Multiple-effect desiccant-based zero liquid discharge desalination systems
Department of Mechanical Engineering-Engineering Mechanics
State-of-the-art zero liquid discharge (ZLD) technologies are currently bound with either intensive use of high-grade electrical energy such as mechanical vapor compressors or high capital cost with environmental concerns such as evaporation ponds. The present study aims to address these issues by an innovative desiccant-based ZLD desalination system in which a multiple-effect distillation (MED) unit is uniquely embedded at the heart of an absorption-desorption system. Here, the MED and absorption systems are inherently coupled enabling both heat and mass transfer processes between a hypersaline brine slurry and a desiccant solution. The proposed technology employs an absorption-based thermally-driven vapor compressor concept to pressurize the vaporized brine of the ZLD crystallizer unit from a low-pressure absorber to a high-pressure desorber module. The low water vapor pressure environment required for the ZLD treatment is established by the strong hygroscopic properties of an aqueous lithium bromide (LiBr) salt capturing a large volume of water vapor from the brine slurry. This eliminates the need for energy-intensive electrically-driven mechanical vapor compressors currently employed in advanced brine crystallizers. Comprehensive thermodynamic modeling is performed to evaluate the energy efficiency and size of the system at different thermohydraulic conditions. The detailed analysis indicates that the thermal energy consumption of the proposed desiccant-based ZLD desalination system is 67 kWh of thermal energy per cubic meter of the distillate produced with a gained output ratio (GOR) of 10. This is 63% and 44% reductions in the ZLD energy consumption compared with advanced mechanical vapor compression and thermal evaporation based ZLD systems, respectively. Insights gained from the present study have a high potential to truly transform thermal desalination and, in particular, ZLD treatment industries.
Multiple-effect desiccant-based zero liquid discharge desalination systems.
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