Comparison of polyelectrolyte coated PVDF membranes in thermopervaporation with porous hydrophobic membranes in membrane distillation using plate-and-frame modules

• Thin-film composite membranes made from polyelectrolyte on porous PVDF membranes.
• In thermopervaporation water permeation rate is only slightly lower than in MD.
• In dewatering of whey they showed performance stability over at least 4 days.
• The thin-film composite membranes can easily be cleaned with NaOH (aq) solution.
• Thermopervaporation is superior to MD without slow pore wetting and fouling.

In thermopervaporation the same economically favorable driving force as in membrane distillation, i.e., a temperature difference between feed and permeate for the transport, is used but with non-porous thin-film composite membranes. Membrane pores cannot be wetted and long-term operational stability can be achieved with the appropriate coating layer, but normally with a decrease of the flux compared to membrane distillation with porous hydrophobic membranes.Porous asymmetric PVDF membranes were made to achieve low permeation resistance and pores which could be overcoated with polyelectrolyte polymers. This coating prohibits pore wetting and strongly reduces adsorption of organic substances.Those membranes showed a high permeation rate for water due to a structure of phase-separated hydrophilic and hydrophobic three-dimensional domains. The permeation rates of these composite membranes for water is between 6 and 12 l/(h m2) at a feed temperature of 60 °C and permeate at a temperature of 40 °C of a 2% saline solution feed depending on the operational parameters. This is only a slight reduction of 10–15% in permeation rate compared to membrane distillation with porous hydrophobic membranes.In whey dewatering experiment this membrane showed a constant performance over 4 days in intermittent operation mode and stability in cleaning with strong alkaline solution.