Industrial Crystallization The process not only requires the product to have high purity and high yield, but also often specifies the crystal form, crystal size, and particle size range. Crystals with large particles and uniform particle size are not only easy to filter and wash, but also greatly reduce the agglomeration phenomenon during storage. The operation mode and control adjustment method of the crystallization equipment are of great significance to maintaining the normal operation of the crystallization equipment and have an important impact on the crystal quality.

I. Crystallization Operation Methods
In the crystallization production process, batch crystallization and continuous crystallization Two different operation methods can be used. When the production scale reaches a certain level, it generally adopts continuous operation . However, sometimes considering the simplicity of operation, intermittent operation can still be reasonably adopted. At present, computer-aided control and operation methods are widely used in the processes of batch crystallization and continuous crystallization. To control the supersaturation level in the crystallizer during batch operation, so that the nucleation and scaling of crystals are reduced to a minimum; for continuous crystallization processes, continuous control of fine crystal elimination is used to stabilize the crystal size.
1.1 Continuous Crystallization
Continuous crystallization operation has the following advantages:
(1) When cooling and evaporation crystallization (except vacuum cooling) adopts continuous operation, the economic effect is good and the operating cost is low.
(2) Compared with batch operation, continuous operation crystallizers have high production capacity and small footprint.
(3) The operating parameters of continuous crystallization are relatively stable compared to batch crystallization.
(4) When the production scale is large, continuous operation can save labor.
(5) The mother liquor of continuous crystallization operation can be fully utilized, and only about 7% of the mother liquor needs to be reprocessed; while batch operation requires 20%~40% of the mother liquor to be reprocessed.
At the same time, continuous crystallization operation also has disadvantages that cannot be ignored:
(1) Compared with well-controlled batch crystallization operation, the average particle size of the product obtained by continuous operation is smaller.
(2) The operation is difficult, and the operators require a high level and rich experience.
(3) Crystal scale is easy to form on the heat exchange surface and the wall in contact with the liquid level, and it continues to accumulate. Therefore, it cannot operate continuously and needs to be shut down regularly for cleaning.
Generally, the operation method of crystallization production is determined according to the amount of material liquid to be processed, the characteristics of the crystalline substance, and the specific conditions of production.
If the crystal growth rate is slow, it is relatively easy to control with batch operation. Batch operation does not have a lower limit of production, but if the amount of material liquid to be processed is greater than 20 m²/h, it is best to choose continuous crystallization operation.

1.2 Batch Crystallization
Batch operation is widely used in small and medium-scale crystallization processes. Its advantages are simple operation and easy control. The crystallization process uses computer-aided control and operation methods to achieve the optimal operation time, that is, continuously adjust its operating parameters according to a certain operating procedure, control the supersaturation in the crystallizer, and reduce the nucleation and scaling of crystals to a minimum.
Batch crystallization operation has two crystallization situations: adding seed crystals and not adding seed crystals. The results can be expressed by the solubility-supersolubility curve.
Figure (a) shows the case of rapid cooling without adding seed crystals. At this time, the state of the solution quickly passes through the metastable zone and reaches a certain point on the supersolubility curve, and primary nucleation occurs. A large number of tiny nuclei suddenly appear in the solution, which belongs to uncontrolled crystallization.

Figure (b) shows the case of slow cooling without adding seed crystals. At this time, the state of the solution will also pass through the metastable zone and reach the supersolubility curve, producing more nuclei. After the supersaturation is consumed due to nucleation, the state of the solution immediately leaves the supersolubility curve, and no more nuclei are generated. Due to crystal growth, the supersaturation decreases rapidly. This method has limited control over the crystallization process, because the primary nucleation rate increases significantly with the increase of supersaturation, and the amount of nuclei generated is impossible to meet the needs, so the particle size range of the obtained crystals is often very wide.

Figure (c) shows the case of rapid cooling with seed crystals. Once the state of the solution crosses the solubility curve, the seed crystals begin to grow, and because the solute crystallizes out, the concentration of the solution in the metastable zone decreases; however, due to rapid cooling, the solution can still quickly reach the unstable zone, so fine nuclei will inevitably be produced.

Figure (d) shows the case of slow cooling with seed crystals. Because there are seed crystals in the solution, and the cooling rate is controlled, the solution remains in the metastable state during the operation and does not enter the unstable zone, and primary nucleation does not occur. Moreover, the crystal growth rate is completely controlled by the cooling rate. This "controlled crystallization" operation method can produce uniform crystals with predetermined particle size and quality requirements.
Many industrial-scale batch crystallization operations use a controlled crystallization operation method with seed crystals. The amount of seed crystals added depends on the mass of solute crystallized in the entire crystallization process, the particle size of the variety, and the particle size of the product. For example, in the sugar industry, microcrystals as small as 5 μm are used as seed crystals in the crystallization process of sucrose, and adding 500 g of such seed crystals to every 50 m³ of syrup is sufficient.

Batch crystallization operation, on the premise of obtaining high-quality crystal products, also requires shortening the operation time as much as possible to obtain as many products as possible. For different crystal systems, a suitable operating procedure should be determined so that a constant maximum allowable supersaturation can be maintained throughout the batch crystallization process, so that the crystals can grow at the specified rate. If the supersaturation exceeds the value, it will affect the product quality; if it is lower than this value, it will reduce the production capacity of the equipment. Although there are only a very small number of crystal surfaces provided by seed crystals in the system, the low energy transfer rate (solvent evaporation rate or solution cooling rate) is sufficient to form a huge supersaturation, causing the operation to deviate from the normal state. As the crystals grow, the crystal surface area increases, and the energy transfer rate can be gradually increased accordingly.

II. Factors Affecting Crystallization Operation and Control Adjustment
The basic control methods for crystallization operations require that the crystallizer operates stably, produces crystal products that meet the requirements of particle size and purity, improves production intensity, reduces energy consumption, reduces fine crystals and scaling, and extends the normal operation cycle of the equipment.

1. Controlling supersaturation The relevant process parameters affecting supersaturation must be strictly controlled. For example, in continuous crystallization operations, when fine crystals appear, the supersaturation should be lowered to prevent the generation of new nuclei; after the fine crystals are removed, it can be adjusted to the upper limit of the specified range to improve crystallization yield as much as possible.

2. Temperature control The supersaturation of the cooling crystallization solution mainly relies on temperature control, and the solution temperature should often operate stably along the optimal conditions. The solution temperature is adjusted by the coolant, so the coolant temperature should be strictly controlled.

3. Pressure control The operating pressure of the vacuum crystallizer directly affects the temperature, so the operating pressure must be strictly controlled. The supersaturation of the evaporative crystallization solution is mainly controlled by the pressure of the heating steam, and the flow rate of the heating steam is an important control index for this type of crystallizer.

4. Controlling the solid-liquid ratio of the slurry When the solvent is removed by vaporization, the supersaturation of the mother liquor in the vacuum crystallizer and the evaporative crystallizer increases rapidly, and it is necessary to add slurry containing particles to make the increased supersaturation disappear as soon as possible. The disappearance of the supersaturation of the mother liquor requires a certain crystal surface area. When the solid-liquid ratio of the slurry is high, the crystal surface area is large, and the supersaturation disappears more completely, which not only allows the existing crystals to grow, but also reduces fine crystals and prevents scaling.

5. Slow control, stable operation This is a significant characteristic of crystallization operation and an important condition for preventing nucleation.

6. Preventing scaling and fouling.

III. Matters to Note During Crystallization

1. Maintain a stable supersaturation to prevent the crystallizer from generating excessive supersaturation in local areas (such as the evaporation surface, cooling surface, and mixing area of two fluids with different concentrations).

2. The liquid level of the crystallizer should be maintained at a certain height. If the liquid level is too low, it will destroy the suspended bed layer, causing the supersaturation to exceed the metastable zone and generate a large number of nuclei.

3. Air entrainment should be prevented, otherwise it will destroy the slurry bed layer, causing the liquid level to surge and serious overflow.

4. The growth rate of crystals should be limited, that is, do not use the method of blindly increasing supersaturation to achieve the purpose of increasing yield.

5. Pre-treat the solution by heating, filtration, etc., to eliminate particles in the solution that may become excessive nuclei.

6. Remove excess fine crystals from the crystallizer in time. The product is discharged according to particle size grading, so that crystals that meet the particle size requirements can be discharged as products in time, and they will not continue to participate in the cycle in the device.

7. After the mother liquor containing excess fine crystals is removed, it is heated or diluted to dissolve the fine crystals, and then sent back to the crystallizer.

8. The temperature difference of the mother liquor should not be too large to avoid excessive supersaturation and an increase in nuclei.

9. Adjust the pH of the feed liquid or add some selective additives to change the nucleation rate.

10. Operators should be responsible and diligent in checking the crystallization operation, stabilizing the process, and ensuring that production is carried out under optimal conditions.