Page 126 - Analysis, Synthesis and Design of Chemical Processes, Third Edition

Example 3.7

For the products A, B, and C in Example 3.3, determine the minimum storage capacities for the single-
product campaign strategy outlined in Example 3.3.

Solution

Table E3.7 shows the results using data given in Example 3.3 and Table 3.1.

Table E3.7 Results for the Estimation of Minimum Storage Volume from Equation (3.7)

It should be noted that the production cycle time is equal to the sum of the campaign times, or (107.5 +
193.5 + 193.5) = 494.5 h, which is slightly less than 500 h. This discrepancy reflects the approximation
of cycle times given by Equation (3.6). The actual cycle times for A, B, and C are found from Example
3.3 and are equal to 2.63, 4.62, and 4.65 h, respectively. The corresponding values of V are 7.79, 9.18,
s
3
and 7.31 m . Clearly, these differences are small, and the approach using Equation (3.6) is acceptable
when the number of production runs per campaign is 10 or more.

3.5.2 Intermediate Storage

Up to this point, it has been assumed that there is no intermediate product storage available. This type of
process is also known as a zero wait, or a zw-process [4]. Specifically, as soon as a unit operation is
completed, the products are transferred to the next unit operation in the sequence, or they go to final
product storage. The concept of storing the final product to match the supply with the demand was
demonstrated in Example 3.7. However, it may also be beneficial to store the output from a given piece of
equipment for a period of time to increase the overall efficiency of a process. It may be possible to store
product in the equipment that has just been used. For example, if two feed streams are mixed in a vessel,
the mixture could be stored until the next process unit in the production sequence becomes available. In

this case, the storage time is limited based on the scheduling of equipment. This holding-in-place method
may not work for some unit operations. For example, in a reactor, a side reaction may take place, and
unless the reaction can be quenched, the product yield and selectivity will suffer. The upper limit of the
intermediate storage concept occurs when there is unlimited intermediate storage (uis) available, and
this is referred to as a uis-process [4]. In general, cycle times can be shortened when intermediate
product storage is available. This concept is illustrated in Figure 3.7, which is based on the information
given in Table 3.2.

Figure 3.7 Multiproduct Sequence (ABC) for Products Given in Table 3.2 Showing Effect of