ABSTRACT
Stirred vessels form the major workhorse in biochemical industry, many of them are operated batchwise and at a very large scale. Whether in a catalytic reaction involving enzymes or in a fermentation process, in order to make the process possible, the components have to be brought in contact at the molecular scale. In a typical reactor, the reacting species are added separately into the vessel in which reaction takes place, either as miscible or immiscible phases. The components from different streams are to be brought in intimate contact with each other if the reaction between them is to proceed. The reactor is, therefore, a three-dimensional space where these components are contacted with each other under either controlled conditions of temperature, pressure, etc., or are left to react under uncontrolled conditions. When the reactants are miscible with each other or are present in the soluble form in a solvent, the reaction is said to homogeneous. On the other hand, when the reactants are present in different phases, the system is heterogeneous. For a soluble enzyme-catalyzed reaction in aqueous phase where the substrate is also soluble, the reaction system is homogeneous. For an aerobic fermentation process, where oxygen transfer from gaseous phase to the liquid medium is necessary for the survival and growth of the microorganisms, the system is heterogeneous with distinct liquid, solid, and gaseous phases. Since in such heterogeneous conditions, the existence of transport processes across an interface characterizes the overall process, the relative rates of kinetic and transfer processes determine the rate-determining step.
