Design versus Retrofitting Problem

This retrofit/expansion is applied when plants are required to satisfy changes in the demand of one or more products. The operational strategy should not be drastically changed, and the bottlenecks of the process must be identified and the feasibility to add new equipment in or out of phase must be considered 

In this case, new products can be added or removed, leading to changes in the production configuration. This condition increases the complexity of the problem, because the addition of new products leads to new production routes 

Several mathematical formulations of the problems can be found in the dedicated literature (Papageorgaki and Reklaitis, 1993 Papageorgaki etal., 1992). 

An important aspect of batch plant modelling relates to the representation of the recipe. The most common representation of recipe is the so-called state task network (STN) that was proposed by Kondili et al. (1993), which involves two types of nodes that is, state nodes and task nodes. The former represent all the materials that are processed in the plant, that is, feeds (raw materials, solvents etc.), intermediate and final products. The latter nodes 

The continuous-time representation has the same ambition events are allowed to take place at any point in the continuous domain of time. This is carried out by introducing the concept of variable event time that can be defined either globally or for each unit. For this purpose, additional variables are necessary to determine the timings of events. Since a major fraetion of the inactive event-time interval assignments is eliminated with the continuoustime approach, the resulting mathematical models require less computational effort for their solution of the associated scheduling task involved in design/retrofit problems. However, the mathematical models involved in the continuous-time approach may embed more complicated structures compared with their discrete-time counterparts. 

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