(249r) Simultaneous Optimization of the Design and Operation of Batch Reactive Distillation Processes

Batch reactive distillation (BREAD) is an integrated process which combines the advantages of reactive distillation and the flexibility of batch processes. In most studies of BREAD processes, a fixed column design is assumed and the operation of the process is optimized by determining parameters such as the arrangement of product cut and off-cut collection, reflux ratio profile and the switching time for collections. However, if new process equipment is going to be designed and purchased, the column design must be considered simultaneously in the optimization to determine the most profitable process.

Therefore simultaneous optimization of equipment design and process operation is studied for BREAD processes in this work. The problem of determining the design and operation with the minimal total annual cost is illustrated for two processes with realistic kinetics and vapor-liquid equilibrium: hydrolysis of methyl lactate and esterification of formic acid. The optimal design is obtained by exhaustive gridding of the column design space coupled with an inner loop that determines the optimal values of the length of the product period and the reflux ratio during the offcut period (Al-tuwaim and Luyben, 1991). Although the optimization is somewhat time consuming, the systematic analysis results give more valuable information than a final set of optimal design and operating variables by gradient-based or stochastic methods (Mujtaba and Macchietto, 1996; Low and Sorensen, 2004).

To determine the minimum total annual cost, the annual product yield is specified and the optimal operation is determined by maximizing the batch capacity. The effect of process design variables including total number of stages, vapor boilup rate, catalyst loading, and process scale on TAC are investigated. The optimization results suggest that the process scale should be specified first. The column should be designed with adequate number of stages so that the CAP improvement by further increasing the number of stages is insignificant. Vapor boilup rate should be specified to make the best use of the available operating time, and the optimal vapor boilup rate is primarily affected by the catalyst loading. This insight into the effect of process design variables on design performance is used to develop an efficient algorithm for determining simultaneously the optimal column design and operating policy.

References

Al-tuwaim, M. S.; Luyben, W. L., Multicomponent batch distillation .3. Shortcut design of batch distillation-columns. Industrial & Engineering Chemistry Research 1991, 30, (3), 507-516.

Mujtaba, I. M.; Macchietto, S., Simultaneous optimization of design and operation of multicomponent batch distillation column - Single and multiple separation duties. Journal of Process Control 1996, 6, (1), 27-36.

Low, K. H.; Sorensen, E., Simultaneous optimal design and operation of multipurpose batch distillation columns. Chemical Engineering and Processing 2004, 43, (3), 273-289.