Page 120 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Thus, 42 batches each of A, B, and C can be run as campaigns in a 500 h period. The cycle times are
t cycle,A = [ (41)(2.5) + 8/(42)] = 2.631 h, t cycle,B = 4.619 h, and t cycle,C = 4.655 h.

Using Equation (3.6) with the approximations t cycle,A = 2.5, t cycle,B = 4.5, and t cycle,C = 4.5,

Equation (3.6) slightly overestimates the number of batches that can be run in the 500 h period but is a
very good approximation. In general, Equation (3.6) will be used to estimate cycle times and other
calculations for single-product campaigns for multiproduct plants.

Running campaigns for the production of the same product is efficient and makes scheduling relatively
easy. However, this strategy suffers from a drawback: The longer the production cycle, the greater the
amount of product that must be stored. The concept of product storage is addressed in the following
section. However, the bottom line is that storage requires additional equipment or warehouse floor space

that must be purchased or rented. On the other hand, a strategy of single-product campaigns may decrease
cleaning times and costs, which generally are greater when switching from one process to another.
Therefore, the implementation of a batch sequencing strategy that uses sequences of single-product
campaigns involves additional costs that should be included in any design and optimization. The extreme
case for single-product batch campaigning occurs for seasonal produce (a certain vegetable oil, for
example), where the feed material is available only for a short period of time and must be processed
quickly, but the demand for the product lasts the whole year.

An alternative to running single-product campaigns (AAA..., BBB..., CCC...) over the production cycle is
to run multiproduct campaigns—for example, ABCABCABC, ACBACBACB, AACBAACBAACB, and
so on. In this strategy, products are run in a set sequence and the sequence is repeated. This approach is
illustrated in Example 3.4.

Example 3.4

Consider the same processes given in Example 3.3. Determine the number of batches that can be produced
in a month (500 h) using a multiproduct campaign strategy with the sequence ABCABCABC....

The Gantt chart for this sequence is shown in Figure E3.4.

Figure E3.4 Gantt Chart Showing the Multiproduct Sequence ABCABCABC...