In addition to the cascading effect, the convective currents in thermogravitational thermal-diffusion column also produce an undesirable remixing effect. Therefore, proper control of the convective strength might effectively suppress this undesirable remixing effect while still preserving the desirable cascading effect, and thereby lead to improved separation. Based on this concept, some improved columns have been developed in the literature. In developing these improved columns, a number of studies of the operating variables in the thermal diffusion column have been made. There is still an important term, plate aspect-ratio which affects separation efficiency and which has never been discussed.