Separation of 1, 3-Propanediol from Aqueous Solutions Using

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Ind. Eng. Chem. Res. 2001, 40, 1952-1959

SEPARATIONS Separation of 1,3-Propanediol from Aqueous Solutions Using Pervaporation through an X-type Zeolite Membrane Shiguang Li, Vu A. Tuan, John L. Falconer,* and Richard D. Noble Department of Chemical Engineering, University of Colorado, Boulder, Colorado 80309-0424

1,3-Propanediol was separated from aqueous solutions by pervaporation through an X-type zeolite membrane on a γ-Al2O3 support. Binary, ternary, and quaternary mixtures and a cell-free fermentation broth were used. At 308 K, the total fluxes for the quaternary solution and the fermentation broth were 2.1 and 1.2 kg/(m2 h), respectively. Pervaporation fluxes of all components increased with temperature, but the water flux increased at the fastest rate. From 308 to 328 K, the 1,3-propanediol/glycerol selectivity for the quaternary model solution increased from 59 to 67; it increased from 61 to 110 for the broth. The 1,3-propanediol/glucose selectivity increased from 1900 to 2200. The 1,3-propanediol/glycerol selectivity was mainly controlled by preferential adsorption, and the 1,3-propanediol/glucose selectivity was mainly controlled by differences in diffusion rates. The membrane was stable during the pervaporation of the quaternary mixture for at least one week. In contrast, after 60 h, the broth started to foul the membrane, and the 1,3-propanediol permeate concentration decreased. The membrane was regenerated by calcination at 653 K for 4 h. Introduction The 1,3-propanediol (C3H8O2, 3G) molecule is an economical source for the production of 3GT (a polymer of 3G and terephthalic acid), which is a novel polyester with good stretch, recovery, and dyeability.1 The 1,3propanediol molecule is fermented from glycerol (1,2,3propanetriol, C3H8O3) or glucose (C6H12O6), and thus, selective separation methods would be valuable for recovery of 1,3-propanediol from glycerol and glucose. Distillation can be used to separate 1,3-propanediol from glycerol, but both compounds have high boiling points, and therefore, separations require significant energy. Moreover, glucose is thermally sensitive, so distillation is neither economical nor suitable. Instead of distillation, a packed bed of zeolite can be used to preferentially adsorb 1,3-propanediol from the mixture. Ethanol can then be used to remove 1,3-propanediol from the zeolite.2 The current study shows that an X-type zeolite membrane (pore size of 0.74 nm) has both high selectivity and high fluxes for continuous separation of 1,3propanediol by pervaporation. Pervaporation has advantages over distillation, 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).3 Pervaporation can also have advantages over a packed bed. Membrane operation is continuous and does not require an additional separating agent such as the ethanol used for a packed bed. Eliminating the ethanol also eliminates the distillation step for the separation and recycling of the ethanol. * Corresponding author. Phone: 303 492-8005. Fax: 303 492-4341. E-mail: [email protected].

Pervaporation using zeolite membranes has concentrated on binary organic/water feed mixtures: for example, organic-selective silicalite4 and water-selective A-type5 and Y-type membranes.5,6 Sano et al.7 carried out the separation of a methanol/MTBE (methyl-tertbutyl ether) mixture using a silicalite membrane and obtained a maximum methanol/MTBE separation factor of 10 at 303 K with a flux of approximately 0.1 kg/ (m2 h). The NaX and NaY zeolite membranes prepared by Kita et al.6 were highly selective for alcohol in the pervaporation of alcohol mixed with benzene, cyclohexane, or MTBE. Nishiyama et al. reported the separation of benzene/p-xylene using a mordenite membrane8 and the separation of cyclohexane/benzene, p-xylene/oxylene, and benzene/p-xylene mixtures using ferrrierite membranes.9 Few pervaporation studies have used more than three components in the feed. Moreover, most studies have used model solution feeds instead of mixtures from a process or from fermentation because of analytical problems10 or membrane fouling by the fermentation medium.11 Qureshi et al.11 used a silicone-silicalite-1 composite membrane to recover acetone, butanol, and ethanol from model solutions and from Clostridium fermentation broth. An optimum ratio of silicalite/ silicone of 1.5 resulted in a flux of 0.090 kg/(m2 h) and a selectivity of 97, whereas a silicone membrane had a flux of 0.044 kg/(m2 h) and a selectivity of 84 under identical conditions. In a previous study,12 Na-ZSM-5 zeolite membranes were used to separate 1,3-propanediol from glycerol and glucose in water solutions by pervaporation. The steadystate total flux was 0.21 kg/(m2 h) at 308 K. The maximum 1,3-propanediol/glycerol separation selectivity was 54, and the maximum 1,3-propanediol/glucose

10.1021/ie000905l CCC: $20.00 © 2001 American Chemical Society Published on Web 03/23/2001

Ind. Eng. Chem. Res., Vol. 40, No. 8, 2001 1953

selectivity was over 2100. The 1,3-propanediol molecule could adsorb in the ZSM-5 pores. The 1,3-propanediol/ glycerol selectivity was controlled by both preferential adsorption and differences in diffusion rates. The 1,3propanediol/glucose selectivity was mainly controlled by diffusion rates, with glucose diffusing through nonzeolite pores. Previously,13 we prepared a series of X-type zeolite membranes by a template-free method on porous supports. The best membrane, which had one layer of zeolite on a γ-Al2O3 support, had a tri-isopropyl benzene pervaporation flux of 2.3 g/(m2 h). This low flux indicated that the amount of flow through nonzeolite pores was small. The fluxes of water and of aqueous mixtures were 3 orders of magnitude higher. At 323 K, the total flux for a 1,3-propanediol/glycerol/water mixture was 4.1 kg/(m2 h), and the 1,3-propanediol/glycerol selectivity was 51. In the current study, this membrane was used for pervaporation of binary, ternary, and quaternary mixtures containing 1,3-propanediol, glycerol, glucose, and water. In addition, a cell-free fermentation broth was used as a feed. In addition to the four components that were in the model solutions, the broth contained at least 12 other compounds, mostly in low concentrations. Pervaporation was carried out from 308 to 328 K. The quaternary model mixture and the fermentation broth were also run for one week to investigate membrane stability. Experimental Methods Membrane Preparation and Characterization. An X-type membrane (Si/Al ) 1.5) was prepared in the sodium form by in situ crystallization on the inside of a porous γ-Al2O3 tube (5-nm pores, U.S. Filter). The support was cleaned by brushing its inner surface and then placing it in an ultrasonic bath that contained deionized water. It was then boiled in distilled water for 1 h and dried at 373 K under vacuum for 30 min. A method similar to that reported by Kita et al.14 was used to seed the inside of the tubes with X-type crystals. Seeding was important for initiating crystal growth and synthesizing high-quality membranes. The X-type powder was prepared using the same gel and conditions as were used for the membrane. The best membranes were obtained using the following gel molar composition: 4.2 Na2O/1.0 Al2O3/3.0 SiO2/150 H2O. The Si source was 27 wt % SiO2 (Ludox AS 40), together with 14 wt % NaOH in water, and the Al source was sodium aluminate (Na2Al2O4). For seeding, the X-type powder was added to deionized water, and about 10 wt % of the crystals remained in suspension and were used for seeding. Large crystals were deposited on the bottom of the container. The porous tubes were soaked in the suspension for 10 min and then dried at 373 K in a vacuum for 30 min. One end of the support tube was wrapped with Teflon tape and plugged with a Teflon cap. The tube was then filled with approximately 2 mL of the synthesis gel and left for 30 min at room temperature. During this time, the porous support soaked up about 30% of the synthesis gel. The tube was filled again with gel, and the other end was then wrapped with Teflon tape and plugged with a Teflon cap. The tube was placed vertically in a Teflon-lined autoclave, and hydrothermal synthesis was carried out at 373 K for 6 h. After synthesis of the zeolite layer, the membrane was washed 5 times with distilled

water and dried in an oven at 373 K for 15 h to remove water occluded in the zeolite crystals. The membrane had the FAU structure, as indicated by XRD analysis for powder collected from the bottom of membrane tube. The quality of the membrane was determined by pervaporation of tri-isopropyl benzene (TIPB, 0.85-nm kinetic diameter) at 300 K, as this molecule has a kinetic diameter that is larger than the zeolite pore diameter. Separation Apparatus. The pervaporation system was similar to that used by Liu et al.15 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. The pump raised the solution temperature so that the minimum temperature was 308 K. A thermocouple was placed in the middle of the membrane, and the temperature was controlled by a temperature controller. The retentate was renewed every 12 h so that the feed concentration changed by less than 5%. A mechanical vacuum pump evacuated the permeate side of the membrane to a pressure of approximately 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 Pa. Permeate concentrations were measured by both offline GC (Hewlett-Packard 5890 Series II) and HPLC (Beckman) analysis. The concentration of glucose could only be measured by HPLC. The GC separation was accomplished using a 6-ft DB-WAX packed column, and the detection was with a flame-ionization detector. The oven and injection temperatures were 333 and 508 K, respectively. The HPLC instrument was equipped with a Beckman 110B solvent delivery module and a Waters 410 differential refractometer. The separation was accomplished using a Shodex HPLC column (SH1011 sugar column, 300 mm × 8 mm). The mobile phase was 0.005 M H2SO4 at 0.6 cm3/min isocratic flow with an operation temperature of 323 K. The permeate concentrations obtained by the two methods agreed to within 5%. Feed Mixtures. Both model mixtures and cell-free broth were used as feed mixtures. For preparation of model mixtures, 1,3-propanediol (Aldrich, 98% purity), glycerol (Aldrich, 99.5% purity), and glucose (Sigma, 99.5% purity) were dissolved in distilled water. Binary (1,3-propanediol/water, glycerol/water, glucose/water), ternary (1,3-propanediol/glycerol/water, 1,3-propanediol/ glucose/water, glycerol/glucose/water), and quaternary (1,3-propanediol/glycerol/glucose/water) solutions were used. For each mixture, the concentrations of 1,3propanediol, glycerol, and glucose were investigated at conditions of industrial interest, which are 100, 5, and 8 g/L, respectively. The fermentation broth was provided by DuPont Corporation. The broth had been filtered with a 10 000 MW cutoff membrane and was essentially protein-free. The broth was brown in color, and in addition to 1,3propanediol, glycerol, and glucose, it contained at least 12 other components. Table 1 lists the main components in the broth. Calibrations were obtained for the 1,3propanediol, glycerol, and glucose components, so that

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Table 1. HPLC Peak Areas and Concentrations of the Fermentation Broth Feed and Permeate

component unknown unknown KH2PO4 glucose unknown unknown succinic acid lactate (Li) glycerol formic acid acetic acid unknown 1,3-propanediol meso-butanediol D/L-butanediol a

elution time (min) 11.52 12.77 14.73 18.16

feed area 0.44 6.0 4.0 0.40 1.5 2.5 0.53

conc (g/L) 0.15 -

permeate at 308 K area conc (g/L)

permeate at 308 Ka area conc (g/L)

0.02 -

-

0.01 -

-

0.09 -

0.46 0.07

0.10 -

19.68

45.5 6.1

10.6 -

0.40 0.12

22.70 24.32 26.58 28.68

5.5 279 2.6 0.38

73.2 -

0.61 152 0.64

40 -

2.90 155 0.68 0.15

41 -

After membrane calcination at 653 K for 4 h.

their concentrations are also given. Some species in the broth were detected by HPLC but are not identified in Table 1. Procedure. The X-type zeolite membrane had a permeable area of about 5.2 cm2. The membrane was boiled in an ethanol/water (50/50) solution for 20 min and dried at 373 K under vacuum for 30 min to remove contaminants prior to each run at a given liquid feed composition. A permeate sample was collected and weighed every hour to determine the total flux. The pervaporation experiments were run at 308 K to reach a steady state, and then the temperature was increased to 313, 318, and 323 K to measure the flux and separation selectivity as a function of temperature. Typically, pervaporation was run for 16 h, and the process was usually interrupted by turning off both the centrifugal and vacuum pumps and keeping the feed mixture inside the system for about 8 h. Both the quaternary model mixture and the fermentation broth were run for approximately one week to investigate long-term stability. Note that the membrane was used for the model solutions for almost two months without calcination, and it was cleaned using the same method as mentioned above for cleaning prior to pervaporation of the broth. The system was run continuously for one week except when the feed solution was replaced. The feed solution was removed and replaced by fresh solution in approximately 1 min, and the total flux and permeate concentrations after this replacement were within 5% of the values before replacement. The composition separation selectivity (β) was calculated as

β1,3-propanediol/glycerol ) (y1,3-propanediol/yglycerol)/ (x1,3-propanediol/xglycerol) where x and y are the weight fractions in the feed and permeate, respectively. The ideal 1,3-propanediol/ glycerol selectivity is the permeate concentration ratio of 1,3-propanediol to glycerol in binary 1,3-propanediol/ water and glycerol/water mixtures. Results and Discussion The main component in the pervaporation feed was water, and the membrane was selective for water permeation, so water had a higher flux than the other species and a higher concentration in the permeate than

in the feed. As for ZSM-5 membranes,12 all mixtures for the X-type membrane reached a steady state in 6-8 h, and the steady-state results are presented here. The vapor pressure of 1,3-propanediol is 138 Pa at 323 K, and glycerol has a lower boiling point (455 K) than 1,3-propanediol (484 K). Thus, both species evaporate during pervaporation. The glucose vapor pressure is too low for significant evaporation, and its kinetic diameter is larger than the XRD-measured diameter of the zeolite pores. Thus, the X-type membrane is expected to sieve glucose. To verify this, a glucose/water solution was used as feed to a Y-type membrane (same pore size of 0.74 nm). After 7 h, the glucose concentration had increased from 8 to 9.9 g/L, and the amount of water that permeated gave a good mass balance. Thus, the low concentrations of glucose on the permeate side of the membranes arises because glucose permeates slowly through the membranes. Total Fluxes. The kinetic diameters of water, 1,3propanediol, glycerol, and glucose are 0.26, 0.61, 0.63, and 0.86 nm, respectively, and the diameter of the X-zeolite pores is 0.74 nm. The pervaporation flux of triisopropyl benzene (TIPB, kinetic diameter of 0.85 nm) for this X-type membrane at 300 K was only 2.3 g/(m2 h). Because of their sizes, water, 1,3-propanediol, and glycerol can permeate through zeolite pores and also through nonzeolite pores that are as large as the molecules. The nonzeolite pores can have a distribution of sizes, and the low TIPB flux indicates only a small number have diameters larger than 0.85 nm. Likewise, the glucose molecule is significantly larger than the zeolite pores, and thus, it is expected to permeate only through nonzeolite pores that are larger than 0.86 nm. Figure 1 shows that the total fluxes increased with temperature for all solutions. The total flux for the quaternary solution [2.1 kg/(m2 h) at 308 K] is approximately 9 times higher than the flux for the ZSM-5 membranes used for this system.12 The total flux was highest for pure water, and the addition of any of the species to the feed decreased the flux. All solutions that contained 1,3-propanediol had lower fluxes than the corresponding solutions without 1,3-propanediol. Apparently, 1,3-propanediol preferentially absorbs in the X-type membrane.13 These results are different from those reported for ZSM-5 membranes; glucose decreased the fluxes of the other species in ZSM-5 membranes.12 Glucose apparently was able to block some pores in


Figure 1. Total flux as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

Figure 2. 1,3-Propanediol permeate concentration as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

Figure 3. Glycerol permeate concentration as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

ZSM-5, but the effect is much less in X-type zeolite membranes. Permeate Concentrations and Fluxes. Figures 2-4 show, respectively, the 1,3-propanediol, glycerol, and glucose permeate concentrations as functions of temperature. The 1,3-propanediol permeate concentrations were almost the same for the different feeds and were lower than the feed concentration of 100 g/L because water permeated faster than 1,3-propanediol. These results show that glycerol and glucose do not significantly affect the permeation of 1,3-propanediol. In contrast, both 1,3-propanediol and glucose decreased the glycerol flux and permeate concentration signifi-

Figure 4. Glucose permeate concentration as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

Figure 5. 1,3-Propanediol flux as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

cantly (Figure 3), and thus, the glycerol permeate concentrations strongly depended on the feed composition. For the glycerol/water and glycerol/glucose/water mixtures, the glycerol permeate concentrations were approximately 2-4% of the glycerol feed concentration of 5 g/L. For the mixtures containing 1,3-propanediol, the glycerol permeate concentrations were only about 0.5% of the feed concentration. Both 1,3-propanediol and glycerol decreased the glucose flux and thus the glucose permeate concentration. For all conditions, however, the glucose concentrations were more than 3 orders of magnitude lower than the feed concentration of 8 g/L and, for some conditions, were 4 orders of magnitude lower. Note in Figure 4 that the glucose concentration is in milligrams per liter. As shown in Figures 5-7, the permeate fluxes of all components increased with temperature, apparently because the rates of diffusion increased. However, the permeate concentrations decreased because the water flux increased faster. The presence of 1,3-propanediol decreased the glycerol fluxes (Figure 6) to about 10% of their values when glycerol was mixed with just water. Similarly, 1,3-propanediol decreased the glucose fluxes (Figure 7) to half of their original values. Apparently 1,3-propanediol preferentially adsorbs in the pores of the X-type zeolite and inhibits permeation of the larger molecules.13 Glycerol did not significantly decrease the 1,3-propanediol flux (Figure 5), but it decreased the glucose flux by a small amount (Figure 7). The fluxes of both 1,3-propanediol (Figure 5) and

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Figure 6. Glycerol flux as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

Figure 7. Glucose flux as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

glycerol (Figure 6) decreased when glucose was in the feed. Moreover, all solutions containing glucose had lower total fluxes (Figure 1), indicating that glucose also inhibited water permeation. O’Brien and Craig16 observed that glucose reduced the total flux through a poly(dimethylsiloxane) membrane in their fermentation/ pervaporation system. Ikegami et al.17 reported that the total pervaporation flux for an ethanol/water solution through a silicalite membrane decreased by 67% with increasing glucose concentration. Washing their membrane surface with distilled water restored the pervaporation performance. They concluded that glucose was not adsorbed within the membrane, but it strongly inhibited water adsorption. At room temperature, the pervaporation fluxes through the X-type membrane for 2,2-dimethylbutane (DMB, kinetic diameter of 0.58 nm), m-xylene (kinetic diameter of 0.685 nm), and tri-isopropyl benzene (TIPB, kinetic diameter of 0.85 nm) are 81, 78, and 2.3 g/(m2 h), respectively. The much lower flux for the TIPB molecule, which is larger than the X-zeolite pore diameter of 0.74 nm, indicates that TIPB permeates through the nonzeolite pores. Thus, if the diffusivities of these three molecules are similar in the nonzeolite pores, then only a few percent of the DMB or xylene permeates through the nonzeolite pores, and thus the membrane is of high quality. The glucose flux through the X-type membrane is almost 3 orders of magnitude lower than the TIPB flux, partly because the glucose feed concentration is 2 orders of magnitude lower than the TIPB concentration. The steady-state total flux for the glucose/water feed is only 65% of the pure water flux at 308 K; some water

Figure 8. 1,3-Propanediol/glycerol and 1,3-propanediol/glucose separation selectivities as a function of temperature for pervaporation of the indicated solutions through an X-type membrane.

permeates through nonzeolite pores, and glucose might diffuse into pores smaller than 0.85 nm but pass through them slowly. Netrabukkana et al.18 found that glucose (0.86 nm) diffused into Y-zeolite (FAU structure, 0.74-nm pores), whereas glucitol (0.97 nm) did not. They concluded that either the cyclic ring of glucose deformed and became smaller in one dimension as it interacted with the pore opening, allowing it to permeate into the intracrystalline matrix, or the acidity of the Y-zeolite opened the cyclic ring to form a linear molecule that could pass through the pore. Chang and Lee19 also found that glucose absorbed in Y-zeolite pores during the separation of glucose from fructose. Separation Selectivities. As shown in Figure 8, both the 1,3-propanediol/glycerol and the 1,3-propanediol/ glucose separation selectivities for quaternary and ternary feed solutions increased slightly with temperature. The highest 1,3-propanediol/glycerol and 1,3propanediol/glucose selectivities were 67 and 2200, respectively, at 323 K. The separation selectivities increased with temperature because the 1,3-propanediol fluxes (Figure 5) increased more than the glycerol or glucose fluxes (Figures 6 and 7, respectively). The presence of glucose increased the 1,3-propanediol/ glycerol selectivity from 41 (ternary mixture) to 59 (quaternary mixture). The ideal 1,3-propanediol/glycerol and 1,3-propanediol/ glucose selectivities were calculated from the permeate concentrations in the binary 1,3-propanediol/water, glycerol/water, and glucose/water mixtures. The 1,3propanediol/glycerol separation selectivity of the ternary 1,3-propanediol/glycerol/water solution is approximately 5 times higher than the ideal selectivity of 8 at 308 K, indicating that the high selectivity is partly due to preferential adsorption of 1,3-propanediol. The glycerol permeate concentration was lower for the quaternary feed than for the ternary 1,3-propanediol/glycerol/water feed (Figure 4). Thus, glucose increased the 1,3-propanediol/glycerol selectivity (Figure 8). Similarly, Ikegami et al.17 reported that the ethanol/water selectivity in pervaporation through a silicalite membrane increased from 23 to 137 when the glucose concentration was increased from 0 to 22.4% (w/v). The 1,3-propanediol/glucose separation selectivity for the ternary 1,3-propanediol/glucose/water solution was 15% higher than the ideal selectivity because of preferential adsorption of 1,3-propanediol. Because the glucose flux decreased in the presence of glycerol (Figure 7), the quaternary solution had higher 1,3-propanediol/


Figure 9. Total flux and 1,3-propanediol/glycerol separation selectivity as functions of temperature for pervaporation of a multicomponent fermentation broth through an X-type membrane.

Figure 10. Permeate concentrations of 1,3-propanediol and glycerol as functions of temperature for pervaporation of a multicomponent fermentation broth through an X-type membrane.

glucose selectivities than the ternary 1,3-propanediol/ glucose/water solution (Figure 8). Note that, at 308 K, glycerol increased the 1,3-propanediol/glucose selectivity from 1600 to 1900. Fermentation Broth Separation. The X-type membrane was also effective in separating 1,3-propanediol from the multicomponent fermentation broth. The feed concentrations for 1,3-propanediol, glycerol, and glucose were 73.2, 10.6, and 0.15 g/L, respectively, and their permeate concentrations are listed in Table 1. The HPLC peak areas are also given in Table 1 for other permeate species that were not calibrated. The glucose permeate concentration was below the detection limit. Figure 9 shows that both the total flux and the 1,3propanediol/glycerol selectivity increased with temperature. This behavior is similar to that observed for the model feed. The 1,3-propanediol/glycerol selectivity was 110 at the highest temperature. At 308 K, the total flux for the fermentation broth was approximately 57% of that for the model solution, indicating that some components in the broth probably adsorb and block pores. As shown in Figure 10, similar to the results for the model solutions, the 1,3-propanediol and glycerol permeate concentrations decreased with temperature. The 1,3-propanediol permeate concentrations were somewhat higher than those of the model solution but were still lower than the feed concentration of 73.2 g/L. The glycerol permeate concentration was less than 1% of its feed concentration of 10.6 g/L. As shown in Figure 11, the 1,3-propanediol permeate flux increased at higher temperatures and was ap-

Figure 11. Fluxes of 1,3-propanediol and glycerol as functions of temperature for pervaporation of a multicomponent fermentation broth through an X-type membrane.

proximately 22% lower than the flux for the model solution. Interestingly, the glycerol flux decreased with temperature, so the 1,3-propanediol/glycerol separation selectivity increased with temperature more dramatically for the broth (Figure 9). The glycerol flux for the broth was higher than that for the model quaternary solution, but it was similar to that for the 1,3-propanediol/glycerol/water solution (Figure 6) because there was almost no glucose in the broth. The permeate flux of water was approximately 45% lower in the broth (relative to the model solution) as a result of pore blocking. The fluxes of 1,3-propanediol and glycerol were also lower for the broth, but their permeate concentrations were higher than for the model solutions because the water flux was even lower for the broth. The 1,3propanediol/glycerol ratio of feed concentrations for the broth was 6.9, whereas it was 20 for the model solution. Therefore, the 1,3-propanediol/glycerol selectivity for the broth was higher than that for the model solution, even though the 1,3-propanediol flux was lower and the glycerol flux was higher for the broth. The feed broth was brown, and the color is probably due to more than one species. However, the permeate liquid was clear. Although D/L-butanediol was the only species with a permeate concentration higher than its feed concentration, its HPLC peak area was still more than 2 orders of magnitude lower than that of 1,3propanediol. The permeate concentrations of some species, including KH2PO4, succinic acid, and lactate (Li), were below the detection limit, indicating that these species were rejected by the X-type membrane. Other species, including formic acid and acetic acid, had approximately an order of magnitude lower permeate than feed concentrations, so they were also rejected by the membrane. Longer-Term Separation. Pervaporation through the X-type membrane was carried out for approximately one week for both the model quaternary solution and the fermentation broth after a steady state had been reached. Figure 12 shows the total fluxes as functions of time at 308 K for the two solutions. The flux of the model solution varied by less than 10% during the week, with a decrease of about 0.06% of its original value per hour. The 1,3-propanediol permeate concentration varied by less than 8% with no obvious trend (Figure 13), and the 1,3-propanediol/glycerol separation selectivity varied by less than 12% (Figure 14). For the fermentation broth, the total flux (Figure 12), the 1,3-propanediol permeate concentration (Figure 13),

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Figure 12. Total fluxes as functions of time at 308 K for pervaporation of a multicomponent fermentation broth and a 1,3propanediol/glycerol/glucose/water model solution through an Xtype membrane.

Figure 13. Permeate concentrations of 1,3-propanediol as functions of time at 308 K for pervaporation of a multicomponent fermentation broth and a 1,3-propanediol/glycerol/glucose/water model solution through an X-type membrane.

pores and fouled the membrane. The fouled membrane was brown, whereas the outside of the tube was white. Sano et al.20 reported long-term separation of acetic acid/water mixtures by pervaporation through a silicalite membrane. They found that the acetic acid permeate concentration decreased by 20% because of acetic acid adsorption in the zeolitic pores. The original permeate concentration was recovered by treating the membrane with pure ethanol. Our X-type membrane was boiled in ethanol/water (50/50) for 20 min and dried at 373 K under vacuum for 30 min, but this treatment did not restore the total flux, the 1,3-propanediol permeate concentration, or the 1,3-propanediol/glycerol selectivity. Because the sodium form of the X zeolite is thermally stable to 933 K,21 the fouled membrane, which was brown, was calcined at 653 K for 4 h. The membrane became white after calcination. Initially, the membrane had a high flux for the broth, and then it approached steady state. Water diffused faster through the X-type membrane but adsorbed more weakly than 1,3-propanediol and glycerol, which diffused more slowly.13 The uptake of mixtures in zeolite crystals often shows a maximum as a function of time for the fast but weakly adsorbing component, whereas at equilibrium, the slowly diffusing but strongly adsorbing component dominates the adsorbed phase.22 Transient pervaporation for the X-type membrane exhibited the same behavior. The water flux was high initially, so the total flux was high (Figure 12) and the 1,3-propanediol permeate concentration was low (Figure 13). The 1,3-propanediol permeate concentration increased with time because the water flux decreased and the 1,3-propanediol flux increased. The 1,3-propanediol flux increased because the adsorbed coverage of 1,3propanediol increased, and thus, the 1,3-propanediol/ glycerol separation selectivity increased. As shown in Figures 12-14, pervaporation reached a steady-state flux in 6 h. As shown in Table 1, the permeate concentrations of all components were close to those measured before fouling. The total flux (Figure 12), the 1,3propanediol permeate concentration (Figure 13), and the 1,3-propanediol/glycerol selectivity (Figure 14) were stable for the next 60 h. The membrane fouled again in the next 15 h, as shown in Figures 12-14. The membrane was regenerated again by calcination at 653 K for 4 h. Summary

Figure 14. 1,3-Propanediol/glycerol selectivity as a function of time at 308 K for pervaporation of multicomponent fermentation broth and a 1,3-propanediol/glycerol/glucose/water model solution through an X-type membrane.

and the 1,3-propanediol/glycerol separation selectivity (Figure 14) were stable for 60 h, but the 1,3-propanediol permeate concentration decreased dramatically in the next 15 h. The total flux decreased by approximately 35%, and the 1,3-propanediol permeate concentration and 1,3-propanediol/glycerol selectivity were approximately 38 and 10%, respectively, of their original values after 80 h. Apparently some component(s) in the broth deposited onto the membrane surface or in the zeolite

An X-type zeolite membrane separated 1,3-propanediol from model aqueous solutions and fermentation broth by pervaporation. The total fluxes for the model solution and the broth were 2.1 and 1.2 kg/ (m2 h), respectively, at 308 K. The corresponding 1,3propanediol/glycerol selectivities were 59 and 61. The 1,3-propanediol/glucose selectivity for the model solution at 308 K was 1900. The total flux and selectivities increased with temperature. The total flux was lower for the broth, but the 1,3-propanediol permeate concentration and the 1,3-propanediol/glycerol separation selectivity were higher than those for the model solution. The membrane was stable with the model solution as the feed for at least a week. The fermentation broth started to foul the membrane after 60 h, and the membrane was regenerated by calcination at 653 K for 4 h.


Acknowledgment We gratefully acknowledge support by the DuPont Corporation. We thank Dr. Tucker Norton from DuPont for his assistance and for the donation of the fermentation broth. Literature Cited (1) Anon, DuPont Unveils Sorona Polymers. Chem. Eng. News 2000, 13, 78. (2) Corbin, D.; Norton, T. (DuPont Corporation). U.S. Patent Application 09/677,121, 2000. (3) Fleming, H. L.; Slater, C. S. Membrane Handbook; Ho, W. S., Sirkar, K. K., Eds.; Van Nostrand Reinhold: New York, 1992; p 105. (4) Sano, T.; Yanagishita, H.; Kiyozumi, Y.; Mizukani, F.; Haraya, K. Separation of Ethanol-Water Mixture by Silicalite Membrane on Pervaporation. J. Membr. Sci. 1994, 95, 221. (5) Kita, H.; Horii, K.; Ohtoshi, Y.; Tanaka, K.; Okamoto, K. Synthesis of a Zeolite NaA Membrane for Pervaporation of Water/ Organic Liquid Mixtures. J. Mater. Sci. Lett. 1995, 14, 206. (6) Kita, H.; Asamura, H.; Tanaka, K.; Okamoto, K. Preparation and Pervaporation Properties of X- and Y-type Zeolite Membranes. Membr. Form. Modif. 2000, 744, 330. (7) Sano, T.; Hasegawa, M.; Kiyozumi, Y.; Yanagishita, H. Separation of Methanol/Methyl-tert-Butyl Ether Mixture by Pervaporation using Silicalite Membrane. J. Membr. Sci. 1995, 107, 193. (8) Nishiyama, N.; Ueyama, K.; Matsukata, M. A Defect Free Mordenite Membrane Synthesized by Vapor Phase Transport Methodology. J. Chem. Soc., Chem. Commun. 1995, 1967. (9) Nishiyama, N.; Ueyama, K.; Matsukata, M. Synthesis of FER Membrane on an Alumina Support and its Separation Properties. Stud. Surf. Sci. Catal. 1997, 105, 2195. (10) Borjesson, J.; Karlsson, H. O. E.; Tragardh, G. Pervaporation of a Model Juice Aroma Solution: Comparison of Membrane Performance. J. Membr. Sci. 1996, 119, 229. (11) Qureshi, N.; Meagher, M. M.; Hutkins, R. W. Recovery of Butanol from Model Solutions and Fermentation Broth using a Silicalite/Silicone Membrane. J. Membr. Sci. 1999, 158, 115.

(12) Li, S.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. Separation of 1,3-Propanediol from Glycerol and Glucose using a ZSM-5 Zeolite Membrane. J. Membr. Sci., submitted for publication. (13) Li, S.; Tuan, V. A.; Falconer, J. L.; Noble, R. D. X-Type Zeolite Membranes: Preparation, Characterization and Pervaporation Separation Performance. AIChE J., submitted for publication. (14) Kita, H.; Asamura, H.; Tanaka, K.; Okamoto, K. Preparation and Pervaporation Properties of X- and Y-Type Zeolite Membranes. ACS Symp. Ser. 2000, 744, 330. (15) Liu, Q.; Noble, R. D.; Falconer, J. L.; Funke, H. H. Organics/Water Separation by Pervaporation with a Zeolite Membrane. J. Membr. Sci. 1996, 117, 163. (16) O’Brien, D. J.; Craig, J. C., Jr. Ethanol Production in a Continuous Fermentation/Membrane Pervaporation System. Appl. Microbiol. Biotechnol. 1996, 44, 699. (17) Ikegami, T.; Yanagishita, H.; Kitamoto, D.; Haraya, K.; Nakane, T.; Matsuda, H.; Koura, N.; Sano, T. Highly Concentrated Aqueous Ethanol Solutions by Pervaporation using Silicalite MembranesImprovement of Ethanol Selectivity by Addition of Sugars to Ethanol Solution. Biotechnol. Lett. 1999, 21, 1037. (18) Netrabukkana, R.; Lourvanij, K.; Rorrer, F. L. Diffusion of Glucose and Glucitol in Microporous and Mesoporous Silicate/ Aluminosilicate Catalysts. Ind. Eng. Chem. Res. 1996, 35, 458. (19) Cheng, Y. L.; Lee, T. Y. Separation of Fructose and Glucose Mixture by Zeolite Y. Biotechnol. Bioeng. 1992, 40, 498. (20) Sano, T.; Ejiri, S.; Yamada, K.; Kawakami, Y.; Yangishita, H. Separation of Acetic Acid-Water Mixtures by Pervaporation through Silicalite Membranes. J. Membr. Sci. 1997, 123, 225. (21) Breck, D. W. Zeolite Molecular Sieves; Wiley: New York, 1974; p 495. (22) Karger, J.; Bulow, M. Theoretical Prediction of Uptake Behavior in Adsorption Kinetics of Binary Gas Mixtures Using Irreversible Thermodynamics. Chem. Eng. Sci. 1975, 30, 893.

Received for review October 19, 2000 Revised manuscript received February 19, 2001 Accepted February 19, 2001 IE000905L

Separation of 1, 3-Propanediol from Aqueous Solutions Using

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