Self optimizing control of a gas oil hydrodesulfurization plant

This work aims to give an appropriate introduction to an innovative technique of plantwide control, based on economics, recently developed by Professor Sigurd Skogestad of NTNU University of Trondheim (NOR); and then its application to a well known post-treatment process: a hydrodesulfurization process plant. The procedure is note as “self-optimizing control” and it is based on the search of some variables which kept near to their optimal point can give acceptable results without too much loss. With this project the main steps of the procedure will be analyzed and then applied and finally a control structure will be developed, leaving to next work the possibility to increase and amply the choice of different controllers or different value of parameters. The unit plant has been modeled in AspenHYSYS®, starting from different examples found in literature; the same procedure has been followed for the kinetics adopted. This model has been the basis on which test the so-called “self-optimizing control”. The structure has been tested on two main disturbances: feed flow and its composition. This choice has been done in order to make it possible a right approaching to the technique, because even if it its simplicity is its main characteristic, it is worth to note that its application on a whole unit plant it is still at the beginning. This approach to plantwide control is innovative because it can give the right orderliness and a mental guideline to follow when whoever starts thinking about the control everything: from the single equipment till the complete plant. Starting from the choice of the main variables to control the procedure ends with the selection of the appropriate control structure, whose nature remains free to be chosen among the different ones present in literature. Reasonable results have been obtained from the point of view of sulfur removed, in line with what European Commission decided and at steady state conditions. However even if the dynamic simulation has been found to be correct, the dynamic behavior of the plant is difficult to control and some numerical problems gives the wrong way to the system.