Food Biochemistry and Food Processing (2 edition)

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778 Part 7: Food Processing

simple distillation, the vapors are recovered by condensation.

In rectification, successive vaporization and condensation are

carried out simultaneously and a part of the condensed liquid,

called the reflux, flows down the column countercurrent to the

flow of vapors for isolating components from a mixture based on

differences in boiling points. The percentage of each constituent

in the vapor phase usually depends on its vapor pressure at a

certain temperature. The principle of vacuum distillation may

be applied to substances, such as oils, that would be damaged

by overheating by the conventional method (Liu et al. 2008).

Several new methods have been developed for the separation

and recovery of minor components from vegetable oils such as

palm oil (Rodr ́ıguez et al. 2007, Shi et al. 2007c).

Distillation and its companion processes, azeotropic and ex-

tractive distillations, are by far the most widely used separation

processes for mixtures that can be vaporized. Vapors are gener-

ated from liquids or solids by heating and are then condensed

into liquid products. However, many mixtures exhibit special

states, known as azeotropes, at which the composition, temper-

ature, and pressure of the liquid phase become equal to those of

the vapor phase. Thus, further separation by conventional distil-

lation is no longer possible. By adding a carefully selected other

component as an entrainer to the mixture, it is often possible

to “break” the azeotrope and thereby achieve the desired sepa-

ration. In azeotropic distillation, a compound is added to form

an azeotrope with at least one of the components of the mix-

ture. That component can then be more readily separated from

the mixture because of the increased difference between the

volatilities of the components. Extractive distillation combines

continuous fractional distillation with absorption. A relatively

high-boiling solvent is used to selectively scrub one or more

of the components from a mixture of components with simi-

lar vapor pressures. Distillation processes are also widely used

for the separation of organic chemicals, usually at cryogenic

temperatures.

Separation Processes in Distillation

When vacuum is applied, there are three major reduced pres-

sure ranges that can be utilized for distillation: (a) distillation at

moderate vacuum or equilibrium distillation, (b) unobstructed

path distillation, and (c) molecular distillation. Distillation at

moderate vacuum is characterized by the use of conventional

distillation equipment as shown in Figure 40.8. Its lowest pres-

sure limit is of the order of 1 Torr, that is, 1 mm Hg. Unobstructed

path distillation is defined as distillation in which the path be-

tween the evaporator and the condenser is not blocked, in other

words there is a free transfer of molecules (Eckles and Benz

1992). When the transfer distance is comparable with the mean

free path of the vapor molecules, the distillation is known as

molecular distillation.

Mean free path is defined as the average distance a molecule

will travel in the vapor phase without colliding with another va-

por molecule (Eckles et al. 1991). This implies that, in molecular

distillation, the vapor molecules can reach the condenser with-

out intermolecular collisions. A dynamic equilibrium can not

be established between the vapor and the liquid. An individual

Figure 40.8.Molecular distillation system.

molecule that has evaporated will be able to travel any distance

without a collision.

Molecular distillation occurs at low temperatures and, there-

fore, reduces the problem of thermal decomposition. High vac-

uum also eliminates oxidation that might otherwise occur in the

presence of air. Unobstructed path and molecular distillations

are often classified together as short-path or high-vacuum dis-

tillation. The difference is in the dimensions and operating con-

ditions. Unobstructed path distillation is carried out at pressures

as low as 10−^2 Torr, while in molecular distillation pressures

of 10−^3 Torr, that is, a mTorr, are used. Another useful distinc-

tion between the methods of vacuum distillation can be made

with reference to the way in which the vapor phase is formed:

(a) Ebullition, (b) evaporative distillation, and (c) molecular

distillation.

Ebullition is accompanied by the formation of bubbles when

the saturated vapor pressure exceeds the pressure of the sur-

rounding gas. The only limit to the rate of evaporation in this

case is the rate at which heat can be transferred to the liquid.

Evaporative distillation occurs when the pressure of the sur-

rounding gas is higher than the vapor pressure of the liquid at a

given temperature, so that bubbles are not formed. The rate of

evaporation is controlled by the temperature of the liquid and

the conditions above the liquid surface. In molecular distilla-

tion, the rate of evaporation is controlled by the rate at which

the molecules escape from the free surface of the liquid and