Batch/continuous processes

FIG. 23-23 Batch and continuous polymerizations, (a) Polyethylene in a tiihiilar flow reactor, up to 2 km long hy 6,4 cm ID, (h) Batch process for polystyrene, (c) Batch-continuous process for polystyrene, (d) Suspension (head) process for polyvinylchloride, (e) Emulsion process for polyvinylchloride, (Ray and Laurence, in Lapidus and Amundson, eds, Chemical Reactor Theory Review, Frentice-Hall, 1977. )... 

Batch continuous processing, in which part of the catalytic solution is removed to a low pressure distillation unit, on the other hand, has recently been commercialised [2-4]. Very little information is available in the public domain concerning this low pressure distillation process, but the main extra cost will be in generating the reduced pressure required for the distillation. The estimated vapour pressures at 110°C of various long chain linear aldehyde products that are commercially desirable are shown in Figure 9.1. This temperature has been chosen because this is the high temperature limit above which the rhodium triphenylphosphine complex starts to decompose. Any commercial process will require to operate the product distillation step at a pressure no higher than those shown for the individual aldehydes. 

None of the alternative strategies for catalyst/product separation has yet reached the point where it can be commercialised for the rhodium catalysed hydroformyation of long chain alkenes and there are very few examples of commercialisation in any catalytic applications. Batch continuous processing with low pressure product distillation has been commercialised but the complexity of the system suggests that alternatives may be able to compete. 

Plant requirements. Is it a single stage, multi stage batch, continuous process or any combination of these Will it use the existing plant or is a new plant required If it is the latter, chemical engineers and a design team will be involved. 

Tiller, F.M. Batch, continuous processes for cake filtration. Chem. Eng. 1974, 29. 

Silverblatt, C. E., Risbud, H., and Tiller, F. M., Batch, Continuous Processes For Cake Filiations, Chemical Engineering, 4 127-136 (1974)... 

FIGURE 22.42 C. E. Silverblatt, Hemant Risbud and Frank M. Tiller, Batch, Continuous Processes for Cake Filtration, Chemical Engineering, April 29, 1974 p. 129, Figure 2. 

Delayed coking is the only main process in a modern petroleum refinery that is a batch-continuous process. The flow through the tube oven is continuous. The feed stream is switched between two drums. One drum is filling with coke while the other drum is being steam-stripped, cooled, decoked and warmed up (see chapter 6). The overhead vapors from the coke drums flow to a rectification unit. The rectification tower has a reservoir in the bottom where the fresh feed is combined with condensed product vapors (recycle) to make up the feed to the coker heater. 

Product rate should be expressed in both mass and molar units. Consider an annual working time of 8000 hours. Decide if the process will be continuous or batch. Continuous processes are suitable for commodities, while batch processes are more advantageous for specialities, the boundary being around 10000 tones/year. 

For a plant of this magnitude, a continuous process would probably be chosen. However, we will return to this issue after considering some process alternatives and see that a hybrid batch/continuous process should also be considered. 

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