Page 113 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

From Figures E3.1(a) and (b), the mole fraction of reactant B is seen to drop to less than the specification
of 0.01 (1 mol%) at a time of approximately 2.3 h. This time, coupled with the heating time of 70.2
minutes, gives a total of 3.5 h. However, note that only about 75% of the product remains in the still to be
recovered in the next step.

Step 5: Cooling and Crystallization of Product

The analysis of the crystallization, filtration, drying, and packaging steps is beyond the scope of this
analysis. Therefore, it is assumed that the times for each of these steps have been determined through
laboratory-scale experiments, and those times are simply stated here. The amount of product crystallized
is 80% of the product recovered from the still, or 60% of the 1057.5 kg produced in the reactor (634.5
kg.) The time required to cool and crystallize is 2 h.

Step 6: Filtration and Drying

The time required for filtration and drying is 4 h.

Step 7: Packaging

The time required for packaging is 1 h.

There are several unique features of batch operations observed in Example 3.1. First, the heating,
reaction, and separations steps are unsteady state, which is different from the typical steady-state analysis
with which most undergraduate chemical engineers are familiar. Secondly, it is observed that no
provision was made to recycle the unreacted raw materials. In Chapter 2, recycle was shown to be a key
element of a steady-state chemical process. Raw materials are almost always the largest item in the cost
of manufacturing; therefore, recycling unreacted raw materials is essential to ensure profitability. So, how
is this done in a batch process? In Example 3.1, the overhead product from the batch distillation contains
unreacted raw material and product in the solvent. This could be sent to a holding tank and periodically
mixed with a stream containing pure solvent and just enough reactants A and/or B to make up a single
charge to the process in Step 1. However, the recycling of product to the reactor would have to be
investigated carefully to determine whether unwanted side reactions take place at higher product
concentrations. Even though an additional tank would need to be purchased, it is almost certain that the