Phase Equilibrium Data for Semiclathrate Hydrate of Synthesized

Jan 23, 2017 - Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China. ABSTRACT: Hydrate-ba...
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Phase Equilibrium Data for Semiclathrate Hydrate of Synthesized Binary CO2/CH4 Gas Mixture in Tetra‑n‑butylammonium Bromide Aqueous Solution Xiaoya Zang†,‡,§ and Deqing Liang*,†,‡,§ †

Guangzhou Institute of Energy Conversion, Guangzhou Center for Gas Hydrate Research, Chinese Academy of Sciences, Guangzhou 510640, China ‡ CAS Key Laboratory of Gas Hydrate, Guangzhou 510640, China § Guangdong Provincial Key Laboratory of New and Renewable Energy Research and Development, Guangzhou 510640, China ABSTRACT: Hydrate-based gas separation(HBGS) technology based on tetran-butyl ammonium bromide (TBAB) semiclathrare hydrate can be utilized in CO2 separation from biogas. The phase equilibria of semiclathrate hydrates are crucial for the successful industrial application of HBGS technology. In this work, the phase equilibrium data of TBAB semiclathrate hydrate with a synthesized binary CO2/CH4 mixed gas (0.5 CO2 and 0.5 CH4 in mole fraction) were reported at five different TBAB concentrations (0.001, 0.01, 0.05, 0.1, and 0.2 mass fraction). The experiments were conducted in the temperature range (278.1 to 292.6) K and pressure range (2.17 to 8.16) MPa using an isochoric method. The agreement between the experimental data and the data reported in the literature indicated the reliability of the apparatus and method. The results showed that there were no obvious thermodynamic promotion effects on the CO2/CH4 mixed gas hydrate phase equilibrium conditions when the mass fraction of TBAB was less than 0.01, while there was a significant affect on the CO2/CH4 mixed gas hydrate phase equilibrium conditions when the mass fraction of TBAB was greater than 0.05. In addition, the results also indicated that the promotion effect of TBAB on the CO2/CH4 gas hydrate formation may be varied as the concentration of TBAB in the aqueous solutions changes.

1. INTRODUCTION Gas hydrates are icelike crystalline compounds formed by guest molecules trapped inside cage structures of water molecules under high pressures and low temperatures.1 The cage structures are formed by water molecules through hydrogen bonding. Gas hydrates are mostly found in sediments under deep sea or permafrost in nature and are considered as a potential source of clean energy. Because of the particularity of the hydrate structure, it also can be utilized in many industrial applications, such as natural gas storage and transportation, seawater desalination, CO2 capture and sequestration, etc.2−6 In addition, the emerging hydrate-based CO2 gas separation technology has gained more attention in recent years.7,8 Biogas is a kind of unconventional natural gas mainly comprising methane (CH4) and carbon dioxide (CO2) along with lesser amounts of water vapor, nitrogen (N2), oxygen (O2), and hydrogen sulfide (H2S) and is considered a renewable energy fuel.9 CO2 gas separation from biogas can reduce environmental pollution from impurity gas emissions, improve energy conversion, and increase utilization efficiency. The foundation of hydrate-based gas separation technology (HBGS) is the discrepancy of the hydrate formation conditions between different gas components. Gas components that easily form hydrates can enter into the hydrate phase. On the contrary, gas components that form hydrate with difficulty © 2017 American Chemical Society

remain in the gas phase. Compared with the conventional gas separation technologies such as physical absorption, chemical absorption, membranes, etc., this innovative separation process can reduce environmental pollution and improve separation efficiency.7,8,10 However, the main drawback of this technology is the high pressures and low temperatures that must be maintained during the hydrate formation and separation process, which involves relatively high energy consumptions. Recent studies have shown that the use of chemical additives could improve the hydrate formation kinetics and increase the CO2 separation efficiency from flue or fuel gas.11−15 Compared to CO2/H2 and CO2/N2 systems, the other problem for CO2 separation from a CO2/CH4 gas mixture by HBGS is that both CO2 and CH4 molecules might be incorporated into the hydrate crystals at the hydrate formation conditions.16,17 In recent years, many researchers have used the HBGS technology for CO2 separation from CH4/CO2 gas mixtures.18−20 Reduction of the hydrate formation conditions or the use of chemical additives (e.g., tetrahydrofuran (THF), tetrahydropalmatine (THP)) could enhance the hydrate formation kinetics and increase the CO2 separation efficiency.21−24 Received: October 11, 2016 Accepted: January 12, 2017 Published: January 23, 2017 851

DOI: 10.1021/acs.jced.6b00876 J. Chem. Eng. Data 2017, 62, 851−856

Journal of Chemical & Engineering Data

Article

Figure 1. Schematic diagram of the experimental apparatus.

Table 1. Sample Description Table

a

chemical name

source

carbon dioxide/methane TBABb water

Guangzhou Puyuan Gas Plant Tokyo Chemical Industry Co.

initial purity

purification method

final purity

analysis method

0.5/0.5 mixture

GCa

>0.98 (mass fraction) distillation

b

GC = gas chromatography. TBAB = tetra-n-butylammonium bromide.

phase was respectively monitored by temperature sensors with an accuracy of 0.1 K. The gas phase pressure was measured by a pressure transducer with an accuracy of 0.025 MPa. Besides, gas pipelines, a temperature-controlled water bath, an agitator, and a vacuum pump were also included in the experimental apparatus. Table 1 lists the specifications and supplier of the chemical materials used in this work. The binary gas mixture of ultrahigh purity CO2 and CH4 was prepared in a 0.5/0.5 molar ratio. TBAB was used as purchased, and distilled water was used in all experiments. 2.2. Procedure. After the experimental system was evacuated, about 250 mL of a desired concentration of the TBAB aqueous solution was transferred to the reactor cell through the liquid injection valve. The extra air was removed again from the cell using a vacuum pump. Then, the binary CH4/CO2 gas was injected into the system. Once the pressure reached the desired value (typically 3−9 MPa) at room temperature (about 300 K), the agitation equipment was turned on with a frequency of 100 R/min to begin the hydrate formation process. The water bath temperature was set at a constant value (about 273 K) during the whole hydrate formation process (more than 10 h). Over the entire reaction period, the reaction cell was disconnected from the gas reservoir. The gas hydrate equilibrium points were measured using an isochoric method. After complete hydrate formation, the temperature was increased stepwise to 300 K to dissociate the hydrate. In the first step, the temperature increased at a rate of about 1 K·h−1 until the hydrate began to dissociate. Then, the temperature was increased slowly at increments of 0.1 K. The system was kept constant at each temperature for a sufficient time (about 4−6 h) to obtain an equilibrium state in the reactor cell. Throughout the hydrate formation and dissociation process, the temperature and pressure changes were recorded and monitored by an Agilent data acquisition unit (model 34970A). Meanwhile, the residual gas was sampled and analyzed by gas chromatography (GC) at the equilibrium

In addition, the quaternary ammonium salts (QASs) also can significantly affect the hydrate formation conditions as thermodynamic additives. QASs, such as tetra-n-butylammonium bromide (TBAB), tetra-n-butylammonium chloride (TBAC), tetra-n-butylammonium fluoride (TBAF), tert-butyl peroxybenzoate (TBPB), form semiclathrate hydrates with water molecules at atmospheric pressure.25−27 In recent years, a few researchers have investigated the equilibrium conditions of gas hydrates mixed with different QASs additives.28−31 Among them, TBAB was the most common used as semiclathrate hydrate formers. In TBAB semiclathrate hydrates, the water molecules and Br− anions formed semiclathrate cages, and tetra-n-butylammonium cations (TBA+) could occupy the cavities structure as guest molecules. TBAB can be used as a chemical additive for CO2 separation from CH4/CO2 gas mixtures. However, the hydrate phase equilibrium data were still inadequate to provide a theoretical basis for the process design to capture CO2 from CO2/CH4 gas mixtures using semiclathrate hydrates. In particular, the thermodynamic conditions for CO2/CH4 hydrate formation with different TBAB concentrations have not been investigated, and if determined, could be used to effectively enhance the gas separation efficiency and process. The main target of this work is to obtain the semiclathrate hydrate phase equilibria data and the enthalpy of hydrate dissociation for CH4/CO2(0.5/0.5) systems in TBAB aqueous solutions with varying mass fractions from 0.001 to 0.2.

2. EXPERIMENTAL SECTION 2.1. Apparatus and Materials. Figure 1 provides a schematic diagram of the experimental setup. The main component of the experimental apparatus was the cylindrical reaction vessel. The maximum working pressure of the reactor cell was 15 MPa. The inner diameter and height of the reaction vessel was 8 and 10 cm, respectively. There were two circle viewing windows inserted at the front and back wall of the reactor cell. The temperature of the gas phase and the liquid 852

DOI: 10.1021/acs.jced.6b00876 J. Chem. Eng. Data 2017, 62, 851−856

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agreement with the data in the literature and compared well with the predicted value from CSMHYD, confirming that our experimental setup and procedure meet the necessary requirements for determining the hydrate phase equilibria. The phase equilibrium conditions for semiclathrate hydrates of the CO2/CH4 binary mixtures in the presence of TBAB at mass fractions (w) of 0.001, 0.01, 0.05, 0.1, and 0.2 are reported herein. Table 2 lists the three phase equilibria data of the

point. When the hydrate dissociated completely, the phase equilibrium data could be determined using a pressure− temperature (P−T) diagram with the isochoric pressure-search method.32 There were two slopes in the P−T profiles: one sharply increased and the other slowly increased because of hydrate dissociation and volume expansion of the gas phase after complete dissociation, respectively. The intersection of the two slopes was taken as the phase equilibrium point. A typical P−T plot of the experimental method was shown in Figure 2.

Table 2. Three Phase Equilibrium Conditions for Binary CO2/CH4 Gas Mixture Hydrates with Different Mass Fractions of TBABa Aqueous Solutionsb TBABa mass fraction (w) 0

0.001

0.01

Figure 2. A representative diagram illustrating the isochoric pressure search method. 0.05

3. RESULTS AND DISCUSSION Five different TBAB concentrations were studied to illustrate the phase equilibrium conditions for the CH4/CO2 + TBAB + water system. In addition, the pure CH4 and binary CO2/CH4 (0.5/0.5) mixture gas hydrate phase equilibrium conditions with distilled water at different pressures was studied and compared with the reported data33,34 and the predicted value of CSMHYD35 to verify the reliability of the experimental apparatus. The equilibrium dissociation conditions for the pure CH4 and CH4 + CO2 hydrates are shown in Figure 3. The equilibrium phase conditions obtained in this work are in good

0.1

0.2

P/MPa

T/K

ΔHdiss/(kJ/mol)

2.76 3.56 3.80 4.46 5.61 3.40 4.52 5.12 6.22 8.16 3.96 4.58 5.26 5.98 6.88 2.92 3.66 4.44 5.50 6.40 7.00 2.86 3.50 4.70 5.58 6.44 2.17 2.82 3.59 4.38 5.08

278.2 280.5 281.0 282.4 284.1 279.3 281.8 283.1 284.5 286.7 280.7 281.9 283.1 283.8 285.1 285.8 286.7 287.4 288.3 288.8 289.0 288.3 288.9 290.1 290.7 291.2 289.3 290.1 291.2 292.0 292.6

254.64 245.41 242.58 234.88 221.21 243.56 230.52 223.75 210.55 188.15 260.99 253.00 244.23 234.48 222.92 576.69 557.98 538.04 510.76 489.19 470.75 589.75 573.84 544.01 521.70 499.79 570.81 556.19 539.15 521.37 505.50

a

Tetra-n-butylammonium bromide. bUncertainties (u) are u(mass fraction) = ±0.002, u(T) = ±0.1 K, u(P) = ±0.025 MPa.

semiclathrate hydrate for the CO2/CH4 binary gas mixtures (0.5/0.5) with different TBAB aqueous concentrations, respectively. Figure 4 shows the corresponding plots of the hydrate equilibrium data for the binary CO2/CH4 gas mixtures with different TBAB aqueous concentrations. Figure 4 shows the shift of the phase boundary to a lower pressure and higher temperature region in the presence of TBAB at a mass fraction greater than 0.05. Thus, the presence of TBAB has a rigorous promotion effect on CO2/CH4 hydrate formation. By increasing the TBAB mass fraction from 0.05 to 0.2, the promotion effect of the TBAB hydrates increased. At a mass fraction of 0.05, the equilibrium temperature of the CO2/ CH4 hydrate increased by 4−6 K at the same pressures. At mass fractions of 0.01 and 0.001, there were no obvious promotion effects because the phase boundary with the TBAB

Figure 3. Three phase equilibrium boundaries of the CH4 and CH4/ CO2 (0.5/0.5) mixture gas hydrates formed from pure water. 853

DOI: 10.1021/acs.jced.6b00876 J. Chem. Eng. Data 2017, 62, 851−856

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Figure 4. Phase equilibria of the semiclathrate hydrates of CO2/CH4 with different mass fractions of tetra-n-butylammonium bromide (TBAB) aqueous solutions. wTBAB: mass fraction of TBAB.

Figure 5. A Clausius−Clapeyron plot based on the hydrate equilibrium data obtained from the CO2/CH4 gas mixture with different mass fractions of tetra-n-butylammonium bromide (TBAB) aqueous solutions.

aqueous solution almost overlapped with that of pure water. This means that the TBAB solution at 0.01 and 0.001 mass fraction does not influence semiclathrate hydrate formation conditions significantly. It may be because very little TBAB hydrate can form and TBAB has no significant effect on the activity of water with low concentration solutions. Therefore, it does not obviously change the phase equilibrium conditions of the hydrate with TBAB mass fraction less than 0.01. The original composition of the CO2/CH4 gas mixture in the gas cylinder is 0.5 CO2 and 0.5 CH4 in mole fractions. However, the composition of CO2 in the vapor phase at the equilibrium state was lower than that in the original gas mixture because the solubility of CO2 in the TBAB aqueous solution is much higher than that of CH4 at the same pressure and temperature. On the basis of the pressure (P, MPa), temperature (T, K), and CO2/CH4 gas molar fractions at the equilibrium state, the dissociation enthalpy (ΔHd, kJ/mol) of the CO2/CH4 hydrate can be calculated using the Clausius− Clapeyron equation: ΔHd d(ln P) = d(1/T ) zR

Figure 6. Phase equilibria of the semiclathrate hydrates containing different molar ratios of CO2/CH4 with different mass fractions of the tetra-n-butylammonium bromide(TBAB) aqueous solution. □, wTBAB = 0.3, CO2/CH4 = 0.45/0.55;36 ○, wTBAB = 0.05, CO2/CH4 = 0.33/ 0.67;29 △, wTBAB = 0.05, CO2/CH4 = 0.4/0.6;37 ▽, wTBAB = 0.1, CO2/ CH4 = 0.4/0.6;37 ◇, wTBAB = 0.2, CO2/CH4 = 0.4/0.6;37 ☆, wTBAB = 0.05, CO2/CH4 = 0.6/0.4;37 ×, wTBAB = 0.1, CO2/CH4 = 0.6/0.4;37 +, wTBAB = 0.2, CO2/CH4 = 0.6/0.4;37 ●, wTBAB = 0.05, CO2/CH4 = 0.5/ 0.5, this work; ▲, wTBAB = 0.1, CO2/CH4 = 0.5/0.5, this work; ⧫, wTBAB = 0.2, CO2/CH4 = 0.5/0.5, this work.

(1)

where z represents the gas compressibility factor and R = 8.314J·K−1·mol−1. A comparison of the CO2/CH4 gas hydrate dissociation conditions in different TBAB mass fraction systems is shown in Figure 5 by the semilogarithmic plot of the equilibrium pressure (ln P) versus the reciprocal temperature (1/T). ΔHd can be obtained from the slope of the straight line fitted to the data. As shown in Figure 5, the slope increased with respect to the TBAB mass fraction. Accordingly, ΔHd increased as the TBAB concentration increased. For a further understanding, the hydrate equilibrium data were compared to those for TBAB/TBAC/TBAF/TBPB/THF + CO2/CH4 + H2O systems.29,36,37 Figure 6 illustrates the hydrate phase equilibria data for different molar ratios of the CO2/CH4 binary mixtures with different mass fractions of the TBAB aqueous solution. For a given mass fraction of TBAB at a given temperature, increasing the mole fraction of CO2 in the feed gas decreased the pressure that was required for hydrate formation. The increasing CO2 molar ratio in the gas phase

caused a shift of the phase equilibrium conditions for the CO2/ CH4 hydrates to higher temperatures and lower pressures. Figure 7 shows that the results are in good agreement with available literature data.17,29,38 The presence of different thermodynamic additives (TBAB, TBAC, TBAF, TBPB, and THF) has a rigorous promotion effect on hydrate formation from the CO2/CH4 binary gas mixtures. At the same mass fraction (w = 0.05), the promotion effect of the THF was strongest. The promotion effect increased according to TBAC < TBAB < TBAF < THF. 854

DOI: 10.1021/acs.jced.6b00876 J. Chem. Eng. Data 2017, 62, 851−856

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Figure 7. Phase equilibria of the semiclathrate hydrates formed from different molar ratios of CO2/CH4 binary mixtures in aqueous solutions with different chemical additives: TBAB, TBAC, TBAF, TBPB, and THF.

4. CONCLUSION Experimental studies were performed to determine the equilibrium pressure and temperature for CO2/CH4(0.5/0.5) binary gas mixture hydrates in an aqueous solution containing mass fractions of TBAB = 0.001, 0.01, 0.05, 0.1, and 0.2 at T = (278.1 to 292.6) K and P = (2.17 to 8.16) MPa. The presence of TBAB (mass fraction > 0.05) has a significant influence on the CO2/CH4 hydrate phase equilibrium conditions. The TBAB promotion effect for hydrate formation increased by increasing the TBAB mass fraction from 0.05 to 0.20. The addition of TBAB at a low mass fraction (