More than 90% of the cumulative world production of heavy water (D20) is by the Hydrogen-sulphide (H2S) water (H20) exchange process, known as the GS process, with the major contribution being from the Canadian plants. Heavy Water Plant at Kota is a solely indigenous effort and is based on the Bithermal H20-H2S exchange process. The plant is located at a distance of 65 KM from Kota Railway Station, adjacent to Rajasthan Atomic Power Plant (RAPP). The Heavy Water Plant is integrated with RAPP for its supply of power and steam.
Process Description :
Water from the nearby Rana Pratap Sagar lake, purified of suspended and dissolved impurities forms the process feed with the D20 enriched from 150 ppm (0.015%) in the feed to 15% D20 by chemical exchange with H2S and later by vacuum distillation to produce 99.8% D20. The exchange unit is arranged in a 3 stage cascade. The first stage handling large quantities of process water and H2S gas, consists of three pairs of cold and hot towers operating at 30 deg. C. and 130 deg. C. respectively. The second and third stage each consist of one pair of cold and hot towers. The purified water enters the top of first stage cold tower and travels down while hydrogen sulphide gas entering the bottom of the tower meets the water in counter current way on tower internals and the exchange of deuterium takes place. In cold tower the water gets enriched with respect to deuterium while gas gets depleted in deuterium concentration. In hot tower the reverse reaction takes place i.e. the gas gets enriched instead of liquid. By proper liquid and gas flow rates with gas in closed circuit in a pair of towers, a small quantity of enriched liquid can be withdrawn from the bottom of the cold tower as a net product. This is further enriched in a similar way in 2nd and 3rd stages. The hot tower bottoms liquid coming from the first stage is divided into two parts. One part is recycled to the top of humidification section located at the bottom of hot tower for heat recovery while the other part constitutes the waste. Before discarding the waste to the environment it is necessary to recover the H2S dissolved in the waste. For this purpose a waste stripper is provided to strip H2S by direct steam stripping and the evolved gas and steam are put back to first stage hot towers. The enriched water from the 3rd stage is stripped off its H2S in a product stripper and fed to the distillation unit for further enrichment upto nuclear grade. As H2S gas is very toxic, corrosive and hazardous in nature and the plant has a dynamic hold up of 200 Te H2S, extreme care has been taken in the design of the plant, selection of equipment and materials, adhering to stringent fabrication procedures and codes. The exchange process operates at about 20 atm. pressure and at a temperature of 30 to 120 Deg.C. while the vacuum distillation plant works at a pressure of 100 mm Hg absolute. The isotopic exchange reaction which is the heart of the technology is carried out with the aid of specially designed efficient trays. The H2S gas required for the process is manufactured in a separate unit at the plant premises by the chemical reaction between sodium sulphide and sulphuric acid. Very elaborate and sensitive H2S detection instrumentation is provided not only in the plant but also in the surrounding areas to monitor the environment.