Membrane separation process for separating an aqueous solution and crystallizing one or more dissolved compounds, comprising passing an aqueous feed comprising said one or more dissolved compounds through a membrane module, wherein said one or more dissolved compounds have an increasing water solubility with increasing temperature, wherein said one or more dissolved compounds are present in the aqueous feed solution in a concentration of the respective compound of at least 90% of the saturation concentration of the respective compound at the operating temperature, wherein the membrane module comprises a feed inlet and a liquid retentate outlet, - wherein the temperature of the liquid in the liquid retentate outlet is higher than the temperature of the liquid in the feed inlet, - and wherein a positive driving force for water vapour permeation through the membrane is created by maintaining the water vapour pressure at the distillate side of the membrane lower than the water vapour pressure at the retentate side of the membrane at corresponding points in the retentate side and the distillate side by other means than by a temperature difference, - such that said one or more dissolved compounds at least partially crystallise in said retentate and not at the membrane surface, - and wherein the relative temperatures of the feed inlet and the liquid retentate outlet are set, - wherein said setting the relative temperatures of the feed inlet and the liquid retentate outlet comprises active heating which involves heating the distillate side of the membrane, heating a spacer optionally present at the retentate side of the membrane, and/or heating the membrane in said membrane module, - and wherein the water vapour pressure at the distillate side of said membrane is maintained lower than the water vapour pressure at the retentate side of said membrane by providing a stripping agent in the distillate or by maintaining the absolute pressure at the distillate side of said membrane lower than the absolute pressure at the retentate side of said membrane. - wherein the temperature of the liquid in the liquid retentate outlet is higher than the temperature of the liquid in the feed inlet, - and wherein a positive driving force for water vapour permeation through the membrane is created by maintaining the water vapour pressure at the distillate side of the membrane lower than the water vapour pressure at the retentate side of the membrane at corresponding points in the retentate side and the distillate side by other means than by a temperature difference, - such that said one or more dissolved compounds at least partially crystallise in said retentate and not at the membrane surface, - and wherein the relative temperatures of the feed inlet and the liquid retentate outlet are set, - wherein said setting the relative temperatures of the feed inlet and the liquid retentate outlet comprises active heating which involves heating the distillate side of the membrane, heating a spacer optionally present at the retentate side of the membrane, and/or heating the membrane in said membrane module, - and wherein the water vapour pressure at the distillate side of said membrane is maintained lower than the water vapour pressure at the retentate side of said membrane by providing a stripping agent in the distillate or by maintaining the absolute pressure at the distillate side of said membrane lower than the absolute pressure at the retentate side of said membrane. <EOS>