Carbon dioxide separation from nitrogen using Y-type zeolite membranes

Carbon dioxide separation from nitrogen using Y-type zeolite membranes

T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Scien...

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T. Inui, M. Anpo, K. Izui, S. Yanagida, T. Yamaguchi (Editors) Advances in Chemical Conversions for Mitigating Carbon Dioxide Studies in Surface Science and Catalysis, Vol. 114 9 1998 Elsevier Science B.V. All rights reserved.

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C a r b o n d i o x i d e s e p a r a t i o n f r o m nitrogen using Y - t y p e zeolite m e m b r a n e s Shigeharu Morooka, Takahiro Kuroda and Katsuki Kusakabe Department of Materials Physics and Chemistry, Graduate School of Engineering, Kyushu University, Fukuoka 812-81, Japan

A polycrystalline Y-type zeolite membrane was formed by hydrothermal synthesis on the outer surface of a porous a - a l u m i n a support tube, which was polished with a finely powdered X-type zeolite for use as seeds. When an equimolar mixture of CO 2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the singlecomponent system, and the N 2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. At 30~ the permeance of CO 2 was higher than 107 mol.m-2.s-l.pa-1, and the permselectivity of CO 2 to N 2 was 20-100. 1. I N T R O D U C T I O N Carbon dioxide is the major compound of greenhouse gases the emission of which should be reduced. Membrane technology is one of the most promising methods for this purpose since it may be able to recover CO 2 at elevated temperatures without losing sensible heat [1-4]. In this study, Y-type zeolite membranes were developed, and their CO2-selective permeation was evaluated. 2. E X P E R I M E N T A L A porous a-alumina tube (2.8 mm o.d. and 1.9 mm i.d.) with an average pore size of 150-170 nm was used as the support of a zeolite membrane. Each support tube was cut to a length of 3 and 20 cm, and the outer surface of the tube was rubbed with NaX zeolite particles of 7 0 - 8 0 gm to implant crystal fragments as nucleation sites. Water glass, sodium aluminate and NaOH were dissolved in distilled water (A1203:SiOz:Na20:H20 1"10:14:798 in a molar basis). The support tube was fixed vertically in a 40 ml Tefloncoated autoclave containing the solution, and hydrothermal synthesis was carried out at 90~ for 6-24 h. After synthesis, the tubes were washed thoroughly with distilled water and dried. Each end of the membrane was then connected to a stainless steel tube with epoxy resin, and permeance was measured at 30-130~ Helium was used as the sweep gas on the permeate side, and ambient pressure was maintained on both sides of the membrane. The partial pressure of each permeant on the permeate side was maintained at less than 10 kPa by dilution with the sweep gas. Permeance was determined for singlecomponent and mixed gases, and selectivity was defined by the ratio of permeances. 3. RESULTS AND DISCUSSION 3.1. Formation of membranes Without implanting seeds, no continuous film was formed even after a 24 h syn-

666 thesis. When seed particles were implanted, however, a continuous layer of zeolite was formed on the outer surface of the tube as shown in Figures 1 (a) and (b). There are two zones of zeolite in the fractured section. The top layer (I) is composed of zeolite polycrystals, and the inner layer (II) is the or-alumina support whose macropores are filled with deposits. The layer of the or-alumina support is the white part below layer I in Figure 1 (b). The crystal size and top layer thickness increased with increasing reaction time. Since the inside of the tube was not rubbed with the zeolite crystals, no continuous film was formed. Membranes were characterized by X-ray diffraction. The XRD pattern of a membrane formed after 12 h was similar to that of the purchased NaX-type zeolite particles. Crystals recovered from the bottom of the reactor showed the same XRD pattern.

3.2. Permeation properties Permeation was determined with single-component gases and equimolar mixed gases. After the membrane was air dried, it was fixed in the permeation test unit. The He carrier was introduced in the permeate side, the temperature was then raised to 130~ in 1 h, and permeating gases were introduced to the feed side. Measurement was started after stabilizing the flow system for 6 h and was completed in about 10 h The temperature was then decreased to 80~ in 1 h, and the measurement was repeated. The temperature was further lowered to 30~ and the procedure was again repeated. As indicated in Figure 2,

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Figure 1. Top (a) and fractured (b) surfaces of a Y-type zeolite membrane.

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667 the permeance of CO 2 was of the order of 10 -7 mol.m-2.s-l-pa -1, which was equivalent to the permeances of H 2 and Ar through an MFI-type membrane reported by Bai et al. [5]. Permeances to CO 2 and N 2 were weakly depended on the CO 2 partial pressure. The Y-type membrane in the present study was unique in that the N 2 permeance was greatly retarded when the mixture of CO z and N 2 was fed at 30~ The permeance to N 2 for single component system was (1-2)x10-8 mol.m-2.s-l.pa -1, and that for mixed gas system was 5x10-10 mol.m-2.s-l.pa-1. The permeance to CH 4 was also greatly decreased when the CO2-CH 4 mixture was fed. The activation energies for permeances to CO2, CH 4 and N 2 were positive. Li and Hwang [6] reported that the activation energy for CO 2 permeance through macropores was negative. Thus, the permeation mechanism of the Ytype membrane is different from that of macroporous membranes. In order to determine the reproducibility of membrane formation, nine membranes of 3 cm length were formed under the same conditions except for reaction time. One membrane was accidentally fractured during the setup procedure. The others obeyed the same relationship between selectivity and permeance as indicated in Figures 3 (a) and (b). The CO2/N 2 selectivity was decreased when the CO 2 permeance exceeded 10 -6 mol-m -2. s-l.pa-1. Figure 4 shows the effect of membrane length on the CO2/N 2 selectivity at 30~ The membranes of 3 cm length showed higher selectivities than those of 20 cm length, but the effect of the membrane length was not serious. These results suggest that the reproducibility of membrane formation was maintained in the present experiment. It was questioned if permeances were affected by desorption of water and adsorption of impurities during the permeation test. Thus, COg and N 2 permeances were determined as a function of time. An air-dried membrane was placed in the permeation test unit, the temperature was maintained at 30~ for 30 min, and an equimolar CO2-N e mixture was introduced. The CO 2 and N 2 permeances increased by the desorption of water in the initial stage of the measurement, and then gradually decreased. However, the CO2/N2 selectivity did not greatly change with time, ranging from 50 at zero time to 75 after 15 h.

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668 The free aperture of the main 100 channels in Y-type zeolite is 0.74 nm [7] and is much larger than the diameter of CO2 and Nz molecules. If the concentrations of CO z and N 2 in the micropores of > the Y-type zeolite membrane are equal to o those in the outside gas phase, these 10 t/) molecules permeate through the membrane r at a low C O z / N 2 selectivity. However, this Z Membrane length o,! was not the case. Carbon dioxide moleO o 3cm cules adsorbed on the outside of the o membrane migrate into micropores by sur9 20cm 1 ........ I ....... face diffusion. Nitrogen molecules, which are not adsorptive, penetrate into micro10-7 10-6 10-5 pores by translation-collision mechanism CO 2 permeance [mol.m-2-s-l-pa -1] from the outside gas phase. The mouth of the micropores may Figure 4. Effect of membrane length be narrowed by adsorbed CO 2 molecules, on separation performance at 30~ which block N 2 molecules from entering the pores. Furthermore, C O 2 diffuses in pores of the zeolite membrane at a faster rate than N z. This selection mechanism is plausible for micropores with a width of up to six molecules [7]. When CO 2 molecules are strongly adsorbed on the pore wall, the CO 2 permeation rate will be low even if CO 2 is concentrated in the pore. If the pore size is close to the size of molecules, CO z molecules cannot pass N 2 molecules. Thus, balances between pore size and molecule size and between adsorptivity and mobility as well as difference in polarity of competitive species are important to attain both high permeance and selectivity. '

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4. CONCLUSIONS A Y-type zeolite membrane was formed on a porous a - a l u m i n a support tube. The membranes produced separated CO 2 from N2 at a permeance of the order of 10 -7 mol.m-2-s-l.pa-1 and a selectivity of 20-100 at 30~ This rapid and selective permeation was due to the pore-size controlled adsorption. REFERENCES 1. 2. 3. 4. 5. 6. 7.

J.-i. Hayashi, H. Mizuta, M. Yamamoto, K. Kusakabe and S. Morooka, Ind. Eng. Chem. Res., 35 (1996) 4176. B.-K. Sca, M. Watanabe, K. Kusakabe, S. Morooka and S.-S. Kim, Gas Sep. Purif., 10 (1996) 187. K. Kusakabe, S. Yoneshige, A. Murata and S. Morooka, J. Memb. Sci., 116 (1996) 39. K. Kusakabe, T. Kuroda, A. Murata and S. Morooka, Ind. Eng. Chem. Res., 36 (1997) 649. C. Bai, M.-D. Jia, J.L. Falconer and R.D. Noble, J. Memb. Sci., 105 (1995) 79. D. Li and S.-T. Hwang, J. Memb. Sci., 66 (1992) 119. D.W. Breck, Zeolite Molecular Sieves, John Wiley & Sons, New York, 1974.