H133-13
Reducing the Energy Consumption for Supercritical Water Desalination (SCWD): A Zero Liquid Discharge Desalination

Monday, 14 December 2020: 04:36
Virtual
Prashant Sharan1, Tae Jun Yoon1, Joshua Domnic Mctigue2, Robert Currier1 and Alp Tugrul Findikoglu1, (1)Los Alamos National Laboratory, Los Alamos, NM, United States, (2)National Renewable Energy Laboratory, Golden, CO, United States
Abstract:
Supercritical water desalination (SCWD) is a zero liquid discharge technology. At supercritical conditions (> 374°C, and 22.1 MPa), the density drastically decreases and water becomes a poor solvent for ions. This leads to a separation of salt from the water. In SCWD the heat required is in the form of sensible heat (heat transferred with varying temperature), as compared to other thermal desalination processes where the heat transfer is in form of latent heat (heat transferred at constant temperature). Due to sensible heat transfer the heat recovery from clean water produced (supercritical water) has the potential to be very efficient.

Wyk et al. (2020) carried out a detailed energy analysis for a SCWD system and calculated the net thermal energy required for treating a 14%w salt solution to be 200 kWhth/m3. This heat needs to be supplied at high temperatures (> 410°C) which has a relatively high exergy content compared to other thermal desalination processes, which require heat at lower temperatures (70-250°C). In addition, SCWD needs around 7 kWhe/m3 for feed pumping. The net equivalent electrical energy required is around 120 kWhe/m3. In comparison, a conventional brine crystallizer requires around 60 kWhe/m3.

To enhance the energy efficiency of the SCWD system, we propose to use a heat pump. Since heat pumps are not typically available at such high temperatures, we use the heat pump concept developed for ‘pumped thermal energy storage' (McTigue et al. 2015). The distillate separated in SCWD is still in supercritical state, and this heat is used for heating the heat pump working fluid (400 °C). Then the compressor compresses the working fluid to a temperature around 430°C, which acts as a heat source for the SCWD. As discussed above, in SCWD the heat transferred is in the form of sensible heat, thus the heat pump can operate at fairly high temperature difference which helps in achieving a heat pump coefficient of performance > 8. This helps to bring down the energy requirement of SCWD to <30 kWhe/m3.

The distillate leaving the system is still at very high pressure, and a pressure recovery exchanger can be used to bring down the pumping power requirement to 1.2 kWhe/m3. The net electrical power requirement is then 31 kWh/m3, which is 75% lower than SCWD, and 50% lower than a commercial brine crystallizer. The overall water cost reduces by 20%.

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