Simulation of a double-effect mechanical vapor recompression sewage desalination technology process
[Objective]With rapid social and economic development,the consumption of industrial water has increased with the soaring production of corresponding wastewater.It is crucial to achieve reasonable water resource recovery.Mechanical vapor recompression(MVR)evaporation is a promising technology that is widely employed in sewage treatment processes.The MVR system utilizes the second steam latent heat for sewage evaporation and achieves stable operation characteristics.However,due to the diverse composition within varied scenarios and the different interactions of the saline wastewater,essential adjustments are required in the MVR wastewater treatment system,which significantly affects the sewage treatment performance and system energy efficiency.Therefore,it is necessary to comprehensively evaluate the material and energy flow characteristics for the alternative operating conditions.[Methods]In this study,a double-effect MVR configuration is developed that focuses on the desalination and resource recovery of oil and gas exploitation processes concomitant with wastewater.This system integrates the evaporation crystallization and salt separation processes,as well as the falling film evaporation and forced circulation evaporation technologies,to achieve multieffect evaporation and strengthen sewage concentration and crystallization.Meanwhile,waste heat recovery technology is applied to achieve system energy savings by using condensate water to preheat the fed sewage.With the assistance of the Aspen Plus platform,the MVR sewage treatment system simulation is implemented,and the system operation and water-salt separation principle were determined by the energy conservation and salt dissolution characteristics.Indicators such as sewage value resource recovery rate,energy consumption,and energy efficiency are adopted for the comprehensive evaluation of the system.In addition,a system-sensitive analysis is performed for the system off-design operation characteristics.[Results]The research findings can be summarized as follows:1)Under the designed conditions,after crystallization evaporation,separation ratios of water purification and inorganic salt reach 90.0％and 38.6％,respectively,and the overall resource utilization of sewage reaches 86.7％.In addition,increasing the initial salt content of sewage is beneficial for promoting the recovery of inorganic salt resources,and no solid salt will be produced with a salt concentration below 4.4％.2)Based on thermodynamic analysis,94.0％of the steam heating heat is provided by the recovered secondary steam heat energy,which results in an evident system energy efficiency ratio(EER)of 15.5.The specific energy consumption rate for wastewater treatment is 212.2 kJ/kg,which realizes favorable thermodynamic economic performances and achieves reasonable energy saving and carbon reduction.3)Based on the sensitivity analysis,the results indicate that the reduced evaporation pressure enhances heat transfer temperature difference and then promotes sewage resource recovery while leading to a decrease in the system EER.[Conclusions]Consequently,the smaller pressure ratio under the appropriate evaporator temperature difference condition contributes to improving the system's energy efficiency and reducing the specific energy consumption for wastewater treatment.These findings provide a reasonable reference to enhance the optimization of the MVR system and wastewater recovery in oilfield areas.
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