An optimisation model for refinery production scheduling

In this paper we describe a production planning and scheduling problem in an oil refinery company. The problem concerns the planning and the utilization of a production process consisting of one distillation unit and two hydro-treatment units. In the process crude oil is transformed to bitumen and naphthenic special oils. The aim of the scheduling is to decide which mode of operation to use in each processing unit at each point in time, in order to satisfy the demand while minimizing the production cost and taking storage capacities into account. The production cost includes costs for changing mode and for holding inventory. We formulate a mixed integer linear programming model for the scheduling problem. The model can be regarded as a generalized lot-sizing problem, where inventory capacities are considered and more than one product is obtained for some modes of operation. A number of modifications and extensions of the model are also discussed. It is shown how the optimization model can be used as a viable tool for supporting production planning and scheduling at the refinery, and that it is possible to analyze scheduling scenarios of realistic sizes. It is also shown that the model can support shipment planning and strategic decisions concerning new products and investments in storage capacity.