Intermediate storage and operation of periodic processes

Intermediate storage has an important role in improving the operating efficiency and reducing the capital cost of batch/semicontinuous processes by decoupling the operation of the adjacent trains. In addition, intermediate storage can mitigate the effects of process parameter variations, such as recipe inaccuracies and operator errors, to which batch operations are particularly susceptible. Viewed over a longer operating horizon, process variations must include not only short term parameter variations but also larger scale processing disruptions such as failure of batches of material in meeting product specifications and failures of the equipment items themselves. In this role too, intermediate storage can be useful by providing a buffer which can allow the properly functioning process trains to proceed with minimum disruption until the failed process components are restored. In this thesis, relations were developed for determining the storage requirements for the building block 1-1 system. Analytical expressions were obtained for the required limiting volume of intermediate storage as a function of the frequency of failure. For equipment failure, the analysis is developed in three parts. First, the 1-1 system is considered in which the upstream equipment is susceptible to failure while the downstream equipment is not. Then results for the converse case are developed, namely, reliable upstream unit and unreliable downstream system. Finally, these results were extended to treat the more usual case involving both types of failure. Next, the effects of variations of the failure occurrence frequency about its mean as well as of the duration of the time period before the resumption of normal production were considered. For the 1-1 case it is possible to develop analytical predictive relations for two cases: the case in which the variations are uniformly distributed over specified intervals and the case in which the variations are normally distributed with given mean and variance. In both cases, the design will be dependent on the desired confidence level for uninterrupted operation. From the above analysis, a dynamic operating policy was proposed which allowed the storage size to be selected less conservatively by exploiting the partial control of storage accumulation afforded by the adjustment in the processing rate of the up- or downstream unit. It was shown that for the same confidence level, the dynamic policy allows operation with a considerably decreased storage volume. Finally extensions of this policy to accommodate general serial process networks were studied. The results are illustrated with suitable examples