The effects of the major design and operation variables on the enrichment by thermal diffusion of aqueous sugar solutions

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Virginia Polytechnic Institute

Continuous, liquid, thermal diffusion is a useful separation method which heretofore has been utilized primarily on a laboratory scale for accomplishing the separation of isotopes, azeotropes, and hydrocarbons of a particular series.

Although the thermal diffusion effect, wherein a gradient of temperature in a body of fluid gives rise to a gradient of concentration, was discovered in 1856, it did not become commercially feasible until 1939 when Clusius and Dickel developed the thermogravitational procedure. In this method, the temperature gradient is applied in a horizontal direction to a vertical layer of liquid, and the convection currents set up by the concentration gradient thus established move the fluids of different concentrations in opposite directions; streams of different concentrations are continuously drawn off from the top and from the bottom of the apparatus.

The important commercial application has been the separation of uranium isotopes contained in liquid uranium hexafluoride. This work was part of the Atomic Energy program; the results have not been published. One major oil company has carried out a detailed research and development program on the separation of petroleum fractions by liquid thermal diffusion.

This investigation was concerned with developing formulae, methods of calculation, and empirical data necessary for designing commercial equipment utilizing liquid thermal diffusion.

Vertical, concentric-tube, thermal diffusion columns were constructed which could be operated with a continuous feed and draw-off of liquid from the diffusion annulus. Steam or hot water was used as the heating medium and tap water was used for cooling purposes. The columns were assembled with a copper center tube through which the cooling water flowed, and a concentric glass tube which formed the hot wall of the diffusion annulus. This set of tubes was jacketed with a pyrex pipe to form a steam or hot water reservoir about the hot wall of the diffusion annulus.

Aqueous solutions of sugar were used as the test liquid. Separation effects were observed in solutions where the initial concentration was between 0.5 and 2.0 gram mols of sugar per liter. All tests were made under steady-state conditions and differences observed between the concentration of the bottom product and that of the top product varied from 0 to 28.3 weight per cent sugar. Curves, together with specific and generalized formulae, were presented to indicate the effect of each of seven variables on the separation ratio.

The feed concentration was varied between 15 and 55 weight per cent sugar, and the separation ration reached a maximum between 26.6 and 44.2 weight per cent sugar. Heating fluid temperatures varying from 77 to 124 degrees centigrade were employed and the separation ratio increased linearly with the heating fluid temperature. Annulus width varying from 3.1 to 19. 7 millimeters were studied and the separation ration reached a maximum for widths between 13.0 and 16. 7 millimeters. The separation ratio decreased as the feed rate was increased from 3.0 to 248 milliliters per hour; however, it increased as the ratio of the bottom draw-off rate to the feed rate was increased from 0.047 to 0.916. The separation ratio reached a maximum when the feed port was located near the middle of the column. Maximum separations were obtained with a column length of only 24.1 centimeters; however, for other column lengths from 54.2 to 114.3 centimeters, the separation ratio increased slightly as the column length was increased.

In addition to the desired temperature gradient across the liquid in the diffusion annulus, large temperature drops occurred in the glass wall of the diffusion annulus (19 to 23 degrees centigrade), and in the water film inside the copper, center tube (8 to 16 degrees centigrade).

Certain fundamental factors relative to continuous, liquid, thermal diffusion have been developed in this investigation. Further research should provide similar data for other liquid systems and for other column designs. This will enable the chemical engineer to design commercial apparatus for specific applications.