G-process

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). 

In the heavy-water plants constmcted at Savannah River and at Dana, these considerations led to designs in which the relatively economical GS process was used to concentrate the deuterium content of natural water to about 15 mol %. Vacuum distillation of water was selected (because there is Httle likelihood of product loss) for the additional concentration of the GS product from 15 to 90% D2O, and an electrolytic process was used to produce the final reactor-grade concentrate of 99.75% D2O. 

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

We close discussion of the GS process with the comment that it is dangerous. A GS plant requires a very large inventory of highly toxic H2S employed at elevated temperature and pressure in a corrosive environment. The safety of operating personnel and of the population in the area surrounding the plant is a major concern. 

This is an isotope exchange system rather than a bona-fide diztal-lation system. Separation is part of the GS process, and runs at molar L/Vof 0.5. 

With the direct current-sulfuric acid technique (GS), final hardness values are between 500 and 600 HV. Lower hardness values of 250-400 HV are reached by a special anodizing technique (NS). The latter procedure is preferred in practice, because, contrary to the GS process, it prevents attack on the base metal of pieces that are not completely aluminum plated, such as a hollow form. 

The first dual-temperature plant for the Manhattan District Heavy Water program was built at Dana and the second at Savannah River. The process was known as GS process (Girdler-Sulfide or Girdler-Spevack). A very simplified flow sheet of this process resembles Fig. 3C. 

Benedict, Pigford, and Levi have carried out mathematical analysis of the GS process. An exhaustive treatment of the process, including calculations for flow rates, dependence of composition on number of stages, effect of solubility and humidity on process analysis, temperature profile in cold towers, simultaneous heat and mass transfer in heat transfer section, concentration reversal in heat transfer section, corrosion, materials of construction, feed purification, and safety, etc. have been reviewed by Dave, Sadhukhan and Novaro. ... 

The main drawback of the GS process is the highly corrosive nature of its aqueous solutions. A 400 Mg/yr GS plant requires an inventory of 800 Mg of H2S, which is an extremely toxic, flammable, and corrosive gas with a distinct, disagreeable smell even at low concentrations. Hence, adequate measures must be taken for material selection, fabrication, feed purification, feed and waste discharges in water and the atmosphere, safety of staff, the surrounding population, and environment. ... 

Indian HWPs. Two plants, one at Kota, Rajasthan, and the other at Manuguru, Andhra Pradesh, are in operation. The Kota plant has a capacity of 85 Mg/yr of 99.8% D2O. It uses GS process for primary enrichment (15%) and vacuum distillation for the final enrichment. The plant was commissioned in 1982. 

Those plants that for primary concentration use water distillation (WD) or the dualtemperature, water-hydrogen sulfide (GS) process are self-contained plants whose sole product is heavy water. 

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 Trail plant was started up in 1943 and began producing heavy water in 1944. It was shut down in 1956 because of the high cost of its heavy water compared with that produced by the GS process (Sec. 11). 

At both Dana and Savannah River the GS process was used for primary concentration of deuterium to 15 percent, with the remaining concentration being effected by distillation of water and electrolysis. 

Pilot-plant investigations of the GS process have been carried out in France [R4] and in Sweden [E2], and a thorough analysis of the process has been published by Weiss [W3]. 

This shows the importance of using a reaction in which the separation factor in the hot tower differs substantially from that in the cold in fact, separation is possible only because the slopes of the two equilibrium lines in Fig. 13.28 are different. For the GS process example of Fig. 13.25, the maximum recovery of deuterium possible is... 

Although the minimum gas flow rate is large, it is much smaller than in the distillation of water [141,000, from Eq. (13.11)]. Moreover, the GS process can be operated at much higher pressure than water distillation, which also helps to reduce the number and diameter of towers. 

Figiue 13.29 Effect of vapor-to-feed ratio on recovery in GS process example, tt/, — 1.80 (Xg ... 

All heat requirements for the process are provided in the form of open steam at 400 psia. Some is used at the bottom of S-1 to strip HjS and the rest is fed to the twelfth plate in HT-1 to control the temperature of the hot towers and to compensate for heat losses and heat exchanger inefficiencies. Steam consumption is 1778/0.28 = 6400 mol/mol of DjO produced. This is much less than the 200,000 mol/mol DjO needed in water distillation. Additional energy in the amount of 680 kWh/kg D2O is used to circulate gas and pump liquid. This, however, is much less than is used in electrolysis or hydrogen distillation (Table 13.7). The low energy consumption of the GS process is due in large measure to the efficient heat recovery obtainable in the flow sheet Fig. 13.30, which follows Spevack s patent [S7]. 

The principal disadvantage of the GS process is the toxic and corrosive character of aqueous solutions of hydrogen sulfide. Extensive corrosion research and experience with the Dana and Savaiuiah River plants has shown what materials of construction can be used to withstand corrosion, without prohibitive cost. The following sununary of recommendations regarding materials of construction is condensed from reference [T4]. 

Utility requirements reported for heavy-water production by the GS process are as follows ... 

The optimum temperature of the cold tower is as low as possible without risking formation of a third phase in addition to vapor and aqueous solution. Table 13.24 gives the temperatures at which solid hydrogen sulfide hydrate or liquid hydrogen sulfide form in the system HjS-HjO. At 300 psi, the minimum safe cold tower temperature is around 30°C. The rapid increase in condensation temperature above 300 psi is another reason for this being the optimum pressure. Before the first pilot plant for the GS process was operated, the possibility of hydrate formation was not recognized, and freeze-ups occuned until the cold tower temperature was raised above 30°C. 

Because of the complexity of the GS process flow sheet, there are a number of opportunities for making improvements in the process that, taken together, should increase deuterium production, reduce the number of separate pieces of equipment, improve energy utilization, and reduce costs. U.S. work on improvements in the early 1960s was described by Proctor and Thayer [P4] and has been used in the first Canadian plants. Later improvements patented by Thayer [T3] have been considered for the newer Canadian plants. 

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. 

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