New distillation calculation method utilizing salient features of both short-cut and tray-by-tray method. Semi-tray-by-tray method.

A distillation calculation method has been developed whereby the theoretical tray model of the tray-by-tray method is applied to sections subject to large fluctuations in vapor/liquid loads and temperatures, and the short-cut model (which makes use of effective stripping factors) to sections subject to small changes of these variables. By applying a combination of both models, calculations of an accuracy approaching that of the tray-by-tray method is attainable without any variation in computer running time or memory capacity, irrespective of increase in the number of trays. Further, in addition to liquid loads and temperatures, the required number of trays can be determined by taking the number of trays within the short-cut model as variables.