Solving a chemical batch scheduling problem by local search

In this paper the following chemical batch scheduling problem is considered : a set of orders has to be processed on a set of facilities. For each orde r a given amount of a product must be produced by means of chemical reactio ns before a given deadline. The production consists of a sequence of proces ses whereby each process has to be performed by one facility out of a given subset of facilities allowed for this process. The processing times depend on the choice of the facility and the processing is done in batch mode wit h given minimum and maximum sizes. The problem is to assign the processes t o the facilities, splitting them into batches, and scheduling these batches in order to produce the demands within the given deadlines. For the scheduling part of the problem we present an approach based on the following steps. First, a procedure to calculate the minimum number of batc hes needed to satisfy the demands is presented. Based on this,the given pro blem is modeled in two different ways: as a general shop scheduling problem with set-up times or as scheduling problem with positive time-lags. Finall y, a two-phase tabu search method is presented which is based on the two di fferent formulations of the problem. The method is tested on some real worl d data.