Page 129 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

From the top chart in Figure 3.8, the limiting piece of equipment is seen to be the crystallizer. The bottom
chart shows the effect of adding a second crystallizer that processes product C. The effect is to reduce the
cycle time from 21 h to 13 h, a considerable improvement in throughput. The determination of whether to
make this change must be made using an appropriate economic criterion, such as net present value (NPV)
or equivalent annual operating cost (EAOC), which are discussed in Chapter 10. The resulting trade-off is
between increased product revenues versus the cost of purchasing a second crystallizer plus additional
operators to run the extra equipment.

3.6 Design of Equipment for Multiproduct Batch Processes

The design of equipment sizes for multiproduct batch processes depends on the production cycle time,
whether single- or multiproduct campaigns are used, the sequence of products for multiproduct
campaigns, and the use of parallel equipment. As an example, the multiproduct process described in
Table 3.4 will be analyzed. It is assumed that each product will be produced using a single-product
campaign. The production cycle will be 500 h (equivalent to one month in a 6000 h year). The production
cycle will be repeated 12 times in a year. The required amount of each product is given in Table 3.4
along with the processing times.

Table 3.4 Data for a Multiproduct Batch Process

By studying Table 3.4, it is apparent that the limiting piece of equipment is the mixing and reaction vessel,
and the cycle times can be found from this piece of equipment. To estimate equipment volume, it is
necessary to determine the volume of each piece of equipment per unit of product produced. To determine
these quantities, descriptions of the method (recipe) for using each piece of equipment for each product
must be known. The procedure to estimate the specific volume of the reactor for Process A in Table 3.4 is
given in Example 3.8.

Example 3.8

The following is a description of the reaction in process A, based on a laboratory-scale experiment. First,
3
10 kg of liquid reactant (density = 980 kg/m ) is added to 50 kg of a liquid mixture of organic solvent