Page 106 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Chapter 3 Batch Processing

Some key reasons for choosing to manufacture a product using a batch process were discussed in Chapter
2. These include small production volume, seasonal variations in product demand, a need to document the
production history of each batch, and so on. When designing a batch plant, there are many other factors an

engineer must consider. The types of design calculations are very different for batch compared with
continuous processes. Batch calculations involve transient balances, which are different from the steady-
state design calculations taught in much of the traditional chemical engineering curriculum. Batch
sequencing—the order and timing of the processing steps—is probably the most important factor to be
considered. Determining the optimal batch sequence depends on a variety of factors. For example, will
there be more than one product made using the same equipment? What is the optimal size of the
equipment? How long must the equipment run to make each different product? What is the trade-off
between economics and operability of the plant? In this chapter, these questions will be addressed, and an
introduction to other problems that arise when considering the design and operation of batch processes
will be provided.

3.1 Design Calculations for Batch Processes

Design calculations for batch processes are different from the steady-state design calculations taught in
most unit operations classes. The batch nature of the process makes all design calculations unsteady state.
This is best demonstrated by example; Example 3.1 illustrates the types of design calculations required

for batch processing.

Example 3.1

In the production of an API (active pharmaceutical ingredient), the following batch recipe is used.

Step 1: 500 kg of reactant A (MW = 100 kg/kmol) is added to 5000 kg of a mixture of organic solvent
(MW = 200 kg/kmol) containing 60% excess of a second reactant B (MW = 125 kg/kmol) in a jacketed
reaction vessel (R-301), the reactor is sealed, and the mixture is stirred and heated (using steam in the
3
jacket) until the temperature has risen to 95°C. The density of the reacting mixture is 875 kg/m (time
taken = 1.5 h).

Step 2: Once the reaction mixture has reached 95°C, a solid catalyst is added, and reaction takes place
while the batch of reactants is stirred. The required conversion is 94% (time taken = 2.0 h).

Step 3: The reaction mixture is drained from the reactor and passed through a filter screen (Sc-301) that
removes the catalyst and stops any further reaction (time taken = 0.5 h).

Step 4: The reaction mixture (containing API, solvent, and unused reactants) is transferred to a
distillation column, T-301, where it is distilled under vacuum. Virtually all of the unused reactants and
approximately 50% of the solvent are removed as overhead product (time taken = 3.5 h). The end-point