Ammonia-hydrogen exchange process

The monothermal ammonia-hydrogen exchange process was developed in France. The first plant was built at Mazingarbe and five plants have been built in India. 

Figure 13.23 Monothermal ammonia-hydrogen exchange process. Flow quantities, kg-mol/h.

Because of the reduced rate of the deuterium exchange reaction at —50°C, the stages of the cold tower (D) are to be of the type developed by Sulzer [LI ] for the ammonia-hydrogen exchange process and used in the Maangarbe plant. Sec. 9.1. For the methylamine-hydrogen system at —50°C, a stage efficiency of 70 percent has been obtained [W6]. 

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). 

Despite the relatively low heat of decomposition of ammonia, 3 x 10 J/kg, cracking of ammonia represents the costliest operation. Hence, bithermal process for the ammonia-hydrogen exchange was proposed. 

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

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. 

Because of the G-S process s limitations, alternatives have been developed. The alternative that has been deployed in India and Argentina relies on ammonia-hydrogen exchange. This needs a catalyst and the potassium salt of ammonia, KNH2, dissolved in the ammonia is used. Because of the high vapor pressure of ammonia, the exchange process is carried... 

The choice of H2S rather than H2 as the circulating gas, in spite of the fact that the latter would give much higher effective separation factors, is based on the fact that to date no stable effective catalysts have been found which will catalyze the H2-H2O exchange with liquid water. The use of water vapor as in the Barr type tower makes the H2-H2O process unfavorable compared to the H2S-H2O process. Only the ammonia-hydrogen process (5) catalyzed in the liquid phase by potassium amide has emerged as competitive with the H2S-H2O exchange. A full... 

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 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. 

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. 

AECL did extensive development of a variant of the ammonia-hydrogen process based on aminomethane (CH3NH2) rather than ammonia. This has better kinetics and a wider envelope of operating temperatures but can only be configured bithermally. This process was superseded by development of processes based on water-hydrogen exchange. 

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