What is Membrane Distillation (MD)?
Thermal distillation and membrane separation are united in a unique hybrid–thermal process with high solute rejection and mild operating conditions.
This process is known as membrane distillation (MD) and utilizes green energy, is low cost, modular, and can handle high-salinity feed with low fouling propensity.
Direct Contact Membrane Distillation
In direct contact membrane distillation (DCMD), a hydrophobic porous film separates hot saline and cold distillate streams. The driving force behind clean water flows through this film is the temperature differential. Its thermal efficiency is reduced due to heat loss.
The feed-membrane interface temperature decreases due to the heat loss, and the latent heat of vaporization reduces the driving force for clean water flow over the film. This temperature drop becomes more prominent with the increase in module size, causing an average flux decline when the process is scaled up. Average flux decline and low heat capacity make the membrane less feasible for large-scale desalination.
Localized Surface Heating
Localized heating lowers the temperature polarization. Therefore, the membrane's energy efficiency is increased by surface heating.
In addition to producing a larger flux, such systems avoid some of the problems associated with heat management, including costs related to the loss of thermal energy, heat exchanger scaling, and metal corrosion.
Localized surface heating is implemented via various techniques. These techniques include induction heating carbon nanotube-coated films, joule heating thin films, and photothermal heating.
Different Techniques Used to Implement Localized Surface Heating
An electrode is not required for induction heating, avoiding costs associated with constant replacement. However, this technique requires a minimum distance between the spacer and coil due to decrease radiowaves amplitude with increased distance, or the operating power needs to be increased, raising operating costs.
Similarly, Joule heating techniques are simple to implement. However, the constant need to replace electrodes due to fouling and corrosion increases the operating costs and time. Likewise, in the polythermal heating technique, solar energy is utilized, but the conversion efficiency of the material used is less than the electrothermal material's conversion efficiency.
Although there are many different methods to deploy localized heating, the success of this technology ultimately relies on how self-heating films and spacers are made and how effectively they scale.
Most fabrication techniques involve the spacers and films coated with electro- or photothermal materials, or the electro- and photothermal materials are combined and cast as mixed matrix membranes.
Issues Associated with Membrane Coating Processes
Coating mixtures comprising the photo- or electrothermal elements must be mixed uniformly to achieve homogeneity. The coating on membranes may also lower pore diameters and permeation flux.
Casting mixed matrix membranes presents a challenge due to mixing particulates and polymer solutions in hazardous solvents of higher viscosity, making it challenging to ensure that the required quantity of photo- or electrothermal material will be present in an even distribution throughout the membrane-making.
The motivation behind this study was to fabricate such functional membranes with surface heating capacity and without reducing flux using a technology that is easier and more scalable.
Significance of the Study
In two steps, the researchers developed a technique for modifying a hydrophilic polyethersulfone (PES) membrane to fabricate Janus membranes with hydrophobic polydimethylsiloxane (PDMS) PES and conductive graphene surface on opposite sides. This improved the energy efficiency of membrane distillation.
Steps of the Fabricating Technique
The first step of the fabricating technique included surface modification by applying the PDMS layer to create a hydrophobic coating suitable for membrane distillation.
The next step involved a laser-induced polyethersulfone photothermal reaction, forming laser-induced graphene (LIG) of several thicknesses. This graphene formation method enables a variety of carbon substrate conversion to graphene, allowing the fabrication of suitable membranes for thermal-driven separation processes beyond membrane distillation.
Material characterization is carried out on the pristine PES membrane to determine amenability for use in membrane distillation. Finally, the researchers evaluated modified membranes' localized heating performance by comparing flux, single-pass heat utilization efficiency (HUEsp) and specific heat energy (QSH).
Overall, this study provided concrete evidence of the practical use of laser-induced graphene for membrane surface localized heating through Joule heating to increase DCMD's energy efficiency.
The energy efficiency of membrane distillation should be enhanced by future research on coupling with spacer changes and other heating techniques, including photothermal heating with laser-induced graphene Janus membranes.
Yong ZenTan, M.S.R. Sridhar Kapavarapu, Jia Zheng Oor, Chi Siang Ong, Jia Wei Chew (2022) Laser-induced graphene Janus membrane for electrothermal membrane distillation. Desalination. https://www.sciencedirect.com/science/article/pii/S0011916422004490