MITIGATION OF TEMPERATURE POLARIZATION AND MINERAL SCALING IN MEMBRANE DISTILLATION: THE IMPACT OF INDUCTION HEATED ELEMENTS

Abstract

In the last decade, there is a growing interest in membrane distillation (MD) technology, a thermally driven membrane separation process, to treat high salinity water or contaminated wastewater. However, MD commercialization is still limited by technical challenges such as temperature polarization (TP), heat losses, scaling, and high energy consumption, as the feed solution requires continuous heating to provide an efficient driving force. The present work aims to develop and optimize a novel, efficient, and robust MD system utilizing an electromagnetic field, i.e., induction heating (IH) as a heat source for MD distillation, specifically aimed to overcome MD limitations. The proposed MD system includes a radio frequency IH system and a metallic component embedded to the hydrophobic membrane. This allows fast and contactless heating of the feed solution at the membrane surface without the need for preheating the bulk feed solution. As a result of the increase in solution’s temperature at the membrane-solution interface, TP reduces and thus, distillate flux increases. In addition, as IH transfers energy directly to electrically and thermally conducting materials with minimal heat loss to the surrounding environment, it leads to significant power savings. Two main concept approaches for the radio frequency heated MD (RF-MD) system were evaluated. First, the RF-MD system included a thermally conducting dual-layer membrane containing a magnetic hydrophilic layer based on iron oxide-carbon nanotubes coated on a hydrophobic membrane. Heating the solution was mainly done directly at the membrane-water interface. The impact of operational conditions on the distillate flux and salt rejection was evaluated while treating high salinity feeds (35-100 g/L NaCl). Following optimization, high distillate flux and 99% salt rejection were measured at a low inlet flow velocity (2.33 cm/min) and low vacuum (20 kPa) conditions. In addition, the specific heating energy of the system was determined to be significantly lower in comparison to the conventional MD system under similar conditions. Finally, numerical simulation tools based on computational fluid dynamics (CFD) coupling heat and mass transport have been used to assess the system’s limitations and potential. In addition, the impact of RF heating on fouling, specifically addressing common inorganic scaling (i.e., CaSO4) as it is a limiting factor in MD processes, was assessed. Scaling was addressed in terms of distillate flux change and crystal formation at the membrane surface and in the liquid medium. Scaling results showed the impact of RF electromagnetic field on salts crystallization, leading to less scaling. Following analysis of membrane surfaces, only sporadic small CaSO4 crystals were detected, while high concentrations of small crystals were detected at the concentrate stream exiting the MD process. The scaling mitigation mechanism is hypothesized to be a result of the high-frequency movement and collision of the ions in the solution. The second approach is based on using RF heated stainless steel thermally conducting feed spacers, thus, coupling heating and mixing. The RF heated spacers were shown (experimentally and by numerical simulations) to reduce TP. The influence of the spacer’s material and geometry were evaluated and results were compared to a conventional MD process using a polymeric spacer. Higher spacer mass and larger porosity led to an increase in distillate flux and the use of thermally conducting spacers heated by RF significantly enhanced the distillate flux while reducing the specific heating energy. Overall, the results are promising as they show the RF-MD systems have the potential to improve MD processes, specifically for hypersaline solutions such as concentrated brine of produced wastewater, where pressure-based applications are limited.