Page 304 - Separation process engineering

cheaper than distillation in many cases.

8.3 Steam Distillation

In steam distillation, water (as steam) is intentionally added to the distilling organic mixture to reduce the
required temperature and to keep suspended any solids that may be present. Steam distillation may be

operated with one or two liquid phases in the column. In both cases the overhead vapor will condense
into two phases. Thus, the system can be considered a type of azeotropic distillation where the added
solvent is water and the separation is between volatiles and nonvolatiles. This is a pseudo-binary
distillation with water and the volatile organic forming a heterogeneous azeotrope. Steam distillation is
commonly used for purification of essential oils in the perfume industry, for distillation of organics
obtained from coal, for hydrocarbon distillations, and for removing solvents from solids in waste
disposal (Ellerbe, 1997; Ludwig, 1997; Woodland, 1978).

For steam distillation with a liquid water phase present, both the water and organic layers exert their own
vapor pressures. At 1 atm pressure the temperature must be less than 100 °C even though the organic
material by itself might boil at several hundred degrees. Thus, one advantage of steam distillation is
lower operating temperatures. With two liquid phases present and in equilibrium, their compositions will
be fixed by their mutual solubilities. Since each phase exerts its own vapor pressure, the vapor
composition will be constant regardless of the average liquid concentration. A heterogeneous azeotrope is
formed. As the amount of water or organic is increased, the phase concentrations do not change; only the
amount of each liquid phase will change. Since an azeotrope has been reached, no additional separation is
obtained by adding more stages. Thus, only a reboiler is required. This type of steam distillation is often
done as a batch operation (see Chapter 9).

Equilibrium calculations are similar to those for drying organics except that now two liquid phases are
present. Since each phase exerts its own partial pressure, the total pressure is the sum of the partial
pressures. With one volatile organic,

(8-14)

Substituting in Eqs. (8-9) and (8-10), we obtain

(8-15)

The compositions of the liquid phases are set by equilibrium. If total pressure is fixed, then Eq. (8-15)
enables us to calculate the temperature. Once the temperature is known the vapor composition is easily
calculated as

(8-16)

The number of moles of water carried over in the vapor can be estimated, since the ratio of moles of
water to moles organic is equal to the ratio of vapor mole fracs.

(8-17)