Semibatch

Semibatch is a mode of operation for reactors where one or more reactants are introduced into the reactor during the reaction, while the remaining reactants are charged before the process begins. The feed can be added continuously or in discrete additions, and the reactor volume may change over time as material is fed in and, in some configurations, withdrawn to keep volume within design limits.

The design and analysis of semibatch systems rely on time-dependent mass and energy balances. Because the feed alters both concentration and volume, the equations include terms for inflow, potential outflow, reaction rates, and heat transfer. Control strategies often seek to manage heat release and maintain desired product specification by adjusting feed rate or composition.

Common applications include polymerization, where gradual addition controls molecular weight and viscosity; fermentation or bioprocessing, where nutrients are fed to sustain growth; and chemical syntheses that benefit from staged addition to improve selectivity or safety by diluting reactive intermediates.

Semibatch lies between batch and continuous operations: in a batch reactor, all feed is present at t = 0 with no inflow during the run; in a continuous reactor, feed and effluent flow at constant rates from the start. The term is sometimes used interchangeably with fed-batch, but some sources reserve semi-batch for configurations with one or more feeds introduced during the reaction while keeping others charged at t = 0.