In crystallization operations, various factors should be considered, including the properties of the system being processed, the influence of impurities, the required particle size and size distribution of the product, the processing volume, energy consumption, equipment costs, and operating costs. Which crystallization equipment should be selected Selecting a crystallizer is a complex task with no simple rules; it largely relies on practical experience. Multiple factors should be considered comprehensively to ensure an efficient crystallization process and that the product quality meets requirements. The following are some key selection principles and considerations:

I. System Properties

First, understand the solubility, temperature dependence, and possible chemical reaction characteristics of the material to determine the most suitable crystallizer type. The relationship between solubility and temperature is the first consideration. The solutes to be crystallized fall into two main categories: the first category shows a large decrease in solubility with decreasing temperature; the second category shows a very small decrease in solubility with decreasing temperature or exhibits inverse solubility. For the first type of solute, a cooling crystallizer or vacuum crystallizer can be selected. For the second type of substance, an evaporative crystallizer is usually required; a salting-out crystallizer can also be used for certain specific substances.

II. Crystal Product Requirements

• Particle size and distribution: If there are strict requirements for crystal size, a crystallizer that can control particle size and distribution, such as a classified crystallizer, should be selected.

• Shape and purity: Consider the final shape and purity requirements of the crystals and select equipment that is conducive to the growth of specific morphologies.

To obtain larger and more uniform crystals , a crystallizer with particle size classification or a mixed crystallizer with product classification and discharge can be selected. The crystals produced by this type of crystallizer are also easy to filter, wash, and dry, thus obtaining a purer crystalline product.

III. Process Conditions

• Temperature control: The crystallization process is sensitive to temperature, and the equipment must be able to precisely control the crystallization temperature.

• Stirring and mixing: Appropriate stirring can promote mass and heat transfer and prevent uneven crystallization caused by local supersaturation.

IV. Equipment Type

• Common crystallization equipment includes OSLO crystallizers, DTB crystallizers, and FC crystallizers, each type suitable for materials and process requirements with different characteristics.

• For specific conditions, such as corrosive systems, equipment made of corrosion-resistant materials should be selected.

V. Operating Efficiency, Economy, and Site Conditions

• Production capacity: The size and design of the equipment should meet the needs of the production scale.

• Energy consumption and cost: Consider the energy consumption, maintenance cost, and initial investment of the equipment. High-efficiency and energy-saving equipment, such as MVR (mechanical vapor recompression) evaporative crystallization equipment, is more suitable in some cases.

• The existing space, layout, and infrastructure (such as energy supply) of the factory are also factors that cannot be ignored when selecting equipment.

Cost and floor space are also important factors to consider. Generally speaking, continuous operation crystallizers are more economical than batch operation crystallizers, especially when the output is large. Evaporative and vacuum crystallizers require considerable headroom, but for the same output, their floor space is much smaller than that of cooling tank crystallizers.

Some simpler cooling crystallizers, especially open ones, are relatively inexpensive, but the cost of mechanical cooling crystallizers (such as long-trough stirred type) is quite high, and their maintenance costs are also considerable. On the other hand, mechanical crystallizers do not require expensive vacuum generation equipment. Another disadvantage of cooling crystallizers is that their heat transfer surface often has crystals agglomerated into crystal scars on the side in contact with the solution, and scale is easily deposited on the side in contact with the cooling water, resulting in both reduced cooling efficiency and increased trouble in removing scars and scale. This type of problem is also encountered in evaporative crystallizers. As for vacuum crystallizers, they do not have a heat transfer surface, so they do not have this problem, but they are not suitable for solutions with a large increase in boiling point, such as the crystallization of sodium hydroxide solution, and the mother liquor temperature should not differ too much to avoid excessive supersaturation and an increase in the number of nuclei.

VI. Post-processing Convenience : Select crystallization equipment that is easy for subsequent filtration, washing, and drying operations to simplify the process and reduce costs.

VII. Special Needs: For systems with special solubility properties or those requiring the generation of solid precipitates through chemical reactions, dedicated crystallization reactors may be needed.

VIII. Environmental factors: Consider the treatment of waste generated during the crystallization process and environmental requirements, and select environmentally friendly equipment.

In summary, the selection of crystallization equipment is a complex process that requires a comprehensive evaluation based on material properties, process objectives, economic feasibility, and technical feasibility.