ARTICLE pubs.acs.org/EF
Precombustion Capture of Carbon Dioxide and Hydrogen with a One-Stage Hydrate/Membrane Process in the Presence of Tetra-nbutylammonium Bromide (TBAB) Xiao-Sen Li,*,†,‡ Zhi-Ming Xia,†,‡,§ Zhao-Yang Chen,†,‡ and Hui-Jie Wu†,‡ †
Key Laboratory of Renewable Energy and Gas Hydrate, Guangzhou Institute of Energy Conversion, and ‡Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, People’s Republic of China § Graduate University of Chinese Academy of Sciences, Beijing 100083, People’s Republic of China ABSTRACT: Trapping CO2 in hydrates is a promising approach to reduce the greenhouse gas emissions. This work presents the efficient separation process of CO2 from the simulated fuel gas (39.2 mol % CO2/H2 binary mixture) based on the hydrate crystallization in the presence of tetra-n-butylammonium bromide (TBAB). The experiments were carried out in the TBAB concentration range of 0.14-1.00 mol %, the temperature range of 275.15-285.15 K, the driving force range of 1.00-4.50 MPa, the gas/liquid phase ratio range of 0.86-6.47, and the hydrate growth time from 15 to 120 min. The results indicate that the increase of the TBAB concentration or the driving force can enhance the separation efficiency, except when the TBAB concentration is above 0.29 mol % or the driving force is above 2.50 MPa. The lower gas/liquid phase volume ratio and the hydrate growth time can also promote gas consumption. However, H2 more competitively encages into the hydrate phase with time. In addition, the temperature change has little effect on the CO2 separation efficiency with the fixed driving force. It is worth noting that the one-stage hydrate formation/decomposition process for the fuel gas in the presence of 0.29 mol % TBAB at 278.15 K and 2.50 MPa driving force could obtain a 96.85 CO2-rich gas and a 81.57 mol % H2-rich gas. The split fraction (SFr) and separation factor (SF) of CO2 are 67.16% and 136.08, respectively. On the basis of the data of the separation efficiency, a hybrid conceptual process for precombustion capture based on only one hydrate formation/decomposition stage coupled with membrane separation is presented.
1. INTRODUCTION Worldwide, fossil fuels impact more than 80% of the total primary energy supply (about 12 billion tons of oil equivalent) and 67% of the whole power generation (about 19 million GWh).1 With global demands for energy from fossil fuels expected to rise, it is therefore important to find ways to efficiently and economically capture and separate CO2 from large fixed-point emission sources, such as power plants. A promising approach to provide the near future electricity from fossil fuels for the worldwide increasing energy requirements with near-zero CO2 emissions is integrated gasification combined cycle (IGCC) technology with precombustion CO2 capture.2 The “precombustion” is, in fact, a separation of CO2 from a CO2/H2 mixture.3 Various traditional methods, including chemical absorption, physical absorption, physical adsorption, membrane technologies, and cryogenic separation, have been developed for this goal. However, these methods have their individual issues of either high corrosion, large energy consumption, high cost, or low capacity.4 Hence, it is urgent to develop the new technologies for CO2 capture to continue to use fossil fuels while leaving a smaller carbon footprint. One such approach is based on gas hydrate crystallization. The U.S. Department of Energy has proposed a high-pressure process for CO2 separation, where a shifted synthesis gas stream (CO2, H2, and other gases) is allowed to form hydrate to obtain a CO2 hydrate slurry and H2rich product gas.5,6 Preliminary economic assessment shows that the cost of the hydrate technique for CO2 separation from the IGCC power plant is much lower than that of other methods.6 r 2011 American Chemical Society
The basis for this approach is the selective partition of the target component between the hydrate phase and the gaseous phase. Because the equilibrium hydrate formation pressure of CO2 is much lower than that of H2 at the same temperature, it is expected that CO2 is preferentially encaged into the hydrate crystal phase. The hydrate crystals are separated and subsequently decomposed to create the CO2-rich stream, while the rest constitute the CO2-lean one. The two-stage hydrate/membrane process for CO2 separation from a fuel gas mixture (40% CO2/60% H2) has been reported by Linga et al.7 However, the operating pressures were 7.5 and 3.8 MPa, respectively, in the first and second stages, at the operating temperature of 273.7 K. That means high energy consumption for high compression work. Accordingly, there is an ongoing effort to search for effective and environmentally friendly addictives that will reduce the operating pressure without affecting the separation efficiency or CO2 and H2 recovery to a greater extent. Some of the promising additives for the fuel gas mixture are cyclopentane (CP),8 tetrahydrofuran (THF),9 and propane (C3H8).10 Lee et al. found that 1 mol % THF was the optional concentration for CO2 capture based on kinetic experiments.9 Zhang et al. found that, in comparison to THF, CP can further reduce the operation pressure of a hydrate-based CO2-capturing process from the precombustion stream at the Received: November 18, 2010 Revised: January 20, 2011 Published: March 04, 2011 1302
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Energy & Fuels same gas composition and proposed that two equilibrium stages of hydrate crystallization and dissociation with CP can enrich CO2 in the vapor phase significantly from a CO2 mole fraction of 0.4 to 0.98 at 282 K. With the thermodynamic analysis, they provided a conceptual design for the development of a new process for precombustion CO2 separation.8 Kumar et al. ameliorated this two-stage process with 2.5 mol % C3H8 as an additive, so that it can be operated at a medium pressure of 3.8 and 3.5 MPa at 373.7 K.10 However, in addition to the relatively high operating pressure and the relatively low operating temperature, this process still has two hydrate stages, which means the complicated process and enormous apparatus and operating costs. The need to simplify the operating flow and ameliorate the operating conditions motivates further research into the subject. As an environmentally friendly compound,11 tetra-n-butylammonium bromide (TBAB) has become more attractive and achieved much attention for gas separation because small gas molecules (CH4 and CO2) can be encaged in the dodecahedral cavities (S-cage, 512) of the TBAB hydrate at favorable stable conditions.12 Duc et al.13 presented thermodynamic data showing that the addition of 0.29 mol % TBAB could considerably decrease the formation pressure of CO2/N2 hydrate. Moreover, the CO2 selectivity in the hydrate phase is at least 4 times higher than in the gas phase. We have also extensively studied the thermodynamics and kinetics of CO2 capture from the formation of semi-clathrate hydrates with TBAB in previous works.14,15 It is critical to note that CO2 capture is more likely an economic problem rather than a scientific problem. Many approaches are known to provide CO2 separation functions, and the critical challenges lie in the technical feasibility under conditions of industrial-scale operation and in cost reduction. Moreover, to the best of our knowledge, there is no research on the separation efficiency for precombustion capture of CO2 and H2 with TBAB as an additive; all of the current separation processes not only contain two hydrate formation/decomposition stages but are also in the research phase. To reduce the total cost for capturing CO2 from IGCC fuel gas, it is therefore important to significantly improve the separation efficiency, optimize the operating conditions, and simplify the process. The objective of the present study is to investigate the effects of the TBAB concentration, the gas/liquid phase volume ratio, the driving force, the experimental temperature, and the hydrate growth time on the separation efficiencies of CO2 from the CO2/ H2 gas mixture based on separation experiments coupled with compositional analysis. With the above information, a hybrid conceptual process for precombustion capture based on only one hydrate formation/decomposition stage coupled with membrane separation is presented.
2. EXPERIMENTAL SECTION 2.1. Materials. A treated synthesis gas coming out of an IGCC power station consists of approximately 40 mol % CO2 and 60 mol % H2 mixture at a total pressure of 2.5-5 MPa.5 Thus, a CO2/H2 gas mixture containing 39.2 mol % CO2 was used in our experimental work to simulate a pretreated fuel gas mixture. The corresponding CO2/H2 gas mixture was purchased from Huate Gas Co., Ltd., China. TBAB with 99.99% purity was purchased from Shanghai Sinopharm Chemical Reagent Co., Ltd., China. The deionized water with the resistivity of 18.25 mΩ cm-1 used in the work was produced by a ultra-pure water system supplied by Nanjing Ultrapure Water Technology Co., Ltd., China.
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Figure 1. Experimental apparatus: T, thermoprobe; P1 and P2, pressure transducers; R, resistance transducer; GC, gas chromatographer; CR, crystallizer; and SV, supply vessel.
2.2. Apparatus and Procedure. The experimental apparatus used in this work is shown in Figure 1. The inner volume and maximum working pressure of the high-pressure crystallizer (CR) are 336 mL and 25 MPa, respectively. The CR has two circular viewing windows on the front and back. Mixing of the CR contents is accomplished using a magnetic stir bar (450 revolutions/min) that is magnetically coupled to a set of the rotating magnet, which is driven by an electric motor (Shanghai Meiyinpu Instrument, Ltd., China). Two Pt1000 thermoprobes (JM6081) with (0.1 K accuracy are used to measure the temperatures in the gas phase and hydrate slurry phase in the CR, respectively. All pressure measurements are determined with Setra smart pressure transducers (model 552, Boxborough, MA), with the pressure range of 0-25 MPa and accuracy of (0.02 MPa. To measure the composition of the gas phase and hydrate slurry phase in the CR, a Wufeng GC522 gas chromatographer (GC) (Shanghai Wufeng Scientific Instrument Co., Ltd., China) is connected online with the CR and automated with a personal computer (PC). The CR and supply vessel (SV) are immersed in a temperature-controlled bath containing a 30:70 ethylene glycol/water mixture. For the separation experiments, the equilibrium hydrate formation conditions for the CO2/H2/TBAB/H2O mixture are required and have been determined in our previous work.14 Each separation experiment was carried out at the individual driving force (Pexp - Peq), which is defined as the difference of the experimental pressure (Pexp) from the equilibrium pressure (Peq). Prior to the experiment, the CR is cleaned using deionized water and allowed to dry. Next, the TBAB aqueous solution prepared at a desired concentration was injected into the highpressure hydrate CR to a desired volume. Subsequently, the hydrate CR with the solution was flushed with the CO2/H2 mixture gas at least 4 times to remove any residual air, and then it was filled with the CO2/H2 mixture gas until the desired pressure was reached. Once the temperature was stabilized (typically within 1 min), the stirrer in the CR was started and the experimental time also began to be recorded. During the experiment, the temperature and pressure in the system were recorded. As the gas in the CR was consumed on account of the hydrate formation, additional gas was supplied and the pressure in the CR was maintained constant with a proportional-integral-derivative (PID) controller. Therefore, the total gas consumed can be calculated with the gas equation of state of Soave-Redlich-Kwong from the pressure difference between the initial and final pressure of SV.16 After the completion of hydrate formation (about 90 min), the stirrer was stopped and the 1303
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Table 1. Experimental Conditions along with Composition Analysis and Separation Efficiencies for Different Systems runs
Ca (mol %)
Rvb
Tc (K)
Pd (MPa)
DFe (MPa)
timef (min)
YGg (mol %)
YLþHh (mol %)
ni (mol)
SFrj (%)
SFk
1
0.29
6.47
278.15
3.00
2.50
90
66.35
96.14
0.0497
23.80
49.11
2
0.29
2.73
278.15
3.00
2.50
90
70.64
96.43
0.0883
40.71
64.98
3
0.29
1.49
278.15
3.00
2.50
90
75.59
96.55
0.0986
55.44
87.81
4
0.29
0.86
278.15
3.00
2.50
90
81.57
96.85
0.1044
67.16
136.08
5
0.29
0.86
277.05
2.62
2.50
90
81.64
96.89
0.1043
64.05
138.53
6
0.29
0.86
279.55
3.84
2.50
90
81.48
96.75
0.1034
64.51
130.97
7
0.29
0.86
281.15
4.84
2.50
90
81.51
96.79
0.1046
65.62
132.92
8 9
0.14 0.21
0.86 0.86
275.15 276.25
3.01 3.00
2.50 2.50
90 90
79.67 80.98
94.42 95.76
0.0897 0.0988
62.70 63.67
66.31 96.16
10
0.50
0.86
280.35
3.00
2.50
90
81.11
96.13
0.0949
56.88
106.66
11
1.00
0.86
282.45
3.02
2.50
90
79.79
94.78
0.0894
54.63
71.68
12
0.29
0.86
278.15
1.50
1.00
90
74.91
92.17
0.0452
50.37
35.15
13
0.29
0.86
278.15
2.00
1.50
90
77.23
93.26
0.0701
56.61
46.93
14
0.29
0.86
278.15
2.50
2.00
90
79.55
95.12
0.0902
57.56
75.82
15
0.29
0.86
278.15
3.50
3.00
90
80.56
95.95
0.1394
63.02
98.18
16 17
0.29 0.29
0.86 0.86
278.15 278.15
4.00 5.00
3.50 4.50
90 90
79.54 78.25
94.87 94.18
0.1419 0.1795
59.73 60.85
71.89 58.22
18
0.29
0.86
278.15
3.00
2.50
15
79.95
97.19
0.0924
67.57
137.92
19
0.29
0.86
278.15
3.00
2.50
45
81.11
96.96
0.1014
67.42
136.94
20
0.29
0.86
278.15
3.00
2.50
120
81.50
96.55
0.1097
58.29
123.29
TBAB concentration. b Gas/liquid phase volume ratio. c Experimental temperature. d Experimental pressure. e Driving force (Pexp - Peq). f Hydrate growth time. g The final H2 composition in the residual gas. h The final CO2 composition in the hydrate slurry. i Amount of gas consumed after hydrate formation was accomplished. j Split fraction of CO2. k Separation factor of CO2. a
residual gas phase was directly sampled and analyzed by the GC. After the gas phase was sampled, the vent valve was opened and the remaining gas was quickly purged.9,11,17 Then, the vent valve was closed, and the vessel was warmed to room temperature to make the hydrate dissociate completely. Finally, the dissociated gas composition was also determined with GC. The measurement of the obtained gas in the cell provided the composition of the hydrate phase. Table 1 shows the initial conditions for all of the separation experiments. Before performing the formal experiments, we determined whether the contribution of CO2 consumption is mainly due to the hydrate formation or the increase in CO2 solubility in this work. With two typical experiments, we have the measurements of the amount of CO2 gas consumed for the cases of the pure water of 180 mL at 3.0 MPa and 278.15 K and the 0.29 mol % TBAB aqueous solution of 180 mL at 3.0 MPa and 281.15 K with the 38.4 mol % CO2/H2 feed gas. It is found that there is no gas hydrate formation at the conditions of the above two cases, and the amount of gas consumed is 0.0242 and 0.02024 mol, respectively. This illustrates that the solubility of CO2 in pure water and the TBAB aqueous solution is quite small. The addition of TBAB has little effect on the solubility. In addition, the total amount of gas consumed is 0.1044 mol for the experiment with 0.29 mol % TBAB aqueous solution of 180 mL at 3.0 MPa and 278.15 K. Thus, the gas encaged into hydrate is approximately 0.08096 mol, which is 4 times more than carbon dioxide consumption because of the gas solubility. Hence, the results show that the amount of CO2 consumed in the TBAB solution is mainly attributed to the hydrate formation. 2.3. CO2 Recovery and Efficiency. In the work, the recovery or split fraction (SFr) and the separation factor (SF) of CO2 are used to assess the hydrate-based separation process. Their expressions are given by Linga et al. as follows:7 SFr ¼
nH CO2 nfeed CO2
ð1Þ
Figure 2. Hydrate formation from systems with 0.86-6.47 gas/liquid phase volume ratio (Rv) at 278.15 K and a driving force of 2.50 MPa: (a) H2 composition in residual gas (YG) and CO2 composition in hydrate slurry (YHþL), (b) amount of gas consumed (n), and (c) split fraction (SFr) and separation factor (SF) of CO2. feed H where nCO2 is the number of moles of CO2 in feed gas and nCO2 is the number of moles of CO2 in the hydrate phase at the end of the experiment
gas
SF ¼
nH CO2 nH2 gas nCO2 nH H2
ð2Þ
,where nH2gas is the number of moles of H2 in the gas phase at the end of the kinetic experiment, nH2H is the number of moles of H2 in the hydrate phase, and nCO2gas is the number of moles of CO2 in the gas phase at the end of the kinetic experiment. 1304
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Figure 3. Hydrate formation from the 0.29 mol % TBAB system at different temperatures and a driving force of 2.50 MPa: (a) H2 composition in residual gas (YG) and CO2 composition in hydrate slurry (YHþL), (b) amount of gas consumed (n), and (c) split fraction (SFr) and separation factor (SF) of CO2.
Figure 4. Hydrate formation from systems with different TBAB concentrations at a driving force of 2.50 MPa: (a) H2 composition in residual gas (YG) and CO2 composition in hydrate slurry (YHþL), (b) amount of gas consumed (n), and (c) split fraction (SFr) and separation factor (SF) of CO2.
3. RESULTS AND DISCUSSION In this work, a total of 20 experimental runs were carried out to investigate the separation efficiencies of hydrate formation at the different conditions, including the gas/liquid phase volume ratio (runs 1-4), the experimental temperature (runs 4-7), the TBAB concentration (runs 4 and 8-11), the driving force (runs 4 and 12-17), and the hydrate growth time (runs 4 and 18-20), to find the suitable operating conditions for CO2 capture from IGCC fuel gas via hydrate formation. Table 1 summarizes the results from the separation experiments with the different conditions. 3.1. Effect of the Gas/Liquid Phase Volume Ratio. The gas/ liquid phase volume ratio (Rv) is defined as the ratio of the volume of the gas phase in the CR (VG) to the volume of the liquid phase (VL). Panels a-c of Figure 2 show H2 composition in the residual gas (YG) and CO2 composition in the hydrate slurry (YHþL), the amount of gas consumed (n), and the split fraction (SFr) and separation factor (SF) of CO2 after hydrate growth for 90 min from the systems with 0.86-6.47 gas/liquid phase volume ratio (Rv) at 278.15 K and a driving force of 2.50 MPa, as shown in runs 1-4. It can be seen from Figure 2a that H2 composition in the residual gas decreases from 81.57 to 66.35 mol %, while CO2 composition in the hydrate slurry holds at approximately 96 mol % with the increase of Rv from 0.86 to 6.47. As shown in panels b and c of Figure 2, with the increase of the gas/liquid volume ratio from 0.86 to 6.47, the amount of gas consumed decreases from 0.1044 to 0.0497 and the split fraction (SFr) and separation factor (SF) of CO2 decrease from 67.16 to 23.80% and from 136.08 to 49.11, respectively. Actually, the decrease in the split fraction for a larger volume ratio is naturally expected even if the same amount of gas is consumed per unit mass of water. Thus, in the following experiments, Rv = 0.86 corresponding to the liquid volume of 180 mL was chosen to obtain the highest amount of forming hydrate, resulting in the highest separation efficiency. 3.2. Effect of the Experimental Temperature. Panels a-c of Figure 3 show the H2 composition in the residual gas (YG) and the CO2 composition in the hydrate slurry (YHþL), the amount of gas consumed (n), the split fraction (SFr) and separation factor (SF) of CO2 after hydrate growth for 90 min from 0.29
mol % TBAB systems at different temperatures and a driving force of 2.50 MPa, as shown in runs 4-7. As seen from panels ac of Figure 3, all of the final phase compositions, moles of gas consumed, and split fraction and separation factors of CO2 do not have an obvious difference, although the experimental temperatures are different. The above phenomena illustrate that the temperature change has little effect on the CO2 separation efficiency with the fixed driving force. 3.3. Effect of the TBAB Concentration. As shown in runs 4 and 8-11, for the systems with the TBAB concentration of 0.14-1.00 mol %, the experiments were carried out at the temperatures whose equilibrium hydrate formation pressure is 0.50 MPa with the above different TBAB concentrations. The data of the equilibrium hydrate formation condition have been measured in our previous work.14 Thus, each experimental pressure is fixed at 3.0 MPa to obtain a driving force of 2.50 MPa. The experimental data are summarized in Table 1 (runs 4 and 8-11) and plotted in Figure 4. Panels a-c of Figure 4 show the H2 composition in the residual gas (YG) and the CO2 composition in the hydrate slurry (YHþL), the amount of gas consumed (n), and the split fraction (SFr) and separation factor (SF) of CO2 after hydrate growth for 90 min from the systems with different TBAB concentrations at a driving force of 2.50 MPa. It can be seen from Figure 4b that the amount of gas consumed increases with the TBAB concentration until 0.29 mol % and turns to a contrary trend when the TBAB concentration is above 0.29 mol %. For example, the amount of gas consumed with the TBAB concentration of 0.14, 0.21, and 0.29 mol % was 0.0897, 0.0988, and 0.1044 mol, which exhibits an increasing trend, while that of 0.50 and 1.00 mol % was only 0.0949 and 0.0894 mol, which exhibits a decreasing trend. It can be explained that the TBAB concentration, when below 0.29 mol %, can more favorably promote the gas encaged into hydrate slurry. However, the gas hydrate sharply forms substantively and agglomerates at the gas/liquid interface when the TBAB concentration is above 0.29 mol %. A similar phenomenon can be found elsewhere.18 The agglomeration of the hydrate at the gas/liquid interface hinders the further mass transfer of the gas into the aqueous solution. On the other hand, as seen from Figure 4a, with the increase of the TBAB concentration from 0.14 to 1.00 mol %, the 1305
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Figure 5. Changes of CO2/H2 consumed ratios with time from systems with different TBAB concentrations at a driving force of 2.50 MPa.
Figure 6. Hydrate formation from the 0.29 mol % TBAB systems at 278.15 K and different driving forces: (a) H2 composition in residual gas (YG) and CO2 composition in hydrate slurry (YHþL), (b) amount of gas consumed (n), and (c) split fraction (SFr) and separation factor (SF) of CO2.
H2 composition of the final gas increases from 79.67 to 81.57 mol % and then turns to 79.79 mol %, while the CO2 composition of the final hydrate slurry increases from 94.42 to 96.85 mol % and then decreases to 94.78 mol %. It is due to the fact that the TBAB concentration of the system affects the CO2 selectivity of the hydrate process. As shown in Figure 5, the CO2 selectivity increases with the TBAB concentration until 0.29 mol % and then decreases with the TBAB concentration at a certain time. As shown above, the more agglomeration results in a smaller amount of CO2 in the solution for the hydrate formation. Comparatively, H2 more easily goes through the layer of the hydrate agglomeration at the gas/liquid interface into the aqueous solution, because of the fact that its molecule is smaller than the CO2 molecule.15,18 Furthermore, this causes the decrease of the H2 composition in the residual gas phase and the increase of the CO2 composition in the hydrate slurry with the high concentration of TBAB. Moreover, it can be seen from Figure 4c that the split fraction (SFr) and separation factor (SF) of CO2 exhibit a similar trend; they increase with the TBAB concentration in the systems, achieve the highest values of 65.35
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Figure 7. Changes of CO2/H2 consumed ratios with time from the 0.29 mol % TBAB systems at 278.15 K and different driving forces.
and 158.29, respectively, and then decrease with the TBAB concentration in the systems. Hence, 0.29 mol % is fixed as the TBAB concentration for the following experiments. 3.4. Effect of the Driving Force. Panels a-c of Figure 6 show the H2 composition in the residual gas (YG) and the CO2 composition in the hydrate slurry (YHþL), the amount of gas consumed (n), and the split fraction (SFr) and separation factor (SF) of CO2 from the 0.29 mol % TBAB systems with different driving forces at 278.15 K, as shown in runs 4 and 12-17. As seen in Figure 6b, with the increase of the driving force from 1.0 to 4.5 MPa, the amount of gas consumed (n) still increases from 0.0452 to 0.1795 mol. This is attributed to the fact that, on one hand, the higher driving force makes more gas go into the aqueous solution and, furthermore, causes more gas hydrate to form, resulting in more gas consumed and, on the other hand, the increase of the amount of the gas going into the solution correlates with the enhancement of the gas hydrate growth rate, which also means the enhancement of the gas consumption rate. However, it is worth noting that the amount of gas consumed, which is caused by the higher driving force, does not makes more CO2 encaged into the hydrate slurry phase. As seen in Figure 6a, the H2 composition in the final gas (YG) increases from 74.91% at 1.0 MPa to 81.57 mol % at 2.50 MPa and then decreases to 78.25 mol % with the increase of the driving force, while the CO2 composition in the hydrate slurry (YHþL) increases from 92.17 to 96.85 mol % and then also turns to the contrary trend when the driving force is above 2.50 MPa. Moreover, the split fraction (SFr) and separation factor (SF) of CO2 exhibit a similar phenomena and achieve the best values of 67.16% and 136.08, respectively, when the driving force is 2.50 MPa, as shown in Figure 6c. In fact, all of the above phenomena can be explained by the different selectivity of CO2 and H2 from the hydrate formation process with different driving forces. As seen in Figure 7, at a certain time, the CO2/H2 consumption ratio increases with the increase of the driving force from 1.00 to 2.50 MPa, achieves the highest value of 23.27 at 2.50 MPa driving force, and then decreases with the increase of the driving force from 2.50 to 4.50 MPa, which attributes to the fact that, in comparison to H2, CO2 is preferential to form CO2 hydrate at a lower driving force; however, H2 can begin to compete with CO2 for hydrate cage (512) occupancy with a higher driving force, and 1306
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Figure 8. Hydrate formation from the 0.29 mol % TBAB systems with different growth times at 278.15 K and a driving force of 2.50 MPa: (a) H2 compositions in residual gas (YG) and CO2 composition in hydrate slurry (YHþL), (b) amount of gas consumed (n), and (c) split fraction (SFr) and separation factor (SF) of CO2.
Figure 9. Changes of moles of single gas consumed and CO2/H2 consumption ratios in the 0.29 mol % TBAB system with different hydrate growth times at 278.15 K and a driving force of 2.50 MPa.
this leads to the increase of the concentration of CO2 in the residual gas phase. A similar experimental phenomenon is also found by Linga et al.18 3.5. Effect of the Hydrate Growth Time. Panels a-c of Figure 8 represent the H2 composition in the residual gas (YG) and the CO2 composition in the hydrate slurry (YHþL), the amount of gas consumed (n), and the split fraction (SFr) and separation factor (SF) of CO2 with hydrate growth times of 15, 45, 90, and 120 min from the 0.29 mol % TBAB system at 278.15 K and a driving force of 2.50 MPa. It can be seen from Figure 8b that the total moles of gas consumed was 0.0924, 0.1014, 0.1044, and 0.1048 mol after 15, 45, 90, and 120 min of hydrate growth, respectively. It demonstrated that the amount of gas consumed mainly occurred in the first 15 min and almost completed after 90 min. This verified that 90 min was adequate for the hydrate growth. However, as seen from Figure 8a, the H2 composition in the residual gas increases with the hydrate growth time, while the CO2 composition in the hydrate slurry decreases with the hydrate growth time. Moreover, the split fraction (SFr) and separation factor (SF) of CO2 also decrease with the hydrate
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growth time, as shown in Figure 8c. Actually, it is not the longer hydrate growth time that obtains the better separation efficiency. Figure 9 shows that the moles of the single gas consumed (CO2 and H2) and the CO2/H2 consumption ratio vary with the sampling time in the system with 0.29 mol % TBAB at 278.15 K and the driving force of 2.50 MPa. It can be seen from Figure 9 that the number of moles of both CO2 and H2 consumed increase with time. However, the CO2/H2 consumption ratio rises up to the maximum value of 23.68 at the 15th minute and then reduces with time. It illustrates that, in comparison to CO2, more H2 can be encaged in the hydrate phase with time. It may be because H2 is difficult to form the hydrate alone; however, it competitively enclathrate in the S-cages with CO2 as some CO2 hydrate forms. A similar phenomenon was also exhibited by Figures 5 and 7, showing that, for any systems, the CO2/H2 consumed ratio increases with time, achieves the highest value at about the 10th minute after the nucleation point, and then reduces with time. 3.6. Hybrid Hydrate/Membrane Process. The above results indicate that, following a one-stage hydrate formation/decomposition process for the CO2/H2 gas mixture in 0.29 mol % TBAB at 3.0 MPa, a CO2-rich gas containing approximately 97.28 mol % CO2 is in accordance with the emission criterion obtained, whereas the residual gas (H2-rich) contains approximately 82 mol % H2. It is worth noting that the CO2 composition of the feed gas phase and the hydrate slurry vary from 39.2 to 97.28 mol %. It means that approximately 65% CO2 can be captured by the hydrate process. However, because the objective is to obtain high-purity H2, a further stage is required to treat the residual gas (H2-rich) stream. However, the operation pressure of hydrate formation increases with the H2 composition in the mixture gas, which means the high energy for the compression work. Thus, it is necessary to find other approaches for the further separation of the residual gas. Membrane separation is believed to be one of the promising methods to separate CO2/H2 mixture gas, but currently, this process is not suitable for such a huge load that comes from a commercial power generation station. A hybrid gas separation process, in combination with the advantage of high selectivity (hydrate crystallization) and small size (membranes), is a worthwhile alternative, especially for an IGCC power station. In the current conceptual process, the membrane separation only takes the load of the residual gas phase from the hydrate process when it is in conjunction with the hydrate separation process. Also, in this case, the gas mixture is available at high pressures, and with high H2 concentrations, a polymeric membrane would give better selectivity and higher flux for industrial application, which will be very efficient and economical to operate.19,20 Therefore, a hybrid hydrate-membrane process is proposed for CO2 recovery from fuel gas, as shown in Figure 10. The fuel gas, which contains about 39.2 mol % CO2, was blown into a CR to form the hydrate with the 0.29 mol % TBAB aqueous solution, and approximately 67% CO2 in the hydrate slurry was encaged. Then, the residual gas containing approximately 82 mol % H2 was further treated by a membrane process to obtain 99 mol % H2 and 99 mol % CO2. While the hydrate slurry was pumped to a high-temperature vessel to decompose, the decomposition gas, which contained approximately 97 mol % CO2, was compressed for disposal. It is worth noting that this flowchart only has a onestage hydrate process. Moreover, the operating pressure (3.0 MPa) happens to be the outlet pressure of the fuel gas, which means no need to compress and just to cool the mixture gas from 1307
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Figure 10. Block flow diagram of a hybrid hydrate-membrane process for CO2 recovery and H2 purification from fuel gas in the presence of TBAB.
288.15 to 278.15 K. In comparison to the two-stage hybrid conceptual flow sheet operated at 3.8 and 3.5 MPa by Klara et al.6 and operated at 7.5 and 3.8 MPa by Kumar et al.,15 it not only simplifies the process flow but also remarkably reduces the equipment and operating costs. In comparison to the IGCC with chilled methanol CO2 recovery, whose power penalty was found to be 124.6 MW, accounting for approximately 44.64% of the power output, and mainly contributed to cooling the solvent and feed gas to -70 °F (216.48 K) and compressing CO2 from 135 °F (330.37 K) and 14.7 psia (0.096 MPa) to 273 °F (407.04 K) and 95 psia (0.65 MPa) (which is far away from the feasible sequestration condition), our conceptual flow only cools the solution and the syngas to 278.15 K (the temperature difference between formation and decomposition is just 10 K) and compresses CO2 from 288.15 K and 3.0 MPa to the sequestration condition (313.15 K and 15.0 MPa) and the power penalty will reduce to about 20%. This process will be more competitive in application for hydrate-based capture from fuel gas and can also be applied to the separation and recovery of other target gases from gas mixtures without changing the basic concept.
4. CONCLUSIONS In this work, we focused on the optional operating condition, the corresponding separation efficiency, and a simple, economical, and feasible process for hydrate-based precombustion capture with TBAB as an additive. We investigated the operating conditions and found that the increase of the TBAB concentration or the driving force can enhance the separation efficiency, except when the TBAB concentration is above 0.29 mol % or the driving force is above 2.50 MPa, and the lower gas/liquid phase volume ratio and the longer hydrate growth time can also enhance the gas hydrate formation. However, H2 more competitively encages into the hydrate phase with time, and the temperature change has little effect on the CO2 separation efficiency with the fixed driving
force. In addition, the CO2 concentration in the hydrate phase slurry reaches approximately 97 mol % after the one-stage hydrate separation at 278.15 K and 3.0 MPa from the 0.29 mol % TBAB system. The split fraction (SFr) and separation factor (SF) are 67.16% and 136.08, respectively. It means that approximately 67% of CO2 is recovered in the hydrate process. Hence, the data allow us to present a conceptual process that consists of the one-stage hydrate process in conjunction with a membrane separation stage to capture CO2 and H2. It is a big merit for simplifying the process and reducing investment and operating costs.
’ AUTHOR INFORMATION Corresponding Author
*Telephone: þ86-20-87057037. Fax: þ86-20-87057037. E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was supported by the National Natural Science Foundation of China (51076155 and 20773133), the Science and Technology Program of Guangdong Province (2009B050600006), and the Chinese Academy of Sciences (CAS) Knowledge Innovation Program (KGCX2-YW-3X6), which are gratefully acknowledged. ’ REFERENCES (1) International Energy Agency (IEA). Energy technology perspectives 2008. Scenarios and Strategies to 2050; Organisation for Economic Co-operation and Development (OECD)/IEA: Paris, France, 2008. (2) Barchas, R.; Davis, R. The Kerr-McGee ABB Lummus Crest technology for the recovery of CO2 from stack gases. Energy Convers. Manage. 1992, 33, 333–340. 1308
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