Pervaporation Membranes That Are Highly Selective for Acetic Acid

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Ind. Eng. Chem. Res. 2003, 42, 582-595

Pervaporation Membranes That Are Highly Selective for Acetic Acid over Water Yingjie Qin, J. P. Sheth, and K. K. Sirkar* Department of Chemical Engineering, Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey 07102

Supported liquid membrane pervaporation (SLMPV) is a pervaporation process for separating volatile organic compounds (VOCs) from their dilute aqueous solution. It simultaneously integrates extraction of the VOCs from the aqueous solution with flash distillation of the VOCs from the organic phase. By using a liquid membrane consisting of reactive extractants, pervaporation of primarily acetic acid from its aqueous solutions was studied. Limited studies of butyric acid were also done. Among various extractants tested, trioctylamine (TOA) and tridodecylamine demonstrated better performances. The SLM is permselective for acetic acid and butyric acid. The acetic acid selectivity can be as high as 33 for a feed of 1 M at 60°C, an order of magnitude higher than that obtained by any solid polymeric membrane reported in the literature. The simple SLM system demonstrated long-term stability; for example, decreases of about 30% in both pervaporation flux and selectivity were observed over an operational period of 500 h. A new technique of continuous on-line regeneration of the LM during operation maintains completely stable operational performance. 1. Introduction Many dilute industrial waste streams contain volatile polar solutes, namely, carboxylic acids of low molecular weight such as formic acid, acetic acid, and acrylic acid. Fermentation also produces dilute solutions of carboxylic acids such as acetic acid, propionic acid, and butyric acid. It is demanding to remove, recover, concentrate, and purify these acidic solutes from their dilute aqueous solutions. The relative volatility between water and acetic acid is close to unity. Thus, their separation by distillation is very energy-intensive. In general, the more dilute the acid solution is, the more attractive solvent extraction becomes. However, in a solvent extraction process, the organic solvent to water feed ratio is generally high, ranging from 2:1 to 5:1 for acetic acid.1 The solubility loss of the solvent to the raffinate is also significant when solvents of low molecular weight (e.g., methyl isobutyl ketone) are used.2 When reactive extractants (amines, phosphates, or phosphine oxides) are used, the distribution coefficient and separation factor are high.2-6 However, back extraction with caustics or distillation is necessary to further recover and purify the final product. In addition, the solubility loss of the extractants and the modifiers to the feed solution leads to further problems of wastewater treatment or toxicity to whole cells in the fermentation broth. Ion exchange and electrodialysis have also been used for the recovery and concentration of acids from fermentation broths or very dilute solutions.7,8 However, ion exchange as well as extraction/back-extraction are less attractive, as they consume a base and an acid to form a salt. Pervaporation, a promising membrane technology, is an energy-saving one-step separation process compared to distillation, especially for the fractionation of liquid mixtures such as azeotropic mixtures, close-boiling mixtures, and mixtures consisting of heat-sensitive * To whom correspondence is to be addressed. Phone: 973596-8447. Fax: 973-642-4854. E-mail: [email protected].

compounds, as well as the separation of a component having a low volatility from its dilute aqueous solutions. At this time, more than 50 articles and patents have been published on many kinds of organic and inorganic membranes for the separation of carboxylic acid-water mixtures by pervaporation. However, most reported membranes were highly water-selective and only suitable for the dehydration of acid-water mixtures.9-12 They are uneconomical for the removal and recovery of acetic acid from wastewater or fermentation broth. Only a few organic/inorganic membranes have been reported for the preferential pervaporation-based separation of acetic acid, formic acid and butyric acid from their aqueous solutions.9-15 The highest selectivity obtained is less than 4.0 for moderate concentrations of acetic acid in the feed and even lower for low feed concentrations. Among polymer membranes synthesized for the pervaporation separation of acetic acid, a few membranes contain basic groups (pyrrolidone, pyridine, or aniline).11,12,16-18 Stimulated by the high selectivity for acetic acid over water achieved when high-molecularweight amines were used in reactive extraction, efforts to combine a reactive extraction process and a backstripping process into a pervaporation process by using the basic groups as fixed carriers were made.12 Nevertheless, such efforts were not successful. The membranes obtained were highly water-selective and useful for the dehydration of acetic acid solutions.11,12,16-18 It was also found that even some organophilic membranes [e.g., poly(4-methyl-1-pentene), poly(styrenebutadiene), poly(1-butyl methacrylate), poly(2-ethylhexyl methacrylate), polystyrene, and poly(vinyl acetal)] are also water-selective over acetic acid.12,18-20 It was explained that the selective adsorption of acetic acid molecules onto the polymer with its hydrophobic moiety in contact with the polymer molecules hydrophilizes the polymer and makes it more compatible with water.19 An additional example is useful: a silicalite zeolite

10.1021/ie020414w CCC: $25.00 © 2003 American Chemical Society Published on Web 01/01/2003

Ind. Eng. Chem. Res., Vol. 42, No. 3, 2003 583 Table 1. Details of Hollow Fiber Membrane Modules Used

a

module no.

1

2

3

4

5

membrane type characteristic no. of fibers porosity o.d., µm i.d., µm effective length, cm mass-transfer area, cm2 shell details

X-10 silicone-coated 75 0.3 290 240 30 205.0 1/ -in.-o.d. 4 SS316

X-20 porous 102 0.3 290 240 18.5 142.3 1/ -in.-o.d. 4 SS316

X-20 porous 78 0.3 290 240 33.5 238.1 1/ -in.-o.d. 4 SS316

X-20 porous 78 0.3 290 240 32 227.4 1/ -in.-o.d. 4 SS316

X-20a porous 48 0.3 290 240 26 94.0 1/ -in.-o.d. 4 SS316

With three shell-side exits.

membrane that could preferentially separate acetone from aqueous solution by a separation factor of 255 was unable to selectively remove acetic acid from aqueous solutions.21 Pervaporation through supported liquid membranes (SLMs) has long been suggested, especially for the separation and concentration of fermentation products such as ethanol, butanol, diacetyl, and acetic acid.22-26 Oleyl alcohol and iso-tridecanol were used as the liquid membranes. The liquid membrane of oleyl alcohol yielded a higher butanol selectivity and flux than did a silicone rubber membrane (a representative solid membrane), as well as an extended stability of over 100 hours. Compared to the large number of pervaporation studies using solid membranes, investigations on liquid membranes for pervaporation are rare. This can be attributed to the well-known instability of supported liquid membranes for practical applications. On the other hand, pervaporation through a polydimethylsiloxane (PDMS) membrane swollen by oleyl alcohol or silicone oil did not provide a butanol selectivity as high as that provided by an SLM. Also, oleyl alcohol swollen membrane did not provide a stable butanol flux.27 A liquid membrane consisting of trioctylamine (TOA) and petrolatum tested for acetic acid separation from its aqueous solutions by pervaporation or vapor permeation22 yielded a selectivity for both processes of less than 6.2. The volatility of acetic acid is very close to that of water. Acetic acid also has the lowest reactive extraction distribution constant because its acidity is lower than that of formic acid and its organophilicity is lower than those of propionic and butyric acids.3 We have chosen to investigate the removal and enrichment of acetic acid from its dilute aqueous solutions by pervaporation through an SLM. The SLM consisted of a fatty amine, as well as its mixtures with a higher fatty alcohol [oleyl alcohol (OA)]. The influence of the liquid membrane composition, liquid membrane thickness, feed concentration, and temperature on the permeability, selectivity, and stability of the supported liquid membrane were studied. We wish to point out, on the basis of runs lasting longer than 500 h, that the membranes were found to be stable and highly selective for acetic acid over water when an on-line liquid membrane regeneration technique was used. We also report the highly encouraging results of pervaporation studies for the removal of butyric acid from water. 2. Experimental Details 2.1. Chemicals Used. Acetic acid (HAc) (99.5%, glacial), butyric acid, trilaurylamine (TLA), (CH3(CH2)11)3N, (95%, GC), hexadecane (99%, GC), and oleyl

alcohol (70%, GC) were obtained from Acros Organics; trioctylamine (∼95%, GC), triisooctylamine (>97%, GC), Amberlite LA-2, di-2-ethyl-hexylamine (97%), and trihexamine (97%) were obtained from Fluka; tri-n-butyl phosphate (TBP) (purified), isooctane (HPLC Grade), and hexane (>99.5%, GC) were obtained from Fisher Scientific; tri-n-octylphosphine oxide (TOPO) was obtained from Sigma; 2-decyl-1-tetradecanol (97%) was obtained from Aldrich; liquid N2 was obtained from JWS Technologies Inc.; and N2 gas (extra dry, 99.9%) was obtained from Matheson. 2.2. Module Dimensions. Hollow fiber membrane modules were fabricated in the laboratory. Microporous hydrophobic polypropylene hollow fiber membranes, supplied by Hoechst Celanese (currently, Celgard Inc., Charlotte, NC), and silicone-coated, microporous hydrophobic polypropylene hollow fiber membranes supplied by Applied Membrane Technology Inc., Minnetonka, MN, were used in the modules. The module details are listed in Table 1. 2.3. Liquid Membrane Immobilization Techniques. 2.3.1 Technique A: Lumen-Side Immobilization. The liquid membrane material was introduced into and immobilized in the pores of the hollow fibers by recirculating the liquid through the fiber lumen with a peristaltic pump. The liquid spontaneously wetted the fiber wall micropores and penetrated the shell side. The circulation was carried out for approximately 30 min. Dry N2 gas was then passed through the lumen side of the module at 3 psig (20.7 kPag) pressure for approximately 1-2 h to ensure complete removal of the liquid membrane from the lumen side, reduce the SLM thickness, and push the liquid membrane closer to the shell side. Excess liquid membrane material was removed from the shell side by draining. At the end of this procedure, a permeation test, described later, was carried out to check the integrity of the immobilized liquid membrane (ILM). This technique was usually employed when TOA was used as the carrier. 2.3.2. Technique B: Shell-Side Immobilization. When the liquid used for the liquid membrane material was very viscous, for example, a solution containing TLA, the liquid membrane material was brought into the shell side of the module and held in that position via a disposable glass pipet. The module was held in that position for more than 1 h. The removal of the excess liquid membrane material and the permeation test were carried out next as in technique A. This technique was also used when pure TLA was used as the carrier. 2.4. Permeation Test and Procedure for Producing SLMs with Decreased Thicknesses. Gas permeation was carried out to check the SLM integrity by measuring the variation in the N2 permeation flux with

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Figure 1. (a) Experimental setup for supported liquid membrane-based pervaporation process. (b) Hollow fiber module with three exits on the shell side.

time. Dry N2 gas was brought to the module lumen from one end while the other end of the lumen was kept closed by a plug. One port on the shell side was connected to a soap bubble meter, while the other shellside port was kept closed with a plug. The pressure in the module lumen was monitored by a pressure gauge fitted between the dry N2 cylinder and the module. Hence, N2 fed to the lumen side had to permeate through the SLM into the shell side, after which it entered the soap bubble meter. The N2 permeation rate was measured with a stopwatch. Measurements were recorded for pressures up to 6 psig (41.3 kPag) at increments of 1 psig. At each pressure, multiple readings were taken, at intervals of 10-15 min, until two successive readings were identical. A low and expected N2 flux ensured a defect-free SLM. To form an SLM having a thickness less than that of the fiber wall, first, the liquid membrane prepared for impregnation of the pores contained a volatile solvent, for example, hexane or isooctane in addition to a nonvolatile amine. Then, after impregnation of the pores with the liquid introduced through the fiber lumen, dry N2 gas was brought to the lumen side from one end while the other end of the lumen side was closed by a plug; the module was put in a vertical position. It was expected to push the membrane liquid flow from the lumen side to the shell side and then out of the shell

side from the port. Continuous lumen-side passing of N2 subsequently removed the volatile SLM component, such as hexane or isooctane, if any, and thereby reduced the LM thickness to a value less than the thickness of the membrane wall. A similar N2 test procedure was used next to test the SLM integrity. 2.5. Experimental Setup and Procedure. The experimental setup is shown in Figure 1a. The hollow fiber module was kept completely immersed in a constant-temperature water bath. A Fisher Scientific temperature controller maintained the temperature of the water bath within (0.1 °C of the set point. Feed containing acetic acid was introduced into the module lumen from a reservoir by a Masterflex peristaltic pump (model 7518-10) at a constant flow rate. The feed-in and feed-out tubing to the module were each fitted with a three-way valve for sampling purposes. The shell side of the hollow fiber module was maintained under vacuum by a Welch GEM 1.0 vacuum pump, monitored by a J-KEM Scientific digital vacuum regulator (model 200), and controlled by a needle valve attached to the bypass loop of the regulator. A glass vacuum trap (Lab Glass, Vineland, NJ) immersed in a liquid N2 well (Dewar flask, Lab Glass) and connected in series to the vacuum pump was used to collect the permeate vapor. 2.5.1. Operation Startup. The hollow fiber module containing a particular SLM was attached to the per-

Ind. Eng. Chem. Res., Vol. 42, No. 3, 2003 585

vaporation system shown in Figure 1a. The feed was introduced to the fiber lumen at a constant flow rate. The vacuum trap for permeate collection was attached to the permeate tubing between the shell side of the hollow fiber module and the vacuum pump. After the lumen side of the module was completely filled with the incoming feed, the vacuum pump was started, and the permeate side of the system was evacuated gradually by controlling the needle valve. After approximately 1015 min, when the shell-side pressure was stable around the preset pressure with a variation of 2 mmHg, the vacuum trap was immersed in a liquid N2 well to collect the permeate vapors by condensation. 2.5.2. Permeate Sampling. Permeate vapors were collected for a fixed interval of time in the attached vacuum trap. This trap was then isolated from the system for sampling by a set of two three-way ball valves as the stand-by vacuum trap was brought online. This was achieved by switching the vacuum pump to the stand-by trap, by one of the three-way ball valves attached between the vacuum pump and the vacuum trap. After stabilization of the vacuum in the stand-by trap, the vacuun trap was immersed in a new liquid N2 well. At the precise changeover time, the second threeway ball valve, attached between the module shell side and the vacuum trap, was switched over to the standby vacuum trap, thus bringing it on-line. The isolated vacuum trap was then removed from the liquid N2 well, and its temperature was allowed to rise to room temperature. The trap was then removed from the system; the liquid permeate concentration was analyzed by acid-base titration using an auto titrator (model DL-12, Mettler, Toledo). Feed-in and feed-out samples were analyzed for feed concentration changes. The weights of each vacuum trap were recorded before and after permeate collection for flux calculations. 2.5.3. Module Washing. A used membrane module containing an SLM could be regenerated by washing with hexane. The lumen side of a module dismantled from the setup was first washed with deionized water circulated by a pump. Then, hexane was pumped to the module lumen to push out the water from the module and to dilute and replace the LM material in the micropores in the membrane wall and any possible LM in the shell side. After both the shell and lumen sides of the module had been washed thoroughly with hexane, the module was filled with fresh hexane and kept overnight to extract any LM material that might have been partitioned into the solid part of the porous polypropylene membrane. After hexane was removed from the module, the module was dried by passing N2 through the shell and lumen sides for 2 h. Then, the module could be used again. 2.5.4. Continuous Regeneration of the LM During Operation. A LM could be also regenerated during an SLMPV experiment on-line. For this purpose, a module with three shell exits, compared to the conventional hollow fiber module with two exits, was fabricated. Two exits near the ends of the module were used as is customary for exits for permeates; the third exit in the middle of the shell was used to add the LM material (TOA) to the shell side (Figure 1b). There were two valves, V1 and V2, on tubing connected to this exit. The volume between the valves can be filled with the LM material, and the valves closed to the environment. When the valve V1 connected to the shell exit was opened, giving the liquid in the tubing access to the shell

side, the high vacuum in the shell side made the gas dissolved in the liquid immediately desorb from the liquid; this would also foam the liquid in the tube and make it enter the shell and contact the fiber bundle. The capillary force would facilitate the refilling of the micropores in the membrane wall if they were only partially filled as a result of evaporation- or dissolvationbased loss. Such on-line regeneration was performed every 2 days. This can be easily automated. 3. Theoretical Considerations 3.1. Background for Extraction of Acetic Acid from Dilute Aqueous Solutions. An appropriate liquid-liquid extraction process promises high economic potential for the separation and concentration of carboxylic acids from their dilute aqueous solutions. Such an extraction system must fulfill two basic requirements: high distribution coefficient and high selectivity. Extraction of an organic acid has been extensively studied using tri-n-octylphosphine oxide (TOPO), tributyl phosphate (TBP), and high-molecular-weight alkylamines as extractants because of their excellent chemical stabilities, high boiling points, and low solubilities in water, compared to physical extractants such as toluene, methyl isobutyl ketone (MIBK), chloroform, or ethyl acetate. Among amines as extractants, trioctylamine (TOA) and trilaurylamine (TLA) are the most widely used. In conventional extraction processes, inert organic solvents such as toluene, benzene, xylene, hexane, heptane, and decane are used as diluents to adjust the viscosity and density of the extractant, and water-insoluble organic acids, fatty amides, and fatty alcohols are used as modifiers to further increase the distribution coefficient.2-4,28-31 3.2. Selection of Extractant and Modifier. As a component of an SLM, the extractant or modifier must have extremely low water solubility and volatility to ensure the stability of the liquid membrane during pervaporation operation. Therefore, TOA, TLA, and TOPO were selected as candidates for the extractant, and dodecanol, oleyl alcohol (OA), TBP, and 2-decyl-1tetradecanol were selected as modifiers. 3.3. Extraction and Mass-Transfer Mechanism. 3.3.1. Forward Extraction. The extraction of a monocarboxylic acid from an aqueous solution into an organic phase is usually characterized by the distribution coefficient Kd

Kd ) [HA]org/[HA]aq

(1)

where [HA]aq and [HA]org are the total concentrations of the carboxylic acid, HA, in the aqueous phase and in the organic phase, respectively. When a reactive extractant, e.g., a fatty amine, phosphate, or phosphine oxide is used to extract the carboxylic acid, Kd is a function of the concentration of the extractant in the diluent; Kd is also strongly influenced by the concentration of the modifier, which is generally a fatty alcohol, an amide, or an organic acid. There is generally an optimum concentration for a mixture of an extractant and a modifier where the value of Kd is the highest.3,4,28 The reactive extraction mechanism of the monocarboxylic acid HA is usually expressed in the literature as

mHAaq + Borg S mHA:Borg

(m ) 1, 2, 3, 4 or 5) (2)

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where Borg is the extractant. When amines (TOA, TLA, or tridecylamine) are used as extractants and fatty alcohols as modifiers for acetic acid extraction, the governing extraction mechanism is by ion-pair formation and H-bonding.32,33 In eq 2, m is the association number in an acid-extractant complex. The value of m can be up to 5.4,28,34,35 Several complexes can exist in the organic phase, and the complex can be stabilized by the modifier. Therefore, the value of Kd is a complicated function of the species and the concentration of the acid in the feed, the species and the concentration of the extractant and the modifier in the organic phase, and the temperature. For example, when pure TOA is used as an extractant without modifier or diluent, Kd increases with increasing acetic acid concentration in the aqueous phase.34 It is rare to consider the coextraction of water during the extraction of carboxylic acid by an extractant.2,34,35 However, it is important for describing a pervaporation process for the separation of acetic acid because the coextraction of water directly influences the selectivity of the pervaporation process. Generally, the solubility of water in the organic phase is taken as physical; however, it is also a complicated function of the composition of the organic phase and, thus, of the species and concentration of the acid in the feed, the species and concentration of extractant and modifier in the organic phase, and the temperature34,35

H2Oaq S H2Oorg

(3)

3.3.2. Stripping by Vacuum. For vacuum stripping in the shell side, the mechanism can be expressed as

mHA:B S mHAvap + Borg

(4)

H2Oorg S H2Ovap

(5)

3.3.3. Mass-Transfer Resistances. It has been reported that, when TOPO in the diluent is used to extract acetic acid in a conventional extraction or supported liquid membrane process, interfacial reaction resistance is negligible compared to the diffusion resistance.36,37 When an extractant such as TOA or TLA is used alone as the SLM material or mixed with a nonvolatile modifier such as OA as the SLM material, the viscosity will be much higher than that in the presence of volatile diluents or/and modifiers of lower molecular weight: the diffusion resistance of the species through the SLM will be more dominant than that observed in the literature. Therefore, it can be thought that the organic-phase species containing acetic acid are in equilibrium with acetic acid in the aqueous phase at the feed-SLM interface. Further, such an equilibrium also exists between the species in the organic phase and the vapor phase at the SLM-vacuum interface. The molar fluxes of acetic acid and water at a given point in the axial direction of the hollow fiber can be given, respectively, as

JHA ) KHA(PHA* - PHA,perm)

(6)

JH2O ) KH2O(PH2O* - PH2O,perm)

(7)

Here, PHA* and PH2O* are the vapor pressures of acetic acid and water, respectively, in equilibrium with the bulk feed. PHA,perm and PH2O,perm are the partial pressures of acetic acid and water, respectively, in the shell side,

which depend on the preset permeation pressure and the selectivity of the liquid membrane. The overall mass-transfer coefficients are KHA and KH2O. The viscosity of OA is 26 cP at 35 °C,23 and that of the mixture of TOA and tridecylamine (TDA) is 23 cP at 40 °C.35 The viscosity of TLA is certainly higher because its molecular weight is greater than those of TOA and TDA. Therefore, the resistance for the diffusion of species in the liquid membrane is much larger than the resistance for the diffusion of acetic acid in the bulk feed. Actually, the mass-transfer coefficients in eqs 6 and 7 reflect the resistance of the SLM. On the other hand, all complexes between acetic acid and extractant have similar molecular sizes and are much larger than free acetic acid molecules. Therefore, it is reasonable to assume that all complexes have the same diffusion coefficient. We assume that the reactions among free acetic acid and all complexes containing acetic acid in the SLM are simultaneous; thus, the mass-transfer rates of acetic acid and water can be further expressed as

JHA ) kHA(CHA,org,f - CHA,org,p) + kcomplex(CmHA:B,org,f - CmHA:B,org,p) (8) JH2O ) kwater(Cwater,org,f - Cwater,org,p)

(9)

where CHA,org,f and CHA,org,p are the concentrations of free acetic acid in the SLM at the feed-SLM interface and at the SLM-vacuum interface, respectively; CmHA:B,org,f and CmHA:B,org,p are the concentrations of complex mHA:B in the SLM at the feed-SLM interface and at the SLM-vacuum interface, respectively; and Cwater,org,f and Cwater,org,p are the concentrations of water in the SLM at the feed-SLM interface and at the SLM-vacuum interface, respectively. The mass-transfer coefficients of species such as free acetic acid, complex mHA:B, and water can be further expressed as

D τ i ki ) r′ ln(R′/r′)

(10)

where Di is the diffusion coefficient of species i (i ) HA, mHA:B, or water); and τ are the porosity and tortuosity, respectively, of the substrate membrane; and r′ and R′ are the inside and outside radii of the hollow fiber, respectively. It must be mentioned that the above description is merely qualitative, as it is difficult to experimentally measure or theoretically predict the values of the parameters used in the above equations. For example, even for the simplest case where only pure TOA was used as the extractant, the concentrations of acetic acid and the complexes in the organic phase were complicated functions of the feed acetic acid concentration.34 3.3.4. Selectivity in SLMPV. The selectivity of SLMPV for acetic acid over water is given by

RHAC-H2O ) [(acetic acid weight fraction in the permeate)/ (acetic acid weight fraction in the feed)]/ [(water weight fraction in the permeate)/ (water weight fraction in the feed)] (11) 4. Results and Discussion 4.1. Vacuum-Stripping Test for Substrate Membranes. Before initiating studies on SLMPV, substrate

Ind. Eng. Chem. Res., Vol. 42, No. 3, 2003 587 Table 2. Acetic Acid Stripping through Various Membranesa RHAc-water

HAc flux, 105 mol/(m2 s)

water flux, 103 mol/(m2 s)

T, °C

porousb

coatedb

porousb

coatedb

porousb

coatedb

porousb

coatedb

25 35 40 45

0.430 0.448 0.527 0.588

0.462 0.522 0.569 0.628

3.40 4.65 7.46 8.39

2.18 2.95 5.01 6.73

4.28 5.63 6.80 7.80

2.27 3.01 3.78 4.87

0.415 0.433 0.512 0.573

0.447 0.507 0.554 0.613

CHAc,permeate, M

a

Cfeed-in ) 1.0 M, feed flow rate V ) 1 mL/min, Pperm ) 2.0 mmHg. b Porous hydrophobic module 2, silicone-coated module 1.

Table 3. Pervaporation of Acetic Acid through Supported Liquid Membrane Consisting of Some Pure Compounds and Mixturesa module used 1 2 3 2 2 2 2 3 3 a

liquid membrane TOA TOA TLA THA TBP TIOA OA 50% TLA/oleic acid 25% TOPO/OA

CHAc,permeate, M 2.20 4.40 3.25 4.66 2.84 1.05 2.62 3.11 4.48

HAc flux, mol/(m2 s) 10-5

1.71 × 3.50 × 10-5 1.22 × 10-5 8.22 × 10-5 1.01 × 10-4 1.02 × 10-5 2.73 × 10-5 2.00 × 10-5 1.25 × 10-4

water flux, mol/(m2 s) 10-4

2.21 × 2.20 × 10-4 1.68 × 10-4 7.06 × 10-4 1.64 × 10-3 1.28 × 10-4 4.87 × 10-4 2.89 × 10-4 1.62 × 10-3

RHAc-water 2.38 5.62 3.81 9.10 3.22 1.06 2.93 3.59 5.76

T ) 45 °C, Cfeed-in ) 1.0 M, V ) 1 mL/min, Pperm ) 5.0 mmHg.

membrane modules (porous or ultrathin silicone-coated porous membranes) were first tested for vacuum-stripping of acetic acid from the feed. The experimental results are shown in Table 2. It can be seen that because of the somewhat lower volatility of acetic acid compared to that of water and also the somewhat lower diffusivity of acetic acid in the gaseous phase, both the porous membrane and the coated membrane are water-selective for such vacuum stripping. It can also be seen that the silicone-coated membrane is slightly less waterselective than the porous membrane, because the nonporous silicone membrane is slightly acetic acid selective.9 The thin coating also provides an additional resistance over the porous membrane, as indicated by the smaller fluxes of both acetic and water through the coated membrane than through the porous membrane. It is noted that, at the same temperature, the pervaporation flux of acetic acid through the porous hydrophobic membrane is much lower than that reported for acetic acid pervaporation with through some nonporous membranes of similar thickness.9,13 This is because of the low porosity and high tortuosity of the porous hollow fiber membrane used. 4.2. Pervaporation of Acetic Acid through SLM of Single Organic Solvents. Commonly used extractants and modifiers such as TOA, TLA, TOPO, TBP, trihexamine (THA), triisooctylamine (TIOA), and LA-2 were tested as liquid membrane materials without the addition of anything else except in two cases. The performances are listed in Table 3. It can be seen that all extractants used were acetic acid selective. TIOA was too weak as an extractant for acetic acid extraction because of its weak basicity. TBP and THA provided good selectivities and acetic acid permeances. However, the TBP/THA SLM was only stable for several hours and then deteriorated rapidly. This is because both of these substances have lower aqueous-organic interfacial tensions, higher solubilities in the aqueous feed, and higher volatilities than TOA, TLA, and OA because of their lower molecular weights. The performances of SLMs from secondary amines, dioctylamine (Amberlite, LA-2), and di-2-ethylhexylamine deteriorated instantaneously after the shell side of the module was subjected to vacuum, especially when the porous membrane was used, because of their higher basicities, and thus higher

water solubilities and lower aqueous-organic interfacial tensions. TOPO provided good performances; however, as a solid at operating temperatures, the low solubility of TOPO in the organic solvents limited further improvement of acetic acid selectivity and permeance. Oleyl alcohol (OA) has been studied as an extractant or modifier for the extraction of fermentation products such as butanol and butyric acid and also as the liquid membrane for the pervaporation separation of several volatile components from their dilute aqueous solutions.23,29 Therefore, it was selected as a modifier and tested as the liquid membrane. From Table 3, it can be seen that the OA liquid membrane gives a selectivity of 2.93 for acetic acid over water; this value is as low as that obtained by solid membranes.9,13 TOA and TLA demonstrated good performances concerning acetic acid selectivity and SLM stability, as shown below. It must be added that, as reported in the literature, the addition of a modifier usually significantly improves the extraction of acetic acid. In the following results, liquid membranes of TOA, TLA, and their mixtures with OA were exhaustively studied. 4.3. Stability of SLMPV Process for Acetic Acid Removal. The performances of a TLA/OA SLM for the pervaporation removal of acetic acid from water are provided in Figure 2a-d. It can be seen that the SLM is rather stable with regard to variations of the acetic acid concentration in the permeate, the acetic acid permeation flux, and the acetic acid selectivity in an experiment lasting more than 500 h. The duration of this experiment was much greater than that of an experiment using OA as an SLM for butanol removal by pervaporation.23 However, there is a general tendency that the acetic acid concentration in the permeate and the permeation fluxes of acetic acid and water gradually decreased with time. Furthermore, we found that a new membrane module provided a better performance than a previously used one (Figures 3 and 2a-d). This phenomenon was also found when TOA was used, as shown in Figure 3a-c. When the hydrophobic porous polypropylene membrane was contacted with the liquid membrane material, the nonporous region of the membrane wall was at least slightly swollen by the solvent, which might have reduced the membrane porosity and enhanced the

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Figure 2. Variation with operation time of the (a) acetic acid concentration in the permeate, (b) permeation flux of acetic acid, (c) permeation flux of water, and (d) selectivity of acetic acid over water (T ) 45 °C, Pperm ) 2.0 mmHg, Cfeed-in ) 1.0 M, Vfeed-in ) 1 mL/min).

mass-transfer resistances of both acetic acid and water. On the other hand, fatty amines such as TOA and TLA, have long organophilic hydrocarbon tails, which might also adsorb onto the surface of the micropores in the membrane wall. The adsorbed amine molecules behave more like fixed carriers. In the literature, dense polymer membrane with fixed carriers such as polypyridine and polyaniline were found to be water-selective.16-19 This would make the membrane less hydrophobic and allow more water to enter the LM and lead to a lower selectivity. This tendency might be more obvious with longer operation. From Figure 2a-d, it can also be seen that the regeneration of a used substrate module by thorough washing with hexane was effective; the reimmobilized liquid membrane could maintain almost the same acetic acid permeate concentration, and furthermore, the fluxes of acetic acid and water were restored to their values near the end of the first use of the new module. We can assume that the hexane washing extracted out almost all of the extractant molecules from the membrane wall and that the establishment of the swelling equilibrium was slower than that of the adsorption onto

the surface of the micropores on the membrane wall. We also found that the performance of the SLM became more stable when the substrate membrane module was used a second time after hexane washing, as indicated by the almost constant acetic acid permeate concentration and acetic acid and water permeate fluxes. We can also see from Figure 2a-d that the composition of the TLA/OA LM significantly influenced the SLM performance. Under our experimental conditions, the presence of a modifier of high molecular weight such as OA was not essential, because we could not use a more efficient modifier of low molecular weight as in conventional extraction. The acetic acid permeation flux in Figure 2b was much lower than those obtained for polymer membranes having low acetic acid selectivities. We increased the permeation rate by reducing the thickness of the SLM by adding the highly volatile solvent isooctane to the membrane liquid during membrane preparation. The solvent evaporated rapidly when a sweep gas was passed through the module or when the module was subjected to vacuum, and as a result, the SLM formed had a reduced thickness compared to the substrate


Figure 3. Dependence on the number of immobilizations of the (a) acetic acid permeation flux, (b) water permeation flux, and (c) acetic acid selectivity (module 3 used, SLM composition ) 60 wt % TOA in OA, T ) 35 °C, Pperm ) 2.0 mmHg, Cfeed-in)1.0 M, Vfeed-in ) 1 mL/min).

membrane wall thickness. From Figure 2b, it can be seen that such an effort increased the acetic acid flux by more than 30% without significantly affecting the performance of the SLM. The results for the TOA/OA LM at 35 °C are given in Figure 3a-c. The TOA/OA LM provided similar acetic acid selectivities and acetic acid permeation fluxes even at a lower temperature. This is because TOA is a stronger base than TLA and TOA has a lower viscosity than TLA. However, it can be seen that the TOA/OA SLM was not as stable as the TLA/OA SLM; both selectivity and permeance decreased more rapidly because of TOA’s higher solubility in water and higher volatility than TLA. It can also be seen that an LM immobilized in a new membrane module had better performances than a reused membrane module after it was washed by hexane and then reimpregnated with the same LM material. Nevertheless, it was found that the difference between the operation performances of the new and regenerated LMs was small, as indicated in Figure 3a-c. Figure 4a-c demonstrates the effect of different compositions of the TOA/OA LM on the SLMPV performance. The conclusion that the loss of TOA from the LM would lead to a decrease in the acetic acid permeate concentration and permeation flux was further verified. It can be seen that pure TOA provided the highest acetic acid permeance, selectivity, and stability; because of the loss of TOA from the LM (mixture of TOA and OA), the PV performance through SLMs of original compositions of 80% and 60% TOA were significantly reduced to the level obtained by an LM of 40% TOA. We can conclude that the synergistic effect of fatty alcohols of lower molecular weight than OA obtained in conventional extraction did not occur when OA was used as a modifier in the SLMPV process. Our further work will focus on pure TOA, because of its simple composition and high permeance. We will also show extraordinarily improved LM stability. 4.4. Effect of Temperature on SLMPV Performance. SLMPV for acetic acid removal from water was tested at different temperatures. Results for module 4 are illustrated in Figure 5a and b. This module had been used previously for other purposes, so the substrate membrane could be assumed to have stable properties. Concerning the thermal stability of the substrate membrane and the possible loss of TOA through evaporation to vacuum at higher temperatures, temperatures higher than 65 °C were not tested. It can be said that the higher the temperature, the higher the acetic acid concentration in the permeate, and the higher the acetic acid selectivity. However, the permeation fluxes of both acetic acid and water decreased sharply with an increase of temperature. When the temperature was increased, the vapor pressures of acetic acid and water increased, which means higher driving forces for pervaporation. Higher temperature also helped to reduce the viscosity and thus increase the diffusivity of species migrating through the liquid membrane. However, higher temperature sharply reduced the partition coefficient of acetic acid, Kd, between the liquid membrane phase and the aqueous phase and the solubility of water in the liquid membrane phase. This same result has been reported for the extraction of acetic acid, as well as other carboxylic acids, by TOA.28,35 The solubility of water was more significantly influenced by the temperature increase than the acetic acid solubility was; this

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Figure 5. Variation with feed temperature of the (a) acetic acid concentration in the permeate and (b) permeation fluxes of acetic acid and water (module 4 used, pure TOA used as LM, Cfeed-in ) 1.0 M, Pperm ) 1.0 mmHg, Vfeed-in ) 1 mL/min).

Figure 4. Dependence on the liquid membrane composition of the (a) permeation flux of acetic acid, (b) permeation flux of water, and (c) acetic acid selectivity (module 2 used, T ) 35 °C, Pperm ) 5 mmHg, Cfeed-in ) 1.0 M, Vfeed-in ) 1 mL/min).

explains why water permeation flux decreased more sharply than acetic acid. This also explains why the acetic acid selectivity went as high as 33 with an increase in temperature. This is an extraordinarily high acetic acid-water selectivity. 4.5. Influence of Permeate Pressure on SLMPV Performance. SLMPV for acetic acid removal from water was also studied at different permeate-side total pressures. The experimental results are given in Figure 6a and b. A higher vacuum (lower permeate pressure) always led to a higher acid concentration in the permeate, a higher acetic acid selectivity, and a higher acetic acid flux. Correspondingly, the acetic acid concentration in the permeate, acetic acid permeation flux, and acetic acid selectivity sharply decreased with an increase of the preset permeate-side pressure. We can analyze these results from the point of view of driving forces and masstransfer resistances through the liquid membrane. For our experimental conditions, the concentration and temperature can be assumed to be essentially uniform through the module because of the low permeation fluxes of acetic acid and water through the LM. At 45 °C and Cfeed-in ) 1.0 M, the vapor pressures of acetic acid and water are 0.86 and 69 mmHg, respec-


Figure 6. Influence of permeate side pressure on the (a) permeate concentration and selectivity and (b) permeation flux (module 4 used, pure TOA as LM, Tfeed-in ) 45 °C, Vfeed-in ) 1 mL/min, Cfeed-in ) 1.0 M).

tively, and the total pressure is around 70 mmHg.38 According to our experimental results, when the permeate pressure was preset at 0.6 mmHg, the permeate concentration was 11.3 M, which meant that acetic acid partial pressure in the permeate was 0.2 mmHg, and the water partial pressure was 0.4 mmHg, if we assumed that no other components existed in the permeate. Therefore, the driving force for acetic acid was only 0.66 mmHg, which is 77% of the maximum driving force (0.86 mmHg). When the permeate pressure was preset at 5 mmHg, the permeate concentration was 4.4 M, which meant that the partial pressure of acetic acid in the permeate was 0.49 mmHg, and the water partial pressure was 4.51 mmHg, if we assumed that no other components existed in the permeate. Therefore, the acetic acid driving force was only 0.37 mmHg, a mere 43% of the maximum driving force. However, the acetic acid flux appeared to have decreased by more than the drop in the driving force: the flux dropped by almost two times for a driving force reduction of 56%. On the other hand, for the permeate pressures tested, the driving force for water permeation was almost constant (64-69 mmHg); therefore, water permeation flux was

less influenced by the permeate pressure, as shown in Figure 6b. As a result, the acetic acid concentration in the permeate sharply decreased with increasing permeate side pressure. It is useful to speculate about the reduction in acetic acid flux with the driving force reduction. Considering the various species containing acetic acid in the liquid membrane, the value of the partition coefficient, Kd, is known to be strongly influenced by the acetic acid concentration in the aqueous feed; it sharply increases with increasing feed concentration.34 Therefore, a much lower preset permeate pressure, and thus a lower acetic acid partial pressure in the permeate side, means a lower acetic acid concentration in the aqueous solution supposed to be in equilibrium with the permeate, or a much lower acetic acid concentration in the LM phase in equilibrium with the gaseous phase on the shell side. This means a larger driving force for species containing acetic acid migrating from the feed side interface to the permeate side interface. However, because of the complexity of the extraction mechanism in the extraction of acetic acid by TOA, the mass-transfer rate (acetic acid permeation flux) was not directly proportional to the driving force. 4.6. Experimental Results for Various Feed Concentrations. SLMPV for acetic acid removal from water was also studied at different feed concentrations. The experimental results are given in Figure 7a and b. It can be seen that the acetic acid concentration in the permeate, the permselectivity, and the permeation fluxes of acetic acid and water all increased when the feed acetic acid concentration increased. This is reasonable because a high feed concentration means a high driving force for the pervaporation of acetic acid through the liquid membrane. Further, Kd is sharply increased when the concentration in the aqueous feed increased in a feed concentration range from 0 to 6 M, for example, for Cfeed ) 0.2, 0.5, 1.0, 2.0, and 4.0 M, Kd ≈ 0.1, 0.3, 2.2, 2.6, and 3.5, respectively.34 Therefore, the masstransfer rate for acetic acid through the LM increased dramatically with increasing feed concentration. The water solubility in the TOA LM also increased dramatically with increasing feed acetic acid concentration as a result of the high acetic acid concentration in the TOA phase;34 therefore, the permeation flux of water increased in this case. We can conclude that pure TOA is a good LM material for acetic acid pervaporation when the feed concentration is moderate (0.5-2 M). However, when the feed concentration is low, the selectivity and acetic acid permeation flux are low. We next added modifiers to the LM because experimental results in the literature demonstrated significant improvement in extraction of acetic acid when feed concentration was lower than 1 M.3,39 We used decanol as a modifier; it did not yield stable results because of its significant solubility loss to the feed and evaporation loss due to its nonnegligible vapor pressure at operating temperature. The results when OA was used as a modifier are shown in Figure 7a and b. The addition of OA did offer significant improvements in the acetic acid concentration in the permeate and permeation flux. 4.7. Results for Butyric Acid. The pervaporation test was also performed using a more organophilic carboxylic acid, butyric acid, with the same SLM as used for acetic acid. As shown in Figure 8a and b, for the same feed concentration and identical experimental

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Figure 7. Variation with feed concentration of the (a) permeate concentration (filled symbols) and selectivity (unfilled symbols) and (b) permeation fluxes of acetic acid (filled symbols) and water (unfilled symbols) (module 4 used, T ) 45 °C, V ) 1 mL/min).

conditions, the SLM yielded a much higher permeate concentration of butyric acid, high permeation fluxes for both acid and water, and a much higher acid selectivity. This is due to the much higher apolarity of butyric acid compared to acetic acid and thus a higher value of Kd, as reported in the literature.34 The butyric acid selectivity was much higher than those obtained by polymer membranes.13,40 However, the performance of the TLA/ OA membrane was not as stable for butyric acid as for acetic acid. The permeate concentration, permeation flux, and selectivity significantly decreased, and the water permeation flux significantly increased overnight. Nevertheless, even without module regeneration and LM reimmobilization, the performance of the LM could be gradually and almost completely restored when 1 M acetic acid was used again as the feed, as indicated by the data shown in Figure 8a and b. 4.8. Experimental Results with On-line LM Regeneration. Pervaporation tests were also performed in an operating mode in which the LM material was provided on-line to the substrate. For such experiments, a module (module 5, see Table 1) having three shell exits was used. The middle exit was occasionally used for TOA injection (once in 2 or 3 days), whereas, most of

Figure 8. Variation with operation time for different feed concentrations and acidic species of the (a) permeate concentration (filled symbols) and selectivity (unfilled symbols) and (b) permeation flux of acidic species (filled symbols) and water (unfilled symbols) (module 4 used; SLM composition ) 60 wt % TLA in OA, with 30 wt % isooctane as a solvent for impregnation of the SLM; T ) 45 °C; V ) 1 mL/min; Pperm ) 2.0 mmHg).

the time, it was used as a third exit for the permeate. The experimental results are shown in Figure 9a and b. As in the case of the other new modules used earlier (see Figure 2a and b), the permeate concentration, permeate flux, and selectivity of acetic acid sharply decreased in the early stages, after which the performances became rather stable. After being in use continuously for about 200 h, the module was dismantled from the setup and put aside for several months even without any treatment except that the shell was filled with pure TOA. When the module was again used under identical experimental conditions, it can be seen that the module demonstrated almost constant performances, even though the permeate concentration, permeate flux, and selectivity of acetic acid decreased slightly compared to the results obtained when the new module was used. However, both the permeate acetic acid concentration and the permeation fluxes of acetic acid and water were much higher than those obtained by the other modules. We have the following explanations: (1) The module was much less packed than other modules; therefore, the possibility of overlapping of the surface areas of the fibers was avoided to a large degree. (2) The module was much shorter than the other modules; the permeate side module length in the module was shorter. (3) The most


Figure 9. Performances of the SLMPV when the LM was regenerated on-line (module 5 used, pure TOA as LM, T ) 45 °C, Cfeed-in ) 1.0 M, V ) 1.0 mL/min, Pperm ) 0.4-0.6 mmHg): (a) permeate concentration (filled symbols) and selectivity (unfilled symbols) and (b) permeation flux of acidic species (filled symbols) and water (unfilled symbols).

important reason is that there were three exits for permeate removal during operation. Therefore, the pressure in the shell side of the module during operation was much closer to the gauge reading. As we have already shown via Figure 6a and b, acetic acid concentration in the permeate and the permeation flux were very sensitive to the permeate pressure because of acetic acid’s low vapor pressure. 5. Concluding Remarks Supported liquid membrane based pervaporation (SLMPV) has been studied for the separation of acetic acid from its dilute solutions. When tested as a liquid membrane in SLMPV, most extractants used for conventional extraction of acetic acid from aqueous solutions were acetic acid selective. The highest selectivity for a 1 M feed of acetic acid were found to reach as high as 33 at 60 °C feed temperature. This is much higher than any value obtained for the acetic acid/water system by pervaporation through any other organic polymer

membranes. Among a variety of extractants, LMs of trioctylamine or trilaurylamine demonstrated defectfree performances for more than 500 h. During this period, the acetic acid concentration in the permeate, the selectivity, and the acetic acid permeation flux decreased gradually. This is because the properties of the substrate membrane, porous hydrophobic polypropylene hollow fiber, changed gradually with operation; it is also due to the loss of extractants from the LM through evaporation to the permeate side and the solubilization to the feed during the operation. When the used module was washed and then reimpregnated with the liquid membrane material, the performances of the SLMPV could be nearly restored. An on-line regeneration technique was used successfully to continuously provide the liquid membrane material to the substrate membrane surface from the permeate side, to make up for the loss of the LM during operation. Therefore, the life of the SLM mainly depends on the life of the substrate membrane module under operating conditions. This on-line regeneration technique is immediately applicable also to SLMPV processes for any volatile organic compound to be removed selectively from water.41 However, the SLMPV offers a smaller acetic acid permeation flux, especially when the feed concentration is low. A major reason for this is that the porous substrate used had a low porosity and high tortuosity. Employing more open substrates will automatically increase the flux by 2-4 times. The low flux is also due to the low partial pressure of acetic acid in the vapor phase in equilibrium with the feed and the sharp decrease in the partition coefficient of acetic acid between the organic phase and the aqueous phase with a decrease of the feed concentration. Acetic acid selectivity and permeation flux were highly sensitive to temperature and the permeate pressure; a high vacuum was needed to obtain high selectivities at the temperatures tested. A good extractant (or a mixture of extractant and modifier) for the LM should have a higher extracting capability, a reasonably low volatility, a low water solubility, a low viscosity, and a high interfacial tension, especially when operated at elevated temperatures and over a wide range of feed concentrations. Supported liquid membrane pervaporation (SLMPV) is actually an integration of solvent extraction with flash distillation in one device and one step, which avoids the temperature swing of the extractant and thus also avoids the thermal degradation or contamination of the extractant. Even though the SLMPV process did not provide as high a selectivity as is obtained in conventional extraction processes, it can avoid the solvent loss encountered in the conventional extraction process. Further, the membrane device can be highly compact and not subject to the equilibrium limitation of solvent extraction. In conventional extraction, a modifier of low molecular weight and moderate polarity is usually used to increase the partition coefficient, and thus, its loss by solubilization to the feed (or raffinate) is inevitable. It might be more economical, when a feed of moderate concentration, e.g., 1-2 M (6-12 wt %), is separated, for SLMPV to be used first to obtain a permeate of 80-90 wt %, followed by further separation of the raffinate by a conventional or membrane-based extraction/stripping process to obtain a product of 50-80%. The highly concentrated acetic acid can be further

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separated into acetic acid and water by distillation or pervaporation dehydration process using water-selective membranes. However, the selectivity from SLMPV is still lower than those obtained from extraction when phase equilibrium is reached.30,34 It is useful to recognize that solvent extraction process requires the subsequent steps, such as solvent dehydration, acid stripping, and acid purification, rather than only dehydration of the permeate for pervaporation. In the extraction of acetic acid from aqueous solution, when reactive extractants were used, the final concentrations of acetic acid and water were determined by their partition coefficients. For an SLMPV process, more parameters influence the separation in such a kinetic process; the diffusivity of water is quite different from those of the complexes between acetic acid and extractants, as the latter are much larger than water. On the other hand, the dissolution of water in the LM can be thought of as physical; however, the reaction kinetics of acetic acid extracted into the LM was more complicated.34 Even if the interfacial resistance is not the dominant controlling step for acetic acid migration through the LM, because of the high viscosity and thickness of the LM, it still contributes to the resistance for the transport of acetic acid through the LM. Acknowledgment The authors acknowledge funding for this research from the following sources: (1) New Jersey Commission of Science and Technology (NJCST) R&D Excellence Grant 98-2890-051-29 to the Center for Membrane Technologies; (2) Membrane Separations and Biotechnology Program at NJIT; and (3) SERDP (Strategic Environmental Research and Development Program) Project EPA-371-94 administered by EPA NHSRC, Newark, NJ. The authors appreciate the assistance and suggestions of Dr. I. Abou-Nemeh and Dr. S. Majumdar. The authors also thank Hoechst Celanese Corporation (now Celgard, Charlotte, NC) and Applied Membrane Technology Corporation (Minnetonka, MN) for providing the appropriate hollow fibers. Literature Cited (1) Aguilo, A.; Hobbs, C. C.; Zey, E. G. Acetic Acid, Ullmann’s Encyclopedia of Industrial Chemistry; VCH Verlagsgesellschaft GmbH: Weinheim, Germany, 1985; Vol. A1, p 45. (2) Ziegenfuss, H.; Maurer, G. Distribution of acetic acid between water and organic solutions of tri-n-octylamine. Fluid Phase Equilib. 1994, 102, 211-255. (3) Wardell, J. M.; King, C. J. Solvent equilibria for extraction of carboxylic acids from water. J. Chem. Eng. Data 1978, 23, 144148. (4) Fahim, M. A.; Qader A.; Hughes, M. A. Extraction equilibria of acetic and propionic acids from dilute aqueous solution by several solvents. Sep. Sci. Technol. 1992, 27, 1809-1821. (5) Matsumoto, M.; Uenoyama, S.; Hano, T.; Hirata, M., Miura, S. Extraction kinetics of organic acids with tri-n-octylphosphine oxide. J. Chem. Technol. Biotechnol. 1996, 67, 260-264. (6) Schierbaum, B.; Vogel, H. Isolation of carboxylic acids from aqueous solutions by extraction with dialkylcarboxylic amides/ trialkylamines. Chem. Eng. Technol. 1999, 22, 37-41. (7) Weier, J.; Glatz, B. A.; Glatz, C. E. Recovery of propionic and acetic acids from fermentation broth by electrodialysis. Biotechnol. Prog. 1992, 8, 479-485. (8) Cloete, F. L. D.; Marais, A. P. Recovery of very dilute acetic acid using ion exchange. Ind. Eng. Chem. Res. 1995, 34, 24642467. (9) Deng, S.; Sourirajan, S.; Matsuura, T. Study of polydimethylsiloxane/aromatic polyamide laminated membranes for

separation of acetic acid/water mixtures by pervaporation process. Sep. Sci. Technol. 1994, 29, 1209-1216. (10) Sano, T.; Ejiri, S.; Yamada, K.; Kawakami, Y.; Yanagishita, H. Separation of acetic acid-water mixtures by pervaporation through silicalite membrane. J. Membr. Sci. 1997, 123, 225-233. (11) Huang, S. C.; Ball, I. J.; Kaner, R. B. Polyaniline membranes for pervaporation of carboxylic acids and water. Macromolecules 1998, 31, 5456-5464. (12) Yoshikawa, M.; Kuno., S. I.; Kitao, T. Specialty polymeric membranes 3. Pervaporation separation of acetic acid/water mixtures through polymeric membranes having a pyridine moiety as a side group. J. Appl. Polym. Sci. 1994, 51, 1021-1027. (13) Hofmann, T.; Hapke, R. L.; Sengupta, A.; Roberts, D. L. Acetic acid and butyric acid recovery from aqueous solutions by pervaporation. Presented at the Conference of the North American Membrane Society, San Diego, CA, May 29, 1991. (14) Bai, J.; Fouda, A. E.; Matsuura, T.; Hazlett, J. D. Study on the preparation and performance of polydimethylsiloxanecoated polyetherimide membranes in pervaporation. J. Appl. Polym. Sci. 1993, 48, 999-1008. (15) Yoshikawa, M.; Kuno, S. L.; Wano, T.; Kitano, T. Specialty polymeric membranes. 4. Pervaporation separation of acetic acid/ water mixtures through modified polybutadiene membranes. Polym. Bull. 1993, 31, 607. (16) Nguyen, T. Q.; Essamri, A.; Clement, R.; Neel, J. Synthesis of membranes for the dehydration of water/acetic acid mixtures by pervaporation. I. Polymer material selection. Makromol. Chem. 1987, 188, 1973. (17) Lee, Y. M.; Oh, B. K. Pervaporation of water-acetic acid mixture through poly(4-vinylpyridine-co-acrylonitrile) membrane. J. Membr. Sci. 1993, 85, 13-20. (18) Lai, J. Y.; Yin, Y. L.; Lee, K. R. Chemically modified poly(4-methyl-1-pentene) membrane for pervaporation separation of acetic acid-water mixtures. Polym. J. 1995, 27, 813-818. (19) Ruckenstein, E.; Sun, F. Anomalous sorption and pervaporation of aqueous organic mixtures by poly(vinyl acetyl) membranes. J. Membr. Sci. 1994, 95, 207-219. (20) Lee, J.-F.; Wang, Y.-C. Dehydration of acetic acid/water mixture by pervaporation through a chemically modified poly(4methyl-1-pentene) membrane. Sep. Sci. Technol. 1998, 32, 187200. (21) 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, 163-174. (22) Komada, H.; Honda, Z. Organic acid permselective membrane. Japanese Patent JP 60-40319, 1985. (23) Matsumura, M.; Kataoka, H. Separation of dilute aqueous butanol and acetone solutions by pervaporation through liquid membranes. Biotechnol. Bioeng. 1987, 30, 887-895. (24) Matsumura, M.; Hataoka, H.; Sueki, M.; Araki, K. Energy saving effect of pervaporation using oleyl alcohol liquid membranes. Bioprocess Eng. 1988, 3, 93-100. (25) Christen, P.; Minier, M.; Renon, H. Ethanol extraction by supported liquid membrane during fermentation. Biotechnol. Bioeng. 1990, 36, 116-123. (26) Ishii, N.; Matsumura, M.; Tanaka, H.; Araki, K. Diacetyl fermentation coupled with pervaporation using oleyl alcohol supported liquid membrane. Bioprocess Eng. 1995, 13, 119-123. (27) Favre, E.; Nguyen, Q. T. Extraction of 1-butanol from aqueous solutions by pervaporation. J. Chem. Biotechnol. 1996, 65, 221-228. (28) Juang, R. S.; Wu, R. T. Effect of a water-insoluble organic acid on amine extraction of acetic acid from aqueous solutions. Equilibrium studies. J. Chem. Biotechnol. 1996, 66, 160-68. (29) Zigova, J.; Vandak, D.; Schlosser, S.; Sturdik, E. Extraction equilibrium of butyric acid with organic solvents. Sep. Sci. Technol. 1996, 31, 2671-2684. (30) Jung, M.; Schierbaum, B.; Vogel, H. Extraction of carboxylic acids from aqueous solutions with the extractant system alcohol/tri-n-alkylamines. Chem. Eng. Technol. 2000, 23, 70-74. (31) Matsumoto, M.; Otono, T.; Kondo, K. Synergistic extraction of organic acids with tri-n-octylamine and tri-n-butyl phosphate. Sep. Purif. Technol. 2001, 24, 337-342. (32) Eyal, A. M.; Canari, R. pH dependence of carboxylic and mineral acid extraction by amine-based extractants: Effects of pKa, amine basicity, and diluent properties. Ind. Eng. Chem. Res. 1995, 34, 1789-1798.

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(38) Ito, T.; Yoshida, F. Vapor-liquid equilibrium of waterlower fatty acid systems. J. Chem. Eng. Data 1963, 8, 315-320. (39) Althouse, J.; Tavlarides, L. L. Analysis of organic extractant systems for acetic acid removal for calcium magnesium acetate production. Ind. Eng. Chem. Res. 1992, 31, 1971-1981. (40) Kabra, M. M.; Netke, S. A.; Sawant, S. B.; Joshi, J. B.; Pangarkar, V. G. Pervaporative separation of carboxylic acid water mixtures. Sep. Technol. 1995, 5, 259-263. (41) Qin, Y. J.; Sheth, J. P.; Sirkar, K. K. Supported Liquid Membrane-Based Pervaporation for VOC Removal from Water. Ind. Eng. Chem. Res. 2002, 41, 3413-3428.

Received for review May 31, 2002 Revised manuscript received November 5, 2002 Accepted November 12, 2002 IE020414W

Pervaporation Membranes That Are Highly Selective for Acetic Acid

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