Cycle time—batch processes

Another reason for using different reactor sizes along the CSTR train is the variation of polymerization rate with monomer conversion. This factor is not a major consideration if the final conversion is modest as in the case of styrene-butadiene rubber (SBR) processes. Normal exit conversions are 55 to 65% in such systems, and the residual monomer is recovered and recycled. If a very high conversion is desired one must deal with the problem that the polymerization rate is low at high conversions. The final reactor in the series needs to be very large if the desired conversion approaches 100%. Likewise, batch reaction cycle times become large if high conversions are desired. 

We must know the process cycle time for a batch or semi-batch process in order to estimate the number of reactors required for a given plant capacity and to estimate the staffing requirement for that plant. By process cycle time, we mean the time between initiating one reaction and the next, subsequent reaction. One process cycle for a batch or semi-batch process involves... 

This chapter discussed process cycle time, which is the total time from charging feed to a batch or semi-batch reactor to the next feed charge of the same reactor, and its optimization. This chapter also discussed the reaction time required to optimize the profitability of a given batch or semi-batch reactor. It finished by presenting scenarios for maximizing product formation and quality through process temperature optimization. 

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. 

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. 

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... 

The Bucher-Guyer horizontal rotary press is a highly automated batch process machine that requires no press aid. The press consists of a horizontal hydrauhc ram inside a rotating cylinder containing many flexible rods covered with a knitted synthetic fabric. The rods have serrated surfaces to allow juice which passes through the fabric to flow to the discharge ends. Hydrauhc pressure is apphed for a preset time, the ram is retracted, and the cylinder is rotated to break up the press cake. This cycle is repeated several times before the press cake is removed from the cylinder and the press is cleaned (16). Juice yield for this horizontal rotary press is 84% with secondary water addition it is increased to 92% (15). 

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... 

Methods have been developed for improving batch process productivity in the manufacture of styrene-butadiene latex by the continuous addition of reactants so the reaction occurs as the reactor is being filled. These are not continuous processes even though the reactants are added continuously during most of a batch cycle. The net result is that reactants can be added almost as fast as heat can be removed. There is relatively little hazardous material in the reactor at any time because the reactants, which are flammable or combustible, are converted to non-hazardous and non-volatile polymer very quickly. 

In contrast, the treatment of industrial steam generation plants is usually more difficult. There is a need to conform to a good working standard and to produce quality waterside conditions for a long period of time without serious upsets, as the systems are always very dynamic and operating conditions can continually vary. This is especially the case with those facilities whose manufacturing operations may employ some form of on-off cycle or up-down batching process, rather than a steady-state, continuous production stream. 

Intermediate storage de-couples the process into periodically operated subprocesses in the sense that each of the subprocesses can be operated with its own limiting cycle time and batch size. If the storage capacity is sufficiently large to accommodate the intermediate product of an entire production campaign, then the subprocesses can be operated completely independently. Alternatively, the storage capacity can be selected so that it de-couples the... 

The cycle time of any stage j is composed of the batch processing times for all products manufactured in that stage and the slacks between consecutive batches ... 

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