polyethersulfone composite membranes for FCC gasoline desulphurization by pervaporation

European Polymer Journal 44 (2008) 3335–3343

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Studies on polyethylene glycol/polyethersulfone composite membranes for FCC gasoline desulphurization by pervaporation Ying Kong a,b,*, Ligang Lin b, Yuzhong Zhang a, Fuwei Lu b, Kekun Xie b, Rongkun Liu b, Lei Guo b, Shuai Shao b, Jinrong Yang b, Deqing Shi b a b

Key Lab of Hollow Fibre Membrane Materials & Membrane Process, Tianjin Polytechnic University, Tianjin 300160, PR China State Key Laboratory of Heavy Oil Processing, China University of Petroleum (East China), Dongying 257061, PR China

a r t i c l e

i n f o

Article history: Received 29 February 2008 Received in revised form 25 June 2008 Accepted 22 July 2008 Available online 29 July 2008

Keywords: Gasoline desulphurization Composite membrane Pervaporation PEG/PES Pre-wetting

a b s t r a c t A polyethylene glycol (PEG)/polyethersulfone (PES) composite membrane that can be applied on a commercial (or scale up) plant for fluid catalytic cracking (FCC) gasoline desulphurization was prepared through pre-wetting combined with double-layer coating methodology. Preparation methodology, morphologies characterization and performance test for the composite membranes were conducted. The results indicated that the pre-wetting method effectively confined the intrusion of PEG solution to porous PES support layer in coating process. The composite membrane had a clear-cut boundary surface between the dense active layer and the porous support layer, which was examined by scanning electron microscope (SEM). Pervaporation (PV) experiments indicated that the membrane, with the crosslinking agent amount of 17% and solids content in active layer solution of 16%, had a stable performance for FCC desulphurization. The sulphur enrichment factor came to 3.63, and the total permeation flux was 3.37 kg/m2 h. It was found that the PV performance of the composite membrane changed slightly when the thickness of active layer varied from 4.25 lm to 33.26 lm. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction Governments in the world have made strict regulations to restrict the sulphur content of gasoline due to environmental concerns. Reducing the sulphur content of gasoline is an irreversible tendency for refiners worldwide [1–3]. Application of membrane technology in the field of gasoline desulphurization has attracted increasing attentions during recent years. The components in gasoline have different solution and diffusion capability through polymeric membrane, which can be used to fulfill the desulphurization task and obtain gasoline with lower sulphur content. The sulphur components have stronger affinity with polymeric membrane, so they dissolve and diffuse through the

* Corresponding author. Address: Key Lab of Hollow Fibre Membrane Materials & Membrane Process, Tianjin Polytechnic University, Tianjin 300160, PR China. Tel.: +86 546 8391029; fax: +86 546 8391971. E-mail address: [email protected] (Y. Kong). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.07.034

membrane faster than the hydrocarbon components in gasoline. As a result, the sulphur deficient retentive fractions enter into the gasoline pool directly, and the sulphur enriched permeate fractions need further treatment by conventional processes [4]. Membrane process offers a number of potential advantages compared to the conventional desulphurization technologies such as hydrotreating process (HDS), including low energy consumption, simplicity in design and operation, little reduction of octane number and easily combined with other technologies [5]. Membrane is the core of the membrane technology for deep desulphurization of FCC gasoline. The chemical property of membrane material and the physical structure of membrane both affect the membrane performance and process results considerably. Most of the existing commercial plants of pervaporation (PV) process worldwide adopt composite membranes, which comprise a dense active layer and a porous support layer. The two layers can be prepared using different materials, besides, through their

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appropriate combination and interaction, the selectivity and permeate flux of composite membrane can be enhanced simultaneously. Moreover, composite membrane is better than homogeneous membrane in mechanical property, corrosion resistance property and swelling resistance property, etc [6]. Considerable patents and articles about the membrane process for FCC gasoline desulphurization have been published since 2002. Grace Davision Company of Germany invented the S-Brane process and constructed an S-Brane Demonstration Plant [7–9]; TransIonics Company of America used the TransSep-S process to remove sulphur components from gasoline by PV process [10]. ExxonMobil Research and Engineering Company of America prepared PVP/PVDF and CTA/PVDF composite membranes for gasoline desulphurization [11], however, the detailed processes were not reported in their patents. The PV performance of polydimethyl siloxane/polyacrylonitrile (PDMS/PAN) composite membranes was reported [12,13] and only simple model compounds were discussed. Therefore, the detailed research about composite membranes using in FCC gasoline desulphurization is quite insufficient. A series of composite membranes were prepared in this study using PEG as active layer and PES as support layer. Preparation methodology, morphologies characterization and performance test for the composite membranes were conducted. And various factors that influenced the membrane performance were discussed. 2. Experimental 2.1. Materials PEG and PES were purchased from the Shanghai Reagent Factory. Gasoline feed was obtained from Shengli Refinery (China), and alkali cleaning process had been carried out for the gasoline used in this study. Other chemicals used were of analytical reagent (A.R.) grade from Tianjin Chemical Reagent Factory and used without further purification.

was filtrated and degassed before cast onto the non-woven fabrics. The support layer was obtained by means of the phaseinversion process on a DKN-40 membrane-coating device, as is shown in Fig. 1. The non-woven fabrics went through the bottom of the casting knife at a certain speed. And the PES solution prepared beforehand was cast onto the nonwoven fabrics uniformly by casting knife. Then the nonwoven fabrics with PES solution went into the coagulation bath containing deionized water, where the PES support layer was formed through a phase separation process. Finally, the support layer was collected on a roller and certain after-treatment was carried out. The thickness of the support layer can be controlled by adjusting the space between non-woven fabrics and casting knife. The overall processes were conducted in a chamber with constant humidity and temperature. 2.2.2. Coating of PEG active layer The PEG (20,000) materials mixed with maleic anhydride as crosslinking agent and trimethylamine as catalyst dissolved in acetone to form a homogenous solution at room temperature. Then the solution was filtrated and degassed before preparing composite membrane. The pre-wetting method was conducted in this study, namely immersed the PES support layer prepared beforehand in glycerol aqueous solution with the glycerol concentration of 25% three days before coating the PEG active layer. Then the PES support was attached to a glass plate and the latter was put in a horizontal position. Excess glycerol solution was wiped off quickly with a filter paper. And then the PEG solution was cast onto the PES support layer using a casting knife. The same procedure was carried out in an hour to form a double active layer composite membrane. The membrane was placed in an oven at 363 K for 60 min to crosslink and to evaporate the solvent, and then it was dipped in water for 48 h to remove the glycerol in the PES support layer. Finally the composite membrane was dried at room temperature in vacuum drying oven for 24 h. All membrane samples were stored in dust-free and dry environment before used in the pervaporation experiments.

2.2. Preparation of composite membrane 2.3. Characterization of the composite membrane by SEM 2.2.1. Preparation of PES support layer The PES materials were placed in an oven at 373 K for 24 h to be dehumidified. And then the dried PES materials mixed with PEG (600) as additive dissolved in N,N-dimethylacetamide (DMAC) to form a homogenous solution of 15 wt.% PES at room temperature. Finally, the solution

The dried composite membrane was cryogenically fractured in liquid nitrogen, and the membrane samples were then coated with gold. The cross-sectional morphologies of the composite membranes were examined with a XL-30 scanning electron microscope (SEM).

Fig. 1. Scheme of DKN-40 membrane-coating device.

Y. Kong et al. / European Polymer Journal 44 (2008) 3335–3343

2.4. Pervaporation experiments The pervaporation performance of the composite membranes was tested on an apparatus whose scheme is shown in Fig. 2. Reference [14] has given more detailed introduction on the apparatus. Permeation flux and sulphur enrichment factor are the typical performance parameters involved, which will be discussed later. And the stability of the membranes under various operation conditions was investigated too. The gasoline in feed tank was heated to constant temperature before entering the PV cell by high-performance oven. The PV cell contains an effective membrane area of 20.45 cm2. Pervaporation experiments were carried out by maintaining on one side of the membrane (feed) at atmospheric pressure and on the other side (permeated) a reduced pressure of not more than 35 mmHg. The permeated vapor was collected in liquid nitrogen trap. And the total sulphur content of feed and permeated samples was analyzed by Micro-Coulometric Analysis Instrument (Jiangsu, China). The total permeation flux, J, at a steady state was obtained by

J¼

M ðkg=m2 hÞ At

ð1Þ

where, M is the total mass permeated during the experimental time interval, t, at steady state and A is the effective membrane area. The sulphur enrichment factor, E, was defined as

E¼

CP CF

ð2Þ

where, CF and CP are the total sulphur content of feed and permeated samples. 3. Results and discussion 3.1. The morphologies of the composite membranes The composite membrane is composed of dense active layer (PEG) and the porous support layer (PES). Fig. 3 shows the morphologies of the composite membrane.

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Fig. 3a and b are the SEM photographs of PES support layer with the magnification of 1000 and 2500, respectively. From those photographs, it is observed that the main structure of PES support layer is composed of irregular loose macropores, whereas the surface is micropore that tightly arranged. Moreover, the non-woven fabrics can be observed clearly in Fig. 3a. As was stated in the Section 2.2.2, the composite membrane was prepared by the coating of PEG active layer onto the PES support layer. Fig. 3c and d are the SEM photographs of composite membrane with the magnification of 1000 and 2500, respectively. As we can see, the composite membrane has a clear-cut boundary surface between the dense active layer and the porous support layer, and the active layer has a thickness of about 16 lm. 3.2. Prevention of pore penetration The intrusion of active layer solution into porous support layer during preparation of composite membrane is defined as pore penetration. Slight pore penetration can enhance the adhesion between active layer and support layer. However, the mass transfer resistance increases remarkably due to pore penetration and the permeation flux through composite membrane decreases, which is unwanted. The pre-wetting method [15] was conducted to confine pore penetration in this study, namely immersed the PES support layer prepared beforehand in glycerol solution three days before casting the PEG active layer. Since the pores of the support layer were filled with glycerol solution, the intrusion of PEG solution during coating process was restricted. Fig. 4 shows the different morphologies of composite membranes when the pre-wetting method was used or unused in membrane preparation process. The pre-wetting method was carried out for the membrane shown in Fig. 4a. From Fig. 4a, obviously, the active layer is distinguishable from the support layer and little intrusion occurred. While in Fig. 4b, the PEG solution was cast onto the dried support layer directly. It is observed that the active layer solution had penetrated into the macropore of support layer. The two layers have a blurry boundary surface. The results fully demonstrate that the

Fig. 2. Scheme of experimental apparatus for pervaporation.

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Fig. 3. Morphologies of the support layer and the composite membrane.

Fig. 4. Morphologies of the composite membranes: (a) pre-wetting; (b) no pre-wetting.

pre-wetting method can confine pore penetration effectively. The pre-wetting method prevented the intrusion of active layer solution into the macropore of support layer. However, the micropore that composing the surface of support layer was hardly to be filled with glycerol solution completely, so the active layer solution would clog the micropore, which can not be observed directly from the SEM photographs. The solids content of active layer solution was enhanced to further reduce pore penetration in this study. Solids content means the

ratio of solute mass (including PEG (20,000) material, crosslinking agent and catalyst) to the total solution mass. The viscosity of the solution increased with the enhancement of solids content, and the intrusion speed of the PEG solution into support layer decreased remarkably [16]. On the other hand, the solution viscosity cannot be too high to form a uniform film. A series of composite membranes with different solids content of PEG solution were prepared by the same procedures in this study. Their SEM photographs are shown in Fig. 5.

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Though pore penetration cannot be observed directly from Fig. 5, it can be revealed by the various thicknesses of active layer for the composite membranes with different solids content of PEG solution. The data of thickness of active layer for the composite membranes in Table 1 were based on considerable measurements from the SEM photographs (Fig. 5). When the solids content of active layer came to 20%, the solution had a high viscosity and was barely to be coated on the support layer uniformly. That yielded a low performance composite membrane, but pore penetration could be prevented as mentioned above. The factor M was defined to estimate the pore penetration degree of the composite membranes. And the factor M of the membrane with 20% solids content was assumed zero as it is so small. For other membranes, the factor M were calculated by

M ¼1

L 20:75  Sð120%Þ 20%ð1SÞ

ð3Þ

where, L is the active layer thickness (lm), S is the solids content of active layer solution. From Eq. (3), when S varies among 10%, 13% and 16%, the factor M is 53.92%, 34.69% and 1.96%, respectively. As we can see, when the solids content of active layer is 16%, the pore penetration occurs at a low level and the viscosity of the solution is appropriate for coating process too. The solids content was fixed on 16% in this study if no special clarification. The PV performance of the composite membranes with different solids content was tested, and the results are

Table 1 The thickness of active layers with different solids content Solids content (S) (wt.%)

Thickness of active layer (L) (lm)

10 13 16 20

4.25 8.10 15.50 20.75

shown in Fig. 6. The PV experiments were conducted at the feed flow rate of 1.2 L/h, feed temperature of 373 K and feed sulphur content of 900 lg/g in this study if no special clarification. From Fig. 6, the permeation flux increased firstly and then decreased with the enhancement of solids content, and met its peak value at solids content of 16%. This is well conformed to the discussion about pore penetration. However, the sulphur enrichment factor had a peak value at solids content of 13% due to the trade-off [17] between permeation flux and sulphur enrichment factor, which means that the increase of permeation flux will bring the decrease of sulphur enrichment factor under some conditions and vice versa. 3.3. The performance of PEG/PES composite membrane 3.3.1. Effect of crosslinking agent amount The separation of organics was frequently involved in the application of PV process, and many membranes

Fig. 5. Morphologies of the composite membranes with different solids content of active layer: (a) 10%; (b) 13%; (c) 16%; (d) 20%.

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3.5

4.5

Enrichment(E)

2.5 2.0

3.5

1.5

E 3.0

J

Flux(J(Kg/m2•h))

3.0 4.0

1.0

2.5

0.5 8

10

12

14

16

18

20

22

Amount of solid content(%) Fig. 6. The effect of solids content on flux and sulphur enrichment factor.

swelled dramatically in organics solution to give large flux but little or no transport selectivity [18–20]. Using crosslinked polymers in the preparation of membranes is an effective method to reduce membrane swelling [21–23]. As for the desulphurization process, the gasoline feed is a complicated mixture with strong swelling effect to the membranes. Hence, crosslinking agent was added into the active layer solution in this study to form reticular spatial structure, and to improve the swelling resistance of membranes in gasoline. The PV performance of the membranes with different crosslinking agent amounts were investigated and the results are shown in Fig. 7. From Fig. 7, the sulphur enrichment factor increased and the permeation flux decreased with the enhancement of crosslinking agent amount. When the crosslinking agent was added to the PEG solution, the chemical connection occurred between macromolecules and reticular spatial structure formed. So the mobility of macromolecules and chain segment weakened with the interchain free volume lessened, which was unfavorable for the permeation of micromolecules. The phenomena were increasingly remarkable with the addition of crosslinking agent, which accounted for the decrease of permeation flux. Although

5.5

the permeation flux of sulphur and hydrocarbon components in gasoline both decreased, the sulphur components decreased more slowly due to their stronger affinity to membrane. Hence, the sulphur enrichment factor increased with the addition of crosslinking agent. The results further demonstrate the trade-off between sulphur enrichment factor and permeation flux. The two parameters can rarely achieve their peak values at the same composite membrane, and the optimization should be conducted in practical application according to the separation task. From Fig. 7, when the amount of crosslinking agent was up to 25%, the permeation flux decreased to less than 0.6 kg/ m2 h, which was unwanted for the scale up of membrane technology. Generally speaking, it is preferable when the permeation flux and sulphur enrichment factor both present high levels. Hence, the crosslinking agent amount was fixed on 17% with the permeation flux of 3.37 kg/m2 h and the sulphur enrichment factor of 3.63 in this study if no special clarification. The stability of composite membrane is an important aspect for practical application. In this study, the long time (500 h) PV experiments were conducted to test the stability of composite membranes with the crosslinking agent amounts of 17% and 8%, respectively. And the results are shown in Figs. 8–11. It is observed from Figs. 8–11 that the PV performance of the composite membranes presented a process of achieving steady step by step since the components in gasoline had a solution and diffusion equilibrium process through polymeric membranes. The sulphur enrichment factor increased firstly and then decreased, while the permeation flux increased gradually in the process, and then both of them became steady. When the crosslinking agent amount was 17%, the PV performance of the composite membrane achieved steady in 6 h at the beginning of experiments and changed slightly at the following 500 h test. However, when the crosslinking agent amount was decreased to 8%, the equilibrium process was shortened to 3 h, but the permeation flux increased while the sulphur enrichment factor decreased obviously after 100 h in the experiments. The phenomenon can be explained as fol-

4.0

4.5

8.0

4.0

4.0

3.5 2.0

Enrichment(E)

4.5

4.0

6.0

J

Flux(J(Kg/m2•h))

Enrichment(E)

E

3.0 2.5

3.5 2.0 E

1.5

3.0

Flux(J(Kg/m2•h))

3.5 5.0

J

3.0

1.0

2.5

0.0 5

10

15

20

25

30

35

Amount of crosslinking agent(%) Fig. 7. The effect of crosslinking agent amount on flux and sulphur enrichment factor.

2.5

0.5 0

5

10

15

20

25

Operation time/h Fig. 8. The stability test of composite membranes (24 h, 17% crosslinking agent).

4.0

4.0

3.8

3.8

3.6

3.6

3.4

3.4

3.2

E

50

150

250

350

3.2

J

3.0 450

Flux(J(Kg/m2•h))

Enrichment(E)

Y. Kong et al. / European Polymer Journal 44 (2008) 3335–3343

3.0 550

Operation time/h Fig. 9. The stability test of composite membranes (500 h, 17% crosslinking agent).

10.0

3.0

8.0

2.6 6.0 2.4

E J

4.0

2.2

2.0

Flux(J(Kg/m2•h))

Enrichment(E)

2.8

2.0 0

5

10

15

20

25

Operation time/h Fig. 10. The stability test of composite membranes (24 h, 8% crosslinking agent).

12.0

2.5

10.0 E J

2.0

8.0

1.5 50

150

250

350

450

Flux(J(Kg/m2•h))

Enrichment(E)

3.0

6.0 550

Operation time/h Fig. 11. The stability test of composite membranes (500 h, 8% crosslinking agent).

lows, the macromolecules of the active layer had a low crosslinking degree when the crosslinking agent amount

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was 8%, and there were excessive interchain free volume so that the membrane was swelled by gasoline after a period of PV experiment. Thus both the sulphur and hydrocarbon components permeated the membrane easily, and the membrane had high permeation flux but low sulphur enrichment factor. According to the experiments, conclusions can be reached that the membrane with crosslinking agent amount of 17% has high stability and present the potential of being applied on a scale up plant. 3.3.2. Effect of active layer thickness It is accepted that composite membranes have a much thinner active layer compared to homogenous membranes with the same selectivity, which conduce to the increase of permeation flux [24]. A series of composite membranes with different active layer thickness were prepared using different casting knives and their PV performance were tested in this study. The results are shown in Table 2. From Table 2, the permeation flux merely reduced by 1.76% and the sulphur enrichment factor increased by 0.55% when the active layer thickness increased by 680% (from 4.25 lm to 33.26 lm). Considering the effect of experimental error, one can conclude that the permeation flux and sulphur enrichment factor retain the same values when the active layer thickness increase. The results are quite different from the views of other researchers that flux is inversely proportional to the thickness of active layer and thinner active layer will bring higher permeation flux [25,26]. In the PV experiments, it was found that the equilibrium process mentioned above was prolonged with the increase of active layer thickness as shown in Fig. 12. Based on the results of experiments, we tentatively put forward that the PV performance of the composite membrane is chiefly determined by the crosslinking degree of active layer in this study, and the increase of active layer thickness has little effect on flux and sulphur enrichment factor but only prolongs the equilibrium process of the composite membrane. Further more, surface deficiency occurred more frequently on the composite membrane with ultrathin active layer, which led to an unstable PV performance or even the gasoline leaked through the membrane with huge flux but no selectivity. To summarize, the active layer of the composite membrane should maintain a proper thickness for practical application. The double-layer coating methodology mentioned above was conducted to prepare composite membranes with no surface deficiency and the active layer thickness was 10-20 lm in this study. 3.3.3. Effect of feed sulphur content level The PV experiments with gasoline of different sulphur contents were conducted to further investigate the desulphurization performance of the composite membrane. And the results shown in Table 3 can be applied as the fundamental data for the design of commercial plant. From Table 3, a higher sulphur content of gasoline feed resulted in a higher flux but a lower sulphur enrichment factor. When the sulphur content in feed increased, an extensive swelling of the membrane occurred due to the strong affinity of sulphur components to the membrane. Consequently, both the sulphur and hydrocarbon components permeated the membrane easily, and the membrane

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Table 2 The PV performance of composite membranes with different active layer thickness Active layer thickness (lm)

Sulphur enrichment factor

Permeation flux (kg/m2 h)

4.25 10.45 15.5 21.28 26.32 33.26

3.62 3.63 3.63 3.63 3.64 3.64

3.41 3.37 3.37 3.37 3.35 3.35

Equilibrium time(t/h)

40

30 t

20

10

0 0

10

20

30

40

Active layer thickness(µm) Fig. 12. The equilibrium time of composite membranes with different active layer thickness.

The pre-wetting method combined with the increase of solids content in active layer solution were conducted in this study. The pore penetration in coating process was confined effectively, and the permeation flux increased remarkably. When the solids content of active layer was 16%, the pore penetration occurred at a low level and the viscosity of the solution was appropriate for coating process too, which was well conformed to the results of PV experiments. The sulphur enrichment factor increased and the permeation flux decreased with the enhancement of crosslinking agent amount. Considering the requirement of practical application, the crosslinking agent amount was fixed on 17% with the permeation flux of 3.37 kg/m2 h and the sulphur enrichment factor of 3.63. And the PV performance of the composite membrane changed slightly at the long time operation (500 h) test. It was found that the increase of active layer thickness had little effect on flux and sulphur enrichment factor but only prolonged the equilibrium process of composite membranes. The double-layer coating methodology was conducted to prepare composite membrane with no surface deficiency and the active layer thickness was 10–20 lm. Further researches should be conducted to enhance the desulphurization performance of composite membrane for the gasoline with ultrahigh sulphur content. Acknowledgements

Table 3 Effect of feed sulphur content level Feed type

Sulphur content of feed (lg/g)

Sulphur enrichment factor E

Flux J (kg/m2 h)

Gasoline-1 Gasoline-2 Gasoline-3

220 900 1580

6.84 3.63 2.50

1.06 3.37 4.11

The authors gratefully acknowledge support of this work by Program for New Century Excellent Talents in University (NCET-06-0605) of Ministry of Education of China. This project was supported by China Postdoctoral Science Foundation (No. 20070420119). References

had high permeation flux but low sulphur enrichment factor. The conclusion corresponded to our previous work [27] about the influence of feed sulphur content level on PV performance for the model gasoline with known concentration (ppm) of sulphur species. Further researches should be conducted to enhance the desulphurization performance of composite membrane for the gasoline with ultrahigh sulphur content. 4. Conclusions A PEG/PES composite membrane that can be applied on a commercial (or scale up) plant for FCC gasoline desulphurization was prepared through double-layer coating methodology. Preparation methodology, morphologies characterization and performance test for the composite membranes were conducted. The composite membrane had a clear-cut boundary surface between the dense active layer and the porous support layer, which was examined by SEM.

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