Page 314 - Introduction to chemical reaction engineering and kinetics

12.2 Batch versus Continuous Operation 295

continuously, the process is classified as continuous, and specific flow rates at the inlet
and outlet of the reactor may be identified.

12.2 BATCH VERSUS CONTINUOUS OPERATION
Some advantages (A) and disadvantages (D) of batch operation compared with con-
tinuous operation are given in Table 12.1.
Batch and continuous processes may also be compared by examining their governing
mass-balance relations. As an elaboration of equation 1.5-1, a general mass balance may
be written with respect to a control volume as:

(12’2-1)

In a batch reactor, the first two terms in equation 12.2-1 are absent. In a semibatch
reactor, one of these two terms is usually absent. In a semicontinuous reactor for a mul-
tiphase system, both flow terms may be absent for one phase and present for another.
In a continuous reactor, the two terms are required to account for the continuous inflow
to and outflow from the reactor, whether the system is single-phase or multiphase.

Table 12.1 Comparison of batch and continuous operation
Batch operation Continuous operation
Usually better for small- Better for indefinitely long
volume production (A) production runs of one product
3r set of products (A)
More flexible for multiproduct
(multiprocess) operation (A)
Capital cost usually relatively Capital cost usually relatively
low (A) high (D)
Easy to shut down and clean
for fouling service (A)
Inherent down-time between No down-time except for
batches (D) scheduled and emergency
maintenance (A); but loss
of production in lengthy
stoppages can be costly (D)

Operating cost may be Operating cost relatively low
relatively high (D) (A)

Unsteady-state operation Steady-state operation means
means process control and process control and obtaining
obtaining uniformity of product uniformity of product less
more difficult (D) (but see difficult (A)
England, 1982)