Modelling the pooling problem at the New Zealand Refining Company

Pooling is usually present throughout an oil refinery right from the processing of raw crudes through to the blending of petroleum products. Pooling occurs when two or more crudes, each with specific properties such as cost, sulphur content and unique distillation yields, are processed through distilling units simultaneously to yield downstream fractions. The decisions required in this problem are to select the quantities of each crude to be processed in each crude distiller and to select the best cut points which produce the desired fractions while minimising total cost of crude. Cut points are temperatures in the distillers at which different output streams are separated. In the proposed model, we introduce the use of cumulative functions for the distillation yields which enables a detailed description of the process in a mathematical model. Preliminary numerical results at the New Zealand Refining Company show that the non-linear model accurately describes the pooling problem and simultaneously is efficiently solvable.