CYCLIC SCHEDULING OF CONTINUOUS PARALLEL-PROCESS UNITS WITH DECAYING PERFORMANCE

The problem of scheduling multiple feeds on parallel units is addresse d The performance of each unit is assumed to decrease with time, there fore requiring planned maintenance to restore its performance. A mixed -integer nonlinear programming (MINLP) model presented uses the proces s information regarding the exponential decay in performance with time to find a cyclic schedule for feed processing. Optimization trade-off is between the performance of the units, and the maintenance costs an d the loss in production due to shutdowns. Three main decisions for th e optimal schedule are: a) assignment of various feeds to different un its; b) processing time of each feed on different units; and c) number of subcycles of each feed on different units. Special mathematical pr operties of the MINLP model are exploited to solve the problem to glob al optimality with a branch-and-bound method. Using several examples a dvantages of the model are illustrated and performances of different s olution algorithms are compared.