Aqueous-organic membrane bioreactors part II. Breakthrough pressure measurement

Process implementation aspects for biocatalytic hydrocarbon oxyfunctionalization

Oxidoreductases catalyze a large variety of regio-, stereo-, and chemoselective hydrocarbon oxyfunctionalizations, reactions, which are important in industrial organic synthesis but difficult to achieve by chemical means. This review summarizes process implementation aspects for the in vivo application of the especially versatile enzyme class of oxygenases, capable of specifically introducing oxygen from molecular oxygen into a large range of organic molecules. Critical issues such as reaching high enzyme activity and specificity, product degradation, cofactor recycling, reactant toxicity, and substrate and oxygen mass transfer can be overcome by biochemical process engineering and biocatalyst engineering. Both strategies provide a growing toolset to facilitate process implementation, optimization, and scale-up. Major advances were achieved via heterologous overexpression of oxygenase genes, directed evolution, metabolic engineering, and in situ product removal. Process examples from industry and academia show that the combined use of different concepts enables efficient oxygenase-based whole-cell catalysis of various commercially interesting reactions such as the biosynthesis of chiral compounds, the specific oxyfunctionalization of complex molecules, and also the synthesis of medium-priced chemicals. Better understanding of the cell metabolism and future developments in both biocatalyst and bioprocess engineering are expected to promote the implementation of many and various industrial biooxidation processes.

Epoxidation of 1,7-octadiene by pseudomonas oleovorans in a membrane bioreactor

A growing cell culture of Pseudomonas oleovorans was used to biotransform 1,7-octadiene to 1,2-epoxy-7,8-octene in a continuous-flow bioreactor with an external membrane module. A dense silicone rubber membrane was used to contact an organic phase, containing both the reactant (1,7-octadiene) and the growth substrate (heptane), with an aqueous biomedium phase containing the biocatalyst. Heptane and octadiene delivery to the aqueous phase, and epoxide extraction into the solvent, occurred by diffusion across the dense membrane under a concentration-driving force. In addition, a liquid feed of heptane and octadiene was pumped directly into the bioreactor to increase the rate of delivery of these compounds to the aqueous phase. In this system 1,2-epoxy-7,8-octene accumulated in a pure solvent phase, thus, product recovery problems associated with emulsion formation were avoided. Furthermore, no phase breakthrough of either liquid across the membrane was observed. In this system, the highest volumetric productivity obtained was 30 U.L-1, and this was achieved at a dilution rate of 0.07 h-1, 70 m2. m-3 of membrane area, and a steady-state biomass concentration of 2. 5 g.L-1. The system was stable for over 1250 h. Decreasing the dilution rate led to an increased biomass concentration, however, the specific activity was significantly reduced, and therefore, an optimal dilution rate was determined at 0.055 h-1. Copyright 1999 John Wiley & Sons, Inc.

A membrane bioreactor for biotransformations of hydrophobic molecules

The Membrane Bioreactor for Biotransformations (MBB) is based on the aqueous/organic two-phase system, and uses a tubular silicone rubber membrane to separate the two liquid phases. This avoids the key problem associated with direct contact two-phase processes, specifically, product emulsification. The baker's yeast mediated reduction of geraniol to citronellol was used as a model biotransformation to demonstrate MBB operation. Values for the overall mass transfer coefficient were determined for geraniol, (2.0 x 10(-5) ms-1), and for citronellol, (2.1 x 10(-5) ms-1) diffusion across the silicone rubber membrane. Using these values, and the specific activity of the biocatalyst (5 nmols-1g biomass-1), a suitable membrane surface area: biomass ratio was determined as 2.4 x 10(-3) m2g biomass-1. The bioreactor was operated at this surface area: biomass ratio and achieved a product accumulation rate 90-95% that of a conventional direct contact two-phase system. The slight reduction in product accumulation rate was shown not to be due to mass transfer limitations with respect to reactant delivery or product extraction. Copyright 1998 John Wiley & Sons, Inc.

Two-phase bioconversion product recovery by microfiltration I. Steady state studies

Recovery of an aqueous bioconversion product from complex, two-phase Pseudomonas putida broths containing 20% (v/v) soybean oil presents a significant challenge for downstream processing. Although not used before in multiple-phase separation for complex biotech products, crossflow filtration employing ceramic filters is one of the most attractive options which allow the design of integrated, continuous bioconversion processes. As a first attempt, we studied multichannel, monolithic ceramic membranes of different nominal pore sizes and lumen diameters under steady-state conditions. The best performance was obtained with 0.2-microm-pore/3-mm-lumen membrane, which completely rejected both cells and oil droplets from the permeate, creating a clear aqueous product stream. Although the same separation was achieved, the 50K molecular weight cut-off (MWCO) ultrafilter showed greater irreversible but similar reversible resistance, in addition to an order-of-magnitude higher membrane resistance. Larger nominal pore microfilters, such as 0.45 and 1.0 microm, experienced both cell and oil leakage even at low transmembrane pressure (10 psig). Attributed to greater shear at the same recirculation rate, smaller lumen filters did provide greater permeate flux. However, for practical purposes, the 0. 2-microm-pore/4-mm-lumen ceramic membrane was chosen for further evaluation. Transmembrane pressures up to 50 psig provided only marginal gains in filtration performance, whereas increasing shear rate resulted in linear increases in steady-state flux, presumably due to formation of shear-sensitive, complex gel/oil/cell layer near the membrane surface. A nominal shear rate of 9200 s-1 and 20 psig transmembrane pressure were chosen as optimal operating conditions. Additional studies in a clean system revealed that as low as 5% (v/v) soybean oil in deionized (DI) water resulted in an order-of-magnitude decline in steady-state permeate flux. Breakthrough of oil droplets occurred at 35 psig transmembrane pressure. The severe fouling and breakthrough phenomena disappeared in the presence of washed cells for transmembrane pressure up to 43 psig, implying an oil/cell layer coating the membrane surface, thus preventing oil penetration. Serious membrane fouling was also experienced in microfiltration of oil-free, cell-free supernatant and oil-free whole broth. Consequently, soluble proteins/surfactants were suspected to be the major membrane foulants. Interestingly, soybean oil up to 30% (v/v) enhanced the flux, presumably through complicated interactions with the major foulants. Regeneration of membrane was best achieved with protease and hot caustic/bleach treatments, supporting the hypothesized fouling mechanisms mentioned above. This work provides process and system information for batch microfiltration runs in the future, to be reported elsewhere as Part II of this work.