Page 464 - Academic Press Encyclopedia of Physical Science and Technology 3rd Chemical Engineering

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Encyclopedia of Physical Science and Technology EN014A-654 July 28, 2001 16:35

26 Reactors in Process Engineering

3. Design Parameters integrated form of the performance equation has varied
significance depending on the particular reaction scheme
The design parameters for a batch reactor can be as simple
being performed. For example, molecular weight distribu-
as concentration and time for isothermal systems. The
tions in polymerization reactions can be controlled more
number of parameters increases with each additional com-
precisely in batch reactors.
plication in the reactor. For example, an additional reactant
One of the traditional disadvantages of the batch reac-
requires measurement of a second concentration, a sec-
tor has been the labor required between runs for emptying
ond phase adds parameters, and variation of the reaction
and filling the tank. With recent advances in computer con-
rate with temperature requires additional descriptors: a
trol, this disadvantage no longer exists. If the advantages
frequency factor and an activation energy. These values
of batch are significant, the capital expense of computer
can be related to the reactor volume by the equations in
control is essentially negligible. Due to computer con-
Section III.
trol, the batch reactor should no longer be looked upon
as something to be avoided. If the kinetics and design pa-
4. Applications rameters indicate that batch is a competitive design, then
use it.
Application of the batch reactor design equations re- The major disadvantage of batch reaction now is the
quires integration over time. Along with the simplicity of hold-up time between batches. Although the actual reac-
cookbook chemistry, this is one of the major advantages tion time necessary to process a given amount of feed may
of the batch reactor: concentrations are not averaged over be substantially less than for a time-averaged reactor such
time. Initially, when concentrations are at their highest, as a CSTR, when the hold-up time is added, the total pro-
the corresponding rates of reaction are also high. This cess time may be greater. Other disadvantages of the batch
gives the greatest amount of conversion in the shortest reactor are dependent on the particular type of reaction be-
time. The integral reactor design form makes the batch ing considered, such as whether the reaction is in parallel
reactor attractive for higher-order reactions. Batch is also or series.
good for reactions in series (if the reaction can be quickly
quenched), where large amounts of an intermediate can
be produced quickly before it has time to react away to a B. Semibatch
by-product.
1. Description
The batch reactor is extremely flexible compared with
continuous reactor configurations. For example, temper- The semibatch reactor is a cross between an ordinary batch
ature can easily be made a function of reaction time. reactor and a continuous-stirred tank reactor. The reactor
Once the reactor is put into service, operational alterna- has continuous input of reactant through the course of the
tives are still available. The tank can be operated half- batch run with no output stream. Another possibility for
full without affecting product quality, or the reaction time semibatch operation is continuous withdrawal of product
can be modified easily. Both of these changes may cause with no addition of reactant. Due to the crossover between
heat and mass transfer problems in fixed-volume contin- the other ideal reactor types, the semibatch uses all of the
uous equipment. This flexibility is worthwhile for prod- terms in the general energy and material balances. This
ucts that are made in various grades, have seasonal de- results in more complex mathematical expressions. Since
mand, or have subjective specifications such as the taste of the single continuous stream may be either an input or
beer. an output, the form of the equations depends upon the
Batch reactors are used extensively in industries where particular mode of operation.
only small quantities of product are made, such as pharma- Physically, the semibatch reactor looks similar to a
ceuticals. For small amounts, the economy of scale hurts batch reactor or a CSTR. Reaction occurs in a stirred tank,
flow reactors, which typically have a higher initial invest- with the following assumptions; (1) the contents of the
ment for controls and plumbing. tank are well mixed, and (2) there are no inlet or outlet
effects caused by the continuous stream.

5. Advantages–Disadvantages
2. Classification
The primary advantages of the batch reactor are simplic-
ity of design, which allows for tremendous flexibility, and The semibatch reactor is one of the primary ideal reactor
integration of the performance equation over time. The types since it can not be accurately described as either
simplicity of design, usually a stirred tank, makes opera- a continuous or a batch reactor. A semibatch reactor is
tion and monitoring easy for the majority of reactions. The usually classified as a type of transient reactor.