Crystallisation

Objective

Crystallisation is the formation of solid crystals from a solution. Crystals solidify in a definite geometric form. The objective of crystallisation is to separate a solute from a solvent. Any impurities in the liquid are usually not incorporated into the lattice structure of the desired crystal. Accordingly crystallisation is also a purification process.

Field of application

Crystallisation is applied in the sugar industry and in the dairy industry (where lactose is produced from cheese whey or casein whey).
Crystallisation (fractionation) is also used in the edible oil industry, to modify the properties of edible oils and fats.

Description of the technique, methods and equipment

Crystals are usually grown by the introduction of nuclei into a super-saturated solution.

a) Crystallisation of sugar in sugar factories

The crystallisation process takes place in vacuum pans in which the juice is boiled under vacuum in order to minimise the temperatures involved. The growth of the sucrose crystal only involves sucrose and water. The non-sugars contained in the sugar juice are not incorporated into the crystal structure, instead most of them remain in the liquid phase while some are released to the vapour phase. The sugar crystals are removed from the liquid phase by centrifugation.

b) Crystallisation of lactose in whey processing

For the production of lactose from whey, whey is normally evaporated to a supersaturated solution (total solids content 60 to 73%). In cooling down the solution, crystallisation starts and the crystals begin to grow. The crystals are removed from the liquid phase by centrifugation. Depending on the required grade, further purification (refining) can take place by washing the crystals, or redissolving them and recrystallising them, followed by treatment with active carbon for the removal of any impurities.

c) Fractionation of edible oils and fats

Objective

Fractionation is based on the principle that the solubilities of the higher melting components in the liquid phase change at different temperatures. This difference can be extended by using an organic solvent which has the effect of decreasing the viscosity and leading to better washing of the crystals.

Description of the technique, methods and equipment

The equipment includes tanks for pre-heating, stirred and cooled tanks for crystallisation, band or membrane filters for the separation of the crystals from the liquor, and distillation vessels for solvent recovery. The oil is heated to 10°C above the melting point of the highest tricylclycerol present, to give a fully liquid starting-material (e.g. the heating point is typically 75°C for palm oil). The molten oil is then cooled and stirred to form crystal nuclei, and the temperature is maintained at a lower temperature to induce crystal growth (typically 12 hours at 28 - 30°C for palm oil). If a solvent is used, it is added to the molten oil prior to cooling. The mixtures containing the crystallised solids and the dissolved liquids are separated by filters. If a solvent is used, it is removed from the fractions by distillation.

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