Mass transfer in osmotic membrane distillation of phycocyanin colorant and sweet-lime juice

Osmotic membrane distillation is a novel membrane process for the removal of water from dilute aqueous solutions, such as liquid foods or natural colors, concentrating them, while retaining the organoleptic and nutritional properties. The effect of various process parameters, such as concentration (2–10 m) and flow rate (25–100 ml min−1) of feed and osmotic agent on the transmembrane flux was evaluated in case of phycocyanin and sweet-lime juice. The increase in the osmotic agent concentration and flow rate resulted in an increase in transmembrane flux. A three-fold concentration in case of phycocyanin and 10-fold in case of sweet-lime juice could be achieved. The magnitude of temperature polarization is small and the maximum temperature difference across the membrane was only 0.8 K that too for 10 m calcium chloride solution. The feed and osmotic agent side mass transfer resistances were estimated based on classical empirical correlation of dimensionless numbers, whereas membrane resistance was estimated using Dusty-gas model. The mass transfer mechanism was found to be in the transition region that is between Knudsen and molecular diffusion. The type of mass transfer mechanism was dominating depending upon the pore size of the membrane. In case of membrane with pore size of 0.05 μm, the contribution of Knudsen diffusion is higher (74% of the membrane resistance), whereas for 0.20 μm the contribution of molecular diffusion is higher (59% of the membrane resistance). The flux across the membrane during the process was predicted using resistances-in-series model. The experimental values were found to correlate well with the predicted values.