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The partition coefficients of ethane between vapor and hydrate phase for methane + ethane + water and methane + ethane + THF + water systems
Fluid Phase Equilibria 225 (2004) 141–144
The partition coefficients of ethane between vapor and hydrate phase for methane + ethane + water and methane + ethane + THF + water systems L.-W. Zhang, G.-J. Chen∗ , X.-Q. Guo, C.-Y. Sun, L.-Y. Yang High Pressure Fluid Phase Behavior and Property Research Laboratory, University of Petroleum, Beijing 102249, PR China Received 18 June 2004; accepted 20 August 2004
Abstract The vapor–hydrate equilibria were measured firstly for CH4 + C2 H6 + water system beyond incipient conditions and the results showed that the partition coefficients of ethane between vapor and hydrate phase were not promising. And then the gas–hydrate equilibria were studied experimentally in detail for CH4 + C2 H6 + tetrahydrofuran (THF) + water systems in the temperature range of 274.15–282.15 K, pressure range of 1.0–3.0 MPa and THF concentration range of 4–14 mol% in initial aqueous solution. The results demonstrated that the partition coefficient of ethane between vapor and hydrate phase increased dramatically because of the presence of THF in water, ethane was remarkably enriched in vapor phase instead of being slightly enriched in hydrate phase for CH4 + C2 H6 + water system. © 2004 Elsevier B.V. All rights reserved. Keywords: Partition coefficient; Vapor; Hydrate; Methane; Ethane; Tetrahydrofuran
1. Introduction When a gas mixture forms hydrate partially, the relative concentration of each component in the hydrate phase and that in residual vapor phase might be different, the component that can form hydrate more easily might be enriched in hydrate phase. Based on this principle, it has been considered to be a possible way to separate gas mixture through forming hydrate. Hydrate technology is promising to the systems for which usual distillation method is not very feasible, such as low-boiling systems and close-boiling systems. There have been some work reported related to the separation technology based on hydrate principles [1–5]. Since methane and ethane are two major components of natural gases, hydrates formed from them have been given special concern. Sloan and coworkers [6–8] have published several papers on the hydrate formed from methane + ethane binary gas mixture with pure water. A series of hydrate phase diagrams have been presented by them.
As methane and ethane are low-boiling components, it is expensive to separate them by usual distillation method because of deep cooling. On the other hand, the separations of methane and ethane are usually occurred in natural gas or oil processing, ethylene producing, etc. Thus it is of practical significance to develop new methods for their separation. It might be a possible choice to separate them through forming hydrate above 0 ◦ C. With respect to hydrate technology, it is very important to increase the separation efficiency of single equilibrium stage as multi-stage operation like distillation is difficult to realize in practice because it takes much more time to reach a vapor–hydrate equilibrium than to reach a vaporliquid equilibrium. It will be demonstrated that the single stage separation efficiency of methane and ethane mixture can be increased remarkably by adding certain quantity of tetrahydrofuran (THF) to water-rich phase in this work. 2. Experimental 2.1. Apparatus
∗
Corresponding author. Tel.: +86 1089733252; fax: +86 1089733252. E-mail address: [email protected] (G.-J. Chen).
0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.08.032
The experimental apparatus used in this work had been described in detail in the previous papers by
142
L.-W. Zhang et al. / Fluid Phase Equilibria 225 (2004) 141–144
this laboratory [9,10]. A brief introduction is given here. The apparatus consists mainly of a cylindrical variablevolume high-pressure (0–25 MPa) transparent sapphire cell (2.54 cm in diameter, effective volume approximately 60 cm3 ) installed in an air-bath and equipped with a magnetic floating piston inside for accelerating the phase equilibrium process. The formation/dissociation of the hydrate crystals in the solution can be observed directly trough the transparent cell wall. The accuracies of temperature and pressure measurements are within ±0.2 K and ±0.025 MPa, respectively. 2.2. Materials and preparation of samples Analytical grade methane (99.9%) and ethane (99.95%) supplied by Beifen Gas Industry Corporation were used in preparing the synthetic binary gas mixtures. A HewlettPackard gas chromatograph (HP 6890) was used to analyze the composition of gas mixtures. The THF used for preparing the aqueous solution was supplied by Beijing Reagents Corporation. An electronic balance with a precision of ±0.1 mg was used in preparing aqueous solution with the required concentration of THF. 2.3. Experimental procedure Firstly, the sapphire cell was washed using distilled water, and then it was rinsed three times with water or the prepared aqueous solution. After the cell was thoroughly cleaned, a suitable quantity of liquid sample was added into the cell. The gas space of the cell was purged with the prepared gas sample four to five times to ensure the absence of air, and then the gas sample was charged into the cell until the given pressure achieved. Subsequently, the air-bath temperature was adjusted to the desired value. Once the cell temperature was kept constant, hydrate nucleation was then induced by agitation of the magnetic floating piston. After all liquid water or aqueous solution was converted to solid hydrate, the residual gas (vapor) was kept in touch with hydrate phase for over 8 h to ensure vapor–hydrate equilibrium. The temperature and pressure were maintained constant during the whole experimental process. When the vapor–hydrate equilibrium was established, the vapor phase was sampled and analyzed at least three times using the gas chromatograph. The average values were then taken as the compositions of vapor phase. The compositions of methane and ethane in hydrate phase were obtained by analyzing the compositions of released gas with the gas chromatograph after hydrate dissociated thoroughly.
Table 1 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + H2 O systems with fixed initial gas–liquid volume ratio of 200 standard volumes of gas per volume of liquid T (K)
P (MPa)
z2 (mol%)
x2 (mol%)
y2 (mol%)
K2
274.15
2.5 3.0 4.0
60.11 60.11 60.11
65.92 65.67 62.53
58.25 57.73 57.48
0.88 0.88 0.92
only the relative compositions of methane and ethane in two phases. z2 , x2 , y2 represent the concentration of ethane in feed gas (the prepared gaseous mixture sample), hydrate phase and vapor in such basis, respectively. K2 is the partition coefficient of ethane between vapor and hydrate phase, K2 = y2 /x2 3.1. CH4 (1) + C2 H6 (2) + H2 O system The vapor–hydrate equilibrium for ternary system of CH4 + C2 H6 + H2 O was firstly measured, where the initial gas–liquid volume ratios were uniformly set to 200 standard volumes of gas per volume of liquid (water). The obtained experimental data were listed in Table 1. Considering the hydrate formation pressure of ethane is much lower than that of methane at same temperature, one should expect a remarkable enrichment of ethane in hydrate phase. But, as shown in Table 1, the ethane was only slightly enriched by 3–5% in hydrate phase compared with feed composition. The partition coefficients of ethane between vapor and hydrate phase are also not promising. 3.2. CH4 (1) + C2 H6 (2) + THF + H2 O system
3. Results and discussion
Considering THF can form hydrate very easily, some researchers [5] used it as a hydrate promoter. Our purpose here is somewhat different. THF was expected to take the role of a selective inhibitor. In our reasoning, THF will dominate the structure of hydrate formed by the guest mixtures concerned in this work because THF can form SII hydrate very easily, especially when methane exists as a help gas. That is to say guests CH4 , C2 H6 and THF together will form SII hydrate. Furthermore, THF molecules again will dominate the occupancy of large cages of SII hydrate and inhibit the occupancy of ethane molecules in them. On the other hand, as ethane molecules cannot occupy small cages,1 the occupancy of small cages will be dominated by methane molecules. Then, as a result, the overall occupancy of ethane molecules in hydrate cages would be obviously lower than that of methane. In order to prove above reasoning, we performed a series of experiments on the vapor–hydrate equilibrium for CH4 + C2 H6 + THF + H2 O system. Firstly, the influences of temperature, pressure and feed composition on the partition coefficient, K2 , of ethane between vapor and hydrate phase were studied
Before presenting the experimental results, it should be noted that the compositions of vapor phase and hydrate phase given here are both in water and THF-free basis, i.e., they are
1 Moita et al. [11] have proved that ethane molecules can occupy small cage by Raman spectroscopy, but this probability is very small at middle or lower pressures and can be neglected.
L.-W. Zhang et al. / Fluid Phase Equilibria 225 (2004) 141–144 Table 2 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + THF + H2 O systems with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid and fixed THF concentration of 6 mol% in initial aqueous solution P (MPa)
T (K)
z2 (mol%)
x2 (mol%)
y2 (mol%)
K2
3.0
276.15 278.15 280.15 282.15 284.15
24.39
14.92 13.18 11.08 8.48 6.77
66.80 66.66 65.45 51.72 44.33
4.48 5.06 5.91 6.10 6.55
2.0
278.15 280.15 282.15 284.15
24.39
9.95 9.17 7.48 5.13
57.44 53.26 45.92 39.63
5.77 5.81 6.13 7.72
3.0
274.15 276.15 278.15 280.15 282.15
56.2
27.03a 26.77a 25.46a 23.44 19.55
72.61a 74.90a 79.71a 87.07 80.74
2.68 2.80 3.13 3.71 4.13
2.0
274.15 276.15 278.15 280.15 282.15
56.2
29.33a 28.89a 27.80 21.37 14.74
68.36a 73.73a 87.68 85.49 80.23
2.69 2.55 3.15 4.00 5.44
2.0
276.15 278.15
92.91
84.14a 73.33
95.19a 96.75
1.13 1.32
a
This datum is in unusual case.
while the concentration of THF in initial aqueous solution was fixed at 6 mol% and the initial gas–liquid volume ratio was specified to 100 standard volumes of gas per volume of liquid. The obtained experimental data were listed in Table 2. Comparing with the data in Table 1, it could be seen that the partition of ethane between vapor and hydrate phase was reversed due to the existence of THF in water. Instead of being slightly enriched in hydrate phase, ethane was enriched in vapor phase remarkably. The partition coefficient of ethane between vapor and hydrate is very fascinating, the biggest value of K2 measured was 7.72. In general, K2 increases with the increasing of temperature or decreasing of pressure. On the other hand, there exist optimized temperature and pressure values corresponding to the highest enrichment of ethane in vapor when mole fraction of ethane in feed gas is relatively high. Here, enrichment means the difference between the mol% of ethane in vapor and that in feed gas. For the feed gas with 56.2 mol% ethane, the biggest enrichment was observed to be 30.87%. The extent of enrichment of ethane is related to feed composition. For the feed gas containing 24.39 mol% ethane, the highest enrichment reached 42%. In order to study the influence of concentration of THF on the partition coefficients, the vapor–hydrate equilibrium data for CH4 + C2 H6 + THF + H2 O system was measured with respect to six different initial concentrations of THF in aqueous solution at fixed temperature of 278.15 K, fixed pressure of 2.5 MPa and fixed initial gas–volume ratio of 100 standard volumes of gas per volume of liquid. The results were listed in Table 3 and depicted in Fig. 1. As shown in them,
143
Table 3 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + THF + H2 O systems with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid but different THF concentration in initial aqueous solution P (MPa) T (K) 2.5
z2 (mol%) THF (mol%)
278.15 79.56
4.0 6.0 8.0 10.0 12.0 14.0
x2 (mol%) y2 (mol%) K2 55.03 51.58 48.53 41.90 33.81 34.09
86.42 89.96 92.92 94.59 95.21 92.75
1.57 1.74 1.91 2.25 2.82 2.72
the partition coefficient of ethane increased until reaching a maximum at a THF content of 12 mol%, then decreased with the increasing of THF content in initial aqueous solution. It implies that the optimized concentration of THF in initial aqueous solution is 12 mol% or so. There is a very interesting phenomenon observed. One may find from Table 2 that the concentration of ethane decreased in hydrate phase while it increasing in vapor phase in lower temperature range when the concentration of ethane in feed is relatively high (56.2 and 92.91 mol%). This phenomenon is unusual compared with the usual case that the decreasing of one component in one phase will be accompanied with the decreasing of this component in another phase, or vice versa. It is not certain why this unusual phenomenon occurred and deserves additional study. We suppose it might be resulted from the formation of SI hydrate accompanying the formation of SII hydrate dominated by THF. Instead of being enriched in vapor phase when SII hydrate is formed with the presence of THF, ethane will be enriched in hydrate phase when SI hydrate is formed. The overall content of ethane in mixed hydrate phase depends on the ratio of two structures. As less SI hydrate would be formed at higher temperature, it was possible that the concentration of ethane in mixed hydrate phase decreased with the increasing of temperature even
Fig. 1. The changing of partition coefficients of ethane between vapor and hydrate with the increasing of THF concentration in initial aqueous solution at fixed temperature of 278.15 K, fixed pressure of 2.5 MPa and fixed initial gas–volume ratio of 100 standard volumes of gas per volume of liquid.
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4. Conclusions The gas–hydrate equilibria were measured firstly for CH4 + C2 H6 + water system and the results showed that the partition coefficient of ethane between vapor and hydrate phase was not promising. And then the gas–hydrate equilibria for CH4 + C2 H6 + tetrahydrofuran (THF) + water systems were studied in more detail. The results demonstrated that the occupancy of ethane in hydrate cages was inhibited extremely by adding THF to water-rich phase, and this inhibition could be enhanced by increasing the concentration of THF in aqueous solution. With the existence of THF in aqueous solution, the ethane was remarkably enriched in vapor phase instead of being slightly enriched in hydrate phase when without the existence of THF. These results might be of industrial significance. Fig. 2. Pseudo-x–y diagram of CH4 + C2 H6 + THF + H2 O system with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid and fixed THF concentration of 6.0 mol% in initial aqueous solution at 2.0 MPa.
it increased in vapor. However, this supposing needs further experimental evidence. For distinction, the data in unusual case were denoted by a superscript “*” in Table 2. The experimental data in usual case and at a fixed pressure of 2 MPa was chosen to produce the pseudo-x–y diagram, i.e., Fig. 2, which implies a very promising separation efficiency for methane + ethane mixtures by forming hydrate with the presence of THF in aqueous solution. From the above experimental study, it can be concluded that two measures can be taken to inhibit ethane forming hydrate and thus increase the separation efficiency for methane + ethane gas mixtures. The first one is to choose a relatively high reaction temperature; the most suitable range of reaction temperature seems to be 278–282 K. If the temperature is too high, the formation of hydrate becomes difficult even for THF. Second one is to increase the THF content in aqueous solution in certain range. The suitable concentration of THF in aqueous solution ranges from 6 to 12 mol% and depends on the feed composition. If the concentration of THF is too high, the dissolving of ethane in aqueous solution will become considerable and have an unfavorable influence on the enrichment of ethane in vapor phase. Initial gas–liquid volume ratio is also an important factor infecting the vapor–hydrate equilibrium behavior for CH4 (1) + C2 H6 (2) + THF + H2 O quaternary systems. However, this factor was specified to 100 in our experiments and its influence was not manifested. It is to be considered in our future work.
Acknowledgements The financial support received from the National Natural Science Foundation of China (Nos. 20176028 and 90210020) and Huo Yingdong Education Foundation (No. 81064) is gratefully acknowledged.
References [1] A.L. Ballard, E.D. Sloan Jr., Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, 19–23 May, 2002, pp. 1007–1011. [2] Y. Yamamoto, T. Komai, T. Kawamura, J.-H. Yoon, S.-P. Kang, S. Okita, Proceedings of the Fourth International Conference on Gas Hydrates, Yokohama, 19–23 May, 2002, pp. 428–432. [3] D.N. Glew, Liquid fractionation process using gas hydrate, US Patent 3 231 630 (1966). [4] D.F. Spencer, US Patent 6 106 595 (2000). [5] S.-P. Kang, H. Lee, C.-S. Lee, W.-M. Sung, Fluid Phase Equilibr. 185 (2001) 101–109. [6] S. Subramnian, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. (2000) 55. [7] S. Subramnian, A.L. Ballard, R.A. Kini, S.F. Dec, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5763–5771. [8] A.L. Ballard, E.D. Sloan Jr., Chem. Eng. Sci. 55 (2000) 5773– 5782. [9] D.-H. Mei, J. Liao, J.-T. Yang, T.-M. Guo, Ind. Eng. Chem. Res. 35 (1996) 4342–4347. [10] D.-H. Mei, J. Liao, J.-T. Yang, T.-M. Guo, J. Chem. Eng. Data 43 (1998) 178–182. [11] K. Moita, S. Nakano, K. Ohgaki, Fluid Phase Equilibr. 169 (2000) 167–175.
The partition coefficients of ethane between vapor and hydrate phase for methane + ethane + water and methane + ethane + THF + water systems L.-W. Zhang, G.-J. Chen∗ , X.-Q. Guo, C.-Y. Sun, L.-Y. Yang High Pressure Fluid Phase Behavior and Property Research Laboratory, University of Petroleum, Beijing 102249, PR China Received 18 June 2004; accepted 20 August 2004
Abstract The vapor–hydrate equilibria were measured firstly for CH4 + C2 H6 + water system beyond incipient conditions and the results showed that the partition coefficients of ethane between vapor and hydrate phase were not promising. And then the gas–hydrate equilibria were studied experimentally in detail for CH4 + C2 H6 + tetrahydrofuran (THF) + water systems in the temperature range of 274.15–282.15 K, pressure range of 1.0–3.0 MPa and THF concentration range of 4–14 mol% in initial aqueous solution. The results demonstrated that the partition coefficient of ethane between vapor and hydrate phase increased dramatically because of the presence of THF in water, ethane was remarkably enriched in vapor phase instead of being slightly enriched in hydrate phase for CH4 + C2 H6 + water system. © 2004 Elsevier B.V. All rights reserved. Keywords: Partition coefficient; Vapor; Hydrate; Methane; Ethane; Tetrahydrofuran
1. Introduction When a gas mixture forms hydrate partially, the relative concentration of each component in the hydrate phase and that in residual vapor phase might be different, the component that can form hydrate more easily might be enriched in hydrate phase. Based on this principle, it has been considered to be a possible way to separate gas mixture through forming hydrate. Hydrate technology is promising to the systems for which usual distillation method is not very feasible, such as low-boiling systems and close-boiling systems. There have been some work reported related to the separation technology based on hydrate principles [1–5]. Since methane and ethane are two major components of natural gases, hydrates formed from them have been given special concern. Sloan and coworkers [6–8] have published several papers on the hydrate formed from methane + ethane binary gas mixture with pure water. A series of hydrate phase diagrams have been presented by them.
As methane and ethane are low-boiling components, it is expensive to separate them by usual distillation method because of deep cooling. On the other hand, the separations of methane and ethane are usually occurred in natural gas or oil processing, ethylene producing, etc. Thus it is of practical significance to develop new methods for their separation. It might be a possible choice to separate them through forming hydrate above 0 ◦ C. With respect to hydrate technology, it is very important to increase the separation efficiency of single equilibrium stage as multi-stage operation like distillation is difficult to realize in practice because it takes much more time to reach a vapor–hydrate equilibrium than to reach a vaporliquid equilibrium. It will be demonstrated that the single stage separation efficiency of methane and ethane mixture can be increased remarkably by adding certain quantity of tetrahydrofuran (THF) to water-rich phase in this work. 2. Experimental 2.1. Apparatus
∗
Corresponding author. Tel.: +86 1089733252; fax: +86 1089733252. E-mail address: [email protected] (G.-J. Chen).
0378-3812/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.fluid.2004.08.032
The experimental apparatus used in this work had been described in detail in the previous papers by
142
L.-W. Zhang et al. / Fluid Phase Equilibria 225 (2004) 141–144
this laboratory [9,10]. A brief introduction is given here. The apparatus consists mainly of a cylindrical variablevolume high-pressure (0–25 MPa) transparent sapphire cell (2.54 cm in diameter, effective volume approximately 60 cm3 ) installed in an air-bath and equipped with a magnetic floating piston inside for accelerating the phase equilibrium process. The formation/dissociation of the hydrate crystals in the solution can be observed directly trough the transparent cell wall. The accuracies of temperature and pressure measurements are within ±0.2 K and ±0.025 MPa, respectively. 2.2. Materials and preparation of samples Analytical grade methane (99.9%) and ethane (99.95%) supplied by Beifen Gas Industry Corporation were used in preparing the synthetic binary gas mixtures. A HewlettPackard gas chromatograph (HP 6890) was used to analyze the composition of gas mixtures. The THF used for preparing the aqueous solution was supplied by Beijing Reagents Corporation. An electronic balance with a precision of ±0.1 mg was used in preparing aqueous solution with the required concentration of THF. 2.3. Experimental procedure Firstly, the sapphire cell was washed using distilled water, and then it was rinsed three times with water or the prepared aqueous solution. After the cell was thoroughly cleaned, a suitable quantity of liquid sample was added into the cell. The gas space of the cell was purged with the prepared gas sample four to five times to ensure the absence of air, and then the gas sample was charged into the cell until the given pressure achieved. Subsequently, the air-bath temperature was adjusted to the desired value. Once the cell temperature was kept constant, hydrate nucleation was then induced by agitation of the magnetic floating piston. After all liquid water or aqueous solution was converted to solid hydrate, the residual gas (vapor) was kept in touch with hydrate phase for over 8 h to ensure vapor–hydrate equilibrium. The temperature and pressure were maintained constant during the whole experimental process. When the vapor–hydrate equilibrium was established, the vapor phase was sampled and analyzed at least three times using the gas chromatograph. The average values were then taken as the compositions of vapor phase. The compositions of methane and ethane in hydrate phase were obtained by analyzing the compositions of released gas with the gas chromatograph after hydrate dissociated thoroughly.
Table 1 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + H2 O systems with fixed initial gas–liquid volume ratio of 200 standard volumes of gas per volume of liquid T (K)
P (MPa)
z2 (mol%)
x2 (mol%)
y2 (mol%)
K2
274.15
2.5 3.0 4.0
60.11 60.11 60.11
65.92 65.67 62.53
58.25 57.73 57.48
0.88 0.88 0.92
only the relative compositions of methane and ethane in two phases. z2 , x2 , y2 represent the concentration of ethane in feed gas (the prepared gaseous mixture sample), hydrate phase and vapor in such basis, respectively. K2 is the partition coefficient of ethane between vapor and hydrate phase, K2 = y2 /x2 3.1. CH4 (1) + C2 H6 (2) + H2 O system The vapor–hydrate equilibrium for ternary system of CH4 + C2 H6 + H2 O was firstly measured, where the initial gas–liquid volume ratios were uniformly set to 200 standard volumes of gas per volume of liquid (water). The obtained experimental data were listed in Table 1. Considering the hydrate formation pressure of ethane is much lower than that of methane at same temperature, one should expect a remarkable enrichment of ethane in hydrate phase. But, as shown in Table 1, the ethane was only slightly enriched by 3–5% in hydrate phase compared with feed composition. The partition coefficients of ethane between vapor and hydrate phase are also not promising. 3.2. CH4 (1) + C2 H6 (2) + THF + H2 O system
3. Results and discussion
Considering THF can form hydrate very easily, some researchers [5] used it as a hydrate promoter. Our purpose here is somewhat different. THF was expected to take the role of a selective inhibitor. In our reasoning, THF will dominate the structure of hydrate formed by the guest mixtures concerned in this work because THF can form SII hydrate very easily, especially when methane exists as a help gas. That is to say guests CH4 , C2 H6 and THF together will form SII hydrate. Furthermore, THF molecules again will dominate the occupancy of large cages of SII hydrate and inhibit the occupancy of ethane molecules in them. On the other hand, as ethane molecules cannot occupy small cages,1 the occupancy of small cages will be dominated by methane molecules. Then, as a result, the overall occupancy of ethane molecules in hydrate cages would be obviously lower than that of methane. In order to prove above reasoning, we performed a series of experiments on the vapor–hydrate equilibrium for CH4 + C2 H6 + THF + H2 O system. Firstly, the influences of temperature, pressure and feed composition on the partition coefficient, K2 , of ethane between vapor and hydrate phase were studied
Before presenting the experimental results, it should be noted that the compositions of vapor phase and hydrate phase given here are both in water and THF-free basis, i.e., they are
1 Moita et al. [11] have proved that ethane molecules can occupy small cage by Raman spectroscopy, but this probability is very small at middle or lower pressures and can be neglected.
L.-W. Zhang et al. / Fluid Phase Equilibria 225 (2004) 141–144 Table 2 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + THF + H2 O systems with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid and fixed THF concentration of 6 mol% in initial aqueous solution P (MPa)
T (K)
z2 (mol%)
x2 (mol%)
y2 (mol%)
K2
3.0
276.15 278.15 280.15 282.15 284.15
24.39
14.92 13.18 11.08 8.48 6.77
66.80 66.66 65.45 51.72 44.33
4.48 5.06 5.91 6.10 6.55
2.0
278.15 280.15 282.15 284.15
24.39
9.95 9.17 7.48 5.13
57.44 53.26 45.92 39.63
5.77 5.81 6.13 7.72
3.0
274.15 276.15 278.15 280.15 282.15
56.2
27.03a 26.77a 25.46a 23.44 19.55
72.61a 74.90a 79.71a 87.07 80.74
2.68 2.80 3.13 3.71 4.13
2.0
274.15 276.15 278.15 280.15 282.15
56.2
29.33a 28.89a 27.80 21.37 14.74
68.36a 73.73a 87.68 85.49 80.23
2.69 2.55 3.15 4.00 5.44
2.0
276.15 278.15
92.91
84.14a 73.33
95.19a 96.75
1.13 1.32
a
This datum is in unusual case.
while the concentration of THF in initial aqueous solution was fixed at 6 mol% and the initial gas–liquid volume ratio was specified to 100 standard volumes of gas per volume of liquid. The obtained experimental data were listed in Table 2. Comparing with the data in Table 1, it could be seen that the partition of ethane between vapor and hydrate phase was reversed due to the existence of THF in water. Instead of being slightly enriched in hydrate phase, ethane was enriched in vapor phase remarkably. The partition coefficient of ethane between vapor and hydrate is very fascinating, the biggest value of K2 measured was 7.72. In general, K2 increases with the increasing of temperature or decreasing of pressure. On the other hand, there exist optimized temperature and pressure values corresponding to the highest enrichment of ethane in vapor when mole fraction of ethane in feed gas is relatively high. Here, enrichment means the difference between the mol% of ethane in vapor and that in feed gas. For the feed gas with 56.2 mol% ethane, the biggest enrichment was observed to be 30.87%. The extent of enrichment of ethane is related to feed composition. For the feed gas containing 24.39 mol% ethane, the highest enrichment reached 42%. In order to study the influence of concentration of THF on the partition coefficients, the vapor–hydrate equilibrium data for CH4 + C2 H6 + THF + H2 O system was measured with respect to six different initial concentrations of THF in aqueous solution at fixed temperature of 278.15 K, fixed pressure of 2.5 MPa and fixed initial gas–volume ratio of 100 standard volumes of gas per volume of liquid. The results were listed in Table 3 and depicted in Fig. 1. As shown in them,
143
Table 3 Vapor–hydrate equilibria data for CH4 (1) + C2 H6 (2) + THF + H2 O systems with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid but different THF concentration in initial aqueous solution P (MPa) T (K) 2.5
z2 (mol%) THF (mol%)
278.15 79.56
4.0 6.0 8.0 10.0 12.0 14.0
x2 (mol%) y2 (mol%) K2 55.03 51.58 48.53 41.90 33.81 34.09
86.42 89.96 92.92 94.59 95.21 92.75
1.57 1.74 1.91 2.25 2.82 2.72
the partition coefficient of ethane increased until reaching a maximum at a THF content of 12 mol%, then decreased with the increasing of THF content in initial aqueous solution. It implies that the optimized concentration of THF in initial aqueous solution is 12 mol% or so. There is a very interesting phenomenon observed. One may find from Table 2 that the concentration of ethane decreased in hydrate phase while it increasing in vapor phase in lower temperature range when the concentration of ethane in feed is relatively high (56.2 and 92.91 mol%). This phenomenon is unusual compared with the usual case that the decreasing of one component in one phase will be accompanied with the decreasing of this component in another phase, or vice versa. It is not certain why this unusual phenomenon occurred and deserves additional study. We suppose it might be resulted from the formation of SI hydrate accompanying the formation of SII hydrate dominated by THF. Instead of being enriched in vapor phase when SII hydrate is formed with the presence of THF, ethane will be enriched in hydrate phase when SI hydrate is formed. The overall content of ethane in mixed hydrate phase depends on the ratio of two structures. As less SI hydrate would be formed at higher temperature, it was possible that the concentration of ethane in mixed hydrate phase decreased with the increasing of temperature even
Fig. 1. The changing of partition coefficients of ethane between vapor and hydrate with the increasing of THF concentration in initial aqueous solution at fixed temperature of 278.15 K, fixed pressure of 2.5 MPa and fixed initial gas–volume ratio of 100 standard volumes of gas per volume of liquid.
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4. Conclusions The gas–hydrate equilibria were measured firstly for CH4 + C2 H6 + water system and the results showed that the partition coefficient of ethane between vapor and hydrate phase was not promising. And then the gas–hydrate equilibria for CH4 + C2 H6 + tetrahydrofuran (THF) + water systems were studied in more detail. The results demonstrated that the occupancy of ethane in hydrate cages was inhibited extremely by adding THF to water-rich phase, and this inhibition could be enhanced by increasing the concentration of THF in aqueous solution. With the existence of THF in aqueous solution, the ethane was remarkably enriched in vapor phase instead of being slightly enriched in hydrate phase when without the existence of THF. These results might be of industrial significance. Fig. 2. Pseudo-x–y diagram of CH4 + C2 H6 + THF + H2 O system with fixed initial gas–liquid volume ratio of 100 standard volumes of gas per volume of liquid and fixed THF concentration of 6.0 mol% in initial aqueous solution at 2.0 MPa.
it increased in vapor. However, this supposing needs further experimental evidence. For distinction, the data in unusual case were denoted by a superscript “*” in Table 2. The experimental data in usual case and at a fixed pressure of 2 MPa was chosen to produce the pseudo-x–y diagram, i.e., Fig. 2, which implies a very promising separation efficiency for methane + ethane mixtures by forming hydrate with the presence of THF in aqueous solution. From the above experimental study, it can be concluded that two measures can be taken to inhibit ethane forming hydrate and thus increase the separation efficiency for methane + ethane gas mixtures. The first one is to choose a relatively high reaction temperature; the most suitable range of reaction temperature seems to be 278–282 K. If the temperature is too high, the formation of hydrate becomes difficult even for THF. Second one is to increase the THF content in aqueous solution in certain range. The suitable concentration of THF in aqueous solution ranges from 6 to 12 mol% and depends on the feed composition. If the concentration of THF is too high, the dissolving of ethane in aqueous solution will become considerable and have an unfavorable influence on the enrichment of ethane in vapor phase. Initial gas–liquid volume ratio is also an important factor infecting the vapor–hydrate equilibrium behavior for CH4 (1) + C2 H6 (2) + THF + H2 O quaternary systems. However, this factor was specified to 100 in our experiments and its influence was not manifested. It is to be considered in our future work.
Acknowledgements The financial support received from the National Natural Science Foundation of China (Nos. 20176028 and 90210020) and Huo Yingdong Education Foundation (No. 81064) is gratefully acknowledged.
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