Intelligent Optimization Research Of The Alumina Double Stream Digestion Based On Exergy Analysis

High-pressure digestion process of alumina production is extremely complex, in which caustic soda is used to dissolve out the alumina in the bauxite. Compared with the single-flow digestion, the double stream digestion applyed in Zhongzhou aluminum plant can avoid scarring the indirect heating surface effectively, increase the heat transfer coefficient and thermal efficiency greatly, reduce energy consumption and operating costs significantly, and extend the running period of the system. However, there are some problems in the double stream digestion: the excess superheated steam is used, the vaporization capacity of the evaporators isn't explored, the exhaust steam wastes a lot, and the temperature of the final evaporator is too high. Therefore, it is very important that exergy analysis and intelligent optimization method are used to reduce the energy consumption of the double stream digestion.Firstly, based on analyzing the principle and the factors of the double stream digestion of the alumina plant, a heat balance model of the system is established to analyze the main factors affecting on energy efficiency. It is shown that (a) the scab of the preheating has limited effects on the reduction in energy efficiency, (b) the loss in energy of the surface of the digesters and evaporators account for a considerable proportion, and (c) the loss in heat of the condensate which is next to that of the surface being used circularly, so the impact of the condensate can be ignored.Secondly, the exergy model of the system is established to find out the energy-saving part. The results show that, the loss in internal exergy caused by the temperature difference mainly concentrates on the digesters, also on the evaporators and the caustic soda pre-heaters, and the loss in external exergy is mainly caused by the condensate effluxing.Thirdly, analyze the relationship between the exergy efficiency with the process operation parameters which include the inlet flow and temperature of the alkali and slurry, and the outlet temperature of the last evaporator. The results show that, the exergy efficiency of the system can be improved by decreasing the flow of alkali and slurry, increasing the inlet temperatures of alkali and slurry, and increasing the outlet temperature of the last evaporator. At last, genetic algorithm (GA) theory is used to optimize the process operation parameters of the double stream digestion system and the allocation of the temperature differences of the evaporators. The results show that, compared to the initial data, the exergy efficiency of the system increases from 30.12% to 30.88%, and the exergy losses reduce 2.7GJ.