Page 299 - Modeling of Chemical Kinetics and Reactor Design

Introduction to Reactor Design Fundamentals for Ideal Systems 269

Batch reactors are charged with reactants, closed, and heated to the
reaction temperature, which may be maintained (i.e., isothermally) for
the duration of the reaction. After the reaction is completed, the
mixture cooled, and the reactor opened, the product is discharged and
the reactor is cleaned for the next batch. In industrial operations, the
cycle time is constant from one batch to the next. The time required
for filling, discharging, heating, cooling, and cleaning the reactor forms
an integral part of the total batch cycle time (t ) and is referred to as
b
the turnaround time (t ). The total batch cycle time t is the reaction
t
b
t time plus the turnaround time t . This is expressed as:
r
t
t =t +t t (5-21)
r
b
From these various estimates, the total batch cycle time t is used
b
in batch reactor design to determine the productivity of the reactor.
Batch reactors are used in operations that are small and when multi-
products are required. Pilot plant trials for sales samples in a new
market development are carried out in batch reactors. Use of batch
reactors can be seen in pharmaceutical, fine chemicals, biochemical,
and dye industries. This is because multi-product, changeable demand
often requires a single unit to be used in various production campaigns.
However, batch reactors are seldom employed on an industrial scale
for gas phase reactions. This is due to the limited quantity produced,
although batch reactors can be readily employed for kinetic studies
of gas phase reactions. Figure 5-4 illustrates the performance equations
for batch reactors.
Following are examples for finding the time of an isothermal batch
reactor for a given conversion of the reactant and other pertinent
variables, and for gas phase reaction.

Example 5-1

Consider an isothermal batch reactor for a given conversion of
the reactant

+
aA bB → cC + dD (5-22)

where component A is the limiting reactant. The stoichiometric coef-
ficients are: a = 1, b = 2, c = 1, and d = 0. The initial moles of
components A, B, C, and D are N = 0.001 gmol, N = 0.003 gmol,
A B