CFD Simulation of Mass Transfer in Membrane Evaporators for Concentration of Aqueous Solutions

Osmotic evaporation (OE) process is usually suggested to concentrate aqueous solutions. OE is a concentration technique based on the use of mesoporous or macroporous and hydrophobic membranes. The difference between the activity of solutions results in a pressure difference between the upstream diluted solution and the downstream (usually concentrated brine) solution. The latter generates a water flow. Osmotic evaporation can be used to selectively extract water from aqueous solutions under atmospheric pressure and at ambient temperature; thus avoiding thermal degradation of the solutions. In spite of the obvious advantages of OE, this technique presents some shortcomings linked to the use of brine such as corrosion and regeneration. In order to develop an alternative and complementary membrane process, the brine was replaced by a sweep flow of a low pressure gas, i.e. generally air. In that case, the flow of water vapor is not condensed but it is taken away by the extracting phase. This new process is named membrane evaporation (ME). Similar to OE, membrane evaporation occurs at room temperature and the driving force of the process is not the thermal gradient but the difference of the partial pressure of the water vapor between the water surface and the dry air. Membrane evaporation presents interesting working conditions including low operating temperature; the latter makes this process attractive for heat-sensitive solutions. Performance of a ME contactor is studied in this work. A two-dimensional mass transfer model was developed to predict the flux of water evaporation in the membrane contactor. The model was based on solving the continuity and momentum equations for water in the membrane contactor. Both axial and radial diffusions were considered in the mass transfer equations. The model equations were numerically solved using finite element method to obtain the concentration distribution of water in the contactor. By obtaining the concentration distribution, the flux of evaporation was determined and compared with the experimental data. The findings of the model were in good agreement with the experimental data. It was also indicated that the proposed model is appropriate for the prediction of membrane evaporator performance.