Batch process semicontinuous steps

As pointed out in Chapter 1, in a batch process the main steps operate discontinuously. This means that temperature, concentration, mass and other properties vary with time. Also as pointed out in Chapter 1, most batch processes are made up of a series of batch and semicontinuous steps. A semicontinuous step operates continuously with periodic start-ups and shutdowns. 

Operating Labor The most reliable method for estimating labor requirements is to prepare a table of shift, weekend, and vacation coverage. For round-the-clock operation of a continuous process, one operator per shift requires 4.2 operators, if it is assumed that 21 shifts cover the operation and each operator works five, 8-h shifts per week. For batch or semicontinuous operation, it is advisable to prepare a labor table, listing the number of tasks and the number of operators required per task, paying particular attention to primary processing steps such as filtration and distillation that may have several items of equipment per step. 

In the thermosol pad batch process, a semicontinuous process, alkali-sensitive disperse dyes can be used if the alkali is applied after the thermosol process. Conversely, if the thermosol process is carried out after the pad batch step, the pH must be lowered before the thermosol process so that the disperse dye is not destroyed. 

Simulating batch and semicontinuous processes to determine rate-controlling steps and investigate batch-to-batch recycles and heat recovery ... 

For commercial applications, an adsorbent must be chosen carefully to give the required selectivity, capacity, stability, strength, and regenerability. The most commonly used adsorbents are activated carbon, molecular-sieve carbon, molecular-sieve zeolites, silica gel, and activated alumina. Of particular importance in the selection process is the adsorption isotherm for competing solutes when using a particular adsorbent. Most adsorption operations are conducted in a semicontinuous cyclic mode that includes a regeneration step. Batch slurry systems are favored for small-scale separations, whereas fixed-bed operations are preferred for large-scale separations. Quite elaborate cycles have been developed for the latter. 

Figure 12.1c. In batch-product removal, the chemicals are fed to the process before processing begins and steps 2 and 3 are combined that is, the product is removed continuously as the processing occurs, as shown in Figure 12.Id. In effect, fed-batch and batch-product removal processes are semicontinuous processes.

This problem does not occur in continuous processing because the batch size can be adjusted by the process time without interfering in the process control. For semicontinuous processes the batch size can be adjusted by the number of subbatches resulting in identical opportunities as for continuous processes. This approach is limited due to economic aspects with respect to the duration of the whole process, also taking into account preceding or following process steps. 

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