Page 80 - Separation process engineering

The K values are used along with the stoichiometric equations which state the mole fractions in liquid and
vapor phases must sum to 1.0.

(2-31)

where C is the number of components. Bubble-point and dew-point calculations are discussed in detail in
Section 5.4.

If only one component is present, then y = 1.0 and x = 1.0. This implies that K = y/x = 1.0. This gives a
i
simple way of determining the boiling temperature of a pure compound at any pressure. For example, if
we wish to find the boiling point of isobutane at p = 150 kPa, we set our straightedge on p = 150 and at
1.0 on the isobutane scale on Figure 2-11. Then read T = –1.5°C as the boiling point. Alternatively, Eq.
(2-30) with values from Table 2-3 can be solved for T. This gives T = 488.68°R or –1.6°C.

For ideal systems Raoult’s law holds. Raoult’s law states that the partial pressure of a component is
equal to its vapor pressure multiplied by its mole fraction in the liquid. Thus,

(2-32a)

where vapor pressure (VP) depends on temperature. By Dalton’s law of partial pressures,

(2-32b)

Combining these equations,

(2-32c)