Design and operation of membrane distillation with feed recirculation for high recovery brine concentration

Thermal-energy-driven desalination processes such as membrane distillation (MD), humidification dehumidification (HDH), and multi-stage flash (MSF) can be used to concentrate water up to saturation, but are restricted to low per-pass recovery values. High recovery can be achieved in MD through feed recirculation. In this study, several recirculation strategies, namely batch, semibatch, continuous, and multistage, are compared and ranked based on flux and energy efficiency, which together influence overall cost. Batch has higher energy efficiency at a given flux than semibatch and continuous recirculation because it spends more operating time treating lower salinity water for the same value of overall recovery ratio. Multi-stage recirculation is a steady-state process that can approach batch-like performance, but only with a large number of stages. Feed salinity rises during the batch operating cycle, and as a result feed velocity may have to be increased to avoid operating above the critical specific area wherein both GOR and flux are low due to significant heat conduction loss through the membrane. Finally, the choice of optimal membrane thickness for batch operation is compared to that of continuous recirculation MD.