An efficient initialization procedure for steady-state distillation simulation problems involving a large number of components is presented. The proposed initialization procedure employs "lumping" and "delumping" of the components present in the mixture. The problem of initialization is then reduced to a system of component mass balance equations for each plate. The procedure generates good initial estimates and is not time consuming. The method initially developed for petroleum fluids has proven to be efficient also for other hydrocarbon mixtures. This is shown with two series of examples of distillation problems, by comparison with traditional methods of initialization. Two optimization problems solved with a "lumped" system are also presented and compared to the optimum solution found with the original system.