Fractional Crystallization Process: A Detailed Explanation

Fractional crystallization is a separation process used in chemistry to separate a mixture of different substances based on their differing solubilities in a solvent. This process is particularly useful for purifying substances and obtaining components in a more concentrated and pure form.

The crystallization process is an operation in which solid particles are formed from a liquid. It is a solid-liquid operation used to separate solutes from solutions in the form of crystals. In this operation, mass is transferred from the liquid phase to the solid phase/crystal surface. 

Crystallization is an important operation in the chemical industry because the number of soluble products (from plants) has to be in the form of crystals (solid particles). 

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It may be carried out at relatively low temperatures and on a scale ranging from a few grams to thousands of tons per day. 

Crystallization usually involves

(i) concentration of a solution (by evaporating a part of the solvent) and

(ii) cooling of solution until the concentration of the solute becomes higher than its solubility at the prevailing temperature. The solute then comes out of the solution in the form of a pure crystal.

Classification of Fractional Crystallisation

Crystallizers may be classified based on the mode of operation. These may be operated batch-wise or continuously. 

(1) Batch crystallizer: stir-tank crystallizer

(2) Continuously crystallizer: Swenson-Walker crystallizer. 

Crystallizers may also be classified according to the method by which supersaturation is achieved and adopted in the actual practice. Thus, agitated thank crystallizers and Swenson-Walker crystallizers are examples of crystallizers wherein supersaturation is achieved by cooling (or temperature reduction) which is a usual practice for materials in which the solubility decreases with temperature decreases. 

Krystal crystallizer is an example of an evaporative crystallizer wherein supersaturation is achieved by evaporating a part of the solvent and is a usual practice for materials whose solubility remains almost constant with temperature variation. 

Vacuum crystallizers are an example of crystallizers wherein supersaturation is achieved by adiabatic evaporation and cooling which is most suitable for heat-sensitive materials. 

This is used for large-scale production in which supersaturation is achieved by introducing the hot solution into a vacuum in which pressure is less than the vapor pressure of the solvent at the temperature at which it is fed, the solvent thus flashes or evaporates and the solution is cooled adiabatically. 

Salting out with the help of a third substance is not often in use. An indirect application of the salting method is found in the evaporation of glycerin soap lyes. 

In this case, the presence of glycerin in high concentration reduces the solubility of the solute, so that as the concentration of the very soluble component increases, the solubility of the less soluble component (in this case sodium chloride) decreases to a point where it crystallizes out. At present, the deliberate introduction of a foreign substance to decrease solubility is rarely found. 

Classification is based on the method of achieving supersaturation. 

(1) Supersaturation by cooling alone

(a) Batch - Agitated tank crystallizers

(b) Continuous - Swenson-Walker crystallizes

(2) Supersaturation by adiabatic evaporation and cooling

(a) Vacuum crystallizers with and without external classifying seed bed

(3) Supersaturation by evaporation

(a) Krystal crystallizers

(b) Draft-tube crystallizers

Crystallizers may also be classified according to the method of suspending the growing product crystals, as (a) where suspension is agitated in a tank, (b) it is circulated through heat exchange (c) it is circulated through a scraped surface exchanger. 

Mechanism of Crystallization/Crystal Formation

Knowledge of the mechanism by which crystals form and grow is required in the design and operation of equipment used for crystallization. The formation of a crystal from a solution is a two-step process. 

The first step is called nucleation (the birth of new small particles or nuclei) and the second one is called crystal growth.

The generation of a new solid phase either on an inert material in the solution or in the solution itself is called nucleation. The increase in the size of these nuclei with a layer-by-layer addition of solute is called crystal growth. Supersaturation is the common driving force for nucleation and crystal growth. Crystals can neither form nor grow unless a solution is supersaturated. 

The number of nuclei and rate of growth of crystals does depend upon the temperature of operation. The number of nuclei and growth rate of crystals increases with an increase in temperature up to a certain point and then decreases. The temperature corresponding to a maximum number of nuclei formation is different than the temperature at which a growth rate is maximum.

Fractional Crystallization Process

Fractional crystallization is a separation process used to separate a mixture of different substances based on their differing solubilities in a solvent. This method is often employed in chemistry and various industries for purification and separation purposes. 

1. Selection of Solvent

Choose a suitable solvent in which the substances to be separated have different solubilities at different temperatures. The choice of solvent is critical to the success of the process.

2. Dissolving the Mixture

Dissolve the mixture containing the desired and undesired components in the selected solvent at an elevated temperature. At this point, both components should be fully dissolved.

3. Cooling

Gradually cool the solution. As the temperature decreases, the solubility of the substances in the solvent generally decreases as well.

4. Crystallization

Crystals of the substance with lower solubility will start to form as the solution cools. These crystals are separated from the remaining liquid. 

5. Isolation

Filter or decant the liquid to separate the formed crystals from the remaining solution. This liquid may contain some impurities and a substance with higher solubility.

6. Repetition

If necessary, the process can be repeated multiple times, with each cycle of cooling and crystallization yielding purer crystals of the desired substance.

7. Drying

The collected crystals are typically dried to remove any remaining solvent, leaving behind the purified substance.

Fractional crystallization is a powerful technique for separating mixtures of substances with different solubilities, and it's commonly used in various industries, including chemical manufacturing, pharmaceuticals, and the production of high-purity materials. 

Take these Notes is, Orginal Sources: Unit Operations-II, KA Gavhane