A new pre-concentration scheme for brine treatment of MED-MVC desalination plants towards low-liquid discharge (LLD) with multiple self-superheating

Abstract

Multi-effect distillation units discard brine at high volume to the environment which results in numerous environmental pollutions, low thermodynamic performance, high exergy destruction rate, and high unit cost for the fresh water. This study aims at introducing a new method of pre-treatment for the brine discarded from each stage of the desalination plant to increase its concentration. The new method per-concentrates the brine by superheating the steam produced in the previous stage via recovering valuable thermal heat of the same effect. Due to a low temperature difference between the heat sink and heat source of this self-superheating scheme (i.e., the temperature difference between the steam and brine at each effect), low electrical power is required. Thermodynamic and component-based cost analysis indicate that, in comparison with the conventional MED-MVC unit, the new scheme lowers the specific work consumption (SWC) and unit cost of distilled water (UCDW) by 25.45% and 1.2%, respectively (under electrical power supply of 1845 kW). In addition, the new desalination plant has 25.57% higher exergy efficiency than the conventional MED-MVC unit under the same electrical power supply. Among all elements, the first effect had the highest exergy destruction rate of 397.6 kW, contributing around 15.67% of the total exergy rate of fuel or 21.78% of the total exergy destruction rate. It is also found that the exergy destruction rate of the last effects increases significantly with the new proposed method since the concentration of brine is substantially accumulated at the final stages. Several new dimensionless metrics are defined in order to quantitatively measure the amount of fresh water increment, discarded brine decrement, brine salinity increment (useful for salt production), and steam cost savings achieved at each stage. The results indicated that the steam cost savings were achieved from the number of effects larger than 3. • A new high-efficient and cost-effective pre-concentration scheme for MED-MVC is proposed. • New indices are defined to better illustrate performance and cost enhancement of the unit. • SWC and unit cost were decreased by 25.45% and 1.2% compared to the reference system, respectively. • Exergy efficiency was improved by 25.57% compared to the reference system. • For number of effects larger than three the steam cost starts saving.