Effect of hollow fiber membrane properties and operating conditions on preventing scale precipitation in seawater desalination with vacuum membrane distillation

Membrane distillation (MD) has attracted attention as a seawater desalination technology because it can treat feed solutions with high osmotic pressures that cannot be treated by reverse osmosis. However, scale precipitation is one of the most significant challenges of MD seawater desalination because it can decrease the vapor permeability and hydrophobicity of MD membranes. Here, the impact of various Reynolds numbers of the feed stream on scaling was investigated. Vacuum MD (VMD) operations with superhydrophobic polyvinylidene fluoride hollow fiber (HF) membranes were performed using seawater at various linear velocities as a feed. Two types of HF membranes with different inner diameters were used. Following 24-h VMD, the amount of scale precipitation on the bore surface of the membrane decreased satisfactorily at a Reynolds number above 1200 due to the decrease in concentration polarization at the bulk/membrane interface. The vapor flux also increased with the Reynolds number. Membranes with larger inner diameters could reduce the amount of scale precipitation and pressure drop, even at the same linear velocity. Thus, it is important for stable VMD seawater desalination to operate at a Reynolds number above 1200 with an HF membrane of large inner diameter.