The deuterium exchange reaction between water and hydrogen sulfide,

H2 0(0 + HDS(g) * HDO(0 + H2 S(g)

proceeds rapidly without catalysis and has an equilibrium constant of 2.32 at 32°C. A monothermal process using this reaction, thus, could concentrate deuterium without the need for the complicated catalyst-recovery steps used in the ammonia-hydrogen exchange process, Fig. 13.23. Moreover, a water-hydrogen sulfide exchange plant can use natural water as feed and thus, unlike hydrogen-fed processes, is not limited in capacity to the amount of deuterium present in other industrial operations.

Despite these advantages of the water-hydrogen sulfide deuterium exchange reaction, it is not economical to use it in a monothermal flow sheet to produce heavy water because of the high cost of chemical reflux in this system. This may be shown by reference to Fig. 13.24.

In this monothermal, water-hydrogen sulfide flow sheet, natural water is fed at the top of a bubble-plate exchange column, and the water becomes progressively enriched in deuterium as it flows down the column in countercurrent contact with upflowing hydrogen sulfide gas. Heavy-water product is drawn off the bottom of the column and hydrogen sulfide gas depleted in deuterium is drawn off the top.

The critical and essential feature of this flow sheet is the D2 S generator at the bottom of the column in which deuterium is transferred from D20 to D2S to provide reflux. Various means for effecting this chemical transfer can be imagined; all are costly. The means assumed here is the reaction between water and aluminum sulfide,

3Dj О + 2AljS3 «*= 3Dj S + Al2 03

Aluminum sulfide may be made by reacting aluminum metal with sulfur:

2A1 + 3S -»■ A12S3

The sulfur needed for this step may be reclaimed from the depleted hydrogen sulfide leaving the top of the column by partial combustion with air:

H2S + j02 ->H20 + S

The overall effect is to separate natural water into D20 and water depleted in hydrogen, with reflux provided by consumption of aluminum metal and production of aluminum oxide. Sulfur and hydrogen sulfide circulate internally and are not consumed by the process. The minimum molar ratio of D2S reflux G to D20 product P may be evaluated from Eq. (12.80):

Because this reflux ratio is much lower than the reflux ratio in the distillation of water derived in Eq. (13.11), the towers of a hydrogen sulfide exchange plant could be much smaller in diameter than the towers of a water distillation plant. Because the separation factor for the exchange process (2.32) is much greater than for water distillation (—1.05), the towers could contain a much smaller number of plates.

However, the cost of providing chemical reflux is so high as to preclude the use of the flow sheet of Fig. 13.24 for heavy-water production. From the preceding chemical reactions it is seen that I mol of aluminum metal is consumed for each mole of D2S reflux. Because aluminum metal costs around $0.50/lb, the minimum cost of aluminum (MW = 27) per pound of heavy-water product (MW = 20) is

Even without allowing for the additional costs of the conversion operations themselves, this is clearly prohibitive. Other possible chemical conversion schemes are similarly uneconomical.