X^^ ci:r///)ppc 'Sii 1/ MDDC - 891 UNITED STATES ATOMIC ENERGY COMMISSION THE DUAL TEMPERATURE PROCESS FOR ISOTOPIC SEPARATION Columbia University This document consists of 2 pages. Date Declassified: February 26, 1947 This document is for official use. Its issuance does not constitute authority for declassification of classified copies of the same or similar content and title and by the same author (s). Technical Information Division, Oak Ridge Directed Operations Oak Ridge, Tennessee ^ THE DUAL TEMPERATURE PROCESS FOR ISOTOPIC SEPARATION The chemical exchange method for isotope separation, as developed by Urey and his co-workers at Columbia University, has been successfully applied to the concentration of isotopes of several elements. An important variation of this method was devised by J. S. Spevack at the Columbia Uni- versity Laboratories in 1942. This procedure, known as the dual temperature process, is essentially an engineering application of the well-known fact that the equilibrium constant for isotopic exchange rea.ctions varies with the temperature. PRINCIPLE OF THE METHOD The system uses two principal components, each of which can exist as a separate phase. These components contain a common constituent (i.e., two isotopic forms of an element) capable of taking part in a reversible exchange reaction. The desired constituent may concentrate in either phase, depending on the equilibrium constant for the reaction. Two exchange towers are required, one of which is maintained at a higher temperature than the other. The phase in which the desired isotope tends to concentrate is fed into that tower, hot or cold, in which the larger equilibrium constant for the exchange reaction prevails. This tower is known as the concentrating tower. The other phase flows countercurrent to the feed stream in the so-called stripping tower, where the smaller equilibrium constant applies. Here the countercurrent stream is equilibrated with the feed. The desired isotope is removed from the stream leaving the concentrating tower and swept back into the opposite phase. Essentially, this operation is a means of producing reflux to the concentrating tower. As a result, the isotope-rich phase that has entered the stripping tower from the concentrating tower is stripped of its high concentration of desired isotope while flowing through the former tower, and approaches an equilibrium concentration with the stream entering at the opposite end of the tower. The latter stream came from the top of the concentrating tower where it was practically in equilibrium with the feed stream. Therefore, since the equilibrium constant prevailing in the stripping tower is smaller than in the concentrating tower, the stripped feed stream will leave the system poorer in the desired isotopic constituent than was present in the original feed. Stated in an equivalent manner, the concentration of desired isotopic constituent in the stripped feed stream under ideal conditions approaches a fraction of the initial feed concentration that is equal to the ratio of the equilibrium constant in the stripping tower to that in the concentrating tower. From this, it follows that operation of the above system is accompanied by an enrichment of the desired isotopic constituent in the region between the two towers. The tower heights required for any desired enrichment of product can be obtained by the methods employed in distillation and absorption practice, i.e., by determination of the lines representing equilibrium conditions in the tower and the material balance relationships. The heights will depend on the rates of diffusion, rates of reaction, types of packing and flow rates used. EFFICIENCY OF THE METHOD When the concentrations of the desired isotope are small, the equilibrium line for the hot tower is Y * = X/Kh MDDC - 891 [1 2 ] MDDC - 891 and for the cold tower Y * = X/Kc where Y * and X are mole fractions of the desired isotope in the gas and liquid phases, respectively; Kh, Kc are equilibrium constants for exchange between the two phases in the hot and cold towers, respectively. In the system, here described, the equilibrium constant for the exchange reaction favors enrich- ment of the desired isotope in the liquid phase, and the cold tower is the concentrating tower. At the top of the cold tower, the exit gas approaches equilibrium with the feed liquid, whereas, at the bottom of the hot tower the waste liquid approaches equilibrium with the entering gas. Since the equilibrium constant prevailing in the hot tower is smaller than that in the cold tower, the waste liquid leaving the system has a lower content of desired isotopic constituent than the original feed stream. When the product stream is much smaller than the feed stream, it follows that the fraction of desired isotopic constituent actually extracted by this process is given by: fe = 1 -Xw/Xf where Xw and Xf are mole fractions in the waste and feed liquids, respectively. The maximum efficiency of extraction in such processes may be obtained by securing equilibrium conditions at the terminals of the two towers. Under these conditions, the fraction of the desired iso- tope in the gas leaving the cold tower is Xf/Kj. and the concentration of the liquid leaving the hot tower is XfKjj/Kc- Since the maximum amount of desired isotope extracted and concentrated in the product is determined by the difference in concentration in the feed and in the waste liquid, the maximum frac- tion of the constituent in the feed which is extracted is f m = 1 - Kh/Kc The fraction extracted in actual practice is given by fe- Its numerical magnitude depends on the degree of attainment of equilibrium at the ends of the two towers. Digitized by tine Internet Arclnive in 2011 witln funding from University of Florida, George A. Smathers Libraries with support from LYRASIS and the Sloan Foundation http://www.archive.org/details/dualtemperaturepOOcolu UNIVERSITY OF FLORIDA 3 1262 08909 7538 !■