Green House” gas (Sloan, 1990b).4.2.1 Crystal StructureClathrate hydrate crystals are formed at suitable pressure and temperatureconditions by the physical combination of water molecules and one or more hydrateforming compounds. The resulting inclusion compounds contain only water and thehydrate former. The cubic lattice like structures of these compounds are formed bythe presence of strong hydrogen bonds between adjacent water molecules. Thesestructures, without the presence of a hydrate former, are thermodynamicallyunstable. However, if hydrate formers occupy the interstitial vacancies of these cubicstructures weak van der Waals type dispersion forces (between the hydrate formerand water molecules) stabilize the structure.The majority of hydrates form one of two structures, referred to as I (bodycentred cubic) or II (diamond lattice). Each of these structures contain two roughlyspherical cavities with different diameters (refer to Figure 4.2). A unit cell ofstructure I is constructed from 46 water molecules and has 2 small (12-Hedron) and6 large (14-Hedron) cavities, and structure II has 16 small (12-Hedron) and 8 large(16-Hedron) cavities formed from 136 water molecules (Sloan, 1990a). Each cavity can56FIGURE 4.2: Cavities and Unit Cellsfor Clathrate Hydrate Structures(Sloan, 1990a)C16-HEDRON57contain only one guest molecule, however the number of cavities that are occupied ina hydrate vary with pressure.4.2.2 Concentration ProcessHydrates can form at several temperatures well above 0°C. These highcrystallization temperatures can result in an energy saving compared with freezeconcentration. Another advantage of the clathrate process is the increased flexibilitydue to its dependence on pressure as well as temperature. Pressure, which has anegligible affect on the freezing point of water, can have a large impact on thetemperature at which hydrates form. This “extra’ process variablemay lead to anincreased ability to control the crystallization process. However, it will also increasethe cost of operating the process and limit the use of hydrate formers that have highformation pressures (due to the energy loss that would result from attaining thesehigh pressures).The operations required for the clathrate concentration process are similar tothose described for freeze concentration. There is, however, one additional operation:the recovery of the hydrate forming substance once the hydrates have decomposed.Refer to Figure 4.3 for a schematic of a conceptual clathrate hydrate concentrationprocess implemented at a pulp mill.This work will explore in-situ concentration and separation. The currentprocedure used in freeze and clathrate processes is to form crystals in a crystallizerand then move the crystal slurry to a separator. The concept of in-situ concentrationis to control the crystal formation conditions in such a way that part of the separationstep can be performed within the crystallizer unit. This process change could result58FIGURE 4.3: Clathrate Hydrate ConcentrationProcess used In theConcentration of Mechanical Pulp Mill EffluentWood ChipsPulpChemicalsMILLEffluentCRYSTALLIZATIONHydrate FormProcessWaterI Hydrates_______SEPARA______T0DECOMPOSITION UNITR —Concentrate4 ToEvaporator59in a decrease in both the capital and operating cost of the separation step.4.2.3 Industrial ApplicationsThe concentration of aqueous streams by clathrate hydrate concentration waspatented by Glew (1962) and was also investigated by Werezak (1969). Formation ofhydrate crystals in seawater was considered as the basis of a process to recover purewater. The process was demonstrated at a pilot plant stage(Knox et al. 1961;Barduhn, 1968; 1975; Tleimat, 1980) but never became commercial due to economicreasons. The application of the process has recently become of interest as a wasteminimization operation as well as in the food product industry (TechCommentary,1988; Heist, 1988; Douglas, 1989).In the early 1960’s research into the application of the clathrate hydrateconcentration process to desalinate water was supported by the US Department ofthe Interior through the Office of Saline Water (OSW). During this time two pilotplants were built, one by the Koppers Company which used F-12 (C12F)as thehydrating agent (Knox, 1961), and the other by the Sweet Water DevelopmentCompany which utilized propane as the hydrating agent(Heem, 1965). One problemthat both of these plants encountered was the difficulty in adequately separating thecrystals from the concentrate. The crystals that formed were very compressible anddifficult to wash using a wash column(Barduhn, 1968). The OSW withdrew theirsupport for the pilot plants in 1968 due to the slow progress in process development(Barduhn, 1968). As a result, an industrial scale desalination unit that utilized theclathrate hydrate concentration process was never built. Rautenbach et al.(1978)determined that it was difficult to produce crystals of adequate size and shape at a60sufficient growth rate, and that the desorption of thehydrating agent(s) from boththe concentrate and the recovered water may poseproblems. This later problem isespecially important in the production of potable water as some hydrating agentsaretoxic at low concentrations. However, this may not be a problemwhen recoveringpulp mill process water because the quality requirementsmay not be as high. In 1985a pilot plant which used a eutectic clathrate hydrate concentration processin whichice and sugar crystals were formed simultaneously(and separated by gravity) wasbuilt by the beet sugar refining industry(Heist, 1988). The results from this plantshowed that if clathrate hydrate concentration replaced evaporation at severalpointswithin the process the overall cost of producing sugar would decrease.4.3 Clathrate Hydrate FormationThermodynamic and kinetic data are required to fully understandtheformation of clathrate hydrates. Thermodynamic experimentsdetermine theconditions, i.e. temperature and pressure, at which hydrateswill form in a specificsystem. And kinetic experiments determine the rate of hydrate formation.4.3.1 ThermodynamicsThe temperature and pressure at which hydrates will form in a crystallizer willdepend on the type of hydrate former(s) that are used, and the chemical make-up ofthe solution that is to be concentrated. If these formation conditions, for agivensystem, are not available in the literature, they can be determined by performingequilibrium experiments. The resulting data points are used to generate phaseequilibrium diagrams.61A partial phase equilibrium diagram is depicted in Figure 4.4. The pointsindicate experimental data for the propane - water system in the hydrate formationregion determined by Kubota et al. (1984), whereas the lines are interpolations ofthese data. Line KL is the vapour pressure line for propane. Line AQ defines thelocus ofhydrate - vapour (rich in propane) - 1iquid (rich in water) phase equilibrium.At a given temperature the pressure that corresponds to a point on this line is theminimum pressure required for hydrate formation. Propane hydrates are formed atconditions corresponding to pressures and temperatures on the left of line AQ. Thetemperature corresponding to point Q is the maximum temperature at which propanecan form hydrates. This point is called the upper quadruple point. At this point fourphases namely, hydrate, vapour, liquid and liquid1)(rich in propane) coexist inequilibrium. Line AQ extends down to another quadruple point (not shown on theplot) at 273.15 K (0 °C). At this lower point hydrate, ice, vapour and liquid watercoexist in equilibrium. Line QC defines the locus of hydrate - liquid - liquidcondensed phase equilibrium. Propane hydrates are formed when the pressure andtemperature correspond to a point on the left of line QC.It is well known that the presence of impurities, also referred to as inhibitors(e.g. electrolytes and dissolved molecular species), in an aqueous solution will alterthe location of the AQ line by depressing hydrate formation. In particular, at a giventemperature, the equilibrium hydrate formation will be higher, or, at a givenpressure, the equilibrium hydrate formation temperature will be lower as comparedto pure water. For example, in Figure 4.4, the incipient hydrate formation conditionsin a 2.5 wt% NaC1 solution are shown. These experimental points are on the lineAl Ql. The extent of deviation from the formation conditions with pure water62FIGURE 4.4: PartialPhase Equilibrium Diagram1.0•CExperimental Data from0.8Kubota et al. 1984Cu_06 01KzC’,0.4LU)0.2-Al0.0-I • I • I • I •270 272 274 276 278 280Temperature (K)63(line AQ) will depend on the type and concentration of the impurities that are presentin the solution of interest.4.3.2 KineticsThe process of hydrate formation can be divided into two separate stages,nucleation and growth. The first stage involves the formation of hydrate nuclei andis often referred to as the induction period. The formation of nuclei is not wellunderstood. The second stage, the growth period, commences afterthe nuclei havereached a critical size and are stable, at which point the liquid becomes turbid andthe growth stage begins. This stage is more quantifiable than nucleation but it is stilldifficult to study. Pinder (1964) exposed the difficulties in studying the kinetics ofhydrate formation. Some progress has been made since then. Englezos(1993b) hasreviewed the literature on clathrate hydrates, including a summary of experimentaland modelling studies on the rate of hydrate formation.The induction time for the nucleation of gas hydrates was experimentally foundto depend on the history of water (Vysniauskas and Bishnoi, 1983a; 1983b), stirringrate (Makogon, 1981; Englezos et al. 1987; Skovborg et al.1993), and the degree ofsupercooling, i.e. the temperature difference between the experimental temperatureand the equilibrium temperature at a given pressure (Glew and Hagget, 1968; Knoxet al. 1969; Scalon and Fennema, 1972; Makogon, 1981; Vysniauskas and Bishnoi,1983a; 1983b; Englezos et al. 1987; Skovborg et al. 1993). Correlations employing thedegree of supercooling have been proposed for the growth of hydrate crystals (Glewand Hagget, 1968; Pangborn and Barduhn, 1970). Recently, mechanistic models havebeen proposed for the growth kinetics (Englezos et al. 1987).64CHAPTER 5: Experimental SectionThe experimental objectives of this work were to:(a) Determine thecharacteristics of interest for the specific mill effluent samples usedin theexperiments (effluent characterization tests);(b) Observe the clathrate hydrateconcentration process (qualitative process characteristics); (c) Determinetheminimum pressure, at a given temperature, at which hydrate crystals form (pressure-temperature hydrate formation experiments); (d) Study the induction periodandgrowth rate of hydrate formation (induction and growth rate experiments); (d)Studyin-situ concentration with the effluents and various salt solutions(in-situconcentration experiments).The first section in this Chapter discusses the effluent sources, and the hydrateformers that were used. The second section outlines the effluentcharacterizationtests. The third section describes the experimental apparatus. And the last sectionpresents the experimental procedures that were used.5.1 Effluent Samples and Other MaterialsEffluent samples were received from four different high-yield mechanical pulpmills in British Columbia. These included a sample of effluent generated by:(1)Fletcher Challenge’s unbleached TMP line at Elk Falls(referred to as TMP1); (2)Howe Sound Pulp and Paper’s bleached TMP line at Port Mellon(referred to asTMP2); (3) Quesnel River Pulp’s combined BCTMP/TMP mill in Quesnel (referred toas CTMP); and (4) Louisiana-Pacific Canada Ltd.’s BCTMP mill in Chetwynd(referred to as BCTMP). Design parameters for these mills are given in Table 5.1.65TABLE 5.1: Design Parameters of the Pulp Mills from which theEffluent Samples Used in this Study were GeneratedMills Process Effluent Wood SpeciesCapacity (t/d) Volume (m3/d)TMP1 1400 170,000 hemlock, fir,(estimated) whitewoodTMP2 600 20,000 hemball, fir,pine, spruceCTMP*950 17,000 pine, spruceBCTMP 500 5,000 aspen*The volume of BCTMP to TMP effluent in the effluent sample used in ourexperiments was approximately 2:1.The types of chemicals that are generally added during the production of pulpat these four mills include the following: (1) The TMP1 mill adds sodium sulphite,sodium dithionite, peroxide, sodium hydroxide (caustic), DTPA, and epson salts (theTMP1 effluent stream also receives landfill site leachate, mill yard run-off, andeffluent from a kraft specialty paper line which adds alum and Rozonsize (a refinedlignin compound)), (Easton, 1992; 1993); (2) The TMP2 mill adds sodium sulphite,talc, DTPA, sodium hydrosuiphite, sodium hydroxide, sodium silicate and peroxide(Ling, 1992); (3) The CTMP mill adds sodium sulphite, DTPA, sodium hydroxide,sodium hydrosulphite, sodium silicate, magnesium sulphate, and hydrogen peroxide(Jackson, 1992); and (4) The BCTMP mill adds sodium suiphite, sodium hydroxide,peroxide and XU82 (a chelating agent) (Morand,1993).Four concentrate samples, generated by evaporation in an open beaker undera fume hood, were also used in this work. It was attempted to concentrate these66samples by seeding them with an ice cube and placing them in the freezer, but thiswas not successful. A 50 and an 85% concentrate (referred to asTMP2/50 andTMP2/85) were generated from a portion of the TMP2 effluent sample, i.e. thesolution volume was reduced by 50 and 85% respectively. And a 50 and an 80%concentrate (referred to as CTMP/50 and CTMP/80) were generated from a portionof the CTMP effluent sample. In addition, a 1 and 5 wt% salt solutions were used.These solutions were prepared with deionized water and laboratory grade sodiumchloride from Fisher Scientific. The appropriate amounts of salt were weighed usingan analytical balance.The hydrate formers selected for this study were: (1) propane (C3H8);(2) carbondioxide (C02); and (3) a 30-70 mol% propane-carbon dioxide mixture. All three wereMedigas industrial grade gases.5.2 Effluent Characteristic TestsThe parameters chosen to characterize the effluent samples used in this studyincluded electrolyte content, solids content, carbon content, colour and pH. Theelectrolyte, solids and carbon content of the effluents and concentrates weremonitored to determine the effect, if any, that these had on the hydrate formationpressure-temperature conditions, and the induction and growth rate of the hydrates.The conductivity, and in some experiments the carbon content, were used todetermine if in-situ concentration was occurring. The colour (which was determinedvisually) and pH were monitored as additional parameters of interest.The electrolyte content of a solution, which is related to conductivity, wasdetermined with an Orion Model 160 Conductivity meter. This meter was designed67for a ± 0.5% accuracy for solutions up to 199.99MS/cm, and has an automatictemperature compensator.The solids content of a solution was determined by the total solids(TS), volatilesolids (VS), fixed solids (FS), and the total suspended solids (TSS) tests. The TS valuerepresents the mass of suspended and/or dissolved solid matter that is left in acrucible after evaporation and drying at 105 °C. The FS value represents the massof the solids that do not burn at 550 °C, i.e. the matter remaining in the crucible. TheVS value refers to the mass of solids that volatilize at 550 °C and is calculated as: VS= TS - FS. The TSS value represents the portion of the solids present in a solutionthat is retained by a filter with a selected porosity. Details of these procedures canbe found in Appendix A.The carbon content of a solution was determined with a Shimadzu TOC 500total organic carbon analyzer. This apparatus determined the total organic carbon(TOC) and total inorganic carbon (TIC) concentration of a solution.The pH of a solution was determined with a digital pH meter, the Cole PalmerModel 0S669-20. This meter was designed for a ± 0.1% full-scale accuracy and hasan automatic temperature compensator. It was calibrated using CanLab pH 4.0, 7.0and 10.0 reference buffer solutions.5.3 Experimental Set-upThe experimental equipment included two equilibrium vessels, a glycol-waterbath, a refrigerator, three thermocouples, two pressure gauges, a thermometer, anda tachometer.685.3.1 ApparatusThe experimental apparatus used to study hydrate formation is illustratedinFigure 5.1. The primary component in this set-up is the vessel in which the hydratesform. This vessel was immersed in a large temperature controlled bath. The bath wasfilled with 100 L of a 50-50 wt% ethylene glycol-water mixture. The temperature ofthe bath liquid was controlled by an external refrigerator/heater which uses acooling/heating coil to transfer heat in and out of the bath. The coil was constructedwith copper tubing. A uniform temperature was maintained within the bath by amotor driven stirring mechanism. The refrigeration unit was a Forma ScientificModel 2095 bath with a 28.4 L capacity. The refrigerator bath was also filled with a50-50 wt% ethylene glycol-water mixture. This set-up can maintain a relativelyconstant temperature(±0.10 K) within the bath over a long period of time. The bathtemperature was measured with a Fisherbrand thermometer with 0.1 °Csubdivisions.5.3.2 Hydrate Formation VesselsTwo different equilibria vessels were used, one for low pressure systems (C3H8and the 30-70 mol% C3H8-C02mixture) and the other for high pressure systems(C02). Both vessels were machined from solid pieces of a 316 stainless steelcylindrical bar.A cross-sectional view of the low pressure vessel is illustrated in Figure 5.2.This vessel, with a capacity of approximately 750 mL, has three sight windows. Twowere situated across from one another near the base of the vessel and a third wasplaced at the top of the vessel (the lid). These windows were machined from 1/2” thickFIGURE 5.1:69Apparatus used for ClathrateHydrate FormationExperimentsON/OFF VALVE‘TTh-COOLING COILTHERMOCOUPLEPUMPPRESSURE REGULATORPRESSURE GAUGEFIGURE 5.2:Drftt 11/323Low Pressure Vessel705/16’—32 Thread316 STAINLESS STEELPLEXIGLASS71Plexiglas plates, sealed with neoprene 0-rings andheld in place by stainless steelbolted studs. Mixing within this vessel was accomplishedwith the aid of a magneticstir bar coupled with a set of magnets rotating beneath the vessel. Theset of magnetswere mounted on an aluminum housing that was connectedto a DC motor (as the setof magnets spin they generate a magnetic force which coupleswith the stir barcausing it to spin thereby mixing the contents in the vessel). This vesselhas two gasand two liquid ports. One of the liquid ports was used to gravity feedthe solution intothe vessel and the other to drain the solution.Figure 5.3 shows a cross-sectional view of the high pressure vessel. This vessel,with a volume of 280 mL, has only two site windows. These windows were situatedacross from one another approximately half way up the vesseland were machinedfrom 1 3/4” thick Plexiglas plates. These windows were friction fit.The lid of thevessel was sealed with a neoprene 0-ring and heldin place by stainless steel bolts.Mixing was accomplished in the same manner as it wasin the low pressure vessel.The high pressure vessel has two inlet and two outlet ports, similarto the lowpressure vessel.The temperature inside both of the vessels was measured with two Omegacopper-constantane thermocouples. One thermocouple was situated in the top halfofthe chamber, to determine the gas phase temperature, while the otherwas positioednear the bottom, to determine the liquid phase temperature. A third thermocouplewas placed outside of the vessel in the bath liquid.The pressure in the low pressure vessel was measured with a 0-300 psiBourdon tube Heisse pressure gauge with 1 psi subdivisions. This gauge wascalibrated against an Ametek Modcal Pressure Model (pneumatic), traceable to NIST.FIGURE 5.3:U—High Pressure Vessel721/2” — 32 Thr’edjJ316 STAINLESS STEEL1/4” — 13 ThreadPLEXIGLASS73The pressure in the high pressure vessel was measured with a 0-14, 000Bourdon tubeHeisse pressure gauge with 50 kPa subdivisions. This gaugewas also calibratedagainst an Ametek Modcal Pressure Model (pneumatic), traceable toNIST. Theaccuracy of the measurements taken with the low pressure gauge are within± 0.5 psior ± 0.003 MPa, and measurements taken with the high pressure gaugeare within± 25 kPa or 0.025 MPa.The speed of the stir bar was approximated with an ADAMSphotoelectricTachometer 5205 set for a 0-1000 RPM range with 10 RPM subdivisions.Thistachometer was calibrated to a known source, its accuracywas within ± 50 RPM. Thetachometer was used in the growth and the concentration experiments.5.4 Experimental ProceduresDifferent procedures were used for each set of experiments, however thevessels were cleaned using the same procedure. The vessels were washed every timethe source of the test solution was changed. The washing procedure includedflushingthe vessel twice with hot water, once with deionized water, and once with the solutionto be tested, prior to injecting the test solution into the vessel.Periodically, the lidof the vessel was removed and the inside of the vessel was wipedwith warm water,with methanol, and finally with deionized water.5.4.1 Qualitative Process CharacteristicsThe observations made about the clathrate hydrate concentration process werevisual, without the aid of any magnifying devices.745.4.2 Pressure-Temperature Hydrate Formation ConditionsThe following outlines the procedure used to determine the hydrate formationpressure at a given temperature in both the low and the high pressure vessels.Approximately 200 mL of the test solution was injected into the vessel. The bathtemperature was set to the desired level and the stir bar was set at the maximumspeed that would not cause decoupling, approximately 200 rpm. The system wasgiven time to equilibrate, such that the bath temperature and the temperature insidethe vessel were stable. At this point the system was purged twice with the chosenhydrate former to remove any remaining air, and the vessel was pressurized(to alevel well above the expected incipient pressure) to induce hydrate nucleation.Subsequently, the hydrates were decomposed by venting the gas out of the system.The reason for this procedure was to eliminate the hysteresis phenomena andenhance the structure in the aqueous solution. After these initial procedures, the firstincipient hydrate phase equilibrium data point was measured as follows.The vessel was pressurized to a level well above the estimated equilibriumvalue to induce hydrate nucleation. The estimated equilibrium value for the firstexperiment was usually obtained by trial and error for a particular effluent. Then,the deviation of this initial value from the value for hydrate formation in pure waterwas calculated and used to obtain an estimate for experiments at other temperatures,with the same effluent. Once hydrates had formed the pressure was dropped to alevel slightly above the estimated equilibrium value, at the system temperature. Thesystem was then given at least 4 hours to equilibrate. If, at that time, a large amountof hydrates were present the pressure was dropped further, if no hydrates werepresent the system was renucleated and set at a higher pressure. This process was75continued until only a small amount of hydrates were present and thetemperatureand pressure had remained constant for at least 4 hours. Once these equilibriumrequirements had been met the incipient hydrate formation pressure,at the systemtemperature, was recorded. The next incipient hydrate equilibriumdata point wasfound by selecting a new temperature and repeating the above procedure.5.4.3 Induction and Growth Rate ExperimentsThe growth experiments were performed with the propane-carbon dioxidemixture in the low pressure vessel. The low pressurevessel was chosen because itcould withstand the experimental pressures required by the C3H8-C02gas mixture,and it was easier to visually observe the formation process in this vessel as comparedto the high pressure vessel. Before induction and growth experimentscould beperformed the driving force had to be established. Driving force can be determinedfrom thermodynamic considerations, and the fact that clathratehydrate formationis a crystallization process. A schematic of a partial three phase diagramfor hydrateformation with a pure compound, e.g. propane, and water is givenin Figure 5.4. Ata given temperature the pressure where the three phases, namely gas, liquid andhydrate coexist is unique. The driving force in a hydrate formation experiment thatis performed at a constant temperature,Texp,is: driving force (AP) experimentalpressure(13exp)- equilibrium pressure (P). One may also define a driving force atconstant pressure,exp’as: driving force (T) = experimental temperature(Texp) -equilibrium temperature (T). Other factors, such as stirring rate,liquid and gasphase volume, and the history of a solution, can also affect the induction period andthe growth rate of hydrates. Therefore, to be able to study the effect of pressure76FIGURE 5.4: Schematic Identifyingthe Driving Force of HydrateFormationa. -Peq ,TexpTemperature (K)77differences the solution volume, temperature, stirring rate, andprior history werekept constant throughout these experiments. The solution volume was200 mL, thebath temperature was set at 100and the stirring rate was set at 200 rpm. At 1 °Cthe hydrate formation pressure for a 30-70 mol% propane-carbon dioxide mixtureindeionized water was expected to be 0.420 MPa, based on experimental resultsobtained in this work.The procedure used was as follows: (1) Clean the vessel; (2) Place the testsample in the vessel; (3) Set the bath temperature to 100,and let the system reachthermal equilibrium; (4) Purge the vessel twice withthe propane-carbon dioxidemixture to remove any remaining air; (5) Set the pressure to the desired value andstart the stop watch; and (6) collect pressure versus time data.The time it took before hydrate crystals could be observed was recorded(induction period), after which pressure versus time data was recorded for thefollowing hour (initial growth period). After this first hour, the growth of the hydratecrystals was stopped by dropping the pressure to 0.15 MPa and maintaining it therefor at least four hours, allowing the hydrates to decompose. The system was thenrenucleated, the pressure was increased to the next desired value, and pressureversus time data was recorded for another hour (second growth period). The aboveprocedure was repeated three times, each time with a “fresh sample of the testsolution.5.4.4 In-Situ Concentration ExperimentsSeveral procedures were attempted to try to systematically determine the effectthat different formation conditions would have on in-situ concentration. The78experiments were performed to try to determine if a part of the hydrate-concentrateseparation step could be attained within the vessel.Three sets of experiments were performed, and two procedures wereestablished, A and B. All of the experiments were performed with the lowpressurevessel, the volume of test solution used varied from 200-400 mL. The initialtwo stepsin both procedure A and B were the same. The first step was to form a suitableamount of hydrate crystals such that when they decomposed theywould result in atleast 50 mL, this is the smallest sample volume required to determineconductivity.The second step was to collect the concentrate by blowing it out ofthe vessel underpressure. The last two steps in procedure A were to decompose thehydrates, andcollect the resultant liquid. The remaining steps in procedureB included venting thegas, i.e. dropping the pressure, removing the lid of the vessel,recovering as much aspossible of the hydrates by scooping them out of the vessel andletting themdecompose. The final step in both procedures was to determine if any concentrationhad occurred. In the first two sets of experiments this was done by testing theconductivity ofboth the concentrate and the decomposed hydrates andcomparing thiswith the conductivity of the original solution. Conductivity was chosen as the initialconcentration parameter because it could be determined immediately.In the last setof experiments conductivity, TOC and TIC were determined for the initial effluentand the concentrates.79CHAPTER 6: Results and DiscussionThe first section in this chapter will present an effluent treatment selectionprocedure. The second section will discuss physical characteristics that weredetermined for the effluent and concentrate samples used in this work. The criteriaused to select the hydrate formers, and characteristics of the chosen hydrate formerswill be given in the third section. The fourth section will summarize and discuss thequalitative characteristics of the clathrate hydrate concentration processthat havebeen observed during the experimental part of this work. The incipient clathratehydrate formation pressure-temperature conditions that have been determined for theeffluents and concentrates, with the various hydrate formers, will be presented in thefifth section. The results from the induction and growth experiments will be given inthe sixth section, and the results from the in-situ concentration experiments will bepresented in the seventh section. In the last section the results that have beenobtained in this work, in addition to the information that was collected during thesurvey of TMP and CTMP effluents, will be discussed with respect to their relativeimportance to the development of the clathrate hydrate concentration process.6.1 Effluent Treatment OptionsThe practice of directly discharging untreated liquid effluent into theenvironment will not be acceptable in the future. This was clearly stated in the 1992amendment to the Fisheries Act. As a result, all of the pulp and paper mills currentlyoperating in Canada will be required to either treat their liquid effluentsappropriately, or avoid the discharge of these effluents by implementing a ZLD80system.The effluent treatment systems that will have the least negative impacton thelocal ecosystem, at a specific site, will be those which implement a ZLD technology(refer to Section 3.5). However, these systems are both expensive and energyintensive in comparison with conventional systems. Therefore, until the volumeofliquid effluent that is generated by a mill can be significantly reduced, therewillcontinue to be a need for conventional treatment systems. The objectiveof thediscussion that is presented here is not to determine which treatmentoption is thebest, but rather to present an effluent treatment selection procedure based on theinformation that was collected during the survey performed on theeffluentsgenerated by TMP and CTMP mills, and the current discharge regulations (refertoChapters 2 and 3).The first step in selecting an effluent treatment system is to determine: 1) Thetype, the point source, and the concentration of the contaminants that will be presentin the untreated effluent. These can not be determined before the pulp quality, woodfurnish(es) and pulping process(es) are selected, refer to Chapter 2; and2) Thesensitivity of the receiving environment and the presence of an adequate receivingwater source. These can not be determined before the selected mill site has beenstudied in-depth, refer to Section 3.1.The determination of the point source(s) from which a contaminant originatesmay aid in minimizing both the volume and pollutant load of the effluent that isgenerated by a mill. There are several different internal measures that can beimplemented. In general, these may include: (1) modifying the unit operation(s) thatgenerate large pollutant loads,and/or have high water consumption rates; and (2)81recycling effluent streams that have low concentrations of contaminants so as todecrease the overall volume of effluent that is generated (this is of particularimportance for ZLD systems).The next step is to select the treatment systems that are capable of attainingthe extent of treatment that they will be required. In the case of a conventionalsystem, the current (and if possible the expected future) discharge regulations willdetermine the extent of treatment that will be required. And, in the case of a ZLDsystem, the extent of treatment (or separation) of the recovered water will bedetermined by the effect that process water has on both product quality and processoperations.The final step in this selection procedure is to determine the capital andoperating costs, and any potential long-term negative environmental impacts thatmay be associated with these systems. Then, based on this information the “best”option can be determined. If the treatment system is part of a proposal for a new millthe anticipated response of the public and local government bodies towards the mill,in particular the environmental impacts, must also be considered. Their opinions areimportant because if these bodies consider the “environmental” cost of implementinga selected treatment system to be too high they can delay, or stop, the permittingprocess of a new mill.6.2 Miii Effluent CharacteristicsIt is known that the extent to which electrolyte and organic compounds inhibithydrate formation depends on their concentration in the solution of interest. As aresult, the electrolyte content (conductivity) and the organic content (total organic82carbon, TOC) of the effluent samples (TMP1, TMP2,CTMP, and BCTMP) and theconcentrate samples (TMP2/50,TMP2/85, CTMP/50 and CTMP!80) were determined.In addition, the solids content, the pH, the colour, and the total inorganiccarbon(TIC) concentration of each of the samples were determined. Total solids (TS), volatilesolids (VS) and total suspended solids(TSS) tests were performed to determine thesolids content. The results from all of the aforementioned tests, with theexceptionof the pH and colour, are shown in Figures 6.1 through 6.6. The arrows, to therightof each test name, indicate which y-axis should be used.Effluent samples. The conductivity, and the TOC and TIC concentrationsofthe effluents are shown in Figure 6.1, and the resultsfrom the solids content testsare presented in Figure 6.2. The conductivity of the BCTMP effluent wasfound to be17,000 tS. Thus, the conductivity of the BCTMP effluent was about fivetimes thatof the CTMP effluent, 20 times greater than the TMP1 effluent, and 40times that ofthe TMP2 effluent.The TOC concentration increased from 60 mg/L in the TMP1 effluent to 7830mg/L in the BCTMP effluent. And the TIC concentration increased from less than 10mg/L in the TMP1 effluent to 490 mg/L in the BCTMP effluent.The TS concentration increased from 710 mg/L in the TMP1 effluent to 26,960mg/L in the BCTMP effluent, and the VS concentrationincreased from 240 mg/L inthe TMP1 effluent to 11,690 mg/b in the BCTMP effluent.The TSS concentrationranged from 10 mg/b for the TMP1 effluent to 980 mg/b for theBCTMP effluent.The electrolyte, carbon, and solids content of a TMP or a CTMP effluent alldepend on the rate of water consumption, the pulping process (in particular, thechemicals and operating conditions) and the wood furnish that are used. The BCTMPO 00 O 0 CD 0 4. CDI(mS)(mg/L)(Thousands)35 30- 25-V 20- 15- 10-5 0Conductivity OrganicCarbon:TOCInorganicCarbon:TIC——108 6 4 20ITMP1TMP2CTMPBCTMPMillEffluentsc)(gIL)TotalSolidsVolatileSolids-___SuspendedSolids(mgIL)35 30 25 2015- 10-5- 0—1200 1000800 600 400 200Ci) C O C 4. C Ci) BTMP1TMP2CTMP0BCTMPMillEffluents85effluent was generated at the mill with the lowest rate of water consumption. Themills that generated the CTMP, TMP2, and TMP1 effluents used 2.8, 3.3, and 28times more water than the mill that generated the BCTMPeffluent respectively(refer to Section 5.1). Therefore, based on these large differences in water usage itwas expected that the electrolyte, carbon, and solids content of these effluents woulddecrease as follows: BCTMP> CTMP >TMP2 >TMP1. The only exception to this wasthe electrolyte content of the TMP1 effluent which was greater than that of the TMP2effluent. The reason for this is not known.Another factor that could be responsible for the difference in the organiccontent and the solids content of these effluents is the use of sodium suiphite(Na2SO3)and/or hydrogen peroxide (H20)in the pulping process. As noted in Section2.4.2, the use of these chemicals will increase the TSS, BOD, and toxicity of aneffluent. In general, an increase in the BODand/or toxicity of an effluent will resultin an increase in its TOC concentration because the contaminants that areresponsible for both BOD and toxicity are predominantly organicin nature.Therefore, the higher TOC and TSS concentrations of the BCTMP and CTMPeffluents, as compared with the TMP1 and TMP2 effluents, may partly result fromthe addition of large amounts of sodium suiphite.The higher TOC and TSS concentrations of the TMP2 effluent, as comparedwith the TMP1 effluent, may be partially due to the addition of peroxide during thebleaching stage at the TMP2 mill. The mill which generated the TMP1 effluent doesnot have a bleaching stage, however, it does add small amounts of peroxide duringits pulping process.The pH of the TMP1, TMP2, CTMP, and BCTMP effluents were 6.3, 6.2, 6.9,86and 6.8 respectively. These are all below the “ideal’pH range of 8.0 to 9.5, at whichthe resin and fatty acids are thought to be least toxic.The BCTMP effluent was anopaque, dark orange-brown solution. The CTMP effluent was an opaque, orangesolution. And both the TMP1 and the TMP2 effluents wereclear, pale orangesolutions.Concentrate Samples. The conductivity, and the TOC and TICconcentrations of the TMP2 concentrates are shown in Figure 6.3, and the resultsfrom the solids content tests are presented in Figure 6.4. The conductivity of theTMP2/50 and the TMP2/85 concentrates were 1.9 and 6 times larger than theconductivity of the TMP2 effluent. The TOC and TIC concentrations of the TMP2/50concentrate were 2 and 3 times greater than those determined for the TMP2 effluent.And the TOC and TIC concentrations of theTMP2/85 concentrate were 8 and 5 timeslarger than those determined for the TMP2 effluent.The TS and VS concentrations of the TMP2/50 concentrate were 2.2 and 2.4times greater than those determined for the TMP2 effluent. And the TS and VSconcentrations of the TMP2/85 concentrate were 3.9 and 4.2 times larger than thosedetermined for the TMP2 effluent. The TSS concentration of the TMP2/50 and theTMP2/85 concentrates were 4 and 16.7 times greater than the TSS concentration ofthe TMP2 effluent.The conductivity, and the TOC and TIC concentrations of the CTMPconcentrates are shown in Figure 6.5, and the results from the solids content testsare presented in Figure 6.6. The conductivity of theCTMP/50 concentrate was 2.5times greater than the conductivity of the CTMP effluent. And theCTMP/80concentrate was 3.5 times greater than the conductivity of the CTMP effluent. It3(mS)(mg/L)(Thousands)500 CD: +.Ci) — CDO 01CD CDr-]Conductivity OrganicCarbon:TOCInroganicCarbon:TIC2.52-1.5-1—0.5-04 3 2TMP2TMP2/50TMP2/85MillEffluents88-J-CoFIGURE 6.4: SolidsContent of the TMP2 Concentrate Samples-J0 0 0 0 0 00 0 0 0 0 0CD It) C) C’] 0It)coC’]F-Co1t)G)F-ztt4C’,Cl) 0D (I)Cl) —V0V= C’)oVrr—=—C—o0DF->Cl)I’llC’]HrIt) C1) C’] 000 +. C20(mS)Conductivity OrganicCarbon(mg/L)(Thousands):TOC•InorganicCarbon:TIC1012 10-8- 6 4 2 08 6 4 2CTMPCTMP/50CTMP!80MillEffluents002, CD OCD35(g/L)(mg/L)500Cl) 0 0 CO C2I’30 25 2015 105 0400 300 200 1000CTMPCTMP/50MillEffluentsCTMP/80cc91should also be noted that the conductivity of the CTMP/80 concentrate was only 1.7times less than that of the BCTMP effluent. The TOOand TIC concentrations of theCTMP/50 concentrate were 2.8 and 2.5 times greater than those determined for theCTMP effluent. And the TOO and TIC concentration of the CTMP/80 concentrate was6.8 and 1.2 times larger than the CTMP effluent. The reason for the low TICconcentration of the CTMP/80 effluent is not known.The TS and VS concentrations of the CTMP/50 concentrate were 1.9 and 1.6times greater than those determined for the CTMP effluent. And the TS and VSconcentrations of the CTMP/80 concentrate were 5.7 and 4.0 times larger than thosedetermined for the CTMP effluent. The TSS concentrations of theCTMP/50 and theCTMP/80 concentrates were 2.5 and 3.6 times greater than theTSS concentration ofthe CTMP effluent.The pH of the TMP2/50, TMP2/85, CTMP/50, andCTMP/80 concentrates were6.6, 5.5, 7.6, and 9.2, respectively. All of the four concentrates were avery darkorange-brown colour, darker than the BCTMP effluent. However, the CTMPconcentrates were appeared clear, in comparison to theTMP concentrates.6.3 Selection of Hydrate FormersSeveral criteria were used to determine which of the over one hundred hydrateformers would be most suitable for this study. The initial criteria, referred to as“gate” criteria, were used in the selection of environmentally acceptable hydrateformers. These criteria included selecting hydrate formers that were nontoxic, had alow ozone depletion potential (ODP), low global warming potential(GWP), lowexplosion potential, and were nonflammable. Flammability was not considered to be92a major issue if extensive knowledge on how to properly handle the particularhydrate former on an industrial scale was available.The next criterion that was used utilized the hydrate formation equilibriumdata for the compound of interest in pure water. This data determined which of theenvironmentally suitable hydrate formers were capable of forming hydrates attemperatures well above 0 °C, and at pressure as close as possible to atmospheric.Thermodynamic models were used to predict these data for those compounds whereexperimental data was not available. This criterion was of particular importancebecause the hydrate formation temperature must be well above the freezing point ofthe solution for the clathrate hydrate concentration process to have an energyadvantage over freeze concentration.The other criteria that were used included the solubility of the hydrate formerin water, its cost and its availability. The solubility of the hydrate former in wateraids in determining what type of a separation step will be required, if at all, torecover the hydrate former from the water phase once the hydrates have beendecomposed. The extent to which the hydrate former must be recovered will dependon both the cost of the hydrate former and the degree of purity that will be requiredfor the recovered water, i.e. the process water. Finally, the cost and the availabilityof the selected compounds are important criteria because they aid in determiningwhether the clathrate hydrate concentration process has the potential to becomeeconomical.Based on the above selection criteria the two hydrate formers that wereinitially chosen for this study were carbon dioxide and propane. Characteristics ofinterest for both of these compounds can be found in Table 6.1.93TABLE 6.1: Hydrate Former Characteristics for Propane and CarbonDioxideCHARACTERISTIC CARBON PROPANEDIOXIDEMaximum equilibrium 10.1 °C at 5.3 °C atT & P in pure water (quadruple 4.5 MPa 0.542 MPapoint)Ozone Depletion Potential 0 0(ODP)Global Warming Potential 1.0 -(GWP)Explosion Potential None Lower level 2.1%Upper level 9.5%Flammability Nonflammable Autoignition temp. 432°CToxicity Asphyxiant AsphyxiantTLV 5000 ppm (no threshold limitgiven)Solubility in water at 6.1 wt. percent 0.06 wt. percentmaximum temperature(quadruple point)Structure of Unit Cell IIIMaximum number of water 7.3 17.95molecules per hydrate formermolecule in the hydrate phaseHydrate Density (g/cm3) 1.112 0.88Heat of hydrate formation - 55.0 - 134.2(kJ/mol hydrate)It is known that propane is flammable and has an explosion potential if not handledproperly. However, due to its wide spread use in other industries it was decided thatprocedures known to minimize both of these risks could be implemented. Carbon94dioxide falls short in three areas. It requires pressures up to 4.5 MPa to formhydrates, it is more soluble in water than propane, and it is a known greenhouse gas.However, it is nontoxic, poses less of an environmental hazard in comparison withpropane, and can form hydrates at temperatures up to 10 °C. Two other hydrateformers were also considered, cyclopropane and the new refrigerant R-134a. It wasfound that the cost of cyclopropane and R-134a were much higher than the cost ofpropane and carbon dioxide. In addition, it was difficult to obtain a small quantityof the R-134a refrigerant for lab purposes.Based on the results from the experiments performed with propane and carbondioxide, a propane-carbon dioxide mixture was chosen as the third hydrate former.It is well known (from thermodynamicconsiderations) that the hydrate formationpressure, at a given temperature, will be less with a propane-carbon dioxide mixture,due to the presence of propane, than with pure carbon dioxide. Furthermore, it wasassumed that hydrates would nucleate more readily in the presence of a propane-carbon dioxide mixture than in the presence of pure propane. This latter point wasverified experimentally.The make-up of the mixture that was determined from pressure formationpredictions. The formation pressures for 100% propane, a 50-50 mol% propane-carbondioxide mixture, a 30-70 mol% propane-carbon dioxide mixture, and 100% carbondioxide in pure water at 3 °C were predicted with thermodynamic models. The PengRobinson Equation of State model was used to model the vapour phase, Henry’s LawConstant was used to model the aqueous phase, and the Van der Waals-Platteeuwmodel was used to model the hydrate phase. The predicted formation pressures were0.32, 0.39, 0.48, and 1.23 MPa for 100% propane, the 50-50 mol% mixture, the 30-7095mol% mixture, and 100% carbon dioxide, respectively. Based on these predictions a30-70 mol% propane-carbon dioxide mixture was chosen.It was predicted that the maximum hydrate formation pressure attainable withthis mixture was between 9 and 10 °C. The formation pressure at 9 °C was predictedto be 1.31 MPa. The propane-carbon dioxide mixture forms structure II hydratecrystals. Carbon dioxide molecules can occupy both the small and the large cavities,whereas propane molecules can only occupy the large cavities. If all of the smallcavities are filled with carbon dioxide and the large cavities are filled with propanethen a unit cell would contain 136 mol H20 : 16 mol C02:8 mol C3H8.6.4 Qualitative Process CharacteristicsThe prime objective in visually observing the formation process was to collectqualitative information on the hydrate crystals that formed. However, changes in thepH, colour, and carbon content of several of the effluent samples were noted. Theresults presented here were observed during the pressure-temperature, induction andgrowth, and concentration experiments that were performed during this study.Hydrate Crystal Formation. The crystals that formed in the presence ofpropane varied in colour. The initial crystals that formed floated on top of the effluentand were white, similar to snow. As the hydrate mass grew all of the visible crystalsappeared to have the same colour as the initial effluent. The reason for this colourchange is most likely due to occlusion of effluent in the vacant space betweenindividual crystals and/or coating of the crystals with the effluent.The crystals that formed in the presence of propane did so in a donut shapedsolid that formed up the walls of the vessel. It was postulated that the solid that had96formed in the bulk of the effluent was not all hydrate crystals but rather a “gel-like”structure which was composed of large amounts of effluent caught in the intersticesbetween individual crystals. This follows from a comparison between the maximumamount of propane that was available in the vessel (determined from the initialpressure - the equilibrium pressure) and the theoretical value, assuming 100% of thelarge cavities are filled with propane molecules. The results for two separate runs inwhich the “gel-like” structure formed are shown in Table 6.2. The theoretical valuewas calculated from the maximum propane to water ratio, i.e. 17 mol C3H8 : 1 molH20, for hydrate formation. The large difference between the theoretical values andthe experimental values show that the structure which was present could not haveused all of the available water molecules in the hydrate structure. Therefore, themost likely conclusion is that water molecules, i.e. liquid effluent, is caught betweenthe crystals.TABLE 6.2: Moles of Propane Gas Required and Available to Convert100% of an Effluent Sample to HydratesVolume Temp Driving Force Moles of Propane Maximumof (K) (AP MPa) Required Number of MolesSample (Theoretical) of Propane(mL) Available(Experimental)300 275 0.4 0.136 0.09350 276 0.19 1.307 0.0497The hydrate crystals that formed in the presence of carbon dioxide at highpressures (i.e. large driving forces) were initially suspended in the liquid phase andappeared to be white, similar to snow. The crystals that formed with small drivingforces were more susceptible to forming on the vessel walls initially, and were similarin colour to the effluent. Foaming was observed during these experiments, howeverthe “gel-like” structure was not apparent.Crystal growth in the presence of the propane-carbon dioxide mixture was verysimilar to that described for carbon dioxide. However, a “gel-like” structure, similarto that which formed in the presence of propane, was apparent during theseexperiments. It should be noted that the “gel-like” structure formed slower in thepresence of the propane-carbon dioxide mixture than in the presence of propane.pH, Colour, and Carbon Content. Changes in the pH and colour of theeffluent that was removed from the vessel, after hydrates had been formed anddecomposed at least once, were observed during several experiments. There was nochange noted in either the pH or the colour of the CTMP effluent after being exposedto propane. Colour changes in the other effluent and concentrate samples, i.e. TMP1,TMP2, BCTMP, CTMP/50 andCTMP/80, that were exposed to propane were notobserved.The colour of the TMP1 and CTMP effluent exposed to carbon dioxide changedfrom an orange to an orange-grey colour, and a black precipitate formed in both cases.There was no colour change or precipitate formation in the experiment with purewater and carbon dioxide. The grey CTMP effluent returned to a slightly darkershade of its original colour and some of the precipitate dissolved when concentratedNaOH was added to a sample of this effluent.98Both pH and colour changes were observed during the five experiments thatwere performed with CTMP effluent samples in the presence of the propane-carbondioxide mixture. The initial pH of these samples ranged from 7.0 to 7.7, whereas thefinal values, after being exposed to the propane-carbon dioxide mixture, ranged from5.6 to 6.0. The colour of each of the CTMP effluent samples changed from an orangeto an orange-grey colour, and a black precipitate was formed. Upon addition ofconcentrated NaOH the colour changed back to an orange hue, slightly darker thanthat of the original, and some of the precipitate dissolved. These results were similarto those obtained with the CTMP effluent in the presence of carbon dioxide.Changes were also noted in the carbon content of a CTMP effluent sample thatwas exposed to the propane-carbon dioxide mixture. The TOC and TIC concentrationsof the sample were determined before the sample was placed in the hydrating vessel,and after it had been exposed to the propane-carbon dioxide mixture for five days,during which hydrates were formed and decomposed twice. The initial TOCconcentration was 1159 mg/L and the final concentration was 552 mgIL. The initialTIC concentration was 182 mg/L and the final concentration was 1008 mg/L.Summary. It was found that hydrate nucleation occurred more readily in purewater than in the effluents in the presence of both carbon dioxide and the propanecarbon dioxide mixture. A “gel-like” structure formed in the presence of both propaneand the propane-carbon dioxide mixture.The pH decreased and the effluent colour changed from an orange to anorange-grey colour in the presence of both carbon dioxide and the propane-carbondioxide mixture. The TIC concentration increased and the TOC concentrationdecreased in the presence of the propane-carbon dioxide mixture. Similar changes99were not noted in the presence of propane. Therefore, these results suggest that achemical reaction may be occurring between the compounds introduced into theeffluent during the pulping process, water, and carbon dioxide.6.5 Pressure-Temperature Hydrate Formation ConditionsIncipient equilibrium hydrate formation pressures, at temperatures above thenormal freezing point of water, were determined for several of the effluent andconcentrate samples in the presence of the three selected hydrate formers. Theresults from these experiments are shown on pressure-temperature diagrams. Datafor the hydrate formation conditions in pure water with the hydrate former of interestare also shown. It should be noted that at a given temperature the points representthe minimum pressure at which hydrate crystals can form in a particular aqueoussolution. Thus, they define the pressure-temperature operating conditions of aclathrate hydrate crystallization unit.Propane. Formation experiments, in the presence of propane, were performedwith each of the four effluent samples and two of the concentrate samples. Theresults are presented in Figure 6.7 and 6.8, and given in Table 6.3. The formationpressures for the TMP1, TMP2, and CTMP effluents were only slightly above thoserequired for hydrate formation in pure water, at a given temperature. Therefore, theconcentrations of the electrolyte and organic compounds present in these effluentswere not large enough to significantly alter the hydrate formation conditions. Thehydrate formation pressure required by the BCTMP effluent sample at 275.15 K was12% higher than the corresponding pressure required to form hydrates in the other100FIGURE 6.7:Pressure-Temperature Hydrate Formation Data fortheEffluent Samples with Propane0.65_________________C Kubota et al. 1984 (pure water)0.55CTMP__• BCTMP•• TMP10.450 TMP20.35Cl)00.250.15• • •273 274 275 276 277 278 279Temperature (K)101FIGURE 6.8: Pressure-TemperatureHydrate Formation Data for theCTMP Concentrates with Propane0.6__________________DKubota et al. 1984 (pure water)aCTMP0.5• CTMP/50ACTMP/80a00.4a)L.0.3C)I- a00.20.1I • I • I • I • I273 274 275 276 277 278 279Temperature (K)102TABLE 6.3: Propane Hydrate Formation Pressure-Temperature DataTemperature Pressure (MPa)(K)CTMP BCTMP TMP1 TMP2CTMP/ CTMP80 /50273.6 0.183274.2 0.224274.5 0.241275.6 0.301276.6 0.377277.6 0.481278.0 0.529275.1 0.287276.3 0.398277.0 0.501274.2 0.225276.1 0.324277.6 0.467274.3 0.225276.5 0.356277.8 0.501273.7 0.212276.3 0.377277.4 0.501274.5 0.230276.0 0.332277.9 0.515103effluent, and at 277 K the required pressure was 26% higher (refer to Figure 6.7).These elevated formation pressures can be attributed to the high electrolyte(conductivity) and organic (TOC) content of the BCTMP effluent, as was discussed inSection 6.2.It was found that propane hydrates were readily nucleated in the TMP1,TMP2, and CTMP effluents, but that they were more difficult to nucleate in theBCTMP effluent, especially at temperatures above 275.15 K. Nucleation was inducedin these experiments by one of the following methods: 1) the pressure was set at avalue that was significantly greater than the equilibrium pressure, therebyincreasing the driving force (AP); 2) the mixing mechanism (the stirrer) wascontinuously turned on and off (this is known to enhance nucleation); or 3) largeamounts of hydrates were formed at a low temperature, then the temperature wasincreased to the desired set point. The last method, the most time consuming of thethree options, was required to nucleate hydrates in the BCTMP effluent attemperatures above 275.15 K.The two TMP concentrates were not tested because their conductivity and TOCconcentrations were less than those determined for the CTMP/80 concentrate. Theresults from the experiments with the two CTMP concentrates, the CTMP effluent,and pure water with propane are shown on Figure 6.8 and given in Table 6.3. It wasfound that the nucleation of hydrates in the CTMP/80 concentrate, like the BCTMPeffluent, was difficult, especially at the higher temperatures. The results presentedin Figure 6.8 show that recovering 50% of the water in the CTMP effluent (CTMP!50)would not affect the formation conditions. However, the recovery of 80% of the water(CTMP/80) would affect the hydrate formation conditions, as shown by the increased104formation pressure required by the CTMPI8O concentrate, at a given temperature, incomparison with that required with the CTMP effluent. As expected, the formationpressure, at a given temperature, increased with increasing electrolyte content, i.e.the formation pressure of the BCTMP effluent was greater than that required by theCTMP/80 concentrate, which was larger than that required for the CTMP effluent.However, a similar correlation was not found with the organic content of thesesolutions (the TOC concentrations of the CTMP/80 concentrate was similar to thatof the BCTMP effluent). These results suggest that hydrate formation in theCTMP/80 concentrate was mostly influenced by its electrolyte rather than its organiccontent.Carbon Dioxide. The experiments that utilized carbon dioxide as the hydrateformer were performed with pure water, the TMP2 and the CTMP effluents. Thesepressure-temperature hydrate formation data are presented on Figure 6.9 and inTable 6.4. The results yielded similar conclusions to those obtained with propane.Both effluents formed hydrates at pressures only slightly above those for pure water,at a given temperature. As expected, the formation pressures required, at a giventemperature, in the presence of carbon dioxide were larger than those required forhydrate formation in the presence of propane. For example, the pressurerequirements for the CTMP effluent at 274.4 K was six times larger with carbondioxide than with propane. The pure water data determined with carbon dioxide werein good agreement with similar data available in the literature. These pure waterdata aid in validating the experimental equipment and procedures that were used inthis study. It was found that the hydrate crystals formed rapidly in the TMP2 andthe CTMP effluents at all of the set point temperatures.105FIGURE 6.9: Pressure-TemperatureHydrate Formation Datafor theTMP2 and the CTMP Effluents, andDeionized Water withCarbon Dioxide5._______________________D This work(pure water)• Robinsonand Mehta, 1971 (pure water)1CTMPo TMP2003Cl)CoC)21!1• I • I • I • I272 274 276 278 280 282 284Temperature (K)106TABLE 6.4: Carbon Dioxide Hydrate Formation Pressure-TemperatureDataTemperature Pressure (MPa)(K)CTMP TMP2 Pure water(This work)274.4 1.440275.9 1.720277.3 2.025278.4 2.360280.0 2.900281.5 3.600282.6 4.290274.5 1.400280.3 2.950282.1 3.800273.6 1.310275.1 1.550276.8 1.900278.2 2.260281.3 3.380282.8 4.350Propane-Carbon Dioxide Mixture. Pressure-temperature data wasdetermined for pure water, the CTMP effluent and theCTMP/80 concentrate in thepresence of the 30-70 mol% propane-carbon dioxide mixture. The results are shownon Figure 6.10 and given in Table 6.5. As expected, the hydrate formation pressure,at a given temperature, in the CTMP effluent was very similar to that determined107FIGURE 6.10: Pressure-Temperature HydrateFormation Data for theCTMP Effluent, the CTMP/80 Concentrateand DeionizedWater with a Propane-Carbon Dioxide Mixture0.80.70pure waterCTMP• CTMP/800.60G)IzCl) 0.5G)0.4•0.3—. • • • •273 274 275 276 277 278 279Temperature (K)108TABLE 6.5: Propane-Carbon Dioxide Mixture Hydrate FormationPressure-Temperature DataTemperature Pressure (MPa)(K)CTMP CTMP/80 Pure water(This work)274.6 0.429277.2 0.656278.0 0.739273.8 0.336275.0 0.429277.5 0.629274.4 0.429276.5 0.598278.2 0.777for pure water. However, it was found that hydrates formed in the CTMP/80concentrate at pressures below those required by the CTMP effluent and pure water,at a given temperature. The reason for this is most likely due to a difference in thesolubility of carbon dioxide and propane in the CTMP/80 concentrate as compared totheir solubilities in the CTMP effluent. A decrease in the solubility of propane in theCTMP/80 concentrate will result in an increase in the relative solubility of carbondioxide in this solution. The reason for this differential change in propane-carbondioxide solubility can be attributed to the “salting-out” effect caused by the presenceof electrolyte and organic compounds. Thus, the propane concentration in the gasphase will be higher in the experiment performed with the CTMP/80 concentrate thanin the one performed with the CTMP effluent. As was mentioned in Section 6.3 the109formation pressure, at a given temperature, will decrease ifthe propane concentrationin the gas phase increases relative to the carbon dioxide concentration.Thermodynamic models predicted that a 50-50 mol% propane-carbon dioxide mixturewould require a lower pressure to form hydrates, at a given temperature, than a 30-70 mol% propane-carbon dioxide mixture.Summary. Hydrate crystals formed more readily in the CTMP effluent withthe propane-carbon dioxide mixture than with propane, but not as readily as theyformed in the presence of carbon dioxide. And, as expected, the hydrate formationpressure, at a given temperature, decreased as the concentration of propane in thehydrate forming mixture increased. For example, the formation pressure at 3 °C inpure water was determined to be 0.323 MPa (Kubota, 1984) with propane, 0.56 MPawith a 30-70 mol% propane-carbon dioxide mixture, and 1.768 MPa with carbondioxide.The overall pressure-temperature results indicate that the TMP1, TMP2 andCTMP effluent samples used in this study did not contain electrolyte and/or organichydrate inhibitors at concentration levels that could significantly alter the hydrateformation conditions, whereas the BCTMP effluent sample did. It was also found that50% of the water present in the CTMP effluent sample could be recovered withoutsignificantly altering the pressure-temperature hydrate formation conditions, whereasrecovery of 80% of the water would affect the formation conditions. As expected, theformation pressure, in the presence of propane at a given temperature, with theCTMP/80 concentrate increased compared with the CTMP effluent. Whereas theformation pressure with the propane-carbon dioxide mixture, at a given temperature,in the CTMP/80 concentrate decreased compared with the CTMP effluent.1106.6 Induction and Growth Rate ExperimentsThese experiments were designed to determine the length of the inductionperiod and the rate of hydrate crystal growth. Because the mill effluents are complexsolutions we wanted to perform a number of kinetic experiments to see if the resultswould agree with the nucleation characteristics of hydrate formation in well definedsystems, i.e. hydrocarbon-water, in which the majority of the previous work has beendone. Experiments were attempted with both propane and the propane-carbon dioxidemixture. The point(time and pressure) at which hydrates formed in each of theexperiments, Figure 6.11 through 6.15, is indicated with an arrow and the runnumber.Propane. The experiments performed with propane were aborted due tooperational problems. Propane hydrates plugged both the gas inlet and pressuregauge lines such that the pressure drop could not be accurately monitored.Propane-Carbon Dioxide Mixture. A temperature of 1 °C was selected toperform the experiments. Higher temperatures could have been chosen but wedecided to work with a small induction time, less than one hour, and have moreflexibility with an increased driving force, i.e. expressed as a pressure difference (P= Texp -Four runs were performed. For each run a fresh 200 mL sample of CTMPeffluent was used, and the stirrer bar was set at 200 rpm. The results from theinduction period and the first growth period (la, 2a, 3a, and 4a) are shown on Figure6.11. The equilibrium pressure,eqat 1 °C for the CTMP effluent is 0.420 MPa,determined from Figure 6.10. The initial pressures,PeXp’were 0.860, 0.846, 0.736, and0.598 MPa for runs la, 2a, 3a, and 4a respectively. And the corresponding AP’s were111FIGURE 6.11: Induction Period andthe First Growth Rate PeriodforRun 1,2,3 and 4 with the CTMP EffluentPressure (MPa)0.95la+2a3aL4a0.85-— Hydrate Nucleation-I10.75:-+--1DD,hla++0.65,3a+0.554aLI’DLILI0.45 I I I0 2040 6080 100Time(minutes)1120.440, 0.426, 0.316, and 0.178 MPa.It can be seen that for runs la, 3a and 4a hydrates formed within12 minutes,whereas it took 20 minutes for hydrates to formin run 2a. Variation in thenucleation time (induction period) is expected since nucleation isnot a completelydeterministic process.In each of these four runs a second growth period was measured (ib, 2b,3b,and 4b). The results are shown on Figure 6.12, 6.13, 6.14, and6.15 for runs 1, 2, 3,and 4 respectively. In run lb and 2b hydrates formed immediately,and in run 3bthey formed after 5 minutes, whereas in run 4b hydratesdid not form within the onehour test period. The immediate formation of hydratesin run lb and 2b can beattributed to the history of the effluents. Hydrates were formedand decomposed inthese effluents prior to the second growth period and, as a result, thesolution hadbecome more structured such that hydrates formed more readily.The reason thathydrates formed later in run 3b and did not form within the one-hourtime period inrun 4b is most probably a result of the small AP values used in these runs,0.288 and0.128 MPa respectively. In summary, these results suggestthat both the history ofthe effluent and the magnitude of the driving force affect the growthpattern ofhydrate formation in a manner similar to that observedwith well defined systems.6.7 In-Situ Concentration ExperimentsThe main objective of these experiments was to try to determine if the hydrateformation conditions (given the configuration of our apparatus) couldbe controlledsuch that in-situ concentration could be realized. It was hypothesized that if theprocess could be adequately controlled a layer of hydrates, that did not contain113FIGURE 6.12: Induction Period, andthe First and Second GrowthRatePeriods for Run 1 with the CTMP EffluentPressure (MPa)la0.95XlbHydrate Nucleation- lb0.85<0.75-0.65><0.55-><0.45 I I I0 20 40 60 80 100Time (minutes)114FIGURE 6.13: Induction Period, and theFirst and Second Growth RatePeriods for Run 2 with the CTMP EffluentPressure (MPa)+ 2a0.950.80.75K2b2bHydrate Nucleation2a++++0.65++++0.55+++0.450 20 40 60 80 100Time (minutes)115FIGURE 6.14: Induction Period,and the First and Second GrowthRatePeriods for Run 3 with the CTMPEffluent0.750.650.550.45Pressure (MPa)0 100* 3aA3bHydrate Nucleation3a/*AAAA***20 40 60 80Time (minutes)116FIGURE 6.15:Induction Period,and the First and SecondGrowth RatePeriods forRun 4 with the CTMP EffluentPressure(MPa)0.59-E4aZ4b0.57Hydrate Nucleation0.550.53z4a0.514bEl ZX No Hydrates0.490.470.45I II0 20 4060 80100Time(minutes)117occlusions and were not coated by impurities,could form on top of the concentrateand grow in a downward motion. In the absenceof stirring, this concept is shownschematically in Figure 6.16. Four tests were performedwith propane, and four withthe propane-carbon dioxide mixture. The stirringrate was set at 200 rpm for each ofthese experiments.Propane. In the first experiment 300 mL of theCTMP effluent was used, thetemperature was set at 2 °C, and the initial pressure was set to0.65 MPa. Thepressure dropped to 0.61 MPa. The effluent formed intoa donut shaped solid up thewalls of the vessel, resembling a “gel-like”structure (refer to Section 6.4). No liquidwas visible in the vessel, and asa result no concentrate could be collected. In thesecond experiment, 350 mL of the CTMP effluentwas used with the temperaturemaintained at 3 °C and an initial pressureof 0.47 MPa. It was thought that byincreasing the effluent volume and the set point temperature,and decreasing theinitial pressure the “gel-like” structure wouldnot form as readily. However, the “gel-like” structure did form. In the third experiment, 400mL of CTMP effluent was used,the temperature was set at 3 °C,and the initial pressure at 0.54 MPa. The same “gel-like” structure formed.Based on the results from these initial experiments it was decidedto use a 5wt% salt solution in place of the effluent. The aim of this experimentwas todetermine whether the “gel-like” structure was associated withthe propane or theeffluent, and to see if in-situ concentration could be attainedwith a less complexsolution. For this test 400 mL of the salt solution wasused, the temperature was setat 2 °C, and the initial pressure was 0.412 MPa. The same “gel-like”structure formedwith this solution. As a result of the limitations caused by the “gel-like”structureFIGURE 6.16: Ideal In-SituConcentration118:Hydrate Forming Gas4 MovingInterface119in-situ concentration in the presence of propane was not observed.Propane-Carbon Dioxide Mixture. In the last four concentrationexperiments hydrates were formed in the presence of the propane-carbon dioxidemixture. The first experiment was performed with 400 mL of a 1% salt solution, thetemperature was maintained at 2 °C, and the initial pressure was set to 0.62 MPa.The pressure decreased to 0.425 MPa, however only a small amount of hydratecrystals had formed. As a result, a pressure of 0.75 MPa was manually maintainedin the vessel to try to promote the growth of hydrates. After five days the majorityof the solution had formed hydrates, and only a small amount of liquid, i.e.concentrate, was visible in the vessel. The “gel-like structure was not apparent inthis experiment. The initial conductivity of this salt solution was 17.6 mS, similar tothat of the BCTMP effluent which was 17.0 mS. The conductivity of the concentratethat was collected was 19.0 mS, slightly higher than that of the original solution.The last three experiments were all performed at a temperature of 2 °C with200 mL of the CTMP effluent. The crystal structure that formed during theseexperiments appeared to be similar to the “gel-like” structure that formed withpropane. However, it did not form as rapidly. As a result, concentrate samples couldbe collected.The conductivity, TOC and TIC concentrations of the initial effluents, theconcentrates, and the final (decomposed hydrate) solutions are given in Table 6.6.These results show that some in-situ concentration did take place. This can beconcluded because the concentration of the electrolyte (conductivity) compounds andthe concentration of the carbon (TOC and TIC) compounds decreased as follows:recovered water samples (final) < effluent samples (initial) < concentrates.120TABLE 6.6: Conductivity, TOC and TIC of CTMP EffluentsDeterminedDuring In-Situ Concentration ExperimentsRun # Conductivity (mS) Carbon Content (mg/L)TOC TIC6 initial : 3.06 1159 182concentrate : 3.20 1399 175final : 2.89 1433 2017 initial : 3.07 1159 182concentrate : 3.29 1433 163final : 2.60 ND ND8 initial : 3.04 1159 182concentrate : 3.32 1400 182final : 2.90 ND NDND: Not DeterminedHowever, the ideal in-situ process that was described in Chapter 5 and shownschematically on Figure 5.5, in which the hydrate crystals formedas a layer on topof the solution without the presence of any impurities (either occludedor coating thecrystals), did not occur. The reason for the lack of significant in-situ concentrationthat occurred during these experiments may resultfrom the limitations posed on theprocess by the formation of the “gel-like’ structure.6.8 Overall DiscussionThe overall scope of this work was to aid in the development of a clathratehydrate concentration process that can recover clean water from the liquideffluentstream that is generated at a TMP or CTMP mill such that it can be reused. Similarwork has not been reported in the literature. The only relevant previous work was121concerned with seawater desalination. However, two major differencesare apparentbetween our system and the systems that are used for desalination:(1) TMP andCTMP effluents solutions are more complex, they contain both organic and inorganiccompounds; and (2) The extent of separation required to produce potable water(theobjective of clathrate hydrate desalination systems) is much higher than thatrequired for the recovery of process water.The motivation for performing the survey presented in Chapters 2 and 3 wasto determine the “environmental” need to treat TMP and CTMP effluentswith a ZLDsystem. It was found that untreated TMP and CTMP effluents can exhibit bothlethaland sub-lethal toxic impacts on organisms present in a receivingwater, whereasconventionally (biological) treated effluents are in general not acutely toxic. However,based on the impact studies performed to date it can not be determinedwhether thecontinuous discharge of treated TMP and CTMP effluents will resultin long-termsub-lethal impacts. Therefore, the “environmental” need forZLD effluent treatmentsystems can not be verified.It has been suggested that the best way to maintaina sustainableenvironment, with respect to liquid effluent streams, is to implementa ZLD system.Because, in addition to alleviating the potential for impacts to occurin the localenvironment as a result of discharging a liquid effluent, these systemsminimize theuse of water, a known commodity. Currently, the only proven ZLD technologyavailable to treat TMP and CTMP effluents is evaporation. Basedon the potentialeconomic and operational advantages of the clathrate hydrate concentration process,in comparison with evaporation, this work was performed.Several observations that will aid in the developmentof the clathrate hydrate122concentration process have been made. Clathrate hydrates can form readily in bothTMP and CTMP effluents. The most concentrated effluent sample, the BCTMPeffluent, has similar characteristics to the type of effluent that a clathrate hydrateconcentration process would be required to treat. The reason being that the millwhich generated this effluent sample has a very low water consumption rate, one ofthe factors that will affect the economics of a ZLD system. Based on environmental,economical, and operational considerations a propane-carbon dioxide mixture is a verysuitable clathrate hydrate former to use in the process. And, results from the batchexperiments that were performed can be used to design and operate a continuoussystem. The specific conclusions of this work, and recommendations for future workare presented in the next chapter.123CHAPTER 7: Conclusions and RecommendationsIn an effort to obtain technical data for the development of a process for theconcentration of TMP and CTMP mill effluents through clathrate hydrateconcentration propane, carbon dioxide and a 30-70 mol% propane-carbon dioxidemixture were selected as suitable hydrate forming substances. Propane hydratesformed readily in all of the mechanical effluents and concentrates thatwereexamined, with the exception of the samples with high electrolyte contents. Carbondioxide was tested with two effluent samples, and the propane-carbon dioxide mixturewas tested with one effluent sample and one concentrate sample. It was found thathydrates formed readily in each of the experiments performed with these hydrateformers.The pressure-temperature hydrate formation conditions (partial phasediagram) for all three gases were determined. It was found that the presence of theelectrolyte and organic compounds, in the less concentrated TMP andCTMPeffluents, altered the pressure-temperature equilibrium locus only slightly comparedwith pure water for each of the three hydrate formers. In the presence of propane thepressure-temperature equilibrium did change with the BCTMP effluent and theCTMP/80 concentrate, which both had relatively high electrolyte and organicconcentrations compared with the other effluents. In both cases the locusshifted toa higher formation pressure at a given temperature. However, the pressuretemperature equilibrium locus shifted to a lower formation pressure at a giventemperature with the CTMP/80 concentrate in the presence of the propane-carbondioxide mixture.124It was found that the induction period and the rate of hydrateformationdepend on the driving force, expressed as a deviation in pressure from theequilibrium hydrate formation pressure at a given temperature, andthe history ofthe effluent sample. Overall, these experimentsdemonstrated that thethermodynamics and kinetics of clathrate hydrate formationin TMP and CTMPeffluents exhibit similar behaviour to those ofwell defined systems, i.e. hydrocarbon-water. 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Sustainable Development or Legislated Decline”, InternationalEnvironmental Symposium, 1:53-65 (1993).Zaidi, A., Buisson, H., and S. Sourirajan, “Ultrafiltration in the Concentration ofToxic Organics from Selected Pulp and Paper Effluents”, TAPPI EnvironmentalConference, 1:453-468 (1991).137Ziering, M.B., Emmermann, D.K., and -W.E. Johnson, “Concentration of IndustrialWastes By Freeze Crystallization”, AIChE Symposium Series: Water-1973,70(136):550-556 (1974).138APPENDIX: Effluent CharacteristicTestsTotal Solids, Volatile Solidsand Fixed Solids (TS, VS,FS):Procedure1. Burn crucibles at 550 ± 50°C for 1 hour, cool to room temperature,andplace in desiccator for 4 hours.2. Weigh the crucibles(A) immediately before use.3. Place the crucibles at 103- 105 °C for 48 hours with a known volumeofsample. The resulting residueshould be between 2.5 to 200mg, if notthe procedure must be redone.4. Let the crucibles cool, andplace them in a desiccator for 4 hours.5. Weigh the crucibles (B).6. Burn the crucibles at 550± 50 °C for 1 hour, cool to room temperature,and place in desiccator overnight.7. Weigh the crucibles(C).8. Perform triplicates for eachsample.CalculationsTS = VS (Volatile Solids) +FS (Fixed Solids)TS = (B - A)! Sample VolumeFS = (C - A)! Sample VolumeVS=TS-FS139II Total Suspended Solids (TSS):Whatman Glass MicroFibre Filters GF/A, 1.6 m, were used.Procedure1. Wash the filter apparatus with distilled water.2. Place the filter paper in the apparatus and wash with three successive20 mL portions of distilled water.3. Remove the filter paper, place in a crucible, and burn at 103 - 105 °C for1 hour.4. Cool to room temperature, and place in desiccator for 4 hours.5. Weigh the crucible plus filter paper (A), immediately before use.6. Place filter paper in apparatus, and wet paper with a small volume ofdistilled water to seat it.7. Filter a known volume of sample through the paper. It may be necessaryto dilute the sample to be within the acceptable residue range of 2.5 to200 mg. If it takes more than 15 minutes to filter the solution, then redothe experiment with a smaller volume.8. Wash the sides of the filter apparatus and the filter paper with threesuccessive 10 mL volumes of distilled water.9. Remove the filter paper and place it in the corresponding crucible anddry at 103-105 °C overnight.10. Let the crucibles cool, then place them in a desiccator for 4 hours.11. Weigh the crucibles (B).12. Perform triplicates for each sample.CalculationsTSS = (B - A)! Sample Volume


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