2.1 Terminology

In analyzing processes for separating isotopes by distillation, it is desirable to select as components those species whose proportions can be varied independently. When each molecule of the mixture being processed contains only one atom of the element whose isotopes are being separated, such as H216 О and H2l80, it is immaterial whether the components be selected as the pair (H2160, H2180) or (160, 180), as the mole fraction of H2180 in (H2160, H2lsO)is identical with the atom fraction of 180 in (160, 180). However, when the molecules of the mixture being processed contain two or more atoms of the element whose isotopes are being separated, such as hydrogen containing H2, HD, and D2, or water containing H20, HDO, and D20, it is necessary to choose as components those species whose proportions can be varied independently. In distilling a mixture of H2, HD, and D2, the amount of any one of the three components can be varied independently of the other two; the mixture is therefore treated as containing the three components H2, HD, and D2, and compositions are expressed as mole fractions of H2, HD, and D2. However, in distilling a mixture of H20, HDO, and D20, equilibrium is continuously maintained in the disproportionation reaction

2HDO ** H3 О + D2 О

so that the amount of only two of the three components can be varied independently. In this case, separation performance equations are simplest if compositions are expressed as atom fractions of deuterium or hydrogen.

All of the processes for separating isotopes of hydrogen or other light elements dealt with in this chapter involve distribution between a liquid and a vapor phase. To remain consistent with standard chemical engineering usage, component fractions in the vapor phase are denoted by у and the liquid phase by x. For a two-component mixture, the symbol у от x will denote the fraction of desired component (e. g., atom fraction deuterium in a mixture of H20, HDO, and DjO) in the vapor or liquid phase. For a mixture containing three or more components, a subscript will be used to designate the component. For example, ^hd denotes mole fraction HD in a vapor mixture of H2, HD, and D2. However, in mixtures of H2, HD, and D2 whose deuterium content is so low that the fraction of D2 can be neglected, the mole fraction of HD will be denoted by у or x without subscript.

In a two-component mixture, the separation factor a is defined as the fraction of desired component in the phase in which it concentrates divided by the fraction of desired component in the other phase. Deuterium, the isotope principally discussed in this chapter, almost always concentrates in the liquid phase. For such deuterium separation processes, the deuterium separation factor a is given by

,*/( 1 — x) УІІ1 — y)

This is the reciprocal of the equation used to define the separation factor in Chap. 12, Eq. (12.1). This change in notation for Chap. 13 is regrettable, but is hard to avoid.