Page 121 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

From Figure E3.4, it can be seen that the limiting equipment for this sequence is the separator. This means
that the separator is used without downtime for the duration of the 500 h. If x batches are produced during
the 500 h period, then

Therefore, an additional five batches of each product can be produced using this sequence compared with
the single-product campaign discussed in Example 3.3, assuming no additional cleaning time. It should be
noted that other sequences, such as BACBACBAC, could be used, and these may give more or fewer
batches than the sequence used here.

3.4.2 Jobshop Plants

The flowshop plant discussed previously is one example of a batch plant that processes multiple
products. When not all products use the same equipment or the sequence of using the equipment is
different for different products, then the plant is referred to as a jobshop plant [4]. Figure 3.5 gives two
examples of such plants. In Figure 3.5(a), all the products move from the left to the right—that is, they
move in the same direction through the plant, but not all of them use the same equipment. In Figure 3.5(b),
products A and B move from left to right, but product C uses the equipment in a different order from the
other two products. The sequencing of multiproduct campaigns for this type of plant is more complex and
is illustrated in Example 3.5. The relative efficiencies of different processing schemes for the plant shown
in Figure 3.5(b) are calculated in Example 3.6.

Figure 3.5 Two Examples of Jobshop Plants for Three Products A, B, and C