WHOLE-CELL BIOCATALYSIS FOR AN OIL DESULFURIZATION PROCESS

Various attempts have been made to develop biotechnological processes based on microbiological desulfurization employing aerobic and anaerob ic bacteria. In order to obtain a biodesulfurization process competiti ve with the chemical-physical one of hydrodesulfurization (HDS), a bio technological process has to follow the three main refinery steps: 1) separation, entailing some pretreatment of crude oil; 2) conversion, i .e. biocatalytic transformation where the biocatalyst favours a select ive desulfurization process without destroying useful products; 3) fin ishing, in which the crude oil is separated from the biocatalyst and t he byproducts. Biocatalysis may be carried out using whole cells or is olated enzymes in the free or immobilized form. The use of isolated en zymes is advantageous since it avoids the formation of undesirable byp roduct mediated by contaminating enzymes as in the case of the aerobic biodesulfurization, which is not a selective process. In fact, the ae robic microorganisms may degrade almost all the compounds which make u p the heavy oil. However, despite this advantage, extraction and purif ication of the enzyme is costly and, frequently, enzymes catalyzing ox idation-reduction reactions require enzyme cofactors which must be reg enerated after the reaction. For these reasons, metabolic conditions c an often be designed by using whole cell biotransformations to promote cofactor regeneration, thus avoiding the problems associated with cof actor recycling and regeneration. Today, the main limitations for the industrial application of a biodesulfurization process are associated with the high cost of the biocatalyst and with the volumetric ratio be tween the organic phase and the aqueous one. In order to overcome thes e problems, cell immobilization is one of the most promising approache s in terms of treatment costs and in finishing step times when compare d to a continuous stirred tank bioreactor for the biodesulfurization p rocess. (C) 1997 Elsevier Science B.V.