The deuterium exchange reaction between water and hydrogen discussed in Sec. 7 is one of a group of deuterium exchange reactions that have been extensively studied and are the basis for most of the world’s heavy-water production. Table 13.17 lists deuterium separation factors between liquid water and gaseous compounds of hydrogen for temperatures in the range 0 to 200°C. The ratio of the separation factor at 25°C to that at 125°C, a2s/ai2s> is also given. The higher this ratio is, the greater is the fractional recovery of deuterium and the smaller is the number of stages needed in the dual-temperature exchange process to be described in Sec. 11.

^A flow sheet like Fig. 13.21 would concentrate deuterium even if electrolysis produced no separation at all.

The reactions of Table 13.17 have been listed in order of increasing values for this ratio. Because water is one of the components of each pair in Table 13.17, processes based on these reactions could use liquid water as feed and thus would not be limited in output by limited feed availability.

Table 13.18 lists deuterium separation factors between gaseous hydrogen and liquid ammonia or methylamine, two compounds of hydrogen proposed for deuterium separation processes. The ratios of separation factors between the temperatures marked by a dagger, which have been proposed for dual-temperature processes based on these reactions, are also given. Both the separation factors and the separation factor ratios of the reactions involving hydrogen are greater than those involving water in Table 13.17. These higher values are what give the reactions of Table 13.18 their practical importance. A disadvantage of the reactions of Table

13.18 is that their deuterium production is limited to the amount present in commercially available hydrogen.

In all systems deuterium tends to concentrate in the phase normally liquid except ammonia-water at high temperature. Separation factors in chemical exchange are much higher than separation factors in distillation for the corresponding materials (cf. Table 13.3) except for ammonia-water. The high value of these separation factors and their strong dependence on temperature are what give the chemical exchange process its importance for separation of deuterium and isotopes of other light elements.

The deuterium exchange reaction between water and ammonia, water and hydrogen sulfide, or water and the hydrogen halides proceeds rapidly in the liquid phase without catalysis, because of ionic dissociation. In the case of a mixture of water and hydrogen sulfide, for example, the ionic equilibria

H20=*H+ + 0H h2s=*h+ + sh-

HDO-H+ + OD — HDS-H+ + SD-

HDO^D+ + OH’ HDS^D+ + SH-

permit rapid exchange of H+ and D+ between the two materials. Deuterium exchange between water and phosphine, water and hydrogen, ammonia and hydrogen, or methylamine and hydrogen does not proceed without catalysis. The water-phosphine reaction can be catalyzed by

Table 13.18 Separation factors in liquid-vapor deuterium exchange reactions involving hydrogen Reactants NH3 + HD CH3NH2 + HD Products NH2D + H2 CH3NHD + H2 а/К Separation factor a at 2 3 1 -50 6.6 7.90+ -25 5.19t 6.04 0 4.25 4.85 25 3.62 — 40 3.32 3.6+ 50 3.15 _ 60 2.99+ — 100 2.55 — 125 2.34 — Ratio at t 1.74 2.19 Reference [PI], [R4] averaged [R7]

strong acids [W4], the water-hydrogen reaction by nickel or platinum-metal catalysts (see Sec. 7), the ammonia-hydrogen reaction by potassium amide dissolved in liquid ammonia [C2], and the methylamine-hydrogen reaction by potassium methylamide.

Solutions used in the ammonia-water, water-hydrogen, and ammonia-hydrogen processes are relatively noncorrosive and may be handled in ordinary steel equipment. Solutions used in all of the other processes are relatively corrosive, and require use of stainless steel or other expensive construction materials.

The constant-boiling mixtures formed by water and the hydrogen halides make it difficult to use these systems in a practical exchange process.

Of the reactions listed, the water-hydrogen sulfide case has the greatest practical impor­tance because it needs no catalysis and has a fairly large change of separation factor with temperature. This case is discussed in detail in Sec. 11. The water-hydrogen reactions discussed in Sec. 7 and the ammonia-hydrogen and methylamine-hydrogen reactions, with their large separation factors and large change of separation factor with temperature, are also of practical importance.

In some cases the separation factors given in these tables have been determined experi­mentally from equilibrium constants К for gas-liquid reactions such as

H2 0(0 + HDS(?) * HDO(I) + H2 S(?)

In other cases, they have been derived from experimental measurements of the equilibrium constants к for gas-phase reactions such as

H2 0(g) + HDSQr) * HDO(g) + H2 S(?)

In still other cases gas-phase equilibrium constants have been computed by statistical mechanics from molecular properties. Procedures for calculating к have been described by Bigeleisen and Mayer [B12]. Varshavskii and Vaisberg [VI] have given a very extensive tabulation of values of к calculated for many deuterium exchange equilibria.

(*hdo+ ^DjoM^HjO +*hdo)

O’HDS + 2yD, s)/(2yHsS +3’hds) When the deuterium content of liquid and vapor is low, under a few percent, Xd3o ^*hdo> ^hdo^^HjO > etc., so that the above equation reduces to *hdo/xh, o

Expressions for the relation between К, к, and the chemical exchange separation factor a will now be derived. Let us consider first the exchange reaction between liquid water and gaseous hydrogen sulfide. As in distillation, the deuterium separation factor in the chemical exchange reaction is defined as the ratio of the abundance ratio of deuterium to light hydrogen in the liquid to the corresponding ratio in the vapor. In terms of the mole fractions of individual compounds in the liquid and vapor, the separation factor is’*’

HDO(I) + D2S(?)^ D2 0(0 + HDS(?)

must be also taken into account. These do not greatly affect the value of a, however. The equilibrium constant к for the gas-phase reaction is defined as

t In this equation the solubility of hydrogen sulfide in the liquid and the vaporization of water in the vapor have been neglected. These effects are treated in Sec. 11.

Because liquid-vapor exchange reaction is the resultant of vapor-phase exchange reaction and the vaporization equilibrium reaction

H, O(0 + HDCfe) ^ HDO(0 + H2OC?)

for which the equilibrium constant is the relative volatility a.*, defined by ^ *hdo/*h3o Унхю/Ун2о (13.75) it follows that K = ka* (13.76) so that a = ka* (13.77)

In the more general exchange reaction

MHm (0 + ZH2 ., Dfr) — MHm., D© + ZHrfr)

in which the liquid compound MHm and the gaseous compound ZH2 contain different numbers of hydrogen atoms, the separation factor is related to the equilibrium constant by

Values of ajK have been listed in Tables 13.17 and 13.18.