Heat and mass transfer in osmotic distillation with brines, glycerol and glycerol-salt mixtures

Osmotic distillation (O.D.) is not a purely mass transfer operation, in that the flux is limited not only by the concentration polarisation, but also by the cooling of the feed and warming up of the extractant due to the phase change at the membrane interfaces (thermal effect). This study presents O.D. experiments with various extractants in a plate and frame module expressly designed to investigate separately the simultaneous effects of the heat and mass transfer. In co-current operation the temperature difference between the two streams approaches an asymptotic value for large residence time within the module, i.e. for long modules or low flow rates. In asymptotic conditions, which correspond to no heat flux through the system, the problem looks like a pseudo isothermal case, this allows to obtain the relevant membrane transport properties and to evaluate the role played by the concentration polarisation. The temperature difference created by the thermal effect, even of few degrees, greatly reduces the driving force for mass transfer through the membrane, the resulting loss of flux is relevant and increases with the temperature. The concentration polarisation plays a comparable role in the O.D. with calcium chloride or glycerol at room temperature, while is less important in O.D. with glycerol-NaCl mixtures, and negligible in the case of sodium chloride. Among the extractant used, CaCl2 is the most effective, however the advantage over the Glycerol-NaCl mixture is not so high as would appear comparing the respective theoretical driving forces for mass transfer. The length of module needed to approach asymptotic conditions (thermal entry length) can be estimated lower than the typical length of industrial apparatuses. The flux achievable in O.D. applications is thus not much higher than the asymptotic value, larger fluxes can only be obtained in lab devices.