Membrane separations and energy efficiency

Membrane processes are usually considered to offer very promising performances in terms of energy efficiency for industrial separations. This statement particularly holds for homogeneous gas and liquid separations which are traditionally performed thanks to unit operations based on a phase change (distillation, evaporation, condensation, crystallization…). The energy efficiency concept can however be addressed through different methodologies, potentially leading to different, if not opposite conclusions. A critical analysis of the energy efficiency concept for membrane separations is proposed. Starting from the most usual minimum work of separation definition, alternative expressions of this key concept are developed in order to better reflect the different types of separation situations encountered for practical purposes (solute purification and/or recovery, process selectivity). In a second step, the real work of separation of a given process, classically evaluated through modern Process Systems Engineering computations, including thermodynamic modelling and irreversible processes computations, is discussed. The interest of the entropy dissipation function, obtained from Irreversible Processes Thermodynamics (IPT) approach is then presented. The methodology is applied to the separation of a perfect gas mixture (air) and non ideal (seawater) liquid mixture. The local entropy dissipation rate offers the opportunity to analyze the impact of fluid distribution in membrane modules, possibly leading to improved designs through the entropy equipartition theory. The largely unexplored possibilities of IPT to provide a predictive evaluation of the overall energy efficiency of a separation process, based on a diffusional mass transfer mechanism, is finally illustrated.