Blend Optimisation inside FORWARD - Constraint Hierarchies and Relaxation on a Real-life Example

In this paper we describe an optimisation package for finished product blending in oil refineries. It is used inside the FORWARD system, an advanced scheduling/simulation tool for the petro-chemical industry. The blend optimiser uses the rational constraint solver of CHIP, a constraint logic programming system. The optimiser handles different linear and non-linear blending laws, as well as different objective functions. As a unique feature it contains special constraint relaxation techniques to handle the over-constrained blending problem with conflicting constraints. The user can define a constraint hierarchy, which the system uses for constraint relaxation. TWo different relaxation objectives are possible: frnd the minimum required quantities of base products which would allow to achieve a given blend, or find the maximum blend quantity which can be obtained with the given base products. The results of the relaxation offers interactive decision support to plan and schedule a series of blend optimisations.