Thermal stability of a CSTR

THERM and THERM PLOT - Thermal Stability of a CSTR System... 

Thermal stability of chemical reactors is a classic yet active area within chemical engineering science. Considerable research has focused on determining safe operating criteria for batch, CSTR, and tubular reactors. Current work has been directed towards understanding thermal stability in the presence of multiple phases (fluid/solid and gas/liquid) and multiple reactions with realistic, complex reaction rates expressions. The advent of computational methods has allowed for this field to continue to thrive. A sound understanding of these principles may help improve industrial reactor performance by reducing waste and costly separation operations and help maintain a clean environment. 

As in all catalytic processes, catalyst stability is an essential feature. We have investigated the stability of acylase in conditions that are pertinent for large-scale processes. Instead of just determining thermal stability, which can be done by measuring storage stability of the enzyme in particular conditions of temperature and pH, we have also determined operational stability. The relevant parameter for operational stability studies of enzymes is the product of active enzyme concentration [E] and residence time t, [E] t. For a CSTR, the quantities [E] and t are linked by Eq. (19.37), where [S0] = initial substrate concentration, % = degree of conversion, and r(x) = reaction rate (Wandrey, 1977 Bommarius, 1992b). 

Another continuous operation that is very popular in industry, but is not used very much in the chemical laboratory, is the continuous stirred tank reactor (CSTR) (it is silently assumed that the CSTR is operated in the steady state). The reactants are fed continuously into a well stirred vessel, and a constant product flow leaves the reactor through an exit port that can be located anywhere in the reactor wall. If the reactor contents are indeed well mixed, the reactants entering the vessel are diluted immediately, and the reaction proceeeds at relatively low reactant concentrations. The pr uct flow leaving the reactor must then have the same composition as the reaction mixture. This may appear to be an illogical way of carrying out a chemical reaction, but it has several distinct advantages, at least for reactions that are intrinsically rapid. The most important advantage is the thermal stability, especially in the case of exothermic reactions. Since the reaction proceeds at low reactant concentrations, the reaction rate per unit volume is relatively low, and so is the heat evolution. These are both constant in time. 

TTie perfectly mixed reactor does not appear to be particularly suitable for this type of reactions, but it may be used when the ratio of the rate constants is very high, and when B is fed in sufficient excess. Also, there may be other reasons for selecting a CSTR, such as good heat transfer and thermal stability. 

A large number of case studies is reported in scientific literature dealing with physical equilibria, design purposes for unit operations, reactor stability, and so on. We have included some below to highlight their individual peculiarities. These include heat exchange in a thermal furnace, vapor-liquid equilibrium calculation, multiple solutions in a continuously stirred tank reactor (CSTR) reactor, and critical nuclear reactor size. Certain special cases are also discussed in Section 7.22. 

---