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Heavy-Water Production Using Amine-Hydrogen Exchange

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HEAVY-WATER PRODUCTION USING AMINE-HYDROGEN EXCHANGE Development of the amine-hydrogen process for heavy-water production involves a large research and development program at CRNL supplemented by contracts with industry and universities. Four major areas of work are: — process design and optimization — process chemistry — deuterium exchange in gas-liquid contactors — materials for construction This article, which is the third in a series to appear in this publication, describes the development of equipment for efficient exchange of deuterium between hydrogen gas and amine liquid. The process flowsheet and the chemistry of aminomethane solu- tions of potassium methylamide catalyst were described previously^ >2). Efficient countercurrent, multistage contacting of liquid amine and hydrogen gas in hot and cold towers are required in this process. The unusually large separation factors possible with amine-hydrogen exchange are an important advantage. A major development program is aimed at the full exploitation of this advantage by achieving rapid exchange in the cold tower. Increasing the difference between the hot and cold tower temperatures decreases the required gas flow rate and number of theoretical plates. As the cold tower temperature is lowered plate efficiency falls, tower fabrication is more expensive and refrigeration costs rise; the range of interest is -100 to 0°F. The upper limit on hot column temperature is set by the vapor pressure of the aminomethane and chemical stability of the catalyst solution. Temperatures up to 160°F are attractive. The exchange reaction HD + CH3NH2 „ » H2 + CH3NHD occurs in the liquid phase a^d consists/of two steps: physical absorption and chemical ^reaction. Mass transfer is controlled by the diffusion of hydrogen in a thin liquid reaction zone near the gas-liquid interface. The mass transfer coefficient, enhanced by chemical Figure 1 Concurrent gas-liquid contactor for amino- reaction, is large but the hydrogen solubility is very methane-hydrogen exchange. Figure 2 Circulating equipment and piping. 3 low. Henry's law constant is 6240 atm ft /lb mole at Part of a column with deep beds is shown in -40°F. The deuterium content of the liquid is many Figure 1. Gas and liquid How cocurrently. providing hundreds of times larger than the dissolved HD contact times of a few seconds for beds which are content so that, to appreciably alter the CH3NHD several feet deep. A packing, such as wire gau/e, in concentration, a long contact time is required to the bed breaks up coalescing gas bubbles, producing allow the deuterium atoms to pass from the gas to small bubbles and large interfacial areas. This type of liquid as HD and then exchange. Pressures of 1000 gas-liquid contactor has been patented by AEClJ-1). psig or more can be used to increase the amount of Note that while flows are cocurrent in each bed, they dissolved hydrogen, but even at these pressures the are countercurrent in the column as a whole. driving force for deuterium exchange is small. As the A test facility for circulating hydrogen and liquid temperature increases, the exchange rate increases aminomethane through a 6-inch diameter column has because both the solubility and the chemical reaction been operated at cold tower conditions. Figure 2 is a rate constant increase. photograph of some of the components. The effects For the hot columns conventional sieve trays of gas and liquid velocity, temperature, catalyst should- be adequate but testing is required to confirm concentration, and bed depths have been studied this. For the cold columns none of the usual types of using packings of wire gau/e, Pall rings, and cylin- gas-liquid contactor performs satisfactorily. Deep drical screens. The deuterium concentration in the beds appear attractive because they provide a high hydrogen is measured by online mass spectrometry at ratio of mass-transfer volume to total column volume. the column inlet and outlet and in equilibrium with Much of the' research and development has been the aminomethane liquid leaving the column to concentrated on cold columns because of the lack of determine bed efficiency. Hydrogen deuteride is information on performance of this type of contactor injected into the hydrogen to maintain a driving force and because the cold columns contribute greatly to between the gas and liquid. The CH3NHD content of the capital cost of the plant. the latter rises very slowly during an experiment. Figure 3 Effect of gas velocity on stage efficiency. 3 - 90 80 70 N 1 RCE UI 0.9 60 Q_ 0.8 0.7 50 P" Figure 4 Effect of temperature on stage effi- 0.6 , ciency. 0.5 40 ENCY o 0.4 I 30 EFF 0.3 UJ CO 20 CO 0.2 TEMPERATURE, °F 0.1 I I I I I 10 20 -20 -40 -60 -80 10 Figures 3 and 4 show the stage efficiency, 77, as a Information obtained with this equipment is being function of superficial gas velocity and temperature. used for the detailed design of a 25 ton per year Tests of sieve trays in a 10-inch diameter column at aminomethane-hydrogen prototype heavy - water temperatures up to 100°F have begun. plant. W.E. Lockerby REFERENCES For further information see: 1. R.S. Jickling, "AECL Research in Engineering", AECL-3787(1971). H.K. Rae, "Heavy Water in Canada", AECL-3866 2. W.J. Holtslander, "AECL Research in Engi- (1971). neering", AECL-3990 (1971). A.R. Bancroft and H.K. Rae, "Heavy Water Pro- 3. J. Chrones and H.K. Rae, Canadian Patent No. duction by Amine-Hydrogen Exchange", AECL-3684 835227 (1970). (1970). 11.
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