Minimizing the thermal energy use of membrane distillation with real-time operating controls

Membrane distillation (MD) has emerged as an alternative desalination process with many advantages, including high-purity product water, water recovery from highly concentrated feed sources, brine minimization, and the ability to make use of low-grade heat sources like renewables and waste heat. One of the challenges with MD however remains its energy intensive nature. Improving the energy efficiency of MD can be achieved by carefully controlling its operating conditions. In this study, we propose a real-time feedback control system that tracks the point of minimum specific heat input by adjusting operating conditions. A simple perturb and observe control strategy is employed to iteratively adjust feed temperature, feed circulation rate, and distillate recirculation rate so that over time the specific heat input to the system is reduced. The concept is evaluated using a laboratory-scale direct contact MD system for seawater desalination. The MD test bench is operated for nearly 6 h, over which period a series of 23 step changes is applied to the operating conditions, successfully reducing the specific heat input by a factor of 7×, from an initial baseline of 1589 kWh/m 3 down to a minimum of 274 kWh/m 3 . The lowest heat input is observed when feed temperature, feed circulation rate, and distillate recirculation rate reach 30.011 ± 0.465 °C, 75.33 ± 0.45 ml/min, and 16.5 ± 0.1 ml/min, respectively. While these conditions are specific to our bench-scale MD setup and short-duration testing, the tracking approach has the potential to be adapted to MD systems with different membranes, module geometries, feed sources, and operating environments. The present work therefore demonstrates a proof-of-concept pathway for improving the energy management of MD systems and can therefore contribute to efforts to make desalination and water reuse more sustainable. • First experimental demonstration of real-time adaptive control for direct contact MD • Uses a perturb-and-observe control strategy to minimize thermal energy use • Identifies optimal feed temperature, feed circulation, and distillate circulation rates • Achieves a 7× reduction in specific thermal energy use for seawater desalination