Page 114 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

cost benefit of recycling the raw materials would far outweigh the cost of the additional tank. Third, it is
observed that, overall, only 60% of the product made in the reactor is crystallized out in Step 5. This
means that the mother liquor (solution containing product to be crystallized after some has crystallized
out) contains significant amounts of valuable product. Additional crystallization steps could recover
some, if not most, of the valuable product. The strategy for accomplishing this could be as simple as
scheduling a second or third cooling or crystallization step, or it could involve storing the mother liquor
from several batch processes until a sufficient volume is available for another cooling or crystallization
step. These additional crystallization steps are tantamount to adding additional separation stages.

3.2 Gantt Charts and Scheduling

Gantt charts (see, for example, Dewar [3]) are tabular representations used to illustrate a series of tasks
(rows) that occur over a period of time (columns). These charts graphically represent completion dates,

milestone achievements, current progress, and so on [3] and are discussed further in Chapter 26 as a
planning tool for completing large design projects. In this chapter, a simplified Gantt chart is used to
represent the scheduling of equipment needed to produce a given batch product. Example 3.2 illustrates
the use of Gantt charts to show the movement of material as it passes through several pieces of equipment
during a batch operation.

Example 3.2

Draw a Gantt chart that illustrates the sequence of events in the production of the API in Example 3.1.

Solution

Gantt charts for this process are shown in Figure E3.2. Note that in both charts, Steps 1 and 2 have been
consolidated into one operation because they occur sequentially in the same piece of equipment. The top

chart shows the row names as tasks, and the bottom figure simply identifies each row with the equipment
number. In general, the notation used in the bottom figure will be adopted.

Figure E3.2 Gantt Chart Showing Sequence of Events for the Manufacture of API in Example 3.1