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thiophene mixtures by pervaporation
Journal of Membrane Science 269 (2006) 94–100
Removal of thiophenes from n-octane/thiophene mixtures by pervaporation Rongbin Qi, Changwei Zhao, Jiding Li ∗ , Yujun Wang, Shenlin Zhu State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 13 April 2005; received in revised form 15 June 2005; accepted 19 June 2005 Available online 20 July 2005
Abstract PDMS/PAN composite membrane was prepared and employed in pervaporation separation of n-octane/thiophene mixtures at laboratory scale. At different initial thiophenes (thiophene and 2-methyl-thiophene) concentrations (corresponding sulfur contents about 500–2500 g/g) and feed temperatures (30–70 ◦ C), experimental results were carried out and average flux, selectivity and activation energy of permeation were calculated. It was found that increasing in temperature yields higher total fluxes and lower selectivity to thiophenes, while changing concentration of thiophens in the range concerned produces insignificant effect on total flux and selectivity. Experiments also showed that PDMS membrane is more selective to thiophene than to 2-methyl-thiophene and it may be mainly because of the difference in molecular size and structure. Total fluxes for thiophene/n-octane mixture and 2-methyl-thiophene/n-octane mixture were measured to be 1.5 and 1.4 kg/m2 h at 30 ◦ C, with the corresponding selectivity to thiophenes 4.9 and 2.5, respectively. Based on simplified solution–diffusion model, the permeation fluxes of individual components in pervaporation of thiophene/2-methyl-thiophene/n-octane ternary mixture had been predicted and showed to be in good agreement with experimental values. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Desulfurization of gasoline; n-Octane/thiophene mixtures; PDMS/PAN membrane
1. Introduction Organosulfur impurities contained in gasoline lead to severe environmental problems. Sulfur present in gasoline results in SOx air pollution and is directly responsible for increased levels of NOx in automotive exhaust, as well as a poison for many of the catalytic converters being introduced to automobiles. The worldwide concerns over environment have promoted an increasing interest both in academic and industry for deep desulfurization of gasoline. Traditionally, hydrotreating process is the most effective technology used for removal of organosulfurs present in gasoline. However, this technology suffers from the significant loss of octane number caused by the accompanying saturation of olefins and the high investment and operating costs. For the ultra low content of sulfur (generally less than 2000 g/g) presenting ∗
Corresponding author. Tel.: +86 10 62782432; fax: +86 10 62770304. E-mail address: [email protected] (J. Li).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.06.022
in gasoline, traditional distillation process alone can hardly meet the pending clean-fuel specifications. Novel and capital-avoiding methodologies are rapidly developed on either bench scale or pilot scale, such as adsorption process [1,2], extraction process [3]. In recent years, pervaporation has been widely studied for the potential feasibility of separating close boiling, azeotropic, isomeric and ultra composition–asymmetric mixtures. Compared to traditional separation processes such as distillation, pervaporation offers many advantages, including high separation efficiency, low energy consumption and simple operation [4,5]. That is so because, according to solution–diffusion model, the separation mechanism in pervaporation is based on the difference in sorption and diffusion properties of the feed compounds, instead of on the difference in relative volatilities, which is the case in distillation. Currently, commercial applications of pervaporation fall into two groups: one is dehydration of alcohols and other solvents and the other is the removal of trace volatile organic
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
substances from water [6,7]. Recently, more and more efforts have been made to separate organic/organic mixtures by pervaporation [8,9]. Pervaporation has been proved a viable technology for the removal of sulfur impurities out of gasoline by different authors [10,11]. However, available information about their work in details is rather limited. PDMS has been extensively investigated for the separation of various mixtures, and attempts have been made to modify PDMS membrane in different ways to enhance the pervaporation performance [12,13]. Semi-organic bonds in PDMS provide highly flexible backbone with large bond angle, long bond lengths and extreme freedom of rotation. This allows for the helix polymeric structure consisting of an inorganic Si O Si backbone with a pendant methyl group. The rapid chain segment motion in the silicone rubber leads to an increasing free volume available for the diffusion of the permeate molecules. In this study, the PDMS composite membrane was prepared and employed to simulate desulfurization process of gasoline. Real gasoline is a rather complex mixture composed of alkanes, olefins, cycloparrafins and aromatics ranging from C5 to C14 . Typical sulfur compounds in gasoline include mercaptans, thiophenes and the ramifications thereof. In this work, n-octane was selected to stand for gasoline, while thiophene and 2-methyl-thiophene were chosen as the representative organosulfurs.
2. Theory According to solution–diffusion model [14], transport of molecule i through dense membrane can be expressed as: Di cim pdi Ji = (1) 1− e δ pi where Ji is the flux of component i, Di the diffusion coefficient, cim the mass concentration of component i inside the membrane, δ the thickness of the membrane and pdi , pei the partial downstream pressure and the equilibrium vapor pressure of component i at the liquid–membrane interface, respectively. Provided that the concentration polarization at the feed side is negligible due to high circulation velocity and sufficiently low flux and the chemical potential of the feed and permeate fluids are in equilibrium with the adjacent membrane surface, the following equation could be applied: cm Ki = iF ci
(2)
with Ki referring the membrane/liquid phase sorption coefficient, ciF the concentration of component i in liquid bulk. Combining Eq. (1) and (2) gives the following expression: Pi ciF Di Ki ciF pdi pdi (3) Ji = 1− e = 1− e δ pi δ pi
95
where Pi is the permeability of component i, defined as: Pi = Di Ki
(4) pd
It is noticeable that the term pie in Eq. (3) is negligible due i to the sufficient vacuum on the downstream side. Hence Eq. (3) can be simplified to: Ji =
Pi ciF δ
(5)
Generally Pi is concentration and temperature dependent. The variation of permeability with temperature is described by the following Arrhenius relationship: Pi = Pi0 e−Epi /RT
(6)
where Pi0 is the pre-exponential factor, Epi the activation energy of permeation for component i, T the feed absolute temperature and R the gas constant.
3. Experimental 3.1. Materials In order to simplify the experiment and to simulate the actual desulfurization process simultaneously, n-octane (China Medicine (Group) Shanghai Chemical Reagent Corporation), thiophene (Tianjin Chemical Company, China) and/or 2-methyl-thiophene (98% pure, Acros) were chosen as the representative components to form the model gasoline. The major properties of n-octane, thiophene and 2-methylthiophene are listed in Table 1. PDMS (viscosity 10 Pa s), ethyl orthosilicate, dibutyltin dilaurate, n-heptane (Beijing Chemical Company, China) were purchased for the preparation of PDMS membrane. Asymmetric microporous PAN membrane (provided by Beijing Megavision Membrane & Engineering Co. Ltd., China) was used as supports. 3.2. Membrane preparation PDMS, cross-linking agent ethyl orthosilicate and catalyst dibutyltin dilaurate were dissolved in n-heptane at ambient temperature. After degassed under vacuum, the solution is Table 1 Representative properties of n-octane, thiophene and 2-methyl-thiophene Compound
n-Octane
Structural formula
CH3 (CH2 )6 CH3
fp (◦ C) mp (◦ C) bp (◦ C) Density at 20 ◦ C (kg/m3 ) Refractive index, n20 D
Thiophene
2-Methyl-thiophene
15.6 −56.795 125.665 702.52
−9 −38.30 84.16 1064.8
7 −63.4 112.6 1019
1.39743
1.5289
1.5200
96
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
Fig. 1. Scheme of the pervaporation apparatus.
cast onto the PAN membrane. The membrane was first vulcanized under room temperature to evaporate the solvent, and then introduced into a vacuum oven to complete crosslinking. Controlling the solution concentration or the coating amount could produce membranes with variable top layer thickness. The thickness of the top skin layer could be determined by means of SEM photographs. All experiments in this study were performed with the same membrane. 3.3. Pervaporation experiments A schematic layout of the experimental set-up used in this study is depicted in Fig. 1. The feed was heated and circulated from the feed tank (volume of 2.5 × 10−3 m3 ) through the upstream side of the membrane cell by a pump with adjustable function of flow rate. In order to minimize the effect of concentration polarization on pervaporation performance, high feed flow rate up to 20 l/h and annular feed chamber (see Fig. 2) were employed in the experiments. A membrane supported by a porous sintered stainless steel in the permeate side was mounted in the pervaporation cell. The feed solution was maintained at temperatures between 30 and 70 ◦ C using a thermostat. The effective area of the membrane was 2.83 × 10−3 m2 . Vacuum on the permeate side was maintained below 500 Pa and was monitored with a digital vacuometer. Two cold traps were set in parallel allowing the collection of permeate without rupture of the vacuum. The permeation rate was determined by measuring the weight of permeate collected in the cold trap and divided by time and
the membrane’s surface area as shown in Eq. (7): J=
m At
(7)
with m denoting weight of the permeate passing through the active membrane area A during the time t. The compositions of the feed solution and permeate were analyzed by gas chromatography (HP6890, USA). Then the selectivity of a membrane in a binary system is obtained as follows: −1 ωiF ωiP (8) α= ωjP ωjF where ωP and ωF refer to the weight fraction of component i (thiophene or 2-methyl-thiophene) or j (n-octane) in the permeate and in the feed, respectively.
Fig. 2. The annular channel in the feed chamber.
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
97
Fig. 3. SEM photographs of PDMS/PAN membrane: (A) top surface; (B) cross-section; and (C) determination of the thickness of the top layer.
4. Results and discussion 4.1. SEM photographs of PDMS membrane The morphology of the PDMS/PAN membrane is shown in Fig. 3. As demonstrated in the SEM photographs, the originally porous surface of the PAN substrate was covered by a flat featureless PDMS layer. The top PDMS layer, functioning as the basis of permselectivity, had a nonporous and tight structure. The thickness of the PDMS layer was determined to be 1.1 × 10−5 m from the SEM photograph of cross-section of the membrane. 4.2. Effect of feed temperature Temperature dependence of flux and selectivity for thiophene/n-octane and 2-methyl-thiophene/n-octane mixtures is illustrated in Figs. 4 and 5, respectively. As expected, all the experimental evidences confirm an increase in
Fig. 4. Temperature dependence of flux and selectivity for noctane/thiophene mixture. Thiophene content in feed: 0.364 wt.%.
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R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100 Table 2 Activation energies and permeability correlations of individual components
Fig. 5. Temperature dependence of flux and selectivity for n-octane/2methyl-thiophene mixture. 2-Methyl-thiophene content in feed: 0.463 wt.%.
Compound
P0i (m2 /h)
Activation energy, EPi (kJ/mol)
Permeability correlation
Thiophene 2-Methyl-thiophene n-Octane
2.11 × 10−5 3.60 × 10−5 3.30 × 10−5
13.15 16.43 18.52
lg Pi =−4.69–686.92/T lg Pi =−4.46–858.47/T lg Pi =−4.49–967.32/T
summarized in Table 2. Activation energies of permeation for the three components in the PDMS membrane decrease in the following order: n-octane > 2-methyl-thiophene > thiophene. The permeability correlations for the three components were then obtained and applied to predict permeate fluxes for individual components in the pervaporation of ternary mixture. 4.3. Effect of feed composition
permeability and decrease in selectivity of membranes with increasing temperature. Flux profiles with feed temperature follow an Arrhenius type relation of the type showing in Eqs. (5) and (6), that is, the permeability of PV membranes decrease exponentially with reciprocal of feed temperature. The increase in total flux with temperature is due to the increase of the mobility of individual permeating molecules caused both by the temperature and by the enhanced mobility of the polymer segments. On the other hand, the increase in the degree of swelling of the membrane with temperature results in more n-octane transport, which leads to the decrease of selectivity to thiophenes. Permeability of thiophene, 2-methyl-thiophene and n-octane can be obtained from the above experimental data by using Eq. (5). Dependence of permeability of the three components on temperature is portrayed in Fig. 6. The permeability of PV membranes decreases exponentially with reciprocal of temperature following an Arrhenius type relation, Eq. (6). By curve fitting of the experimental data, the pre-exponential factor and the activation energy of permeation Ep for the three components are calculated and
Fig. 7. Effect of feed composition on flux and selectivity for noctane/thiophene mixture. Feed temperature: 60 ◦ C.
Fig. 6. Permeability profiles with feed temperature.
Fig. 8. Effect of feed composition on flux and selectivity for n-octane/2methyl-thiophene mixture. Feed temperature: 60 ◦ C.
Impact of feed composition on pervaporation performance was depicted in Fig. 7 and Fig. 8. The corresponding sul-
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
Fig. 9. Dependence of partial fluxes on feed compositions. Feed temperature: 60 ◦ C.
fur contents in the feed under consideration were in the vicinity from 500 to 2500 g/g. It can be found that variation of feed composition had nearly negligible influence on total fluxes and selectivity, while the partial fluxes of thiophenes are proportional to the concentrations of thiophenes in the feed, which is illustrated in Fig. 9. This situation may be attributed to the ultra low feed concentration and the narrow range thereof. According to the solution–diffusion theory, the separation performance of this process is dominated by the sorption and diffusion characteristics of the individual components. The feed processed in this work is an ultra composition–asymmetric mixture, that is, the thiophene content was very low and fell into a relatively narrow range. On one hand, the change in thiophene concentration in feed yields very limited impact on the transport of n-octane through the membrane. On the other hand, the partial fluxes of thiophenes are still proportional to the concentrations of thiophenes in the feed. The former results in nearly unchanged total fluxes, while the latter leads to nearly unchanged selectivities to thiophenes. 4.4. Prediction of fluxes for pervaporation of ternary mixture Pervaporation of n-octane/thiophene/2-methyl-thiophene ternary mixture was performed to confirm the ability of the PDMS membrane to remove thiophenes from n-octane. The individual fluxes of the three components were illustrated in Fig. 10 by varied scattered symbols. It is evident that the flux of thiophene is higher than that of 2-methyl-thiophen due to the difference of the molecular size and structure. From the viewpoint of solution–diffusion theory, both the differences in sorption and diffusion of the species in the membrane might be contributing factors to this situation. A decrease in diffusivity with the increase in penetrant size has been reported by different researchers. In rubbery polymer, which is the case of PDMS, increasing penetrant size tends to improve the heat
99
Fig. 10. Experimental data and theoretical predictions of fluxes for noctane/thiophene/2-methyl-thiophene mixture. Feed: 0.326 wt.% thiophene and 0.362 wt.% 2-methyl-thiophene in n-octane.
of sorption, which would facilitate the generation of sites for permeant molecules to occupy [15]. It follows that larger permeant molecules would be readily sorbed resulting in higher degree of plasticization of the polymer. Consequently, the overall effect of the additional methyl of 2-methyl-thiophene on the transport of its molecules would be a balance between an increased solubility and a decreased diffusivity. That is, the lower flux of 2-methyl-thiophene is the result of a decreased permeability, which is related to both the solubility and the diffusivity. By using the permeability correlations obtained from the binary experiments, the fluxes of the three components were estimated theoretically. The predicted results were illustrated by dot lines in Fig. 10 and shown to be in good agreement with the experimental data, especially in the case of low feed temperature.
5. Conclusion Pevaporation separation of n-octane/thiophene mixtures was investigated by a composite PDMS/PAN membrane using a PV cell at laboratory scale. Experimental results were carried out at different initial thiophene concentrations and feed temperatures. Average flux, selectivity and activation energy of permeation were calculated. It was found that increasing in temperature yields higher total fluxes and lower selectivity to thiophenes, while changing concentration of thiophenes in the range concerned produces insignificant effect on total flux and selectivity. Experimental results also showed that PDMS membrane is more selective to thiophene than to 2-methyl-thiophene and it may be mainly because of the difference in molecular size and structure. Total fluxes for thiophene/n-octane mixture and 2-methyl-thiophene/noctane mixture were measured to be 1.5 and 1.4 kg/m2 h at 30 ◦ C, with the corresponding selectivity to thiophenes 4.9 and 2.5, respectively. Based on solution–diffusion model, the permeation fluxes of individual components in pervaporation
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
100
of thiophene/ 2-methyl-thiophene/n-octane ternary mixture had been calculated and showed to be in good agreement with experimental results.
Acknowledgement The financial support of National 973 Project of PR China (No. 2003CB615701) is gratefully acknowledged.
Nomenclature A ciF cim Di Epi J Ji Ki m pdi pei Pi Pi0 R t T
area of the membrane (m2 ) mass concentration of component i in feed (kg m−3 ) mass concentration of component i in membrane (kg m−3 ) diffusion coefficient of component i (m2 h−1 ) activation energy of permeation for component i (kJ/mol) total flux of permeation (kg m−2 h−1 ) partial flux of component i, (kg m−2 h−1 ) membrane/liquid phase sorption coefficient weight of permeate (kg) partial downstream pressure of component i (Pa) equilibrium vapor pressure of component i at the liquid–membrane interface (Pa) permeability of component i (m2 h−1 ) pre-exponential factor in Eq. (6) (m2 h−1 ) gas constant permeating time (h) temperature (K)
Greek symbols α selectivity of a membrane in a binary system δ thickness of the membrane (m) ωiF weight fraction of component i in the feed ωiP weight fraction of component i in the permeate
References [1] X. Ma, C. Song, A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultraclean fuels and for fuel cell applications, Catal. Today 77 (2002) 107. [2] C. Covert, T.C. Sherpherd, M.W. Thompson, How Phillips s-zorb sulfur removal technology quickly came to life, World Refin. 11 (2001) 56. [3] L. Zhou, J. Mowry, D. Hamm, FCC naphtha desulphurization by solvent extraction, prepared for Presentation at AIChE 2002 Spring National Meeting, 10–14 March 2002. [4] Han Binbing, Li Jiding, Chen Cuixian, R. Wickramasinghe, Computer simulation and optimization of pervaporation process, Desalination 145 (1–3) (2002) 187. [5] Li. Jiding, Chen. Cuixian, Han. Binbing, Peng. Yong, Zou. Jian, Jiang Weijun, Laboratory and pilot-scale study on dehydration of benzene by pervaporation, J. Memb. Sci. 203 (2002) 127. [6] Toraj Mohammadi, Abdolrza Aroujalian, Ali Bakhsai, Pervaporation of dilute alcoholic mixtures using PDMS membrane, Chem. Eng. Sci. 60 (2004) 1875. [7] A. Jonquieres, R. Clement, P. Lochon, et al, Industrial state-of-the art of pervaporation and vapor permeation in the west countries, J. Membr. Sci. 206 (2002) 87. [8] J.P. Garcia Villaluenga, A. Tabe-Mohammadi, A review on the separation of benzene/cyclohexane mixtures by pervaporation process, J. Membr. Sci. 169 (2000) 159. [9] N. Tanihara, K. Tanaka, K. Hidetoshi, et al., Pervaporation of organic liquid mixtures through membranes of polyimide containing methylsubstituted phenylenediamine moieties, J. Membr. Sci. 95 (2) (1994) 161. [10] J. Balko, N. Wynn, R. Glaser, R. Wormsbecher, Reduce your Tier2 gasoline compliance cost with new membrane technology, World Refin. 12 (4) (2002) 22. [11] B.S. Minhas, M.R.Chuba, R.J. Saxton, Membrane process for separating sulfur compounds from FCC light naphtha, US Patent 6,649,061. [12] Li. Lei, Xiao. Zeyi, Tan. Shujuan, et al., Composite PDMS membrane with high flux for the separation of organic from water by pervaporation, J. Membr. Sci. 243 (2004) 177. [13] A. Hasanoglu, Y. Salt, S. Keleser, et al., Pervaporation separation of ethyl acetate–ethanol binary mixtures using polydimethylsiloxane membranes, Chem. Eng. Proc. 44 (2005) 375. [14] J.G. Wijmans, R.W. Baker, The solution–diffusion model: a review, J. Membr. Sci. 107 (1995) 1. [15] Soney C. George, Sabu Thomas, Transport through polymeric systems, Prog. Polym. Sci. 26 (2001) 985.
Removal of thiophenes from n-octane/thiophene mixtures by pervaporation Rongbin Qi, Changwei Zhao, Jiding Li ∗ , Yujun Wang, Shenlin Zhu State Key Laboratory of Chemical Engineering, Department of Chemical Engineering, Tsinghua University, Beijing 100084, China Received 13 April 2005; received in revised form 15 June 2005; accepted 19 June 2005 Available online 20 July 2005
Abstract PDMS/PAN composite membrane was prepared and employed in pervaporation separation of n-octane/thiophene mixtures at laboratory scale. At different initial thiophenes (thiophene and 2-methyl-thiophene) concentrations (corresponding sulfur contents about 500–2500 g/g) and feed temperatures (30–70 ◦ C), experimental results were carried out and average flux, selectivity and activation energy of permeation were calculated. It was found that increasing in temperature yields higher total fluxes and lower selectivity to thiophenes, while changing concentration of thiophens in the range concerned produces insignificant effect on total flux and selectivity. Experiments also showed that PDMS membrane is more selective to thiophene than to 2-methyl-thiophene and it may be mainly because of the difference in molecular size and structure. Total fluxes for thiophene/n-octane mixture and 2-methyl-thiophene/n-octane mixture were measured to be 1.5 and 1.4 kg/m2 h at 30 ◦ C, with the corresponding selectivity to thiophenes 4.9 and 2.5, respectively. Based on simplified solution–diffusion model, the permeation fluxes of individual components in pervaporation of thiophene/2-methyl-thiophene/n-octane ternary mixture had been predicted and showed to be in good agreement with experimental values. © 2005 Elsevier B.V. All rights reserved. Keywords: Pervaporation; Desulfurization of gasoline; n-Octane/thiophene mixtures; PDMS/PAN membrane
1. Introduction Organosulfur impurities contained in gasoline lead to severe environmental problems. Sulfur present in gasoline results in SOx air pollution and is directly responsible for increased levels of NOx in automotive exhaust, as well as a poison for many of the catalytic converters being introduced to automobiles. The worldwide concerns over environment have promoted an increasing interest both in academic and industry for deep desulfurization of gasoline. Traditionally, hydrotreating process is the most effective technology used for removal of organosulfurs present in gasoline. However, this technology suffers from the significant loss of octane number caused by the accompanying saturation of olefins and the high investment and operating costs. For the ultra low content of sulfur (generally less than 2000 g/g) presenting ∗
Corresponding author. Tel.: +86 10 62782432; fax: +86 10 62770304. E-mail address: [email protected] (J. Li).
0376-7388/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2005.06.022
in gasoline, traditional distillation process alone can hardly meet the pending clean-fuel specifications. Novel and capital-avoiding methodologies are rapidly developed on either bench scale or pilot scale, such as adsorption process [1,2], extraction process [3]. In recent years, pervaporation has been widely studied for the potential feasibility of separating close boiling, azeotropic, isomeric and ultra composition–asymmetric mixtures. Compared to traditional separation processes such as distillation, pervaporation offers many advantages, including high separation efficiency, low energy consumption and simple operation [4,5]. That is so because, according to solution–diffusion model, the separation mechanism in pervaporation is based on the difference in sorption and diffusion properties of the feed compounds, instead of on the difference in relative volatilities, which is the case in distillation. Currently, commercial applications of pervaporation fall into two groups: one is dehydration of alcohols and other solvents and the other is the removal of trace volatile organic
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
substances from water [6,7]. Recently, more and more efforts have been made to separate organic/organic mixtures by pervaporation [8,9]. Pervaporation has been proved a viable technology for the removal of sulfur impurities out of gasoline by different authors [10,11]. However, available information about their work in details is rather limited. PDMS has been extensively investigated for the separation of various mixtures, and attempts have been made to modify PDMS membrane in different ways to enhance the pervaporation performance [12,13]. Semi-organic bonds in PDMS provide highly flexible backbone with large bond angle, long bond lengths and extreme freedom of rotation. This allows for the helix polymeric structure consisting of an inorganic Si O Si backbone with a pendant methyl group. The rapid chain segment motion in the silicone rubber leads to an increasing free volume available for the diffusion of the permeate molecules. In this study, the PDMS composite membrane was prepared and employed to simulate desulfurization process of gasoline. Real gasoline is a rather complex mixture composed of alkanes, olefins, cycloparrafins and aromatics ranging from C5 to C14 . Typical sulfur compounds in gasoline include mercaptans, thiophenes and the ramifications thereof. In this work, n-octane was selected to stand for gasoline, while thiophene and 2-methyl-thiophene were chosen as the representative organosulfurs.
2. Theory According to solution–diffusion model [14], transport of molecule i through dense membrane can be expressed as: Di cim pdi Ji = (1) 1− e δ pi where Ji is the flux of component i, Di the diffusion coefficient, cim the mass concentration of component i inside the membrane, δ the thickness of the membrane and pdi , pei the partial downstream pressure and the equilibrium vapor pressure of component i at the liquid–membrane interface, respectively. Provided that the concentration polarization at the feed side is negligible due to high circulation velocity and sufficiently low flux and the chemical potential of the feed and permeate fluids are in equilibrium with the adjacent membrane surface, the following equation could be applied: cm Ki = iF ci
(2)
with Ki referring the membrane/liquid phase sorption coefficient, ciF the concentration of component i in liquid bulk. Combining Eq. (1) and (2) gives the following expression: Pi ciF Di Ki ciF pdi pdi (3) Ji = 1− e = 1− e δ pi δ pi
95
where Pi is the permeability of component i, defined as: Pi = Di Ki
(4) pd
It is noticeable that the term pie in Eq. (3) is negligible due i to the sufficient vacuum on the downstream side. Hence Eq. (3) can be simplified to: Ji =
Pi ciF δ
(5)
Generally Pi is concentration and temperature dependent. The variation of permeability with temperature is described by the following Arrhenius relationship: Pi = Pi0 e−Epi /RT
(6)
where Pi0 is the pre-exponential factor, Epi the activation energy of permeation for component i, T the feed absolute temperature and R the gas constant.
3. Experimental 3.1. Materials In order to simplify the experiment and to simulate the actual desulfurization process simultaneously, n-octane (China Medicine (Group) Shanghai Chemical Reagent Corporation), thiophene (Tianjin Chemical Company, China) and/or 2-methyl-thiophene (98% pure, Acros) were chosen as the representative components to form the model gasoline. The major properties of n-octane, thiophene and 2-methylthiophene are listed in Table 1. PDMS (viscosity 10 Pa s), ethyl orthosilicate, dibutyltin dilaurate, n-heptane (Beijing Chemical Company, China) were purchased for the preparation of PDMS membrane. Asymmetric microporous PAN membrane (provided by Beijing Megavision Membrane & Engineering Co. Ltd., China) was used as supports. 3.2. Membrane preparation PDMS, cross-linking agent ethyl orthosilicate and catalyst dibutyltin dilaurate were dissolved in n-heptane at ambient temperature. After degassed under vacuum, the solution is Table 1 Representative properties of n-octane, thiophene and 2-methyl-thiophene Compound
n-Octane
Structural formula
CH3 (CH2 )6 CH3
fp (◦ C) mp (◦ C) bp (◦ C) Density at 20 ◦ C (kg/m3 ) Refractive index, n20 D
Thiophene
2-Methyl-thiophene
15.6 −56.795 125.665 702.52
−9 −38.30 84.16 1064.8
7 −63.4 112.6 1019
1.39743
1.5289
1.5200
96
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
Fig. 1. Scheme of the pervaporation apparatus.
cast onto the PAN membrane. The membrane was first vulcanized under room temperature to evaporate the solvent, and then introduced into a vacuum oven to complete crosslinking. Controlling the solution concentration or the coating amount could produce membranes with variable top layer thickness. The thickness of the top skin layer could be determined by means of SEM photographs. All experiments in this study were performed with the same membrane. 3.3. Pervaporation experiments A schematic layout of the experimental set-up used in this study is depicted in Fig. 1. The feed was heated and circulated from the feed tank (volume of 2.5 × 10−3 m3 ) through the upstream side of the membrane cell by a pump with adjustable function of flow rate. In order to minimize the effect of concentration polarization on pervaporation performance, high feed flow rate up to 20 l/h and annular feed chamber (see Fig. 2) were employed in the experiments. A membrane supported by a porous sintered stainless steel in the permeate side was mounted in the pervaporation cell. The feed solution was maintained at temperatures between 30 and 70 ◦ C using a thermostat. The effective area of the membrane was 2.83 × 10−3 m2 . Vacuum on the permeate side was maintained below 500 Pa and was monitored with a digital vacuometer. Two cold traps were set in parallel allowing the collection of permeate without rupture of the vacuum. The permeation rate was determined by measuring the weight of permeate collected in the cold trap and divided by time and
the membrane’s surface area as shown in Eq. (7): J=
m At
(7)
with m denoting weight of the permeate passing through the active membrane area A during the time t. The compositions of the feed solution and permeate were analyzed by gas chromatography (HP6890, USA). Then the selectivity of a membrane in a binary system is obtained as follows: −1 ωiF ωiP (8) α= ωjP ωjF where ωP and ωF refer to the weight fraction of component i (thiophene or 2-methyl-thiophene) or j (n-octane) in the permeate and in the feed, respectively.
Fig. 2. The annular channel in the feed chamber.
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
97
Fig. 3. SEM photographs of PDMS/PAN membrane: (A) top surface; (B) cross-section; and (C) determination of the thickness of the top layer.
4. Results and discussion 4.1. SEM photographs of PDMS membrane The morphology of the PDMS/PAN membrane is shown in Fig. 3. As demonstrated in the SEM photographs, the originally porous surface of the PAN substrate was covered by a flat featureless PDMS layer. The top PDMS layer, functioning as the basis of permselectivity, had a nonporous and tight structure. The thickness of the PDMS layer was determined to be 1.1 × 10−5 m from the SEM photograph of cross-section of the membrane. 4.2. Effect of feed temperature Temperature dependence of flux and selectivity for thiophene/n-octane and 2-methyl-thiophene/n-octane mixtures is illustrated in Figs. 4 and 5, respectively. As expected, all the experimental evidences confirm an increase in
Fig. 4. Temperature dependence of flux and selectivity for noctane/thiophene mixture. Thiophene content in feed: 0.364 wt.%.
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R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100 Table 2 Activation energies and permeability correlations of individual components
Fig. 5. Temperature dependence of flux and selectivity for n-octane/2methyl-thiophene mixture. 2-Methyl-thiophene content in feed: 0.463 wt.%.
Compound
P0i (m2 /h)
Activation energy, EPi (kJ/mol)
Permeability correlation
Thiophene 2-Methyl-thiophene n-Octane
2.11 × 10−5 3.60 × 10−5 3.30 × 10−5
13.15 16.43 18.52
lg Pi =−4.69–686.92/T lg Pi =−4.46–858.47/T lg Pi =−4.49–967.32/T
summarized in Table 2. Activation energies of permeation for the three components in the PDMS membrane decrease in the following order: n-octane > 2-methyl-thiophene > thiophene. The permeability correlations for the three components were then obtained and applied to predict permeate fluxes for individual components in the pervaporation of ternary mixture. 4.3. Effect of feed composition
permeability and decrease in selectivity of membranes with increasing temperature. Flux profiles with feed temperature follow an Arrhenius type relation of the type showing in Eqs. (5) and (6), that is, the permeability of PV membranes decrease exponentially with reciprocal of feed temperature. The increase in total flux with temperature is due to the increase of the mobility of individual permeating molecules caused both by the temperature and by the enhanced mobility of the polymer segments. On the other hand, the increase in the degree of swelling of the membrane with temperature results in more n-octane transport, which leads to the decrease of selectivity to thiophenes. Permeability of thiophene, 2-methyl-thiophene and n-octane can be obtained from the above experimental data by using Eq. (5). Dependence of permeability of the three components on temperature is portrayed in Fig. 6. The permeability of PV membranes decreases exponentially with reciprocal of temperature following an Arrhenius type relation, Eq. (6). By curve fitting of the experimental data, the pre-exponential factor and the activation energy of permeation Ep for the three components are calculated and
Fig. 7. Effect of feed composition on flux and selectivity for noctane/thiophene mixture. Feed temperature: 60 ◦ C.
Fig. 6. Permeability profiles with feed temperature.
Fig. 8. Effect of feed composition on flux and selectivity for n-octane/2methyl-thiophene mixture. Feed temperature: 60 ◦ C.
Impact of feed composition on pervaporation performance was depicted in Fig. 7 and Fig. 8. The corresponding sul-
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
Fig. 9. Dependence of partial fluxes on feed compositions. Feed temperature: 60 ◦ C.
fur contents in the feed under consideration were in the vicinity from 500 to 2500 g/g. It can be found that variation of feed composition had nearly negligible influence on total fluxes and selectivity, while the partial fluxes of thiophenes are proportional to the concentrations of thiophenes in the feed, which is illustrated in Fig. 9. This situation may be attributed to the ultra low feed concentration and the narrow range thereof. According to the solution–diffusion theory, the separation performance of this process is dominated by the sorption and diffusion characteristics of the individual components. The feed processed in this work is an ultra composition–asymmetric mixture, that is, the thiophene content was very low and fell into a relatively narrow range. On one hand, the change in thiophene concentration in feed yields very limited impact on the transport of n-octane through the membrane. On the other hand, the partial fluxes of thiophenes are still proportional to the concentrations of thiophenes in the feed. The former results in nearly unchanged total fluxes, while the latter leads to nearly unchanged selectivities to thiophenes. 4.4. Prediction of fluxes for pervaporation of ternary mixture Pervaporation of n-octane/thiophene/2-methyl-thiophene ternary mixture was performed to confirm the ability of the PDMS membrane to remove thiophenes from n-octane. The individual fluxes of the three components were illustrated in Fig. 10 by varied scattered symbols. It is evident that the flux of thiophene is higher than that of 2-methyl-thiophen due to the difference of the molecular size and structure. From the viewpoint of solution–diffusion theory, both the differences in sorption and diffusion of the species in the membrane might be contributing factors to this situation. A decrease in diffusivity with the increase in penetrant size has been reported by different researchers. In rubbery polymer, which is the case of PDMS, increasing penetrant size tends to improve the heat
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Fig. 10. Experimental data and theoretical predictions of fluxes for noctane/thiophene/2-methyl-thiophene mixture. Feed: 0.326 wt.% thiophene and 0.362 wt.% 2-methyl-thiophene in n-octane.
of sorption, which would facilitate the generation of sites for permeant molecules to occupy [15]. It follows that larger permeant molecules would be readily sorbed resulting in higher degree of plasticization of the polymer. Consequently, the overall effect of the additional methyl of 2-methyl-thiophene on the transport of its molecules would be a balance between an increased solubility and a decreased diffusivity. That is, the lower flux of 2-methyl-thiophene is the result of a decreased permeability, which is related to both the solubility and the diffusivity. By using the permeability correlations obtained from the binary experiments, the fluxes of the three components were estimated theoretically. The predicted results were illustrated by dot lines in Fig. 10 and shown to be in good agreement with the experimental data, especially in the case of low feed temperature.
5. Conclusion Pevaporation separation of n-octane/thiophene mixtures was investigated by a composite PDMS/PAN membrane using a PV cell at laboratory scale. Experimental results were carried out at different initial thiophene concentrations and feed temperatures. Average flux, selectivity and activation energy of permeation were calculated. It was found that increasing in temperature yields higher total fluxes and lower selectivity to thiophenes, while changing concentration of thiophenes in the range concerned produces insignificant effect on total flux and selectivity. Experimental results also showed that PDMS membrane is more selective to thiophene than to 2-methyl-thiophene and it may be mainly because of the difference in molecular size and structure. Total fluxes for thiophene/n-octane mixture and 2-methyl-thiophene/noctane mixture were measured to be 1.5 and 1.4 kg/m2 h at 30 ◦ C, with the corresponding selectivity to thiophenes 4.9 and 2.5, respectively. Based on solution–diffusion model, the permeation fluxes of individual components in pervaporation
R. Qi et al. / Journal of Membrane Science 269 (2006) 94–100
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of thiophene/ 2-methyl-thiophene/n-octane ternary mixture had been calculated and showed to be in good agreement with experimental results.
Acknowledgement The financial support of National 973 Project of PR China (No. 2003CB615701) is gratefully acknowledged.
Nomenclature A ciF cim Di Epi J Ji Ki m pdi pei Pi Pi0 R t T
area of the membrane (m2 ) mass concentration of component i in feed (kg m−3 ) mass concentration of component i in membrane (kg m−3 ) diffusion coefficient of component i (m2 h−1 ) activation energy of permeation for component i (kJ/mol) total flux of permeation (kg m−2 h−1 ) partial flux of component i, (kg m−2 h−1 ) membrane/liquid phase sorption coefficient weight of permeate (kg) partial downstream pressure of component i (Pa) equilibrium vapor pressure of component i at the liquid–membrane interface (Pa) permeability of component i (m2 h−1 ) pre-exponential factor in Eq. (6) (m2 h−1 ) gas constant permeating time (h) temperature (K)
Greek symbols α selectivity of a membrane in a binary system δ thickness of the membrane (m) ωiF weight fraction of component i in the feed ωiP weight fraction of component i in the permeate
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