The equipment that sends the solution heated to the boiling point into the container for evaporation, concentration and crystallization is called an evaporation crystallizer. Its principle and structure are similar to general evaporators, and are mainly divided into the following types of crystallizers.

1.Vacuum crystallizer

Operation principle: send the hot concentrated liquid into a closed and insulated container, maintain a high degree of vacuum in the container, make the boiling point of the solution in the container lower than the feed temperature, flash evaporation of the concentrated liquid, and heat insulation cool to the container the equilibrium temperature corresponding to the pressure. This type of crystallizer has both a cooling effect and a small amount of concentration. The latent heat of vaporization consumed by the solvent evaporation is exactly in balance with the sensible heat released by the cooling of the solution and the crystallization heat of the solute. In this type of crystallizer, the solution is cooled without contacting the cooling surface, and the solution is evaporated without the need for a heat transfer surface, which can avoid the generation of crystal scale in the container.

2. Oslo crystallizer

        OSLO type evaporative crystallizer, also known as Oslo crystallizer, was proposed by Norwegian Jeremiassen in the 1920s, and is also often called Krystal crystallizer or particle size fractional crystallizer. It has been widely used in industry. OSLO type crystallizer is a fluidized bed crystallizer with particle size classification, which belongs to the mother liquor circulation type continuous crystallizer. Its main feature is that the supersaturation area and the crystal growth area are located in different places of the crystallizer. The fluidized suspension in the circulating mother liquor stream provides a good condition for crystal growth. On the basis of continuous operation, it can grow into large and uniform crystals.

        OSLO crystallizer is divided into two categories: evaporative crystallizer and cooling crystallizer. OSLO evaporative crystallizer is heated by an external heater, and then enters the vacuum flash chamber to evaporate to supersaturation, and finally enters the suspended bed through a vertical pipe to grow the crystal. If the heating chamber of the OSLO evaporative crystallizer is replaced by a cooling chamber and the evaporative chamber etc. are removed, an OSLO cooling crystallizer is constructed. The main disadvantage of the OSLO cooling crystallizer is that the solute is easy to deposit on the heat transfer surface, and the operation is troublesome, so it is not widely used. Due to the special structure of the OSLO crystallizer, the larger particles first contact the supersaturated solution and grow first, followed by the smaller solution. Therefore, the crystals produced by the OSLO crystallizer have the advantages of large volume, uniform particles, large production capacity, continuous operating and low labor intensity.

The figure below shows the OSLO evaporative crystallizer. The fluidized bed crystallization chamber is located at the bottom of the equipment, and the upper part of the equipment is the evaporation chamber or vapor-liquid separation chamber. The heated material liquid first enters the evaporation chamber to flash, and the vapor-liquid separation forms a supersaturated solution, and then goes straight to the bottom of the crystallizer along the downcomer in the middle of the crystallizer, and then turns to rise, and the supersaturated material liquid during the ascent Pass through the fluidized bed crystallization chamber at a slower speed, and fully contact the suspended crystal nuclei, so that the crystal nucleus can grow and the supersaturation can be eliminated. The fluidized bed crystallization chamber can perform hydraulic classification of crystal particles, with large crystals on the bottom and small crystals on the top, and large and uniform crystal products are discharged from the bottom of the fluidized bed. The fine particles in the fluidized bed flow into the circulation pipe with the mother liquor, mix with the circulating mother liquor and the raw material liquid in the pipe, and are pumped to the heating chamber. The small crystals are reheated in the heater to dissolve the tiny crystals. The heated solution is then reheated. Return to the evaporation chamber to evaporate and reach supersaturation, and enter the next cycle.

Main Features：

●Due to the special structure of the OSLO crystallizer, the products produced have the advantages of larger particles and narrower particle size distribution.

● The bottom of the crystallizer is arc-shaped, which improves the flow state of the material liquid in the crystallizer and does not form a dead zone.

● The upper part of the crystallizer is an enlarged section, which can make the crystal settle down without participating in the circulation, reducing the chance of secondary nucleation.

● The circulation volume of the solution is large, the supersaturation of the solution is small, and the mother liquor basically contains no crystal grains, so as to avoid the nucleation phenomenon between the circulating pump impeller and the crystal grains.

● It can be produced continuously, and the output can be large or small.

● Adopt the mother liquor circulation, the crystal does not participate in the circulation, the crystal is not easy to be broken, the crystal size is large, up to 6-20 mesh.

● The crystallizer does not have a stirring device, and the crystal does not contact any stirring device. There are few fine particles to be processed and the energy consumption is minimal.

In the process of evaporation and crystallization, the rate of generation of supersaturation depends on the evaporation rate, that is, the evaporation intensity of the equipment. The greater the evaporation intensity, the faster the supersaturation is generated, and the higher the supersaturation is easily formed. The elimination of supersaturation mainly depends on the spontaneous nucleation and growth of crystals. If there is enough crystal surface and fast growth rate in the crystallizer, all the supersaturation produced by evaporation can grow on the crystal surface. The supersaturation of the solution is mainly eliminated by crystal growth in time, and it will not cause Solute nucleation process to eliminate supersaturation, so as not to produce large number of crystal nuclei.

        If the surface of the crystals in the solution is insufficient, the growth of the crystals is not enough to eliminate the supersaturation due to evaporation, making the supersaturation of the solution too high, and in the unstable region, the supersaturation of the solution will be consumed by the spontaneous nucleation process Supersaturation, thus forming large number of fine crystal particles. At this time, it is necessary to control the evaporation intensity during the evaporation crystallization process, so that the supersaturation of the crystallization system is always within the crystallization metastable zone, as to ensure that the designed evaporation crystallization equipment can produce products that meet the design task requirements. The longer the residence time of the crystal in the growth zone, the longer the crystal growth time, the larger the crystal grain size, and the larger the crystal surface. The formation of large-size crystals requires sufficient growth time.

        On basis of the original OSLO evaporative crystallizer, Jiangsu Heide Company has made a lot of improvements to the crystallizer. Based on accurate crystallization kinetics data, through software simulation, the crystal growth rate is calculated, and the size is designed according to the user's requirements. A suitable OSLO crystallizer was developed to make it more in line with actual production conditions.

        In addition, the raw material liquid is fed from the lower part of the evaporation chamber, and the upper center tube is discharged, which reduces the loss of short-circuit temperature difference. At the same time, there is a process of dissolving the crystallites when the material liquid rises in the evaporation chamber, and the material liquid changes from unsaturated to saturated. , Can reduce the number of crystallites, thereby ensuring the grain size of the crystals.

        According to needs, a mother liquor discharge pipe can be set on the upper cone of the crystallizer. The mother liquor can be discharged to control the crystal content in the tank, or some fine crystals can be discharged out of the system to keep the crystal nucleus in the tank relatively stable, which is beneficial for crystal growth.

Application range:

The OSLO type evaporative crystallizer designed by Jiangsu Heide Company has been widely used in salt making, MSG, metallurgy, aquatic product processing, soft water manufacturing, sewage treatment, high-concentration brine treatment and other industries.

1.Diversion tube (DTB) Crystallizer

        TB (Draft Tube Baffle) was invented in 1957 by Svensson Evaporation Company in New York, USA, and completed the experiment of manufacturing large-scale potassium chloride crystals in Carlsbad, New Mexico, and later for the chemical, food, pharmaceutical and other industrial sectors used widely. It is a kind of crystal slurry internal circulation crystallizer. Its typical characteristics are that there are stirrers, deflector cylinders and baffles inside the crystallizer, as well as supporting heat exchange devices and microcrystal elimination devices.

        The structure diagram is shown in the figure below. There is a diversion tube in the middle and a cylindrical baffle around it. There is a propeller (internal circulation axial flow pump) near the lower end of the guide tube, which rotates at a lower speed. Propelled by the propeller, the suspension rises to the surface of the liquid in the cylinder, then turns to the bottom, flows to the bottom of the device along the annular channel between the deflector and the baffle, and is again sucked into the lower end of the deflector. Form good mixing conditions. The cylindrical baffle separates the crystallizer into a crystal growth zone and a clarification zone. The annulus between the baffle and the vessel wall is the clarification zone. The effect of its stirring has actually disappeared, making the crystals settle and separate from the mother liquor. Only the excess crystallites are discharged out of the top of the clarification zone with the mother liquor, as to realize the Control of crystal quantity. The upper part of the crystallizer is a vapor-liquid separation space to prevent mist entrainment. The hot concentrated material is added to the bottom of the guide tube, and the crystal slurry is discharged from the bottom of the crystallizer. In order to make the crystals have a narrower particle size distribution, this kind of crystallizer is sometimes equipped with elutriation legs at the lower part.

        On basis of the original DTB crystallizer, combined with software simulation, Jiangsu Heide Company has made a lot of improvements to the crystallizer to make it more in line with actual production conditions.

Scope of Application:

The DTB crystallizer designed by Jiangsu Heide Company has been widely used in chemical products such as potassium compounds, potassium chloride, potassium sulfate, potassium permanganate; sodium compounds, such as sodium chloride, sodium carbonate, and sodium bromide; ammonia compounds, such as Evaporative crystallization of ammonium sulfate, ammonium nitrate, ammonium phosphate, ammonium chloride and other products.

1、Forced circulation (FC) Crystallizer

        FC (Forced Circulation) crystallizer is also called forced circulation crystallizer, also called growth crystallizer. This kind of crystallizer has the characteristics of simple structure and easy operation. As shown in the figure below, the FC crystallizer is composed of a crystallization chamber, a circulation pipe, a circulation pump, and a heat exchanger. The crystallizer has a conical bottom. After the crystal slurry is discharged from the cone bottom, it is sent to the heat exchanger by the axial flow circulation pump through the circulating pipe. After being heated, it enters the crystallization chamber along the tangential direction and circulates in this way. It belongs to the circulation type of crystal slurry. The crystal slurry discharge port is close to the bottom of the cone of the crystallization chamber, and the feed port is on the inlet pipeline of the circulating pump. The crystal growth area is at the lower end of the crystal separator, and the bottom of the crystallizer is generally equipped with a crystal elutriator. When designing the FC crystallizer, Jiangsu Heide must fully consider the appropriate circulating feeding method to avoid crystal breakage or abrasion of the wall, as well as the anti-short circuit measures of the circulating feed liquid and sufficient growth space for the crystal.

        The FC crystallizer designed by Jiangsu Heide Company can be combined with MVR technology or multi-effect evaporation technology to form a forced circulation evaporation crystallizer, which has superior energy-saving effects and is of great value to environmental protection and reducing production costs.

Scope of application:

        The FC forced circulation crystallizer designed by Jiangsu Heide Company is generally used in occasions where the crystal size is not high or the crystal is easy to grow, such as the evaporation and crystallization of liquids such as sodium chloride, ammonium chloride, sodium sulfate, and ammonium sulfate.