n-dodecane mixtures

Journal of Membrane Science 343 (2009) 119–127

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Desulphurization of kerosene: Pervaporation of benzothiophene/n-dodecane mixtures I. Bettermann, C. Staudt ∗ Department of Organic and Macromolecular Chemistry, Heinrich-Heine-University, Universitätsstr. 1, 40225 Düsseldorf, Germany

a r t i c l e

i n f o

Article history: Received 15 April 2009 Received in revised form 8 July 2009 Accepted 10 July 2009 Available online 18 July 2009 Keywords: Pervaporation Kerosene Desulphurization Copolyimides Benzothiophene

a b s t r a c t Different fluorinated copolyimides have been synthesized using 6FDA (4,4 -(hexafluoroisopropylidene)diphthalic anhydride), DABA (3,5-diaminobenzoic acid), 4MPD (2,3,5,6-tetramethyl-1,4phenylenediamine) and 3MPD (2,4,6-trimethyl-1,3-phenylenediamine). The copolyimides with different compositions of monomers were used as membrane materials in order to remove benzothiophene from benzothiophene/n-dodecane mixtures by pervaporation. This is especially of interest in fuel cell applications where sulphur components are poisoning the catalyst and therefore reducing the life time of the system. In order to figure out which operation parameters, e.g. temperature, pressure and membrane material are necessary for the enrichment of the sulphur-aromatic component and sufficient transmembrane fluxes, different pervaporation experiments have been performed. Feed temperatures have been varied between 353 and 413 K and permeate pressures between 19 and 45 mbar, average fluxes and enrichment factors ␤ were determined. Activation energies for permeation were calculated for benzothiophene and n-dodecane in order to understand the temperature-dependent separation characteristics. The influence of the different diamine structures on the separation characteristics was investigated. It was found out that slight differences in structure, e.g. an additional methyl group on the polymer backbone does not have a significant effect on the pervaporation properties. Total fluxes for 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1 membranes were 15.2 and 10.3 kg ␮m/(m2 h) at 393 K, with the corresponding enrichment factor of benzothiophene of 3.6 and 3.3, respectively. With increasing temperature, enhanced fluxes as well as enhanced enrichment factors were observed. Furthermore it was found that higher permeate pressures led to a decrease of the enrichment factor with no significant change in flux. © 2009 Elsevier B.V. All rights reserved.

1. Introduction Desulphurization of gasoline diesel fuel and jet fuel is of great industrial and research interest, which is due to the strict regulations for sulphur concentrations in oil products. The Environmental Protection Agency (EPA) set the maximum sulphur content in gasoline to 30 ppmw (0.003 wt.%) as of the year 2006 and the sulphur content in highway diesel is regulated to a current limit of 15 ppmw (0.0015 wt.%) [1]. The desulphurization of gasoline using pervaporation units is a newly emerged technology, which is already commercially used in Grace Davison’s S-brane process since 2002 [2]. In addition several research groups have investigated the removal of thiophene and its methylated derivatives from hydrocarbons in binary, ternary and multicomponent mixtures [3–10]. For example, an enrichment factor of 3.31 in combination with

∗ Corresponding author. Tel.: +49 211 8115362; fax: +49 211 8110696. E-mail address: [email protected] (C. Staudt). 0376-7388/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2009.07.017

a flux of 33.3 kg ␮m/(m2 h) was achieved with PEG membranes using a binary thiophene (1200 ppmw)/n-hexane mixture, at 358 K and a permeate pressure of 1.3 mbar [3]. Conversely, with PDMS composite membranes the enrichment factor of thiophene in thiophene/n-octane mixtures was 4.4 at 304 K with a corresponding flux of about 15.6 kg ␮m/(m2 h) [8]. Furthermore, Kong et al. investigated the pervaporation of FCC gasoline and its sulphur compounds through polyethylene glycol (PEG) membranes and polyethylene glycol/polyurethane blend membranes, respectively [3,4,11–13]. The latter ones showed at permeate pressure of 1.3 mbar and a sulphur content of 1200 ppmw (0.12 wt.%) in FCC gasoline an enrichment factor of 4.03 and a flux of 2.5 kg/(m2 h) at 383 K. So far, mostly hydrodesulphurization (HDS) and selective adsorption processes are commercially available for the desulphurization of higher boiling refinery streams such as kerosene, middle distillate and light cycle oil (LCO) and improvement of those technologies is a current topic focus of research [14]. In contrary to the strict regulations for gasoline and diesel fuels, currently, a maximum sulphur content of 3000 ppmw is tolerable in jet fuel. Jet

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fuels are manufactured predominantly from straight run kerosene, which is obtained directly from crude oil atmospheric distillation. Typical sulphur compounds in jet fuels are alkylated thiophenes, e.g. C2- and C3-thiophenes, benzothiophene and also alkylated benzothiophenes [14,15]. In addition, concerning jet fuels at some refineries, an increasing tendency is observed to incorporate proportions of products produced by hydrocracking processes [16]. Generally JET A1 is specified with 3000 ppmw sulphur compounds (0.3 wt.%) [17], however most straight run kerosene streams in the refinery and also most military jet fuels in the US have sulphur contents below 1500 ppmw (0.15 wt.%) because of the refining processes used [14]. Due to the fact that in the past decades the sulphur content in US crude oil increased constantly [18–20] and because of increasing environmental awareness, it is expected that in future the reduction of jet fuel sulphur content will also be necessary. As a consequence, refinery streams with steadily increasing sulphur contents have to be desulphurized to lower and lower limits. Thus refineries will have to undertake increasing efforts in obtaining clean fuels combined with higher capital and operating costs. Here pervaporation, e.g. as part of a hybrid process including conventional HDS, could be a promising alternative, as it is already in the desulphurization of gasoline. In addition the desulphurization of jet fuels is decisive for on-board use of fuel cell systems in aircraft since the sulphur compounds are poisonous to the catalysts in the reformer as well as having detrimental effects on the fuel cell itself [1]. Thus the sulphur content has to be reduced to less than 10 ppmw. Peters et al. evaluated desulphurization processes with respect to their application in fuel cell auxiliary power units (APUs). Among various processes like adsorption, extraction with ionic liquids, selective oxidation and pervaporation a two step concept of pervaporation combined with an adsorption process was considered as a very promising approach [15,21]. As mentioned above, most of the research so far focussed on the pervaporation of FCC gasoline and the removal of its sulphur compounds such as thiophene and methylated thiophenes. In order to find out if the removal of binuclear sulphur-aromatics, e.g. benzothiophene by pervaporation is also possible, especially in low concentrations, the development of novel membrane material is necessary. Recently, successful investigation of the removal of dinuclear aromatics such as naphthalene from binary mixtures with n-decane

using copolyimide membranes was reported [22–25]. Therefore, the aim of this project was to investigate this class of polymers for the removal of dinuclear sulphur containing aromatics. Temperature-dependent pervaporation experiments were carried out using copolyimide membranes with different structures in order to show which experimental parameters are necessary for the enrichment. The binary mixture investigated in this work consisted of benzothiophene as a dinuclear, high boiling sulphur-aromatic component and n-dodecane as a typical aliphatic compound of jet fuels. The copolyimides synthesized and investigated in pervaporation experiments in this work were 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1. The structures of the synthesized copolymers are given in Fig. 1. The 3MPD and 4MPD based copolyimide structures were chosen since several papers were published before on 6FDA–4MPD and 6FDA–3MPD. For example Liu et al. [26] investigated the gas permeation of H2 /N2 and O2 /N2 systems through 6FDA–3MPD and 6FDA–4MPD membranes and associated the results with the fractional free volumes (FFV) of the different membrane materials calculated based on the group contribution method after Bondi [27]. The calculations have shown FFV values of 0.167 for the 6FDA–3MPD and 0.192 for the 6FDA–4MPD. The higher FFV calculated for the 6FDA–4MPD is due to the additional methyl group on the diamino monomer. Due to the smaller fractional free volume of the 6FDA–3MPD in comparison to the 6FDA–4MPD, lower gas permeabilities for hydrogen and oxygen were found. Whereas the 6FDA–4MPD obtains a hydrogen permeability of 344 barrer (10−10 cm3 (STP)/(cmscmHg)), a permeability of only 434 barrer was found for the 6FDA–3MPD. Similar effects have been observed for oxygen permeabilites, which were 87 barrer for the 6FDA–4MPD and 62.5 barrer for the 6FDA–3MPD, respectively. For the 6FDA–3MPD compared to the 6FDA–4MPD membrane higher selectivities were observed for H2 /N2 (21.7 for 6FDA–4MPD and 25.6 for 6FDA–3MPD) as well as for O2 /N2 (4.35 for 6FDA–4MPD and 4.46 for 6FDA–3MPD). Furthermore polyimides made from 3MPD and 4MPD as diamino monomers have been investigated in pervaporation before [28]. In this case, BP (BP: 3,3 -4,4 biphenyltetracarboxylic dianhydride) was used as dianhydride in the polymerization reaction. Pervaporation experiments were performed using a benzene (50 wt.%)/cyclohexane mixture. At a feed temperature of

Fig. 1. Structure of synthesized copolyimides.

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323 K the BP-3MPD polyimide membrane shows smaller fluxes (9.1 kg ␮m/(m2 h)) at higher selectivities (˛ = 7.1) than the BP-4MPD polymer (14 kg ␮m/(m2 h), ˛ = 5.7). Due to the fact that polyimide structures using 3MPD and 4MPD as diamino compounds have been used successfully in gas separation and pervaporation experiments, new polyimide structures based on these monomers have been synthesized and characterized in this work and parameters for the removal of sulphur containing aromatics from aromatic/aliphatic mixtures have been worked out. 2. Experimental

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6FDA was added afterwards in one portion. The solution was stirred over night, during which the viscous polyamic acid was formed. The imidization was conducted by dehydration of the polyamic acid by adding a mixture of triethylamine and acetic anhydride, respectively. The amount of triethylamine and acetic anhydride was a 3 equiv. surplus with regard to the amount of 6FDA used in the synthesis. The reaction mixture was stirred for 30 min at 393 K oil bath temperature. After cooling to room temperature the copolyimide was precipitated in ethanol/waterdest 1:1. Long, stiff, white fibres of copolyimide were obtained, homogenized in a blender and washed several times with ethanol. The copolyimide powder was dried overnight at room temperature and afterwards for 3 days at 80 mbar and 423 K.

2.1. Materials 2.2. Membrane preparation Membrane materials investigated in this work were copolyimides made of (4,4 -(hexafluoroisopropylidene)diphthalic anhydride) (6FDA), 3,5-diaminobenzoic acid (DABA), 2,3,5,6-tetramethyl-1,4-phenylenediamine (4MPD) and 2,4,6trimethyl-1,3-phenylenediamine (3MPD), respectively. The monomers and the solvent used were purified before the polymerization reaction. All diamines and the anhydride were sublimed at 0.1 mbar whereas the oil bath temperature was kept at 488 K for 6FDA (Lancaster, purity 99%), 373 K for 4MPD (Fluka, purity >99%), 348 K for 3MPD (Aldrich, purity 96%) and 473 K for DABA (Merck, purity >99%). The purified monomers were stored in brown glass ware. In order to avoid traces of water in the solvent, dimethyl acetamide (DMAc) was boiled under reflux over CaH2 as desiccant, followed by a fractional distillation. The copolyimide synthesis was performed in a two step polycondensation reaction [29] as shown schematically in Fig. 2. The water residuals were excluded by heating the flask and the reaction was carried out under nitrogen atmosphere. Then the diamine monomers were dissolved in DMAc at room temperature and the

The synthesized copolyimide was dissolved in absolute tetrahydrofuran (THF), filtered to avoid defects in the film caused by dust particles and then cast onto a smooth metal plate. The solvent was evaporated over night at room temperature. The membranes obtained were removed from the metal plates with water, and dried for 24 h at 423 K at 30 mbar to ensure that no residual solvents remained in the films. Average membrane thicknesses were 25–35 ␮m. The cross-linked membranes used for swelling experiments have been prepared as previously reported [24] by esterification with different diols. 2.3. Characterization The average molecular weight measurements of the synthesized copolyimides were performed at the Max-Planck-Institute for Polymer Research, Mainz by size exclusion chromatography with an attached multiangle light scattering detector.

Fig. 2. Synthesis of copolyimides 6FDA–4MPD/DABA m:n.

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Differential thermal analysis (DTA) in combination with thermogravimetric analysis (TGA) (NETZSCH STA 449C) was used to determine thermal characteristics of the copolyimide’s glass transition temperature Tg and decomposition behaviour. The heating rate was 5 K/min and the setup was flooded with nitrogen during measuring. The density of the membranes was determined by using CaCl2 solutions with different concentrations. Thereby in each case 3 samples of a 6FDA–4MPD/DABA 9:1 membrane and a 6FDA–3MPD/DABA 9:1 membrane were immersed in a solution of CaCl2 -dihydrate and distilled water. The density of the solution was increased and decreased by adding CaCl2 -monohydrate and distilled water, respectively until the membrane floated. In this case the density of the solution and those of the sample are equal. After reaching equilibrium the density of the solution was determined with an Anton Paar DMA 35n portable density meter (error +/− 0.0005 g/ml). The obtained densities were averaged and the percentage standard deviation  was determined. The size of membrane samples was 2 cm in length and 0.5 cm in width with membrane thicknesses of 25–35 ␮m. The temperature of the solution was kept at 298 K. 2.4. Stress strain measurements In order to investigate the influence of temperature and sorbed feed components on membrane stability, stress strain measurements were performed (Zwick 2.5N tensile tester with temperature control). It was especially of interest to learn, whether the components sorbed in the membrane material act as plasticizers and decrease the glass transition temperature of the copolyimide dramatically. In this case the membrane material is expected to be no longer in the glassy state. In order to investigate this, the membrane was pre-treated with the mixture at the same temperature used in pervaporation experiments. The mixture contained benzothiophene (0.25 wt.%) and n-dodecane and the temperature was kept at 403 K for about 24 h. Stress strain measurements of the pretreated membrane materials were carried out with the pre-treated membranes immediately after removing them from the mixture. The oven temperature of the tensile tester was kept at 403 K, during the stress strain experiments. Furthermore the influence of temperature on the mechanical stability of the membrane was studied. This was achieved by examining the membranes as native samples at room temperature, 373 and 403 K, respectively. All samples used were 15 mm in width with a length of 45 mm. Membrane thicknesses were between 40 and 60 ␮m. The distance between the sample brackets was adjusted to 35 mm and tension speed was 10 mm/min. Each measurement was repeated three to four times, the data obtained were averaged, and the percentage of standard deviation  of the measurements determined. 2.5. Swelling measurements In many cases membrane materials with a high affinity to certain feed components show a high degree of swelling. It is also well known that aromatic 6FDA containing copolyimides have a high affinity to aromatic compounds. Therefore swelling measurements can be used as preliminary tests to estimate the interactions of different membrane materials with the aromatic compounds in a mixture. Due to the fact that the ultimate task was to desulphurize jet fuels, the solubility tests were performed with Jet A1 in order to find the copolyimide material with the best sorption properties and on the other hand to ensure that the evaluated membrane materials are resistant to the mixture to be separated. The composition of Jet A1 is heavily dependent on the origin of the crude oil [30], yielding products containing hydrocarbons

Table 1 Composition of hydrocarbon compounds in jet fuels. Hydrocarbon group Aliphatics Cyclic aliphatics Aromatics Polycyclic aromatic compounds (PAC)

Content in jet fuels (vol.%) 30–75 [44] 10–65 [44] 5–22 [44] 11.9–15.8 [45]

ranging from C7 up to C20. In Table 1 various jet fuels and their compounds are listed. The typical sulphur content of Jet A1 in the EU is about 700 ppmw (0.07 wt.%) but depending on the source of crude oil the sulphur content in Jet A1 can also be much higher [15]. To assure that a sufficient stability at elevated temperatures of the feed mixture will be obtained, non-cross-linked materials as well as cross-linked membranes were investigated. The membrane samples were weighed and immersed in Jet A1 at room temperature for the sorption measurements. In intervals of several days the samples were taken out, dapped with tissue and weighed again to ensure that the equilibrium of swelling was reached. In order to test the reproducibility, all different membrane types were analyzed twice, each time with a new sample. The maximum deviation of the average of both measurements was 18.6%. Eq. (1) gives the calculation for the degree of swelling (DS) DS (%) =

MW − MD × 100 MD

(1)

wherein MW is the mass of the swollen membrane and MD is the mass of the dry membrane. The copolyimide films with the highest sorption level were applied for the pervaporation experiments. 2.6. Pervaporation experiments The removal of benzothiophene from a benzothiophene/ndodecane mixture was performed with the pervaporation equipment shown in Fig. 3. The membrane area in the pervaporation experiments was 33.2 cm2 . In order to investigate the influence on permeate pressure on the separation characteristics, experiments with varying permeate pressures were performed. The pressure on the backside of the membrane was kept between 19 and 25 mbar in one experiment and between 39 and 45 mbar in a second experiment. The feed temperature was varied between 353 and 413 K. Since the advanced aim of this project was to desulphurize straight run kerosene, the sulphur content of the investigated binary mixture was similar to the maximum sulphur limits of straight run kerosene. Thus, the feed concentration of benzothiophene in the mixture was about 0.25 wt.% (±0.04 wt.%). The compositions of the feed and permeate streams were determined by gas chromatography (Shimadzu GC 2010). The flux quantifies the mass of permeate mp transported through a certain membrane area A during a time period t. In order to compare different membrane materials, the flux is normalized to the membrane thickness ıM and given in kg ␮m/(m2 h) (Eq. (2)). Jnorm =

mp ıM tA

(2)

The enrichment factor ˇ of benzothiophene/n-dodecane is calculated using Eq. (3), wherein wi characterizes the weight percentage of the considered component in permeate and feed, respectively. permeate

ˇ=

wi

wifeed

(3)

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Fig. 3. Schematic diagram of experimental pervaporation equipment.

In some of the papers cited in this work the separation factor ˛ is given, which can be calculated from Eq. (4). Thereby wi and wj characterize the weight percentages of components i and j in permeate and feed, respectively and i is the preferentially permeating component. permeate

˛=

wi

permeate

/wj

(4)

wifeed /wjfeed

9:1, respectively. The difference in densities of non-cross-linked and cross-linked membranes is marginal. Pithan determined the densities of non-cross-linked 6FDA–4MPD/DABA 2:1 and 6FDA–4MPD/DABA 2:1 cross-linked with dodecanediol to be 1.35 and 1.32 g/cm3 , respectively. The smaller density for the crosslinked polymer is explained by the spacer effect of the long-chain dodecanediol [32].

3.2. Influence of temperature and swelling on stress strain characteristics

3. Results and discussion 3.1. Characterization The average molecular weights of the polymer batches used are given in Table 2. Many research groups established a connection between the glass transition temperature and the FFV of different polyimides [26,31]. Generally an increase in Tg is combined with an increase in FFV. For example, a lower Tg as well as a lower FFV were found for 6FDA–3MPD (Tg = 669 K, FFV = 0.167) in comparison with 6FDA–4MPD (Tg = 683 K, FFV = 0.192). This behaviour was ascribed to the higher chain rigidity and therefore larger matrix openness of the 6FDA–4MPD [26]. DTAs were performed in order to investigate, whether a similar behaviour occurs with the copolyimides examined in this work concerning the differences in Tg and therefore to get an indication of the differences in the free volumes of the 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1. For the 6FDA–4MPD/DABA 9:1 polymer a glass transition temperature Tg of 698 K was observed, whereas the analysis of the 6FDA–3MPD/DABA 9:1 polymer resulted in two endothermal steps, so that Tg could not be precisely determined. In TGA the mass loss due to polymer decomposition at 743 K is 4% and 3% for 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1, respectively. Significant mass losses for 6FDA–4MPD/DABA 9:1 as well as 6FDA–3MPD/DABA 9:1 were observed only above this temperature. The determination of the densities showed identical values for both membrane structures at 1.31 g/cm3 ( = 0.07%) and 1.31 g/cm3 ( = 0.04%) for 6FDA–3MPD/DABA 9:1 and 6FDA–4MPD/DABA Table 2 MW of the polymer batches synthesized. Polymer

MW (g/mol)

6FDA–4MPD/DABA 9:1 batch1 6FDA–4MPD/DABA 9:1 batch2 6FDA–3MPD/DABA 9:1

48,800 45,200 115,100

It is well known that the trend of the curves in stress strain measurements yields information about the characteristics of the polymers. According to this, glassy, ridged polymers, e.g. Kapton, show rampant curves with high tensile strengths (165 MPa) at low ultimate elongations (40%) [33], whereas rubbery polymers such as vulcanized polyisoprene obtain low tensile strengths of about 20 MPa and high ultimate elongations up to 850% [34]. Thus, the curve of stress strain measurements enables to judge whether the membrane material is in the rubbery or in the glassy state. The plasticization caused by sorbed components can lead to a considerable loss in selectivity. This was shown, e.g. for CO2 /CH4 mixtures at different feed pressures. In addition, a significant depression of the membranes’ glass transition temperature is observed [35]. In order to investigate whether the membrane material is still in the glassy state during pervaporation conditions, stress strain measurements were carried out in this work with pre-treated membrane materials. Fig. 4 shows the stress strain measurements of native and pretreated 6FDA–4MPD/DABA 9:1 membranes at 403 K (pre-treatment with benzothiophene (0.3 wt.%)/n-dodecane mixture). The curves in Fig. 4 for the 6FDA–4MPD/DABA 9:1 show the typical characteristics for glassy polymers, with high tensile strength (60.1 MPa), low ultimate elongation ( = 4.1%) and a high tensile modulus of 1876 MPa. After swelling in benzothiophene/ndodecane (0.3 wt.%/99.7 wt.%) the membrane’s behaviour is quite different (Fig. 4, right). Although in this case a rampant run is observed as well, the curves reach a yield point at a tensile strength of 45.1 MPa ( = 4.9%). In contrast to the native membranes the ultimate elongations are clearly different. Therefore, measurements were not averaged. The ultimate elongations for the various measurements were 21.1%, 11%, 8.9% and 15.4%. With regard to the tensile modulus, swollen membranes show clearly decreased values of 1375 MPa ( = 4.2%) in comparison to 1876 MPa for the untreated membrane material. Since the appearance of a yield point in stress strain measurements generally just occurs for amorphous polymers below their glass transition temperatures [36]

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Fig. 4. Stress strain measurements at 403 K, 6FDA–4MPD/DABA 9:1 membranes native (left) and pre-treated in benzothiophene/n-dodecane (0.3 wt.%/99.7 wt.%) (right) at 403 K. Table 3 Temperature-dependent mechanical data of 6FDA–4MPD/DABA 9:1. Temperature (K)

Tensile modulus (MPa)

Tensile strength (MPa)

Ultimate elongation (%)

296 373 403

2322 (0.5%) 1968 (3.8%) 1876 (5.2%)

80.5 (4.8%) 74.1 (4.0%) 60.1 (5.3%)

4.0 (10.0%) 4.9 (10.6%) 4.1 (17.1%)

Standard deviation of 3 membrane samples investigated in percent is given in parenthesis.

a change from glassy to rubbery state due to swelling was not observed—exclusively an increase in durability. In order to investigate the influence of temperature on the native membrane material, stress strain measurements were performed at room temperature, 373 and 403 K for the native non-crosslinked 6FDA–4MPD/DABA 9:1 copolyimide (see Table 3). Due to increased chain mobility and decreased stiffness of the polymer with rising temperature, a decrease in tensile modulus is observed. In addition, the tensile strength is decreased. The ultimate elongation shows an indifferent behaviour with rising temperature. 3.3. Material selection based on swelling measurements In order to find the most promising copolyimide structure for both, the pervaporation experiments with binary mixtures and subsequent measurements with kerosene, preliminary swelling experiments with Jet A1 were performed. Fig. 5 presents the degree

of swelling (DS) depending on the copolyimide structure, the content of DABA and the cross-linking agent. As shown in Fig. 5 non-cross-linked 6FDA–4MPD/DABA 9:1 (DS = 1.8%) and 6FDA–3MPD/DABA 9:1 (DS = 1.3%) membranes have the highest DS of all tested membrane materials. Compared to the cross-linked membranes a decrease of the DS with decreasing chain length of the diol is observed. In order to test the mechanical stability of the non-cross-linked membranes at enhanced temperatures, they were immersed in benzothiophene/n-dodecane (0.25 wt.%/99.75 wt.%) mixtures as well as in Jet A1 at 393 K. Due to the fact that non-cross-linked 6FDA containing membranes are stable in both mixtures at high temperatures and cross-linked membrane materials usually have lower fluxes, 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1 copolyimides were chosen for pervaporation experiments. 3.4. Pervaporation experiments 3.4.1. Reproducibility tests In order to interpret the results of the different measurements and therefore the observed effects correctly, reproducibility tests were performed. Fig. 6 shows the results of temperature-dependent pervaporation experiments. The employed membrane material was 6FDA–4MPD/DABA 9:1 and films made of two different polymer batches were prepared for pervaporation experiments. During the measurements with sample 1 at higher temperatures, decomposition of the feed mixture occurs. This was obvious from the darkening of the mixture. In addition decomposition products were

Fig. 5. Degree of swelling (DS) of 6FDA–4MPD/DABA 9:1 membranes non-cross-linked and cross-linked with diols of different chain length (left) and non-cross-linked copolyimides with different monomers and contents of DABA (right), Jet A1 at 293 K.

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Fig. 6. Temperature-dependent reproducibility tests for different batches of 6FDA–4MPD/DABA 9:1 using benzothiophene/n-dodecane (0.25 wt.%/99.75 wt.%), permeate pressure: 19–25 mbar.

found in gas chromatographic analyses. Furthermore, at temperatures above 403 K, small particles were found in the feed mixture, resulting from decomposition. Thus, measurements of the second membrane sample were performed at 393 K. In summary, for membrane sample 2 compared to membrane sample 1 slightly higher enrichment factors with reduced fluxes were observed. The measurements showed good reproducibility with a maximum error of ±4.4% for the enrichment factor ˇ and ±7.5% for the flux. Due to the very low fluxes measured at 353 K the deviation is higher there. It has been shown before that the deviation in flux and enrichment factor is in the same range for membranes made from the same batch as well as membranes, investigated in different pervaporation equipment [37]. 3.4.2. Influence of the polymer structure and operation temperature Different diamine monomers were used in copolyimide syntheses in order to investigate the influence of the polymer structure on the enrichment factor for benzothiophene. Thereby it was expected that the 6FDA–3MPD/DABA 9:1 shows slightly lower fluxed compared to the 6FDA–4MPD/DABA 9:1 since the 3MPD diamine contains less methyl substituents compared to the 4MPD. This is also expected due to the fact that the fractional free volume (FFV) which can be calculated by the group contribution method after Bondi is lower for the 3MPD containing copolyimides compared to the 4MPD containing copolyimides. In order to show the influence of temperature on the polymer structure, the temperature was varied between 353 and 393 K.

Fig. 8. Temperature dependency of the normalized fluxes for benzothiophene/ndodecane mixture (0.25 wt.%/99.75 wt.%) using 6FDA–4MPD/DABA 9:1 (batch 1).

At 393 K the pervaporation experiments showed a maximum flux of 10.3 kg ␮m/(m2 h) for 6FDA–3MPD/DABA 9:1 and 15.2 kg ␮m/(m2 h) for 6FDA–4MPD/DABA 9:1 (Fig. 7). As expected the fluxes were higher for the 6FDA–4MPD/DABA 9:1 due to the higher free volume compared to the 6FDA–3MPD/DABA 9:1. However at 383 K and lower temperatures no difference in fluxes was observed. With regard to the enrichment of benzothiophene the

Fig. 7. Temperature-dependent influence of polymer structure for 6FDA–3MPD/DABA 9:1 and 6FDA–4MPD/DABA 9:1 with benzothiophene/n-dodecane (0.25 wt.%/99.75 wt.%), permeate pressure: 19–25 mbar.

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Fig. 9. Influence of permeate pressure for 6FDA–3MPD/DABA 9:1 with benzothiophene/n-dodecane (0.25 wt.%/99.75 wt.%).

data of 6FDA–3MPD/DABA 9:1 and 6FDA–4MPD/DABA 9:1 are within the range of error. Apparently, the differences between 4MPD and 3MPD in structure are not influencing the enrichment factor significantly. As also illustrated in Fig. 7, increasing fluxes are obtained for both polymer structures with rising temperature which is due to the increased motion of the polymer chains as well as the higher diffusion coefficients at higher temperatures. In addition, Fig. 7 shows the high dependency of the enrichment factor ˇ on the temperature. For the benzothiophene/n-docane mixture, increasing enrichment factors were observed with increasing operating temperature. This is quite unusual since most of the experimental results in aromatic/aliphatic separation confirm an increase in flux and a decrease in selectivity with increasing temperature. Thereby the aromatic compound, e.g. toluene, has a lower activation energy EJ than the aliphatic compound, e.g. hexane. Thus, the permeation of the aliphatic compound is affected more by the increase in temperature than the permeation of the aromatic compound, leading to its enhanced permeation and resulting in a decrease of selectivity [38,39]. Ren investigated the pervaporation characteristics of benzene/cyclohexane mixtures (50 wt.%/50 wt.%) with non-cross-linked 6FDA–4MPD/DABA 4:1 membranes in dependency to the temperature. With increasing temperature a decrease of the separation factor ˛ from 4.9 at 333 K to 3.1 at 383 K was observed. The calculated activation energies for benzene and cyclohexane were 27.3 and 35.8 kJ/mol, respectively [40]. In order to find the reason for the different behaviour of the enrichment factor for the benzothiophene/n-dodecane mixture investigated in this work, the EJ of these components were calculated. Thereby the corresponding normalized partial fluxes are plotted versus 1/T (Fig. 8). According to Eq. (5) the activation energies for the permeation of benzothiophene and n-dodecane can be calculated. The partial flux through the 6FDA–4MPD/DABA 9:1 membrane (batch 1) shows a typical linear temperature dependency on the Arrhenius-type plot [41]:

 J = J0 exp

−EJ RT

 (5)

with J0 describing the pre-exponential factor, EJ the activation energy of permeation (kJ/mol), R the gas constant (J/(mol K)) and T as the temperature (K). The activation energies of permeation are 53.9 kJ/mol for benzothiophene and 40.2 kJ/mol for n-dodecane through a 6FDA–4MPD/DABA 9:1 membrane. In contrast to the aforementioned studies, the aromatic compound has a higher EJ than the aliphatic compound. The higher EJ for benzothiophene is likely

due to its bulky and more rigid structure which leads to an increase of the enrichment factor with temperature as shown in Fig. 7. The results obtained in this work are in good agreement with data from Katarzynski [25], where the separation performances of various DABA containing 6FDA-copolyimides with mixtures from naphthalene (5 wt.%) and n-decane were investigated at different temperatures. That work showed that all investigated membrane materials also exhibited an enhanced enrichment of the bulky aromatic compound with increasing temperatures. In addition the exemplary calculation of the activation energy of a non-cross-linked 6FDA–4MPD/DABA 9:1 membrane showed higher activation energy for naphthalene than for ndecane. The normalized partial fluxes through the 6FDA–3MPD/DABA 9:1 did not show the expected linear temperature dependency. Thus, the calculated activation energies are afflicted with substantial errors and were not compared with the observed data obtained from the 6FDA–4MPD/DABA 9:1 membrane. However, the 6FDA–3MPD/DABA 9:1 membrane also showed higher activation energies for the aromatic compound as in the case of the 6FDA–4MPD/DABA 9:1 copolyimide. 3.4.3. Influence of permeate pressure It is well known that the permeate pressure is decisive for the separation performance [42]. However, in industrial applications for a very low permeate pressure a high energy input is required [43]. Thus the influence of the permeate pressure especially the comparison of separation characteristics at very low and moderate permeate pressure is an important topic. In order to investigate this, measurements with two different samples of 6FDA–3MPD/DABA 9:1 membranes were performed between 373 and 393 K at permeate pressures in the range of 19–25 and 39–45 mbar, respectively. As shown in Fig. 9, an increased permeate pressure results in lower enrichment factors ˇ whereas the fluxes did not change significantly. The increase of permeate pressure leads to a decline of separation performance by 15.6%. However, in order to obtain the highest possible enrichment factors, the desulphurization by means of pervaporation should be performed at permeate pressures as low as possible. 4. Conclusion The membrane materials were tested by differential thermal analysis (DTA), yielding a glass transition temperature of the 6FDA–4MPD/DABA 9:1 membrane of 698 K. Stress strain measurements with membranes pre-treated with benzothiophene/ndodecane (0.25 wt.%/99.75 wt.%) at 403 K showed that the

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membranes are still in the glassy state during pervaporation experiments. Swelling experiments in Jet A1 using different cross-linked and non-cross-linked 6FDA containing copolyimides were carried out. Non-cross-linked 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1 membranes showed the highest degrees of swelling in combination with adequate mechanical stability in benzothiophene/ndodecane (0.25 wt.%/99.75 wt.%) and Jet A1 at high temperatures. Pervaporation experiments were performed with a binary benzothiophene/n-dodecane (0.25 wt.%/99.75 wt.%) mixture and 6FDA–4MPD/DABA 9:1 and 6FDA–3MPD/DABA 9:1 membranes, respectively. The variation of diamines in the membrane material resulted in nearly the same pervaporation characteristics. At higher temperatures, an increase of flux due to increasing chain mobility and increased diffusion coefficients are observed for both polymer structures. At a temperature of 393 K a triplication of the weight percentages of benzothiophene in permeate was achieved compared to the weight percentages of benzothiophene in the feed for both the 6FDA–4MPD/DABA 9:1 and the 6FDA–3MPD/DABA 9:1 membrane. An increase of 20 mbar in permeate pressure resulted in nearly the same flux at slightly lower enrichment factors at a given temperature. In summary, the pervaporation experiments have shown that a significant enrichment of dinuclear aromatic sulphur compounds in combination with an adequate flux is possible. Acknowledgements This work was kindly supported by the DECHEMA (Max-Buchner scholarship (Abreicherung substituierter Thiophen-Derivate aus SAromaten/Aliphaten Mischungen MBFSt-Kennziffer 2755) and the Forschungszentrum Jülich, Institute for Energy Research. References [1] X. Ma, L. Sun, C. Song, A new approach to deep desulfurization of gasoline, diesel fuel and jet fuel by selective adsorption for ultra-clean fuels and for fuel cell application, Catal. Today 77 (2002) 107–116. [2] H.E.A. Brüschke, N. Wynn, J. Balko, Desulphurization of Gasoline by Pervaporation, Aachener Membrankolloquium 2006, Aachen, Germany, 2006. [3] L. Lin, Y. Kong, G. Wang, H. Qu, J. Yang, D. Shi, Selection and crosslinking modification of membrane material for FCC gasoline desulfurization, J. Membr. Sci. 285 (2006) 144–151. [4] Y. Kong, L. Lin, J. Yang, D. Shi, H. Qu, K. Xie, L. Li, FCC gasoline desulfurization by pervaporation: Effects of gasoline components, J. Membr. Sci. 293 (2007) 36–43. [5] R. Qi, C. Zhao, J. Li, Y. Wang, S. Zhu, Removal of thiophenes from noctane/thiophene mixtures by pervaporation, J. Membr. Sci. 269 (2006) 94–100. [6] R. Qi, Y. Wang, J. Li, C. Zhao, S. Zhu, Pervaporation separation of alkane/thiophene mixtures with PDMS membrane, J. Membr. Sci. 280 (2006) 545–552. [7] L. Wang, Z. Zhao, J. Li, C. Chen, Synthesis and characterization of fluorinated polyimides for pervaporation of n-heptane/thiophene mixtures, Eur. Polym. J. 42 (2006) 1266–1272. [8] R. Qi, Y. Wang, J. Li, S. Zhu, Sulfur removal from gasoline by pervaporation: the effect of hydrocarbon species, Sep. Purif. Technol. 51 (2006) 258–264. [9] R. Qi, Y. Wang, J. Chen, J. Li, S. Zhu, Removing thiophenes from n-octane using PDMS–AgY zeolite mixed matrix membranes, J. Membr. Sci. 295 (2007) 114–120. [10] J. Chen, J. Li, R. Qi, H. Ye, C. Chen, Pervaporation performance of crosslinked polydimethylsiloxane membranes for deep desulfurization of FCC gasoline. I. Effect of different sulfur species, J. Membr. Sci. 322 (2008) 113–121. [11] L. Lin, G. Wang, H. Qu, J. Yang, Y. Wang, D. Shi, Y. Kong, Pervaporation performance of crosslinked polyethylene glycol membranes for deep desulfurization of FCC gasoline, J. Membr. Sci. 280 (2006) 651–658. [12] L. Lin, Y. Kong, J. Yang, D. Shi, K. Xie, Y. Zhang, Scale-up of pervaporation for gasoline desulphurisation. Part 1. Simulation and design, J. Membr. Sci. 298 (2007) 1–13.

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