S.B. Sawant

Lipase-Catalyzed Esterification

Lipases are versatile catalysts. In addition to their natural reaction of fat hydrolysis, lipases catalyze a plethora of other reactions such as esterification, amidation, and transesterification of esters as well as organic carbonates. Moreover, lipases accept a wide variety of substrates while maintaining their regioselectivity and stereoselectivity. Lipases are highly stable even under adverse conditions such as organic solvents, high temperatures, and so forth. Applications of lipases include production of food additives, chiral intermediates, and pharmaceutical products. Among these, synthesis of various chiral intermediates in pharmaceutical industry and cocoa butter substitutes is being commercially exploited currently. Lipase-catalyzed esterification and transesterification in anhydrous media (e.g., organic solvents and supercritical fluids) has been an area of major research activity in the past decade or so. Absence of water eliminates the competing hydrolysis reaction. Moreover, substrate specificity, regioselectivity, and stereoselectivity of the enzyme can be controlled by varying the reaction medium. Although organic solvents, which are generally used for lipase-catalyzed reactions, are nearly anhydrous; they contain water in trace quantities. This water content can be controlled over a range and has a profound effect on the activity of lipases. Water not only affects the enzyme but also acts as a competing nucleophile. Enzyme activity has been correlated with thermodynamic activity of water in the medium rather than with the concentration of water. Because lipases are not soluble in most organic solvents, the method of preparation of the enzyme has a strong influence on the enzymatic activity. The major factors are the pH of the aqueous solution in which the enzyme last existed, additives used during preparation, and method of removal of water (e.g., freeze-drying, evaporation, extraction of enzyme into solvent, etc.). Immobilization of lipases allows easy recovery and reuse of the enzyme. Various immobilization techniques have been studied for lipases and some of them have been shown to enhance the activity and stability of the enzyme. Enzyme stability is an important parameter determining the commercial feasibility of the enzymatic process. Various factors, such as temperature, reaction medium, water concentration, as well as the method of preparation, affect the stability of the lipases. This review deals with fundamental as well as practical aspects of lipase catalysis. A discussion has been presented on various factors affecting lipase activity and stability. Moreover, a brief account of current and potential applications of lipases has been given.

Sorption and permeation of binary water-alcohol systems through PVA membranes crosslinked with multifunctional crosslinking agents

Sorption and permeation of aqueous alcohol (C1–C4) through PVA membrane cross-linked with a multifunctional cross-linker is reported. The effect of temperature, feed concentration, cross-linker loading and the shape and size of the permeating species on pervaporation characteristics is investigated. Sorption isotherms of cross-linked PVA are obtained for the aqueous alcohol systems namely water+methanol, +ethanol, +n-propanol, +isopropanol, +n-butanol and +isobutanol. Values of activation energy for permeation (ΔEp), diffusion (ΔEd) and heat of sorption (ΔHs) for water through cross-linked PVA are estimated. The value of ΔEd for water was found to vary for all the water-alcohol systems (20–50 kJ mole-1) covered in this work.

Effect of Sparger Design and Height to Diameter Ratio on Fractional Gas Hold-up in Bubble Columns

The combined effect of sparger design and dispersion height on fractional gas hold-up (−ɛG) was investigated in a 0.385 m i.d. bubble column. Perforated plates were used as -spargers. Free areas of sparger plates (F. A.) and hole diameters were varied in the range of 0.13% to 5.0% and 0.8 mm to 87 mm, respectively. The height to diameter ratio (HD/D) was varied in the range of 1–8. In all the cases, the superficial gas velocity was covered in the range of 30-300 mm s−1. In order to investigate the combined effects of sparger design and the HD/D ratio together with the coalescing nature of the liquid phase, three liquid systems were considered, namely, water, an aqueous solution of an electrolyte and an aqueous solution of carboxymethyl cellulose (CMC). The comparative behaviour of these systems has been presented. The effects of sparger design, HD/D ratio and the gas- liquid system have also been analysed on the basis of drift flux model. A predictive procedure has been given for the drift flux constants by simulating the flow pattern in bubble columns using Computational Fluid Dynamics (CFD).

Dehydration of acetic acid by pervaporation

Pervaporative dehydration of acetic acid over the entire concentration range of 0-100% is studied using four copolymer membranes of acrylonitrile and high performance Nafion and polyimide membranes. From each copolymer of acrylonitrile, three different membranes were produced with three different copolymer compositions. Polyimide showed high water selectivity but very low flux, while Nafion showed highest flux but lowest selectivity. Copolymers of acrylonitriles showed reasonable flux and selectivity behavior. Among the acrylonitrile copolymers, copolymers with hydroxyethyl methylacrylate yielded water selectivity comparable to that of polyimide with much higher flux.

Cellulase deactivation in a stirred reactor

During the production or downstream processing of an enzyme it is always subjected to shear stress, which may deactivate the enzyme. This susceptibility of enzymes to shear stress is a major concern as it leads to the loss of enzyme activity and is, therefore, a major consideration in the design of the processes involving enzyme production and its application. In the present work the cellulase enzyme was subjected to shear stress in a stirred reactor with an objective of investigating its deactivation under various conditions such as different agitation speeds, concentrations of enzyme, concentrations of buffer, pH ranges, buffer systems and the presence of gas-liquid interface. It was found that the extent of deactivation depends upon the conditions under which the enzyme was subjected to shear.

Pervaporative Dehydration of Organic Solvents

Pervaporative separation of acetone/water and isopropanol (IPA)/water systems has been studied in the water-lean range of composition of the feed mixtures. Poly(vinyl alcohol) (PVA) membranes crosslinked with citric acid, adipic acid, maleic acid, glutaraldehyde, and glyoxal were used for this purpose. The sorption characteristics of all the membranes indicate that these membranes have a good sorption selectivity for water in view of the hydrophilic nature of PVA. The type of crosslinker used for crosslinking has been shown to have an important bearing on the permeation characteristics of the membranes. Thus, the trifunctional citric acid yields the highest selectivity but lowest flux. A comparison of the productive capacities of the various membranes indicates that the glutaraldehyde crosslinked membrane has the maximum productive capacity for IPA dehydration whereas maleic acid crosslinked membrane yields the highest productive capacity for acetone dehydration.

Sorption and permeation of acetic acid through zeolite filled membrane

Separation of acetic acid from water using silicalite filled PDMS membrane has been studied. The selectivity of the membrane for acetic acid has been found to increase with (i) increasing wt% of silicalite in the membrane and (ii) increasing hydrophobicity of the silicalite. The flux of acetic acid is however, found to behave in an opposite manner. An attempt has been made to propose a Dual Mode model for permeation in filled membranes and explain the results of this investigation as well as those available in the literature in the light of this model.

Enzyme and protein mass transfer coefficient in aqueous two-phase systems. I: Spray extraction columns

Fractional dispersed phase hold-up and dispersed side mass transfer coefficients for bovine serum albumin (BSA) and amyloglucosidase were measured in 22, 34, 50, 56, 70 and 95 mm i.d. spray columns using salt-polyethylene glycol (potassium phosphate-PEG and sodium sulphate-PEG) systems. The effect of distributor design and column height were investigated. The effect of phase compositions of the aqueous phase system was also studied. Empirical and semi-empirical correlations have been developed for fractional dispersed phase hold-up and dispersed side mass transfer coefficients.

Dehydration of ethylene glycol by pervaporation using hydrophilic IPNs of PVA, PAA and PAAM membranes

Abstract Pervaporative dehydration of ethylene glycol-water mixture was carried using PAAPVA and PAAMPVA interpenetrating polymer network (IPN) membranes. PAAPVA IPN and PAAMPVA IPN membranes were prepared by sequential IPN technique. The effects of concentration of water in the feed, and feed temperature on the sorption and permeation of water were studied. The effects of content of PAA in PAAPVA IPN membrane on the permeation of water and mechanical strength of the membrane were also studied. The data obtained from sorption and permeation experiments were used to estimate the energy of activation for permeation ( ΔE p ), energy of activation for diffusion ( ΔE d ) and heat of sorption ( ΔH s ) of water in the membrane. A model has been proposed to estimate the diffusion coefficient of water and ethylene glycol. It was found that PAAPVA IPN membrane shows selective water transport compared to that of a PAAMPVA IPN membrane.

Immobilization of Mucor miehei lipase on ion exchange resins

An attempt was made to immobilize Mucor miehei lipase on ion exchange resins and other supports. Indion 850, a styrene-divinyl benzene polymer resin, proved to be the best among the supports tested. The procedure for immobilization is simple adsorption of the soluble enzyme from dilute aqueous solution onto the resin beads. The immobilized version was found to have 1.4 times higher activity compared with its soluble counterpart and an enhanced thermal stability.