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Fluid-flow theory
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Heat-transfer theory

separation processes


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Biological raw materials are usually mixtures, and to prepare foodstuffs it may be necessary to separate some of the components of the mixtures.

One method, by which this separation can be carried out, is by the introduction of a new phase to the system and allowing the components of the original raw material to distribute themselves between the phases.
For example, freshly dug vegetables have another phase, water, added to remove unwanted earth; a mixture of alcohol and water is heated to produce another phase, vapour, which is richer in alcohol than the mixture. By choosing the conditions, one phase is enriched whilst the other is depleted in some component or components.
The maximum separation is reached at the equilibrium distribution of the components, but in practice separation may fall short of this as equilibrium is not attained. The components are distributed between the phases in accordance with equilibrium distribution coefficients which give the relative concentrations in each phase when equilibrium has been reached. The two phases can then be separated by simple physical methods such as gravity settling.

This process of contact, redistribution, and separation gives the name contact equilibrium separations. Successive stages can be used to enhance the separation.

An example is in the extraction of edible oil from soya beans. Beans containing oil are crushed, and then mixed with a solvent in which the oil, but not the other components of the beans, is soluble. Initially, the oil will be distributed between the beans and the solvent, but after efficient crushing and mixing the oil will be dissolved in the solvent. In the separation, some solvent and oil will be retained by the mass of beans; these will constitute one stream and the bulk of the solvent and oil the other. This process of contacting the two streams, of crushed beans and solvent, makes up one contact stage. To extract more oil from the beans, further contact stages can be provided by mixing the extracted beans with a fresh stream of solvent.

For economy and convenience, the solvent and oil stream from another extraction is often used instead of fresh solvent. So two streams, one containing beans and the other starting off as pure solvent, can move counter current to each other through a series of contact stages with progressive contacting followed by draining. In each stage of the process in which the streams come into contact, the material being transferred is distributed in equilibrium between the two streams. By removing the streams from the contact stage and contacting each with material of different composition, new equilibrium conditions are established and so separation can proceed.

In order to effect the desired separation of oil from beans, the process itself has introduced a further separation problem - the separation of the oil from the solvent. However, the solvent is chosen so that this subsequent separation is simple, for example by distillation. In some cases, such as washing, further separation of dissolved material from wash water may not be necessary and one stream may be rejected as waste. In other cases, such as distillation, the two streams are generated from the mixture of original components by vaporization of part of the mixture.

The two features that are common to all equilibrium contact processes are the attainment of, or approach to, equilibrium and the provision of contact stages. Equilibrium is reached when a component is so distributed between the two streams that there is no tendency for its concentration in either stream to change. Attainment of equilibrium may take appreciable time, and only if this time is available will effective equilibrium be reached. The opportunity to reach equilibrium is provided in each stage, and so with one or more stages the concentration of the transferred component changes progressively from one stream to the other, providing the desired separation.

Some examples of contact equilibrium separation processes are:
Gas absorption
Extraction and washing
Membrane separations

In addition, drying and humidification, and evaporation can be considered under this general heading for some purposes, but it seemed more appropriate in this book to take them separately.

For the analysis of these processes, there are two major sets of quantitative relationships; the equilibrium conditions which determine how the components are distributed between the phases, and the material flow balances which follow the progression of the components stage by stage.

Contact-Equilibrium Processes - THEORY > CONCENTRATIONS

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Unit Operations in Food Processing. Copyright © 1983, R. L. Earle. :: Published by NZIFST (Inc.)
NZIFST - The New Zealand Institute of Food Science & Technology