Integrated membrane distillation–crystallization: Process design and cost estimations for seawater treatment and fluxes of single salt solutions

Abstract

The goal of this research is to design an integrated membrane distillation–crystallization (MDC) process for desalination of seawater with pure water and dry salts as the only products. The process is based on a combination of membrane distillation (MD) and osmotic distillation (OD) steps with integrated crystallization. In the first MD step seawater is concentrated up to the saturation point of CaCO3. In subsequent MD or OD steps, pure water and salts (a.o. CaCO3, NaCl, KCl) are separated until complete dewatering and crystallization. The equipment and energy costs for this process are approximately € 1.09/m3.Laboratory research with single salt solutions showed that after three days of MD operation with a saturated CaCO3 solution (77°C) a complete flux decline occurred. This could be prevented by adding CaCO3 crystals to the system to promote crystallization in the bulk liquid. The treatment of a saturated NaCl solution (80°C) with MD resulted in a complete flux decline within 1h. In an OD system NaCl crystallization (64°C) was more successful when NaCl seeds were added to the system and a draw solution was used at the condensate side. During 3h of operation the flux decreased from 3 to 1Lm−2h−1.