Use of Membrane Reactors to Enhance Selectivity
In addition to species leaving through the sides of the membrane reactor, species can also be fed to the reactor through the sides of the membrane. For example, for the reaction
A + B → C + D
species A could be fed only to the entrance, and species B could be fed only through the membrane as shown here.
As we will see in Chapter 8, this arrangement is often used to improve selectivity when multiple reactions take place. Here, B is usually fed uniformly through the membrane along the length of the reactor. The balance on B is
where RB = FB0/Vt with FB0, representing the total molar feed rate of B through the sides and Vt the total reactor volume. The feed rate of B can be controlled by controlling the pressure drop across the reactor membrane.5 This arrangement will keep the concentration of A high and the concentration of B low to maximize the selectivity given by Equation (E8-2.2) for the reactions given in Section 8.6.
6.5 Unsteady-State Operation of Stirred Reactors
In Chapter 5 we discussed the unsteady operation of one type of reactor, the batch reactor. In this section, we discuss two other aspects of unsteady operation: startup of a CSTR and of semibatch reactors. First, the startup of a CSTR is examined to determine the time necessary to reach steady-state operation [see Figure 6-4(a)], and then semibatch reactors are discussed. In each of these cases, we are interested in predicting the concentration and conversion as a function of time. Closed-form analytical solutions to the differential equations arising from the mole balance of these reaction types can be obtained only for zero- and first-order reactions. ODE solvers must be used for other reaction orders.
Figure 6-4. Semibatch reactors. (a) Reactor startup, (b) semibatch with cooling, and (c) reactive distillation.
Excerpted by special permission from Chem. Eng., 63(10) 211 (Oct. 1956). Copyright © 1956 by McGraw-Hill, Inc., New York, NY 10020.
There are two basic types of semibatch operations. In one type, one of the reactants in the reaction
A + B → C + D
(e.g., B) is slowly fed to a reactor containing the other reactant (e.g., A), which has already been charged to a reactor such as that shown in Figure 6-4(b). This type of reactor is generally used when unwanted side reactions occur at high concentrations of B (see Section 8.1) or when the reaction is highly exothermic (Chapter 11). In some reactions, the reactant B is a gas and is bubbled continuously through liquid reactant A. Examples of reactions used in this type of semibatch reactor operation include ammonolysis, chlorination, and hydrolysis. The other type of semibatch reactor is reactive distillation and is shown schematically in Figure 6-4(c). Here, reactants A and B are charged simultaneously and one of the products vaporizes and is withdrawn continuously. Removal of one of the products in this manner (e.g., C) shifts the equilibrium toward the right, increasing the final conversion above that which would be achieved had C not been removed. In addition, removal of one of the products further concentrates the reactant, thereby producing an increased rate of reaction and decreased processing time. This type of reaction operation is called reactive distillation. Examples of reactions carried out in this type of reactor include acetylation reactions and esterification reactions in which water is removed.