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Title: Evaluation and comparison of two-liquid phase hydrolytic reaction in a stirred tank and membrane biocatalytic reactor
Author: Cunnah, Philip Jeffrey
Awarding Body: University of London
Current Institution: University College London (University of London)
Date of Award: 1995
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The introduction of a second liquid phase as a means to improving the productivity of bioreactions involving poorly water soluble compounds is now a fairly well established technique. It is particularly applicable for the generation of chiral intermediates via enzyme catalysed hydrolytic resolution. The work presented in this thesis attempts to address issues which will aid in the understanding of elements involved with the commercial development of such processes. Hydrolytic reactions can be facilitated by both esterases and lipases. In this study a purified pig liver esterase and a crude porcine pancreatic lipase were found to catalyse hydrolysis of a model substrate benzyl acetate, in a two liquid phase medium, in a stirred tank reactor. In previous study pig liver esterase was found to catalyse hydrolysis solely in the aqueous phase, in contrast porcine pancreatic lipase was found to catalyse reaction at the interface. In the stirred tank reactor, the activity of the esterase was dependent on the mass transfer conditions in the reactor as influenced by operating variables of phase ratio and agitation rate. Using data from previous studies and that obtained in this study mass transfer conditions were evaluated and optimum conditions of operation in the stirred tank reactor were identified. The optimal mass transfer conditions in the reactor did not support large aqueous phase concentrations of the pig liver esterase. The lipase activity was influenced by the amount of interface in the reactor. The optimal interfacial operating conditions supported large aqueous phase concentrations fo the lipase preparation at maximum activity. The esterase was prone to interfacial effects and product inhibition and thus had limited stability. Resultingly overall conversion of substrate was low. In contrast the lipase was comparatively more stable and the overall conversion of substrate using lipase as the catalyst was much greater. Although reaction could be carried out in the stirred tank reactor, difficulties were forseen in the downstream separation of reactor contents for product recovery and catalyst reuse. Conditions in the stirred tank reactor, necessary to maintain optimal enzyme activity, led to the irreversible destruction of the enzyme. To overcome some of these problems a further reaction device was studied, a membrane bioreactor. Fundamentally the two reactors differed in the way in which contact of the two phases was facilitated, in the stirred tank reactor this was by dispersion, in the membrane reactor by contact across a membrane surface, and the enzyme location, in the stirred tank reactor solubilised in the aqueous phase and in the membrane bioreactor immobilised onto the membrane surface. The lipase catalysed hydrolysis was compared in the membrane reactor with its hydrolysis in the stirred tank reactor. The activity of the enzyme in the membrane reactor was severely limited in comparison to activity in the stirred tank reactor. Limitations were greatest at high enzyme loads. Several ideas were proposed as to why this might be. Although the activity of the enzyme was limited, the stability was much improved. The time required to achieve the same degree of conversion in the membrane reactor as achieved in the stirred tank reactor was greater. The membrane reactor facilitated phase separation throughout the course of the reaction and enzyme was reused in several subsequent reactions with some loss of activity between runs. The results and methods used to obtain them provide useful tools and guidelines and a scientific basis which can be applied to the evaluation of similar reaction systems in order to identify the most effective process option.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available