Acoustic field-assisted osmotic membrane distillation

Osmotic membrane distillation (OMD) is a novel membrane process that facilitates the concentration of aqueous solutions under mild operating conditions. The main drawback of OMD, like any other membrane process, is relatively low flux. In the present work acoustic field has been applied for the enhancement of transmembrane flux. Acoustic field, using transducer, of the frequency 1.2 MHz was applied perpendicularly to the membrane. Encouraged by the initial results of reasonable enhancement in the transmembrane flux (by 20–30%), further experiments were carried out for 5 M NaCl/pure water, 5 M CaCl2/pure water, NaCl/sugarcane juice and CaCl2/sugarcane juice systems both in the presence and absence of acoustic field. It was observed that there was about 22–205% enhancement in transmembrane flux with the application of acoustic field. All the experiments have been carried out in a membrane cell using different hydrophobic membranes polytetrafluoroethylene and polypropylene). The effect of various parameters such as concentration (2, 3, 4, 5 M), stirring speed (0, 198, 250, 450 rpm) and temperature (40, 50, 60°C) was studied on transmembrane flux in the absence of acoustic field, and the effect of acoustic field was studied at the best conditions observed. PTFE and PP membrane fluxes for CaCl2 were both higher in the case of sugarcane juice and water than those for NaCl and K2HPO4 solutions. Results obtained from these experiments were correlated using a modified Nernst film model.

The OMD process has been carried out with three different hydrophobic membranes, and the influence of the parameters such as osmotic solution concentration, stirring speed and temperature has been studied. The OMD coefficient (C) and mass transfer coefficient (k) were estimated based on vapour pressure data and model equations reported in the literature. The experimental values of fluxes were well within the range of predicted values. Application of the acoustic field enhanced the transmembrane flux for pure systems as well as real systems. Further studies need to be undertaken for the enhancement of transmembrane flux using real systems in the presence of the acoustic field on a larger scale.