- Email: [email protected]
Preparation of supported ionic liquid membranes (SILMs) for the removal of acidic gases from crude natural gas
Desalination 236 (2009) 342–348
Preparation of supported ionic liquid membranes (SILMs) for the removal of acidic gases from crude natural gas You-In Parka*, Beom-Sik Kima, Yong-Hoon Byuna, Sang-Hak Leeb, Eun-Woo Leea, Jung-Min Leea a
Environment & Energy Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejeon 305-606, Korea email: [email protected] b Hankook Jungsoo Industries, Co. Ltd, 400, Moknae, Ansan, Gyeonggi 425-100, Korea Received 30 June 2007; revised accepted 7 October 2007
Abstract Through a multi-phase separation process with the use of room temperature ionic liquid (RTILs) and polymer, new supported ionic liquid membranes (SILMs) were developed for the removal of acidic gases from crude natural gas. PVDF (poly vinylidene fluorolide) and BMImBF4 were used as polymer materials for membrane and RTILs respectively. The structure of SILMs was characterized with SEM. To investigate the permeation properties, the SILMs were tested with CO2, H2S and CH4 at various operating conditions. Since CO2 and H2S have higher affinity toward RTILs than CH4, the permeability coefficients of these two acidic gases were found to be considerably high at 30–180 and 160–1100 barrer, respectively. Moreover, the selectivity of CO2/CH4 and H2S/CH4 were found to be 25–45 and 130–260, respectively. Keywords: Supported ionic liquid membranes (SILMs); Room temperature ionic liquids (RTILs); Gas separation; Natural gas
1. Introduction The separation of acidic gases such as CO2 and H2S from crude natural gas using membrane has received considerable attention in recent years for its potential to become the most energy and
process efficient separation process. Membranes used for most natural gas separation were composed of glassy polymers such as cellulose acetate and polyimide because they possess higher membrane stability and performance than rubbery polymer [1,2]. Although cellulose acetate membranes have been used for natural gas
*Corresponding author. Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2007.10.085
Y.-I. Park et al. / Desalination 236 (2009) 342–348
separations on limited bases, they failed to become commercialized due to insufficient selectively and permeability for large scale gas separation process. The membrane material is required to have CO2/CH4 selectivity of 40 in order for membrane separation process to merit competitive advantage in practice over other alternate separation methods such as amine scrubbing. Many polyimide membranes with high gas permeability and stable mechanical properties were tested in vain as their low CO2 permeability and low CO2/CH4 selectivity made them unsuitable for the separation of natural gas in large scale operation [3,4]. In this study, we offer the breakthrough in search of a suitable membrane for separation of natural gas through the development of new supported ionic liquid membranes (SILMs) which offer increased selectivity and permeability over polymer membranes. The new SILMs, which consist of a solid polymer matrix (support) and room temperature ionic liquid (RTIL), improve upon the following disadvantages of conventional SILMs: loss of ionic liquid, narrow range of application temperature and excessive thickness. They were prepared by multi-phase separation process combined with the low temperature phase separation (LTPS) and the high temperature phase separation (HTPS) to control the domain size of RTIL in the polymer matrix. RTIL used in this study is 1-butyl-3-methylimidazolium tetrafluororate (BMImBF4), a salt with low melting point as well as superior affinity to acidic gases, and poly(vinylidene fluoride)(PVDF) was used as polymer matrix for its hydrophobicity as well as stable physical and chemical properties. The permeability of new SILMs for the following acidic gases were tested at various operating conditions: CO2, H2S, and CH4.
Inc.(Pierre-Benite, France) and Junsei Chemical Co. (Tokyo, Japan) respectively. BMImBF4, ionic liquid, was prepared in our laboratory and the chemical structure of BMImbF4 is shown in Fig. 1 [5]. N-methyl-2-pyrrolidone (NMP) and all organic solvent were an extra pure grade, and used without any further purification. 2.2. Membrane preparation SILMs were prepared by multi-phase separation process combined with the low temperature phase separation (LTPS) and the high temperature phase separation (HTPS) using PVDF as membrane matrix and RTIL. The casting solution was prepared by dissolving RTIL and PVDF in solvent at various ratios. The weight ratio of PVDF to solvent was kept constant at 1.5:8.5 throughout the experiment. Equal mixture of NMP and 1,4-Dioxane (1:1 ratios) was used as a solvent with a ratio of 5–5 for easier control of solvent evaporation rate. The polymer solution was stirred mildly at 80 C for at least 5 h. The casting solution was poured into a Petri dish and then dried in an evaporating chamber in which relative humidity was controlled at 50% for solvent evaporation step. LTPS was for 1 h at 30 C and then the cast film was placed in an oven at 100 C for 2 h to HTPS that proceed phase separation of cast film into two isolated phase, RTIL rich phase and PVDF rich phase, and quench the phase separation appropriate domain size. Finally, the membrane was kept in a vacuum oven at 50 C for 10 h to remove any residual solvent. The thickness of the resulting dry membranes were measured to be in the range of 30–50 C.
2. Experimental 2.1. Materials Poly(vinylidene fluorolide)(Kynar 500) and 1,4-Dioxane were purchased from Elf Atochem
343
N
N Bu
– BF4
Me
Fig. 1. The chemical structures of BMImBF4.
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2.3. Gas permeation apparatus
1200
The gas permeance of the membranes was measured with CO2, H2S and CH4 using GPA60 (SeparTek Co., Incheon, Korea). It allows for precise and rapid on-line measurements of both permeation transient and permeation composition during gas permeation. Permeability as well as diffusion and solubility coefficient were derived from the permeation transient curve. The description of gas permeation apparatus and the determination principle of respective parameters from the transient curve can be found in the literature [6].
1000 Permeability coefficient barrer
(A) 800 CH4
600
CO2 H2S
400 200 6 3 0 0.0
0.5
1.0
1.5
2.0
2.5
(B) 200
With a constant permeate pressure below 2 mbar, three types of feed gas (CO2, H2S and CH4) were permeated through the new SILMs using GPA-60 at feed pressure ranging from 2 to 5 bar and operating temperature ranging from 35 to 65 C.
Selectivity
2.4. Gas permeations 150
CO2/CH4 H2S/CH4
60
30
0 0.0
3. Result and discussion 3.1. Effects of RTIL content on permeation performance through new SILMs Fig. 2 shows the permeability coefficient of feed gases and the ideal selectivity of CO2/CH4 and H2S/CH4 through new SILMs with RTIL content in the membranes at 35 C. The weight ratio of the RTIL/PVDF (WL/WP) in the SILMs varied from 0.5 to 2.0. As the RTIL content in the membrane increased, the permeability coefficient of gases showed drastic increase, especially for acidic gases with an affinity to RTIL. The H2S/CH4 selectivity (with similar pattern of behavior shown for CO2/CH4 selectivity) also increased as a result of increasing RTIL content, peaking at WL/WP ¼ 1.5 before decreasing. This behavior can be explained by plasticization of RTIL molecules on the PVDF phase as membrane matrix. In other words, when the RTIL
0.5
1.0
1.5
2.0
2.5
Weight ratio of RTIL to PVDF in the membane, WL/WP
Fig. 2. Permeability coefficients (A) and selectivity of permeate gases (B) with weigh ratio of RTIL to PVDF in the membrane (WL/WP). Operating pressure: 3 bar, Operating temperature: 35 C.
content is higher, amount of RTIL that exists as molecular form in the membrane increases, because the degree of phase separation between RTIL and PVDF decreases, so that PVDF phase becomes more rubbery by RTIL molecules by the so-called plasticization effect. Consequently, the permeation resistance of permeate gases through PVDF phase, especially for CH4 that has more diffusion selective property toward permeate gases than solubility selective property, decreases resulting in the decrease of selectivity. Fig. 3 exhibits the diffusion and solubility coefficients of H2S, CO2 and CH4 with RTIL
Y.-I. Park et al. / Desalination 236 (2009) 342–348 100
Diffusion coefficient ( × 107), cm2/s
(A)
10
CH4 1
CO2 H2S
Solubility coefficient ( × 103),cm3/(cm3, cm Hg)
0.1 0.0
100
0.5
1.0
1.5
2.0
2.5
(B)
10
CH4
345
previous research, the kinetic diameters of CO2, H2S and CH4 are found to be 3.3, 3.6 and 3.8 C respectively [7]. However, the diffusion coefficient of H2S in SLMs is the highest among gases in the given range of WL/WP, as shown in Fig. 3. This result implies that the RTIL phase contributes more to the transfer of permeant than the PVDF phase does as proved by the permeability coefficients of gases toward WL/WP in Fig. 2. In other words, the higher diffusion coefficient of permeate gas is attributed to its higher affinity toward RTIL in the SLMs. Thus, the order of diffusion coefficients of permeate gases was in a good accordance with the order of their solubility coefficients as shown Fig. 3. Findings from this study suggest that the permeation behavior of acidic gases through new SLMs is dominated by sorption of acidic gases.
CO2 H2S
3.2. Effect of temperature on permeation performance
1
0.1 0.0
0.5
1.0
1.5
2.0
2.5
Wieght ratio of RTIL to PVDF in the membrane, WL/WP
Fig. 3. Diffusion (A) and solubility (B) coefficients of permeate gases with weigh ratio of RTIL to PVDF in the membrane (WL/WP). Operating pressure: 3 bar, Operating temperature: 35 C.
content in the SLMs at 35 C. The diffusion coefficients of all permeate gases increased with increasing RTIL content in the membrane. The diffusion coefficient of a permeant in a polymeric membrane is generally expressed as a function of both its molecular size and its affinity toward the membrane material. These two factors have opposite effects on the diffusion coefficient. Large molecules interact more with segments of the polymeric chains than small molecules, so that the passage of small molecules through polymeric chains is easier. According to
Fig. 4 shows the Arrhenius plot of permeability and ideal selectivity of permeate gases at 4 bar of the trans-membrane pressure difference against various temperatures. It is interesting to note that the permeability coefficient of all permeate gases increased while selectivity decreased with the increase in operating temperature. Generally, the volume of a polymeric membrane increases as the temperature increase and more gas molecules are able to permeate through the increased volume, causing an increase in permeability. This is especially true for the new SLMs in which temperature increase can cause increase in volume during the PVDF phase and result in higher permeability coefficient of all permeate gases at higher temperature. On the other hand, the permeability coefficient shows opposite correlation with temperature. The diffusion and solubility coefficient data clearly demonstrates the reason behind opposite correlation between permeability coefficients and the selectivity with temperature. Fig. 5 displays the relationship of
346
Y.-I. Park et al. / Desalination 236 (2009) 342–348 100
(A)
(A) Diffusion coefficient ( × 107), cm2/s
Permeability coefficient, barrer
1200 1000 800
CH4
600
CO2 H2S
400 200 9 6 3 0 2.9
3.0
3.1
3.2
Solubility coefficient ( × 103), cm3/(cm3, cm Hg)
(B) 200
Selectivity
150
CO2/CH4 H2S/CH4
40
20
0 2.9
3.0
3.1 3
3.2
3.3
1/T ( × 10 ), K
–1
CH4
1
CO2 H2S
0.1 2.9
3.3
250
10
3.0
3.1
3.2
3.3
(B) 100
10
CH4 CO2 H2S
1
0.1 2.9
3.0
3.1
3.2
3.3
1/T ( × 103), K–1
Fig. 4. Arrhenius plots of permeability coefficients (A) and selectivity of permeate gases. (B) Operating pressure: 4 bar, WL/WP ¼ 1.5.
Fig. 5. Arrhenius plots of diffusion (A) and solubility coefficients (B) of permeate gases. Operating pressure: 4 bar, WL/WP:1.5.
diffusion and solubility coefficient of every permeate gas with respect to the operating temperature. The diffusivity of every permeate gas increased as a result of increasing temperature due to creation of larger free volumes in the membrane by more vigorous motion of polymeric chains and enhanced membrane mobility. On the contrary, the solubility coefficient of acidic gases slightly decreased with increasing temperature due to declined interaction between acidic gases and RTIL at higher temperature. However, the solubility coefficient of CH4
showed an increase with increasing temperature because its low affinity to RTIL in the membrane is likely to make the solubility more dependent on the membrane structure rather than interaction between CH4 and membrane. Thus, more CH4 molecules could be accommodated by the membrane with more free volume generated by the temperature increase. Consequently, the reason for the decreasing selectivity with increasing temperature is that at high temperature solubility coefficient of CH4 increases while those of acidic gases decrease.
Y.-I. Park et al. / Desalination 236 (2009) 342–348
3.3. The stability of new SLMs under high operating pressure Despite of their excellent separation performance for gas separation, the conventional SILMs have been known to have a serious problem on the membrane stability. The SILMs impregnated in the micropores of the membrane is easy to be evaporated or to be leaked from the micropores under severe operating conditions. Successful implementation of SILMs into practice would require the new SILMs to display mechanical stability while maintaining superior separation performance. Fig. 6 exhibits the
347
relationship of the permeability and selectivity of permeate gases through new SILMs and operating pressure. The permeability coefficient of H2S increased while that of CH4 decreased with increasing operating pressure. The opposite behavior of two gases can be explained by the difference in affinity toward the membrane. That is, H2S has high affinity toward the membrane while CH4 has low affinity. Therefore, the selectivity of CO2/CH4, H2S/CH4 had increased with increasing pressure. The result of this study confirmed the mechanical stability of new SILMs at high operating pressure.
1200
Permeability coefficient, barrer
1000
(A)
4. Conclusions
800 600
CH4
400
H2S
CO2
200 9 6 3 0
300
2
3
4
5
(B)
Selectivity
250 200 CO2/CH4 150
H2S/CH4
100 50 0
2
3 4 Operating pressure, bar
5
Fig. 6. Permeability coefficients (A) and selectivity of permeate gases (B) with operating pressure. Operating temperature: 35 C, WL/WP:1.5.
Using room temperature ionic liquid(RTILs) and polymer, the new SILMs for separation of acidic gases from crude natural gas were prepared through a multi-stage phase separation process combined with the low temperature phase separation(LTPS) and the high temperature phase separation(HTPS). PVDF was used as a polymer for the membrane and BMImBF4 was used for RTILs. CO2, H2S, and CH4 were used to permeate through the new SILMs at various operating conditions. The permeability coefficient of gases increased dramatically with increasing the RTIL content in the membrane, especially for acidic gases with an affinity to RTIL. However, when the RTIL content in the membrane was higher, the selectivity declined as a result of the plasticization of RTIL molecules on the PVDF phase as membrane matrix. As the temperature increased, the permeability coefficient of permeate gases increased, while the selectivity decreased due to increase of membrane mobility. Furthermore, the new SILMs demonstrated high permeation performance to acidic gases while maintaining mechanical stability at high operating pressure.
348
Y.-I. Park et al. / Desalination 236 (2009) 342–348
References [1] R.W. Baker, Future directions of membrane gas separation technology, Ind. Eng. Chem. Res., 41 (2002) 1393–1411. [2] A.Y. Houde, B. Krishnakumar, S.G. Charati and S.A. Stern, Permeability of dense (homogeneous) cellulose acetate membranes to methane, carbon dioxide, and their mixtures at elevated pressures, J. Appl. Polym. Sci., 62 (1996) 2181–2192. [3] A-J.Mohammed and W.J. koros, performance of natural gas membranes in the presence of heavy hydrocarbons, J. Membr. Sci., 274 (2006) 227–243. [4] L.C-G Martha, P-A. Joaquin and J.M. Dominguez, On the limits of gas separation in CO2/CH4, N2/CH4
and CO2/N2 binary mixtures using polyimide membranes, J. Membr. Sci., 293 (2007) 53–65. [5] Y.H. Byun, B.S. Kim, Y.I. Park, S.H. Lee, J.M. Lee and K.K. Koo, Behaviors of ionic conductivity with temperature for high-temperature PEMFC containing room temperature ionic liquids under non-humidified condition, Membr. J., 16 (4) (2006) 268–275. [6] C.K. Yeom, B.S. Kim and J.M. Lee, Precise on-line measurements of permeation transients through dense polymeric membranes using a new permeation apparatus, J. Membr. Sci., 161 (1999) 55. [7] D.W. Breck, Zeolite molecular sieves, structure, chemistry, and use, John Wiley & Sons, 1974.
Preparation of supported ionic liquid membranes (SILMs) for the removal of acidic gases from crude natural gas You-In Parka*, Beom-Sik Kima, Yong-Hoon Byuna, Sang-Hak Leeb, Eun-Woo Leea, Jung-Min Leea a
Environment & Energy Research Center, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejeon 305-606, Korea email: [email protected] b Hankook Jungsoo Industries, Co. Ltd, 400, Moknae, Ansan, Gyeonggi 425-100, Korea Received 30 June 2007; revised accepted 7 October 2007
Abstract Through a multi-phase separation process with the use of room temperature ionic liquid (RTILs) and polymer, new supported ionic liquid membranes (SILMs) were developed for the removal of acidic gases from crude natural gas. PVDF (poly vinylidene fluorolide) and BMImBF4 were used as polymer materials for membrane and RTILs respectively. The structure of SILMs was characterized with SEM. To investigate the permeation properties, the SILMs were tested with CO2, H2S and CH4 at various operating conditions. Since CO2 and H2S have higher affinity toward RTILs than CH4, the permeability coefficients of these two acidic gases were found to be considerably high at 30–180 and 160–1100 barrer, respectively. Moreover, the selectivity of CO2/CH4 and H2S/CH4 were found to be 25–45 and 130–260, respectively. Keywords: Supported ionic liquid membranes (SILMs); Room temperature ionic liquids (RTILs); Gas separation; Natural gas
1. Introduction The separation of acidic gases such as CO2 and H2S from crude natural gas using membrane has received considerable attention in recent years for its potential to become the most energy and
process efficient separation process. Membranes used for most natural gas separation were composed of glassy polymers such as cellulose acetate and polyimide because they possess higher membrane stability and performance than rubbery polymer [1,2]. Although cellulose acetate membranes have been used for natural gas
*Corresponding author. Presented at the International Membrane Science and Technology Conference, IMSTEC 07, 5–9 November 2007, Sydney, Australia 0011-9164/09/$– See front matter # 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2007.10.085
Y.-I. Park et al. / Desalination 236 (2009) 342–348
separations on limited bases, they failed to become commercialized due to insufficient selectively and permeability for large scale gas separation process. The membrane material is required to have CO2/CH4 selectivity of 40 in order for membrane separation process to merit competitive advantage in practice over other alternate separation methods such as amine scrubbing. Many polyimide membranes with high gas permeability and stable mechanical properties were tested in vain as their low CO2 permeability and low CO2/CH4 selectivity made them unsuitable for the separation of natural gas in large scale operation [3,4]. In this study, we offer the breakthrough in search of a suitable membrane for separation of natural gas through the development of new supported ionic liquid membranes (SILMs) which offer increased selectivity and permeability over polymer membranes. The new SILMs, which consist of a solid polymer matrix (support) and room temperature ionic liquid (RTIL), improve upon the following disadvantages of conventional SILMs: loss of ionic liquid, narrow range of application temperature and excessive thickness. They were prepared by multi-phase separation process combined with the low temperature phase separation (LTPS) and the high temperature phase separation (HTPS) to control the domain size of RTIL in the polymer matrix. RTIL used in this study is 1-butyl-3-methylimidazolium tetrafluororate (BMImBF4), a salt with low melting point as well as superior affinity to acidic gases, and poly(vinylidene fluoride)(PVDF) was used as polymer matrix for its hydrophobicity as well as stable physical and chemical properties. The permeability of new SILMs for the following acidic gases were tested at various operating conditions: CO2, H2S, and CH4.
Inc.(Pierre-Benite, France) and Junsei Chemical Co. (Tokyo, Japan) respectively. BMImBF4, ionic liquid, was prepared in our laboratory and the chemical structure of BMImbF4 is shown in Fig. 1 [5]. N-methyl-2-pyrrolidone (NMP) and all organic solvent were an extra pure grade, and used without any further purification. 2.2. Membrane preparation SILMs were prepared by multi-phase separation process combined with the low temperature phase separation (LTPS) and the high temperature phase separation (HTPS) using PVDF as membrane matrix and RTIL. The casting solution was prepared by dissolving RTIL and PVDF in solvent at various ratios. The weight ratio of PVDF to solvent was kept constant at 1.5:8.5 throughout the experiment. Equal mixture of NMP and 1,4-Dioxane (1:1 ratios) was used as a solvent with a ratio of 5–5 for easier control of solvent evaporation rate. The polymer solution was stirred mildly at 80 C for at least 5 h. The casting solution was poured into a Petri dish and then dried in an evaporating chamber in which relative humidity was controlled at 50% for solvent evaporation step. LTPS was for 1 h at 30 C and then the cast film was placed in an oven at 100 C for 2 h to HTPS that proceed phase separation of cast film into two isolated phase, RTIL rich phase and PVDF rich phase, and quench the phase separation appropriate domain size. Finally, the membrane was kept in a vacuum oven at 50 C for 10 h to remove any residual solvent. The thickness of the resulting dry membranes were measured to be in the range of 30–50 C.
2. Experimental 2.1. Materials Poly(vinylidene fluorolide)(Kynar 500) and 1,4-Dioxane were purchased from Elf Atochem
343
N
N Bu
– BF4
Me
Fig. 1. The chemical structures of BMImBF4.
344
Y.-I. Park et al. / Desalination 236 (2009) 342–348
2.3. Gas permeation apparatus
1200
The gas permeance of the membranes was measured with CO2, H2S and CH4 using GPA60 (SeparTek Co., Incheon, Korea). It allows for precise and rapid on-line measurements of both permeation transient and permeation composition during gas permeation. Permeability as well as diffusion and solubility coefficient were derived from the permeation transient curve. The description of gas permeation apparatus and the determination principle of respective parameters from the transient curve can be found in the literature [6].
1000 Permeability coefficient barrer
(A) 800 CH4
600
CO2 H2S
400 200 6 3 0 0.0
0.5
1.0
1.5
2.0
2.5
(B) 200
With a constant permeate pressure below 2 mbar, three types of feed gas (CO2, H2S and CH4) were permeated through the new SILMs using GPA-60 at feed pressure ranging from 2 to 5 bar and operating temperature ranging from 35 to 65 C.
Selectivity
2.4. Gas permeations 150
CO2/CH4 H2S/CH4
60
30
0 0.0
3. Result and discussion 3.1. Effects of RTIL content on permeation performance through new SILMs Fig. 2 shows the permeability coefficient of feed gases and the ideal selectivity of CO2/CH4 and H2S/CH4 through new SILMs with RTIL content in the membranes at 35 C. The weight ratio of the RTIL/PVDF (WL/WP) in the SILMs varied from 0.5 to 2.0. As the RTIL content in the membrane increased, the permeability coefficient of gases showed drastic increase, especially for acidic gases with an affinity to RTIL. The H2S/CH4 selectivity (with similar pattern of behavior shown for CO2/CH4 selectivity) also increased as a result of increasing RTIL content, peaking at WL/WP ¼ 1.5 before decreasing. This behavior can be explained by plasticization of RTIL molecules on the PVDF phase as membrane matrix. In other words, when the RTIL
0.5
1.0
1.5
2.0
2.5
Weight ratio of RTIL to PVDF in the membane, WL/WP
Fig. 2. Permeability coefficients (A) and selectivity of permeate gases (B) with weigh ratio of RTIL to PVDF in the membrane (WL/WP). Operating pressure: 3 bar, Operating temperature: 35 C.
content is higher, amount of RTIL that exists as molecular form in the membrane increases, because the degree of phase separation between RTIL and PVDF decreases, so that PVDF phase becomes more rubbery by RTIL molecules by the so-called plasticization effect. Consequently, the permeation resistance of permeate gases through PVDF phase, especially for CH4 that has more diffusion selective property toward permeate gases than solubility selective property, decreases resulting in the decrease of selectivity. Fig. 3 exhibits the diffusion and solubility coefficients of H2S, CO2 and CH4 with RTIL
Y.-I. Park et al. / Desalination 236 (2009) 342–348 100
Diffusion coefficient ( × 107), cm2/s
(A)
10
CH4 1
CO2 H2S
Solubility coefficient ( × 103),cm3/(cm3, cm Hg)
0.1 0.0
100
0.5
1.0
1.5
2.0
2.5
(B)
10
CH4
345
previous research, the kinetic diameters of CO2, H2S and CH4 are found to be 3.3, 3.6 and 3.8 C respectively [7]. However, the diffusion coefficient of H2S in SLMs is the highest among gases in the given range of WL/WP, as shown in Fig. 3. This result implies that the RTIL phase contributes more to the transfer of permeant than the PVDF phase does as proved by the permeability coefficients of gases toward WL/WP in Fig. 2. In other words, the higher diffusion coefficient of permeate gas is attributed to its higher affinity toward RTIL in the SLMs. Thus, the order of diffusion coefficients of permeate gases was in a good accordance with the order of their solubility coefficients as shown Fig. 3. Findings from this study suggest that the permeation behavior of acidic gases through new SLMs is dominated by sorption of acidic gases.
CO2 H2S
3.2. Effect of temperature on permeation performance
1
0.1 0.0
0.5
1.0
1.5
2.0
2.5
Wieght ratio of RTIL to PVDF in the membrane, WL/WP
Fig. 3. Diffusion (A) and solubility (B) coefficients of permeate gases with weigh ratio of RTIL to PVDF in the membrane (WL/WP). Operating pressure: 3 bar, Operating temperature: 35 C.
content in the SLMs at 35 C. The diffusion coefficients of all permeate gases increased with increasing RTIL content in the membrane. The diffusion coefficient of a permeant in a polymeric membrane is generally expressed as a function of both its molecular size and its affinity toward the membrane material. These two factors have opposite effects on the diffusion coefficient. Large molecules interact more with segments of the polymeric chains than small molecules, so that the passage of small molecules through polymeric chains is easier. According to
Fig. 4 shows the Arrhenius plot of permeability and ideal selectivity of permeate gases at 4 bar of the trans-membrane pressure difference against various temperatures. It is interesting to note that the permeability coefficient of all permeate gases increased while selectivity decreased with the increase in operating temperature. Generally, the volume of a polymeric membrane increases as the temperature increase and more gas molecules are able to permeate through the increased volume, causing an increase in permeability. This is especially true for the new SLMs in which temperature increase can cause increase in volume during the PVDF phase and result in higher permeability coefficient of all permeate gases at higher temperature. On the other hand, the permeability coefficient shows opposite correlation with temperature. The diffusion and solubility coefficient data clearly demonstrates the reason behind opposite correlation between permeability coefficients and the selectivity with temperature. Fig. 5 displays the relationship of
346
Y.-I. Park et al. / Desalination 236 (2009) 342–348 100
(A)
(A) Diffusion coefficient ( × 107), cm2/s
Permeability coefficient, barrer
1200 1000 800
CH4
600
CO2 H2S
400 200 9 6 3 0 2.9
3.0
3.1
3.2
Solubility coefficient ( × 103), cm3/(cm3, cm Hg)
(B) 200
Selectivity
150
CO2/CH4 H2S/CH4
40
20
0 2.9
3.0
3.1 3
3.2
3.3
1/T ( × 10 ), K
–1
CH4
1
CO2 H2S
0.1 2.9
3.3
250
10
3.0
3.1
3.2
3.3
(B) 100
10
CH4 CO2 H2S
1
0.1 2.9
3.0
3.1
3.2
3.3
1/T ( × 103), K–1
Fig. 4. Arrhenius plots of permeability coefficients (A) and selectivity of permeate gases. (B) Operating pressure: 4 bar, WL/WP ¼ 1.5.
Fig. 5. Arrhenius plots of diffusion (A) and solubility coefficients (B) of permeate gases. Operating pressure: 4 bar, WL/WP:1.5.
diffusion and solubility coefficient of every permeate gas with respect to the operating temperature. The diffusivity of every permeate gas increased as a result of increasing temperature due to creation of larger free volumes in the membrane by more vigorous motion of polymeric chains and enhanced membrane mobility. On the contrary, the solubility coefficient of acidic gases slightly decreased with increasing temperature due to declined interaction between acidic gases and RTIL at higher temperature. However, the solubility coefficient of CH4
showed an increase with increasing temperature because its low affinity to RTIL in the membrane is likely to make the solubility more dependent on the membrane structure rather than interaction between CH4 and membrane. Thus, more CH4 molecules could be accommodated by the membrane with more free volume generated by the temperature increase. Consequently, the reason for the decreasing selectivity with increasing temperature is that at high temperature solubility coefficient of CH4 increases while those of acidic gases decrease.
Y.-I. Park et al. / Desalination 236 (2009) 342–348
3.3. The stability of new SLMs under high operating pressure Despite of their excellent separation performance for gas separation, the conventional SILMs have been known to have a serious problem on the membrane stability. The SILMs impregnated in the micropores of the membrane is easy to be evaporated or to be leaked from the micropores under severe operating conditions. Successful implementation of SILMs into practice would require the new SILMs to display mechanical stability while maintaining superior separation performance. Fig. 6 exhibits the
347
relationship of the permeability and selectivity of permeate gases through new SILMs and operating pressure. The permeability coefficient of H2S increased while that of CH4 decreased with increasing operating pressure. The opposite behavior of two gases can be explained by the difference in affinity toward the membrane. That is, H2S has high affinity toward the membrane while CH4 has low affinity. Therefore, the selectivity of CO2/CH4, H2S/CH4 had increased with increasing pressure. The result of this study confirmed the mechanical stability of new SILMs at high operating pressure.
1200
Permeability coefficient, barrer
1000
(A)
4. Conclusions
800 600
CH4
400
H2S
CO2
200 9 6 3 0
300
2
3
4
5
(B)
Selectivity
250 200 CO2/CH4 150
H2S/CH4
100 50 0
2
3 4 Operating pressure, bar
5
Fig. 6. Permeability coefficients (A) and selectivity of permeate gases (B) with operating pressure. Operating temperature: 35 C, WL/WP:1.5.
Using room temperature ionic liquid(RTILs) and polymer, the new SILMs for separation of acidic gases from crude natural gas were prepared through a multi-stage phase separation process combined with the low temperature phase separation(LTPS) and the high temperature phase separation(HTPS). PVDF was used as a polymer for the membrane and BMImBF4 was used for RTILs. CO2, H2S, and CH4 were used to permeate through the new SILMs at various operating conditions. The permeability coefficient of gases increased dramatically with increasing the RTIL content in the membrane, especially for acidic gases with an affinity to RTIL. However, when the RTIL content in the membrane was higher, the selectivity declined as a result of the plasticization of RTIL molecules on the PVDF phase as membrane matrix. As the temperature increased, the permeability coefficient of permeate gases increased, while the selectivity decreased due to increase of membrane mobility. Furthermore, the new SILMs demonstrated high permeation performance to acidic gases while maintaining mechanical stability at high operating pressure.
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