Studies on scaling of membranes in desalination by direct contact membrane distillation: CaCO_3 and mixed CaCO_3/CaSO_4 systems

摘要

Scaling of membranes by CaCO_3 and CaSO_4-CaCO_3 is of considerable concern in membrane desalination processes. It is particularly relevant for porous crossflow hollow fiber-based membrane distillation (MD) processes which can achieve high water recovery and can encounter heavy precipitation of scaling salts. Therefore an analysis of the scaling potential for CaCO_3 and mixed CaSO_4-CaCO_3 systems is presented first in terms of the saturation index profiles throughout the crossflow hollow fiber membrane module as a function of the location in the module for feed solutions resulting from high water recovery. Scaling experiments during DCMD with tap water, CaCO_3 and mixed CaSO_4/CaCO_3 were conducted over a wide range of values of saturation index (SI) (10 < SI_(calcite) < 64, 1.1 < SI_(Gypsum) < 1.5) using porous fluorosilicone coated crossflow hollow fiber membrane desalination modules. The effects of flow rates, flow patterns (cross vs. parallel flow) and the nature of the membrane surface on possible scaling scenarios were further investigated for the scaling salt CaSO_4. Experimental results at high saturation indices show that even when the precipitation rate was fast in the CaCO_3 system at elevated temperatures or high concentrations, no significant loss in water vapor permeation was observed suggesting no effect of scaling on membrane flux. However, for a few of the mixed CaSO_4-CaCO_3 systems, the water vapor flux dropped somewhat. Possible explanations have been provided and a method to solve this problem has been illustrated. Fast feed flow rate resulted in a shortened induction period. Crossflow flow pattern and the nature of the hydrophobic porous coating on the membrane surface were proven to be helpful in developing the resistance to scaling. Results of modeling show that concentration polarization effects are far more important than temperature polarization effects.