According to the different ways of solid precipitation, crystallization can be divided into various types such as solution crystallization, melt crystallization, sublimation crystallization, and precipitation crystallization. The most widely used method in industry is solution crystallization, which is achieved by cooling or removing solvents to achieve a solution that
In a saturated state, precipitate solutes as products. In addition, crystallization operations can also be divided into intermittent and continuous based on whether the operation is continuous, or into stirred and non stirred based on the presence or absence of a stirring device.
1. Method of solution crystallization
Solution crystallization refers to the process in which crystals precipitate from a solution. The basic condition for solution crystallization is the supersaturation of the solution, which generally goes through the following process: unsaturated solution → saturated solution → supersaturated solution → formation of crystal nuclei → crystal growth.
1. Cooling method
Cooling method, also known as cooling method, is a method of achieving supersaturation of a solution by cooling down. Cooling crystallization basically does not remove the solvent, but reduces the temperature by removing the heat from the solution, allowing the solution to reach a supersaturated state and proceed with crystallization. This method
Suitable for situations where solubility significantly decreases with decreasing temperature. Cooling can be divided into natural cooling, wall cooling, and direct contact cooling. The natural cooling method is to cool and crystallize a solution in the atmosphere, and its equipment structure and operation are the simplest, but the cooling rate is the same
Slow, low production capacity, and difficult to control crystal quality. Wall cooling method is a widely used crystallization method in industry, which relies on indirect heat transfer and cooling of crystallization through jackets or tube walls. This method consumes less energy and is widely used, but the cooling heat transfer rate is low and cold
However, crystals often precipitate on the wall surface, forming crystal scale or scars on the device wall, which affects the cooling effect. Direct contact with the cooler to cool with air or refrigerant in direct contact with the solution. This method overcomes the drawbacks of wall cooling, has high heat transfer efficiency, and is not complicated
Scar problem, but the equipment is bulky; When using this operation, it is important to note that the selected cooling medium should not be miscible with the solvent in the crystallization mother liquor or, although miscible, should be easy to separate and not contaminate the crystallization product.
2. Evaporation method
The evaporation method is a method of crystallization that achieves supersaturation of a solution by removing some solvents, and is suitable for situations where solubility does not change significantly with temperature. Evaporative crystallization consumes more energy and also has the problem of easy scaling on the heating surface, but it is not conducive to
The crystallization process of solvent recovery is still cost-effective. Evaporative crystallization equipment is often operated under low vacuum pressure in order to lower the operating temperature, facilitate the stability of thermosensitive products, and reduce thermal energy loss.
3. Vacuum cooling method
Vacuum cooling method, also known as flash cooling crystallization method. It is a crystallization method in which a solvent undergoes flash evaporation under vacuum conditions to adiabatic cool the solution. Essentially, it combines cooling and evaporation methods simultaneously. This method is applicable as the temperature rises
Substances with high solubility that increase at a moderate rate, such as ammonium sulfate, potassium chloride, etc. The main equipment of this method is simple, without heat exchange walls, with fewer crystal scars, and can take a longer maintenance time. The corrosion prevention problem of the equipment is also easy to solve, making it the first choice in large-scale crystallization production
Method.
4. Salt precipitation method
The salt precipitation method is a method of establishing supersaturation for crystallization by adding a certain substance to the solution to reduce the solubility of the solute in the solvent. The added substance is called a salt precipitation agent or precipitant, and it is required to be miscible with the original solvent, but not soluble
The substance to be crystallized requires easy separation between the added substance and the original solvent. The reason why it is called salt precipitation method is because sodium chloride is the most common additive. For example, in the combined alkali production method, adding sodium chloride to a low-temperature ammonium chloride solution can make the solution
The ammonium chloride crystallized out. Water, alcohols, and ketones can also be used as additives to cause salt crystallization in certain solutions, sometimes also known as solution crystallization. The salt precipitation process is simple and easy to operate, suitable for the crystallization of thermosensitive materials and drug crystallization; The disadvantage is that it often requires
Set up recycling equipment to process crystallization mother liquor, in order to recover solvents and salt precipitation agents.
5. Reactive crystallization
Reaction crystallization is the use of chemical reactions between gases and liquids or liquids and liquids to produce products with low solubility. This situation is a combination of reaction and crystallization processes. As the reaction progresses, the concentration of reaction products increases and reaches supersaturation
In solution, crystal nuclei are generated and gradually grow into larger crystal particles. In addition, there are pressure crystallization and isoelectric point crystallization methods that reduce solubility by changing pressure or controlling pH.
2.
Crystallizer There are many types of crystallizers, which can be divided into cooling crystallizers and evaporating crystallizers according to the method of obtaining saturation state of the solution; According to the flow mode, it can be divided into mixed slurry crystallizer, graded crystallizer, mother liquor circulation crystallizer, and slurry circulation crystallizer; By Yes
Non stirred crystallizers are divided into stirred crystallizers and non stirred crystallizers; According to the operation mode, it can be divided into continuous crystallizer and intermittent crystallizer.
1. Cooling crystallizer
1) Air cooled crystallizer
Air cooled crystallizer is the simplest open crystallization tank, which cools in the atmosphere and gradually lowers the temperature in the tank, while a small amount of solvent vaporizes. Due to the intermittent operation and slow cooling, salts containing polycrystalline water often
High quality and large crystals can be obtained. But it occupies a large area and has low production capacity.
2) Kettle crystallizer
The cooling required for the crystallization process is supplied by a jacket or an external heat exchanger, and the choice of crystallizer mainly depends on the demand for heat exchange capacity. Currently, the widely used ones include internal circulation cooling crystallizers with stirring and external circulation cooling crystallizers
Device, as shown in the following figure. The external circulation cooling crystallizer can be operated intermittently or continuously. If producing large particle crystals, intermittent operation is recommended, while continuous operation is better for preparing small particle crystals. External loop operation can strengthen the structure
The uniform mixing and heat transfer inside the crystal have the advantages of a large cooling heat exchanger area and a high heat transfer rate, which is conducive to the control of solution supersaturation. However, it is necessary to choose a suitable circulation pump to avoid the wear and breakage of suspended particle crystals.
2. Evaporative crystallizer
1) Krystal Olso growth type evaporative crystallizer
Krystal Olso growth type (forced circulation type) evaporative crystallizer, which consists of an evaporation chamber and a crystallization chamber. The evaporation chamber is located above and the crystallization chamber is located below, connected by a central downcomer in the middle. The body of the crystallization chamber is equipped with
A certain taper, with a small lower section and a larger upper section. After the raw material liquid is preheated by an external heater, it enters the evaporation chamber through a recirculation tube and is quickly evaporated. The solvent is extracted and the solution is cooled, causing the solution to quickly enter the metastable zone and precipitate in the crystallization chamber
Produce crystals. Larger crystal particles are enriched at the bottom of the crystallization chamber, and the supersaturation of the solution flowing out of the downcomer gradually decreases. When the solution reaches the top layer of the crystallization chamber, there are basically no grains left, and the supersaturation is completely consumed. The clear mother liquor crystallizes
The overflow from the top of the room enters the circulation pipeline. This operation method is a typical mother liquor circulation type, which has the advantage that the circulating liquid basically does not contain crystal particles, thereby avoiding excessive secondary nucleation caused by collision between the pump impeller and the grains, as well as crystallization
The particle size grading effect of the room produces crystalline products with large and uniform particles. The disadvantage of this crystallizer is its low operating flexibility, limited circulation of mother liquor by the settling speed of product particles in saturated solution, and easy formation of the inner wall surface of the heating tube in the crystallizer
Crystal scale causes a decrease in the heat transfer coefficient of the heat exchanger
2) DTB type evaporative crystallizer
DTB type (also known as shielded type) evaporative crystallizer. It can be used in conjunction with evaporative heaters or separated from heaters. The crystallizer is currently the most commonly used type as a vacuum evaporative cooling crystallizer. Its characteristic is steaming
There is a guide tube in the generator room, which is equipped with a stirrer with a propeller. It quickly pushes the saturated solution with small crystals to the evaporation surface. Due to the vacuum state of the system, the solvent produces flash evaporation, resulting in mild supersaturation, and then
When a saturated solution flows downwards along the annular area, its supersaturation is released, allowing the crystal to grow. There is a grading leg at the bottom of the device, and the extracted product slurry needs to pass through it first, mix with the raw material liquid, and then circulate through the central guide pipe. Crystal growth
After reaching a certain size, it precipitates in the grading legs, and the product is also washed. Finally, it is separated outside the crystal slurry pump to ensure the quality and uniform particle size of the crystalline product, so that the product is not mixed with fine crystals.
DTB type crystallizer is a typical slurry internal circulation crystallizer with excellent performance, high production intensity, and the ability to produce large particle crystalline products. It is not easy to scale inside the crystallizer and has become one of the main forms of continuous crystallizer, which can be used for vacuum cooling and evaporation methods
Crystallization and reaction crystallization operations.
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