The main objective of this paper is to develop a nonlinear planning model for refinery production. The model is able to represent a general refinery topology and allows the implementation of nonlinear process models as well as blending relations. A real-world application is developed for the planning of diesel production in the RPBC refinery in Cubatao (SP). The resulting optimization model is solved with the-generalized reduced gradient method The optimization results were compared to the current situation, where no computer algorithm is used and the stream allocation is made based on experience, with the aid of manual calculations. Considering the market limitations for each kind of diesel oil usually supplied by the refinery, the optimization algorithm : was able to define a new point of operation, increasing the production of more valuable oil, while satisfying all specification limits. This new operating point represents an increase in profitability of about US$ 6,000,000 per year.