Ind. Eng. Chem. Res. 2001, 40, 4577-4585
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Pervaporation of Water/THF Mixtures Using Zeolite Membranes Shiguang Li, Vu A. Tuan, Richard D. Noble, and John L. Falconer* Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424
Four types of hydrophilic zeolite membranes (A-type, Al-ZSM-5, mordenite, and Y-type) were used to dehydrate water/THF mixtures by pervaporation. At 303 K, the total fluxes increased with zeolite pore size, and membranes with lower Si/Al ratios had higher water/THF separation selectivities. The Y-type membrane, which was prepared using a template-free seeding technique, had the highest flux and selectivity; at 333 K, the total flux was 2.4 kg m-2 h-1 with a water/ THF selectivity of 290. The total flux increased with water feed concentration, but the selectivity decreased. Separation was based on both preferential adsorption of water and differences in diffusion rates. During continuous operation, the water permeate concentrations were higher than 95 wt %. Hydrophobic silicalite-1 and Ge-ZSM-5 membranes were used to remove THF from water. The Ge-ZSM-5 membrane had a total flux of 0.3 kg m-2 h-1 and a THF/water separation selectivity of 890 at 333 K for a 5 wt % THF aqueous solution. Introduction Water/tetrahydrofuran (THF) has an azeotropic concentration of 6.7 wt % water1 and, thus, cannot be separated by conventional distillation. In contrast, pervaporation had the potential to separate azeotropes because vapor-liquid equilibrium is not the controlling mechanism for separations in membranes. During pervaporation, the feed is placed in contact with one side of a membrane, and vapor permeate is removed from the opposite side, which is kept under vacuum. The chemical potential gradient across the membrane is the driving force for transport. Pervaporation has other advantages over distillation,2 including reduced energy demand (only a fraction of the liquid is vaporized) and relatively inexpensive equipment (only a small vacuum pump is needed to create a driving force). Azeotropic water/THF mixtures have been separated by pervaporation using polymer membranes. Neel et al.3 used a polytetrafluoroethylene-poly(vinylpyrrolidone) membrane to obtain a water/THF separation selectivity of 18.4 and a total flux of 0.94 kg m-2 h-1 for a 5.7 wt % water feed. Nguyen et al.,4 using a polyacrylonitrilepoly(vinylpyrrolidone) membrane and a 5.9 wt % water feed concentration, obtained a water/THF selectivity of 10.4 with a total flux of 0.4 kg m-2 h-1 at 293 K. A selectivity of 1518 was obtained by Oikawa et al.5 with a polyacylhydrazone membrane; their total flux was 0.125 kg m-2 h-1 for a 6.7 wt % water feed at 298 K. Although polymer membranes separate water from THF by pervaporation, they have limitations,6 and zeolite membranes have advantages such as better chemical and thermal stabilities. Organic-selective silicalite zeolite membranes7 and water-selective A-type8 and Y-type zeolite membranes9 have been used to separate liquid mixtures by pervaporation. The current study shows that a Y-type zeolite membrane has both high selectivity and high flux for continuous separation of water from THF by pervaporation. Zeolites are inorganic crystalline structures with uniform-sized pores of molecular dimensions. Since Suzuki10 patented the first preparation of zeolite mem* Corresponding author: John L. Falconer. Phone: 303 4928005. Fax: 303 492-4341. E-mail:
[email protected].
branes in 1987, many types of zeolite membranes have been prepared and used for gas separations and pervaporation. For example, small-pore (A-type),11 mediumpore (MFI-, MEL-, and FER-type)7,12,13 and large-pore (MOR-, X-, and Y-type)9,14,15 zeolite membranes have been reported. Membrane preparation has also been extended to related materials such as crystalline silicoaluminophosphates (SAPOs).16 Preparing zeolite membranes with high aluminum contents has been reported to be more difficult than preparing low-aluminumcontent membranes.17 Most zeolite membranes are prepared using organic templates for structure direction.18 Some alumina-rich zeolites, such as A, X, Y, and MOR and MFI with Si/Al ) 15-30 can be prepared from gels without templates.18 In general, the hydrophilicity/ hydrophobicity of zeolite membranes is controlled by the Si/Al ratio in the framework; alumina-rich membranes are hydrophilic and water-selective, whereas all-silica membranes (e.g., silicalite-1) are hydrophobic and organic-selective. Silicalite-1 and ZSM-5 zeolites have the MFI structure, which has a system of straight channels (0.53 × 0.56 nm) interconnected by zigzag channels (0.51 × 0.55 nm). Cook and Conner19 reported, however, that molecules that are significantly larger than these dimensions could fit into the ZSM-5 pores, which they calculated to be 0.62 nm in diameter. Isomorphous substitution has been shown to be an effective method for modifying the MFI structure, and membranes with germanium substituted into the MFI framework (GeZSM-5) were hydrophobic and effective at separating ethanol and 2-propanol from aqueous solutions by pervaporation.20 Mordenite (MOR, Si/Al ) 5) has an ordered distribution of Si and Al in the framework structure, which consists of two major channels: 0.65 × 0.70-nm pores and 0.26 × 0.57-nm pores. Matsukata et al.21 prepared mordenite membranes on alumina supports by a vaporphase transport method. These membranes had a benzene/p-xylene separation selectivity greater than 160 during pervaporation. The Y-type zeolite (Si/Al ) 1.5-3) has a faujasite (FAU) structure. This zeolite is strongly hydrophilic, and its pores are 0.74 nm in diameter. Y-type membranes
10.1021/ie010140x CCC: $20.00 © 2001 American Chemical Society Published on Web 08/30/2001
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Table 1. Membrane Preparation Conditions membrane
type
structure
M1 M2 M3 M4 M5 M6 M7 M8
Y-type Y-type mordenite Al-ZSM-5 A-type A-type silicalite-1 Ge-ZSM-5
FAU FAU MOR MFI LTA LTA MFI MFI
support
Si/Me ratio
γ-Al2O3 2.5 γ-Al2O3 2.5 R -Al2O3 6 R-Al2O3 25 γ-Al2O3 1 γ-Al2O3 1 stainless steel >1000 stainless steel 25
number of layers 1 1 2 4 1 1 2 2
have been used for gas separation22 and pervaporation.9,23 The NaY membranes of Kita et al.9 had a total flux of 1.59 kg m-2 h-1 and a water/ethanol separation selectivity of 125 at 348 K for a 10 wt % water feed mixture. The channels of zeolite A (LTA, Si/Al ) 1) are 0.42 nm in diameter. Oriented A-type membranes have been used for separation of H2, CO2, and CH4 gases17 and for dehydration of organic solvents by pervaporation.8 The current study focuses on the dehydration of an azeotropic water/THF mixture and also presents some results on the removal of THF from aqueous solutions. Hydrophilic (Y-type, mordenite, Al-ZSM-5, and A-type) and hydrophobic (silicalite-1 and Ge-ZSM-5) zeolite membranes were prepared and characterized by XRD, SEM, and pervaporation of 1,3,5-triisopropyl benzene (TIPB). The hydrophilic membranes were used to separate water from THF, whereas the hydrophobic membranes were used to recover THF from water. Experimental Methods Membrane Preparation. Membranes were prepared by in situ crystallization onto tubular porous supports. Table 1 lists the type, structure, support, Si/ Me ratio, and number of layers. The Al-ZSM-5 and mordenite membranes were on R-Al2O3 supports (0.2µm pores, U.S. Filter), and the Y-type and A-type membranes were on γ-Al2O3 supports (5-nm pores, U.S. Filter). Silicalite-1 and Ge-ZSM-5 membranes were synthesized on stainless steel tubes (0.5-µm pores, Mott Company) to avoid substitution of Al from the support into the membrane. About 1 cm on each end of the alumina supports was glazed, and stainless steel tubes were welded onto each end of the stainless supports to provide sealing surfaces for the O-rings. Before membrane preparation, the supports were cleaned by brushing their inner surfaces and then placing them in an ultrasonic bath that contained deionized water. The support tubes were then boiled in distilled water for 1 h and dried at 373 K under vacuum for 30 min. The membranes were deposited on the supports by hydrothermal synthesis. One end of the support was sealed with Teflon tape and a Teflon end cap. The support was then filled with the synthesis gel, and the other end was taped and capped. The support was sealed inside a Teflon-lined autoclave and then placed in an oven for synthesis. For the MFI and mordenite membranes, Ludox AS 40 (silica sol) and sodium aluminate (Na2Al2O4) were used as the Si and Al sources, respectively. For the Y-type and A-type membranes, sodium silicate (14 wt % NaOH, 27 wt % SiO2) was used. A detailed description of the synthesis procedures for the Al-ZSM-5 (Si/Al ) 25) membrane was given previously.24 The molar gel composition used for the AlZSM-5 membrane was 9.1 Na2O:1.0 Al2O3:50 SiO2:3.0 TPAOH:3213 H2O, where TPAOH (tetrapropylammo-
nium hydroxide) was used as the template. The gel composition for the silicalite-1 membrane was the same as that for Al-ZSM-5 membrane, but it did not contain Na2O and Al2O3. Hydrothermal synthesis was carried out at 458 K for 24 h. Four synthesis layers were required for ZSM-5 and two layers for silicalite-1 membranes. The gel molar composition for preparation of the Ge-ZSM-5 membrane was 0.78 Ge(C2H5O)4:19.5 SiO2:1.0 TPAOH:438 H2O. Hydrothermal synthesis was carried out at 458 K for 24 h, and two synthesis layers were required. After synthesis, the membranes were washed twice with distilled water, dried in a vacuum oven at 373 K, and then calcined in air at 753 K for 8 h with heating and cooling rates of 0.011 and 0.015 K/s, respectively. The mordenite membranes were prepared using the molar gel composition: 3.2 Na2O:1.0 Al2O3:12 SiO2:210 H2O. Two synthesis layers (24 h for each layer) were crystallized at 438 K. The Y-type membranes were prepared by growing one layer onto seed crystals. The alumina tubes were seeded with X-type powder, as both X- and Y-type zeolites have the same FAU structure. The seeding process, preparation procedure, and conditions were similar to those used for the X-type membrane reported previously,15 except that the gel composition was different. The molar gel composition in this case was 14.0 Na2O:10.6 SiO2:1.0 Al2O3:900 H2O. One synthesis layer was grown at 373 K for 6 h. For preparation of A-type membranes, γ-Al2O3 supports were first seeded with A-type crystals. The gel composition was 4.25 Na2O:2.5 SiO2:1.0 Al2O3:111 H2O. The gel was filtered, and the filtrate was diluted with the same amount of water used to prepare the gel. Hydrothermal synthesis was carried out at 373 K for 5 h. After synthesis, the membrane was washed five times with distilled water and dried in an oven at 373 K for 15 h to remove water occluded in the zeolite crystals. Membrane Characterization. The powders collected from the bottom of the membrane tubes were analyzed by X-ray diffraction (XRD) with a Scintag PAD-V diffractometer with Cu KR radiation. Some membranes were broken and analyzed by SEM (ISI-SX30) operated at 30 keV. Single-gas permeation rates were measured at 300 and 473 K for n-C4H10 (0.43-nm kinetic diameter) and i-C4H10 (0.50-nm kinetic diameter) as an indication of the membrane quality.24 The membranes were sealed in a stainless steel module by Viton O-rings with a feed pressure of 222 kPa and a permeate pressure of 84 kPa. The membrane qualities were also characterized by pervaporation of 1,3,5-triisopropyl benzene (TIPB, 0.85-nm kinetic diameter) at 300 K, as TIPB has a kinetic diameter that is larger than the zeolite pore. Pervaporation Measurements. The pervaporation system was similar to that used by Liu et al.25 The membrane was sealed in a brass module with Viton O-rings, and the liquid feed (250 cm3) flowed through the inside of the membrane at a flow rate of 20 cm3/s. The retentate was recirculated by a centrifugal pump to the feed side of the membrane. The system lines near the membrane module were wrapped with heating tape and insulated so that pervaporation could be carried out at elevated temperatures. A thermocouple was placed in the liquid in the center of the membrane and the temperature was controlled by a temperature controller. A mechanical vacuum pump evacuated the permeate side of the membrane to a pressure of approximately
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Figure 1. XRD spectrum of powder collected from the bottom of Al-ZSM-5 membrane M4.
0.2 kPa, and the pump was then valved off during pervaporation measurements. A liquid nitrogen cold trap condensed the permeate vapor and maintained the vacuum on the permeate side below 0.5 kPa. The membranes had permeable areas of approximately 5.2 cm2. The THF was HPLC grade. For THF dehydration, most experiments used a water feed concentration of 6.7 wt %. The feed was replaced when the feed concentration changed by more than 5% of its initial value. A 10 wt % water/THF mixture was also used for the best membrane (Y-type) for approximately 53 h at 333 K to investigate stability. For separation of THF from water, most experiments were done with a 5 wt % THF/water feed, which was also replaced when the feed concentration changed by more than 5% of its initial value. The process was interrupted for about 9 h by turning off both the centrifugal and vacuum pumps and keeping the feed inside the system. A permeate sample was collected and weighed every 1-2 h to determine the total pervaporation flux. Permeate concentrations were measured by an off-line GC (HewlettPackard 5730A) equipped with a 6-ft PORAPAK-Q packed column and a thermal conductivity detector. The oven and injection temperatures were 453 and 473 K, respectively. The separation factor (same as separation selectivity) was Rwater/THF ) (ywater/yTHF)/(xwater/xTHF), where x and y are the weight fractions in the feed and permeate, respectively. The ideal selectivity is the ratio of purecomponent fluxes. The vapor-liquid equilibrium selectivity is defined in the same way as the separation factor, but in that case, x and y denote the equilibrium mass fractions in the liquid and vapor phases, respectively. Results and Discussion Membrane Characterization. Membranes M1 and M2 were prepared under the same conditions, as shown in Table 1, and thus, their properties are expected to be similar. The same is true for membranes M5 and M6. The XRD pattern of the powder collected from the bottom of the Al-ZSM-5 membrane M4 is shown in Figure 1. All peaks match those reported by Szostak18 for ZSM-5 crystals, and no additional peaks are observed. The high intensity of the XRD lines and the low background intensity indicate high degrees of crystal-
linity. The Y-type, mordenite, A-type, silicalite-1, and Ge-ZSM-5 membranes also had the XRD patterns expected for these structures. The SEM images (Figure 2) of the surfaces of these membranes indicate that crystals have intergrown to form a continuous layer. The diameters of the intergrown crystals vary from 1 to 20 µm for different zeolite structures (Table 2). The SEM cross section of these membranes (Figure 3) indicates that the zeolite layer was 15-40 µm thick, as shown in Table 2. Our previous study26 indicated that the stainless steel-supported silicalite-1 (M7) and Ge-ZSM-5 (M8) membranes were approximately 80 and 30 µm thick, respectively. As shown in Table 2, the TIPB pervaporation flux at 300 K was 3.1 g m-2 h-1 for the A-type membrane M5. Because the TIPB molecule (0.85 nm) is significantly larger than the LTA pores (0.42 nm), this A-type membrane contains nonzeolite pores. The other membranes had TIPB fluxes no higher than 0.7 g m-2 h-1, indicating that these membranes have few nonzeolite pores. The n-C4H10/i-C4H10 ideal selectivities for the MFI membranes were 5.4-21 at 473 K, further indicating that these membranes were of high quality. For the mordenite and Y-type membranes, the n-C4H10/i-C4H10 ideal selectivities were close to 1. Because both butane molecules are significantly smaller than these zeolite pores, differences in permeance might not be expected. Pervaporation through Hydrophilic Membranes. Pervaporation of an Azeotropic Mixture. The total pervaporation flux for the 6.7 wt % water/THF solution increased with temperature for all membranes (Figure 4); water preferentially permeated through these membranes, and both the water and THF fluxes increased with temperature (Table 3). The flux for the Y-type membrane increased most rapidly with temperature. The highest total flux at 333 K, obtained for the Y-type membrane M1, was 2.35 kg m-2 h-1. Figure 5 shows that for all membranes the water/THF separation selectivity was greater than 1, and except for the A-type membrane, the selectivity increased with temperature because the rate of water permeation increased faster than the THF rate as the temperature increased (Table 3). The selectivity of the Y-type membrane increased fastest, and its maximum selectivity was 290 at 333 K. The selectivities for mordenite and Al-ZSM-5 membranes were less than 12 and 4, respectively. The kinetic diameters of water and THF are 0.26
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Figure 2. Top-view SEM micrographs: (a) Y-type membrane M2, (b) mordenite membrane M3, (c) Al-ZSM-5 membrane M4, (d) A-type membrane M6. Table 2. Membrane Properties morphology
c
membrane
pore size (nm)b
crystal size (µm)
thickness (µm)
Y-type (M2) mordenite (M3) Al-ZSM-5 (M4) A-type (M6) silicalite-1 (M7) Ge-ZSM-5 (M8)
0.74 0.65 0.56 0.42 0.56 0.56
5-10 2-20 1 5-10 -
15 25 30-40 25 80 30
n-/i-C4H10 selectivitya 300 K 473 K 1.1 1.1 10 13 30
1 1 6.7 5.4 21
TIPB flux at 300 Kc (g m-2 h-1) 0.7 0.5
