Thermal diffusion process isotopes separated

A number of special processes have been developed for difficult separations, such as the separation of the stable isotopes of uranium and those of other elements (see Nuclear reactors Uraniumand uranium compounds). Two of these processes, gaseous diffusion and gas centrifugation, are used by several nations on a multibillion doUar scale to separate partially the uranium isotopes and to produce a much more valuable fuel for nuclear power reactors. Because separation in these special processes depends upon the different rates of diffusion of the components, the processes are often referred to collectively as diffusion separation methods. There is also a thermal diffusion process used on a modest scale for the separation of heflum-group gases (qv) and on a laboratory scale for the separation of various other materials. Thermal diffusion is not discussed herein. 

The thermal diffusion factor a is proportional to the mass difference, (mi — mo)/(mi + m2). The thermal diffusion process depends on the transport of momentum in collisions between unlike molecules. The momentum transport vanishes for Maxwellian molecules, particles which repel one another with a force which falls off as the inverse fifth power of the distance between them. If the repulsive force between the molecules falls off more rapidly than the fifth power of the distance, then the light molecule will concentrate in the high temperature region of the space, while the heavy molecule concentrates in the cold temperature region. When the force law falls off less rapidly than the fifth power of the distance, then the thermal diffusion separation occurs in the opposite sense. The theory of the thermal diffusion factor a is as yet incomplete even for classical molecules. A summary of the theory has been given by Jones and Furry 15) and by Hirschfelder, Curtiss, and Bird 14), Since the thermal diffusion factor a for isotope mixtures is small, of the order of 10", it remained for Clusius and Dickel (8) to develop an elegant countercurrent system which could multiply the elementary effect. 

We have observed that countercurrent separation devices achieve considerable separation under driving forces such as chemical potential gradient and the external force of a centrifugal force field. As illustrated in equations (3.1.44) and (3.1.50), there is another type of force, the thermal diffusion force. The separation achieved thereby in a closed two-bulb cell has been illustrated in Section 4.2.5.I. We have already illustrated conceptually how thermal diffusion can achieve separation in a countercurrent column via Figures 8.1.1(a)-(h). For UF isotope separation, however, the radial separation factor in a two-bulb cell is much smaller than for other isotope separation processes. In a countercurrent column, this value is reduced by about 50%. As a result, thermal diffusion columns are not used at all for any practical/large-scale separation. More details on thermal diffusion columns are available in Pratt (1967, chap, viii) and Benedict et al. (1981, pp. 906-915), where one can find information on the primary references. 

Many challenging industrial and military applications utilize polychlorotriduoroethylene [9002-83-9] (PCTFE) where, ia addition to thermal and chemical resistance, other unique properties are requited ia a thermoplastic polymer. Such has been the destiny of the polymer siace PCTFE was initially synthesized and disclosed ia 1937 (1). The synthesis and characterization of this high molecular weight thermoplastic were researched and utilized duting the Manhattan Project (2). The unique comhination of chemical iaertness, radiation resistance, low vapor permeabiUty, electrical iasulation properties, and thermal stabiUty of this polymer filled an urgent need for a thermoplastic material for use ia the gaseous UF diffusion process for the separation of uranium isotopes (see Diffusion separation methods). 

Enrichment, Isotopic—An isotopic separation process by which the relative abundances of the isotopes of a given element are altered, thus producing a form of the element that has been enriched in one or more isotopes and depleted in others. In uranium enrichment, the percentage of uranium-235 in natural uranium can be increased from 0.7% to >90% in a gaseous diffusion process based on the different thermal velocities of the constituents of natural uranium (234U, 235U, 238U) in the molecular form UF6. 

Clusius A process for separating isotopes by a combination of thermal diffusion and thermal siphoning. Invented in 1938 by K. Clusius and G. Dickel. 

The thermal diffusion method requires large quantities of power and is therefore primarily of interest for preparation of laboratory scale samples. As such, it has been developed by Clusius among others, and is a very effective separation process. Overall separations as high as 10,000,000 have been achieved by the Clusius group. A summary of the evolution of the thermal diffusion column in Clusius laboratory is given in Table III (JO). Of particular note is the enrichment of Ar, a middle isotope, from a natural abundance of 0.064% to a final isotopic purity of 99.984%. 

With the development of specific equipment and processes by which thermal be diffusion is now carried out, separation of substances from their mixtures can often carried out more cheaply by this method, if applicable, than by other separation techniques. Amongst the successful separations effected by thermal diffusion are those of the isotopes of helium and the isotopes of chlorine gas. The method had also been used to effect separation of the isotopes of uranium during the years of World War II in the U.S.A. Constituent hydrocarbons can easily be separated from their mixture by liquid-phase thermal diffusion, because interaction between molecules of different hydrocarbons is practically non-existent and, consequently, each hydrocarbon molecule of the mixture acts independently under the influence of the applied temperature gradient. Another use of thermal diffusion of special interest is its applicability to the separation of mixtures of liquids of close boiling points and of mixtures of isomers, into their respective components. 

The degree of separation obtainable in thermal diffusion (the difference in composition between hot and cold walls) is much less than in other diffusion processes, so that use of a column to multiply the composition difference is practically essential. The stage type of thermal diffusion has been used only to measure the thermal diffusion coefficient and is never used for practical separations. In some thermal diffusion columns, htu s are as low as 1.5 cm, and as many as 800 stages of separation have been obtained from a sin e column. Even with such a great increase in separation, it is often necessary to use a tapered cascade of thermal diffusion columns for isotopic mixtures, to minimize hold-up of partially enriched isotopes and to reduce equilibrium time. 

Thermal diffusion is a convenient way of separating isotopes on a small scale. It is a very inefficient process for large-scale use because of its high energy consumption. 

The foregoing discussion fairly well demonstrates that on a small scale any desired Isotope can be separated either by the electromagnetic or the thermal diffusion method. In contrast to these laboratory-scale processes, the separations of the heavier Isotope D, of the lightest element, hydrogen, and of the lighter Isotope 235u, of the heaviest natural element uranium, are carried out on a literally enormous Industrial scale. 

Sections 0 and 0 demonstrate that almost any desired isotope can be separated by thermal diffusion or electromagnetic methods on a small scale. For large-scale separation, enormous amounts of material must be processed and energy consumption, cascade design, and other such factors become important. Tables 51.3 and 51.4 list theoretical upper limits of single-stage separation factors for H/D, and for several different separa-... 

A number of other processes for separating isotopes are documented. A partial list includes membrane pervaporation, thermal diffusion of liquids, mass diffusion, electrolysis and electro migration, differential precipitation, solvent extraction, biological microbial enrichment, and more. Although not discussed in this chapter, some are suitable for small-scale laboratory separations, and others have been applied on reasonably large scale to D/T, H/T, and i/ Li and presumably to other pairs of isotopes. 

Thermal diffusion utilizes the transfer of heat across a thin liquid layer to accomplish isotope separation. The process is based upon the fact that lighter molecules diffuse toward a hot surface and heavier molecules diffuse toward a cold surface. 

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