Retrofit Design of Multiproduct Batch Plants

In a retrofit batch design, we optimize the batch plant profitability defined as the total production value minus the cost of any new equipment. The objective is to obtain a modified batch plant structure, an operating strategy, the equipment sizes, and the batch processing parameters. Discrete decisions correspond to the selection of new units to add to each stage of the plant and their type of operation. Continuous decisions are represented by the volume of each new unit and the batch processing variables which are allowed to vary within certain bounds. 

New units may be added to any stage j in parallel to existing units. These new units at stage j are denoted by the index k, and binary variables yjk are introduced so as to denote whether a new unit k is added at stage j. Upper bounds on the number of units that can be added at stage j and to the plant are indicated by Zj and Zu, respectively. 

The operating strategy for each new unit involves discrete decisions since it allows for the options of 

Option Bm operate in phase with existing unit m to increase its capacity 

Option C operate in sequence with the existing units to decrease the stage cycle time 

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Part 3 of this book presents a number of major developments and applications of MINLP approaches in the area of Process Synthesis. The illustrative examples for MINLP applications, presented next in this section, will focus on different aspects than those described in Part 3. In particular, we will consider the binary distillation design of a single column, the retrofit design of multiproduct batch plants, and the multicommodity facility location/allocation problem. 

R. Fletcher, J. A. J. Hall, and W. R. Johns. Flexible retrofit design of multiproduct batch plants. Comp. Chem. Eng., 15(12) 843, 1991. 

Fletcher, R., Hall, J.A.J., and Johns, W.R. (1991) Elexible retrofit design of multiproduct batch plants. Computers and Chemical Engineering, 15 (12), 843-852. 

Vaselenak, JA., Grossmann, I.E., and Westerberg, A.W. (1987) Optimal retrofit design of multiproduct batch plants. Industrial Engineering Chemistry Research, 26 (4), 718-726. 

The MINLP model for the retrofit design of a multiproduct batch plant takes the following form ... 

Whatever the case of batch plants, either of multiproduct or multipurpose nature, few works have addressed the retrofit problem compared with the batch plant design problem. Most of the mathematical formulations used to model the retrofit problem usually lead to optimization-oriented problems. It must be also emphasized that scheduling is intrinsically associated with batch plant design/retrofit, and is part and parcel of the formulation of both issues. It can be considered as a sub-problem of the design/retrofit problem. As highlighted in the compared formulation of these companion problems, the retrofit problem appears to be a more constrained problem. Yet, the basic solution strategy involves the same mathematical approaches and tools. 

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