International Conference on Gas Hydrates (ICGH) (6th : 2008)


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Proceedings of the 6th International Conference on Gas Hydrates (ICGH 2008), Vancouver, British Columbia, CANADA, July 6-10, 2008.  THE GAS HYDRATE PROCESS FOR SEPARATION OF CO2 FROM FUEL GAS MIXTURE: MACRO AND MOLECULAR LEVEL STUDIES Rajnish Kumar Peter Englezos Department of Chemical & Biological Engineering University of British Columbia 2360 East Mall, Vancouver, BC, V6T 1Z3 CANADA John Ripmeester The Steacie Institute for Molecular Sciences National Research Council of Canada 100 Sussex Drive, Rm 111 Ottawa, Ontario, K1A OR6 CANADA ABSTRACT The “Integrated Coal Gasification Combined Cycle” (IGCC) represents an advanced approach for green field projects for power generation. This process requires separation of carbon dioxide from the shiftedsynthesis gas mixture (fuel gas). Treated fuel gas consists of approximately 40% CO 2 and rest H2. Gas hydrate based separation technology for hydrate forming gas mixtures is one of the novel approaches for gas separation. The present study illustrates the gas hydrate-based separation process for the recovery of CO2 and H2 from the fuel gas mixture and discusses relevant issues from macro and molecular level perspectives. Propane (C3H8) is used as an additive to reduce the operating pressure for hydrate formation and hence the compression costs. Based on gas uptake measurement during hydrate formation, a hybrid conceptual process for pre-combustion capture of CO2 is presented. The result shows that it is possible to separate CO2 from hydrogen and obtain a hydrate phase with 98% CO2 in two stages starting from a mixture of 39.2% CO2. Molecular level work has also been performed on CO2/H2 and CO2/H2/C3H8 systems to understand the mechanism by which propane reduces the operating pressure without compromising the separation efficiency. Keywords. Hydrogen, carbon dioxide, gas hydrates, gas separation, IGCC, Raman spectroscopy  INTRODUCTION Current estimates suggest that coal, the most abundant fossil fuel would last for over 500 years at current usage rate [1]. However, in order to be able utilize the energy from coal without carbon emissions, carbon capture and storage is needed. IGCC is a process that converts low value fuels such as coal, petroleum coke, biomass and municipal waste into a high value environmentfriendly natural gas type fuel also called synthesis gas. This synthesis gas can be converted (by a shift reaction) into a mixture of CO2 and H2. In such an   integrated gasification combined cycle in a coalbased power station, the fuel gas exits the gasifier at 1000-2600 oF [2]. This gas is treated for removal of particulate matter and hydrogen sulfide. This gas clean up involves cooling (cold gas cleanup at 311 K) or no cooling (hot gas cleanup at 810 K) [3]. Treated synthesis gas from an IGCC power station consists approximately of a 40 % CO2 & 60 % H2 mixture at a total pressure of 2.5 to 5 MPa [2]. This process involves separation of carbon dioxide from a carbon dioxide/hydrogen mixture [4]. The resultant stream of H2 could be  Corresponding author: Phone: +1 604 822 6184 Fax +1 604 822 6003 E-mail:  used in fuel cells or in a gas turbine and the carbon dioxide can be sequestered. There is an ongoing effort to reduce operation cost of an IGCC plant, particularly for CO2 capture [4,5]. Alkaline sorbents and scrubbing solutions are often employed to separate carbon dioxide from various gas matrices. Other processes, which have potential include cryogenic fractionation, selective membrane separation etc. [6,7]. Liquid absorption using amines was considered the most promising current method while some other methods are promising but too new for comparison. There is continued interest in the development of less energy intensive processes for the selective removal of CO2 from multi-component gaseous streams. Novel methods have the potential to reduce the cost of carbon dioxide capture and sequestration. This work is concerned with one of those new concepts, the hydrate crystallization process to separate the CO2/H2 mixture into its constituent gases. A hybrid gas separation process, combining the advantage of high selectivity (hydrate crystallization) and small size (membranes) has been proposed for separating CO2 and H2 from an IGCC power station. A membrane process has the potential to be less energy intensive and less voluminous than absorption based processes [8]. Hydrates are ice-like crystals formed by water molecules and a number of gas components such as CO2, CH4 and volatile liquids like tetrahydrofuran (THF) [9,10]. These molecules are enclosed in a network of cavities formed by water molecules linked through hydrogen bonding. Hydrate based separation process When gas hydrate crystals are formed from a mixture of gases the concentration of these gases in the hydrate crystals is different from that in the original gas mixture. This is the basis for the utilization of hydrate formation-decomposition as a separation process. Figure 1 illustrates the basic idea behind the gas separation using gas hydrate technology. A CO2/H2 mixture is fed in to the process where it comes into contact with water at suitable temperature and pressure conditions and forms hydrate crystals. Due to a sharp difference in hydrate formation pressure between CO2 and H2 it  is expected that CO2 preferentially will go into the hydrate crystal phase. The crystals are separated and subsequently decomposed to create the CO2rich stream while the rest constitute the CO2-lean one. Linga et al. [11] employed a CO2/H2 mixture containing 39.2-mol% CO2 corresponding to a typical composition of a fuel gas mixture from an integrated coal gasification cycle to form hydrate. It was found that the hydrate crystals contained a higher percent of CO2 on a water-free basis. These crystals were then decomposed and the resulting gas was used to form hydrate crystals. The hybrid process consisting of two hydrate stages and one membrane separation unit was found suitable for obtaining 99% CO2 and almost pure H2 as two separate streams. However, it was felt that the high operating pressure required for hydrate formation with CO2/H2 mixtures would require significant energy for compression and would not be feasible for industrial application. One way to alleviate this is the use of additives that will lower the hydrate formation conditions without affecting the kinetics and the separation efficiency or CO2 & H2 recovery. One such additive is a small amount of propane added during the hydrate formation process. Kumar et al. [12] reported that hydrate formation pressure at any given temperature reduced by almost half on addition of just 3% of propane to the CO2/H2 mixture. More recently using THF as an additive, Linga et al [13] presented a hybrid process having three hydrate stages and one membrane separation unit to recover CO2 from a flue gas mixture at moderate pressure range. CO2-lean  Fuel gas  Gas Hydrate Process  H2O  CO2-rich  Figure 1. Hydrate based gas separation process  The objective of the current study is to determine the CO2 and H2 separation efficiencies and recoveries from a fuel gas (CO2/H2) mixture in the presence of 2.5-mol % propane. Based on this information a medium-pressure process will be proposed. Molecular level studies have been done to find the mechanism by which the use of additives lower the hydrate formation conditions without affecting the kinetics and the separation efficiency or CO2 & H2 recovery. CO2 recovery and efficiency Linga et al. [14] defined the following quantities relevant to hydrate based separation processes. These quantities are calculated based on the experimental information. The CO2 recovery or split fraction (S.Fr.) of carbon dioxide in gaseous and hydrate phase is calculated as follows  reduces the separation factor in comparison to CO2/H2 system, which is closer to 100 as reported in Linga et al [11]. It would be interesting to know the mechanism by which propane reduces the hydrate formation pressure without compromising on the CO2 recovery and separation efficiency. This is discussed later.  CO2 Composition (mol %) in stage 1 Initial gas phase composition  Hydrate phase composition  Final gas phase composition  38.1  83.6  24.1  CO2 Composition (mol %) in stage 2 80.0  98.0  64.6  H  S .Fr. =  n CO 2  (1)  Feed  n CO2  where n is defined as number of moles of CO2 in feed gas and n is the number of moles of CO2 in hydrate phase at the end of the experiment. In addition, the separation factor (S.F) is determined Feed CO2  H CO2  n CO  n H  2  n H  n CO  2  H  S .F.   gas  2  H  (2)  gas  2  where n is the number of moles of CO2 in the gas phase at the end of the kinetic experiment, n is the number of moles of H2 in the gas phase at the end of the kinetic experiment and n is the number of moles of H2 in the hydrate phase. gas CO2  gas H2  H H2  The minimum pressure to form hydrate crystals from the gas mixture containing 2.5% C3H8, 38.1% CO2 and the rest hydrogen was determined by following the procedure explained in the literature [12] and was found to be 2.1 MPa at 273.7 K. The separation efficiency of this process was determined by calculating the split fraction (Eq. 1) and separation factor (Eq. 2). The split fraction or CO2 recovery and the separation factor were found to be 0.475 and 27.841 at 3.8 MPa and 273.7 K. This shows that the presence of propane does not affect the CO2 recovery in comparison to the CO2/H2 mixture [14]. The CO2 recovery value reported here for CO2/H2/C3H8 is better than for the CO2/H2 system where CO2 recoveries of 0.42 and 0.38 were obtained for stage one and two respectively. The presence of propane slightly  Table 1. CO2 content in the gas phase (initial composition), hydrate phase and residual gas phase (final composition) for two stages. The first stage operates at 3.8 MPa and the second at 3.5 MPa. The above results indicate that following a one stage hydrate formation/decomposition process for fuel gas mixtures with 2.5 mol% of propane, a CO2-rich gas is obtained which contains approximately 83.6 mol % CO2 as shown in table 1, while the composition of propane and hydrogen are 6.5 % and 9.9 % respectively. Since the objective is to obtain a highly concentrated CO2 and pure H2 streams a second hydrate formation/decomposition stage is required to treat this CO2-rich stream. In addition a membrane stage is proposed to treat the CO2-lean one. The feed to the second stage of hydrate crystallization will be a mixture that consists of approximately 80% CO2/ 18.8% H2 and 1.2% C3H8. The above gas mixture is obtained by recycling part of gas mixture from the residue side of the membrane separation unit and part from the CO2-lean stream from the second crystallizer. Figure 2 shows the proposed flow sheet. Hydrate formation for the second stage was carried out at 3.5 MPa at 273.7 K. Hydrate phase equilibria for 80% CO2/ 18.8% H2 and 1.2% C3H8 mixture was determined by following the procedure explained in literature [12] and was found to be 1.6 MPa at 273.7 K. Following the separation experiment from the  second stage, separation efficiency were calculated using equation 1 and 2. The split fraction or CO2 recovery and the separation factor were found to be 0.325 and 91.91 respectively at 3.5 MPa and 273.7 K. It is also clear from table 1 that, in two stages CO2 and H2 can be separated from a fuel gas mixture.  CO2 -lean 23 % CO2 38.1 % CO2 59.4 % H2  ~ 96.0% H2 ~ 3.0 % CO2 ~ 1.0% C3H8 Membrane Process  CO2/H2/C3H8  Gas Hydrate Stage 1 CO2 / C3H8 Recycle  (3.8 MPa) 65 % CO2  H2O CO2 -rich 80 % CO2  Intensity (a.u.)  2.5 % C3H8  3 shows the stretching mode of H2 in the small cages of structure I as well as structure II hydrate. In the hydrate phase, the H2 signature appears as doublet at 4120 cm-1 and 4126 cm-1 which suggests that hydrogen is present in CO2/H2 hydrate which forms structure I as well as in CO2/H2/C3H8 hydrate which forms structure II.  CO2/H2  Gas Hydrate Stage 2 (3.5 MPa)  H2O  ~ 98.0 % CO2 ~ 1.0 % C3H8 ~ 1.0 % H2  Figure 2. Block flow diagram for a hybrid hydratemembrane process for CO2 recovery from fuel gas in presence of propane. Figure 2 is a similar flow sheet as presented in [14] for CO2/H2 mixture, however it can be seen that with small amount of propane as an additive this process operate at a medium pressure of 3.83.5 MPa. The reduction in operating pressure reduces the compression cost for hydrate formation. Molecular level studies Hydrates from CO2/H2 and CO2/H2/C3H8 gas mixtures were synthesized from ice powder and stored at liquid nitrogen temperature for further analysis. Powder XRD diffraction shows that in the presence of 2.5% propane, CO2/H2 mixture forms structure II hydrate at much lower pressure compared to a CO2/H2 mixture which forms a structure I hydrate. Raman spectroscopic observations were performed for both hydrate forming systems. The Raman spectra for rotational and stretching regions of hydrogen were specifically recorded to find the signature of hydrogen in the hydrate cages. Figure  4180  4160  4140  4120  4100  -1  Wavenumber (cm )  Figure 3. Raman spectra from the H-H stretch for H2 incorporated into CO2/H2 and CO2/H2/C3H8 hydrate.  It has been observed previously that Raman spectra of pure CO2 hydrate does not show peak splitting [15] even though CO2 has been said to occupy both the small and large cages in resultant structure I hydrate [16]. In the present study Infrared spectroscopy has been used to find the occupancy ratio of CO2 in large and small cages for structure I hydrate as well as for structure II hydrate. Figure 4 shows FTIR spectra of CO2/H2 mixture which forms structure I hydrate, CO2 in the large cages can be seen at 2337 cm-1 whereas the CO2 signature in the small cages comes at 2346 cm-1. It is evident that for the CO2/H2 hydrate, which forms structure I, most of the CO2 in the hydrate phase is present in the large cage and a very small amount of CO2 is present in the small cage. The insert in figure 4 shows the infrared spectra for the CO2/H2/C3H8 hydrate which forms structure II. It is evident from the figure that a significant amount of CO2 occupies the small cages and that it shares the large cages  4080  Absorbance  with propane. Loss of some of the large cages to propane is counter balanced by CO2 occupying a significant number of small cages in the resultant structure II hydrate. This explains why in the presence of propane the separation efficiency of the hydrate based separation process is not compromised.  2380  2400  2380  2360  2340  2320  2360  2300  2340  2320  2280  and large cages in the structure II hydrate formed in the presence of propane.  2300  2260  -1  Wavenumer (cm )  Figure 4. FTIR spectra from the CO2 peak in small & large cages of CO2/H2 hydrate. (Insert) CO2 peak in small & large cages of CO2/H2/C3H8 hydrate. Conclusions A hybrid gas hydrate/membrane process for the recovery of CO2 and H2 from a fuel gas mixture is presented. The fuel gas mixture is mixed with propane to achieve 2.5-mol % propane content and then is subjected to gas hydrate crystallization. The role of propane is to reduce the hydrate formation pressures from 7.5 MPa (in a system without propane) to 3.8 MPa thus reducing the compression costs in a hydrate based separation process operating at 273.7 K. A membrane separation unit has been used to aid the separation process and recover pure H2 from the CO2-lean stream. Two hydrate formation and decomposition stages together with a membrane separation unit enable the recovery of 98 % pure CO2 and 96 % pure H2 streams. It was also found that propane does not compromise the split fraction compared to a system without any propane. Molecular level studies show that while CO2 mostly occupies the large cages in the resultant structure I hydrate (from a CO2/H2 mixture) it occupies both small  References: [1] Williams RH. Toward zero emissions for transportation using fossil fuels. 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