How do molecules get together

In order to make a chemical reaction possible between two or more reactants that were originally separate, the reactants have to be contacted on a molecular scale. This can happen before the reaction starts, or while the reaction is proceeding. We can distinguish two cases  

I The reactants are premixed on a molecular scale under conditions where no reaction occurs, outside the reactor, or, in the case of a batch process, in the reactor before the reaction is started. 

II The reactants are introduced separately into the reactor, either as mutually miscible or as immiscible phases, so that the mixing or contacting takes place while the reaction is proceeding. 

Chapter 3 generally dealt with the first case. For the perfectly mixed CSTR it does not make any difference whether or not the reactants are premixed, since it is assumed that the mixing rate inside the reactor is infinite anyway. 

However, many chemical reactions are so fast that reaction in a premixed condition is impossible to exclude. Therefore, in most practical situations, the reactants are introduced separately into the reactor (case II). In those situations the reactor has to serve as a contacting device, Uiat is, in the case of single-phase reactions it has to be a mixer, and in the case of multi-phase reactions the reactor has to serve as mass exchanger (this term, though uncommon, is used to indicate the analogy with a heat exchanger). 

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