"Applied Science, Faculty of"@en . "Chemical and Biological Engineering, Department of"@en . "DSpace"@en . "UBCV"@en . "Gaarder, Cathrine"@en . "2009-02-20T19:46:21Z"@en . "1993"@en . "Master of Applied Science - MASc"@en . "University of British Columbia"@en . "This work presents data toward the development of a new zero-liquid discharge\r\n(ZLD) system, the aim of which is to concentrate a liquid effluent thereby recovering\r\nprocess water. The crystallization technology used in this work is clathrate hydrate\r\ncrystal formation. A screening of clathrate hydrate formers was performed and three\r\nwere chosen, propane, carbon dioxide, and a 30-70 mol% propane - carbon dioxide\r\nmixture. Experiments were carried out with these substances in a vessel that was\r\nimmersed in a temperature controlled bath. The effluent samples used in this study\r\nwere generated at four pulp mills, an unbleached and a bleached thermo-mechanical\r\npulp mill (TMP1, TMP2), a bleached chemi-thermo-mechanical pulp mill (BCTMP),\r\nand a combined bleached BCTMP/TMP mill (CTMP).\r\nThe aim of this work was two-fold. The prime objective was to study\r\nexperimentally the formation of clathrate hydrate crystals in these effluents.\r\nExperiments were designed to measure: (1) The temperature and pressure conditions\r\nat which hydrates form in mechanical pulp mill effluent; (2) The induction time and\r\nthe growth rate of crystal formation; and (3) The ability of this apparatus to\r\nconcentrate the effluents in-situ. In addition, effluent characteristic tests were\r\nperformed, and qualitative process characteristics were noted. A secondary objective\r\nwas to perform a survey on the liquid effluents generated at TMP and CTMP mills.\r\nThis survey focused on the type, concentration, source, and environmental impacts\r\nassociated with the contaminants typically present in these effluents, on the current\r\ndischarge regulations, and on the available treatment options.\r\nIt was determined that hydrate crystals can form in these effluents at\r\ntemperatures above the normal freezing point of water. The measurements were\r\ncompared with similar hydrate formation data in pure water. It was also found that\r\nthe presence of impurities did not cause any appreciable change in the hydrate\r\ncrystal pressure and temperature formation conditions with the TMP and CTMP\r\neffluent samples, but that the BCTMP effluent, which had a high electrolyte and\r\norganic content, did affect these conditions. It was found that the induction period\r\nand growth rate of hydrate crystals are dependent on the driving force and the prior\r\nhistory of the effluent. And finally, that some level of in-situ concentration was\r\nattainable with our apparatus."@en . "https://circle.library.ubc.ca/rest/handle/2429/4856?expand=metadata"@en . "2914379 bytes"@en . "application/pdf"@en . "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\u00C2\u00A9 Cathrine Gaarder, 1993In presenting this thesis in partial fulfilment of the requirements for an advanceddegree at the University of British Columbia, I agree that the Library shall make itfreely available for reference and study. I further agree that permission for extensivecopying of this thesis for scholarly purposes may be granted by the head of mydepartment or by his or her representatives. It is understood that copying orpublication of this thesis for financial gain shall not be allowed without my writtenpermission._______________________Department of Ck( LrjThe University of British 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 CTMP mills.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 Recommendations 123REFERENCES 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 of the 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\u00E2\u0080\u009DFIGURE 6.8: Pressure-Temperature Hydrate Formation Data for theCTMP Concentrates with Propane 101FIGURE 6.9: Pressure-Temperature Hydrate Formation Data for theTMP2 and the CTMP Effluents, and Deionized Water withCarbon Dioxide 105FIGURE 6.10: Pressure-Temperature Hydrate Formation Data for theCTMP Effluent, the CTMP/80 Concentrate and DeionizedWater with a Propane-Carbon Dioxide Mixture 107FIGURE 6.11: Induction Period and the First Growth Rate Period for Run1, 2, 3 and 4 with the CTMP Effluent 111FIGURE 6.12: Induction Period, and the First and Second Growth RatePeriods for Run 1 with the CTMP Effluent 113FIGURE 6.13: Induction Period, and the First and Second Growth RatePeriods for Run 2 with the CTMP Effluent 114FIGURE 6.14: Induction Period, and the First and Second Growth RatePeriods for Run 3 with the CTMP Effluent 115FIGURE 6.15: Induction Period, and the First and Second Growth RatePeriods for Run 4 with the CTMP Effluent 116FIGURE 6.16: Ideal In-Situ Concentration 118ixACKNOWLEDGEMENTSI would like to express my sincere appreciation to the following persons andorganizations:Prof. P. Englezos for his patience and all his support during the past two years @ !!!Prof. S. Duff for his input on the effluent survey portion of this thesis.The Natural Sciences and Engineering Research Council of Canada (NSERC) for theirfinancial support.PAPRICAN for their financial support through the 1992 PAPRICAN Merit Award.MacMillan Bloedel for their financial support through the 1993 MacMillan BloedelFellowship Award.Fletcher Challenge, Howe Sound Pulp and Paper, Quesnel River Pulp, and LouisianaPacific Canada Ltd. for happily donating effluent samples.The staff in the Chemical Engineering Department and the Pulp and Paper Centrefor all their help.Rob S. and the Civil Engineering Lab technicians for their advice and help with theanalytical procedures.YeeTak N. for his help with the initial experiments.Prof. C. Oloman\u00E2\u0080\u0099s \u00E2\u0080\u009Cbleaching group\u00E2\u0080\u009D, the CFB group, and in particular RobertFreundlich and Assoc. Ltd for allowing me to use their computers/printers.My office mates Ian H., Susan N., Y-S. Perng, and Rory T. for all of the philosophicaldiscussions held in our office, with a special thankyou to 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, \u00E2\u0080\u009COur Common Future\u00E2\u0080\u009D, 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 \u00E2\u0080\u009Cenvironmental\u00E2\u0080\u009D 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?iisMWEffluentProcess IWaterConcentrate5are 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 \u00E2\u0080\u009Cwet\u00E2\u0080\u009D 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 \u00C2\u00B0C (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 I IiiiBLEACHINGiiPRESSiiiDRYERSfWastewaterPulpIChips10solution (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 fingerprint for thedifferent species. Extractive compounds can be divided into four classes (Sj\u00C3\u00B6str\u00C3\u00B6m,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 (Sj\u00C3\u00B6str\u00C3\u00B6m, 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 (Sjostr\u00C3\u00B6m, 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 \u00C2\u00B0C 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\u00E2\u0080\u0099Connor et al. 1992). Another juvenile hormone analog ofinterest is lignan which has been isolated in hemlock (O\u00E2\u0080\u0099Connor 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\u00E2\u0080\u0099Connor 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\u00E2\u0080\u0099-dehydrojuvabione, s\u00E2\u0080\u009Ddehydrojuvabiol, dihydrojuvabione todomatuicacid,_3\u00E2\u0080\u0099-deoxy,3\u00E2\u0080\u0099 -hydro-todomatuicOther neutrals Minor: DAbienol, 12 E-abienol, 13-epimanool, 3\u00E2\u0080\u0099,4-divanillyltetrahydrofuranM: refers to mechanical pulpingD: refers to debarkingSW: softwoodHW: hardwoodhemlock> white pine> \u00E2\u0080\u0094 black spruce \u00E2\u0080\u0094 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\u00E2\u0080\u0099Connor, 1992)Wood Species Total Resin Acids Free Fatty Acids* 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 rotating disks) 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 toxicity decreasedas 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\u00C2\u00B0C (Idner and Norberg, 1976; Stenberg and Norberg, 1977; J\u00C3\u00A4rvinen 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 \u00E2\u0080\u009Ctrue\u00E2\u0080\u009D 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 effluent has 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 effects on 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 light in areceiving water. This may decrease the photosynthetic activity of algae, and disturbthe predator-prey interactions of aquatic organisms that depend on vision. Thedischarge of suspended solids can also result in stress, secondary infections, and/orsuffocation in fish (if enough material gets trapped within their gills). 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 dwelling plants andcreatures, as well as interfere with the growth of aquatic plants (if the blanket is notconducive to good root attachment). In the case where the blanket becomes thickenough anaerobic conditions may develop and undesirable organisms such as sludgeworms may become prevalent. These anaerobic conditions could lead to the death ormigration of desirable life forms such as shellfish due to a lack of oxygen, and theproduction of toxic and/or foul smelling gases, e.g. reduced sulphur compounds. Theproduction of a gas may cause the sludge mats to float to the surface resulting in anunpleasant sight. The dilution of the non-settleable portion of the suspended solidsby the receiving water is, in general, sufficient to ensure that they will have aminimal impact on the environment.BOD. The major impact resulting from the discharge of BOD into theenvironment is a decrease in the dissolved oxygen level of a receiving water. In theextreme case, where the BOD load is very high, the level of dissolved oxygen can bedepleted to such low levels that both motile fauna which do not leave the area andnonmotile fauna die. At lower BOD loads some aquatic organisms may 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 (tissue structure).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 TSS and/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, fathead minnows, andCerioclaphnia affinis. The 1C50 test was performed with Photobacteriumphosphoreum, and the 1C25 was determined for fathead minnows and Ceriodaphniaaffinis. The results from these tests can be found in Table 3.2.TABLE 3.2: Acute and Sub-lethal Toxicity Data for Several DifferentAquatic Species Exposed to TMP Effluent (Kovacs et al.1992)Species Toxicity TestsLC5O 1C50 1C25*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 increase ineffluent concentration. Nestmann et al. (1979; 1983) found that the resin acidsneoabietic (tested with yeast and bacteria) and 7-oxodehydroabietic (tested withyeast) exhibit mutagenic activity.Summary. The results found in this section present evidence that untreatedTMP and CTMP effluents can be acutely toxic to organisms that are typically 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 TMP and CTMP millsthat generate toxic effluents so as to preserve the integrity of the environment. Theselection of a treatment system that can prevent all of the potentially negativeimpacts from occurring within a particular ecosystem will depend on the results oflong-term impact studies.3.2.3 Long-term Impact StudiesSeveral researchers have noted that a general effects model and/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 within a specificecosystem. These studies should be performed in both the field and in simulatedstream channels or pools, include organisms from different levels of the food-web, andbe performed for a long enough period of time so as to include several life cycles ofthe test species being considered. In addition, the sub-lethal effects that a 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 should be determined(Lehtinen, 1991; Hall, 1992; Owens, 1991; Swanson, 1992). At present no in-depth,long-term ecosystem studies have been performed with TMP or CTMP mill effluent.However, this will soon change as a result of the new discharge regulations that havebeen set forth by the Federal government (refer to 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 pulp and paper mill effluents(Environment Canada and Department of Fisheries and 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 Effluent Regulations (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 program istwo-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-design Requirements* 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 previous impacts,information assist in sampling designEffluent quality Evaluation of effluent quality 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 (for thepresence 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 and sedimentationlagoons 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 require primarytreatment 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 in theeffluent. 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 not able 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 separation and/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 that this 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 \u00E2\u0080\u009Ccleaner\u00E2\u0080\u009D than that of the receiving water and as a result would change theoverall ecosystem (albeit in a \u00E2\u0080\u009Cpositive\u00E2\u0080\u009D 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 this work is clathrate hydrateconcentration. This process is a variant of freeze concentration and will 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 product quality. Thedisadvantages associated with the implementation and operation of 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 environment because 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 technologies and/or more adequate disposal42treatments for sludge (both of these will increase the cost of biological systems); (4)These mills are easier to site because they do not depend on a 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 result inan increase in the air pollution load (due primarily to the incineration of theconcentrate that is produced); and (5) There is a lack of long-term operatingknowledge about these systems as they have been on-line for less than two years.Wearing et al. (1985) found that the energy and associated equipment sizerequirements could be reduced substantially if mills were capable of decreasing theirwater-use to flow rates as low as 1.7 m3/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 that theeffluent flow will get much below 10 m3/tonne pulp unless new more innovativeinternal recirculation technologies are developed (Cornacchio et al. 1988).Several simulations have recently been 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 would be comparableto the costs associated with a conventional biological system. Jantunen (1993)simulated a TMP mill utilizing an evaporationJmembrane ZLD system at an effluentflow rate of 6.9 m3/tonne pulp and compared the capital and operating costs with asecondary biological treatment system. The results showed that the capital cost of theZLD system was greater than for the biological system, whereas the operating costsfor the ZLD system were less than those required by the biological system (the authornotes that these numbers are very dependent on the volume of water that is to betreated). Gerbasi et al. (1993) also compared the capital and operating costs of asimulated TMP mill with the costs for a conventional biological system. Threedifferent ZLD systems were simulated, a biological/membrane system, a freezeconcentration system, and an evaporation system, with the effluent flow rate varyingfrom 5 to 20m3/tonne pulp. In contradiction to Jantunen and Paleologou, Gerbasi etal. found that conventional biological treatment was the most economical treatmentoption in each case.Summary. The main advantage of a ZLD system is the removal of allpotentially negative impacts that may occur as a result of continually discharging aconventionally treated effluent. At present the major drawback of ZLD systems istheir high capital and operating costs. However, as effluent flowrates decrease,discharge regulations become more stringent, and the ZLD technologies are furtherdeveloped specifically for the recovery of water from TMP and CTMP effluents thesecosts will decrease relative to conventional systems. Therefore, it can be inferred 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, and aliterature review on the clathrate hydrate process and its applications. And the lastsection 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 because the 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 from freezing pointdepression data.454.1.1 Basic ProcessThere are several process configurations which can be formulated to recoverwater by freeze concentration. A general process based on desalination is shownschematically in Figure 4.1. The first step in the process is to cool the originalsolution to a temperature close to its freezing point via a feed-heat exchanger. 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 increase the overallconversion. The crystals are removed from the separator and fed into a melter inwhich they are melted upon contact with a hot refrigerant stream. Heat is removedfrom the crystallizer and transferred to the melter so as to increase the energyefficiency of this process. The resulting liquid refrigerant is recycled back into thecrystallizer, and the product water is removed after exchanging heat with the feedin the feed-heat exchanger. The most important unit operations, when considering theeconomics of the overall separation process, are the crystallization stage (theformation of the crystals), and the separation and purification stage (the recovery ofpure crystals) (Hahn, 1986; Heist, 1979).4.1.2 Unit OperationsThe formation of ice crystals occurs in the crystallizer, and proceeds in twosteps: nucleation and growth. The crystals that are formed in the crystallizer may becoated by the impurities that are present in the effluent. Some of the concentrate 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)COMPRESSORFeed Concentrate 4 I4Product waterSeawater Cooling water47because the degree to which they occur will determine the ease and extent ofcomplete separation. The reason that the crystals are coated with these impuritiesis probably due to the low interfacial tension between the crystals and theconcentrate (Tleimat, 1980). This surface attraction can be minimized if the surfacearea to volume ratio of each crystal is small, i.e. crystals are shaped like spheresrather than snowflakes, and both the surface tension and viscosity of theconcentrated solution are low (Gorgol, 1991; Douglas, 1989; Barduhn, 1975). It isthought that the extent of occlusion can be minimized if large crystals 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 concentration in 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 transfer processes(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 separation unitsin desalination are wash columns. In these units, which operate by 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 crystallization process by preventing effectiveseparation or exhibiting an excessive freezing point depression. Therefore, it isnecessary to select the extent of concentration required by the process at a level lowenough such that these maximum limits will not be attained. To concentrate theeffluent beyond this limit evaporation must be 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 solution will depend on the characteristics of thesolution and the end-use of the product water.Indirect systems use a refrigerant to lower the solution temperature througha heat exchanger surface in a closed refrigeration cycle. The hot side contains the feedsolution and the cold side contains the refrigerant. These systems, which areexpensive when compared with the other freeze options, are commonly used by thefood industry so as to avoid contamination of the product solution with therefrigerant.In direct systems a heat exchange surface is not required because therefrigerant acts as the heat-transfer medium. There are two types of direct systems:(a) triple point (vacuum freezing), and (b) secondary refrigerant. The triple pointsystems use water as the refrigerant by operating the process at the triple point ofwater. As water evaporates it removes heat from the system allowing the solvent tocrystallize. In secondary refrigerant systems the refrigerant is mixed with the49solution and acts as the heat transfer medium by evaporation. The refrigerant thatis used must be nearly insoluble in water such that it can be removed from both theproduct water and the concentrate, and be recycled back into the process. Thesesystems are considered for applications where large volumes of water are to betreated and a slight degree of contamination is acceptable. The clathrate hydrateconcentration process resembles a secondary refrigerant system.4.1.4 Industrial ApplicationsFreeze concentration is currently used in the food industry and in theproduction of high purity organics. It is well suited to the food industry because it canseparate water from a solution without the loss of flavour and aroma often associatedwith evaporation and distillation processes (Douglas, 1989; Chowdhury, 1988).However, commercial freeze units in the food industry process less 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 produce high 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 110 L/min ofpotable water from a freeze concentration process (Douglas, 1989; HPD, 1990).Several researchers have performed small scale testing programs to 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 built inOntario at Tembec\u00E2\u0080\u0099s BCTMP mill during 1991. Kenny et al. (1992) determined thatthe indirect freeze process used at this pilot plant could reduce the concentration oflow molecular weight organics (Cl through C3), 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\u00E2\u0080\u0099sdesalination plant in Yanbu were used to design an industrial scale freezeconcentration system. This system was designed by HPD Inc. and built in the fall of1991 at Louisiana-Pacific Canada Ltd.\u00E2\u0080\u0099s (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 to be lost throughevaporation during the drying of the pulp. The make-up water was to be supplied byon-site wells, as there are no other available water sources at the mill site. It wasproposed that the concentrate be sold to kraft mills and used for energy and/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 at a 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 the L-P mill limitedthe overall recovery of an adequate volume of clean process water. It was found thatonce ice started to form in the refrigeration units it was difficult to control the growthsuch that the whole system would not freeze up. Scale formation in the refrigerationunit was also noted. It was found that the wash columns could not adequatelyseparate the ice crystals from the concentrate. This may have been partly due to thepresence of colloidally suspended material which enhanced surface attraction and thedegree of occlusion. As a result, the wash columns were replaced with belt-press units(Gorgol, 1992). However, the freeze system was still not capable of recovering morethan 40 to 50% of the water present in the effluent (Arac, 1993). Due to all of theseproblems the mill converted to an evaporation system similar to that used by MillarWestern\u00E2\u0080\u0099s 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 and pharmaceuticalwaste 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 advantage of acrystallization process, e.g. freeze concentration, is that at a given temperature andpressure only one component will crystallize and it will do so as a 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 (6.008 J/molvs 40.66 J/mol). However, this energy advantage is substantially reduced when 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 and CTMPmill effluents. These problems become more prominent at higher temperatures, andas a result more costly materials will be required for the construction of the unitsused in evaporation.Freeze concentration has two main advantages over membrane separation.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 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 BOD and/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 organic in 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 \u00E2\u0080\u009Cideal\u00E2\u0080\u0099 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 were clear, 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 the TMP2/85 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 the CTMP/50 concentrate was 2.5times greater than the conductivity of the CTMP effluent. And the CTMP/80concentrate was 3.5 times greater than the conductivity of the CTMP effluent. It3(mS)(mg/L)(Thousands) 500CD:+.Ci)\u00E2\u0080\u0094 CDO 01CD CDr-]ConductivityOrganicCarbon:TOCInroganicCarbon:TIC2.5 2-1.5-1\u00E2\u0080\u00940.5-04 3 2TMP2TMP2/50TMP2/85MillEffluents88-J-CoFIGURE 6.4: Solids Content of the TMP2 Concentrate Samples-J0 0 0 0 0 00 0 0 0 0 0CD It) C) C\u00E2\u0080\u0099] 0It)coC\u00E2\u0080\u0099]F-Co1t)G)F-ztt4C\u00E2\u0080\u0099,Cl) 0D (I)Cl) \u00E2\u0080\u0094V 0 V= C\u00E2\u0080\u0099)o Vrr\u00E2\u0080\u0094=\u00E2\u0080\u0094C\u00E2\u0080\u0094o 0 DF-> Cl)I\u00E2\u0080\u0099llC\u00E2\u0080\u0099]HrIt) C1) C\u00E2\u0080\u0099] 000+. C2 0(mS)ConductivityOrganicCarbon(mg/L)(Thousands):TOC\u00E2\u0080\u00A2InorganicCarbon:TIC1012 10-8-6 4 2 08 6 4 2CTMPCTMP/50CTMP!80MillEffluents002, CD OCD35(g/L)(mg/L) 500Cl)0 0 C O C 2 I\u00E2\u0080\u009930 25 20 15 10 5 04003002001000CTMPCTMP/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 TOO and 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 the CTMP/50 and theCTMP/80 concentrates were 2.5 and 3.6 times greater than the TSS concentration ofthe CTMP effluent.The pH of the TMP2/50, TMP2/85, CTMP/50, and CTMP/80 concentrates were6.6, 5.5, 7.6, and 9.2, respectively. All of the four concentrates were a very darkorange-brown colour, darker than the BCTMP effluent. However, the CTMPconcentrates were appeared clear, in comparison to the TMP 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\u00E2\u0080\u009Cgate\u00E2\u0080\u009D 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 \u00C2\u00B0C, 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 \u00C2\u00B0C at 5.3 \u00C2\u00B0C 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\u00C2\u00B0CToxicity 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 I IIMaximum 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 \u00C2\u00B0C. 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 thermodynamic considerations) 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 \u00C2\u00B0C were predicted with thermodynamic models. The PengRobinson Equation of State model was used to model the vapour phase, Henry\u00E2\u0080\u0099s 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 \u00C2\u00B0C. The formation pressure at 9 \u00C2\u00B0C 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 \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009Dstructure 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 \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D 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 \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D 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 \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure, similarto that which formed in the presence of propane, was apparent during theseexperiments. It should be noted that the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D 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 and CTMP/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 \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D 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 for theEffluent Samples with Propane0.65_________________C Kubota et al. 1984 (pure water)0.55CTMP__\u00E2\u0080\u00A2 BCTMP\u00E2\u0080\u00A2\u00E2\u0080\u00A2 TMP10.45 0 TMP20.35Cl)00.250.15 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2273 274 275 276 277 278 279Temperature (K)101FIGURE 6.8: Pressure-Temperature Hydrate Formation Data for theCTMP Concentrates with Propane0.6__________________D Kubota et al. 1984 (pure water) aCTMP0.5 \u00E2\u0080\u00A2 CTMP/50A CTMP/80 a00.4a)L.0.3C)I- a00.20.1 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I273 274 275 276 277 278 279Temperature (K)102TABLE 6.3: Propane Hydrate Formation Pressure-Temperature DataTemperature Pressure (MPa)(K)CTMP BCTMP TMP1 TMP2 CTMP/ 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-Temperature Hydrate Formation Data for theTMP2 and the CTMP Effluents, and Deionized Water withCarbon Dioxide5._______________________D This work (pure water)\u00E2\u0080\u00A2 Robinson and Mehta, 1971 (pure water) 1CTMPo TMP2003Cl)CoC)21!1 \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 I \u00E2\u0080\u00A2 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 the CTMP/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 Hydrate Formation Data for theCTMP Effluent, the CTMP/80 Concentrate and DeionizedWater with a Propane-Carbon Dioxide Mixture0.80.70 pure waterCTMP\u00E2\u0080\u00A2 CTMP/800.6 0G)IzCl) 0.5G)0.4\u00E2\u0080\u00A20.3\u00E2\u0080\u0094 . \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2 \u00E2\u0080\u00A2273 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 \u00E2\u0080\u009Csalting-out\u00E2\u0080\u009D 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 if the 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 \u00C2\u00B0C 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 \u00C2\u00B0C 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, eq at 1 \u00C2\u00B0C for the CTMP effluent is 0.420 MPa,determined from Figure 6.10. The initial pressures, PeXp\u00E2\u0080\u0099 were 0.860, 0.846, 0.736, and0.598 MPa for runs la, 2a, 3a, and 4a respectively. And the corresponding AP\u00E2\u0080\u0099s were111FIGURE 6.11: Induction Period and the First Growth Rate Period forRun 1,2,3 and 4 with the CTMP EffluentPressure (MPa)0.95 la+2a3aL 4a0.85-\u00E2\u0080\u0094 Hydrate Nucleation-I10.75:-+--1DD,hla ++0.65,3a+0.55 4aLI\u00E2\u0080\u0099DLILI0.45 I I I0 20 40 60 80 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 within 12 minutes,whereas it took 20 minutes for hydrates to form in run 2a. Variation in thenucleation time (induction period) is expected since nucleation is not 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, and 6.15 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 hydrates did not form within the onehour test period. The immediate formation of hydrates in run lb and 2b can beattributed to the history of the effluents. Hydrates were formed and decomposed inthese effluents prior to the second growth period and, as a result, the solution hadbecome more structured such that hydrates formed more readily. The reason thathydrates formed later in run 3b and did not form within the one-hour time 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 suggest that both the history ofthe effluent and the magnitude of the driving force affect the growth pattern ofhydrate formation in a manner similar to that observed with 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) could be 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, and the First and Second Growth RatePeriods for Run 1 with the CTMP EffluentPressure (MPa)la0.95X lbHydrate 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 the First and Second Growth RatePeriods for Run 2 with the CTMP EffluentPressure (MPa)+ 2a0.950.80.75K 2b2bHydrate Nucleation2a+ + ++0.65++++0.55+++0.450 20 40 60 80 100Time (minutes)115FIGURE 6.14: Induction Period, and the First and Second Growth RatePeriods for Run 3 with the CTMP Effluent0.750.650.550.45Pressure (MPa)0 100* 3aA 3bHydrate Nucleation3a/*AA A A ***20 40 60 80Time (minutes)116FIGURE 6.15: Induction Period, and the First and Second Growth RatePeriods for Run 4 with the CTMP EffluentPressure (MPa)0.59- E 4aZ 4b0.57 Hydrate Nucleation0.550.53 z4a0.51 4bEl Z X No Hydrates0.490.470.45 I I I0 20 40 60 80 100Time (minutes)117occlusions and were not coated by impurities, could form on top of the concentrateand grow in a downward motion. In the absence of stirring, this concept is shownschematically in Figure 6.16. Four tests were performed with propane, and four withthe propane-carbon dioxide mixture. The stirring rate was set at 200 rpm for each ofthese experiments.Propane. In the first experiment 300 mL of the CTMP effluent was used, thetemperature was set at 2 \u00C2\u00B0C, and the initial pressure was set to 0.65 MPa. Thepressure dropped to 0.61 MPa. The effluent formed into a donut shaped solid up thewalls of the vessel, resembling a \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure (refer to Section 6.4). No liquidwas visible in the vessel, and as a result no concentrate could be collected. In thesecond experiment, 350 mL of the CTMP effluent was used with the temperaturemaintained at 3 \u00C2\u00B0C and an initial pressure of 0.47 MPa. It was thought that byincreasing the effluent volume and the set point temperature, and decreasing theinitial pressure the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure would not form as readily. However, the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure did form. In the third experiment, 400 mL of CTMP effluent was used,the temperature was set at 3 \u00C2\u00B0C, and the initial pressure at 0.54 MPa. The same \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure formed.Based on the results from these initial experiments it was decided to use a 5wt% salt solution in place of the effluent. The aim of this experiment was todetermine whether the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure was associated with the propane or theeffluent, and to see if in-situ concentration could be attained with a less complexsolution. For this test 400 mL of the salt solution was used, the temperature was setat 2 \u00C2\u00B0C, and the initial pressure was 0.412 MPa. The same \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structure formedwith this solution. As a result of the limitations caused by the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D structureFIGURE 6.16: Ideal In-Situ Concentration118: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 \u00C2\u00B0C, 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 \u00E2\u0080\u009Cgel-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 \u00C2\u00B0C with200 mL of the CTMP effluent. The crystal structure that formed during theseexperiments appeared to be similar to the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u009D 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 Effluents DeterminedDuring 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 formed as a layer on topof the solution without the presence of any impurities (either occluded or coating thecrystals), did not occur. The reason for the lack of significant in-situ concentrationthat occurred during these experiments may result from the limitations posed on theprocess by the formation of the \u00E2\u0080\u009Cgel-like\u00E2\u0080\u0099 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 liquid effluentstream 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 differences are 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 \u00E2\u0080\u009Cenvironmental\u00E2\u0080\u009D need to treat TMP and CTMP effluents with a ZLDsystem. It was found that untreated TMP and CTMP effluents can exhibit both lethaland sub-lethal toxic impacts on organisms present in a receiving water, whereasconventionally (biological) treated effluents are in general not acutely toxic. However,based on the impact studies performed to date it can not be determined whether thecontinuous discharge of treated TMP and CTMP effluents will result in long-termsub-lethal impacts. Therefore, the \u00E2\u0080\u009Cenvironmental\u00E2\u0080\u009D need for ZLD effluent treatmentsystems can not be verified.It has been suggested that the best way to maintain a sustainableenvironment, with respect to liquid effluent streams, is to implement a ZLD system.Because, in addition to alleviating the potential for impacts to occur in the localenvironment as a result of discharging a liquid effluent, these systems minimize theuse of water, a known commodity. Currently, the only proven ZLD technologyavailable to treat TMP and CTMP effluents is evaporation. Based on 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 development of 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 that wereexamined, 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 and CTMPeffluents, 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 locus shifted 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 hydrate formationdepend on the driving force, expressed as a deviation in pressure from theequilibrium hydrate formation pressure at a given temperature, and the history ofthe effluent sample. Overall, these experiments demonstrated that thethermodynamics and kinetics of clathrate hydrate formation in TMP and CTMPeffluents exhibit similar behaviour to those of well defined systems, i.e. hydrocarbon-water. In addition, it was found that some in-situ concentration could be achieved inour batch apparatus.Based on the above work, the recommendations for the next step includedesigning and building a lab-scale continuous clathrate hydrate concentration unit,studying the separation of the crystals from the concentrate, and determining thecause of the reactions that occur in the presence of carbon dioxide and the effluent.The design of a continuous unit should include the following: (a) A mechanical mixerrather than a magnetic stirrer such that the gel-like structure can be more readilycontrolled and in-situ concentration can be further studied; (b) A data acquisitionsystem such that the gas consumption rate during hydrate formation can bemeasured; (c) A pressure control system such that isobaric experiments can beperformed; and (d) A unit in which the gas and effluent can be mixed prior toentering the hydrate formation vessel such that hydrates will not plug the gas inletlines.125REFERENCESAndersson, P.E., Gunarsson, L., Olsson, G., Welander, T., and A. 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Burn the crucibles at 550 \u00C2\u00B1 50 \u00C2\u00B0C for 1 hour, cool to room temperature,and place in desiccator overnight.7. Weigh the crucibles (C).8. Perform triplicates for each sample.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 \u00C2\u00B0C 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 \u00C2\u00B0C 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"@en . "Thesis/Dissertation"@en . "1994-05"@en . "10.14288/1.0058505"@en . "eng"@en . "Chemical and Biological Engineering"@en . "Vancouver : University of British Columbia Library"@en . "University of British Columbia"@en . "For non-commercial purposes only, such as research, private study and education. Additional conditions apply, see Terms of Use https://open.library.ubc.ca/terms_of_use."@en . "Graduate"@en . "Crystallization of mechanical pulp mill effluents through hydrate formation for the recovery of water"@en . "Text"@en . "http://hdl.handle.net/2429/4856"@en .