Robert Y. M. Huang

Estimation of activation energy for permeation in pervaporation processes

Abstract The effect of operating temperature on permeation flux in pervaporation was analyzed. In the standard pervaporation processes, both the permeability coefficient of a membrane and the driving force for mass transport are influenced by temperature. The so-called “activation energy” of permeation, EJ, conventionally obtained from ln J vs. 1 T plot is a compounded parameter characterizing the overall temperature dependence of permeation flux. The activation energy, EP, characterizing temperature dependence of membrane permeability should be evaluated from the log of driving force-normalized flux vs. 1 T data. When the transmembrane driving force is expressed in terms of partial pressure, EP can be evaluated from ln ( J Δp ) vs. 1 T plot. A rule of thumb for estimating EP is to subtract the molar heat of vaporazation from EJ.

Characteristics of sodium alginate membranes for the pervaporation dehydration of ethanol–water and isopropanol–water mixtures

Alginate membranes for the pervaporation dehydration of ethanol–water and isopropanol–water mixtures were prepared and tested. The sodium alginate membrane was water soluble and mechanically weak but it showed promising performance for the pervaporation dehydration. To control the water solubility the sodium alginate membrane was crosslinked ionically using various divalent and trivalent ions. Among them the alginate membrane crosslinked with Ca2+ ion showed the highest pervaporation performance in terms of the flux and separation factors.

Pervaporation dehydration of isopropanol with chitosan membranes

Homogeneous and composite chitosan based membranes were prepared by the solution casting technique. The membranes were investigated for the pervaporation dehydration of isopropanol-water systems. The effects of feed concentration and temperature on the separation performance of the membranes were studied. In terms of the pervaporation separation index (PSI), the composite membrane was more productive than the homogeneous membrane for pervaporation of feed with high isopropanol content. It was observed that permeation increased and the separation factor decreased with the temperature. Modification of the homogeneous chitosan membrane by chemical crosslinking with hexamethylene diisocyanate improved the permselectivity but reduced the permeation rate of the membrane.

Pervaporation separation of aqueous mixtures using crosslinked poly(vinyl alcohol)(pva). II. Permeation of ethanol-water mixtures

Abstract The pervaporation separation of ethanol-water mixtures was carried out on a series of chemically crosslinked poly(vinyl alcohol) (PVA) membranes which had been developed in our laboratory. For the preparation of these membranes, PVA was crosslinked with amic acid; subsequently the crosslinking agent, amic acid was imidized for an additional hour at 150°C. Optimum pervaporation results were obtained with crosslinked PVA membranes containing 12 wt.% crosslinking agent, giving separation factors of 70–380 and permeation rates of 30–1600 g/(m2-hr) depending on the operating temperature and feed mixture composition. Deviation of permeation rates from ideal rates was discussed with respect to the permeation ratio. These phenomena were explained in terms of both the plasticizing effect of water and the interaction between permeants within the polymer membrane. We have also defined a pervaporation separation index (PSI) as the product of permeation rate and separation factor; this could become a new measure of the pervaporation separation ability of a membrane for a binary mixture. The temperature dependence of the permeation rate for binary mixtures was expressed by an Arrhenius-type relation and activation energies of 5.21–9.55 kcal/mol were calculated for ethanol-water mixtures using the crosslinked PVA membrane with 12 wt.% crosslinking agent. The pre-exponential factor as well as the activation energy were found to be functions of the permeant concentrations.

Crosslinked chitosan composite membrane for the pervaporation dehydration of alcohol mixtures and enhancement of structural stability of chitosan/polysulfone composite membranes

Abstract Chitosan composite membranes having a microporous polysulfone substrate were prepared and tested for the pervaporation dehydration of aqueous isopropanol mixtures. When the composite membrane experienced excessive swelling at the feed mixture of high water content, the composite membranes were found to be segregated in structure due to the opposite characteristics to water of chitosan and polysulfone. Efforts to enhance the structural stability under various pervaporation operational conditions were made. The polysulfone substrate was immersed into hydrophilic binding polymer solutions such as polyvinyl alcohol, polyacrylic acid, and hydroxyethylcellulose before the casting of chitosan layer to increase the affinity between the thin chitosan layer and porous polysulfone layer which resulted in increased geometrical stability of the chitosan/polysulfone composite membranes. The chitosan layer was crosslinked with glutaraldehyde and H 2 SO 4 in acetone solution to control the permselectivity.

Pervaporation dehydration of aqueous ethanol and isopropanol mixtures through alginate/chitosan two ply composite membranes supported by poly(vinylidene fluoride) porous membrane

Abstract Composite membranes consisting of an active alginate layer and supporting chitosan layer on top of the base porous blended polyvinylidene fluoride (PVDF) membrane were prepared and tested for pervaporation dehydration applications. Efforts to enhance the surface properties of PVDF by blending with relatively hydrophilic polymethyl methacrylate (PMMA) were carried out and evaluated by contact angle measurements. Various modifications for the alginate/chitosan composite membranes such as converting to the free acid form and cross-linking with cobalt ion were investigated. They were then compared for the pervaporation dehydration of ethanol/water and isopropanol/water mixtures and the temperature effect on the permeation flux was also investigated.

Pervaporation with chitosan membranes. I. Separation of water from ethylene glycol by a chitosan/polysulfone composite membrane

Abstract A chitosan/polysulfone composite membrane was prepared. The preparation procedure involved dissolution of chitosan in dilute aqueous acetic acid to form chitosan salt, coating of the chitosan salt solution on a porous polysulfone substrate, and regeneration of chitosan by alkaline treatment. The membrane was tested for selective removal of water from aqueous ethylene glycol solutions by pervaporation. The effects of operating parameters, including feed concentration, temperature, and downstream pressure, on the separation performance of the membrane were investigated. At 35°C and 60 Pa downstream pressure, a permeation flux of 0.3 kg/m2 h and permeate water concentration higher than 92 wt% was achieved at a feed water content of 10 wt%. This study demonstrated the potential of membrane pervaporation as an alternative to conventional distillation for the given separation. It was also shown that conditioning of the membrane in the pervaporation system at the maximum operating temperature led to a quasi-permanent change in membrane permselectivity.

Pervaporation separation of aqueous mixtures using crosslinked polyvinyl alcohol membranes. III, Permeation of acetic acid-water mixtures

Abstract The pervaporation separation of acetic acid-water mixtures was carried out over the full range of compositions at temperatures varying from 30 to 70°C, using chemically crosslinked poly (vinyl alcohol) (PVA) membranes which had been developed in our laboratory. For the preparation of these membranes, PVA was crosslinked with amic acid and then the crosslinking agent, amic acid was imidized for an additional one hour at 150°C. The best condition for preparing the crosslinked PVA was found to be ca. 12 wt.% amic acid content. Pervaporation separation of acetic acid-water mixtures using the crosslinked PVA membrane containing 12 wt.% crosslinking agent gave separation factors of 13-42 and permeation rates of 79-2285 g/(m 2 -hr) depending on the operating temperature and feed mixture composition. Deviation of permeation rates from ideal rates was discussed through the permeation ratio concept. These phenomena were explained in terms of both the plasticizing effect of permeants and the interaction between permeants in the polymer membrane. We have also defined a new term, the pervaporation separation index (PSI) as the product of permeation rate and separation factor, which could be a measure of the pervaporation separation ability of a membrane for a binary mixture under the specified experimental conditions. The temperature dependence of the permeation rate for binary mixtures was expressed by the Arrhenius-type relation and activation energies of 5.73-8.21 kcal/mol were calculated for acetic acid-water mixtures through the crosslinked PVA with 12 wt.% crosslinking agent. The pre-exponential factor as well as the activation energy were found to be functions of the permeant concentrations.

Pervaporation with chitosan membranes II. Blend membranes of chitosan and polyacrylic acid and comparison of homogeneous and composite membrane based on polyelectrolyte complexes of chitosan and polyacrylic acid for the separation of ethanol-water mixtures

Abstract Three different types of blend membranes based on chitosan and polyacrylic acid were prepared from homogeneous polymer solution and their performance on the pervaporation separation of water-ethanol mixtures was investigated. It was found that all membranes are highly water-selective. The temperature dependence of membrane permselectivity for the feed solutions of higher water content (>30 wt%) was unusual in that both permeability and separation factor increased with increase in temperature. This phenomenon might be explained from the aspect of activation energy and suggested that the sorption contribution to activation energy of permeation should not always be ignored when strong interaction occurs in the pervaporation membrane system. A comparison of pervaporation performance between composite and homogeneous membranes was also studied. Typical pervaporation results at 30°C for a 95 wt% ethanol aqueous solution were: for the homogeneous membrane, permeation flux = 33 g/m 2 h, separation factor = 2216; and for the composite membrane, permeation flux = 132 g/m 2 h, separation factor = 1008. A transport model consisting of dense layer and porous substrate in series was developed to describe the effect of porous substrate on pervaporation performance.

Pervaporation separation of ethyl butyrate and isopropanol with polyether block amide (PEBA) membranes

Abstract Polyether block amide (PEBA) membranes were prepared by the solution casting technique. The membranes were investigated for the pervaporation separation of isopropanol–water and ethyl butyrate–water mixtures. The effects of feed concentration and temperature on the separation performance of the membranes were studied. The overall performance of pervaporation separation was evaluated in term of the pervaporation separation index (PSI), which is a composite parameter combining permeation flux and separation factor. It was shown that under the same operating conditions, the pervaporation separation of aqueous ethyl butyrate solution was more efficient than the separation of aqueous isopropanol solutions. It was observed that both permeation flux and separation factor increased with an increase in feed ethyl butyrate content, while an increase in temperature resulted in an increase in permeation flux and a reduction in separation factor.