A crude distillation unit model suitable for online applications

A steady state, multicomponent distillation model particularly suited for fractionation of crude oil has been developed based on equilibrium stage relations. For a mixture of C components, the present formulation uses C+3 iteration variables namely the mole fractions of the components, temperature, total liquid and total vapor flow rates on each stage. This choice of variables makes the present model numerically stable and robust rendering a separate initial guess computation unnecessary. An improved scheme of numbering the equilibrium stages when side strippers are present, was found to be advantageous with respect to computation time. Selected example problems have been included from literature as well as industry to demonstrate the efficacy and usefulness of the method. The accuracy of predictions and speed of solution of the model equations are particularly suited for online applications such as online optimization.