Pervaporation of aqueous solutions with hydrophobic membranes
Dr.-Ing. Lorenz Krätz
Pervaporation is a separation process where matter is transfered from a liquid feed mixture to the vapour phase through nonporous polymeric membranes. After passing the membrane the components will evaporate requiring heat of evaporation only for that permeate flux. If at the permeate side of the membrane the partial pressure of the permeating molecules is kept below their saturation pressures, a driving force of these components through the membrane is established. Separation occurs according their permeation rates through the membrane which are complex functions of a variety of parameters. As a separation process for liquid mixtures, pervaporation relies on differences in membrane permeability, on thermodynamic activity of the solution compounds as well as operating parameters.
The objective of our studies is to examine the effect of operating conditions on the pervaporation process of aqueous solutions containing dilute organic solvents with different degree of water solubility. The membranes are usually commercial polymeric composite membranes with hydrophobic active layer made of PDMS.
The pervaporation of single organic solutes from water is very different from multicomponent transport because activity coefficient of water immiscible solutes are controlled by miscible solvents. As an example, the mutual interference of the permeants with a system of water, ethanol, toluene and xylene are described. Chemically similar toluene and xylene appear to permeate independent from each other through the membrane. Toluene exerts the controlling influence on the permeation of the polar components, water and ethanol. Water flux is remarkably influenced by the enhancement of toluene concentration though total organic content is the same.
In general, the pervaporation of aromatic or halogenated hydrocarbons' leads to phase separation thus simplifying downstream processing of the permeates. Obviously a higher organic-removal could be achieved if further improvements can be made both on the materials and the optimization of the operation parameters.
From the point of view of mass separation, one should primarily optimize enrichment of organic pollutants instead of flux, allowing a larger membrane area.