INTRODUCTION
The potential benefits of optimization for process operations in oil refineries have long been observed, with applications of linear programming in crude blending and product pooling (Symonds, 1955). In the past 20 years, the implementation of advanced control systems in oil refineries has allowed significant production increases at the plant units. The resulting savings have created a growing interest for more powerful systems that take explicitly into account production objectives.
Petroleum refineries are increasingly concerned with the improvement of their planning operations. The existing commercial software for refinery production planning, such as RPMS (Refinery and Petrochemical Modeling System - Bonner and Moore, 1979) and PIMS (Process Industry Modeling System - Bechtel, 1993) are based on very simple models which are mainly composed of linear relations. The production plans generated by these tools are interpreted as general trends as they do not take into account more complex process models and/or nonlinear mixing properties.
On the other hand, process unit optimizers based on nonlinear complex models, which determine optimal values for the process operating variables, have become increasingly popular. However, they are restricted to only a portion of the plant. Furthermore, single-unit production objectives are conflicting and therefore contribute to suboptimal and even inconsistent production objectives.
It has also been recognized that the integration of new technologies for process operations is an essential profitability factor and that it can only be achieved through appropriate planning (Cutler and Ayala, 1993; Macchietto, 1993).
In this work, we develop a general representation for refinery process units in which nonlinear equations are considered. The unit models are composed of the blending relations and process equations. The general framework is then applied to two different production-planning situations of two real-world oil refineries and the results are compared to the current situation where there is no extensive use of planning tools.