Immobilised enzyme technology (IET) is attractive to process industries in which either the enzymes (biocatalysts) are expensive or a large throughput of substrate is required. In a bioreactor using IET the enzymes are constrained to remain within the reactor. The advantages of IER include: the easy re-use of the enzymes, easier product recovery and purification, the use of high enzyme loads, prolonged enzyme activity and the production of large amounts of product with relatively small amounts of catalyst (Katchalski-Katzir, 1993). When operated as a continuous process additional advantages include reductions in costs and energy, the ability to recycle products and to operate at high flow rates. This last point is particularly important in the food industry, especially in the treatment of perishable commodities (D'Souza, 1999). The high yield of pure material resulting from the use of IET is associated with a reduction in the production of waste products. Finally, immobilization often protects the enzyme from inactivation. This allows reactions to occur under harsher environmental conditions, such as pH or temperature or even allowing the use of organic solvents, than when the non-immobilized enzyme is used.
We are examining simple kinetic schemes for biologically catalysed reactions in an immobilised enzyme reactor (IER). The IER is treated as a well-stirred flow reactor, with the restriction that bounded and unbounded enzyme species are constrained to remain within the reaction vessel by a membrane. Our aim is to find operating conditions for the reactor which optimises either the yield of the desired product or the productivity of the desired product.
We are investigating the effective of negative feedback mechanisms, such as preferential filling of active sites by the desired product, effect the reaction yield. We are also investigating mechanisms whereby the active species catalyses the degradation of the desired product. Although the models are generic the motivation for this work are problems in food engineering.
The use of membrane bioreactors to reduce the emission of pollutants in industrial wastewaters was investigated by Nelson et al (2008). It was found that in many cases the optimal performance of a cascade of two membrane bioreactors outperforms that of a single reactor by two orders of magnitude.
In (Nelson et al; 2008b) we investigated the behaviour of a reaction described by Michaelis-Menten kinetics in an immobilised enzyme reactor (IER). The IER is treated as a well-stirred flow reactor, in which the bound and unbound enzyme species are immobilized and therefore constrained to remain within the reaction vessel. The product species leaves the bioreactor either in the reactor outflow or by permeating through the semi-permeable reactor wall.
We showed that at low residence times membrane extraction through the reactor walls increases the total product concentration recovered whereas at high residence times membrane extraction decreases the total product concentration. We also show that the reactor productivity is maximised at high residence times. For reactor productivity the key control variable is the ratio of the reactor volume to the jacket volume. If this ratio is greater than one, then membrane extraction increases the productivity. If this ratio is less than one, then membrane extraction decreases the productivity.
In (Nelson et al; 2009) we investigated the behaviour of a reaction described by Michaelis-Menten kinetics in an immobilised enzyme reactor (IER). The IER is treated as a well-stirred flow reactor, in which the bound and unbound enzyme species are immobilized and therefore constrained to remain within the reaction vessel. The product species leaves the bioreactor either in the reactor outflow or by permeating through the semi-permeable reactor wall. Before the substrate can react with the enzyme, the enzyme must first be activated by absorption of an activator.
We use steady state analysis to identify the best operating conditions on the reactor. To this end, we show that the concentration of product is maximised at low residence time whereas the productivity of the reactor is maximised at high residence times.
This paper is available at http://www.bepress.com/cppm/vol4/iss3/14.