Compared to traditional single tank intermittent crystallization, continuous crystallizer can achieve continuous crystallization production. The crystallization mode varies depending on the physical properties of different products, mainly including the following:
First, adiabatic vacuum crystallization
After the high-temperature solution enters the insulated container, a portion of water flashes out under vacuum, thereby increasing the concentration of the solution, reducing its temperature, and creating a supersaturated state. The adiabatic vacuum crystallizer mainly adopts DTB vacuum crystallizer, OSLO vacuum crystallizer, etc. The main application products include ammonium chloride, potassium nitrate, nickel sulfate, cobalt sulfate, cobalt chloride, methionine, etc.
2. Evaporative Crystallization
Evaporative crystallization is a method of producing supersaturation by heating and evaporating water to increase the concentration of the solution. During this process, the solution has a lower boiling point under negative pressure conditions, and the boiling point of the solution decreases with the increase of vacuum degree. Under negative pressure conditions, a large amount of heat is carried away by the boiling and vaporization of the solution, resulting in a low-temperature solution. After the solute reaches saturation, a solid is precipitated. Evaporative crystallizers are mostly operated continuously, and the main crystallizers include Oslo evaporative crystallizers, FC evaporative crystallizers, etc. Sodium chloride, sodium sulfate, and ammonium chloride all use this crystallization method. More energy-efficient evaporative crystallizers include MVR evaporators, multi effect evaporators, etc.
III. Cooling Crystallization
Cooling crystallization is a crystallization method that reduces solution temperature and increases supersaturation by exchanging heat with external cooling water, chilled water, and other cold sources. Sodium sulfate decahydrate, sodium carbonate decahydrate, and cobalt sulfate can all use this crystallization method. The commonly used forms of continuous cooling crystallizers are DTB crystallizers and OSLO crystallizers, as well as vacuum cooling crystallizers and continuous heat exchange crystallizers. A relatively complete vacuum continuous crystallization system generally consists of a vacuum crystallizer, condenser, and vacuum pump. High temperature and high concentration materials are continuously fed into the crystallizer. Under the action of the vacuum pump and condenser, negative pressure is maintained in the crystallizer, and the materials boil and vaporize to take away heat. The temperature inside the crystallizer is relatively constant, and crystals continue to precipitate.
4、 Reaction crystallization
Reaction crystallization refers to the crystallization method of adding reactants to the target solution to increase the concentration of the target product, thereby producing supersaturation. This process is usually accompanied by heat exchange, which can be gas gas reaction, gas-liquid reaction, liquid-liquid reaction, solid-liquid reaction, gas-solid reaction, etc. Regardless of the reaction process, the basic principles of crystal growth must be followed during the crystal formation stage.
The implementation of crystal suspension and particle classification in the crystallizer is a typical feature of continuous crystallizers. For example, in the OSLO continuous crystallizer, the system continuously sends the clarified mother liquor to an external heat exchanger for cooling to obtain supersaturation concentration, and then returns it to the crystal suspension bed. During the process of supersaturated solution passing through the crystal bed from bottom to top, the supersaturation gradually disappears and the crystal gradually grows. At the same time, the upward mother liquor has a washing and grading effect on the crystals, with large crystal particles falling down and small crystal particles floating upward. In this way, large crystal particles are preferentially extracted and filtered, while small crystal particles remain and continue to grow.
The reason why continuous crystallizers can achieve rapid cooling of high-temperature materials without producing a large number of small crystals is that they fully utilize the solubility, deterioration zone, supersaturation characteristics, and crystal cultivation principles of different components in the solution. At the same time, the continuous operation of continuous crystallizers has the advantages of high automation, small footprint, high production efficiency, large production capacity, and low manpower, and has a wide range of applications in fine chemicals, pharmaceuticals, inorganic salts, and other fields.
