Combining Different Additives with TBAB on CO2 Capture and CH4

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Cite This: J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Combining Different Additives with TBAB on CO2 Capture and CH4 Purification from Simulated Biogas Using Hydration Method Gang Yue,†,⊥ Yu Liu,‡ Yang Luo,§,⊥ Ai-Xian Liu,†,∥ Bo Chen,†,⊥ Qiang Sun,†,⊥ Xing-Xun Li,†,⊥ Bo Dong,†,⊥ Lan-ying Yang,†,⊥ and Xu-Qiang Guo*,†,∥ †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China Zhejiang Ningbo City Fenghua District Power Supply Bureau, Ningbo, Zhejiang Province 315500, China § Sinopec Research Institute of Petroleum Processing, Beijing 100083, P. R. China ∥ China University of Petroleum-Beijing at Karamay, Karamay, Xinjiang Province 834000, P. R. China ⊥ China University of Petroleum-Beijing, Beijing 102249, P. R. China

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‡

ABSTRACT: CH4 purification and CO2 removal by hydrate-based separation method from the simulated biogas (64 mol % CH4/CO2) were investigated in tetrabutylammonium bromide (TBAB) solution. Three different hydrate additives were combined with TBAB to study the synergistic influence on thermodynamics and kinetics of hydrate formation. These additives were 1butyl-3-methylimidazolium hexafluorophosphate ([EMIM]BF4), sodium dodecyl benzene sulfonate (SDBS), and propylene carbonate (PC). The thermodynamic experiment results showed that [EMIM]BF4 and PC have an inhibition effect on CO2 and CO2/CH4 hydrate formation. SDBS has a little influence on phase equilibrium. As for the results of hydrate kinetic and biogas hydrate separation, CH4 mole fraction in the remaining gas could reach 79.20 mol % from the initial 64.0 mol %, and CH4 recovery was 94.21% in TBAB + [EMIM]BF4 solution when the operation conditions were 3.5 MPa and 277.15 K. CO2 mole fraction in the hydrate phase could reach 50 mol % or even higher from the initial 36.0 mol %. The gas storage in the hydrate phase did not change dramatically. Basically, the gas storage in the hydrate phase was 0.07 mol on account of addition of TBAB. The CO2 hydrate ratio was about 55%, and it did not change widely. On the whole, the overall effect of biogas hydrate separation in TBAB solution was [EMIM]BF4 > PC > SDBS.

1. INTRODUCTION The gas hydrates were non-stoichiometry cage crystal compounds formed by the hydrogen-bond network between water molecules at appropriate conditions.1 When the guest molecule was gas, it formed the gas hydrate. However, some liquid compound molecules and solid compound molecules could also form hydrates, such as propane (CP)2 and tetrabutylammonium bromide (TBAB).3 The categories of formed hydrates were clarified by the size of the gas molecule.4 There are three main structures: structure I (sI), structure II (sII), and structure H (sH).5 The structures of sI and sII were appropriate for small gas molecule, such as methane, carbon dioxide, and so on. sH has a hydrate structure that is proper for big gas molecules, such as cyclooctane and cyclohexane.6 A unit volume of the hydrate phase could contain about 180 volumes of gas according to the calculation at the standard conditions. In the 21st century, natural gas, primarily methane, is considered to be one of the major resources for future because of its considerable reserve on earth.7 However, lots of CH4 resources existed in the mixture gases, and these gases did not have immediate commercial specifications. Therefore, the corresponding gas separation methods were studied to purify © XXXX American Chemical Society

the CH4 gas. Recently, the gas hydrate-based separation technology has attracted a lot of attention from researchers, especially in the area of gas storage in hydrate and gas transportation. Additionally, compared with the traditional gas separation method, such as the chemical or physical absorption,8 pressure swing adsorption,9 cryogenic separation,10 and membrane separation,11 the gas hydrate separation technology would be an economical and environment-friendly method after a survey of the literature. Simultaneously, the use of hydrate-based separation technology is at the laboratory stage and does not have industrial application. The hydratebased technology has been applied in many fields, such as gas separation, desalination, refrigeration, gas storage, and transportation. The study of gas hydrate separation has been used for many kinds of gases, such as biogas (CH4/CO2),12 coal bed gas (CH4/N2),13 flue gas (CO2/N2),14 and IGCC flue gas (CO2/H2).15 The mechanism of hydrate gas separation was based on different conditions of hydrate formation pressure of mixture gas. The components of mixture gas, which formed the Received: December 11, 2018 Accepted: March 18, 2019

A

DOI: 10.1021/acs.jced.8b01188 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

Journal of Chemical & Engineering Data

Article

Figure 1. Apparatus for the hydrate experiments. (1) Temperature controller. (2) Gas chromatography. (3) Rotary valve. (4) Air bath. (5) Highpressure reactor. (6) Stirrer. (7) Hand pump. (8) Pressure display. (9) Pressure and temperature display of reactor. (10) Gas cylinder. (11) Vacuum pump.

thermodynamic additives to lower phase equilibrium conditions in relative literature of gas hydrate studies. TBAB is one of the quaternary ammonium salts that consists of Br− and TBA+. The TBAB molecules could form semiclathrate hydrate.25 TBAB was one of the important thermodynamic promoters, and it could significantly decrease gas hydrate formation pressure. Many researchers make the experiments about imidazolium-based ionic liquid on hydrate formation and hydrate separation.26,27 Sodium chloride (NaCl), methane (MeOH), and ethylene glycol (EG) were three hydrate inhibitors. Researchers studied the hydrate of CH4 + H2O + TBAB. The results showed that MeOH is observed to be a more effective inhibitor than NaCl and EG.28,29 Relevant studies have shown that inhibitors along with lower concentrations of promoters may be suitable for efficient hydrate formation and gas storage in hydrate phase. [BMim]BF4 has been used to combine with TBAB to study the effect of thermodynamic and kinetic additives on biogas hydrate formation and separation. CH4 in simulated biogas increased from initial feeding gas 67−83.1 mol %. The combination of [C8min]BF4 with TBAB and THF has been studied in the separation of simulated biogas, CH4 can be purified from initial feeding gas 66−77 mol % in residual gas.30 In order to promote more CO2 to form the gas hydrate phase, this paper presents the hydrate formation studies carried out using different combinations of two types of additives: kinetic and thermodynamic. The aim was to analyze the combination effect on gas enclathration kinetics, gas storage ability in hydrate phase, and CH4 separation efficiency from the simulated biogas. The surfactant sodium dodecyl benzene sulfonate (SDBS), biological ionic liquid ([EMIM]BF4), and physical absorbent propylene carbonate (PC) were studied on biogas hydrate separation in this work. The surfactant sodium dodecyl sulfate (SDS) is one well-known anionic surfactant. Low dosage of SDS additive could shorten the hydrate nucleation time and increase the hydrate formation rate greatly.31,32 Methane hydrate formed from pure water with SDS shows higher moles of gas consumption per mole of water as compared to other aqueous systems at higher pressure, such

gas hydrate easily, would be enriched in the hydrate phase. The other mixture gas would stay in the residual gas phase. Biogas is one of the green energy resources that has an important developmental potential. It consists of methane (CH4), carbon dioxide (CO2), and sulfureted hydrogen (H2S).16 However, the sulfureted hydrogen was one of the poisonous gases, and it is dangerous to people’s health. CO2 is the main greenhouse gas, and it occupies a major percentage in biogas. CH4 is considered to be one of the major energy sources for future energy usage because of its comparative abundance on earth.17 In order to obtain higher purity methane and have a better commercial application, the component of CO2 and H2S should be removed from the biogas. Recently, many studies on hydrate-based separation have been applied on CO2 separation from flue gases and power plant gas.18 Theoretically, the hydrate gas separation method seems attractive for biogas separation. However, there are a few studies on CO2 separation from biogas by this method in practice. The basic reason is the proximity of hydrate formation pressure of CH4 and CO2, so it is quite difficult to effectively separate CO2 from the biogas using the hydrate-based method.19 However, many studies have shown that the addition of some chemical additive into the hydrate reaction liquid could efficiently separate CO2 gas, such as the ionic liquids, surfactants,20 and physical and chemical absorbents. These additives could shorten the induction time of hydrate formation and accelerate the hydrate formation rate. They have the selective enclathration ability for CO2 to enrich in the hydrate phase. Among the many additives reported in the literature on hydrate application, the hydrate chemicals added can be classified into two categories, which are thermodynamic and kinetic additives.21 Thermodynamic additives were used to decrease hydrate formation pressure under a certain temperature and could shift the hydrate-phase equilibrium condition to higher temperature and lower pressure. Kinetic additives were used to promote hydrate formation rate, shorten the induction time, and increase gas storage in hydrate phase. There are some common hydrate promoters. Tetrahydrofuran (THF),22 TBAB,23 and cyclopentane (CP)24 were effective B



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as the aqueous solution of pure water, TBAB, (TBAB + SDS), THF, and THF + SDS.33 SDBS is an anion surfactant and it has similar characteristics as TBAB.34 Some researchers have studied that SDBS is an excellent kinetic promoter in hydrate formation. However, there are a few studies about the combination of TBAB with SDBS in biogas hydrate separation. [EMIM]BF4 is an imidazolium ionic liquid and it has been used in some environmental-friendly reactions.35 However, [EMIM]BF4 has not been investigated in the area of gas hydrate formation and gas separation. Therefore, the influence of TBAB and [EMIM]BF4 on biogas separation was investigated in this work. PC is a sour gas absorbent that is used in physical absorption process and it has good solubility in aqueous solution.36 Because CO2 in biogas was about 30−40 mol %, the addition of PC may promote more CO2 to dissolve in the solution and to form gas hydrate. Therefore, the combination of TBAB with PC was studied on biogas hydrate separation in this work. The hydrate separation method would provide a theoretical foundation for economical and scaled-up industrial application.

(1) First, the reactor was cleaned by deionized water three times. Then, 80 mL reaction liquid was put into the reactor by vacuum pump. (2) The reactor was pumped to vacuum by hand pump. Then, the reactor was flushed by feeding gas three times. The gas cylinder was opened to make feeding gas to arrive at the desired pressure. The gas pressure was high enough to make sure that the hydrate would form under this condition. (3) The agitation was initiated when temperature and pressure did not change any more. The electromagnetic agitation rate was controlled at 1000 rpm. The liquid− gas interfacial area was increased and renewed to improve the hydrate formation rate. (4) The reactor temperature was controlled at a certain value for 1 h. When no hydrate particles were formed in the reactor, the pressure of feeding gas was increased slowly by hand pump sufficiently to make sure that the hydrate particles formed on the gas−liquid interface. It means that there were little hydrate particles that were like grains of ice. The length and width of hydrate particles were about 0.3 and 0.1 cm. (5) When hydrate particles formed under high gas pressure, the pressure was decreased slowly by the hand pump until only a little gas hydrate particles remained on the gas−liquid interface. (6) When a little hydrate particles existed on the gas−liquid interface for 3 h, the corresponding gas pressure was hydrate thermodynamic phase equilibrium point under this temperature. If the hydrate particles disappeared after 1 h, the fifth step was repeated. 2.2. Experimental Process of Hydrate Kinetics Study. (1) The first and second steps were the same as the steps of hydrate thermodynamic study. (2) The air bath was used to control hydrate reactor temperature. When the temperature was maintained stably for 2 h, the hydrate kinetic experiments were studied. The reaction volume was 246 mL. The temperature and volume were kept constant. The change in temperature and pressure was recorded timely. (3) When the pressure of the remaining gas, which was unhydration gas, did not change any more, it shows that the hydrate formation process has arrived at equilibrium. Then the reaction liquid and the remaining gas were drained out by hand pump. (4) Continually, the condition of experiments changed and the above steps were repeated, so that the chronological changes in pressure and temperature could be recorded. 2.3. Experimental Process of the Simulated Biogas Separation Process. The mixture gas separation experiments were similar to the kinetic experiments. The main difference was that the feeding experiment gas was simulated biogas. In the experiments, the gas storage in the hydrate phase, CH4 recovery, phase equilibrium constant, gas storage in the hydrate phase, and CH4 hydrate ratio were calculated. (1) The first four steps were the same as the kinetic study steps of pure gas kinetic experiments. (2) When the gas pressure of the reactor arrived at equilibrium, the remaining gas was analyzed by GC. Also, the volume of 40 mL remaining gas was analyzed three times. Then, the values were averaged.

2. MATERIALS AND EXPERIMENTAL PROCESS The schematic of experimental apparatus is shown in the Figure 1. The high-pressure reactor was made by Jiangsu Hai’An Oil Research Instruments Co. Ltd. The tolerable pressure of the reactor was 25 MPa and its volume was 246 mL. There existed two viewing windows on the two sides of the reactor. The hand pump was supplied by Nantong Huaxing Petroleum Instrument Co. Ltd. The air bath type was WD2050, which was supplied by Shanghai Experimental Instrument Factory Co. Ltd. The temperature control range of the air bath was 253−403 K. The pressure sensor was supplied by Fujian Shun Chang Hong Yun Precision Instruments Co. Ltd. The uncertainty of pressure was ±0.02 MPa. The temperature sensor was supplied by Hangzhou Meacon Automation Technology Co. Its temperature range was 223−523 K and its sensitivity was ±0.01 K. A gas chromatograph (GC) 7890B was supplied by the Agilent Technologies of America and precision was 0.01 mol %, which was used to analyze the constitution of mixture gas. TBAB was provided by Tianjin Nankai Share Compounds Co. Ltd. and its purity was 0.999 wt. The mass fraction of TBAB in this experiment was 0.05; this value was determined from the reference of correlative experiments and it was the optimized concentration.37 The deionized water (with 18.25 mΩ·cm−1 resistivity) was made by our own laboratory. The [EMIM]BF4 was purchased from Zhengzhou Ekam Chemical Co. Ltd. and its purity was 0.999 wt. SDBS was supplied by Tianjin Beichen Fangzheng Reagent Factory and its purity was 0.9 wt. PC was supplied by Shanghai Zhanyun Chemical Co. Ltd. The purity of CO2 gas and CH4 gas was 99.9 mol % and they were supplied by Beijing Bei Temperature Gas Company. The simulated biogas (64.0 mol % CH4/CO2) with precise components was supplied by Beijing Bei Temperature Gas Company. 2.1. Experimental Process of Hydrate Thermodynamic Phase Equilibrium. As for the thermodynamic phase equilibrium experiments, it was measured by the isochoric pressure method. The brief procedures are listed as follows: C



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Figure 2. Pictures of hydrates growth process in different time periods: [(1) 5 (2) 10 (3) 15 (4) 20 (5) 25 (6) 30 (7) 35 (8) 40 min].

3.4. Calculation of CH4 Recovery. The parameter of CH4 recovery was important for the mixture gas hydrate separation. The CH4 recovery was calculated by the following equation. nCH4, x nx ·xCH4 R CH4 = × 100% = × 100% nCH4, f nf · xf (4)

(3) When the hydrate formation arrived at equilibrium, the reactor was cooled down to 268.15 K and then the remaining gas was pulled out by hand pump. Then, when the reactor was pulled to the vacuum and the hydrate particles were heated to dissolve to the liquid, the releasing gas from the hydrate phase was analyzed by GC.

where x is the gas constitution in the hydrate phase, f is the gas constitution of the feeding gas, and nx and nf are CH4 moles in the hydrate phase and feeding gas phase. nx and ny were calculated by the following equation. The parameters of vr and v0 stand for the volume of gas in the hydrate phase and the feeding gas.

3. CALCULATION OF RELATIVE GAS HYDRATE SEPARATION CONSTANT 3.1. Calculation of Gas Storage in the Hydrate Phase. The reaction gas that was consumed in the hydrate phase was calculated by the following equation. PV PV Δn = n0 − nt = 0 0 − t t z 0RT zt RT (1)

nx = nf =

where n is the total moles of gas, 0 and t stand for the starting time and the reaction time, P is the pressure, R is the gas universal constant, and V is the volume of hydrate reaction. In this experiment, V0 was the same as Vt. Z is the compressible factor that was calculated by P−R equation. 3.2. Calculation of Phase Equilibrium Constant. The parameter of phase equilibrium constant is important for mixture gas hydrate separation. The separation results would be better when the phase equilibrium constant increases. The constant was calculated by the following equation. K1 = x1/y1

PxVr zxRTx

(5)

Pf V0 z 0RT

(6)

3.5. Calculation of CO2 Hydrate Ratio. CO2 hydrate ratio is the ratio of CO2 mole in the hydrate phase with CO2 mole in the feeding gas. The calculation was as follows xn a= 11 x0n0 (7) In the equation, x1 represents CO2 mole fraction in the hydrate phase, n1 represents the total gas moles in the hydrate phase, x0 represents CO2 mole fraction in the feeding gas, n0 represents the mole of the feeding gas, and n1 and n0 were calculated by equations (5) and (6).

(2)

where x is the CO2 percentage in the hydrate phase, And y is the CO2 percentage in the remaining gas phase. 3.3. Calculation of Separation Factor. The parameter of separation factor was important to reflect CH4 separation efficiency from biogas. The separation factor was calculated by the following equation. n1, xn2, y x1y2 SF = = n1, yn2, x y1x 2 (3)

4. RESULTS AND DISCUSSION 4.1. Experimental Phenomenon of Hydrate Formation. In this part, the hydrate formation phenomenon was visualized by a digital camera. These photos reflected the formation process of hydrate and the growth of hydrate. The glaring light in the photos was the cold light source and it did not affect the observation of hydrate reaction. With the increase in time, the gas hydrate particles increased greatly in the reactor and the hydrate particles was visible through the front window. Figure 2(2) shows the beginning of hydrate

where n1,x and n1,y are CO2 moles in the hydrate phase and the un-reaction equilibrium gas phase, n2,x and n2,y are CH4 moles in the hydrate phase and the remaining gas phase. D



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Figure 3. Hydrate formation equilibrium conditions of three gases at different temperatures. [(A) Single CO2 gas. (B) Simulated biogas (64.0 mol % CH4/CO2)].

Figure 4. Kinetic study of CO2 hydrate in the different additive solutions [(A) Pressure change. (B) Temperature change].

high. The addition of pure TBAB into deionized water could greatly decrease CH4/CO2 hydrate formation pressure. The addition of [EMIM]BF4 and PC has a slight inhibition on biogas hydrate in TBAB solution. The addition of SDBS did not have obvious change with the results in pure TBAB solution. However, the thermodynamic promotion effect of TBAB could ignore the inhibition effect of PC and [EMIM]BF4 on hydrate formation. 4.3. Study of CO2 Hydrate Kinetic. CO2 in simulated biogas was about 36 mol %, which was the major component gas needed for removal. Thus, the hydrate reaction process of CO2 in different additive solutions should be studied in detail. Figure 4A reflects the pressure change of CO2 when the time increased. Figure 4B reflects the temperature change in hydrate solution. As shown in Figure 4A, the additives of TBAB, [EMIM]BF4, PC, and SDBS were added into the deionized water. However, the initial feed gas pressure was 3.05 MPa, and the pressure decreased greatly within the first 2 min. The pressure decreased from 3.05 to 2.35 MPa in deionized water. It was obvious that the CO2 gas pressure in PC solution decrease from the initial pressure of 3.05 to 2.54 MPa. Comparing the hydrate kinetic progress in the four additive solutions, the addition of [EMIM]BF4 could promote more CO2 to be absorbed and form the gas hydrate. However, compared with the pure additives solution, more CO2 was absorbed and formed hydrate in the deionized water. The time to arrive at CO2 hydrate phase equilibrium in deionized water was 30 min, which was six times longer than the phase equilibrium time (5 min) in other three additive solutions. In the study of CO2 kinetic formation progress, there existed two periods that have obvious changes. The first period was CO2

formation and the hydrate particles assembled on the gas− liquid interface. Figure 2(2−7) shows the period of hydrate growth process quickly. More hydrate particles aggregated and the crystalline solids formed. The hydrate formation rate slowed down with the increase in time, because the hydrate particle existed on the gas−liquid interface and it prevented the reaction gas from contacting with the aqueous liquid. Figure 2(8) shows that the hydrate formation arrived at phase equilibrium and almost no liquid existed in the reactor. 4.2. Thermodynamic Study of Hydrate Formation. In this part, the study of hydrate thermodynamic experiments of CO2 and simulated biogas was investigated. In Figure 3A, we explored the thermodynamics of CO2 hydrate formation in deionized water. Compared with the results of deionized water in literature values,38 the values that were measured in experiments were consistent. Moreover, the deviation was permitted and so the experiment steps were proper. When the three different additives were added into deionized water, the thermodynamic results showed that [EMIM]BF4 and PC have the inhabitation effect on CO2 hydrate formation. The CO2 hydrate thermodynamic phase equilibrium pressure in PC solution was about 0.4 MPa higher than the phase equilibrium pressure in deionized water. The hydrate phase equilibrium in [EMIM]BF4 solution was 0.3 MPa higher than the results obtained in deionized water. The addition of SDBS did not have obvious effect on CO2 hydrate formation compared with the results in deionized water. In Figure 3B, the thermodynamics of simulated biogas was studied in TBAB solution when three additives were added. The experimental data and the literature data are in preferable accordance.39 The pressure of biogas hydrate formation in the deionized water was too E



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Figure 5. Various [EMIM]BF4 concentration (1000/2000/3000 ppm) experiments were studied on CH4/CO2 hydrate formation process and gas separation results in TBAB solution. [(A) Pressure change. (B) Temperature change. (C) Results of the CH4 mole fraction in the remaining gas and CH4 recovery. (D) Results of K and SF].

Table 1. Summary of Experimental Conditions and Simulated Biogas (64 mol % CH4/CO2) Separation Results in 4 wt % TBAB Solution SF

CH4 recovery (%)

gas storage (mol)

CO2 hydrate ratio (%)

number

additives

T (K)

P (MPa)

t90 (s)

1 2

TBAB TBAB + 1000 ppm [EMIM]BF4 TBAB + 2000 ppm [EMIM]BF4 TBAB + 3000 ppm [EMIM]BF4 TBAB + 0.003 wt SDBS TBAB + 0.005 wt SDBS TBAB + 0.01 wt SDBS TBAB + 0.05 wt PC TBAB + 0.1 wt PC TBAB + 0.2 wt PC

277.15 277.15

3.5 3.5

50 55

79.37 78.08

41.62 56.74

2.02 2.41

2.74 4.35

92.5 92.39

0.073 0.073

57.24 56.21

277.15

3.5

68

79.29

58.51

2.83

5.41

94.21

0.075

55.43

277.15

3.5

76

78.26

56.98

2.71

4.93

91.41

0.072

54.61

277.15

3.5

120

77.59

42.04

1.89

2.55

91.17

0.071

53.19

277.15

3.5

104

78.17

48.61

2.23

3.38

91.31

0.073

54.89

277.15

3.5

89

78.82

50.75

2.39

3.82

91.45

0.074

56.32

277.15 277.15 277.15

3.5 3.5 3.5

80 93 120

78.14 78.47 78.49

52.25 49.03 48.64

2.39 2.28 2.26

3.91 3.51 3.45

90.81 93.22 92.27

0.071 0.073 0.068

55.09 54.83 54.53

3 4 5 6 7 8 9 10

CH4 (mol %, remaining gas)

CO2 (mol %, hydrate phase)

K

dissolution in liquid, and the second period was CO2 hydrate reaction progress. The decrease in CO2 was mainly in the first gas dissolution period from the curves of temperature and pressure. Thus, we could make the relative biogas separation by hydrate method after the study of CO2 hydrate formation. 4.4. Conclusion of Simulated Biogas Separation Results. In this part, various concentrations (1000/2000/ 3000 ppm) of [EMIM]BF4 were added into the TBAB solution. From Figure 5A, it can be seen that the concentration change of [EMIM]BF4 did not have obvious influence on gas storage in the hydrate phase. The initial feeding pressure was 3.5 MPa; when the process of gas absorption and hydrate formation arrived at equilibrium, the equilibrium pressure was

about 2.7 MPa. [EMIM]BF4 was one common CO2 absorb agent, so more CO2 would be absorbed and would enter the hydrate phase when the concentration increased compared with the pure TBAB solution. Figure 5B shows the temperature change of the gas absorption and hydrate formation process. The temperature curve showed a decreasing trend at 1.2 min and then the temperature rose quickly. The point of 1 min meant the end of gas absorption and the start of hydrate formation. From Table 1 and Figure 5C,D, the combination of 2000 ppm [EMIM]BF4 and TBAB could get higher CH4 percentage in the remaining gas, which was 79.29 mol %. Compared with the pure TBAB solution, the addition of ionic liquid could improve CO2 percentage in the hydrate F



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Figure 6. Various SDBS concentration (0.003/0.005/0.01 wt) experiments were studied on CH4/CO2 hydrate formation process and gas separation results in TBAB solution. [(A) Pressure change. (B) Temperature change. (C) Results of the CH4 mole fraction in the remaining gas and CH4 recovery. (D) Results of K and SF].

Figure 7. Various PC concentration (0.05/0.1/0.2 wt) experiments were studied on CH4/CO2 hydrate formation process and gas separation results in TBAB solution. [(A) Pressure change. (B) Temperature change. (C) Results of the CH4 mole fraction in the remaining gas and CH4 recovery. (D) Results of K and SF].

trend when [EMIM]BF4 concentration increases. In summary, the optimized [EMIM]BF4 concentration for biogas hydrate separation was 2000 ppm for gas separation results. In this part, the combination of SDBS and TBAB on biogas hydrate separation was invested. The initial feeding gas pressure was 3.5 MPa. As shown in Table 1, the time of t90

phase. The maximum CO2 percentage in the hydrate phase was 58.51 mol %; the phase equilibrium constant and separation factor were 2.83 and 5.41, respectively. The CH4 recovery was 94.21% and the gas storage in the hydrate phase was 0.075 mol when the [EMIM]BF4 concentration was 2000 ppm. The CO2 hydrate ratio in Table 1 shows a declining G



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formation pressure. It increases the driving force for nucleation and thus shifts the hydrate−liquid−vapor equilibrium curve to a most stable region. Similarly, the addition of [EMIM]BF4 and PC has a thermodynamic inhibition effect on hydrate formation; the addition of SDBS did not have thermodynamic effect on biogas hydrate formation. The kinetic study of CO2 hydrate formation process was studied in the relative experiments. The results showed that the addition of [EMIM]BF4 and PC could increase the CO2 gas storage in hydrate phase. The addition of SDBS could greatly increase the gas hydrate formation rate and shorten the hydrate formation induction time. The second part discussed the studies on simulated biogas hydrate separation process. The addition of [EMIM]BF4, SDBS, and PC was applied to promote CH4 purification in biogas. In the experiments of TBAB + [EMIM]BF4 hydrate separation process, CH4 in remaining gas and CO2 in hydrate phase would increase to the maximum values, which were 79.29 and 58.51 mol %, respectively. The phase equilibrium constant and the separation factor were 2.83 and 5.41, respectively. The time of t90 increased with the increase in [EMIM]BF4 concentration. The separation results would be better with the increase in SDBS concentration. The time to arrive at phase equilibrium shortened with increase in SDBS concentration. When the SDBS concentration was 0.01 wt, the separation results were better. CH4 concentration in the remaining gas reached 78.82 mol %, and CO2 concentration in the hydrate phase reached the maximum value, which was 50.75 mol %. The combination of PC + TBAB on biogas hydrate separation experiments showed that CH4 mole fraction in the remaining gas increased with the increase in PC concentration, from 78.14 to 78.49 mol %. Although the PC additive was a CO2 absorbent, CO2 in hydrate phase could not increase with the increase in PC concentration. When PC concentration was 0.05 wt, the phase equilibrium constant and separation factor reached the maximum values, which were 2.39 and 3.45, respectively. CH4 recovery was 93.22% when the PC concentration was 0.1 wt. In all the experiments, the gas storage in the hydrate phase did not change dramatically. Basically, the gas storage in hydrate phase was 0.07 mol on account of addition of TBAB. TBAB could form the semiclathrate with water molecules, and more gas molecules could not form the gas hydrate. On the whole, because of CH4 in remaining gas and separation factor, the synergistic effect on simulated biogas of TBAB + [EMIM]BF4 was better than that of other two combination additives. The CO2 hydrate ratio was about 55%, and it did not change widely. However, the combination of TBAB + SDBS did not have a significant influence on CH4 recovery and CO2 capture compared with the separation results in pure TBAB solution. SDBS could be used as one of the hydrate promoters to accelerate the hydrate formation rate and shortened the induction time. However, many studies are still needed to optimize the parameters of hydrate gas separation technology to provide a theoretical basis for industrial application.

decreased from 120 to 89 s slightly with the increase in SDBS concentration. Thus, SDBS could shorten the time for hydrate formation to arrive at phase equilibrium. As shown in Figure 6A,B, the time point of 1.7 min was the demarcation point of the end of the gas absorption process and the beginning of gas hydrate formation. The addition of surfactant SDBS could decrease the surface tension and the adhesion force between the two hydrate particles could prevent the agglomeration of gas hydrates. The resistance of gas molecules entering into the gas−liquid interface decreased. As shown in Figure 1, gas storage in hydrate phase and CO2 hydrate ratio increased with the increase in SDBS concentration. Their maximum values were 0.074 mol and 56.32%, respectively. As shown in Figure 6C,D, CH4 in the remaining gas was 78.82 mol % and CO2 in the hydrate phase was 50.75 mol %. The phase equilibrium constant and separation factor were 2.39 and 3.82, respectively. Thus, the induction time was shortened and the mass transformation rate was promoted. The rate of hydrate formation and gas storage in the hydrate phase increased. The CH4 recovery was 91.45% when the SDBS concentration was 0.01 wt. In this part, various concentrations of PC were added into the TBAB solution to study simulated biogas hydrate separation. As shown in Figure 7A, the gas storage in hydrate phase has the trend of slight increase with the increase in PC concentration. As shown in Table 1 and Figure 7C, when PC concentration was 0.1 wt, CH4 mole fraction in the remaining gas was the maximum, which was 78.48 mol %. When the concentration of PC was 0.2 wt, CH4 recovery was maximum, which was 93.22%. As shown in Figure 7A,B, the time point of 2.7 min was the demarcation point of gas absorption and gas hydrate formation. As shown in Table 1, CO2 in hydrate phase and the hydrate ratio of CH4 showed a declining trend with the increase in PC concentration. The maximum values were 52.25 mol % and 55.09%. Thus, the sour gas absorber cannot increase gas storage in hydrate formation progress. However, the maximum phase equilibrium constant and maximum separation factor were 2.39 and 3.91, respectively. The gas storage in hydrate phase was maximum in the TBAB + 0.1 wt PC solution, which was 0.073 mol. PC was one of the acid gas absorbents. The addition of PC could increase the hydrate formation pressure under identical temperature. The fugacity of water decreased when PC was added. The formation pressure of CO2 increased in the solution. At the same time, the addition of PC increased the formation pressure of CO2 and TBAB. Thus, the addition of PC could not increase the capacity of gas storage during hydrate formation.

5. CONCLUSIONS In this paper, the study of simulated biogas separation was employed in different additive TBAB solutions. The experiments were about hydrate formation thermodynamic study and kinetic study of simulated biogas. As for thermodynamic study of CO2 hydrate formation, the addition of [EMIM]BF4, PC has an inhibition effect on hydrate phase equilibrium, and the hydrate formation pressure increased with the increase in additive concentration. The addition of SDBS seems to have no effect on hydrate thermodynamic formation compared with the results with deionized water. As for the study of simulated biogas hydrate formation experiments, CH4/CO2 hydrate formation pressure was too high under the experiment temperature conditions. The addition of TBAB could greatly decrease biogas hydrate

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xu-Qiang Guo: 0000-0002-0781-1477 H



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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Science Foundation of China University of Petroleum-Beijing at KARAMAY Science Foundation of CUPBK RCYJ2017A-02-001 RCYJ2017A-03-001, which is greatly acknowledged.

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Combining Different Additives with TBAB on CO2 Capture and CH4

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