water mixtures through silicalite membrane

water mixtures through silicalite membrane

Desalination 234 (2008) 286–292 Pervaporation of ketone/water mixtures through silicalite membrane Xiangshu Chena, Xiao Linb, Pei Chenc, Hidetoshi Ki...

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Desalination 234 (2008) 286–292

Pervaporation of ketone/water mixtures through silicalite membrane Xiangshu Chena, Xiao Linb, Pei Chenc, Hidetoshi Kitac* a

College of Chemistry and Chemical Engineering, Jingxi Normal University, Nanchang 330027, P.R. China b State Key Laboratory of Materials-oriented Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P.R. China c Environmental Science and Engineering, Graduate School of Science and Engineering, Yamaguchi University, Ube, Yamaguchi 755-8611, Japan Tel. þ81-836-859661; Fax þ81-836-859601; email: [email protected] Received 31 July 2007; accepted revised 25 September 2007

Abstract Silicalite membranes were prepared by in situ crystallization on the outer surface of porous tubular mullite supports with tetraethoxysilane (TEOS) and tetrapropylammonium hydroxide (TPAOH) as silica source and organic structure directing agent, respectively. The outer-surface of the porous support was completely covered with randomly oriented, intergrown silicalite crystals and the thickness of the dense intermediate layer was estimated about 5–10 mm, judging from the scanning electron microscopy observation. Pervaporation of organic/water mixtures through silicalite membranes was investigated. The silicalite membranes showed high organic selectivity from organic/water mixtures. For example, the highest acetone/water and methyl ethyl ketone (MEK) /water separation factors of 934 and 32,000 with fluxes of 0.20 and 0.25 kg m2 h for a feed of 1 wt.% acetone and 5 wt.% MEK at 30 C, respectively, were obtained for the silicalite membrane prepared at 185 C for 40 h of hydrothermal treatment. An n-butane/i-butane ideal separation factor of 73 at 200 C and at pressure difference of 101 kPa also indicated that the silicalite membrane prepared under such synthesis conditions had fewer defects. Keywords: Organic/water mixture; Pervaporation separation; Zeolite membrane; Silicalite

1. Introduction Pervaporation (PV) is an attractive process for removal of organics from water, water from *Corresponding author.

organics, and for organic–organic mixture separations, especially when an azeotrope is involved. PV can have advantages over distillation for separation of liquid mixtures for its lower energy consumption [1]. PV is proposed to be one of

Presented at the Fourth Conference of Aseanian Membrane Society (AMS 4), 16–18 August 2007, Taipei, Taiwan. 0011-9164/08/$– See front matter # 2008 Published by Elsevier B.V. doi:10.1016/j.desal.2007.09.096

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the most challenging techniques for continuous organic component recovery from its aqueous solution. Silicalite membranes may be good candidates for such applications. Sano et al. [2,3] studied PV of alcohol/water mixtures through silicalite membranes prepared on stainless steel and -Al2O3 flat disks and obtained separation factors of 60–120 for feed solutions of 5 vol% alcohol. They also investigated PV separation of acetone/water and MEK/water mixtures [4]. The highest acetone/water and methyl ethyl ketone (MEK)/water seperation factors of 106 and 266 with fluxes of 1.06 and 0.41 kg/m2 h for a feed of 5 wt.% acetone and 5 wt.% MEK at 30 C, respectively, were obtained for the silicalite membrane prepared on flat stainless steel disks. Liu et al. also reported their separation results for acetone/water and MEK/water mixtures through stainless steel supported silicalite membranes [5]. Tuan et al. studied boron isomorphously substituted MFI zeolite membrane (B-ZSM-5) for PV separation of organic/water mixtures [6]. For a feed of 5 wt.% acetone/water mixtures, they obtained the separation factors varying from 200 to 440 with total fluxes 0.04–0.16 kg m2 h, respectively, in the PV temperature range from room temperature to 60 C. For a 5 wt.% MEK feed concentration, separation factors lied in 220–380 range with the PV temperature varying from room temperature to 60 C. Both of the separation factor and the flux increased with feed temperature. In 1993 [7], we first developed a seeding method to prepare zeolite NaA membranes on tubular -Al2O3 supports and first presented their excellent PV properties. Now, a tubular type dehydration module of NaA zeolite membrane is on sale [8]. Up to date now, this seeding method has been successfully adopted to synthesize not only NaA membranes [9,10], but also zeolite X [11], Y [12] and T [13] membranes. In addition, silicalite membranes with high PV performance had also been prepared with this

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seeding method [14,15]. The quality of silicalite membranes was further improved by in situ crystallization with colloidal silica (CS) [16]. One objective of this study is to obtain a better understanding on the effect of synthesis conditions, especially silica sources and supports, on a membrane quality. Another objective is to investigate the PV separation of acetone/water and MEK/water mixtures through silicalite membranes.

2. Experimental 2.1. Silicalite membrane preparation Silicalite membranes were prepared by in situ crystallization on the outer surface of the porous tubular supports with 10 cm in length. Two types of porous tubes were used as supports: mullite tube (Nikkato Corp, 12 mm outer diameter, 1.5 mm thickness, 1.0 mm average pore size). -Al2O3 tube (Mitsui Grinding Wheel Corp, 9.2 mm outer diameter, 1.2 mm thickness, 1.0 mm average pore size). Most of the silicalite membranes were prepared onto mullite tubes, while a few of them were synthesized on -Al2O3 tubes in order to study the effect of supports on the membrane performance. The synthesis solution with the molar ratio of 1SiO2: 0.17TPAOH: 120 H2O was prepared by first mixing tetrapropylammonium hydroxide (TPAOH, 20–25 wt.% in water, Tokyo Kasei) with distilled water, then adding tetraethoxysilane (TEOS, 98 wt.%, Shin-etsu Chem. Ind. Co.) into the above solution. The resultant solution was initially turbid. After about 1 h aging at room temperature under stirring, the mixture became clear. The 200 g of the clear sol was poured into a stainless steel autoclave. Two support tubes were vertically placed in the autoclave. The autoclave was moved into convection oven preheated to the synthesis temperature of 185 C. After hydrothermal synthesis for a given

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time, the autoclave was removed from the oven and cooled to room temperature. The samples were collected, washed thoroughly with distilled water and dried at room temperature and then 100 C in the oven each for several hours. Finally, they were calcined at 500 C in air for 10 h at a heating and cooling rate of 0.15 C min1 and 0.25 C min1, respectively. 2.2. Characterization and permeation experiments SEM observation of the membrane surface and cross section was carried out using field emission scanning electron microscopy (FE-SEM, JEOL JSM 6335F). Pervaporation experiments were carried out with a batch system as described elsewhere and illustrated in Fig. 1 [15]. The inside of the membrane tube was evacuated by a vacuum pump. The permeate vapor was collected by a cold trap cooled with liquid nitrogen. The downstream pressure was maintained below 13.3 Pa. The effective membrane areas were about 28 cm2 (mullite) and 22 cm2 (-Al2O3). The amount of the feed solution was about 1100 g. During PV, a proper amount of organic component was added to the feed solution at intervals of 30 min to keep the feed concentration constant due to the loss of organic by high flux of the silicalite membranes. The compositions of the feed and permeate were analyzed by a gas chromatograph Pirani gauge N2 Line Vacuum pump Cold trap Heater heater

Cold trap

Feed solution Water bath

Membrane stirrer Stirrer

Fig. 1. Schematic illustration of a batch PV system.

(GC, Shimadzu GC8A) equipped with 3 m column packed with polyethylene glycol-1000. The flux was calculated by weighing the condensed permeate. The separation factor was determined as A=B ¼ ðYA =YB Þ=ðXA =XB Þ; where XA, XB, YA and YB denote the mass fractions of components A (organic) and B (water) in the feed and the permeate sides, respectively. Single-component gas permeation experiments were carried out for n-C4H10 and i-C4H10 gases at pressure difference of 101 kPa and at 50–200 C by means of a vacuum method described as Cui et al. [17].

3. Results and discussion 3.1. Membrane preparation and characterization Fig. 2 shows FE-SEM surface and crosssectional views of the membrane prepared on an unseeded mullite tube at 185 C for 40 h of hydrothermal treatment. After hydrothermal treatment, the unseeded mullite tube was fully covered with randomly intergrown silicalite crystals. The particles in the surface layer were hexagonal and their sizes were in the range of 15–20 mm. The thickness of the top surface layer was estimated to be 15–20 mm. It was also found that silicalite crystals were formed inside the support. It was very difficult to determine the interface between the mullite support and the silicalite crystal layer because of not only the very rough surface of the support, but also formation of the silicalite crystals inside the support. The thickness of the dense intermediate layer was estimated to be 5–10 mm, which indicated a good interaction (adherence) was present between the top surface layer and the support. In order to explore the optimal synthesis condition for silicalite membrane preparation, the

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Table 1 PV performance through silicalite membranes prepared by in situ crystallization for different hydrothermal treatment times at 185 C (5 wt.% acetone/water mixture at 60 C) No.

Support

Synth. time (h)

Flux (kg m2 h)

Sep. factor

M1 M2 M3 M4 M5

Mullite Mullite Mullite Mullite -Al2O3

24 30 40 60 40

1.36 1.11 1.30a 1.38 2.47b

290 410 500a 360 207b

a

Average value of two membranes. Average value of three membranes.

b

Fig. 2. FE-SEM views of (A) surface and (B) crosssection of silicalite membrane prepared on mullite support at 185 C for 40 h.

effect of synthesis time on the membrane PV performance was examined. Table 1 shows PV separation performance for a feed of 5 wt.% acetone/water mixture at 60 C through silicalite membranes prepared on the mullite tubes at 185 C as a function of the different synthesis times. Obviously, 30–40 h of hydrothermal treatment was suitable for synthesis of high-quality silicalite membranes. The PV performance of the silicalite membranes prepared on the unseeded -Al2O3 tubes for 40 h of hydrothermal treatment is also shown in Table 1. The membranes prepared on the unseeded -Al2O3 tubes displayed the lower

separation factor than those prepared on the mullite tubes. This phenomenon is consistent with that in our previous study [16]. As expected, the framework of MFI grown on porous alumina supports could contain Si–O–Al instead of full Si– O–Si bond due to the leach of Al from the Al2O3 support under the harsh synthesis condition (at 185 C for 40 h, and in the alkali media). The presence of aluminum increased the hydrophilicity of the membranes formed on the unseeded -Al2O3 tubes, so that the membranes had the lower separation factor than those formed on the unseeded mullite tubes. Therefore, the mullite tubes are more suitable for preparation of silicalite membranes with the higher separation factor. As shown in Table 1, the membranes prepared on the mullite support tubes for 40 h of hydrothermal treatment exhibited the higher separation factor of 460–540, along with the total flux of 1.43–1.16 kg m2 h. Therefore, the following silicalite membranes were all prepared under such synthesis conditions for study on reproducibility of silicalite membrane synthesis. Then, the averaged acetone concentration in the permeate side was 95.3 wt.%, the separation factor 400, the total flux 1.26 kg m2 h at 5 wt.% acetone feed solution and 60 C. The corresponding relative

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3.2. Pervaporation performance 3.2.1. Temperature dependence of PV performance Figs. 3 and 4 show the temperature dependence of PV separation performance through the silicalite membrane for PV of 5 wt.% acetone/ water and 5 wt.% MEK/water mixtures, respectively. With increasing temperature, both of the 1.2 800

0.8

600

0.6 400 0.4 200

Separation factor

Flux (kg/m2 h)

1

0.2 0

25

35

45

55

65

0

Temperature (°C)

Fig. 3. Temperature dependence of the flux and the separation factor through the silicalite membrane for pervaporation of 5 wt.% acetone/water mixtures. Note: (1) ( ) and (^) represented the flux of acetone and the separation factor, respectively; (2) ( ) represented the flux of water.





105

0.8

0.6

104

0.4

0.2

0

25

35

45

55

65

Separation factor

standard deviations of the acetone permeate concentration, total flux and separation factor were only 0.71, 17.8 and 14.3%, respectively, which suggested these different membranes had a uniform masses and thickness of the crystal layers. They also revealed that the preparation of the silicalite membranes was highly reproducible in the term of their separation performance. The membrane with the highest separation performance (the total flux of 1.16 kg m2 h and the separation factor of 540) was then used for study on dependence of PV performance on organic feed concentration, operational temperature, and time.

Flux (kg/m2 h)

290

103

Temperature (°C)

Fig. 4. Temperature dependence of the flux and the separation factor through the silicalite membrane for pervaporation of 5 wt.% MEK/water mixtures. Note: (1) ( ) and (^) represented the flux of MEK and the separation factor, respectively; (2) ( ) represented the flux of water.





water and organics fluxes increased, while the separation factors decreased for both of the mixtures. At 30 C for a feed concentration of 5 wt.% acetone/water mixtures (Fig. 3), the membrane had the separation factor of 801 with the total flux of 0.52 kg m2 h. On the other hand, at 30 C for a feed concentration of 5 wt.% MEK/ water mixtures (Fig. 4), the highest separation factor (>30,000) was obtained. It is indicated that the silicalite membrane was more effective for removing MEK from aqueous solutions than for acetone. The performance of the silicalite membrane synthesized in this work is much better than any other membrane reported in PV permeation [4–6,18–20]. It is known that the overall selectivity of a PV process is controlled by mobility difference and sorption selectivity [21,22]. For silicalite membranes, the preferential adsorption of acetone and MEK presumably determines the PV performance [20]. The permeation of water is hindered by the presence of acetone and MEK. The kinetic diameter of MEK is 0.504 nm, larger than that of acetone (0.470 nm). MEK more effectively blocked the permeation of water than that for acetone. Hence, the higher separation factor

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could be obtained in the MEK/water system than that in acetone/water mixtures. On the other hand, the apparent activation energies of acetone, MEK and water were 22, 30 and 34–64 kJ mol1, respectively. The increasing PV temperature was more favorable for water permeation than for acetone and MEK permeation. Therefore, the separation factors decreased with increasing temperature. 3.2.2. Feed concentration dependence of PV performance Fig. 5 shows the acetone feed concentration dependence of PV performance through the silicalite membrane. As the acetone feed concentration increased, the separation factor decreased, while the total flux increased. The separation factor as high as 934 was obtained at the acetone feed concentration of 1 wt.%. In the low concentration region of 1–10 wt.% acetone, both the separation factor and the total flux changed rapidly. The highest total flux of 0.65 kg m2 h was obtained at the highest feed concentration of 20 wt.% acetone/water mixture. 0.8

1000

800

600 0.4 400

Separation factor

Flux (kg/m2 h)

0.6

0.2 200

0

0

5

10

15

20

25

0

Acetone (wt.%)

Fig. 5. Acetone feed concentration dependence of the flux and the separation factor for silicalite membrane at 30 C. Note: Open and close symbol represented flux and separation factor, respectively.

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Table 2 Pure gas permeation results of n-C4H10 and i-C4H10 through the silicalite membrane Temperature Gas permeance  (K) 1010(mol m2 s Pa)

323 373 423 473

n-butane

i-butane

714.3 2040.8 2449.0 1802.7

6.1 8.2 18.0 24.5

Ideal separation factor (n-C4/i-C4) 120 250 140 74

3.2.3. Gas permeation Single gas permeation of n-C4H10 and i-C4H10 was measured as a function of temperature at a pressure difference of 101 kPa to evaluate the membrane quality. Vroon et al. [23] suggested that high-quality, defect-free MFI membranes can be defined as those with an ideal separation factor greater than 10 for n-C4H10 over i-C4H10 at 200 C. Table 2 shows gas permeation results for our silicalite membrane. It can be seen that the membrane had high ideal separation factors for n-butane over i-butane, even at 200 C (ideal separation factor ¼ 74), which was much higher than the ideal separation factor of 10. They were also higher than those for our previously reported silicalite membranes [15,16]. All results indicate that silicalite membranes with fewer defects should have high PV performance. 4. Conclusions Silicalite membranes prepared on the outer surface of porous tubular supports by in situ crystallization preferentially permeated organics in PV of organics/water mixtures. The silicalite membranes showed high acetone and MEK selectivities. Gas permeation of n-C4H10 and i-C4H10 also indicated that the silicalite membrane prepared under such synthesis conditions had fewer defects.

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Acknowledgement This work was partly supported by a Grant-inAid for Scientific Research from Japan Society for the Promotion of Science (No. 18360377).

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