Mixing Issues Associated with Batch, Semibatch, and Continuous Operation

3 Mixing Issues Associated with Batch, Semibatch, and Continuous Operation 

Pipe mixing E = (APVs)/pL, where Vs is the superficial velocity. See  

Continuous and semicontinuous flow reactors are sometimes used in fine chemical and pharmaceutical applications, primarily because some reactions require the high intensity of mixing that can only be achieved in in-line mixers (see Examples 13-3, 13-7, and 13-8a and discussion in Chapter 17). 

An important question for the design of continuous flow systems is When can the classic perfectly mixed assumption (ideal CSTR) be used in a continnons flow stirred tank reactor The blend time concept can be used here. If the blend time is small compared to the residence time in the reactor, the reactor can be considered to be well mixed. That is because the residence time is proportional to the characteristic chemical reaction time. A 1 10 ratio of blend time to reaction time is often used, but often, larger values result because the mixer must do other jobs, which lead to even smaller blend times. Frequently, residence time distributions are used to determine whether a reactor is well-mixed. It is usually easy to achieve well-mixed conditions in continuous flow, turbulent stirred vessels unless the reactions are very fast, such as acid-base neutralizations. Even in laminar systems the blend time can be made much less than the required residence time for the chemical reaction mainly because required residence times are so long for high viscosity reactants. For discussions of residence time distribution analysis, see Chapter 1, Levenspiel (1972), and Nauman (1982). 

In Chapter 9 it is suggested that if the batch blending time is less than one-tenth the residence time and the inlet and outlet are separated in such a way that a line drawn from the inlet to the outlet passes through the impeller, fiiUy back mixed conditions will be achieved. Even in the case of a perfectly backmixed vessel, mixing effects on selectivity must also be checked. 

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