Vacuum distillation II: Performance of a Vigreux column and a spinning band column over a wide range of pressures

Abstract

Performance figures referring to a 11 mm diameter Vigreux column and a 6 mm diameter spinning band column over a pressure range of about 0·13 kN/m 2–101·5 kN/m 2 (about 1–760 mm Hg) are presented. In order to gain an insight into the mechanisms of material transfer involved, analyses of separate vapour and liquid phase resistances were made. In the case of the Vigreux column it was found that liquid phase resistance was about half the total resistance at atmospheric pressure. A reduction in pressure causes the liquid phase resistance to be increased by increasing liquid viscosity on the one hand and to be decreased by the higher vapour rates on the other hand. These two compensating factors cause a total efficiency loss of the Vigreux column of about 25% when the pressure is reduced from atmospheric value to about 0·13 kN/m 2 (1 mm Hg). at this top pressure the maximum pressure drop along the Vigreux section is about 0·3 kN/m 2 (3–4 mm Hg) per metre. In a spinning band column the main resistance to material transfer is offered by the vapour phase. This resistance is reduced by creating turbulence through rotation of the spinning band. The present study disclosed the existance of two limiting values of rotational speed: a lower limit below which rotation does not yet cause turbulence, and a higher limit above which back mixing in the liquid phase causes the separating power to be independent of—or to decrease with—speed of rotation. Due to the fact that the extent to which turbulence can be effected in the vapour phase proves to depend on rotational speed and vapour density to the same degree, the sensitiveness to pressure variations of the spinning band column is rather great. Reduction of the pressure from atmospheric value to about 1·3 kN/m 2 (10 mm Hg) causes an efficiency loss of about 70%; in fact at this pressure the separating power of the spinning band column was not better than that of a wetted wall column of the same dimensions.