Scaling mitigation in radio-frequency induction heated membrane distillation

Calcium sulfate (CaSO4) scaling is a limiting factor in membrane distillation as a result of concentration polarization and elevated temperatures. We assessed the influence of radio-frequency magnetic fields, used as a method of heating the membrane's surface, as part of a membrane distillation (RF-MD) process. The influence of RF heating on CaSO4 scaling is addressed in terms of distillate flux and crystal formation, and the results are compared to conventional MD systems. The RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface, and scaling was assessed at different feed flow velocities using feed solutions containing elevated concentrations of CaSO4 and NaCl and at ionic composition simulating RO brine. Results show only sporadic small CaSO4 crystals on the membrane surface following RF-MD, while high concentrations of small crystals were detected at the concentrate stream exiting the filtration module. Furthermore, the RF-MD system was able to produce enhanced distillate flux. The scaling mitigation mechanism is discussed in detail and hypothesized to be a result of the high-frequency movement and collision of the ions in the solution.