Heat pump assisted air gap and direct contact membrane distillation configurations: Comparative simulations of industrial scale cases at the water-energy nexus

This work presents comparative simulations of a multi-stage continuous and in-series desalination system using direct contact membrane distillation (DCMD) and air gap membrane distillation (AGMD) configurations. Industrial-scale cases were defined and focused on determining a realistic approach at the water-energy nexus by integrating heat pumps to recover the heat transferred in the process. The work assessed multistage hollow fiber membrane distillation modules for desalinating 67.5 L/s seawater reverse osmosis brines using a phenomenological model to study the effect of different operating conditions. The results showed that AGMD presented a 90.7 % higher thermal efficiency than DCMD despite requiring 7.1 % more in-series stages. AGMD presented the lowest Capex and Opex, reaching an average capital cost of 240.3 US$/(m3/y). In addition, the specific electrical energy consumption decreased from 363 in DCMD to 182 kWh/m3 in the AGMD configurations, which ultimately resulted in a decrease in operating costs of up to 2.1 US$/m3 of freshwater produced by using renewable energy sources, and with energy consumption for heating and cooling the system streams three times lower through the assistance of a heat pump with recovery heat. These values support that the AGMD configuration at the industrial scale is a promising, competitive, and sustainable option compared to the current costs of desalinated water supplied in several industries in Chile, which are around 5 US$/m3.