Abstract: A method of designing or optimizing a column for a separation process includes the computer implemented steps of, in a digital processor, providing vapor-side and liquid-side mass transfer coefficient expressions and a mass transfer area expression relevant for a subject column, the vapor-side and liquid-side mass transfer coefficient expressions and the mass transfer area expression having been derived from defining a column average height equivalent to a theoretical plate HETP) as a mathematical relationship in which HETP is proportional to a vapor flow rate, is inversely proportional to effective packing area participating in mass transfer, has a first correction factor with respect to liquid-side mass transfer, and has a second correction factor with respect to vapor-side mass transfer.