Phase Equilibrium Conditions and Carbon Dioxide Separation

Article pubs.acs.org/jced

Phase Equilibrium Conditions and Carbon Dioxide Separation Efficiency of Tetra‑n‑butylphosphonium Bromide Hydrate N. Ye and P. Zhang* Institute of Refrigeration and Cryogenics, MOE Key Laboratory for Power Machinery and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China ABSTRACT: In the present study, the phase equilibrium conditions of tetra-n-butylphosphonium bromide (TBPB) hydrate forming with carbon dioxide (CO2) (0.17 mole fraction)/N2 and CO2 (0.6171 mole fraction)/N2 gas mixtures and TBPB aqueous solution at mass fractions w = 0.05, 0.10, and 0.15 are measured in the pressure range from (0.6 to 4.3) MPa, respectively. The CO2 separation efficiencies in a two-stage separation with TBPB hydrate are measured at 277.5 K in the initial pressure range of (2.0 to 4.0) MPa. Furthermore, three quaternary salts, i.e., tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), and tetra-nbutylphosphonium chloride (TBPC), are adopted in the first-stage separation for comparison.

molecules such as CO2, N2, and CH4, etc.5,6 When the gas hydrates form in the presence of gas mixture, the mole fractions of these gases in hydrate are different from those in original gas mixture. For example, the CO2 fraction in hydrate-encaged gas mixture from flue gas is much higher than that in original flue gas. After several rounds of sequential CO2 separations with gas hydrate, CO2 in flue gas can be further purified. The separated gas with high CO2 fraction can be buried underground or used in chemical engineering. The above environmental and economic benefits make the hydrate-based separation proceed on the way for industrial application. Theoretically, a high pressure is necessary for gas hydrate formation, but the flue gas from the power plant usually consists of CO2 from (0.15 to 0.2) mole fraction with very low pressure (about atmospheric pressure),7 so the operation cost for compressing the flue gas for hydrate formation will be very high. For example, the equilibrium pressures for gas mixture containing CO2 of 0.1761 mole fraction are (7.60 and 11.0) MPa at (274.0 and 277.0) K, respectively, which will apparently hamper the application of hydrate technology. To overcome the technique difficulty, some additives are added to reduce the hydrate formation pressure in which tetrahydrofuran (THF) has been shown remarkability for decreasing the equilibrium pressure of gas hydrate formation for gas separation application. For example, the addition of THF may reduce the operation pressure largely from (7.8 to 2.5) MPa at 273.75 K, and the kinetics of hydrate formation was also improved.7−12 In addition, the mole fraction of CO2 can reach 0.99 by three hydrate separation stages in the presence of THF. Even THF is a promising additive for gas

1. INTRODUCTION The progress of human civilization shows too much reliance on fossil fuels, and such a trend will continue with the development of economy and human society, which consequently leads to severe greenhouse gases emission. Gases like carbon dioxide, methane, nitrous oxide, etc., are able to absorb infrared radiation to increase the atmospheric temperature, and too high concentrations of these gases in atmosphere cause global warming. Among these gases, CO2 is responsible for about 64% of the temperature increase due to its amount.1 As a result, this anthropogenic climate change is a world-widely concerned issue today as it may endanger the survival of human being, and the urgent demands of reducing CO2 emission and CO2 concentration in atmosphere using novel methods are required. Large quantities of flue gas mainly containing carbon dioxide and nitrogen (N2) are emitted from the power plants burning fossil fuels every year. To separate CO2 in flue gas in order to reduce the greenhouse effect, some methods are used for CO2 separation and have worked very well, which contain cryogenic fractionation, selective adsorption, gas absorption, and membrane separation. However, there are many deficiencies in these methods such as high corrosion, high energy consumption, high cost, and low separation capacity.2 Due to the above deficiencies, people are searching for the alternative ways, which are basically cheap and reliable for CO2 separation. In recent years, gas hydrate has been considered as one of potential phase change material for application in secondaryloop refrigeration due to its large cold storage capacity and good heat transfer performance.3,4 At the same time, its strong capacity for gas absorption has also received intensive concerns as a promising material for gas separation. The gas hydrates are ice-like crystals formed by water molecules and some gas © XXXX American Chemical Society

Received: July 6, 2014 Accepted: August 6, 2014

A

dx.doi.org/10.1021/je500630r | J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

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fraction)/N2 gas mixture is supplied by Shanghai Wei Chuang Standard Gas Analysis Technology Co., Ltd.. All of the abovementioned chemical substances are directly used without further purification. 2.2. Apparatus and Experimental Preparations. The experimental apparatus used for hydrate formation in the present study is presented in Figure 1 and described in detail in

separation, its negative aspects are apparent; i.e., a very low temperature (273.75 K) is required in the separation process, and THF is harmful to the health and environment. TBAB shows environmental friendly characteristics and can form semiclathrate hydrate with water molecules and some small gas molecules, which also functions for gas separation.2,13 As a typical ionic promoter, TBAB is the most studied substance, and it can shift hydrate phase equilibrium conditions toward lower pressure at the same temperature. In addition, the efficiency of CO2 separation with TBAB is larger than that with THF in one-stage separation.14 Furthermore, other quaternary salts like TBAF and TBPB have also been studied for gas separation,15,16 in which Li et al.16 experimentally obtained a larger CO2 separation efficiency in one-stage separation with TBPB than that with TBAB. To increase the purity of CO2 separated from the flue gas, it seems that two-stage separation can reach a larger separation efficiency with TBPB. In addition, the system temperature for hydrate-based separation must balance the economic benefits and experimental needs for hydrate formation. For example, the system temperature for hydration must be below the phase equilibrium temperature of hydrate, but an excessively low system temperature will not be energy-efficient, which is not applicable to large-scale industrial application. Thus, the equilibrium conditions of gas hydrate forming with CO2 and quaternary salts are necessary for the determination of the experimental conditions. In the present study, the phase equilibrium conditions of TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture and CO2 (0.6171 mole fraction, corresponding to the maximum CO2 fraction of gas mixture in hydrate phase with TBPB in the first-stage separation)/N2 gas mixtures and TBPB aqueous solution at mass fractions w = 0.05, 0.10, and 0.15 are measured. Moreover, on the basis of the above phase equilibrium conditions, a two-stage CO2 separation experiment with TBPB is conducted at 277.5 K in the pressure range of (2.0 to 4.0) MPa at w = 0.05, 0.10, and 0.15. Besides, three additional quaternary salts, TBAB, TBAC and TBPC, are adopted in the first-stage separation for the separation efficiency comparison.

Figure 1. Schematic diagram of the experimental apparatus. PT, pressure transducer; T, thermometer; V1 to V5, valves.17

our previous paper.17 The high pressure up to 15.0 MPa is maintained by a stainless cylindrical crystallizer (CR). The CR has a 1050.0 mL inner volume, and the inner CR can be clearly observed with two embedded optic windows made of aluminosilicate glass. The CR is immersed in a thermostatic bath, and the temperature is controlled by a thermostat (DC2020, Shanghai Hengping Instrument Co., Ltd., China) with circulating the ethylene glycol/water mixture (40:60 wt %) as coolant. An electric motor (H03-A, Shanghai Meiyinpu Instrument, Ltd., China) is placed directly beneath the thermostatic bath to drive the magnetic stir bar in CR. The temperatures of the gas and liquid phases in CR are measured by two PT100 thermometers (WZP-270S, Shanghai Institute of Process Automation Instrumentation) with an uncertainty of 0.1 K, and the pressure in CR is measured by a pressure transducer (YSZK-311, Dinkey Instrument Co. Ltd., China, (0 to 15.0) MPa F.S., uncertainty: 0.1%). A stainless steel cylinder with 25.0 mL inner volume is used to sample gas mixture from CR and convey it into a gas chromatography analyzer (GC1690, Hangzhou Jiedao Scientific Instrument Co., Ltd., China) for composition analysis. A vacuum pump (2XZ-2, Shanghai HEDE Laboratory Equipment Co. Ltd., China) is adopted to evacuate the sample cylinder and pipes before sampling. An electronic balance (JM10002, Zhejiang Yuyao Jingming Weighing Sclae Co., Ltd., China) with an uncertainty of 0.01 g is used to weigh quaternary salts and distilled water for aqueous solution preparation with the designated mass fraction. 2.3. Experimental Procedure. The mass fractions of quaternary salt aqueous solutions used in the present study are w = 0.05, 0.10, and 0.15, respectively. The CR is rinsed with distilled water before loading the aqueous solution in the experiments. After the aqueous solution with about 350.0 mL in volume is loaded, the CR is flushed by CO2 gas to remove the residual gas, and the aqueous solutions are not degassed by vacuum. Once the pressure in CR reaches a designated value in CO2 charging, the CR is sealed, and the preparation procedure is completed.

2. EXPERIMENTS AND METHODS 2.1. Materials. Table 1 shows the purities and suppliers of quaternary ammonium/phosphonium salts used in the present study. Distilled water is used as solvent for salt aqueous solution preparation. The simulated flue gas of CO2 (0.17 mole Table 1. Quaternary Ammonium/Phosphonium Salts Used in the Present Studya symbol TBAB TBAC TBPB TBPC

molecular formula

purity (mass fraction)

(nC4H9)4NBr (nC4H9)4NCl (nC4H9)4PBr (nC4H9)4PCl

0.99 0.99 0.99 0.99

supplier Wuxi B-Win Chemical Industry Co., Ltd., China Shanghai Richness Chemical Co., Ltd., China Shanghai Richness Chemical Co., Ltd., China Shanghai Weifang Fine Chemical Co., Ltd., China

a

The IUPAC (International Union of Pure and Applied Chemistry) systematic names for TBAB, TBAC, TBPB, and TBPC are as follows: TBAB = tetra-n-butylammonium bromide, TBAC = tetra-n-butylammonium chloride, TBPB = tetra-n-butylphosphonium bromide, and TBPC = tetra-n-butylphosphonium chloride. B



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2.3.1. Measurements of the Hydrate Phase Equilibrium Condition. After CO2 is charged into CR, the magnetic stir bar begins to agitate the aqueous solution in CR. When the hydration process is completed, the temperature in CR is increased to dissociate the hydrate. The temperature increase rate is about (0.2 to 0.3) K·h−1 until a small amount of hydrate is left in CR, and then the temperature increase rate is adjusted to be about 0.1 K h−1. When only an infinitesimal amount of hydrate is left in CR, the temperature increase rate of 0.0333 K· h−1 is adopted until the end of hydrate dissociation. The pressure and temperature in CR are then identified as the equilibrium data of the gas hydrate when all of the hydrate vanishes. 2.3.2. CO2 Separation with Hydrate Formation and Dissociation. We explain the measurements by taking the experiments with TBPB aqueous solution at w = 0.10 as examples. When the pressure and temperature of CO2 (0.17 mole fraction)/N2 gas mixture stabilize, experiment starts with agitation at 700 r·min−1. Shown in Figure 2 is a typical example

When all of the hydrate vanishes at stage 7, it is ready for sampling with the same procedure again. 2.4. CO2 Separation Factor. To analyze the composition of the released gas from the hydrate and determine the separation efficiency, the CO2 separation factor (S.F.) is used and formulated as follows:7 S.F. =

H nCO n gas 2 N2 gas n NH2nCO 2

(1)

gas where ngas CO2 and nN2 are the moles of CO2 and N2 in the gas phase at the end of hydration process, respectively. nHCO2 and nHN2 are the moles of CO2 and N2 encaged into the hydrate, respectively.

3. RESULTS AND DISCUSSION 3.1. Phase Equilibrium Conditions of CO2 + N2 + TBPB Hydrate. According to the results of Li et al.,16 larger CO2 separation efficiency from simulated flue gas of CO2 (0.17 mole fraction)/N2 with TBPB can be reached by a one-stage separation than that with TBAB. To find an optimum temperature to form hydrate with TBPB, the phase equilibrium condition of TBPB hydrate with flue gas is necessary. For example, the temperature of the system for hydration must be below the phase equilibrium temperature of hydrate, but an excessively low system temperature will not be energy efficient, which is not applicable to large-scale industrial application. In the present study, the phase equilibrium condition of CO2 + N2 + TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture and TBPB aqueous solution at w = 0.05, 0.10, and 0.15 are measured in the pressure range of (0.6 to 4.3) MPa. The phase equilibrium data of CO2 + N2 + TBPB hydrate forming in the presence of CO2 (0.17 mole fraction)/N2 gas mixture are seldom reported in the literature. In addition, it is found that the phase equilibrium conditions of CO2 + TBAB hydrate and CO2 + TBPB at w = 0.10 and 0.20 are very close, respectively, according to our previous study.17 Thus, we deem that the phase equilibrium conditions of CO2 + N2 + TBAB hydrate and CO2 + N2 + TBPB at w = 0.05, 0.10, and 0.15 might be very close, too. The phase equilibrium data of CO2 + N2 + TBAB hydrate from Lu et al.18 and Meysel et al.19 are adopted in comparison with those of CO2+N2+TBPB hydrate in the present study. Although the compositions of CO2 in CO2/N2 gas mixture used by Lu et al.18 and Meysel et al.19 are 0.159 and 0.2 mole fractions, respectively; they are very close to 0.17 mole fraction in the present study. The phase equilibrium data of CO2 + N2 + TBPB hydrate with CO2 (0.17 mole fraction)/N2 gas mixture are presented in Figure 3 and Table 2. It is observed that the phase equilibrium conditions of CO2 + N2 + TBAB hydrate of Lu et al.18 at w = 0.05 and 0.1532 are close to the phase equilibrium condition of CO2 + N2 + TBPB hydrate in the present study at w = 0.05 and 0.15, respectively. The presence of even a small amount of TBPB can lower the equilibrium pressure and raise the equilibrium temperature of gas hydrate forming without TBPB. For example, the equilibrium pressure of CO2 hydrate at T = 278.15 is 13.68 MPa according to the data of Lu et al.18 but 1.12 MPa in the presence of TBPB at w = 0.05 in the present study. As shown in Figure 3, the phase equilibrium temperature increases at a specified pressure with the increase of the mass fractions of TBPB in aqueous solution at w = 0.05, 0.10, and 0.15; i.e., the CO2 + N2 + TBPB hydrate becomes more stable.

Figure 2. Variation of gas pressure and liquid temperature in the hydrate-based separation process (w = 0.10).

of the experiments. The dissolution of gas into aqueous solution at 284.0 K begins at t = 0 h with pressure drop at stage 1. The system temperature of 284.0 K for gas dissolution is set very close to the phase equilibrium temperature of CO2 + N2 + TBPB forming with CO2 (0.17 mole fraction)/N2 gas mixture; i.e., the subcooling is too small for hydration. After the gas dissolution finishes at the end of stage 1, a much lower temperature (277.5 K) than the phase equilibrium temperature of CO2 + N2 + TBPB forming with CO2 (0.17 mole fraction)/ N2 gas mixture is set for hydration. The dissolution of gas with a secondary pressure drop at stage 2 due to temperature decrease is proceeded until the hydration process begins with temperature rise at the beginning of stage 3. Stage 4 is a steady state where no more gas is consumed. After the system is stabilized for enough long time, the agitation is stopped, and the sample cylinder is connected to the experimental apparatus for gas sampling. To make sure the accuracy of result, the sample cylinder and pipings are flushed by the gas from CR before sampling. Then the gas sample is conveyed to GC for composition analysis, as shown at stage 5. Afterwards, the pressure in CR is reduced quickly to atmospheric pressure and isolated again by closing the vent valve. Then, the temperature of CR is increased to about 300.0 K to make sure all of the hydrate is dissociated with agitation. As shown at stage 6, the pressure in CR increases with the increase of temperature. C



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equilibrium condition of CO2 + N2 + TBPB hydrate forming in the second-stage separation. The phase equilibrium conditions of CO2 + N2 + TBPB hydrate forming with CO2 (0.6171 mole fraction)/N2 gas mixture are presented in Figure 4 and Table 3. As shown in Figure 4, the phase equilibrium

Figure 3. Phase diagram of CO2 + N2 + TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture. ●, w = 0.05, present study; ■, w = 0.10, present study; ▲, w = 0.15, present study; ○, w = 0.05 for TBAB, Lu et al.;18 △, w = 0.1532 for TBAB, Lu et al.;18 ☆, w = 0.05 for TBAB, Meysel et al.;19 □, w = 0.1 for TBAB, Meysel et al.19 Figure 4. Phase diagram of CO2 + N2 + TBPB hydrate forming with CO2 (0.6171 mole fraction)/N2 gas mixture. ●, w = 0.05, present study; ■, w = 0.10, present study; ▲, w = 0.15, present study.

Table 2. Equilibrium Data for CO2 + N2 + TBPB Hydrate with CO2 (0.17 Mole Fraction)/N2 Gas Mixturea w

T/K (± 0.1 K)

P/MPa

0.15

286.17 285.81 285.42 284.86 283.84 283.24 282.26 284.56 284.20 283.64 282.82 282.02 281.08 280.20 281.74 281.30 280.62 279.97 278.98 277.59

4.2605 3.7788 3.1859 2.5934 1.7931 1.2907 0.7248 4.2548 3.7599 3.0653 2.3254 1.6950 1.0381 0.6636 4.2163 3.5863 2.7822 2.2424 1.5251 0.7097

0.10

0.05

Table 3. Equilibrium Data for CO2 + N2 + TBPB Hydrate with CO2 (0.6171 Mole Fraction)/N2 Gas Mixturea w

T/K (± 0.1 K)

P/MPa

0.15

288.88 288.41 287.90 287.27 286.61 285.37 283.76 287.59 287.14 286.31 285.64 284.65 283.57 282.26 285.19 284.77 284.12 283.22 282.27 280.92 279.60

4.2771 3.6459 3.0804 2.4205 1.8951 1.2472 0.7588 4.2544 3.6167 3.0275 2.3524 1.6497 1.1885 0.7248 4.3536 3.6953 3.0842 2.3971 1.7743 1.1516 0.7361

0.10

0.05

a

Relative uncertainties for equilibrium temperature, u(T), equilibrium pressure, u(P), and mass fraction of aqueous solution, u(w), are estimated to be less than 0.036 %, 2.27 %, and 0.0038 %.

a Relative uncertainties for equilibrium temperature, u(T), equilibrium pressure, u(P), and mass fraction of aqueous solution, u(w), are estimated to be less than 0.036 %, 2.07 %, and 0.0038 %.

The CO2 separation efficiencies in different separation processes need to be compared at the same temperature in a two-stage separation. Thus, the system temperature of 277.5 K for hydration is also adopted in the second-stage separation. In order to ensure that the temperature of 277.5 K is low enough for hydration in the second-stage separation, the phase equilibrium data of CO2+N2+TBPB hydrate at w = 0.05, 0.10, and 0.15 in the second-stage separation are measured. The initial composition of CO2 in CO2/N2 gas mixture in the second-stage separation is 0.6171 mole fraction which is equal to the maximum CO2 composition of gas mixture in hydrate phase with TBPB in the first-stage separation. Besides, the system temperature for adequate gas dissolution before hydration can be determined according to the phase

temperature of CO2 + N2 + TBPB hydrate increases at a specified pressure with the increase of the mass fractions of TBPB in aqueous solution at w = 0.05, 0.10, and 0.15. However, at a specified mass fraction of TBPB and pressure, the equilibrium temperature of CO2 + N2 + TBPB hydrate forming with CO2 (0.6171 mole fraction)/N2 gas mixture is higher than that with CO2 (0.17 mole fraction)/N2 gas mixture. The phase equilibrium temperature of N2 + TBAB hydrate is much lower than that of CO2 + TBAB hydrate at a specified mass fraction of TBAB and pressure;20 i.e., CO2 molecules D



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Figure 5. Separation factor versus initial pressure for CO2 (0.17 mole fraction)/N2 gas mixture in the presence of quaternary salts at 277.5 K.

equilibrium temperature of CO2+N2 + TBPB at 4.0 MPa. Furthermore, the system temperatures for gas dissolution with TBAB, TBAC, and TBPC before hydration in the first-stage separation are the same as those with TBPB at the specified pressure and mass fraction. The separation factors of CO2 in the presence of TBAB, TBAC, TBPB, and TBPC at w = 0.05, 0.10, and 0.15 are presented in Figure 5. The range of initial pressure is from (2.0 to 4.0) MPa, and the system temperature for hydration is 277.5 K. As defined in eq 1, the larger separation factor indicates a stronger capability for CO2 separation from the CO2/N2 gas mixture. All of the separation factors shown in Figure 5 increase with the increase of initial pressure from 2.0 MPa and then reduce from (3.0 or 3.5) MPa. It is indicated that higher initial pressure, i.e., higher driving force, can enhance the competition of CO2 molecules with N2 molecules for dodecahedral cage occupancy, but it becomes a disadvantage when the initial pressure is larger than a specified value. Thus, for a specified quaternary salt aqueous solution with a specified mass fraction, there is a largest separation factor in the range of (2.0 to 4.0) MPa, which is defined as the optimum CO2 separation factor. Besides, the largest value in those optimum CO2 separation factors for a specified quaternary salt is defined as the maximum CO2 separation factor. As presented in Figure 5, the optimum CO2 separation factors at a specified mass fraction of aqueous solution are mainly at initial pressure of (3.0 and 3.5) MPa. The maximum separation factors of TBAB, TBAC and TBPB are 12.81, 11.23, and 13.25 at w = 0.05, respectively. In addition, for TBPC aqueous solution, the maximum separation factor is 11.60 at w = 0.15. At 277.5 K and w = 0.05, Fan et al.15

show higher competition over N2 molecules for dodecahedral cage occupation in TBAB hydration. Similarly, more CO2 molecules enter into the dodecahedral cages in TBPB hydrate due to the higher composition of CO2 in initial gas mixture in the second-stage separation than that in the first-stage separation. Thus, the phase equilibrium temperature of CO2 + N2 + TBPB hydrate forming in the second-stage separation increases. In addition, the system temperature of 277.5 K is low enough for hydration in the second-stage separation. 3.2. Separation Characteristics. Two hydrate-based CO2 separations with TBPB are discussed in the present study, in which CO2 + N2 + TBPB hydrate forms with CO2 (0.17 mole fraction)/N2 in the first-stage separation and with CO2 (0.6171 mole fraction)/N2 in the second-stage separation. In addition, three additional quaternary salts, i.e., TBAB, TBAC, and TBPC, are adopted in the first-stage separation for comparison purposes at the same mass fraction and initial pressure. The above separation experiments are all conducted at w = 0.05, 0.10, and 0.15 in the initial pressure range from (2.0 to 4.0) MPa, and the system temperatures for hydration in separations are all at 277.5 K according to the above phase equilibrium data. However, only dissolved gas mixture in aqueous solution can be encaged and separated by hydrate; thus, the adequate gas dissolution before hydration is important for gas separation. In order to attain an adequate dissolution before hydration, different system temperatures are adopted according to the phase equilibrium data of hydrate. For example, TBPB aqueous solutions at w = 0.05 and 0.10 are fully dissolved by CO2 (0.17 mole fraction)/N2 gas at (281.0 and 284.0) K, respectively, in which (281.0 and 284.0) K are very close to the phase E



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obtained the optimum separation factor of about 10.0 at Pini = 4.03 MPa for TBAB aqueous solution which is less than 12.81 obtained in the present study. Moreover, the corresponding composition of CO2 in hydrate phase is 0.3653 mole fraction which is much smaller than 0.6091 mole fraction in the present study. However, Li et al.16 presented the optimum separation factor of about 12.80 at T = 275.15 K and Pini = 3.3 MPa at w = 0.105 for TBAB aqueous solution, which is very close to 12.69 in the present study at w = 0.10, T = 277.5 K, and Pini = 3.5 MPa. Meanwhile, the corresponding mole fraction of CO2 in the residual gas phase is about 0.088 which is very close to 0.0891 mole fraction at T = 277.5 K and Pini = 3.5 MPa in the present study. We believe that the volume ratio of gas phase to liquid phase in CR is a main factor to cause the above difference. For example, the volume ratios of gas phase to liquid phase of Fan et al.,15 Li et al.,16 and the present study are about 0.67, 1.65, and 2.0, where the similar volume ratio values of Li et al.16 and the present study result in a closer S.F. and closer mole fractions of CO2 in the residual gas phase. Less gas will be consumed for larger volume ratio of gas phase to liquid phase, and a smaller pressure variation provides a higher driving force for hydrate formation, which might cause a larger CO2 composition of gas mixture in hydrate phase. The maximum separation factors for TBAB, TBAC, TBPB, and TBPC are listed in Table 4, where the maximum separation

Figure 6. Separation factor versus initial pressure for CO2 (0.6171 mole fraction)/N2 gas mixture in the presence of TBPB at 277.5 K.

Table 4. Mass Fraction and Initial Pressure with Maximum S.F. for Quaternary Salts with CO2 (0.17 Mole Fraction)// N2 Gas Mixturea salt

w

Pini/MPa

maximum S.F.

TBAB TBAC TBPB TBPC

0.05 0.05 0.05 0.15

3.5 3.0 3.5 3.0

12.81 11.23 13.25 11.60

Figure 7. Mole fractions of CO2 in initial gas and hydrate phase for a two-stage separation with TBPB aqueous solution at 277.5 K.

Table 5. CO2 Mole Fractions in Hydrate Phases Forming by CO2 (0.17 and 0.6171 Mole Fractions)/N2 Gas Mixture with TBPB Aqueous Solutiona

a

Relative uncertainties for S.F., u(S.F.), is estimated to be less than 3.47 %.

CO2 mole fraction

factor of 13.25 for TBPB aqueous solution is the largest. In addition, the corresponding CO2 mole fraction in hydrate phase of 0.6171 for TBPB aqueous solution is the largest for all quaternary salts in the present study. Thus, with excellent CO2 separation capacity for TBPB aqueous solution in the first-stage separation, TBPB is adopted to separate CO2 in the secondstage separation with the initial CO2 (0.6171 mole fraction)/N2 gas mixture. The experiments for second-stage CO2 separation with TBPB are carried out at w = 0.05, 0.10, and 0.15 from (2.0 to 4.0) MPa at 277.5 K. As presented in Figure 6, with the increase of initial pressure from (2.0 to 4.0) MPa, the separation factor increases at first and then reduces in the similar manner to that in Figure 5c. The condition with maximum separation factor for TBPB in the second-stage separation is at w = 0.10 and Pini = 3.0 MPa which is different from that at w = 0.05 and Pini = 3.5 MPa in the first-stage separation. Furthermore, the maximum separation factor in the second-stage separation can reach 16.23 with TBPB aqueous solution, and the corresponding CO2 mole fraction in hydrate phase is 0.9128. The CO2 mole fraction in hydrate phase processed by TBPB aqueous solution are shown in Figure 7 and Table 5. Therefore, it can be seen that a stream with more than 0.9 mole fraction CO2 can be obtained from a CO2 (0.17 mole fraction)/N2 gas mixture by only two hydrate-based separation processes with TBPB.

w

Pini/MPa

in initial gas

in hydrate phase

0.05 0.10

3.5 3.0

17.0 61.71

61.71 91.28

a

Relative uncertainties for the mole fraction of CO2 in gas mixture are estimated to be less than 2.74 %.

4. CONCLUSIONS In the present study, the phase equilibrium conditions of CO2 + N2 + TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture and CO2 (0.6171 mole fraction)/N2 gas mixtures are measured at w = 0.05, 0.10, and 0.15 in the pressure range of (0.6 to 4.3) MPa. Moreover, the CO2 separation efficiencies by a two-stage separation with TBPB aqueous solution are measured at 277.5 K in the initial pressure of (2.0 to 4.0) MPa at w = 0.05, 0.10, and 0.15. Besides, TBAB, TBAC, and TBPC are adopted in the first-stage separation for comparison. The conclusions are drawn as follows: (1) Compared to that of CO2 hydrate, the formation condition of CO2 + N2 + TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture is more mild. With the increase of mass fraction from w = 0.05 to 0.15, the phase equilibrium temperature of CO2 + N2 + TBPB hydrate forming with CO2 (0.17 mole fraction)/N2 gas mixture and CO2 (0.6171 mole fraction)/N2 gas mixture increases at a specified pressure. However, the equilibrium temperature of CO2 + N2 + F



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TBPB hydrate with CO2 (0.17 mole fraction)/N2 gas mixture is lower than that with CO2 (0.6171 mole fraction)/N2 gas mixture at a specified mass fraction and pressure. (2) Compared to the maximum separation efficiencies in the first-stage separation with the four quaternary salts, they are summarized as TBPB > TBAB > TBPC > TBAC. With TBPB aqueous solution, the mole fraction of CO2 in the hydrate phase in the first-stage separation can reach 0.6171 and comes to 0.9128 in the second-stage separation.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +86-21-34205505. Fax: +86-21-34206814. Funding

This research is jointly supported by the National Natural Science Foundation of China under the Contract No. 51176109 and the NSFC-JSPS cooperative project under the Contract No. 51311140169. Notes

The authors declare no competing financial interest.

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REFERENCES

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Phase Equilibrium Conditions and Carbon Dioxide Separation

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