Optimisation of Vacuum System at Vacuum Distillation Unit

audiovizuální tvorba

angličtina

Optimisation of parameters of both particular equipment and overall process plays important role in process industries. The optimal design of particular equipment is usually well known and easily feasible, but serious problems may arise at equipment with phase change of multicomponent mixtures. To optimise overall process we must properly design and optimise particular equipment and moreover adjust them to optimal working process. This paper contains a design of vacuum system (VS) at vacuum tower in petroleum refinery. This system should maintain vacuum at column with minimum investment and operating costs. Purchase price of equipment at process can be hundreds thousand of U.S. dollars and at these equipment is used big amount of cooling water and motive steam. Their costs compose main part of operating costs. The main task of this work is to optimise the vacuum system from the economical point of view. We must find the suitable objective function, estimate its value and minimise it. But there are several problems. Getting total investment costs for VS is too complicated, there are too many unknowns, and its necessary to simplify the procedure. The objective function will not allow to use any regular optimising solver, because of very complicated engineering calculations. It is necessary to simulate VS again and again for different settings and compare results. Then analysis of these results can be done and optimal setting of VS should be determined. To fulfil this objective it was necessary to dispose with accurate mathematical models of all equipment of the process. Although ChemCad is a very powerful tool in this field, it does not contain mathematical model of vacuum ejector, which is in addition to the vacuum condenser the most important equipment of vacuum system. It was necessary to implement this model. Also methods for estimation of equipment cost, operating costs for transport of steam and cooling water costs had to be implemented. Another problem consists in a determination of suitable configurations of steam ejectors and vacuum condensers, which involves both the capital investments (dimensions of precondensers and vacuum ejectors) and operating costs (consumption of cooling water and motive steam). As the key process parameter the condensing ratio was identified, which is the quotient of condensed vapours mass flow in precondensers and mass flow of entering overhead vapours. The objective function is expressed in terms of condensing ratio.

Optimisation, vacuum distillation, equipment design.

ŠÁLEK, J.; KOHOUTEK, J.; STEHLÍK, P.

1. 1. 2002

Prague, Czech Republic

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