Surface heated membrane with laser-induced graphene technique for membrane distillation

Freshwater scarcity has become a major concern due to accelerating population growth that increases the demand from the agricultural and industrial sectors. To address the increasing freshwater supply and demand, we need to rely on different desalination techniques. However, existing technologies face challenges while treating waters having high TDS (> 70000 mg/L). Membrane distillation (MD) is a promising technology that can be used to treat hypersaline water. MD is a newer desalination technology requiring a relatively lower operating temperature and is less prone to fouling than pressure-driven processes. An efficient MD process operation requires a porous hydrophobic membrane that prevents liquid intrusion to the permeate side while only allowing water vapor to diffuse through the membrane. However, the lack of a robust hydrophobic membrane and lower energy efficiency requirements limits MD’s true potential. In this context, exploring more robust hydrophobic membranes and devising more innovative solutions to reduce the energy consumption of the MD process have been the subject of extensive research. The primary focus of the research outlined in this dissertation is to propose a novel membrane fabrication technique for developing electroconductive surface-heated membranes and integrating innovative heating techniques that can be used in MD applications. The synthesis approach for the new membrane includes the phase inversion step to fabricate the polyethersulfone (PES) membrane substrate, followed by CO2 laser ablation. Different membrane substrates with different PES concentrations were fabricated with casting thicknesses of 700 µm. PES membrane substrates were soaked in 10 wt.% CaCl2 after being taken out of the coagulation bath. After drying, CO2 laser ablation was performed while controlling various laser parameters such as power, speed, pulse per inch (PPI), and line per inch (LPI – also known as image density). Different types of laser patterns were studied, which can give electroconductive membranes suitable for surface heating techniques integrating radiofrequency (RF) and Joule heating (JH). During the laser ablation, the polymeric surface of the membrane converts to a three-dimensional, porous conductive graphene, referred to as laser-induced graphene (LIG). Laser parameters significantly impact surface-wetting properties as these parameters control the formation of graphene on the PES membrane substrate. SEM-EDS was performed on the membranes for surface and cross-sectional morphological study, which confirmed a preserved porous structure underneath the LIG. Water contact angles were measured, which show an increase from 60° to 143.6° with increasing laser power. Raman spectroscopy and XPS analysis were also carried out to confirm graphene formation with laser ablation. The prepared LIG membrane was used in conventional VMD first before integrating surface heating. A successful VMD performance was observed, and then surface heating with RF was investigated, which eliminates MD's bulk feed heating requirement. LIG heating capability was studied first in the RF field. LIG materials showed excellent thermal response in the RF field with a maximum steady-state temperature of 500 °C. Unprecedented heating rates, as high as 502 °C/s, were observed when LIG was exposed to RF fields at 200 MHz frequency and 4.6 W power. RF parameter was further optimized for application specific to the VMD membrane module in this work , showing surface average temperature of ~140 °C. The PES-LIG membrane's mechanical and thermal properties are characterized by an investigation of its feasibility for MD application. VMD with RF heating shows permeate flux up to 13.5 L m-2 h-1 with >99% salt rejection. Cyclic thermal and mechanical stability tests and long-term VMD tests were performed, which showed stable performance of the LIG membranes. This work demonstrates a novel MD technique strategy that can potentially address challenges impeding its commercialization. The surface heating study of VMD was further extended with the JH technique for the PES-LIG membranes. JH requires a different type of laser pattern for enhanced electrical properties of the membrane, which gives a hydrophilic LIG layer. To obtain a hydrophobic surface, PDMS-SiO2 coating was introduced with drop-casting, which gave a water contact angle of 152°, making a superhydrophobic LIG surface. JH in two modes, with direct current (DC) and alternating current (AC), was investigated. It has been found that electrode corrosion on DC JH can be overcome with AC JH. JH technique with the PES-LIG membrane prepared in this work showed successful application in VMD with permeate flux as high as 11.9 L m-2 h-1 with >99% salt rejection.