Short-term Scheduling of Refinery Operations Based on a Continuous Time Formulation

The problem addressed in this work is to develop a comprehensive mathematical programming model for the efficient scheduling of oil-refinery operations. Our approach is first to decompose the overall problem spatially into three domains: the crude-oil unloading and blending, the production-unit operations and the product blending and delivery. In particular, the first problem involves the crude-oil unloading from vessels, its transfer to storage tanks and the charging schedule for each crude oil mixture to the distillation units. The second problem consists of the production unit scheduling which includes both fractionation and reaction processes and the third problem describes the finished product blending and shipping end of the refinery. Each of those sub-problems is modeled and solved in a most efficient way using continuous time representation due to relatively small number of variables and constraints. Our last step is to address the integration of these sub-problems by applying heuristic based Lagrangian decomposition iterative methodology. The proposed methodology is applied to realistic case studies and significant computational savings can be achieved compared with existing discrete time models.