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Thermal Diffusion Separation of gases by thermal diffusion is based upon the tendency of components of differing molecular weight in a gas mixture to partially separate when subjected to a temperature gradient. This effect arises as a consequence of conservation of molecular momentum and energy equilibrium relationships in accordance with the kinetic theory of gases. The lighter component moves toward the warm zone at a rate dependent upon the properties of the gases and the temperature differences, thereby establishing a concentration gradient between the warm and cool zones. The concen tration gradient results in an ordinary diffusion of the lighter component in the reverse direction at a rate proportional to the "steepness" of the gradient. When the two rates become equal, a steady state condition with a partial separation of components is established. In usual practice, many stages of separation are combined by situating the warm and cool surfaces vertically so that natural convection produces a relative upward motion of the gas at the warm surface and a downward motion at the cool surface. The upper portion of the column becomes richer in the lighter component, thereby allowing further separation to occur. Figure 36-1 schematically illustrates the temperature, velocity, and concentration profiles which exist in this type of device. Possible configurations of thermal diffusion equipment are presented in Figure 36-2. A source of comprehensive discussion of the theory for design of thermal diffusion gas separators are references 36-1 through 36-5. The calculation methods for concentration gradients in a thermal diffusion column are given in reference 36-1. Velocity and thermal gradient calculations are treated in reference 36-5. Concentric-cylinder geometry, or a hot wire in a cylinder, is frequently used be cause greater economy of materials is obtained. However, for the preliminary cal culations used in this program, flat-plate geometry was assumed because the results will be sufficiently close for the "factor of 2" estimates desired and because the heat, mass, and momentum fluxes can more readily be determined. Other basic assumptions were: 1. Density, viscosity, and the thermal expansion coefficient for chlorine was used for the gas. Again, this assumption is adequate for estimates, and the time-consuming calculation of mixed gas properties was avoided. 2. Plate-end effects were ignored. 3. Effects of concentration on density profiles were ignored. 36-5PDF Image | Development of the Zinc-Chlorine Battery for Utility
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