Discussion on evaporation devices for crystalline solutions
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This post was last edited by kmmhj on 2010-6-5 at 11:37. In the design of crystallization solution evaporation systems, crystallization has a significant impact on the heat transfer coefficient of the heat exchange surfaces; in some cases it even causes complete blockage of the pipes. How can it be ensured that no evaporation occurs within the heat exchange tubes while allowing the crystals to be removed online? Are there any evaporation devices with similar processes? Please advise. I am referring specifically to the carbonate removal evaporators used in the alumina industry; currently, forced-circulation evaporators are employed in these applications. However, when the carbonate content in the sodium aluminate solution is high, it can easily lead to crystallization on the tube walls and blockages. The idea of evaporating at higher temperatures and allowing crystallization at lower temperatures is good, but the solubility of carbonates in sodium aluminate solutions is directly related to the alkali concentration, with little correlation to temperature. The above approach is not suitable.
A falling-film evaporator is used. After being heated in the tubes of the falling-film evaporator, the material enters the evaporation chamber where it is evaporated and concentrated.
Scraper evaporators should have a wide range of applications; products similar to them have shown good concentration results
In a production process based on evaporation crystallization with the product as the goal, the crystallization step should be the primary one; evaporation serves to create supersaturation in the system. Therefore, for evaporators used in the crystal formation process, evaporation methods such as falling film evaporation should generally not be chosen. The specific form of the crystallizer should be determined based on the product requirements and the laws governing the crystallization process, so as to define the manufacturing process and consequently select the appropriate evaporation crystallization equipment.
In a falling-film evaporator, a properly designed vacuum is utilized to raise the temperature, prevent crystal formation, create a supersaturated solution, then cool it to induce crystallization, followed by centrifugation to obtain the crystals
The heating tubes in evaporation crystallizers are quite critical; in falling-film or rising-film evaporators, due to the high evaporation heat transfer coefficient and the large temperature difference for heat transfer, excessive superheat can easily lead to crystallization.
For forced circulation, the flow rate should be set relatively high to prevent pipe blockage; at the same time, efforts should be made to remove as much crystal as possible from the evaporator, so that only a small amount of crystals remain in circulation.