Operation of pipeless batch plants - I. MILP schedules

Pipeless batch plants are an emerging technology for multipurpose chemical batch production. Optimization formulations for the design, layout and scheduling of pipeless batch plants have been developed. These mixed integer linear programming (MILP) formulations make certain assumptions about the operation of this new type of batch plant. For example, the detailed movement of vessels is left up to a control algorithm, and the travel times are assumed to be long enough to allow conflicts to be resolved. In this paper we examine the implementation of plant schedules generated by the MILP. A discrete event simulator (DES) has been coupled to the output of the MILP to simulate plant operation while allowing for variations in its operation such as vessel travel times and task durations. First, the paper describes how the MILP schedule is implemented in the DES, given that DES are event driven rather than time driven. Second, the performance of two different plant layouts, with the same equipment, is compared. The DES is able to differentiate the performance of these layouts whereas the differences were invisible to the MILP due to the assumptions involved in the formulation. Third, small pertubations to processing and travel times are introduced in the plant. The overall plant performance does not degrade significantly indicating that the tightly coupled MILP schedules are not hypersensitive to the variations. (C) 1998 Elsevier Science Ltd. All rights reserved.