Optimization Of Evaporation And Crystallization System Of Diaphragm Caustic Soda

Diaphragm electrolysis is one of the main methods for production of caustic soda in our country. The evaporation and crystallization is the key procedure in the preparation of caustic soda, aiming at the removal of NaCl and increasing the concentration of NaOH solution. Concerning the problem of lower NaOH concentration in the product of Chlorine-alkali Company of Qilu Petrochemical Corporation, the crystal size distribution and mass variation in the devices of the evaporation and crystallization system were measured, and the main factors affecting the product quality were analyzed in this thesis. The improvement schemes were proposed based on the industrial crystallization theory, which would provide the fundamental for the optimization of the evaporation and crystallization system and the improvement of caustic soda qualityBased on the spot measured technological data, the variations of crystal weight and particle size distribution in the crystallizers of the evaporation and crystallization system were analyzed. It showed that the selection of the crystal separation sites of the system was reasonable. The crystallization was mainly occurred in the evaporation and crystallization procedure, and the crystallization degree was low in the cooling crystallization procedure.The crystal quantity was maximum in the fourth crystallizer, and the formed crystal showed a normal particle size distribution. The fourth, third and second crysatllizers had no significant influence on the quality of product.The crystals grew well in the first flash evaporator. The crystals with greater size were seriously crashed in the second flash evaporator, and the crash degree exceeded 10%. The crystals were also crashed in DV309, and the separation efficiency was poor. The improvement schemes were proposed for the second flash evaporator and DV309.The cooling capacity was finished by the former three cooling crystallizers, and 76.6% crystal in cooling crystallization procedure was formed in these crystallizers.The crystals with greater size were seriously crashed in DE404-DE408. Two modified technological procedures were proposed according to the principle of first slow cooling and then fast cooling. Few devices would be changed for the first procedure, but it could not achieve the optimized cooling rate. The optimized cooling rate would be achieved using the second procedure, but it had the shortcoming of changing more devices.