Temperature Water-Hydrogen Exchange Processes

14 DUAL-TEMPERATURE WATER-HYDROGEN EXCHANGE PROCESSES 

Section 7.4 described the development in Canada [S8] of a catalyst for the deuterium exchange reaction between hydrogen and liquid water that is not inactivated when submerged in water. 

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. 

Miller and Rae [M7] have suggested process conditions for a dual-temperature process using this catalyst at 69 atm pressure and temperatures of S0°C for the cold tower and HO C for the hot. These conditions have been used to estimate optimum flow rates and numbers of theoretical stages for dual-temperature water-hydrogen processes using these two flow schemes. The results are tabulated in Table 13.28 and compared with similar data for the other dual-temperature processes discussed previously. 

With synthesis-gas feed, the water-synthesis-gas exchange process appears to be at a disadvantage relative to the ammonia and methylamine exchange processes because the water process has the highest flow rates and the largest number of stages. 

---

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.

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.

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 This process was superseded by development of processes based on water-hydrogen exchange. 

Temperature-dependent chemical shifts arise when an atom in a molecule is involved in an unsymmetrical intra-molecular re-orientation, e.g. the population of the methyl groups in methyl nitrite in cis and trans forms varies with temperature (Phillips, 1958), or when there is an intermolecular exchange process between distinct sites on two molecules, e.g. the —OH signal in a mixture of hydrogen-bonding molecules, such as methanol and water. A consideration of the time scales involved in observation of temperature-dependent chemical shifts will be delayed until later. 

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. 

---