Energy Saving Research On Evaporation Crystallization Process Of Bittern Based On Heat Pump Technology

Based on the stable phase equilibrium of K⁺,Na⁺,Mg²⁺//Cl^-,SO₄^2---H₂O,the crystallization path was obtained for the bitter brine system.And Aspen Plus（Version 7.3）was used to simulate the processes of the electrolyte-containing system under the ELECNTRAL thermodynamic model.On those bases,the products which were magnesium sulfate monohydrate and magnesium chloride hexahydrate were respectively obtained through two-stage conventional evaporation-cooling methods.For the purpose of energy saving,the heat integration,multi-effect evaporation,thermal vapor recompression heat pump technology and mechanical vapor recompression heat pump technology were applied to the conventional evaporation concentration process.Meanwhile,for the better evaluation of various evaporation concentration processes,some important evaluation indexes,such as energy consumption,annual total cost and exergy loss were chosen as objective functions.The research results were as follows:（1）The law of crystallization in the process was:When the bittern system was heated to evaporated at a high temperature,the magnesium sulfate monohydrate was separated crystals out the earliest.After cooling,part of NaCl precipitated.Then,the reheated liquor continued to be evaporated,the certain KCl precipitated as impurities.In the meantime,the magnesium chloride hexahydrate was crystallized as pure as possible when the liquor was cooled in saturated condition.The target products,the magnesium sulfate monohydrate and the magnesium chloride hexahydrate,were qualified products with each purity coming to 0.999and 0.947.（2）In the conventional evaporation concentration process,the energy consumption of the bittern system was about 524.1kW/ton.As for the whole device which came to 45t/h,and the total annual cost is about 69million CNY,which was equated to the standard coal with 20859.6 t/year.Under the conventional evaporation concentration process optimizing by coupling with heat integration technology,the energy consumption was reduced by about 15.0%,the annual total cost was saved by about15.3%,and the standard coal consumption was 17716.2 t/year.（3）On the basis of the heat integration evaporation concentration process,the multi-effect evaporation process was applied to the evaporation system.As what the research results showed was that the double-effect evaporation concentration process consumed standard coal with 12387.6t/year,which is 30.1%less than the heat integration evaporation concentration process,and annual total cost was saved by 30.0%.The three-effect evaporation concentration process has more advantages in economical than the double-effect evaporation concentration process,which consumed standard coal with 8242 t/year,reducing energy consumption by 33.5%,and annual total cost was saved by 31.0%.The multi-effect evaporation concentration process was simulated with higher thermodynamic efficiency.（4）Due to the low grade,the secondary steam in the double-effect evaporation concentration process cannot be fully used,which caused the massive loss of latent heat.Therefore,the mechanical vapor recompression heat pump and thermal vapor recompression heat pump technology were applied to the double-effect evaporation concentration process.The research results showed that the mechanical vapor recompression heat pump evaporation concentration process saved energy by 60.5%,the double-effect coupled mechanical vapor recompression heat pump evaporation concentration process reduced the energy consumption by 18.1%compared with the mechanical vapor recompression heat pump evaporation concentration process;the double-effect coupled thermal vapor recompression heat pump evaporation concentration process reduced the energy consumption by61.5%compared with the double-effect evaporation concentration process;the doubled-effect coupled thermal vapor recompression heat pump with mechanical vapor recompression heat pump evaporation concentration process reduced the energy consumption by 25.8%compared with the doubled-effect thermal vapor recompression heat pump evaporation concentration process.（5）The energy consumption of the double-effect coupled mechanical vapor recompression heat pump yevaporation concentration process was closed to that of the double-effect thermal vapor recompression heat pump coupled mechanical vapor recompression heat pump evaporation concentration process.Compared with the double-effect evaporation concentration process,the energy consumption of those processes could be reduced by about 71.4%,and the annual total cost of the above two processes were also relatively approached.Still,the annual total cost of the double-effect coupled mechanical vapor recompression heat pump evaporation concentration process was about 5%less than that of the coupled process,and whose thermodynamic efficiency was also higher.