Page 118 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

For the overlapping scheme, when the number of batches (n) to be produced is large, the cycle time is
approximated by

(3.5)

Therefore, for Example 3.1, the nonoverlapping and overlapping cycle times are 14.5 h and 4 h,
respectively.

3.4 Flowshop and Jobshop Plants

Thus far, the discussion has focused on the production of only a single product. However, most batch
plants produce multiple products. All these products may require the same processing steps, or more often
will require only a subset of all possible steps. Moreover, the order in which a batch process uses

different equipment might also differ from product to product.

3.4.1 Flowshop Plants

Consider a plant that must make three products: A, B, and C. Figure 3.4 shows an example of the
sequence of equipment used to produce these three products. In Figure 3.4, all the products use the same

equipment in the same order or sequence, but not necessarily for the same lengths of time. This type of
plant is sometimes referred to as a flowshop plant [4]. The total time for operation of overlapping
schedules depends on the number of runs of each product and how these runs are scheduled. One
approach to scheduling multiple products is to run each product in a campaign during which only that
product is made. Then the plant is set up to run the next product in a campaign, and so on. The case when
multiple products, using the same equipment in the same order, are to be produced in separate campaigns
is considered first. If the corresponding numbers of batches for products A, B, C in a campaign are n , n ,
B
A
and n , respectively, then the total processing time, or production cycle time, can be found by adding the
C
operation times for each product. If the number of batches per campaign is large (for example, >10), then
the production cycle time can be approximated by

Figure 3.4 An Example of a Flowshop Plant for Three Products A, B, and C