Dual-temperature exchange processes

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. 

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

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. 

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... 

Dual Temperature Process. A unit of a dual temperature cascade is shown schematically in Figure 8. The dual temperature system operates on the principle that isotope exchange reactions, like all chemical reactions, change their equilibrium constants with temperature. The general, but far from universal (22), rule is that in systems with large isotopic... 

All of the previously mentioned plants except those employing distillation of water were parasitic to a synthetic anunonia plant. Their deuterium-production rate is limited by the amount of deuterium in ammonia synthesis gas. To produce heavy water at a sufficient rate, a growing industry of heavy-water reactors requires a deuterium-containing feed available in even greater quantity than ammonia synthesis gas. Of the possible candidates, water, natural gas, and petroleum hydrocarbons, water is the only one for which an economic process has been devised, and the dual-temperature hydrogen sulfide-water exchange process is the most economic of the processes that have been developed. 

DUAL-TEMPERATURE WATER-HYDROGEN SULFIDE EXCHANGE PROCESS... 

The dual-temperature principle for providing reflux for the ammonia-hydrogen deuterium exchange process was proposed by the British firm Constructors John Brown [C12], has been tested in pilot-plant experiments conducted by Friedrich Uhde Gmbh at the plant of Farbwerke Hoechst in Germany [W2], and is to be used in a commercial plant at Talcher, India (item 19, Table 132), being constructed by Uhde. 

Figure 13.37 Material flow sheet for dual-temperature ammonia-hydrogen exchange process. Flow units, kg-mol/h.

Table 13.26 Comparison of hydrogen exchange processes monothermal and dual-temperature ammonia-...

Figure 13.40 Material flow sheet for first stage of Sulzer dual-temperature methylamine-hydrogen exchange heavy-water process. [AT] = deuterium content of hydrogen relative to natural water containing 135 ppm. Flow quantities, kg-mol/h.

DUAL-TEMPERATURE WATER-HYDROGEN EXCHANGE PROCESSES... 

Availability of this catalyst has led to interest in its possible use in dual-temperature water-hydrogen exchange processes. With liquid-water feed and recirculated hydrogen gas, this catalyst could be used in a dual-temperature process similar in principal to the GS process, with a schematic flow sheet like Fig. 1325. With ammonia synthesis-gas feed and recirculated water, this catalyst could be used in a dual-temperature process similar to the ammonia-hydrogen process flow scheme of Fig. 13.37, provided that impurities in synthesis-gas feed that would poison the catalyst can be recovered sufficiently completely. 

The dual-temperature, methylamine-hydrogen exchange process described in Sec. 13 could also be used to concentrate deuterium from ammonia synthesis gas produced from natural gas and steam containing the normal abundance of deuterium instead of the enriched steam used in the Sulzer flow sheet. Fig. 13.40. Figure 13.42 is a flow sheet for such a process giving the deuterium content of each stream in the first stage of the plant. 

Figure 13.42 Primary concentration step in dual-temperature methylamine-hydrogen exchange process fed with synthesis gas made from normal water. Flow rates G and L in kg-mol/h. [ DY) = 135 parts deuterium per million parts deuterium -h hydrogen.

In parallel with this, dual temperature processes were described by Rideal and Hartley in 1942. The earliest chemical engineering contributions therefore were made by men who would at that timie have described themselves as applied chemists but would, in their later careers, describe themselves as chemical engineers. During the period 1941-43 they had the advantage of exchange of information with the United States, until the latter withdrew their earlier cooperation. 

As it turned out, the dual temperature H2O-H2 exchange process was never used because the exchange was too slow, and a much better exchange reaction was found in the system H2O-H2S which does not require a catalyst. 

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