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Several reactor designs have been described in the recent literature for continuous organic-phase enzymatic esterification reactions. While these designs have excellent performance characteristics, there are operational constraints in their use. The present article describes a new reactor design, the gas-phase hollow fiber reactor (GPHFR), which does not suffer from any such limitations. The reactor consists of, commonly available, hollow fiber dialyzer modules with enzyme immobilized on the lumen of the hollow fiber membranes by ultrafiltration. Substrate mixtures are passed through the fiber lumens and subjected to esterification with a constant humidity gas phase recirculated through the shell of the reactor, acting as the medium used to control water activity. The simplicity of the device renders it suitable for use over a wide range of water activities, and its modular nature facilitates easy scale-up. The use of the reactor for the fixed water activity esterification of an equimolar mixture of dodecanol and decanoic acid has been described. Under optimum conditions the reactor was found to give yields of ester as high as 97%. In continuous operation the immobilized enzyme was found to have a half-life of about 70 days.

The yield of organic-phase enzymatic esterification reactions can be improved by continuous removal of product water. When water is the only volatile component of the reaction system, this can be accomplished by carrying out the reaction under a partial vacuum. The performance in such reaction systems can be further improved by employing a controlled leak of air into the headspace of the reactor. This improvement is achieved at a lower vacuum than would be required in an ideal evacuated reactor delivering the same performance. The theory of air-leak effects has been analyzed in this paper. Experiments done to verify this theory have also been presented. Air-bleed evacuated-headspace reactors (ABEHRs) can produce extremely high synthetic yields. For instance, during the lipozyme-catalyzed esterification of a solvent-free solketal-decanoic acid mixture, a yield of 96% ester was obtained in a reactor operated under a vacuum of 0.7 bar and a temperature of 50°C when air at 20°C with a relative humidity of 54% was leaked into the headspace.

Surface-active components, both reactants and products, are frequently encountered in two-phase, aqueous-organic, biocatalytic reactions, When such reaction are carried out in a membrane reactor, employing a membrane selectively wetted by one of the two reactants, changes in the content of these surfactants--as a consequence of the progress of the reaction--can lead to wetting transitions at the two membrane-liquid interfaces as a result of adsorption of the tenside. This can lead to a decrease in the pressure required to cause the, initially, nonwetting phase to break through the membrane. Such effects render difficult the operation of two-phase membrane bioreactors. Hence, it is necessary to make a careful selection of the membrane material and type by considering factors such as UF versus MF and low MWCO versus high MWCO to enable the reactor to be operated without breakthrough, but without significantly compromising the reaction rates that can be maintained.The phenomena leading to breakthrough effects are discussed in this paper, and experimental results for the hydrolysis of ethyl laurate by lipase from Candida rugosa in a batch flat sheet membrane reactor are presented with the reactor operated with a variety of membranes. An experimental result showing the decrease in the pressure required to cause breakthrough of the organic phase (for the system ethyl laurate-lauric acid-water) as the content of the highly surface-active lauric acid in the organic phase is increased is also presented for an asymmetric, hydrophilic meta-aramid ultrafiltration membrane.