Principles of Reactor Design for Mixing-Sensitive Systems

13-2 PRINCIPLES OF REACTOR DESIGN FOR MIXING-SENSITIVE SYSTEMS 

1 Mixing Time Scales Calculation of the Damkoehler Number 

The Damkoehler number requires characteristic time scales for both mixing and reaction. Calculation of the reaction time scale is relatively straightforward, although the necessary data may be difficult to obtain. Many choices for the mixing time have been proposed, and data are available for many common semibatch geometries. 

1 Characteristic Reaction Time. As shown in Example 13-1, mixing can affect the selectivity of a reaction, not just the rate. Reactions that show selectivity are usually two-step reactions which are either consecutive or parallel. One reaction is usually so fast that it is mixing controlled. The second reaction has a characteristic time constant of the order of the local mixing time. The reaction time is usually given by 

Vessel blend times are typically about 2 s in a 1 L vessel and about 20 s in a 20 000 L vessel for low viscosity liquids. 

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