Temperature exchange, dual

In the hotter column, at 130°C, the exchange equilibrium constant Is 1.69, and deuterltmi Is returned from the aqueous phase to the gaseous phase. Thus, dual-temperature operation avoids the need for a chemical reaction to return the desired Isotope from the phase In which It enriches to the phase In which it Is depleted. 

In operation, feed water at the natural deuterium abundance of 145 ppm Is fed to the top of the cold column, depleted water at 120 ppm deuterium Is discarded from the bottom of the hot column, and enriched D2O Is withdrawn between the cold and hot columns. 

ACS Symposium Series American Chemical Society Washington, DC, 1975. 

For many years attempts have been made to use photochemical processes for the separation of Isotopes. The basic idea is to utilize the difference in the absorption spectra of different isotopic species, and by use of sufficiently monochromatic light of an appropriate wavelength to excite only one of the species to an upper energy level. The excited species may then be separated by chemical or physical means from its isotopic partners the separating process need not have any Inherent isotopic selectivity. A particularly successful application of the method was to the separation of Hg isotopes by Gunning et al. (36,37). For example. 

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Dual Temperature Exchange The GS Process for Deuterium Enrichment... 

Section 8 summarizes separation factors obtainable in isotope exchange reactions and their temperature dependence. The latter is the key property in dual-temperature exchange processes. Section 9 develops equations to be used for calculating the number of theoretical stages needed in exchange separation towers. 

Dual-temperature exchange processes using ammonia and hydrogen, methylamine and hydrogen, and water and hydrogen are described in Secs. 12, 13, and 14, respectively, and are compared with the GS process in Sec. 14. 

This figure brings out an important characteristic of dual-temperature exchange processes The recovery (or production rate) of a given plant is very sensitive to the gas-to-liquid flow ratio. There is only a narrow range of flow ratios within which optimum performance is obtained. In the example of Fig. 13.29, the minimum value of XY/jxp, 0.8563, is obtained at G/F=2.03. If G/F is less than 1.85 or greater than 2.25, Xy/lxp becomes greater than 0.90, and the recovery of deuterium is decreased by 30 percent or more. 

In many respects, the case history presented by D. W. Jones and J. B. Jones of the DuPont Company is typical, They reported studies conducted on a pair of columns in use at Dana, Indiana, and Savannah River, Georgia, for a heavy-water process using dual temperature exchange of deuterium between water and hydrogen sulfide at elevated pressures. 

Figure 11 Illustrates a simplified arrangement of components and the magnitude of operating parameters for the concentration of deuterium by dual-temperature exchange between hydrogen sulfide gas and liquid water. H2S Is circulated In a closed loop counter-current to a descending stream of water. In the colder column deuterium concentrates In the liquid phase the equilibrium constant for the exchange at the cold temperature, 30°C, is 2.20.

Figure 11. Simplified flow sheet for a dual-temperature exchange system for concentrating deuterium...

In the dual-temperature H2O/H2S process (61,62), exchange of deuterium between H20(l) and H2S(g) is carried out at pressures of ca 2 MPa (20 atm). At elevated temperatures deuterium tends to displace hydrogen in the hydrogen sulfide and thus concentrates in the gas. At lower temperatures the driving force is reversed and the deuterium concentrates in H2S in contact with water on the tiquid phase. 

The deuterium exchange reactions in the H2S/H2O process (the GS process) occur in the tiquid phase without the necessity for a catalyst. The dual-temperature feature of the process is illustrated in Figure la. Dual-temperature operation avoids the necessity for an expensive chemical reflux operation that is essential in a single-temperature process (11,163) (Fig. lb). 

A variant of the H2/NH2 chemical exchange process uses alkyl amines in place of ammonia. Hydrogen exchange catalyzed by NH2 is generaHy faster using alkyl amines than ammonia, and a dual-temperature flow sheet for a H2/CH2NH2 process has been developed (69). 

Chemical exchange between hydrogen and steam (catalyzed by nickel—chromia, platinum, or supported nickel catalysts) has served as a pre-enrichment step in an electrolytic separation plant (10,70). If the exchange could be operated as a dual-temperature process, it very likely... 

To avoid the high cost of chemical reflux the dual temperature H2S/H20 exchange was independently suggested by Geib (1946) and Spevack (1957) (GS). The method exploits the fact that the equilibrium constant for isotope exchange is temperature dependent. The scheme is illustrated in Fig. 8.13. To carry out the exchange... 

Fig. 8.13 Schematic diagram of one stage of a GS dual temperature chemical exchange system...

The chemical exchange system employed for lithium-6 enrichment is lithium amalgam and aqueous lithium hydroxide. It also employs paired dual-temperature columns. 

Zagorodni, A. A. Muhammed, M. Explanation of the Zn/Cu Dual Temperature Separation on Amber-lite IRC-718 Ion-Exchange Resin, Sep. Sci. Technol. 1999,34,2013-2021. 

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