Unit Operations in Food Processing

Major uses of distillation in the food industry are for concentrating essential oils, flavours and alcoholic beverages, and in the deodorization of fats and oils.

The equilibrium relationships in distillation are governed by the relative vapour pressures of the mixture components, that is by their volatility relative to one another. The equilibrium curves for two-component vapour-liquid mixtures can conveniently be presented in two forms, as boiling temperature/concentration curves, or as vapour/liquid concentration distribution curves. Both forms are related as they contain the same data and the concentration distribution curves, which are much the same as the equilibrium curves used in extraction, can readily be obtained from the boiling temperature/concentration curves.

A boiling temperature/concentration diagram is shown in Fig. 9.10. Notice that there are two curves on the diagram, one giving the liquid concentrations and the other the vapour concentrations.

If a horizontal (constant temperature) line is drawn across the diagram within the limit temperatures of the two curves, it will cut both curves. This horizontal line corresponds to particular boiling temperature, the point at which it cuts the lower line gives the concentration of the liquid boiling at this temperature, the point at which it cuts the upper line gives the concentration of the vapour condensing at this same temperature. Thus the two points give the two concentrations which are in equilibrium. They give in fact two corresponding values on the concentration distribution curves, the point on the liquid line corresponding to an x point (that is to the concentration in the heavier phase) and the point on the vapour line to a y point (concentration in the lighter phase). The diagram shows that the y value is richer in the more volatile component of the mixture than x, and this is the basis for separation by distillation.

It is found that some mixtures have boiling-point diagrams that are a different shape from that shown in Fig. 9.10. For these mixtures, at another particular temperature and away from the pure components at the extremes of composition, the vapour and liquid composition lines come together. This means that, at this temperature, the liquid boils to give a vapour of the same composition as itself. Such mixtures constitute azeotropes and their formation limits the concentration attainable in a distillation column. The ethanol-water mixture, which is of great importance in the alcoholic beverage industry, has a minimum boiling-point azeotrope at composition 89.5 mole% (95.6% w/w) ethanol and 10.5 mole% (4.4% w/w) water, which boils at 78.15°C. In a distillation column, separating dilute ethyl alcohol and water, the limit concentrations of the streams are 100% water on the one hand in the "liquid" stream, and 95.6% ethyl alcohol, 4.4% water by weight in the "vapour" stream, however many distillation stages are used.

A multi-stage distillation column works by providing successive stages in which liquids boil and the vapours from the stage above condense and in which equilibrium between the two streams, liquid and vapour, is attained. Mass balances can be written for the whole column, and for parts of it, in the same way as with other contact equilibrium processes.

EXAMPLE 9.13. Distillation of alcohol/water mixture
In a single-stage, continuous distillation column used for enriching alcohol/water mixtures, the feed contains 12% of alcohol, and 25% of the feed passes out with the top product (the "vapour" stream) from the still. Given that, at a boiling temperature of 95.5°C, 1.9 mole% of alcohol in the liquid is in equilibrium with 17 mole% of alcohol in the vapour, estimate the concentration of alcohol in the product from the still.

From the equilibrium data given and since the mole fraction of alcohol is small we may assume a linear equilibrium relationship. The equilibrium curve passes through (0,0) and (1.9, 17) so that over this range we can say y = x(17/1.9) or x = y(1.9/17).

From the operating conditions given, as the feed is equal to liquid + vapour phases,(L + V) we can write:

Continuous fractional distillation columns can be analysed in rather similar ways to continuous extraction systems, They generally have a reboiler at one end of a column and a condenser at the other (head). A feed stream normally enters somewhere away from the end points of the column and there is often provision of reflux which is a distillate return flow from the condenser section at the head of the column. Full analysis of such columns can be found in standard chemical engineering texts.

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 to 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 (P - p_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

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, for example from edible fats and oils.

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.

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.

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:

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.

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 Fig. 9.11.

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. 9.11(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.

Contact-Equilibrium Processes - SUMMARY, PROBLEMS