paraffin separation

Journal of Membrane Science 232 (2004) 107–114

Super selective membranes in gas–liquid membrane contactors for olefin/paraffin separation Kitty Nymeijer, Tymen Visser, Rijanne Assen, Matthias Wessling∗ Department of Science and Technology, Membrane Technology Group, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands Received 1 July 2003; accepted 10 October 2003

Abstract In the present paper, selective composite membranes containing sulfonated poly(ether ether ketone) (SPEEK) layers on top of a hydrophobic, polypropylene support are applied as absorber and desorber in a gas–liquid membrane contactor system for the separation of paraffins and olefins. The water present in the absorption liquid swells the hydrophilic polymer sufficiently, making the membranes olefin-selective. As a result, even at high liquid velocities where the membrane determines the selectivity of the process, high selectivities can be obtained in combination with high productivities. Continuous contact between the absorption silver nitrate solution and the SPEEK layer prevents the layer from drying out and subsequent loss of selectivity. Previously, unknown high ethylene/ethane selectivities (>2700) are obtained in combination with reasonable ethylene productivities (7.6 × 10−10 cm3 /cm2 s Pa (1 × 10−6 cm3 /cm2 s cmHg)). © 2003 Elsevier B.V. All rights reserved. Keywords: Olefin-selective membrane; SPEEK; Olefin/paraffin separation; Olefin–silver complexation; Gas–liquid membrane contactor

1. Introduction Olefin/paraffin separations are one of the most important separations in the petrochemical industry nowadays [1]. Traditional systems used for this separation, e.g. low temperature distillation and extractive distillation, are expensive, energy consuming and only attractive for streams containing high amounts of olefins. This provides an incentive to develop cost effective separations, e.g. membrane-based separations like gas–liquid membrane contactors. Gas–liquid membrane contactors offer a unique way to perform gas–liquid absorption processes in a controlled way: gas and liquid flow can be controlled independently, giving large operational flexibility as opposed to state-of-the-art packed column devices [2]. Separation of the olefin/paraffin mixture is achieved by the selective absorption of olefins in a concentrated silver salt solution, a separation based on the ability of silver ions to complexate with the double bond of the olefin [3]. Gas–liquid membrane contactors frequently suffer from wetting of the microporous membrane. Therefore, stabilization layers of highly gas permeable elastomeric materials at ∗

Corresponding author. Tel.: +31-53-4892950; fax: +31-53-4894611. E-mail address: [email protected] (M. Wessling).

0376-7388/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.memsci.2003.10.045

the liquid-side of the membrane can be applied and potentially prevent this wetting of the porous structure [4–9]. Shelekhin and Beckman [10] presented a theoretical model to describe gas transport phenomena in a membrane absorber having the configuration given in Fig. 1. The model assumes that mass transport consists of two separate steps: (1) Diffusion of the gaseous components through the membrane. The maximum amount of gas that can permeate through the membrane is given by the feed gas pressure and the productivity of the membrane. (2) Dissolution of the gaseous components into the absorption liquid. The maximum amount of gas that can dissolve in the liquid phase is given by the liquid flow rate and the maximum solubility of the gas in the liquid. According to this model there are two limiting cases for the olefin/paraffin separation factor obtainable in circulatory membrane absorbers: • At low liquid flow rates the selectivity is determined by the (high) selectivity of the silver nitrate solution, but the total productivity is low due to insufficient supply of absorption liquid.

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K. Nymeijer et al. / Journal of Membrane Science 232 (2004) 107–114 y = LM a + LMd Permeate

y = LMa

y = LM a Feed

Desorber

y= 0 Absorber

Reject

Absorbent

Fig. 1. Schematic picture of a membrane absorber used to describe gas transport. y, axis; LMa , length of membrane absorber, LMd , length of membrane desorber.

• At high liquid flow rates the productivity is higher, but the selectivity is determined by the (mostly lower) selectivity of the membrane. We hypothesize that a combination of high separation factors and high ethylene productivities is only obtainable if olefin-selective membranes are applied. Although efforts have been undertaken to develop nonporous, polyimidebased gas separation membranes for olefin/paraffin separation, the work of Staudt-Bickel and Koros [11] shows that the application of such membranes is not very attractive for industrial purposes because of the relatively low separation factors obtained. Other approaches to obtain olefin-selective membranes include the incorporation of silver salts into a polymeric matrix. Shukla and Peinemann [12] were one of the first who recognized the importance of the incorporation of silver ions into a polymer matrix as a method to enhance olefin transport and to obtain high olefin/paraffin separation factors. Composite membranes with Ag+ -Nafion top layers were prepared by dip coating. Single gas permeation experiments resulted in 1-butene productivities of 2.2 × 10−9 cm3 (STP)/cm2 s Pa (2.9 × 10−6 cm3 (STP)/cm2 s cmHg) and ideal selectivities >1000. 1-Butene productivity strongly increased with increasing AgBF4 :Nafion ratio, whereas n-butane showed the opposite behavior. Obviously, 1-butene mainly permeates through the AgBF4 phase, whereas n-butane mainly chooses the Nafion phase. Mixed gas permeation experiments showed a maximum selectivity of 129 for 1-butene (9%)/n-butane (91%). Pinnau and Toy [13] used a PEO matrix to incorporate AgBF4 , whereas Müller et al. [14] and Morisato et al. [15] described the use of nylon-12/tetramethylene oxide block copolymer (Pebax® 2533) as matrix material for AgClO4 and AgBF4 to separate paraffins and olefins. However, to obtain reasonable olefin productivities in combination with high selectivities, the addition of a swelling mediator (e.g. water vapor) to transport the silver–olefin complex is inevitable [14]. The work of Sungpet et al. [16] describes the incorporation of a silver salt into graft polymers prepared by pre-irradiation graft methods. Other approaches include the use of supported liquid membranes [17] and flowing liquid membranes [18]. Tsou et al. [19] investigated the silver-facilitated transport of ethylene in a liquid membrane contactor system. They

applied a supported liquid membrane in the absorption part of the contactor, whereas desorption is established using a flash pot swept with helium. Although separation factors as high as 300–400 are obtained, preparation into thin layers is a challenging, yet unsolved track. Furthermore, liquid membrane systems often suffer from stability problems and decrease in selectivity due to loss of the physically entrapped carrier solution. Cation-exchange membranes able to exchange H+ for Ag+ like sulfonated poly(ether ether ketone) (SPEEK) [20], sulfonated polysulfone [20,21], and Nafion® [22–24] are more stable since Ag+ is not only physically but also ionically bond to the sulfonated membranes, due to the exchange of H+ with Ag+ (Fig. 2). Fixation of the silver ions takes place via electrostatic forces between the negatively charged SO3 − groups and the positively charged Ag+ ions. Sungpet et al. [24] investigated the performance of Ag+ -form Nafion® 117 in ethylene/ethane separations. Separation factors as high as 350 in combination with reasonable permeabilities are obtained (1.6 × 10−10 cm3 cm/cm2 s Pa). However, although high olefin/paraffin selectivities are obtained, high olefin fluxes through these membranes are only observed when swollen or hydrated polymers are used which allow transport of the silver–olefin complex [20,22,24]. Unfortunately, also these cation-exchange membranes do not retain water sufficiently long resulting in a decrease in olefin fluxes and olefin/paraffin selectivity. This could only be prevented if water is added continuously to the feed mixture to substitute the loss from the membrane. However, in many olefin/paraffin separations water-free processing is a must. In the present work, composite hollow fiber membranes with olefin-selective stabilizing layers of sulfonated poly(ether ether ketone) are prepared by dip coating and applied in a gas–liquid membrane contactor system for the separation of paraffins and olefins using an aqueous silver nitrate solution as absorption liquid. The water present in the absorption liquid swells the polymer sufficiently and prevents drying out of the membrane. This means that even

C O

O

O

SO3

-Ag +

C

O

O

O

x

1-x

Fig. 2. Repeat unit of sulfonated poly(ether ether ketone) after exchange of H+ with Ag+ .

K. Nymeijer et al. / Journal of Membrane Science 232 (2004) 107–114

at high liquid velocities where the membrane determines the selectivity of the process, high selectivities may be obtained in combination with high productivities. The performance of the SPEEK-coated membranes is compared with the performance of hollow fiber composite membranes with a nonselective top layer of ethylene propylene diene terpolymer (EPDM), as presented in previous work [6]. Interaction between EPDM and silver ions, as occurs between the sulfonic acid groups of SPEEK and silver ions, does not take place in this case.

2. Materials and methods 2.1. Materials Poly(oxy-1,4-phenylene-oxy-1,4-phenylene-carbonyl-1, 4-phenylene), also called poly(ether ether ketone) or PEEK (specialty grade 450PF, from Victrex, UK) is used as precursor polymer for sulfonation. Concentrated H2 SO4 (95–98 wt.%, extra pure), methanol, sodium chloride and sodium hydroxide are purchased from Merck, The Netherlands, and used as received. Ultra pure water (resistivity >18.0 M cm at 298 K (25 ◦ C)) is used. The gas mixture used for membrane contactor experiments (ethane (20%)/ethylene (80%)) is obtained from Praxair, Belgium. Nitrogen (99.9%) is supplied by Hoekloos, The Netherlands. AgNO3 (analytical grade) is supplied by Merck, The Netherlands. All chemicals are used as received. 2.2. Polymer modification Sulfonated poly(ether ether ketone) is prepared by random sulfonation of poly(ether ether ketone) according to the method proposed by Shibuya and Porter [25] and Bailley et al. [26]. After drying in a vacuum oven at 303 K (30 ◦ C) for 24 h, 36.6 g PEEK is dissolved in 200 ml concentrated H2 SO4 at 343 K (70 ◦ C). During reaction, the solution is stirred and kept at the desired reaction temperature. To stop the reaction after 5 h, the reaction vessel is immersed in an ice bath. The polymer-sulfuric acid solution is precipitated in ultra pure water at 278 K (5 ◦ C) and washed until pH = 5. After washing the sulfonated polymer is dried in air over night on a glass plate and subsequently in a vacuum oven at 303 K (30 ◦ C) for 48 h. 2.3. Film formation and characterization Thin SPEEK films with an average thickness in the dry state of about 35 ␮m are prepared by solvent evaporation and characterized in terms of ion exchange capacity (IEC), degree of sulfonation (X), fixed charge density (cFIX ), water permeation and degree of swelling [27,28]. After swelling, the films are dried again in a vacuum oven of 303.15 K (30 ◦ C) for 72 h and the weight of the dry films is determined again. The difference between the dry weight of the

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Table 1 Characteristics of Accurel® S6/2 polypropylene hollow fibers used as support for composite membrane preparation Trade name Outer diameter (mm) Inner diameter (mm) Pore size (␮m) Overall porosity (%)

Accurel® S6/2 2.7 1.8 0.2 (nominal) 69

films before and after submersion accounts for the weight increase due to the exchange of H+ (Mw = 1 g/mol) with Ag+ (Mw = 108 g/mol). The difference between the weight of the swollen film and the weight of the film that is dried after submersion accounts for the real water uptake due to swelling. 2.4. Composite membrane and module preparation Composite hollow fiber membranes with a SPEEK top layer at the outside are prepared by dip coating. Accurel® S6/2 polypropylene hollow fibers are used as support material. Characteristics of these fibers, as given by the manufacturer, are presented in Table 1. Fibers used in the gas–liquid membrane contactor experiments are coated for four times using a 5 wt.% solution of SPEEK in methanol. The fibers are dried in air overnight after every coating step. Gas permeation experiments are performed at room temperature using nitrogen as feed gas to determine the quality of the composite fibers. It is assumed that the prepared composite fibers are defect free if the gas permeability of the fibers is too low to be determined accurately in the gas permeation set-up described in previous work [6]. Scanning electron microscopy (SEM) is used to investigate the morphology of the prepared fibers. Only defect-free membranes are used to prepare membrane modules. The module housing is made of PVC tubing. The fibers are potted in modules using an epoxy resin. Each module contains 10 fibers, resulting in an effective membrane area of 102 cm2 . Gas permeation experiments are performed using nitrogen as feed gas to test the quality of the modules and to determine if the modules are defect free. In order to estimate the counter permeation of water from the absorption liquid into the ethylene permeate stream, pervaporation experiments with the prepared modules are performed. A 3.5 M silver nitrate solution circulates at the outside at a flow rate of 50 ml/min, whereas nitrogen flows at the inside of the fibers with a flow rate of 80 ml/min and collects the water vapor. The permeate is collected in a cold trap cooled with liquid nitrogen. 2.5. Membrane contactor experiments Membrane contactor experiments are performed in the set-up described elsewhere [6]. A mixture of ethane (20%) and ethylene (80%) is fed to the absorption module at a pressure of 1 or 3 bars. The gas is fed to the inner side

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Table 2 Reaction conditions, ion exchange capacity (IEC), degree of sulfonation (X), fixed charge density (cFIX ), pervaporation characteristics, and swelling behavior of SPEEK Property

Value

Error

IEC (mol charge/kg dry polymer) X (–) cFIX (mol/l)

2.39 0.85 2.71

0.50 0.04 0.52

Permeability (Barrer) Water AgNO3(aq)

290000 120000

60000 30000

Swelling (%) Water AgNO3(aq) Real water uptake

88 51 11

21 10 3 Fig. 3. SEM picture of the cross-section of a composite hollow fiber membrane with a SPEEK top layer. Magnification: 2000×.

of the fibers because the outer side of the fibers is coated with SPEEK. The retentate and permeate stream are fed to a GC (Varian 3300) and the amount and composition of the streams are analyzed automatically. A 3.5 M solution of silver nitrate is used as absorption liquid. Liquid flow rates of 50–600 ml/min are applied, resulting in estimated Reynolds numbers between 19 and 224. Nitrogen is used as sweep gas in the desorption module. All experiments are performed at room temperature (298 K (25 ◦ C)).

3. Results and discussion 3.1. Polymer modification and film characterization The maximum degree of sulfonation is found to be 1.0 indicating that per repeat unit of polymer one hydrogen atom is replaced by a sulfon group [29]. Sulfonation only takes place on the phenyl ring flanked by two ether groups. Due to the electron-withdrawing effect of the carbonyl group, the other two phenyl rings are de-activated for electrophilic sulfonation. Higher degrees of sulfonation at the same phenyl ring are also not expected because of the electron-withdrawing effect of the introduced SO3 − group [25]. To be soluble in methanol, SPEEK with a sulfonation degree of at least 0.70 is required. Experimentally determined characteristics of the prepared polymer (films) are given in Table 2. SPEEK with a degree of sulfonation of 0.85 is prepared. Because the fixed charge density is lower than the concentration of the silver nitrate solution used in membrane contactor experiments (3.5 M), co-ion exclusion of NO3 − will be negligible in this application and both silver and nitrate ions can freely move in and out the SPEEK layer. For control purposes, the silver nitrate concentration in the permeate of pervaporation experiments is analyzed by AAS. It is negligibly low with a value of 3.7 × 10−4 ± 2.2 × 10−4 mol/l. Probably these small amounts of silver ions present are impurities due to handling.

3.2. Composite membrane and module preparation A SEM picture of the prepared fibers (cross-section) is given in Fig. 3. The thickness of the SPEEK layer is ±10 ␮m. Pore penetration of the coating layer into the porous support is not visible in this picture. Despite that, the adhesion between support and coating layer is good. 3.3. Membrane contactor experiments The prepared SPEEK membrane modules are used in a membrane contactor bench-scale process as absorption and desorption module for more than 10 weeks without any change in membrane performance or leakage of absorption liquid to the feed or sweep gas side. After 10 weeks, the modules are still defect free, as detected by gas permeation in the dry state. No visible interaction between SPEEK and silver ions occurred and the membranes still have the same color as before the experiments (no reduction of Ag+ ). The concentration or color of the silver nitrate solution used has not changed during the experiments as well. The water permeability of the SPEEK-coated membrane modules is determined by pervaporation before and after the contactor experiments. In both cases the same value of 1.6 × 10−7 cm3 /cm2 s Pa is found. This value is in the same order of magnitude as the value found for EPDM-coated membranes [6] and more than 10 times lower than the value determined by Bessarabov et al. [4] for PDMS-coated membranes. The performance of the prepared SPEEK membrane modules in a bench-scale membrane contactor process for olefin/paraffin separation is investigated and the results are compared to the corresponding results found for EPDM-coated membrane modules [6]. Fig. 4 gives the ethylene productivity for both SPEEK- and EPDM-coated membranes as a function of the flow rate of the absorption liquid. For EPDM one observes that the ethylene productivity has a maximum at 150 ml/min with increasing liquid flow rate. It is believed [6] that at low liquid flow rates, the

111

10000

80

2) SPEEK-SPEEK Minimum selectivity

2

Ethylene productivity (⋅10 cm /cm ⋅s⋅cm Hg)

K. Nymeijer et al. / Journal of Membrane Science 232 (2004) 107–114

1000

-7

3

Separation factor (-)

60

1) EPDM-EPDM

40

20

100

1) EPDM-EPDM

10

2) SPEEK-SPEEK 1

0 0

100

200

300

400

500

600

700

Liquid flow rate (ml/min)

Fig. 4. Ethylene productivity as a function of the absorption liquid flow rate for SPEEK- and EPDM-coated hollow fibers. Total feed pressure 3 bars. Experiments performed at T = 298 K (25 ◦ C). (1) Absorber: EPDM–desorber: EPDM; (2) absorber: SPEEK–desorber: SPEEK.

productivity is limited by the liquid phase boundary layer resistance and the supply of silver ions. At higher liquid flow rates, the productivity is limited by the reduced time for decomplexation and subsequent diffusion of ethylene to the membrane surface. Although SPEEK swells much more in the absorption liquid solution, it has a lower ethylene permeability than EPDM and no maximum in ethylene productivity is observed. Apparently, the supply of free ethylene at the membrane surface exceeds the maximum possible ethylene productivity through the membranes in the liquid flow rate regime investigated. The presence of a silver salt solution in the dense top layer can have various effects [12,13,30,31]: (1) It may reduce the solubility of the gases into the film due to the salting-out effect. (2) It may change the transport of gases through the swollen polymer film. (3) It might diminish the transport of ethane through the polymer film. (4) It can enhance and facilitate the transport of ethylene through the swollen polymer film. Two different mechanisms can play a role in this transport of ethylene: the Ag+ -olefin complex can freely diffuse through the swollen polymer and the ion-paired silver sites can pass olefins from one site to the next, using the ‘wagging’ motion of the polymer side chains to which the silver ions are ‘attached’ [30]. Fig. 5 gives the ethylene over ethane separation factor for both SPEEK and EPDM membrane modules. The ethane productivity for SPEEK-coated membranes is too low to analyze with the GC and the minimum selectivity is given by the lower detection limit of the analytical method to quantify the ethane concentration in the sweep gas. Hence we must

0

100

200

300

400

Liquid flow rate (ml/min)

Fig. 5. Ethylene over ethane separation factor as a function of the absorption liquid flow rate for SPEEK- and EPDM-coated hollow fibers. Total feed pressure 3 bars. Experiments performed at T = 298 K (25 ◦ C). (1) Absorber: EPDM–desorber: EPDM; (2) absorber: SPEEK–desorber: SPEEK.

conclude that within the experimental detection limits, we succeeded to prepare a membrane configuration permeating only ethylene. Because SPEEK-coated membranes are swollen and saturated with silver ions, they are highly olefin-selective, which results in extremely high selectivities (>2700, compared to a maximum value of 72 for EPDM-coated membranes) and high purities of the ethylene permeate (>99.99%), even at high liquid flow rates. The remarkably high selectivities obtained for SPEEKcoated membranes in membrane contactor experiments (Fig. 5) are either caused by a reduction in ethane productivity, by an increase in ethylene productivity or by a combination of both. However, it is currently impossible to accurately quantify a potential decrease in ethane permeability and a potential increase in ethylene permeability. Most probably both effects play an important role. Fig. 6 shows the effect of feed pressure on the ethylene productivity at various liquid flow rates for the systems SPEEK–SPEEK and EPDM–EPDM. In the case of EPDM membranes, the pressure normalized ethylene productivity is independent of the feed pressure [6], whereas in the case of SPEEK membranes the feed pressure strongly affects the pressure normalized ethylene productivity. This is due to a difference in transport mechanism between the EPDM and the SPEEK system. In the case of EPDM, the partial pressure of the gas in the liquid in the desorber is in equilibrium with the applied feed pressure and a three times increase in feed pressure therefore indicates at the same time a three times increase in driving force for permeation, resulting in a three times increase in ethylene flow but the same pressure normalized ethylene productivity [6]. For SPEEK-coated membranes, the driving force for ethylene permeation depends on the ethylene loading of the

K. Nymeijer et al. / Journal of Membrane Science 232 (2004) 107–114 10000

80

1) SPEEK-SPEEK

2

Ethylene productivity (⋅10 cm /cm ⋅s⋅cmHg)

112

3

Separation factor (-)

60

-7

1) EPDM-EPDM 3 bar (filled) and 1 bar (open) 40

2) SPEEK-SPEEK 1 bar

20

1000 2) EPDM-SPEEK 100 3) SPEEK-EPDM

10 4) EPDM-EPDM

3) SPEEK-SPEEK 3 bar 1

0 0

200

400

0

600

100

200

300

400

Liquid flow rate (ml/min)

Liquid flow rate (ml/min)

Fig. 8. Ethylene over ethane separation factor as a function of the absorption liquid flow rate. Total feed pressure 3 bars. T = 298 K (25 ◦ C). (1) Absorber: SPEEK–desorber: SPEEK; (2) absorber: EPDM–desorber: SPEEK; (3) absorber: SPEEK–desorber: EPDM; (4) absorber: EPDM–desorber: EPDM.

Fig. 6. Ethylene productivity as a function of the absorption liquid flow rate and the feed pressure for SPEEK- or EPDM-coated hollow fibers. Measurements performed at T = 298 K (25 ◦ C). (1) Absorber: EPDM–desorber: EPDM; (2) absorber: SPEEK–desorber: SPEEK; (3) absorber: SPEEK–desorber: SPEEK.

similar behavior. SPEEK as absorber and EPDM as desorber hardly has any effect on the productivity, compared to the combination SPEEK–SPEEK. On the other hand, if EPDM is used as absorber and SPEEK as desorber, the supply of ethylene in the absorber highly exceeds the removal of ethylene in the SPEEK desorber. In that case the desorber becomes the limiting part. This is visible in an increase in the ethylene productivity. The productivity of the combination EPDM–EPDM is of course even higher at the liquid flow rate range investigated because of the higher permeability of the EPDM desorber. Consequently, in the combination SPEEK–EPDM, the transport of ethylene through the SPEEK layer in the absorber limits the ultimate ethylene productivity that leaves the desorber, whereas in the combinations EPDM–EPDM and EPDM–SPEEK the desorber limits the productivity. In the combination SPEEK–SPEEK,

liquid. However, there exist a nonlinear relationship between the olefin partial feed pressure and the silver–ethylene complex concentration in the solution. This results in higher pressure normalized ethylene productivities at lower ethylene feed pressures. To increase productivity and at the same time keep high selectivity the performance of two mixed configurations is investigated: absorber: SPEEK, desorber: EPDM and absorber: EPDM, desorber: SPEEK. The results are compared to the previous described experiments using the same membranes for both absorber and desorber. Fig. 7 gives the ethylene (a) and ethane (b) productivity as a function of the liquid flow rate for these four situations. EPDM–EPDM and SPEEK–EPDM membranes show similar behavior concerning the ethylene productivity and SPEEK–SPEEK and EPDM–SPEEK membranes show

2

Ethylene productivity (⋅10 cm /cm ⋅s⋅cmHg)

a

b

70 60

3

6

50

-7

-7

3

2

Ethane productivity (⋅10 cm /cm ⋅s⋅cmHg)

8

4

1) EPDM-EPDM

2 3) SPEEK-EPDM

2) EPDM-SPEEK

1) EPDM-EPDM 40 30 2) EPDM-SPEEK

20

3) SPEEK-EPDM 10

4) SPEEK-SPEEK

0 0

100

200

300

Liquid flow rate (ml/min)

400

0

100

200

300

400

500

600

Liquid flow rate (ml/min)

Fig. 7. (a) Ethane and (b) ethylene productivity as a function of the absorption liquid flow rate. Total feed pressure 3 bars. T = 298 K (25 ◦ C). (1) Absorber: EPDM–desorber: EPDM; (2) absorber: EPDM–desorber: SPEEK; (3) absorber: SPEEK–desorber: EPDM; (4) absorber: SPEEK–desorber: SPEEK.

K. Nymeijer et al. / Journal of Membrane Science 232 (2004) 107–114 10000

1 2 3 4 5 6 7 8 9 10 11 12 K1 K2 K3 K4

K2

Separation factor (-)

1000 12

6 K1

100

7

3

5

9

K3

8

10

10

K4

2

4

11

1

1 0.01

0.1

1

10 -6

3

100

113

Staudt Bickel et al. [11] Sungpet et al. [16] Pinnau et al. [13] Morisato et al. [15] Müller et al. [14] Eriksen et al. [23] Tsou et al. [19] Bessarabov et al. [4] Teramoto et al. [17] Teramoto et al. [18] Creusen et al. [5] Sungpet et al. [24] This work, EPDM-SPEEK This work, SPEEK-SPEEK This work, EPDM-EPDM This work, EPDM-EPDM

1000

2

Productivity (⋅10 cm /cm ⋅s⋅cmHg)

Fig. 9. Survey of the results published in literature and described in the present work for ethane/ethylene separations.

both absorber and desorber probably determine the overall ethylene productivity. The ethane productivity increases with increasing liquid flow rate. Ethane productivities for SPEEK–EPDM and EPDM–SPEEK membranes are almost equal and it is difficult to distinguish between both, whereas the combination EPDM-EPDM gives a 10 times increase in ethane productivity due to the higher ethane permeability of EPDM compared to swollen SPEEK. The corresponding ethylene over ethane separation factors are given in Fig. 8 (the minimum separation factor for SPEEK–SPEEK is given by the analytical method to quantify the ethane concentration in the sweep gas). Extremely high separation factors are obtained with the combination SPEEK–SPEEK, although the productivity is low. The combination EPDM–SPEEK also results in very high separation factors (∼300) and purities of the ethylene permeate (>99.9%) in comparison to EPDM–EPDM and SPEEK–EPDM (15 and 4 times higher selectivities are obtained, respectively). If the results described in the present work are compared to the work described in literature (Fig. 9), it is obvious that previously unknown high ethylene/ethane selectivities are obtained if SPEEK-coated membranes are used in membrane contactor applications. This makes the application of olefin-selective membranes in a membrane contactor for the separation of paraffins and olefins very attractive. Corresponding productivities, however, are an order of magnitude lower than most productivities described in literature. However, taking into account the effective thickness of the prepared composite SPEEK membranes, it is obvious that the thickness of the membranes prepared in the present work is 2–6 times the thickness of the membranes described in literature [4,5,13,14]. So in order to increase productivity and at the same time keep selectivity, the application of thinner top layers of highly permeable, olefin-selective membrane materials is required.

4. Conclusions We succeeded in the preparation of composite hollow fiber membranes with a 10 ␮m thick, olefin-selective stabilizing layer of sulfonated poly(ether ether ketone) that, when applied in a gas–liquid membrane contactor for the separation of paraffins and olefins, combine previously unknown high selectivities (>2700) with reasonable good productivities (7.6 × 10−10 cm3 /cm2 s Pa (1 × 10−6 cm3 /cm2 s cmHg)). The water present in the absorption liquid swells the polymer sufficiently, allowing the silver salt to partition into the top layer and making it olefin-selective and preventing drying out of the membrane. Fixation of the silver ions takes place via electrostatic forces between the negatively charged SO3 − groups and the positively charged Ag+ ions. Membrane performance did not change during a period of 10 weeks and significant leakage of absorption liquid to the feed or sweep gas side was not observed.

Acknowledgements This research was financially supported by the Centre for Separation Technology, a cooperation between the University of Twente, The Netherlands, and TNO, The Netherlands Organisation for Applied Scientific Research.

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