Batch Cycle Times

For batch reactors, account has to be taken of the time required to achieve a given conversion. Batch cycle time is addressed later. 

The step with the longest time limits the cycle time. Alternatively, if more than one step is carried out in the same equipment, the cycle time is limited by the longest series of steps in the same equipment. The batch cycle time must be at least as long as the longest step. The rest of the equipment other than the limiting step is then idle for some fraction of the batch cycle. 

Clearly, the time chart shown in Fig. 4.14 indicates that individual items of equipment have a poor utilization i.e., they are in use for only a small fraction of the batch cycle time. To improve the equipment utilization, overlap batches as shown in the time-event chart in Fig. 4.15. Here, more than one batch, at difierent processing stages, resides in the process at any given time. Clearly, it is not possible to recycle directly from the separators to the reactor, since the reactor is fed at a time different from that at which the separation is carried out. A storage tank is needed to hold the recycle material. This material is then used to provide part of the feed for the next batch. The final flowsheet for batch operation is shown in Fig. 4.16. Equipment utilization might be improved further by various methods which are considered in Chap. 8 when economic tradeoffs are discussed. 

Figure 4.15 Overlapping batches in Example 4.5 reduces the batch cycle time.

The batch cycle time has been reduced from 2.6 to 1.3 hours. This means that a greater number of batches can be processed, and hence, if there are two reactors each with the original capacity, the process capacity has increased. However, the increase in capacity has been achieved at the expense of increased capital cost for the second reactor. An economic assessment is required before we can judge whether the tradeoff is justified. 

Introducing parallel operations to the steps which limit the batch cycle time. 

Increasing the size of equipment in the steps which limit the batch cycle time to reduce the dead time for those steps which are not limiting. 

Whether parallel operations, larger or smaller items of equipment, and intermediate storage should be used can only be judged on the basis of economic tradeoffs. However, this is still not the complete picture as far as the batch process tradeoffs are concerned. So far the batch size has not been varied. Batch size can be varied as a function of cycle time. Overall, the variables are... 

Optimization of Cycle Times. In batch filters, one of the important decisions is how much time is allocated to the different operations such as filtration, displacement dewatering, cake washing, and cake discharge, which may involve opening of the pressure vessel. Ah. of this has to happen within a cycle time /. which itself is not fixed, though some of the times involved may be defined, such as the cake discharge time. 

The total cycle time for most production batches is 2.5—3.5 min, considerably shorter than the cold- or warm-coating processes. Although a few... 

From diese various estimates, die total batch cycle time t(, is used in batch reactor design to determine die productivity of die reactor. Batch reactors are used in operations dial are small and when multiproducts are required. Pilot plant trials for sales samples in a new market development are carried out in batch reactors. Use of batch reactors can be seen in pharmaceutical, fine chemicals, biochemical, and dye industries. This is because multi-product, changeable demand often requues a single unit to be used in various production campaigns. However, batch reactors are seldom employed on an industrial scale for gas phase reactions. This is due to die limited quantity produced, aldiough batch reactors can be readily employed for kinetic studies of gas phase reactions. Figure 5-4 illustrates die performance equations for batch reactors. 

Assume that in Example 7-10, the overall cycle time for a batch reaction is 8 hrs. The cycle time will include 2 hrs for heat-up and 3 hrs for cool-down. The batch will be heated from 20°C to the reaction temperature of 60°C, then cooled to 35°C. Using a hot-water jacket temperature of 80°C, it took 15 min to heat the batch from 20°C and 30°C. Calculate the jacket temperatures required for heat-up and cool-down. 

Optimum cycle time during batch filtration... 

Theoretically, the volume of a batch reactor is equal to that of a plug flow reactor however, batch reactors are never operated at 100% capacity and cycle times are always greater than reaction times. The combination of these leads to the conclusion that switching from B2C gives an inherent reduction in the reactor volume. 

In the campaign context of a batch plant, it is important to note that throughput and cycle time should include time required for cleaning, transportation and maintenance as these can dramatically affect the overall times and the utilization of any part of a plant. For example, a simple model developed from the AstraZeneca survey calculates the time for a... 

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