6.Plants for the production or concentration of heavy water, deuterium and deuterium compounds and equipment especially designed or prepared therefor

INTRODUCTORY NOTE
Heavy water can be produced by a variety of processes. Five processes are demonstrated here. Older processes that have proven to be commercially viable are the waterhydrogen sulphide exchange process, the Girdler-Sulphide (GS) process, and the ammonia-hydrogen exchange process. Three newer processes first demonstrated in the early 2000s, are based on catalysed hydrogen-water exchange and have been shown to have the potential for production or upgrading of heavy water on an industrial scale with favourable economics. These processes are: Combined Electrolysis and Catalytic Exchange (CECE), Combined Industrial Reforming and Catalytic Exchange (CIRCE) and Bithermal Hydrogen-Water exchange (BHW).
The GS process is based upon the exchange of hydrogen and deuterium between water and hydrogen sulphide within a series of towers which are operated with the top section cold and the bottom section hot. Water flows down the towers while the hydrogen sulphide gas circulates from the bottom to the top of the towers. A series of perforated trays are used to promote mixing between the gas and the water. Deuterium migrates to the water at low temperatures and to the hydrogen sulphide at high temperatures. Gas or water, enriched in deuterium, is removed from the first stage towers at the junction of the hot and cold sections and the process is repeated in subsequent stage towers. The product of the last stage, water enriched up to 30% by weight in deuterium, is sent to a distillation unit to produce reactor grade heavy water; i.e., 99.75% by weight deuterium oxide (D₂O).
The ammonia-hydrogen exchange process can extract deuterium from synthesis gas through contact with liquid ammonia (NH₃) in the presence of a catalyst. The synthesis gas is fed into exchange towers and to an ammonia converter. Inside the towers the gas flows from the bottom to the top while the liquid NH₃ flows from the top to the bottom. The deuterium is stripped from the hydrogen in the synthesis gas and concentrated in the NH₃. The NH₃ then flows into an ammonia cracker at the bottom of the tower while the gas flows into an ammonia converter at the top. Further enrichment takes place in subsequent stages and reactor grade heavy water is produced through final distillation. The synthesis gas feed can be provided by an ammonia plant that, in turn, can be constructed in association with a heavy water ammonia-hydrogen exchange plant. The ammonia-hydrogen exchange process can also use ordinary water as a feed source of deuterium.
Many of the key equipment items for heavy water production plants using GS or the ammonia-hydrogen exchange processes are common to several segments of the chemical and petroleum industries. This is particularly so for small plants using the GS process. However, few of the items are available ‘off-the-shelf’. The GS and ammoniahydrogen processes require the handling of large quantities of flammable, corrosive and toxic fluids at elevated pressures. Accordingly, in establishing the design and operating standards for plants and equipment using these processes, careful attention to the materials selection and specifications is required to ensure long service life with high safety and reliability factors. The choice of scale is primarily a function of economics and need. Thus, most of the equipment items would be prepared according to the requirements of the customer.
Finally, it should be noted that, in both the GS and the ammonia-hydrogen exchange processes, items of equipment which individually are not especially designed or prepared for heavy water production can be assembled into systems which are especially designed or prepared for producing heavy water. The catalyst production system used in the ammonia-hydrogen exchange process and water distillation systems used for the final concentration of heavy water to reactor-grade in either process are examples of such systems.
Of the three main heavy water production processes employing hydrogen-water exchange, two (CECE and CIRCE) are only practical when integrated into large hydrogen production processes where hydrogen is being made for other commercial uses. The third process Bithermal Hydrogen-Water exchange (BHW) could potentially be used in a stand-alone plant. All these processes require large quantities of specialised wet-proofed platinised catalysts installed in long columns to provide good contact with the water flowing down. The CECE process requires such wet-proofed platinised catalyst exchange columns to be provided with hydrogen from a water electrolyser that receives its water feed from the exchange columns. In this way, the heavier isotope (deuterium) will build up a concentration in the electrolyser that receives its water feed from the exchange columns. The electrolyser system may potentially build up its deuterium concentration to almost pure heavy water. In practice the process will be staged and the large first stage typically raises the deuterium concentration by a factor between 5 and 20. The CIRCE process is similar, but uses a steam-hydrocarbon reformer as the source of hydrogen, providing the reformer with its source of water for steam. In all these plants, the CECE process is typically used as the final stage to produce reactorgrade heavy water. It should be noted that the largest hydrogen production plants in the world produce enough hydrogen to extract about 20-60 Mg per year of heavy water using a CECE or CIRCE process. A BHW process is conceptually the same as the GS, but using hydrogen instead of hydrogen sulphide with a catalyst to promote the deuterium transfer. In an arrangement analogous to the GS process, the BHW process exploits the effect of temperature on the equilibrium ratio of deuterium between water and hydrogen. The equilibrium falls with rising temperature. As water flows down through upper cold and lower hot towers, deuterium is enriched between them while hydrogen is circulated up through the hot and cold towers in turn. Water taken from between cold and hot towers is sent on to higher stages for further deuterium enrichment. A BHW process could be built for any scale of production.
The key component in these processes is clearly the specialised wet-proofed platinised catalyst that has proven to be relatively difficult to manufacture on a large scale at reasonable cost. Operating conditions are benign, with non-toxic fluids and catalysts, pressure between atmospheric and about 4 MPa and temperatures in the range 293 K (20 °C) to 473 K (200 °C). None of the equipment is significantly different from that used in various part of the chemical process industry other than the wet-proofed platinised catalyst.
The items of equipment which are especially designed or prepared for the production of heavy water utilising any of the technologies described above include the following: