NaY형 제올라이트 멤브레인의 헥산 이성질체 투과특성

화학공학의 이론과 응용 제8권 제1호 2002년

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

정병헌, 하세가와 야수히사, 소토와 켄이치로, 쿠사카베 카츠키, 모로오카 시게하루

일본 큐슈대학 대학원 응용화학과

Permeation Properties of Hexane Isomers through an NaY-Type Zeolite Membrane

Byeong-Heon Jeong, Yasuhisa Hasegawa, Ken-Ichiro Sotowa,

Katsuki Kusakabe, Shigeharu Morooka

Dept. of Applied Chemistry, Graduate School of Engineering,

Kyushu University, Fukuoka 812-8581, Japan

Introduction

Inorganic membranes are stable in the presence of organic solvents, which often cause swelling of polymeric membranes. Thus, inorganic membranes are highly appropriate for the separation of organic compounds. Many investigators have attempted to separate hydrocarbons using MFI-type zeolite membranes. Gade et al. [1] investigated the permeation properties of n-hexane (n-C6) and 2,2-dimethylbutane (DMB) through B-ZSM-5 zeolite membranes. n-C6 permeance was found to be high, and DMB permeance was low. The separation was based on the selective adsorption of n-C6. In contrast, Flanders et al. [2] reported that the separation between n-C6 and DMB through H-ZSM-5 zeolite membranes was based on molecular sieving.

A few studies on the permeation properties of hydrocarbons using FAU-type zeolite (NaX and NaY) membranes have been published. Nikolakis et al. [3] and Jeong et al. [4] measured permeances to benzene and cyclohexane and obtained a high separation factor for benzene (B) over cyclohexane (C) using NaX- and NaY-type zeolite membranes, respectively. To our knowledge, no studies on the permeation of branched hydrocarbons through FAU-type zeolite membranes have been reported to date. Thus, an investigation of the permeation properties of branched hydrocarbons would be relevant for further optimization of the permeation properties of FAU-type zeolite membranes.

In the present study, therefore, the vapor permeation properties of an NaY-type zeolite membrane were investigated. Permeation experiments were performed with single-component n-C6, 3-methylpentane (MP) and DMB, binary mixtures of n-C6 with MP or DMB. The permeation and separation mechanisms were discussed on the basis of adsorption properties.

Experimental Section

The procedures used for the membrane preparation, as well as the permeation and adsorption tests, are the same as those reported in a previous paper [4]. An NaY-type zeolite membrane was prepared on the outer surface of a porous alumina support tube (NOK Corp., Japan) by means of hydrothermal synthesis. For implantation of seeds for nucleation, the outer surface of the support tube was rubbed with NaY zeolite crystals (Tosoh Corp., #HSM-320NAA, Si/Al = 2.8, crystal size = 0.5 □m). The support was then placed in a tubular autoclave, which was maintained in a horizontal position. An aqueous solution of water glass, sodium aluminate, and sodium hydroxide was homogenized by stirring at room temperature for 240 min. The initial composition was Al2O3:SiO2:Na2O:H2O = 1:12.8:17:975 on a molar basis. The hydrothermal synthesis was performed at 363 K for 24 h. The outer surface of the synthesized NaY-type zeolite membrane, except for the permeating portion, was sealed with a polytetrafluorocarbon tube. An NaY-type zeolite powder was also prepared by the same procedure as that used for the membrane.

The vapor permeation properties of the membrane were determined using single- and binary-component hexane isomers at permeation temperatures of 358-413 K. The hydrocarbon vapor was diluted in a flow of nitrogen and introduced onto the feed side. Nitrogen was also used as the sweep on the permeate side. The total pressure on both sides was maintained at 101.3 kPa throughout the experiments. The permeance to permeant i, □i, is defined as moles of the component permeated per unit time and unit membrane area divided by the partial pressure difference between the feed and permeate sides. The separation factor, □(i/j), is defined as the ratio of the permeances, i/j.

Adsorption properties of single- and binary-component hexane isomers were determined for the NaY-type zeolite powder at 373 K.

Results

Fig.1 shows the permeances to single-component MP and DMB as a function of the permeation temperature. The partial pressure on the feed side was 9.5 kPa for MP and 7.3 kPa for DMB. The sweep flow rate was maintained at 10 ccm [cm3(STP)/min], and the partial pressure on the permeate side was 0.22 kPa for MP and 0.06 kPa for DMB. The partial pressures on the permeate side were much lower than the partial pressures on the feed side. The permeances to branched hexanes were much lower than the permeance to n-C6 over the entire temperature range. All of the permeances increased with increasing temperature, indicating that an activated transport mechanism was operative.

Figs.2 and 3 show the permeances and separation factors for binary systems of n-C6/MP and n-C6/DMB, respectively, as a function of the permeation temperature. The sweep flow rate was also maintained at 10 ccm. Partial pressures for the mixture of n-C6/MP were 3.9 kPa for n-C6 and 3.7 kPa for MP on the feed side and 0.05 kPa for n-C6 and 0.04 kPa for MP on the permeate side. Similarly, the partial pressures for the mixture of n-C6/DMB were 3.5 kPa for n-C6 and 3.3 kPa for DMB on the feed side and 0.03 kPa for n-C6 and 0.02 kPa for DMB on the permeate side.

Fig.1. Permeances to single-component       Fig.2. Permeances and separation factors

hexane isomers as a function of             for a binary system composed of

the temperature.                           n-C6 and MP.

Fig.3. Permeances and separation factors        Fig.4. Adsorption isotherms for single-

for a binary system composed of               ponent hexane isomers on the

n-C6 and DMB.                              NaY-type zeolite at 373 K.

The partial pressures on the permeate side were also much lower than the partial pressures on the feed side. For the mixtures, the permeances to branched hexanes were the same as the value determined for the single-component systems. However, the n-C6 permeance for the mixtures decreased to the values of branched hexanes. Furthermore, the permeances to all components were not significantly affected by the permeation temperature.

Fig.4 shows the adsorption isotherms for single-component hexane isomers (n-C6, MP, and DMB) on the NaY-type zeolite at 373 K. For all components, adsorbed amounts were similar to each other, and the adsorption increased with increasing pressure.

화학공학의 이론과 응용 제8권 제1호 2002년

Discussion

For the permeation of single-component systems, the permeances through both of the FAU- and MFI-type zeolite membranes were in the order of n-C6 >> MP > DMB and were in good agreement with the extent of chain branching (i.e., molecular width) of the components as shown in Table 1. The permeants n-C6, MP, and DMB were similar in adsorptivity and different in diffusivity. Thus, the higher permeance of n-C6 can be ascribed to its larger diffusivity. Millot et al. [5] examined the diffusion of n-C6 and MP in ZSM-5-supported membranes based on the Maxwell-Stefan equations and calculated the diffusion coefficients and diffusion activation energies. The diffusion coefficients were determined to be 1.0 x 10-11 and 5.0 x 10-14 m2/s for n-C6 and MP, respectively, at 373 K. The diffusion coefficient decreased with increasing molecular size because large molecules interact more strongly with segments of the zeolites than small molecules. The activation energies for the diffusion of n-C6 and MP were 34.7 and 50.0 kJ/mol, respectively. Thus, larger molecules diffuse through the zeolite pores at lower rates than small-sized molecules.

The n-C6 permeance through the FAU-type zeolite membrane was similar to that through the MFI-type zeolite membrane. However, the permeances to branched hexanes through the FAU-type zeolite membrane were 20-100 times higher than those through the MFI-type zeolite membrane. This behavior appears to be due to the fact that the size of branched hexane molecules is similar to or larger than the MFI-type zeolite pore size (=0.55 nm), whereas the size of branched hexane molecules is much smaller than the pore size of the FAU-type zeolite (=0.74 nm). The adsorption of hexane isomers on an NaY-type zeolite was similar, as shown in Fig.4. For the single-component permeation, therefore, the permeation mechanism of hexane isomers through the FAU-type zeolite membrane appears to be based on the differences in diffusion rates in the zeolite channels.

The FAU-type zeolite membrane was unsuitable for use in the separation of n-C6 from binary mixtures with MP or DMB, as shown in Table 1. For binary systems composed of n-C6/MP and n-C6/DMB, the branched hexanes inhibited the permeation of n-C6 and the permeance to n-C6 was equal to that of the branched hexanes, although the diffusivity of n-C6 was much higher than that of branched hexanes for single-component systems. Thus, the slowly permeating species restrained the diffusion of the other species. This behavior appears to be due to the single-file transport mechanism. Meanwhile, the MFI-type zeolite membrane was highly selective for n-C6 permeation in the mixtures. The molecular diameter of DMB (=0.62 nm) is significantly larger than the pore size of the MFI zeolite (=0.55 nm), whereas the molecular diameter of MP (=0.5 nm) is approximately the same as the pore size of the MFI zeolite. The differences in separation of n-C6 from branched hexanes between the FAU- and MFI-type zeolite membranes appear to be due to differences in the zeolite structures or the orientation of the polycrystalline layers.

Table 1. Permeances properties through FAU- and MFI-type zeolite membranes at 373 K.

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Permeance x 108 [mol m-2 s-1 Pa-1]

---------------------------------------------------------      Separation

Hydrocarbon   Width   Length         FAU               MFI*        factor,□(n-C6/i)

(i)         [nm]    [nm]     Single    Binary     Single    Binary    FAU  MFI*

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n-C6	0.43     1.02     25.0	10.0

MP	0.50     0.90      2.7	n-C6: 1.9     0.1    n-C6: 3.6     1.3  >500

MP: 1.5	MP: 0.07

DMB	0.62     0.78      1.2    n-C6: 1.9     0.01   n-C6: 8.0     1.9  >1000

DMB: 1.0	DMB: 0.07

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*MFI data are taken from Funke et al. [6]

References

[1] Gade, S. K.; Tuan, V. A.; Gump, C. J.; Noble, R. D.; Falconer, J. L., Chem. Commun., 2001, 601.

[2] Flanders, C. L.; Tuan, V. A.; Noble, R. D.; Falconer, J. L., J. Membr. Sci., 2000, 176, 43.

[3] Nikolakis, V.; Xomeritakis, G.; Abibi, A.; Dickson, M.; Tsapatsis, M.; Vlachos, D. G., J. Membr. Sci., 2001, 184, 209.

[4] Jeong, B.-H.; Hasegawa, Y.; Kusakabe, K.; Morooka, S., Sep. Sci. Technol., 2002, 37, 1225.

[5] Millot, B.; M□thivier, A.; Jobic, H.; Moueddeb, H.; Dalmon, J. A., Microporous Mesoprous Mater., 2000, 38, 85.

[6] Funke, H. H.; Argo, A. M.; Falconer, J. L.; Noble, R. D. Ind. Eng. Chem. Res. 1997, 36, 137.