Strategies for dealing with industrial liquid wastes involve reducing both the quantity produced and their environmental impact. Recent Irish and EU statutory provisions require consideration of waste minimisation and environmental performance. Innovative solutions are required for reduction, recovery and recycling of hazardous liquid wastes in particular.

We have been using pervaporation for the separation of liquid solutions. This process has advantages – including improved selectivity, reduced energy consumption, and separation of azeotropic mixtures and mixtures of components with close boiling points. (Azeotropic mixtures are those whose composition does not change on boiling and thus the components cannot be separated by simple distillation.)

Pervaporation is a membrane separation process, which uses a dense polymeric membrane for selective permeation of one or more components from a liquid mixture. In the process, a concentration/vapour pressure gradient is established across the membrane, and one component preferentially permeates the membrane. A vacuum applied to the permeate side is coupled with immediate condensation of the permeated vapours.

The choice of membrane is most important for selective separation of specific components. We have used hydrophilic membranes for the separation of azeotropic solvent mixtures containing water. Hydrophobic membranes can be used for the preferential permeation of organic solvents from organic/ water solutions or organic/ organic mixtures.

Experiments are performed on a laboratory-scale unit incorporating a membrane cell. Samples of permeate and retentate are analysed using gas chromatography and Karl-Fisher titration. The variations of permeate concentration and flux with feed concentration and temperature are studied in order to generate data to be used for the design of pervaporation and hybrid pervaporation/distillation systems for the separation, recovery and recycling of organic solvents.

Contact: Paul Ashall,
School of Chemistry, DIT;
E-mail: paul.ashall@dit.ie