Modelling the pooling problem at the New Zealand Refining Company
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Modelling the pooling problem at the New Zealand Re®ning Company F Amos1, M RoÈnnqvist1 and G Gill2 1
The University of Auckland and 2The New Zealand Re®ning Company
Pooling is usually present throughout an oil re®nery right from the processing of raw crudes through to the blending of petroleum products. Pooling occurs when two or more crudes, each with speci®c 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 Re®ning Company show that the non-linear model accurately describes the pooling problem and simultaneously is ef®ciently solvable. Keywords: modeling; non-linear programming; oil; optimisation; practice of OR
Introduction There are many processes that take place when crude oil is turned into higher value products such as petrol, jet fuel, gas oil and bitumen. At an oil re®nery these processes can be split into three distinct areas: Separation, Re®ning, and Blending. Crude oil is not a homogeneous substance but is made up of many different hydrocarbon compounds. These compounds can contain any number of carbon atoms between one and about two hundred. As a general rule, the more carbon atoms there are in a compound, the higher the boiling point will be. This boiling point property is utilised in the separation process known as distillation. Crude is pumped from storage tanks into a distillation column which is then heated to a very high temperature (typically one thousand degrees centigrade). Most of the crude will be gaseous at this temperature, except for the compounds with many carbon atoms. This collection of compounds that has a very high boiling point is called the residue fraction and on its own is of little value and will need to be further re®ned at a later stage in order to be a useful component for blending into a ®nal product. A simple schematic of a distillation column is illustrated in Figure 1. Further up the distillation column, the temperature will be lower and some of the crude that had previously
Correspondence: Dr M RoÈnnqvist, Division of Optimisation, Department of Mathematics, LinkoÈping University, 58183 LinkoÈping, Sweden. E-mail: [email protected]
evaporated will condense again. The material that has condensed here is called the fuel±oil fraction, and if the separation in the distillation column was perfect then all of the fuel±oil fraction would evaporate be
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