Theoretical research on ship desulfurization wastewater freezing desalination system driven by waste heat

Abstract

This study proposes a novel two-stage freezing and pre-desalination system for desulfurization wastewater driven by the waste heat of marine diesel engine exhaust. Ice crystals and concentrated wastewater are separated and flow to the thermal desulfurization branch and the wet desulfurization branch respectively, so as to realize the preliminary desalination of the desulfurization wastewater, as well as provide the air conditioning and refrigeration which is recovered from the freezing and pre-desalination system. This combined system can effectively reduce the desalination costs on board. In this paper, both the thermodynamic model and economical model for the two-stage freezing pre-desalination subsystem, the absorption refrigeration subsystem and the thermal wet desulfurization branch are established, and the effect of freezing crystallization is experimentally studied. The thermodynamic performance of the proposed system is analyzed under typical onboard working conditions. Besides, the economic performance of this system is compared with traditional desulfurization technology. The results show that the pre-desalination output increases with the increase in temperature, and the salinity of pre-desalinated desulfurization wastewater can be reduced from 11 ppt to 6.8 ppt, the primary crystallization stage reaches stable a desalination rate at 20 %–23 %, and the total desalination rate involving the secondary crystallization stage reaches 30 %–40 %. The distribution rate of cold capacity in the two stage crystallizers exists an optimized value of 0.74. Simultaneously, the pre-desalination system reaches the highest desalination rate at 37.5 % with the ammonia-water concentration at 0.5 and the crystallization temperature in the primary/secondary stage at −15 °C/−9 °C, respectively. The economic performance of the system shows a downward trend as the temperature rises. The increase in initial ammonia-water concentration is benefit for the system economy. The comprehensive cost of the system with refrigeration cycle is greatly reduced by 5 %–30 % compared with that without refrigeration cycle.