Energy saving and production enhancement through scheduling optimization of displacement batch digesters

Displacement batch digester house for pulp production involves many digesters sharing common resources. In addition to this, since the digesters interact with each other in terms of using the hot liquors from previous batches, the problem of scheduling becomes a challenging task. The objective of scheduling optimization, in such cases, is to reduce the idle waiting time between various phases of the digesters so that this time can be used for production maximization and/or energy minimization. This study provides mathematical programming based optimization models and computational results for short-term scheduling of displacement batch digesters in a pulp industry. The scheduling problem involves development of an optimal solution in terms of batch's start and end times that yield the best sequence of operations in each of the batch digesters sharing common resources. The constraints are imposed on meeting the demand of pulp production within a specified time horizon. The problem comprises of both fixed-time and variable time durations of the tasks, different storage policies for states: zero-wait and finite wait times; and handling of shared resources. The scheduling problem is formulated using a state-task-network (STN) representation of production recipes, based on discrete time representation. This results in a mixed-integer linear programming (MILP) problem which is solved using GAMS software. Various case studies involving parallel digesters in multiple production lines are considered to demonstrate the effectiveness of the proposed formulation.