Effects of Dimethyl Sulfoxide on Phase Equilibrium Conditions of CO2

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Effects of Dimethyl Sulfoxide on Phase Equilibrium Conditions of CO2 and IGCC Fuel Gas Hydrate in the Presence and Absence of Tetra‑n‑butyl Ammonium Bromide Yang Luo,† Zhen-Yu Yan,† Jing Li,† Guang-Juan Guo,‡ Xu-Qiang Guo,*,† Qiang Sun,† and Ai-Xian Liu† †

State Key Laboratory of Heavy Oil Processing, China University of Petroleum, Beijing 102249, P. R. China Petrochemical Research Institute of PetroChina, Beijing 102206, P. R. China

‡

ABSTRACT: This work presents the study to explore the thermodynamic conditions of CO2 separation from the simulated integrated gasification combined cycle (IGCC) fuel gas via hydrate formation assisted by dimethyl sulfoxide (DMSO) and tetra-n-butyl ammonium bromide (TBAB). The measurements of the equilibrium hydrate formation conditions in the pressure range of (0.45 to 7.60) MPa and temperature range of (275.55 to 285.45) K were explored by the T-cycle method, and the gas phase composition at the equilibrium point was sampled and analyzed. In addition, the effect of DMSO upon CO2 hydrate and CO2 + TBAB was further investigated. The experimental results showed that while DMSO has a thermodynamic inhibition effect on the formation of CO2 hydrate, but a kinetic promotion effect on the solubility of CO2 in water solution, the mixture of TBAB and DMSO can reduce the pressure needed for CO2 hydrate formation obviously as well as promote the solubility of CO2 remarkably.

1. INTRODUCTION The upsurge of carbon dioxide levels in the atmosphere is a major environmental concern, as this powerful greenhouse gas is strongly linked to global warming and climate change. Particularly, increasing energy demand has resulted in a rapidly growing number of electric power plants that burn fossil fuels, which are releasing approximately one-third of the carbon dioxide worldwide.1,2 In recent years, the integrated gasification combined cycle (IGCC) process, which is characterized by converting fossil fuels into syngasa mixture of hydrogen and carbon monoxidethrough partial oxidation in a gasifier and then producing the mixture of hydrogen and carbon dioxide through a water gas shift (WGS) reaction in the following WGS column, has been widely utilized in coal-fuel electric power plants. The gas mixture of H2 + CO2 coming out of WGS column usually contains a high concentration of CO2 (0.4 to 0.5 mole fraction) and should be purified before combustion, which is usually referred to as IGCC fuel gas. Generally, the pressure range of this stream is from 2.0 to 7.0 MPa.3 Higher CO2 partial pressure of IGCC fuel gas offers a more efficient opportunity to capture CO2 from it compared to that from flue gas (mixture of CO2, N2, and O2) emitted by a conventional power plant. As known widely, there are three conventional methods, i.e., membrane separation, adsorption, and absorption, to be used for CO2 capture from the fuel gas mixture. Despite their advantages, these methods also have their own disadvantages.4−7 Lately, a method for separating CO2 from gas mixtures via hydrate formation has been put forward6,8,9 and shed light for highly efficient CO2 capture with low energy consumption. Gas © XXXX American Chemical Society

hydrates are nonstoichimetric crystalline compounds formed when suitably sized gas molecules (guest molecules), such as CH4, CO2, and N2, are encapsulated in various hydrogenbonded cages made up of water molecules (host molecules) at a specific temperature and pressure.10,11 Gas hydrates have been widely explored by researchers from energy and environment fields. Natural gas hydrates deposits (NGHD) distributed in permafrost or oceans are considered as an alternative energy source in the future.12 In particular, with increasing depletion of the limited coal and petroleum reserves, the production of natural gas from NGHD has been receiving much attention, and a series of national programs striving to realize this have been established in many countries. Moreover, as gas hydrates formed in transmission pipelines and production facilities always bring about blockage and considerable economic loss, many petroleum and gas industries are trying hard to avoid the formation of hydrates.13−15 What’s more, due to the excellent performance of large storage capacity and high safety, clathrate hydrates have been believed to be a promising method to transport gases, such as natural gas transportation and hydrogen storage.16 Note that the thermodynamic condition required to form a corresponding hydrate with guest molecules is largely dependent on the properties of guest molecules, such as shape and size; the component with milder condition for corresponding hydrate formation can be enriched in the hydrate phase during the Received: June 30, 2016 Accepted: November 18, 2016

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DOI: 10.1021/acs.jced.6b00558 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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CO2 + H2 gas mixtures with TBAB + DMSO aqueous solution were investigated. Meanwhile, the gas phase composition at the equilibrium point was sampled and analyzed. Based on the experimental results, the thermodynamic effects of TBAB + DMSO additive on hydrate formation were determined, which is certainly beneficial for further study of the potential IGCC fuel gas (CO2 + H2) separation process and lays the foundation for later industrial application.

hydrate formation from the gas mixture. Based on this mechanism, the hydrate-based method has been extensively applied for gas separation, such as separating CH4 from coal bed gas,17−20 refrigerant R134a from air,21 C2 from refinery gas,22,23 and CO2 from CO2 + H2 mixtures,9,24−26 all of which achieved remarkable accomplishments. For example, since, at the same temperature, the pressure needed for hydrate formation of CO2 is much lower than that of H2, it is expected that gas separation of CO2 + H2 mixtures can be realized with a high selectivity via hydrate formation. It should be borne in mind that the formation of gas hydrates cannot be rationalized completely on a thermodynamic basis (regulation of temperature and pressure) alone on account of drastic increasing energy consumption with the pretty harsh formation requirements. In fact, a promising solution to this problem would be to introduce some gas hydrate promoters into the system. The process of gas hydrate formation can be realized relatively easily with mild conditions assisted by these additives, which generally reduce the required pressure and increase the rate to form hydrate greatly by selectively modifying hydrate cages.27 For instance, tetra-n-butyl ammonium bromide (TBAB), as an environmental friendly additive, as its quaternary ammonium salt can form semiclathrate hydrates with water molecules at atmospheric pressure. At favored stable conditions, the remarkable feature of TBAB hydrate cavities is the encapsulation of the TBA+ in large pentakaidecahedra and tetrakaidecahedra (50% of each), while in small dodecahedral cavities (S-cage, 512), small guest gas molecules (such as H2S, N2, CO2, and CH4) are encaged within.28 Therefore, TBAB semiclathrate hydrates have been deeply applied for gas storage as well as separation.24,29,30 However, it has been reported that even though the TBAB semiclathrates could dramatically lower the thermodynamic formation condition, some kinetic factors (such as gas storage capability and hydrate formation rate) were not satisfactory. Consequently, previous researchers have devoted great efforts to overcome these bottlenecks and envisioned that a faster hydrate formation rate with lower mass transfer resistance can be obtained by the assistance of a higher solubility of guest molecules in host molecules and subsequent larger contact area between water and guest gas molecules.31−33 That is to say, it is possible to improve the gas dissolution and diffusion process through coupling traditional hydrate thermodynamic additives with physical gas solvent. Dimethyl sulfoxide (DMSO), which has been reported as a crucial gas solvent in industry, can remarkably enhance the solubility of CO2 in aqueous solution due to its properties.34 Xia et al.8,35,36 found that DMSO as kinetic additive could improve both the rate, gas consumption, and selectivity of CO2 for landfill gas purification. However, to the best of our knowledge, the thermodynamic effect of DMSO upon CO2 hydrate formation was rarely reported. It is expected that if the mixture of TBAB and DMSO is used as a gas hydrate promoter, not only the equilibrium formation pressure for CO2 will be reduced by TBAB but both the solubility of CO2 in water and the CO2 hydrate formation rate will be promoted. Hence, on the basis of these observations, in this article, the study to explore the thermodynamic conditions of CO2 separation from the simulated IGCC fuel gas via hydrate formation with a combination of TBAB and DMSO was presented. First, the equilibrium formation conditions of CO2 hydrates in DMSO aqueous solution were measured and the effect of DMSO upon hydrate was discussed as well. Then, the equilibrium thermodynamic properties of hydrate formation for

2. EXPERIMENTAL SECTION 2.1. Materials. Two CO2 + H2 gas mixtures (M1 and M2) and the analytical-grade carbon dioxide that was employed as the feed gas in the following experiments were obtained from Beijing Bei Temperature Gas Company. The compositions of two gas samples and pure CO2 provided by the supplier and determined by gas chromatography are listed in Table 1. Table 1. Mole Fraction of Gas Samples Used in This Work Composition, mol %a

a

Gas Sample

CO2

H2

Pure CO2 Mixture 1 (M1) Mixture 2 (M2)

99.99 40.6 15.5

59.4 84.5

The uncertainty in the composition is 0.02 mol %.

Deionized water was made in our laboratory with a SZ-93 water distillation unit. TBAB (purity of 99 mol %) and DMSO (purity of 98 mol %) were purchased from Beijing Reagents Corporation. 2.2. Experimental Apparatus. Figure 1 presents the schematic diagram of the experimental setup, by which the equilibrium condition to form hydrates and separation experiments was systematically explored. This device mainly includes a high-pressure vessel (Haian Special Equipment, China), a volume-variable buffer tank, a hand pump, a gas cylinder, a vacuum pump, and an air bath. As a main part of the apparatus, the high-pressure vessel has a magic stirrer (fixed at the top to agitate the liquid and gas mixture) and two viewing windows. The design pressure and effective internal volume of the vessel are 25 MPa and 246 mL, respectively. Another important part is a volume-variable buffer tank with a minimum volume of 370 mL and a maximum volume of 590 mL, in which the feed gas is prestored. A piston that connects to a hand pump is installed at the top, and the hand pump is used to precisely regulate the initial pressure. The air bath (Shanghai Instrument, China) with a compressor power of 1.5 kW is used to supply the vessel with a constant temperature ranging from 243.15 to 323.15 K. The temperatures in the vessel and buffer tank are measured with Platinum 100 resistance thermometers with an accuracy of 0.05 K, and the gas pressure is measured with a pressure transducer (DG1300, Guangzhou Senex Instrument Ltd., China) with an uncertainty of 0.02 MPa. Additionally, the residual gas of the vessel is evacuated with a vacuum pump connected to the gas outlet. The composition of the gas mixture is determined with a gas chromatograph (7890B, Agilent Technoligies, USA) with an uncertainty of 0.02 mol %. 2.3. Experimental Procedure. In this work, the “T-cycle method”9,37−39 was employed to obtain reliable data of the equilibrium hydrate formation pressures and temperatures for the gas−liquid−hydrate phase equilibrium. B

DOI: 10.1021/acs.jced.6b00558 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 1. Schematic diagram of the experimental setup: 1, Data acquisition and computer; 2, Magnetic driver; 3, Valve; 4, Hydrator; 5, Outlet valve; 6, Volume-variable buffer tank; 7, Air bath; 8, Inlet valve; 9, Hand pump; 10, Gas cylinder.

Prior to the experiment, the vessel and each joint were cleaned with deionized water and rinsed with the prepared solution with a desired concentration. Subsequently, a certain amount of the aqueous solution prepared previously was introduced into the vessel, which was then evacuated using a vacuum pump to deprive the residual gas and flushed with gas sample three times to ensure the inner space of the vessel was air free. First, the vessel was pressured up to the desired value by injecting the experimental gas sample. After that, the inlet valve was shut off to isolate the vessel from the gas cylinder and the stirring rate was set to 250 rpm. The air bath was adjusted to the intended value to cool the vessel. Subsequently, the system temperature was reduced gradually with a temperature gradient of 0.5 K/h through the air bath until a drastic pressure depression was observed. Then, the temperature was kept constant at this point over a period of 60 min. Finally, the temperature was increased again with the gradient of 0.3 K/h until the equilibrium point with an infinitesimal amount of gas hydrate left was reached.

Figure 2. Comparison of phase equilibrium conditions for CO2 hydrates formed in pure water (●, this work; □, Adisasmito;40 Δ, Ohgaki41) and 0.62 mol % TBAB solution (+, Mohammadi;37 ◆, this work).

3. RESULTS AND DISCUSSION 3.1. Validation of Apparatus. In order to ensure that the experimental apparatus mentioned in section 2.2 is suitable to measure the equilibrium hydrate formation conditions, there is a need to conduct an initial study to establish the experimental methodology. Herein, following the experimental procedure mentioned in section 2.3, a series of experiments toward measuring the equilibrium conditions of CO2 hydrates formed in the pure water and 0.0062 mole fraction of TBAB solution in the temperature range of (274.05 to 278.75) K were carried out. Figure 2 shows a direct comparison between the experimental data obtained in this work and previous literature reported by Adisasmito et al.,40 Ohgaki et al., 41 and Mohammadi et al.37 The excellent consistence between the two indicates the good reliability of our apparatus that reliable data of equilibrium hydrate formation conditions can be obtained using the experimental apparatus and following the procedure mentioned above. 3.2. Effects of DMSO on the Phase Equilibrium Conditions of CO2 Hydrate in the Presence and Absence of TBAB. To provide some insights toward studying the phase

equilibrium conditions of the simulated IGCC fuel gas hydrate, the influence of gas solvent DMSO upon CO2 hydrate and TBAB + CO2 semiclathrate hydrate formation should be determined first. For this reason, the equilibrium conditions for the CO2 + DMSO hydrate system in the pressure range of (1.59 to 3.07) MPa and temperature range of (273.15 to 279.35) K were systematically measured. For the CO2 + DMSO + H2O system, the mole fractions of DMSO in aqueous solutions were fixed at 0.01, 0.02, and 0.05, respectively, and the phase equilibrium data (temperature and pressure) obtained from these measurements were listed in Table 2. As can be seen from Figure 3, the introduction of DMSO to the liquid phase results in the increase of the equilibrium pressure needed for CO2 hydrate formation at a certain temperature. For example, the equilibrium formation pressure for CO2 + H2O hydrate is 1.544 MPa at 275.37 K,41 while the pressure goes up to 2.88 MPa at the similar temperature (275.55 K) with only 0.05 mole fraction of DMSO in the liquid phase. Additionally, it should be noted that the equilibrium hydrate formation pressure increases along with the concenC

DOI: 10.1021/acs.jced.6b00558 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Table 2. Phase Equilibrium Temperatures and Pressures for CO2 Hydrate with DMSO (1) + TBAB (2) + H2O Mixturea x1 = 0.010, x2 = 0

a

x1 = 0.020, x2 = 0

x1= 0.050, x2 = 0

x1= 0.014, x2 = 0.011

x1= 0.029, x2 = 0.011

T/K

p/MPa

T/K

p/MPa

T/K

p/MPa

T/K

p/MPa

T/K

p/MPa

274.55 276.95 278.15 279.35

1.59 2.02 2.33 2.72

273.45 275.85 276.95 278.05 278.85

1.72 2.18 2.39 2.68 3.07

273.15 274.35 275.55

2.57 2.62 2.88

282.15 284.25 286.35 288.25

0.52 1.03 1.64 2.55

282.05 284.45 286.65

0.96 1.49 2.42

Standard uncertainties u are u(T) = 0.05 K and u(p) = 0.02 MPa.

Table 3. Hydrate−Liquid−Vapor Equilibrium Temperature, Pressure, and Equilibrium Vapor-Phase Mole Fraction of CO2 for CO2 (1) + H2 (2) + TBAB (3) + DMSO (4) + H2O (5) Mixture Hydratea Mole fraction, mol %

Figure 3. Effects of DMSO (1) on the phase equilibrium conditions of CO2 hydrate in the presence and absence of TBAB (2): ●, x1 = 0.050, x2 = 0; ▲, x1 = 0.020, x2 = 0; ▼, x1 = 0.010, x2 = 0; ■, x1 = 0, Adisasmito;40 ○, x1 = 0.029, x2 = 0.011; Δ, x1 = 0.014, x2 = 0.011; □, x1 = 0, x2 = 0.011, Mohammadi37

CO2

TBAB

DMSO

Temp, K

Pressure, MPa

40.6

0.29

2.00

0.62

2.00

0.29

2.00

0.62

0

0.62

2.00

276.55 277.35 278.55 279.55 280.55 281.85 283.45 278.95 280.55 282.45 283.45 285.45 275.55 277.05 278.15 279.15 280.05 281.15 280.85 282.25 283.55 284.45 285.15 280.25 281.45 282.35 283.65 284.55

0.62 0.85 1.42 2.06 2.76 3.96 6.23 0.45 1.09 2.11 3.03 4.68 0.75 1.67 2.74 4.23 5.66 7.55 0.73 1.97 3.56 5.43 7.54 1.11 2.41 3.70 5.82 7.61

15.5

tration of DMSO rising in the solution. These results indicate that DMSO has a thermodynamic inhibition effect on the formation of CO2 hydrate, which may be attributed to the fact that both the activity and chemical potential of water become lower with the addition of DMSO to the system. Furthermore, Figure 3 displays a direct comparison between the equilibrium conditions (temperature and pressure) for CO2 + DMSO in the presence and absence of TBAB; that is, milder equilibrium hydrate formation conditions (higher temperatures and lower pressures) could be realized in the presence of TBAB. Combined with results from previous research on TBAB, the drastic thermodynamic promotion effect of TBAB on the formation of CO2 hydrate (no matter with or without DMSO in the system) can be easily confirmed. 3.3. Equilibrium Conditions for CO2 + H2 + H2O Mixture Hydrate Formation in TBAB + DMSO Aqueous Solutions. Seeking to shed light on the operating conditions of separating CO2 from IGCC fuel gas via hydrate formation, the measurements of equilibrium hydrate formation conditions for M1 assisted by TBAB + DMSO aqueous solutions in the temperature range of (276.55 to 285.45) K and pressure range of (0.45 to 6.23) MPa were carried out. Beyond that, to evaluate the solubility of simulated IGCC fuel gas in TBAB + DMSO solution, the gas phase compositions at each equilibrium condition were carefully analyzed. These experimental data (equilibrium temperatures, pressures, and gas phase compositions) were shown in Table 3 and Figure 4.

a b

Vapor-phase mole percentage of CO2, mol %b 32.44 32.69 32.77 32.75 32.64 32.55 32.97 32.88 32.91 32.66 32.71 32.79 10.07 10.13 10.01 10.32 10.52 10.61 13.06 12.95 13.13 12.99 13.35 10.63 10.68 10.05 10.02 10.10

Standard uncertainties u are u(T) = 0.05 K and u(P) = 0.02 MPa. The uncertainty in the composition is 0.02 mol %.

Figure 4 shows that the equilibrium hydrate pressures for M1 in TBAB or TBAB + DMSO solutions are remarkably lower than that in pure water. For example, compared to the equilibrium hydrate pressure of CO2 + H2 + H2O mixture hydrate (e.g., 11.01 MPa at 278.75 K),24 that of CO2 + H2 + H2O + TBAB and CO2 + H2 + H2O + TBAB + DMSO mixture hydrate at a similar temperature (278.65 and 278.55 K) reduces to 0.7124 and 1.42 MPa, respectively. As presented in these experimental data, the introduction of only 0.0029 mol fraction of TBAB can lead to approximately 93% reduction of the equilibrium pressure needed for hydrate formation. Moreover, it is worth mentioning that the addition of DMSO weakens the D

DOI: 10.1021/acs.jced.6b00558 J. Chem. Eng. Data XXXX, XXX, XXX−XXX

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Figure 4. Equilibrium hydrate formation pressures for Mixture 1 [CO2 (1) + H2 (2)] in the presence of TBAB (3) + DMSO (4) + H2O (5): ▲, x3 = 0.00, x4 = 0.00, Li et al.;24 ●, x3 = 0.0029, x4 = 0.02; ■, x3 = 0.0029, x4 = 0.00, Li et al.;24 ▼, x3 = 0.0062, x4 = 0.02.

Figure 5. Equilibrium hydrate formation pressure for Mixture 2 in the presence of TBAB (3) + DMSO (4) + H2O (5): ■, x3 = 0.0029, x4 = 0.020; ●, x3 = 0.0029, x4 = 0, Li et al.;24 ▲, x3 = 0.0062, x4 = 0.020.

4. CONCLUSION In this paper, the thermodynamic conditions of CO2 separation from the simulated IGCC fuel gas via hydrate formation assisted by a combination of TBAB and DMSO were systematically investigated. The gas phase composition at the equilibrium point was sampled and analyzed, and the effects of DMSO upon CO2 hydrate formation with or without TBAB were further evaluated. The experimental results illustrate that (1) DMSO has a thermodynamic inhibition effect on the formation of CO2 hydrate and a kinetic promotion effect on the solubility of CO2 in water solution and that (2) the mixture of TBAB and DMSO can still obviously reduce the pressure needed for CO2 hydrate formation, but the pressure drop effect of this mixture is slightly inferior to that of TBAB.

pressure drop effect of TBAB to 87%, which indirectly confirms that DMSO has a thermodynamic inhibition effect on the formation of CO2 hydrate (consistent with results mentioned in section 3.2). From another perspective, even though DMSO alone has this thermodynamic inhibition effect, the mixture of TBAB + DMSO can still reduce the equilibrium hydrate pressure remarkably. In addition, it is noteworthy that, as observed from follow-up experiments (not shown), the mole fraction of CO2 in the residual gas of the first-stage separation is approximately 16%, which indicates there is a necessity to perform a second-stage separation. To provide some guidance for further separation using the residual gas from the first-stage separation as feed gas, a series of experiments that measured the equilibrium hydrate formation conditions for a CO2 + H2 gas mixture (M2) containing 15.5% mole fraction of CO2 assisted by TBAB with the mole fraction of (0.0029 and 0.0062) and DMSO with the mole fraction of 0.02 solution in the temperature range of (275.55 to 285.15) K and pressure range of (0.75 to 7.61) MPa were carried out. The results in Figure 5 show that the temperature needed for hydrate formation at a certain pressure increases along with the mole fraction of TBAB rising in the liquid phase, which demonstrates that the equilibrium hydrate formation conditions for M2 and M1 gas mixtures have similar characteristics. Furthermore, the gas phase compositions at each equilibrium point were also shown in Table 3, which can be seen as a sign of the relative solubility of CO2 + H2 in the different systems. The mole fractions of CO2 in the equilibrium gas phase of the mixture hydrate system assisted by TBAB and the mixture of TBAB + DMSO are ∼13.0% and ∼10.5%, respectively. Considering these experimental results, it should be pointed out that not only DMSO but also its mixture with TBAB can drastically promote the solubility of CO2 in the corresponding solution, which proves that DMSO can be chosen as one excellent gas absorbent for CO2 in the water solution. Potentially, DMSO may have great industrial application for assisting the formation of a CO2 mixture hydrate as a synergic kinetic promoter in the future.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: +86 10 89731003; fax: +86 10 89731003. E-mail address: [email protected]. ORCID

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

We gratefully acknowledge financial support from National 973 Project of China (No. 2012CB215005) and Science Foundation of China University of Petroleum, Beijing (YJRC2013-09). Notes

The authors declare no competing financial interest.

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REFERENCES

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DOI: 10.1021/acs.jced.6b00558 J. Chem. Eng. Data XXXX, XXX, XXX−XXX