Comprehensive experimental and theoretical studies on material-gap and water-gap membrane distillation using composite membranes

Biomass-Derived Carbon and Carbon Nanofiber-Integrated Electrospun Janus Membranes: A New Frontier in Membrane Distillation

Membrane distillation (MD) is an emerging desalination technique that uses low-grade energy to extract water vapor from saline solutions. In a thermally driven MD system, achieving a lower heat transfer and a higher mass transportation rate is desirable. To balance the trade-off between heat transfer and mass transportation, we developed novel dual-layered electrospun Janus nanofibrous membranes in this study, showing asymmetric wettability on each layer. The developed Janus membrane was constructed with a bottom hydrophilic layer composed of PVDF-co-HFP/biomass-derived jute carbon (JC) particles, and the top hydrophobic layer was formed using PH/carbon nanofibers (PH/CNF). The effect of distinct carbon nanoparticles on the prepared membranes was investigated by analyzing their chemical structure, morphology, water contact angle (WCA), pore size, porosity, thickness, liquid entry pressure, and mechanical and thermal stability. The hydrophobic layer of the optimized Janus membrane exhibited a WCA of 138 ± 1°, and the hydrophilic surface showed 72 ± 4°. Additionally, the optimized Janus membrane composed of a hydrophobic PH/0.5 wt % CNF layer and PH/0.5 wt % JC hydrophilic layer experienced an outstanding improvement in water flux (with 70 g L-1 of NaCl content), reaching to a value of 71.72 kg m-2 h-1 (∼162% improvement compared to the pristine PH membrane), while maintaining a salt rejection of >99.99% for 24 h of water gap membrane distillation. Notably, the optimum Janus PH-0.5CNF/PH-0.5JC membrane demonstrated an astonishing long-term stability with real seawater, exhibiting a remarkable flux of 78.42 kg m-2 h-1, which is ∼547% higher than commercially available PVDF membranes, while maintaining a salt rejection of 99.98% after 50 h. The proposed strategies provide a novel approach to fabricate an electrospun Janus membrane, and their performance highlights a strong potential candidate to be used in commercial water desalination plants.

Productivity and Thermal Performance Enhancements of Hollow Fiber Water Gap Membrane Distillation Modules Using Helical Fiber Configuration: 3D Computational Fluid Dynamics Modeling

Although hollow fiber water gap membrane distillation (HF-WGMD) units offer certain advantages over other MD desalination systems, they still require enhancements in terms of distillate flux and productivity. Therefore, this work proposes a novel configuration by incorporating the helical turns of HF membranes within the water gap channel of the HF-WGMD modules. A fully coupled 3D CFD model is developed and validated to simulate the multifaceted energy conservations and diffusion mechanisms that are inherent to the transport phenomena in the proposed HF-WGMD module. Single and double helical HF membrane designs with different numbers of turns are compared to the reference modules of single and double straight HF membrane designs under various operational conditions. At a feed temperature of 70 °C, a noteworthy 11.4% enhancement in the distillate flux is observed when employing 20 helical turns, compared to the single straight HF membrane module. Furthermore, the specific productivity revealed a maximum enhancement of 46.2% when using 50 helical turns. The thermal performance of the proposed HF-WGMD module shows higher energy savings of up to 35% in specific thermal energy consumption for a one-stage module. Using three stages of single helical modules can increase the gain output ratio from 0.17 for the single stage to 0.37, which represents an increase of 117.6%. These findings indicate the high potential of the proposed approach in advancing the performance of HF-WGMD systems.

Experimental Investigation of a Plate-Frame Water Gap Membrane Distillation System for Seawater Desalination

This study presented a detailed investigation into the performance of a plate-frame water gap membrane distillation (WGMD) system for the desalination of untreated real seawater. One approach to improving the performance of WGMD is through the proper selection of cooling plate material, which plays a vital role in enhancing the gap vapor condensation process. Hence, the influence of different cooling plate materials was examined and discussed. Furthermore, two different hydrophobic micro-porous polymeric membranes of similar mean pore sizes were utilized in the study. The influence of key operating parameters, including the feed water temperature and flow rate, was examined against the system vapor flux and gained output ratio (GOR). In addition, the used membranes were characterized by means of different techniques in terms of surface morphology, liquid entry pressure, water contact angle, pore size distribution, and porosity. Findings revealed that, at all conditions, the PTFE membrane exhibits superior vapor flux and energy efficiency (GOR), with 9.36% to 14.36% higher flux at a 0.6 to 1.2 L/min feed flow rate when compared to the PVDF membrane. The copper plate, which has the highest thermal conductivity, attained the highest vapor flux, while the acrylic plate, which has an extra-low thermal conductivity, recorded the lowest vapor flux. The increasing order of GOR values for different cooling plates is acrylic < HDPE < copper < aluminum < brass < stainless steel. Results also indicated that increasing the feed temperature increases the vapor flux almost exponentially to a maximum flux value of 30.36 kg/m2hr. The system GOR also improves in a decreasing pattern to a maximum value of 0.4049. Moreover, a long-term test showed that the PTFE membrane, which exhibits superior hydrophobicity, registered better salt rejection stability. The use of copper as a cooling plate material for better system performance is recommended, while cooling plate materials with very low thermal conductivities, such as a low thermally conducting polymer, are discouraged.

Plasmonic Phenomena in Membrane Distillation

Water scarcity raises important concerns with respect to human sustainability and the preservation of important ecosystem functions. To satisfy water requirements, seawater desalination represents one of the most sustainable solutions. In recent decades, membrane distillation has emerged as a promising thermal desalination process that may help to overcome the drawbacks of traditional desalination processes. Nevertheless, in membrane distillation, the temperature at the feed membrane interface is significantly lower than that of the bulk feed water, due to the latent heat flux associated with water evaporation. This phenomenon, known as temperature polarization, in membrane distillation is a crucial issue that could be responsible for a decay of about 50% in the initial transmembrane water flux. The use of plasmonic nanostructures, acting as thermal hotspots in the conventional membranes, may improve the performance of membrane distillation units by reducing or eliminating the temperature polarization problem. Furthermore, an efficient conversion of light into heat offers new opportunities for the use of solar energy in membrane distillation. This work summarizes recent developments in the field of plasmonic-enhanced solar evaporation with a particular focus on solar-driven membrane distillation applications and its potential prospects.