Synthesis and Permeation Properties of a DDR-Type Zeolite

Sep 13, 2007 - ... Tomita, Kenji Suzuki, Kunio Nakayama, Kenji Yajima, and Shuichi Yoshida ... Center, NGK Insulators, Ltd., 2-56 Suda-cho, Mizuho-ku,...
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Ind. Eng. Chem. Res. 2007, 46, 6989-6997

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Synthesis and Permeation Properties of a DDR-Type Zeolite Membrane for Separation of CO2/CH4 Gaseous Mixtures Shuji Himeno* Department of CiVil and EnVironmental Engineering, Nagaoka UniVersity of Technology, 1603-1, Kamitomioka-cho, Nagaoka, Niigata 940-2188, Japan

Toshihiro Tomita, Kenji Suzuki, Kunio Nakayama, Kenji Yajima, and Shuichi Yoshida NCM Project, New Products DeVelopment Center, NGK Insulators, Ltd., 2-56 Suda-cho, Mizuho-ku, Nagoya 467-8530, Japan

Using hydrothermal synthesis, a highly hydrophobic DDR-type zeolite membrane was prepared on the outer surface of a porous R-alumina tube. The results of this study show that it is useful for CO2/CH4 separation. Single-gas permeation for CO2, CH4, H2, He, N2, and O2, and CO2/CH4 binary gas permeation, were measured, respectively, at pressures up to 5 and 2 MPa. The respective single-gas permeances of CO2 and CH4 at 298 K at a feed pressure of 0.2 MPa and a permeate pressure of 0.1 MPa were 4.2 × 10-7 and 1.2 × 10-9 mol m-2 s-1 Pa-1; the ideal selectivity for CO2/CH4 was 340. The permeances were in the following order: CO2 > H2 > He > O2 > N2 > CH4. Single-gas permeance was dependent on the relative molecular size of the DDR pore diameter. However, CO2 permeance dominated the adsorption affinity to the pore wall of DDR zeolite. In mixed-gas permeation experiments using the sweep method, the DDR-type zeolite membrane showed high selectivity for CO2/CH4 mixtures of 200 and high CO2 permeance of 3.0 × 10-7 mol m-2 s-1 Pa-1 at 298 K with a feed pressure of 0.2 MPa. Furthermore, the DDR zeolite membranes were compared to other zeolite membranes for evaluation of their CO2/CH4 selectivity and CO2 permeance; the DDR-type zeolite membranes show better CO2/CH4 separation and CO2 permeance. 1. Introduction The separation and recovery of CO2 are of great interest, from the perspectives of global warming and energy production and conservation. The separation of CO2 from CH4 is important for the processing of natural gas and for the processing of digestive gas from the anaerobic treatment of biomass. A widely used technology for CO2 removal is amine adsorption.1 However, amine plants are complex and costly. Membrane separation is energy-efficient; many membranes have been developed for CO2 separation. Furthermore, polymeric membranes might be less stable for CO2/CH4 separation at high temperatures and high CO2 pressures.2 Inorganic membranes, especially zeolite membranes, offer the potential of a breakthrough development that obviates those disadvantages.3 Zeolites are inorganic crystalline structures with pores of uniform molecular dimensions. They have superior thermal mechanical, chemical, and high-pressure stability. For those reasons, zeolite membranes were prepared with large pores (FAU,4,5 MOR6), medium pores (MFI,7 MEL,8 FER9), and small pores (LTA,10 CHA,11-13 DDR14). Most studies of CO2 separation have examined medium-pore zeolites such as MFI-type and FAU-type zeolites. Their respective selectivities would be low, because the pores of these zeolites are too large to separate CO2 from CH4 with high selectivity. Numerous eight-membered-ring zeolites with small pores have been examined as candidates for adsorbents that are useful for the separation of small molecules.15-20 Small-pore zeolites, such as zeolite T, SAPO-34, and DDR, have pores that have a size that is similar to that of the CH4 molecule, but which are * To whom correspondence should be addressed. Tel./Fax: +81 258479623. E-mail: [email protected].

larger than the CO2 molecule. For that reason, they are suitable for CO2/CH4 separation. High-silica types offer the advantage of hydrophobicity: they retain their adsorption capacity in water.21 The all-silica zeolite deca-dodecasil 3R (DD3R) is a clathrasil. In a pioneering work, Gies synthesized the DD3R clathrasil.22,23 The respective sizes of the adsorbate molecule and cavity serve important purposes: matching the pore-material diameter with the critical dimensions of gas molecules might provide higher separation factors. A DDR-type zeolite with small eightmembered-ring windows can separate small-molecule gas mixtures. The DD3R critical diameter with eight-memberedring windows closely matches the diameters of light hydrocarbons and carbon dioxide. Zhu et al.24 reported high adsorption selectivity of DD3R for propene and propane. However, few studies have specifically addressed adsorption for DD3R.25 High CO2/CH4 selectivities were displayed by SAPO-34 membranes with pore sizes estimated as 0.38 nm. Poshusta et al.12 reported a CO2/CH4 selectivity of 30 at 300 K and 4.7 at 470 K with a SAPO-34 membrane. Using a SAPO-34 membrane, Li et al.26 obtained a CO2/CH4 selectivity of 90 and a CO2 permeance of 2.4 × 10-7 mol m-2 s-1 Pa-1 at 297 K for a pressure drop of 0.138 MPa. For pressures up to 3 MPa, the CO2/CH4 selectivity was 180 for a 50%/50% feed mixture at 253 K. The highest CO2 flux measured in a mixture was 21 kg m-2 h-2 at 295 K, with a pressure drop of 3 MPa. Recently, Li et al.27 reported a SAPO-34 membrane that improved the CO2 permeance and CO2/CH4 selectivity, thereby obtaining CO2/ CH4 selectivity of 170 and a CO2 permeance of 1.2 × 10-7 mol m-2 s-1 Pa-1 at 295 K for a pressure drop of 140 kPa. The SAPO-34 membranes with higher CO2 permeance exhibited lower CO2/CH4 separation selectivity (115) but higher CO2

10.1021/ie061682n CCC: $37.00 © 2007 American Chemical Society Published on Web 09/13/2007

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permeance (4.0 × 10-7 mol m-2 s-1 Pa-1) at 295 K and a pressure drop of 140 kPa. Zeolite T is an intergrowth-type zeolite of erionite and offretite, of which the respective pore sizes are 0.36 nm × 0.51 nm and 0.67 nm × 0.68 nm. Using a T-type zeolite membrane, Cui et al.28 obtained a CO2/CH4 selectivity of 400 and respective CO2 permeance values of 4.6 × 10-8 and 4 × 10-8 mol m-2 s-1 Pa-1 at 308 K for a pressure drop of 0.1 MPa and under vacuum on the permeate side. The selectivity decreased as the temperature increased, but showed a CO2/CH4 selectivity of 52, even at 473 K. Tomita et al.14 first reported that DDR-type zeolite membranes have CO2/CH4 selectivities of 220 and 100 at 301 K and 373 K, respectively, and respective CO2 permeance values of 7 × 10-8 and 4 × 10-8 mol m-2 s-1 Pa-1 with a pressure drop of 0.5 MPa. Most studies of zeolite membranes have used low feed pressures; however, feed pressures might be as high as 7 MPa in industrial applications such as natural gas processing. Using SAPO-34 membranes with a pressure drop of 3 MPa, Li et al.26 found that the CO2/CH4 separation selectivity at 295 K was ∼60. Both CO2 and CH4 permeances in an equimolar CO2/CH4 mixture decreased as the feed pressure increased; the permeance decreased proportionally more for CH4 than for CO2. Consequently, the separation selectivity was increased. On the other hand, for a zeolite T membrane, the CO2/CH4 selectivity decreased from 400 to 150 and the CO2 permeance decreased from 4.6 × 10-8 mol m-2 s-1 Pa-1 to 1.3 × 10-8 mol m-2 s-1 Pa-1 as the feed pressure increased from 0.1 MPa to 0.5 MPa.28 Natural gas and biogases contain impurities such as H2O, N2, H2S, and hydrocarbons, in addition to CO2. Although zeolite membranes have shown potential for CO2 separation, most studies include only pure gas mixtures. A few studies have investigated the effect of impurities on the CO2/CH4 separation performance of the zeolite membrane. For a SAPO-34 membrane, Poshusta et al.29 reported that 0.6-0.9 mol % humidity (1.2-2.0 kPa) strongly affected gas permeation through SAPO34 membranes. However, Li et al.30 found that CO2 permeance decreased 12% and that CO2/CH4 selectivity was stable after 12 days of exposure to 170 ppm of water vapor with SAPO-34 membranes on stainless steel. For a FAU-type zeolite membrane with a humidified CO2/ N2 mixture (water partial pressure of 2.64 kPa at 296 K), the CO2 permeance was decreased drastically from 2 × 10-8 mol m-2 s-1 Pa-1 to 3.5 × 10-10 mol m-2 s-1 Pa-1.31 For a DDR-type zeolite membrane, Tomita et al.14 found that the permeation properties were unaffected by water addition at 373 K; however, at 301 K, the CO2 permeance was slightly lower with 3% water. Nevertheless, the effects of impurities for a DDR-type zeolite membrane have not been investigated in detail. In the present study, DDR-type zeolite membranes were prepared using a different type of alumina tube from those reported previously;14 higher CO2/CH4 selectivity and higher CO2 permeances were obtained. Single-gas permeances were measured for CO2, CH4, H2, He, N2, and O2 at a feed pressure up to 5 MPa. The CO2/CH4 mixed gas permeances were measured at total feed pressures up to 3 MPa, and the effects of feed pressure, temperature, and selectivity were examined. High-pressure adsorption isotherms were measured for CO2 and CH4 on DDR zeolite crystals to assess the influence of adsorption affinity on permeation characteristics. The effects of H2O, N2, and n-C3H8 impurities on CO2/CH4 separations were also studied.

Figure 1. X-ray diffraction (XRD) patterns of DDR crystals and a DDRtype membrane.

2. Experimental Section 2.1. Synthesis and Adsorption Isotherms of an All-Silica DDR Crystal. All-silica DDR crystals were synthesized at NGK Insulators, Ltd. Figure 1 presents X-ray diffraction (XRD) patterns of the DDR crystal used in this study. The XRD patterns are characteristic of DDR zeolite compositions.32,33 A conventional static-volumetric adsorption apparatus was used to measure pure gas adsorption isotherms.34 A 2-g sample of the all-silica DDR crystals was used for adsorption experiments. Before measurement, the sample was outgassed at 523 K for 12 h to remove adsorbed impurities. The adsorption isotherm data of CO2 and CH4 were measured accurately at pressures of 0.01-3000 kPa at 298 K. 2.2. Membrane Preparation. The DDR-type zeolite membranes were prepared using hydrothermal synthesis on the outside surface of porous R-alumina tubes (NGK Insulators, Ltd.). To improve the CO2 permeability, an alumina tube with high porosity and asymmetric structure was used for this study, although alumina with an average particle size of 0.6 µm had been used in a previous study.14 The 1.5-mm-thick supports had outer diameters of 11.8 mm and were structured asymmetrically, with a pore diameter of 200 nm in the outer-layer alumina. All membranes used in this work were obtained using seeded substrates to promote nucleation and growth of the zeolite layer. A mixture with a molar ratio of 1-adamantanamine:silica: ethylenediamine:water ) 6:100:95:3500 was prepared in a perfluoro container. The synthesis gel differed slightly from that previously reported.14 The 1-adamantanamine:silica ratio was decreased from 0.09 to 0.06, and the water:ethylenediamine ratio was decreased from 40 to 35. The porous alumina tube, with zeolite crystals as seeds, was immersed in the mixture. The mixture was heated in a pressure vessel at 423 K for 48 h. The membrane then was calcined at 1073 K for 4 h. Because of the high permeance of the DDR-type zeolite membrane, we cannot measure the permeance of the total length of the 15-cm DDRtype zeolite membrane accurately; the 15-cm-long support tube was cut to 2.3 cm segments for permeation tests (permeate area of 8.7 cm2). The membrane was analyzed using XRD analysis. The surface and the cross-section of the membrane were observed using scanning electron microscopy (SEM). 2.3. Gas Permeation and Separation Measurement. Using the constant-pressure method, single-gas permeation was measured as a function of feed pressure. Figure 2 shows the permeation experimental apparatus of the constant pressure method. The membrane was mounted in a stainless steel module, and then was sealed at each end, using an epoxide-based adhesive. The feed gas was introduced into the outside of the membrane. The pressure on the retentate side of the membrane

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Figure 2. Schematic diagram of the separation experimental apparatus with the constant-pressure method. LEgend: 1, feed gas cylinder; 2, mass flow controller; 3, gas mixing vessel; 4, zeolite membrane cell; 5, pressure transducer; 6, back-pressure regulator; 7, TCD gas chromatograph; 8, soap film flow meter; and 9, rotary vacuum pump.

Figure 3. Schematic diagram of the membrane module inside of the sweep method.

was controlled using a pressure regulator; the permeate pressure was maintained at atmospheric pressure. The pressures were measured using an electronic pressure transducer. The module was heated using an electric oven; the temperature was measured using thermocouples inside the feed of the membrane. A soapfilm bubble flowmeter was used to measure the gas flux. The measured permeance in all pressure ranges was accurate to within (2%. Single-gas permeances for He, H2, CO2, O2, N2, CH4, and n-C3H8 were measured at pressures up to 5 MPa. Figure 3 shows a schematic diagram of the membrane holder that was used for the sweep gas method. Each end of the membrane was connected to the stainless steel using epoxy resin. Helium was used as the sweep gas on the permeance side, and ambient pressure was maintained. In this study, a CO2/CH4 50/ 50 mol % mixture gas was used to evaluate the permeance characteristics of the DDR-type zeolite membranes. Permeances for CO2 and CH4 and mixtures of CO2/CH4 were measured at four temperatures: 298, 348, 373, and 423 K. For separation experiments using the sweep gas method, both the retentate and permeate streams were analyzed using a gas chromatograph that was equipped with a thermal conductivity detector. The ideal selectivity is the ratio of the single-gas permeance. The selectivity for binary mixtures RA/B is calculated as follows:

RA/B )

yA/yB xA/xB

In this equation, x and y represent the respective mole fractions of each component in the feed and permeate sides. The subscripts “A” and “B” refer, respectively, to components A and B. 3. Results and Discussion 3.1. Adsorption Isotherms. The DDR zeolite had a BrunauerEmmett-Teller (BET) surface area of 304 m2/g and a micropore volume of 0.153 cm3/g, as determined by the N2 adsorption

Figure 4. Adsorption isotherms of CO2, CH4, O2, and N2 on a DDR zeolite crystal at 298 K.

isotherm at 77 K. Figure 4 shows adsorption isotherms for CO2, CH4, N2, and O2 on DDR zeolite crystals. The ranking of the adsorption capacities was as follows:

CO2 > CH4 > O2 g N2 The amounts of CO2 adsorbed at high pressure were 1.8-4.8 times greater than the amounts of CH4 adsorbed. An accurate Henry’s Law constant is imperative for determining the interaction between the adsorbate molecules and the surface at infinite dilution.35 The virial isotherm provides a simple method of determining the Henry constant. According to the virial isotherm, a plot of ln(p/n) vs n should be linear at low loading, thereby providing a straightforward extrapolation to determine the Henry’s law constant KH.36 The Henry’s law constants for CO2 and CH4 at 298 K for DDR zeolite crystals were, respectively, 0.0421 and 0.00569 mol kg-1 kPa-1. The Henry’s law constants of CO2 were 7.4 times greater than those of CH4, indicating the high selectivity of DDR toward the adsorption for CO2 from CH4.37 Gas permeation was measured as a function of feed pressure, using the constant pressure method shown in Figure 6 (presented later in this paper). 3.2. Membrane Morphology. The permeance of CO2 on the alumina supports used for this study was 5 × 10-5 mol m-2 s-1 Pa-1, whereas the CO2 permeance of alumina supports used in a previous study were 1 × 10-6 mol m-2 s-1 Pa-1.14 The alumina supports used in this study are comparable to the

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Ind. Eng. Chem. Res., Vol. 46, No. 21, 2007 Table 2. Permeance Values of Three DDR-Type Zeolite Membranes (M1, M2, and M3) under Similar Conditions Value

Figure 5. Microstructure of a DDR-type zeolite membrane observed using electron microscopy: (a) surface and (b) cross section. Table 1. Single-Gas Permeance of Each Cut Sample (2.3 cm in Cross Section,15 cm in Length): DDR-Type Zeolite Membranes M1-M5 Value parameter

M1

M2

M3

M4

M5

temperature, T (K) PCO2 (× 108 mol m-2 s-1 Pa-1) PCH4 (× 1010 mol m-2 s-1 Pa-1) R ∆P (MPa)

298 26 9 275 0.1

298 33 17 183 0.1

298 29 7 422 0.1

298 30 10 300 0.1

298 30 12 250 0.1

previous supports, but have much higher permeability (a factor of 10 greater), because these alumina supports have high porosity. Figure 5 shows the top surface and cross section of the DDRtype zeolite membrane that was synthesized in this study. The membrane was formed uniformly, with a membrane thickness of ca. 2-3 µm. The scanning electron microscopy (SEM) image of the cross section (Figure 5b) confirmed definitively that the thickness of this membrane was less than in the previous membrane. The SEM image of the membrane surface shows intergrown zeolite crystals that have a diameter of 1 µm. Figure 1 shows the XRD patterns of the DDR crystals and DDR-type membrane. Both patterns indicate a crystal diffraction pattern of the DDR zeolite, but the membrane peaks are broad, indicating low crystallinity. 3.3. Single-Gas Permeation. Table 1 shows the single-gas permeance of CO2 and CH4 for each sample cut from the 15cm-long DDR-type zeolite membrane. The permeances of the five cut samples mutually corresponded. Therefore, it was confirmed that the membrane was uniform. Membrane M3 was used for permeation tests of single gases and equimolar CO2/ CH4 mixed gases in this study. Table 2 lists the permeance of three 15-cm DDR-type zeolite membranes made on the same condition. The permeance performances of three membranes were approximately equal; this method showed reproducibility. Figure 6 shows that each single-gas permeated flux through the membrane at pressures up to 5 MPa at 298 and 373 K with the constant pressure method. The permeance of each gas was calculated from Figure 6 at a feed pressure of 0.2 MPa. Singlegas permeances of He, H2, CO2, N2, O2, CH4, and n-C3H8 at 298 and 348 K at 0.2 MPa feed pressure are shown in Figure 7, as a function of the kinetic diameter. Membrane quality was

parameter

M1

M2

M3

temperature, T (K) PCO2 (× 108 mol m-2 s-1 Pa-1) PCH4 (× 1010 mol m-2 s-1 Pa-1) R ∆P (MPa)

298 12 12 98 0.1

298 12 15 75 0.1

298 13 19 68 0.1

characterized by measuring the single-gas permeance for n-C3H8 (kinetic diameter of 0.43 nm). In Figure 7, the n-C3H8 singlegas permeances at 298 K were He > H2 > N2 > O2 > CH4 Z n-C3H8 With increasing molecular diameter from 0.33 nm for CO2 to 0.38 nm for CH4, the permeance decreased; permeances of n-C3H8 (0.43 nm), such as those of hydrocarbons whose molecular diameters were larger than the CH4, were almost equal. The CO2 permeance was more than three orders greater than the permeance of CH4, which indicates that the DDR-type membrane had few defects and that their gas permeation was controlled by zeolite pores of DDR (0.36 nm × 0.44 nm). On the other hand, the CO2 permeance was greater than the permeance of H2 and He, but the CO2 molecules were larger than H2 (0.29 nm) and He (0.26 nm). Actually, den Exter et al. measured the adsorption isotherm of CO2 and H2 on DD3R at 296 K at 101 kPa: their results showed that the respective adsorbed amounts of CO2 and H2 were 1.764 and 0.014 mol/ kg.38 Furthermore, the adsorbed H2 amount was 1/100 that of CO2 at equal pressure. Moreover, there was only a slight adsorption of He to the DDR zeolite at room temperature. These gases only contribute slightly to the adsorption of the zeolitic pore wall for permeation of the DDR-type zeolite membrane. The kinetic diameter dependence of permeance through the membrane is similar to that reported previously by Poshusta et al., using a SAPO-34 membrane,3 and by Cui et al., using a zeolite T membrane.28 Those results indicate that the respective CH4 permeances at 298 K and 0.2 MPa were 23 and 12 times less than those of O2 and N2 in the DDR-type zeolite membrane, despite the high adsorption capacity of CH4. For a zeolite membrane with larger zeolitic pores such as the 10-membered

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Figure 6. Single-gas permeated fluxes at 298 and 373 K as a function of feed pressure for the DDR type membrane: (b) 298 K and (O) 373 K.

Figure 7. Plots of permeance versus kinetic diameter of gas molecules for a DDR-type zeolite membrane. Closed symbols indicate data obtained at 298 K, and open symbols indicate data obtained at 373 K.

ring and the 12-membered ring, the CH4 permeances are similar to or slightly larger than those of N2. Cui et al.28 reported that high N2/CH4 selectivity must be caused by a molecular sieving effect of zeolite with eight-membered rings, such as the SAPO34 membrane and zeolite T membrane. The SAPO-34 zeolite membrane3 and zeolite T membrane28 were reported to be less permeable to CH4 than N2, and that N2/CH4 ideal selectivities were 3-4 and 10 times. The value of N2/CH4 ideal selectivities of DDR-type zeolite membrane was greater than for the SAPO-

34 membrane and the zeolite T membrane. Therefore, the presence of high N2/CH4 and O2/CH4 selectivity is evidence for the molecular sieving contribution in the DDR-type zeolite membrane. Great differences were observed in the single-gas flux of CO2 and CH4 at permeance temperatures of 298, 348, 373, and 423 K, which are shown in Figure 8. Both CO2 and CH4 single-gas fluxes through the DDR-type zeolite membrane increased as the feed pressure increased at all temperatures. Figure 9 shows the temperature dependence of the CO2 and CH4 single-gas permeance and CO2/CH4 ideal selectivities. The CO2/CH4 ideal selectivities decreased as the temperature increased: The CO2 permeated flux decreased as the temperature increased from 298 to 423 K, although the CH4 permeated flux decreased only slightly. Consequently, the high permeance of CO2 for DDRtype zeolite membrane is attributable both to the high adsorption affinity of the DDR zeolite pore wall and the easy diffusion of CO2 molecules through the zeolitic pore. 3.4. Mixture Permeation. Figure 9 also shows the temperature dependence of permeance and the selectivity of CO2/CH4 for 50/50 mol % mixture separations. Similar to the single-gas behavior, the CO2 permeance for a CO2/CH4 mixture decreased with temperature, whereas the CH4 permeance was almost independent of temperature. Consequently, the separation selectivity decreased as the temperature increased, and this value was less than the ideal selectivity. The difference between ideal

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Figure 10. CO2 and CH4 fluxes for single-gases and a CO2/CH4 mixture at 298 K for a DDR-type zeolite membrane.

Figure 8. Single-gas permeated flux of CO2 and CH4 for DDR-type zeolite membranes at 298, 348, 373, and 423 K.

Figure 11. Permeances and selectivity of a CO2/CH4 mixture (50%/50%) at 298 K, as a function of feed partial pressure, for a DDR-type zeolite membrane.

Figure 9. Temperature dependence of CO2 and CH4 permeances and selectivity of CO2/CH4 for single components (closed symbols) and 50/50 mol % mixture (open symbols) for a DDR-type zeolite membrane at a pressure drop of 0.1 MPa.

and mixture selectivities was attributed to separation by competitive adsorption. Separation experiments at high pressure (up to 3 MPa) were conducted to investigate the conditions that might be useful on

a large scale. Figure 10 shows the CO2 and CH4 fluxes for a CO2/CH4 mixture (50/50) at 298 K, as a function of partial pressure for a DDR-type zeolite membrane with the sweep gas method. Both CO2 and CH4 fluxes in equimolar CO2/CH4 mixtures increased concomitant with feed pressure, but CO2 flux at the same CO2 pressure is less in the mixture than for the single gas; the CH4 flux was also lower in a mixture. Li et al., using the SAPO-34 membrane, observed a similar behavior: they found that (i) the slower-diffusing CH4 slows the fasterdiffusing CO2, and (ii) CO2 inhibits CH4. Therefore, both the CO2 and CH4 fluxes were less in the mixture than the singlegas flux.26 Figure 11 shows the permeance and selectivities of an equimolar CO2/CH4 mixture for the DDR-type membrane, as a function of partial pressure at 298 K with the sweep gas method. For CO2/CH4 (50/50 mol %) mixture separation, the maximum value of separation selectivities was 200 at 0.2 MPa and the selectivities decreased concomitant with increasing feed pressure. The CO2 permeance for a CO2/CH4 mixture decreased with feed pressure, whereas the CH4 permeance was almost independent of feed pressure. Therefore, the separation selectiv-

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Figure 12. Permeances and selectivity of a CO2/CH4 mixture containing 3% H2O at 298 K, as a function of time, for a DDR-type zeolite membrane.

ity decreased as the feed pressure increased. However, the DDRtype zeolite membrane had a CO2/CH4 selectivity of 80 and a high CO2 permeance (1.1 × 10-7 mol m-2 s-1 Pa-1) for a 50%/ 50% feed at 3 MPa. 3.5. Effect of Impurities. The effects of H2O, N2, and n-C3H8 impurities on CO2/CH4 separation were investigated for DDRtype zeolite membranes. First, the effects of humidity on gas permeation were studied for DDR membranes. The system attained a steady state for a CO2/CH4 mixture at 298 K for 4 h; a CO2/CH4 mixed gas that contained water vapor then was introduced to the feed. The CO2/CH4 mixture gas flowed through two bubblers (with a volume of 1 L); the water vapor was saturated (3 kPa) at 298 K. Figure 12 shows that the CO2 permeance through the DDR-type zeolite membrane decreased rapidly from 2.5 × 10-7 mol m-2 s-1 Pa-1 to 1.1 × 10-7 mol m-2 s-1 Pa-1, although the CH4 permeance was almost equal to that for a dry mixture gas. The CO2/CH4 separation selectivity decreased to half that of the dry CO2/CH4 mixture. However, the CO2 permeance and CO2/CH4 selectivity were restored rapidly afterward without water in the feed. The CO2 permeance and the CO2/CH4 selectivity with saturated water vapor were retained respectively as 1.1 × 10-7 mol m-2 s-1 Pa-1 and 100. For a SAPO-34 membrane, Poshusta et al.29 reported that the CO2 permeance through the SAPO-34 membrane decreased from 1 × 10-7 mol m-2 s-1 Pa-1 to 8 × 10-10 mol m-2 s-1 Pa-1 with 0.6% water in the feed. In contrast, for a feed of an equimolar CO2/CH4 mixture that contained 170 ppm water, the CO2 permeance through SAPO-34 membrane decreased slightly, by 15%, from its original value. However, the CO2 permeance and selectivity were not restored after 3.5 days without water in the feed. The CO2 recovered the original permeances and selectivity after calcination at 573 K for 24 h.30 Water had a strong effect on gas permeation through SAPO-34 membranes: water almost completely blocked the SAPO pores. Figure 13 shows the adsorption isotherm of water vapor on DD3R crystal at 298 K. The upper limit of the relative pressure of water vapor was 0.92, and the maximum amount adsorbed was ∼19 mg/g at 298 K at a water vapor pressure of 2.93 kPa (p/p0 ) 0.92). The maximum amount of adsorbed water vapor was compared to the accessible volume of the intracrystalline space of the DDR. The accessible volume was estimated as 0.14 mL/g, using liquid nitrogen adsorption, and the degree of volume filling was 0.13. A unit cell of DDR is comprised of six decahedra, nine dodecahedra, and six 19-hedra. The 19-hedra can only absorb adsorbate molecules of DDR;39 a 19-hedron cage can theoretically accommodate ∼10 water molecules. The number of molecules per 19-hedron cage was ∼1. Therefore, water vapor was considered to influence permeation through DDR-type zeolite membranes only slightly.

Figure 13. Adsorption isotherm of water vapor on a DDR crystal at 298 K.

Figure 14. Permeances and selectivity of a CO2/CH4 mixture that contains 3% N2 at 298 K, as a function of time, for a DDR-type zeolite membrane.

Figure 14 shows that the presence of 3% N2 in the equimolar CO2/CH4 mixture feed had almost no effect on the CO2 and CH4 permeances or CO2/CH4 separation selectivity on the DDR zeolite membrane. The CO2 permeance and CO2/CH4 separation selectivity were each 104% and 104% of their original values. Li et al. reported that 3% N2 in the CO2/CH4 mixture had no effect on the separation performance of the SAPO-34 membrane, because the amount of N2 adsorbed at the partial pressure of the feed mixture was very slight.30 For pure components at the partial pressure used in this feed gas, the respective amounts of adsorbed N2, CH4, and CO2 on the DD3R crystal were 0.008, 0.1, and 1 mol/kg. (See Figure 14.) As shown in Figure 15, the presence of 1% n-C3H8 in the equimolar CO2/CH4 mixtures had also almost no effect on the CO2 and CH4 permeance and the CO2/CH4 separation selectivity. They were, respectively, 94%, 97%, and 95% of the original values. The n-C3H8 permeate concentration was below the gas chromatography (GC) detection limit. The n-C3H8 decreased the CO2 permeance more than the CH4 permeance. In addition, the CO2 permeance and CO2/CH4 selectivity were also restored rapidly afterward, without n-C3H8 in the feed. (See Figure 15.) 3.6. Comparison to Other Zeolite Membranes. Two other small-pore zeolite membranes (Zeolite T28 and SAPO343,13,26,27,29,30) and a DDR-type zeolite membrane14 reportedly have high selectivities for CO2/CH4 separations. Table 3 shows a comparison of CO2 separation performances of zeolite membranes for CO2/CH4 systems. Although a strict comparison of separation performance among these membranes is inappropriate, because of the difference in experimental methods, the following were reasonably deduced. Cui et al. reported that the zeolite T membrane had very high CO2/CH4 selectivity of 400 at 308 K and a CO2 permeance of 4.6 × 10-8 mol m-2 s-1

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Figure 15. Permeances and selectivity of a CO2/CH4 mixture that contains 1% n-C3H8 at 298 K, as a function of time, for a DDR-type zeolite membrane. Table 3. Comparison of CO2/CH4 Separation Performances of DDR-Type Zeolite Membranes and Other Zeolite Membranes temp, T (K)

system

CO2 permeance, PCO2 (× 108 mol m-2 s-1 Pa-1)

R

∆P (MPa)c

reference

298 298 373 301 373

50/50 50/50 50/50 50/50 50/50

DDR-Type Membranes 30 200a 11 80a 10 82a 7 220b 4 100b

308 308 473

50/50 50/50 50/50

Zeolite T Membranes 4.6 400a 1 150a 1.5 52a

0.1 0.5 0.1

28 28 28

300 297 295 295 295 295

50/50 50/50 50/50 50/50 50/50 50/50

SAPO-34 Membranes 20 36b 16 67b 24 95a 10 55b 35 120b 12 170b

0.138 0.138 0.138 3.0 0.138 0.138

3 13 26 26 27 27

0.1d 3.0d 0.1d 0.5 0.5

this work this work this work 14 14

a Calculated by the mole fraction ratio. b Calculated by the ratio of the permeance. c Pressure difference between the feed side and the permeance side. d With sweep gas.

Pa-1 for a feed pressure of 0.1 MPa and a vacuum on the permeate side.28 The feed pressure increased to 0.5 MPa, and the CO2/CH4 selectivity and CO2 permeance decreased to 150 and 1.3 × 10-8 mol m-2 s-1 Pa-1, respectively. Li et al.26 noted that the vacuum, which was used on the permeate side flux, is expected to be lower when the permeate pressure is 0.1 MPa or higher. The SAPO-34 membrane exhibited a CO2/CH4 separation selectivity of 90 and a CO2 permeance of 2.4 × 10-7 mol m-2 s-1 Pa-1 at 295 K for a permeance pressure of 0.084 MPa with a pressure drop of 0.138 MPa,26 which represents a slightly lower selectivity than that of the DDR-type zeolite membrane. Similar to DDR, the SAPO-34 demonstrated CO2/CH4 separation selectivities; furthermore, the CO2 permeance at 295 K was decreased to 60 and 1.0 × 10-8 mol m-2 s-1 Pa-1 at pressure drops up to 3.0 MPa.26 Recently, SAPO-34 membranes with higher separation performance were obtained by modifying the gel composition and synthesis procedures used by Li et al.27 The SAPO-34 membranes, whose CO2/CH4 separation selectivity is 170, have slightly lower CO2 permeance (1.2 × 10-7 mol m-2 s-1 Pa-1). In addition, a membrane with a CO2 permeance of 3.5 × 10-7 mol m-2 s-1 Pa-1 is high, but the CO2/CH4 separation selectivity is slightly lower (120) at 295 K with a pressure drop of 140 kPa. The membrane, which has high

separation selectivity, had a high CO2/CH4 selectivity of 100 for a 50/50 feed at 295 K at 7 MPa. In contrast to those membranes, the DDR-type zeolite membranes, with an effective pore size of 0.36 nm × 0.43 nm, have very high CO2 separation performances. The DDR-type zeolite membrane used in this study had similar CO2/CH4 selectivity (200) and higher CO2 permeance (3 × 10-7 mol m-2 s-1 Pa-1 at feed pressure of 0.2 MPa), compared to the zeolite T membrane and the SAPO-34 membranes. The DDR-type zeolite membrane retained a high selectivity of 80 and a high permeance of 1.1 × 10-7 mol m-2 s-1 Pa-1, even at feed pressures as high as 3 MPa. The DDR-type zeolite membrane has higher separation selectivities of N2/CH4 and O2/CH4 than the zeolite T membrane and SAPO-34 membrane, which indicates that the DDR-type zeolite membrane has high selectivity for CO2 separation, mainly because of the molecular sieving effect. It is noteworthy that, because DDR is a hydrophobic zeolite that consists only of silica and oxygen, water vapor only slightly influences permeation through DDR zeolite membranes, in contrast to those of hydrophilic zeolite membranes such as zeolite T and SAPO-34 membranes. 4. Conclusions The DDR-type zeolite membranes used in this study were synthesized as 2-3 µm layers on porous R-alumina tube supports. For those membranes, single-gas permeances decreased in the following order:

CO2 > He > H2 > N2 > O2 > CH4 The permeation of light gases through the DDR-type zeolite membrane is strongly related to molecular size, but the adsorption affinity of CO2 on the DDR-type zeolite strongly affects CO2 permeance and selectivities in mixtures containing CO2. The best CO2/CH4 separation selectivity, that at 298 K, was 200, with a CO2 permeance of 3 × 10-7 mol m-2 s-1 Pa-1 with the sweep gas. The separation selectivities and CO2 permeances decreased as the feed pressure increased. The DDRtype zeolite membrane retained higher CO2/CH4 selectivity (80) and higher CO2 permeance (1 × 10-7 mol m-2 s-1 Pa-1) at pressures up to 3.0 MPa. The DDR-type zeolite membranes are only slightly affected by water vapor: the CO2 permeance and the CO2/CH4 selectivity with saturated water vapor at 298 K remained 1.1 × 10-7 mol m-2 s-1 Pa-1 and 100, respectively. Acknowledgment This work was supported partly by the New Energy and Industrial Technology Development Organization (NEDO) and the Research Institute of Innovative Technology for the Earth (RITE). Literature Cited (1) Barker, R. W. Future Directions of Membrane Gas Separation Technology. Ind. Eng. Chem. Res. 2006, 41, 1393-1411. (2) Koros, W. J.; Mahajan, R. Pushing the limits on possibilities for large scale gas separation: which strategies? J. Membr. Sci. 2000, 175, 181-196. (3) Poshusta, J. C.; Tuan, V. A.; Pape, E. A.; Noble, R. D.; Falconer, J. L. Separation of light gas mixtures using SAPO-34 membranes. AIChE J. 2000, 46, 779-789. (4) Kusakabe, K.; Kuroda, T.; Murata, A.; Morooka, S. Formation of a Y-Type Zeolite Membrane on a Porous-Alumina Tube for Gas Separation. Ind. Eng. Chem. Res. 1997, 36, 649-655.

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ReceiVed for reView December 28, 2006 ReVised manuscript receiVed July 28, 2007 Accepted July 30, 2007 IE061682N