The high cost of recovering heavy water from electrolytic hydrogen by exchange with steam is due largely to the cost of making and condensing steam and to the large mass of catalyst needed for this vapor-phase reaction at low pressure. These difficulties would be avoided if the reaction could be carried out at an acceptable rate in the presence of liquid water. Becker [BIO] developed a colloidal platinum-on-charcoal catalyst, suspended in liquid water, which was circulated in countercurrent flow to gaseous hydrogen through a conventional sieve-plate column. This catalytic exchange system was tested on a semicommercial scale at Dortmund,

Figure 13.19 Flow sheet for one ex­change tower and electrolytic cell group.

Germany [Wl], in the early 1960s, using a dual-temperature flow sheet. Even with this finely divided catalyst and a pressure of 200 bar, the plate efficiency at 30°C was only around 1 percent. The resulting large column and catalyst volume made the process appear to be only marginally economical.

The low plate efficiency is due to the low solubility of gaseous hydrogen in liquid water, which results in a low mass-transfer coefficient for hydrogen to and from the catalyst surface, which is wetted by liquid water. Stevens [S8], in Canada, has recently developed a catalyst for the deuterium exchange reaction that is not wetted by liquid water and is much more active. The catalyst consists of nickel or platinum deposited on a conventional support such as silica gel and then coated with a thin layer of a water-repellent resin through which hydrogen can rapidly diffuse. Liquid water and gaseous hydrogen flow countercurrent through a column packed with particles of such a catalyst. The gas-phase deuterium exchange reaction between water and hydrogen takes place on the catalyst surface, while H20 and HDO are transferred simultaneously between the gas and liquid phases. From experiments described in Stevens’ patents, transfer-unit heights of around 1.5 m are predicted [H2] at a superficial gas velocity of 3 m/s evaluated at standard conditions of 0°C and 1 atm for actual conditions of 60°C and pressures in the range of 14 to 40 bar.

Figure 13.20 shows how exchange towers packed with such a catalyst permitting counterflow of hydrogen and water might be used to increase the recovery of deuterium from

the cascade of electrolytic cells shown in Fig. 13.16. Exchange towers are used to reduce the deuterium content of hydrogen leaving electrolytic stages 2, 3, and 4 to 0.0563 a/о, the same value as in hydrogen leaving the first electrolytic stage. In this flow sheet it has been assumed that the exchange towers are so designed and operated that water and hydrogen leaving them have the same deuterium content as the corresponding streams leaving the electrolytic stages with which they are mixed. This ideal cascade condition minimizes exchange tower volume.

Comparison of Fig. 13.20 with Fig. 13.16 shows that the use of exchange towers would increase heavy-water production from 0.3553 to 0.930 mol and deuterium recovery from 23.8 to 62.2 percent.

To reduce the number of electrolytic stages to three, Hammerli and co-workers have suggested [H2] the flow sheet of Fig. 13.21. The big advantage of this flow sheet is the great simplification of interstage connections compared with Fig. 13.20. The principal disadvantage of Fig. 13.21 is its much greater catalyst volume. Hammerli [HI] estimates, however, that the cost of catalyst and catalyst towers for a flow sheet like Fig. 13.21 is only 15 percent of the cost of the electrolytic cells, so that it is cost-effective to simplify the flow sheet at the expense of increased catalyst volume. The dimensions of the catalyst towers of Fig. 13.21 for a superficial hydrogen gas velocity of 3 m/s at standard conditions are

In comparing Figs. 13.20 and 13.21, the following should be noted. Hydrogen product rates are substantially equal, 10,000 kg-mol/h. The total amounts of water electrolyzed are about the same. Use of an exchange tower on hydrogen from the first cell coupled with the closer approach to exchange equilibrium at the top of the towers of Fig. 13.21 permits reduction in deuterium content of depleted hydrogen from 0.0563 to 0.050 percent and

Natural water
Feed

10,000 9 8569 0.0149%

increases heavy-water production from 0.930 to 0.982 kg-mol/h. More separative work is performed in the exchange towers of Fig. 13.21 than in those of Fig. 13.20, primarily to compensate for the loss of separative work in Fig. 13.21 where the water recycled from each burner is mixed with water of quite a different composition from an exchange tower. Other factors increasing the separative work demand on the towers in Fig. 13.21 are the lower electrolytic separation factor of 6^ used in that figure compared with 7 in Fig. 13.20 and the lower deuterium content of hydrogen product.

One possible difficulty with Fig. 13.21 is the much higher average deuterium content of water in the electrolytic cells compared with Fig. 13.20. This requires that cell leak rates and water holdup be kept small.