Crude distillation unit (CDU) operations are often defined in several operating modes, which are still being used by quite a few refineries for their planning. However, these operational modes cannot reflect all the ever-changing operating conditions of a refinery. As a result, the CDU operation condition may be sub-optimal and potential profit may be lost. This paper proposes a CDU model to decide the weight transfer ratio (WTR) ranges of CDU fractions from the operation modes. These WTR ranges are integrated into the refinery planning model to obtain the optimal CDU operation condition. Unlike the traditional linear CDU models that often omit the optimal solutions, or the rigorous simulation models that are time consuming to solve, the proposed CDU model can be easily solved with much higher accuracy than traditional linear models and better solution speed than the rigorous simulation models.