Agitated Batch Crystallizer Working Principle and Uses | Agitated Tank Crystallizer

It is also known as a stir-tank crystallizer Or agitated batch crystallizer. This is the simplest and most economical unit. In this crystallizer, supersaturation is generated by cooling so it is a cooling crystallizer.

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This type of crystallizer is commonly employed in small-scale production or batch processing due to several advantages, such as low initial cost, simplicity in construction, and flexibility. These have capacities more than tank crystallizers.

Construction

The agitated tank crystallizer consists of a cylindrical tank provided with a low-speed agitator and a cooling coil. The tank has a conical bottom through which the product is withdrawn. The agitator improves the heat transfer rate keeps the temperature of the solution uniform and keeps the fine crystals in suspension which is essential for the uniform growth of the crystals, and simplicity of operation. 

Working Principle

A known quantity of hot solution is charged to the crystallizer, cooling is applied by circulating a coolant through the coil, and the agitator is started. The mass in the crystallizer cools due to heat transfer to the circulated coolant and as the temperature decreases, crystals are formed due to a decrease in the solubility of the solute. The mass is cooled to a pre-decided temperature and finally, a product stream containing crystals plus mother liquor is withdrawn from the bottom of the crystallizer. 

One of the drawbacks of this crystallizer is that the solids deposited on the surface of the coil add resistance to heat transfer so that it ceases to function efficiently. High supersaturation at a cooling surface is unavoidable and because of this, the cooling surface gets rapidly fouled with adhering crystals so the surface might therefore require frequent washing and scrapping. Other disadvantages include difficulty in controlling nucleation and size of crystals and high labour costs. 

This type of crystallizer is used to produce fine chemicals, pharmaceutical products, and dye intermediates.

Agitated Batch Crystallizer Uses

Agitated batch crystallizers are widely used in the chemical, pharmaceutical, and food industries for the controlled precipitation and crystallization of solids from a liquid solution. These crystallizers utilize mechanical agitation to promote crystal growth and control the size, shape, and quality of the crystals produced. 

1. Agitated batch crystallizers are used to crystallize active pharmaceutical ingredients (APIs) from solution. This is crucial for obtaining pure and stable drug crystals with specific properties for drug formulation.

2. They are used to produce high-purity chemical compounds, including speciality chemicals and fine chemicals, through crystallization from solution.

3. In the production of sugar and confectionery products, agitated batch crystallizers are used to control sugar crystallization for desired product textures and qualities.

4. Agitated crystallizers are used in the production of paraffin wax from crude oil fractions, where controlled crystallization is crucial for product quality.

5. In hydrometallurgical processes, agitated batch crystallizers are used for the recovery of valuable metals from solution by crystallization.

6. Crystallizers are used to produce the salts necessary for detergent formulations.

7. In the production of fertilizers like ammonium sulfate, crystallization is employed to obtain the final product in a solid crystalline form.

8. Agitated batch crystallizers are used in research and development laboratories to study and optimize crystal growth processes, aiding in the development of new materials and formulations.

9. They are used in the production of food additives, flavourings, and colourants, where precise control over crystal size and morphology is important.

Agitated batch crystallizers are valuable tools in industries where the control of crystal size, shape, and purity is essential for product quality and performance. 

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