Method for the purification of a liquid by membrane distillation, in particular for the production of desalinated water from seawater or brackish water or process water

Method for the purification of a liquid by membrane distillation, comprising:

• passing a relatively warm vaporising stream of liquid (retentate stream (3)) over a porous membrane (4), vapour flowing via the pores of the membrane to the other side of said membrane,
• condensing said vapour on a relatively cool condenser surface (2) to give a distillate/condensing stream (10), said condenser surface forming the non-porous separation between a feed stream (1) to be purified and said distillate stream (10), which feed stream (1) is in counter-current with the retentate stream (3) so that an appreciable proportion of the latent heat will be transferred via vapour to the feed stream (1),
• a gas gap (5) with the width of less than 5 mm being present between the porous membrane (4) and the condenser surface (2),
• a pressure which is lower than the atmospheric pressure and higher than the vapour pressure of the feed stream being maintained in the gas gap (5),
• wherein that the porosity ε of the porous membrane (4) is higher than 0.7, porosity being understood to be the ratio of the open volume to the total volume of the porous membrane 4,
• the surface area of the condenser surface (2) is 1.2 to 6 times the surface area of the porous membrane (4),
• the effective local vapour pressure difference between the retentate stream (3) and the condensing stream (10) is less than 10 kPa (0.1 bar),
• the perceptible heat of the distillate stream (10) is released by heat exchange to the feed stream (1) and/or the retentate stream (3),
• the relationship between the porosity ε of the porous membrane (4), the ratio S between condenser surface area and membrane surface area, the local vapour pressure difference D between retentate and feed in kPa, the width L of the gas gap in cm and the ratio P between the absolute pressure in the gas gap and the local water vapour pressure of the retentate is as follows:
• ϵ . S 6 D . L . P 2 > 1
• the perceptible heat loss of the retentate stream (3) is less than 300 KJ/kg condensate (less than 12% of the latent heat) and the specific flow rate is higher than 0.5 kg condensate/m2 membrane/hour/kPa difference in water vapour pressure.