6.6.1 Motivation for Using a Semibatch Reactor

One of the best reasons to use semibatch reactors is to enhance selectivity in liquid-phase reactions. For example, consider the following two simultaneous reactions. One reaction produces the desired product D

with the rate law

and the other produces an undesired product U

with the rate law

The instantaneous selectivity S_D/U is the ratio of these two rates

6-8

and guides us how to produce the most of our desired product and least of our undesired product (see Section 8.1). We see from the instantaneous selectivity that we can increase the formation of D and decrease the formation of U by keeping the concentration of A high and the concentration of B low. This result can be achieved through the use of the semibatch reactor, which is charged with pure A and to which B is fed slowly to A in the vat.