Chapter 5 - Ideal Flow Reactors

Abstract

There is one important parameter used for flow reactors, τ, the reactor space time or its inverse, D , the dilution rate. The space time is the time needed to feed a reactor full of reaction mixture through the reactor. There are two basic types of ideal flow reactors: PFR (plug flow reactor) and CSTR (continuous stirred tank reactor). PFR represents a one dimensional flow reactor, like a piston. At steady state, the concentration of reactants as well as products changes along the direction of flow in a PFR. The concentrations of the reactants are the highest in a PFR presented to reaction. The analysis to a PFR is similar to that for a batch reactor, the only difference being the independent variable: the residence time or distance along the reactor flow direction, rather than the time for a batch reactor. CSTR represents a reactor that is well mixed such that no concentration gradient exits. CSTR is also termed chemostat. At steady state, only algebraic equations are involved in CSTR analysis. The concentrations of reactants in a CSTR are at their lowest level in a CSTR and thus least available to reaction. Graphical and analytical solution methods are demonstrated with examples for CSTR and PFR. In flow reactor design, our interest is not only on the reactor volume, but type of reactors. Usually, a whole bioprocess system optimization (economic analysis) is applied to determine whether CSTR or PFR is employed. In some cases, one can save time by knowingly selecting the right type of reactors. Selection of PFR or CSTR depends on the nature of the reaction mixture and type of reaction (kinetics). The reactor feed strategy greatly affects the product mixture. A reactor with optimum feed distribution and / or product separation can yield significantly more desired product. Methods of how these “complicated” flow reactors as well as simple flow reactors are demonstrated in the chapter.