FOOD ENGINEERING

Module 5. Food processing equipments and unit operations

Lesson 44
DISTILLATION

44.1 Introduction

Distillation is a separation process, separating components in a mixture by making use of the fact that some components vaporize more readily than others. When vapours are produced from a mixture, they contain the components of the original mixture, but in proportions which are determined by the relative volatilities of these components. The vapour is richer in some components, those that are more volatile, and so a separation occurs.

In fractional distillation, the vapour is condensed and then re-evaporated when a further separation occurs. It is difficult and sometimes impossible to prepare pure components in this way, but a degree of separation can easily be attained if the volatilities are reasonably different. Where great purity is required, successive distillations may be used. The following types of distillation processes are in use

44.2 Steam Distillation

In some circumstances in the food industry, distillation would appear to be a good separation method but it cannot be employed directly as the distilling temperatures would lead to breakdown of the materials. In cases in which volatile materials have to be removed from relatively non-volatile materials, steam distillation may sometimes be used to effect the separation at safe temperatures.

A liquid boils when the total vapour pressure of the liquid is equal or more than the external pressure on the system. Therefore, boiling temperatures can be reduced by reducing the pressure on the system; for example by boiling under a vacuum, or by adding an inert vapour which by contributing to the vapour pressure, allows the liquid to boil at a lower temperature. Such an addition must be easily removed from the distillate, if it is unwanted in the product, and it must not react with any of the components that are required as products. The vapour that is added is generally steam and the distillation is then spoken of as steam distillation.

If the vapour pressure of the introduced steam is p_s and the total pressure is P, then the mixture will boil when the vapour pressure of the volatile component reaches a pressure of (Pp_s), compared with the necessary pressure of P if there were no steam present. The distribution of steam and the volatile component being distilled, in the vapour, can be calculated. The ratio of the number of molecules of the steam to those of the volatile component, will be equal to the ratio of their partial pressures -

p_A/p_s = (P - p_s)/p_s = (w_A/M_A)/(w_s/M_s)

and so the weight ratios can be written:

w_A/w_S = (P - p_s)/p_s x (M_A/M_s)

where p_A is the partial pressure of the volatile component, p_s is the partial pressure of the steam, P is the total pressure on the system, w_A is the weight of component A in the vapour, w_s is the weight of steam in the vapour, M_A is the molecular weight of the volatile component and M_s is the molecular weight of steam.

Very often the molecular weight of the volatile component that is being distilled is much greater than that of the steam, so that the vapour may contain quite large proportions of the volatile component. Steam distillation is used in the food industry in the preparation of some volatile oils and in the removal of some taints and flavours, from edible fats and oils.

44.3 Batch Distillation

Batch distillation is the term applied to equipment into which the raw liquid mixture is admitted and then boiled for a time. The vapours are condensed. At the end of the distillation time, the liquid remaining in the still is withdrawn as the residue. In some cases the distillation is continued until the boiling point reaches some predetermined level, thus separating a volatile component from a less volatile residue. In other cases, two or more fractions can be withdrawn at different times and these will be of decreasing volatility. During batch distillation, the concentrations change both in the liquid and in the vapour.

Let L be the mols of material in the still and x be the concentration of the volatile component. Suppose an amount dL is vaporized, containing a fraction y of the volatile component.

Then writing a material balance on component A, the volatile component:

y ΔL = Δ(Lx) = LΔx + xΔL

y ΔL - xΔL = LΔx

(y – x)ΔL = LΔx

ΔL/L = Δx/(y - x)

and this is to be integrated from L₀ moles of material of concentration x₀ up to L moles at concentration x.

To evaluate this integral, the relationship between x and y, that is the equilibrium conditions, must be known.

If the equilibrium relationship is a straight line, y = mx + c, then the integral can be evaluated:

Log_eL/L_o = 1 Log_e (m - 1)x + c
(m - 1) (m - 1)x_o + c

or

L/L_o = [(y - x)/(y_o - x_o)]^1/(m-1)

In general, the equilibrium relationship is not a straight line, and the integration has to be carried out graphically. A graph is plotted of x against 1/(y - x), and the area under the curve between values of x₀ and x is measured.

44.4 Vacuum Distillation

Reduction of the total pressure in the distillation column provides another means of distilling at lower temperatures. When the vapour pressure of the volatile substance reaches the system pressure, distillation occurs. With modern efficient vacuum-producing equipment, vacuum distillation is tending to supplant steam distillation. In some instances, the two methods are combined in vacuum steam distillation.

44.5 Distillation Equipment

The conventional distillation equipment for the continuous fractionation of liquids consists of three main items: a boiler in which the necessary heat to vaporize the liquid is supplied, a column in which the actual contact stages for the distillation separation are provided, and a condenser for condensation of the final top product. A typical column is illustrated in Figure 44.1 .

The condenser and the boiler are straightforward. The fractionation column is more complicated as it has to provide a series of contact stages for contacting the liquid and the vapour. The conventional arrangement is in the form of "bubble-cap" trays, which are shown in Fig. 44.1 (b). The vapours rise through the bubble caps. The liquid flows across the trays past the bubble caps where it contacts the vapour and then over a weir and down to the next tray. Each tray represents a contact stage, or approximates to one as full equilibrium is not necessarily attained, and a sufficient number of stages must be provided to reach the desired separation of the components.

In steam distillation, the steam is bubbled through the liquid and the vapours containing the volatile component and the steam are passed to the condenser. Heat may be provided by the condensation of the steam, or independently. In some cases the steam and the condensed volatile component are immiscible, so that separation in the condenser is simple.

44.6 Application

a) Manufacture of Whisky

Whisky is a spirit produced by the distillation of a mash of cereals, which may include barley, corn, rye and wheat, and is matured in wooden casks. There are three types of Scotch and Irish whisky: malt whisky produced from 100% malted (germinated) barley, grain whisky produced from unmalted cereal grains and blended whisky which contains 60– 70%grain whisky and 30–40% malt whisky.

b) Manufacture of Neutral Spirit

A multicolumn distillation plant is used for producing neutral spirits from fermented mash. A typical system would be comprised of five columns: a whisky-separating column, an aldehyde column, a product-concentrating column, an aldehyde-concentrating column and a fusel oils concentrating column.The whisky-separating column is fitted with sieve plates, with some bubble cap plates near the top of the column. The other four columns are fitted with bubble cap plates. The fermented mash containing 7% (v/v) of alcohol is fed to near the top of the whisky-separating column. The overhead distillate from this column is fed to the aldehyde column. The bottom product from this column is pumped to the middle of the product-concentrating column. The end product, neutral spirit, is withdrawn from near the top of this column.

c) Recovery of solvents from oil after extraction

Most of the solvent can be recovered by evaporation using a film evaporator. However, when the solution becomes very concentrated, its temperature rises and the oil may be heat-damaged. The last traces of solvent in the oil may be removed by steam distillation or stripping with nitrogen.

d) Concentration of Aroma Compounds from Juices and Extracts

By evaporating 10–30% of the juice in a vacuum evaporator, most of the volatile aroma compounds leave in the vapour. This vapour can be fed to a distillation column. The bottom product from the column is almost pure water and the aroma concentrate leaves from the top of the column. This is condensed and may be added back to the juice or extract prior to drying. Fruit juices and extract of coffee may be treated in this way.

e) Extraction of Essential Oils from Leaves, Seeds, etc.

This may be achieved by steam distillation. The material in a suitable state of subdivision is placed on a grid or perforated plate above heated water. In some cases the material is in direct contact with the water or superheated steam may be used. If the oil is very heat sensitive distillation may be carried out under vacuum.