Theoretical guidance for fabricating higher flux hydrophobic/hydrophilic dual-layer membranes for direct contact membrane distillation

Engineered eco-friendly composite membranes with superhydrophobic/hydrophilic dual-layer for DCMD system

Considerable advancements have been made in the development of hydrophobic membranes for membrane distillation (MD). Nonetheless, the environmentally responsible disposal of these membranes poses a critical concern due to their synthetic composition. Herein, an eco-friendly dual-layered biopolymer-based membrane was fabricated for water desalination. The membrane was electrospun from two bio-polymeric layers. The top hydrophobic layer comprises polycaprolactone (PCL) and the bottom hydrophilic layer from cellulose acetate (CA). Additionally, silica nanoparticles (SiO2 NPs) were electrosprayed onto the top layer of the dual-layered PCL/CA membrane to enhance the hydrophobicity. The desalination performance of the modified PCL-SiO2/CA membrane was compared with the unmodified PCL/CA membrane using a direct contact membrane distillation (DCMD) unit. Results revealed that silica remarkably improves membrane hydrophobicity. The modified PCL-SiO2/CA membrane demonstrated a significant increase in water contact angle of 152.4° compared to 119° for the unmodified membrane. In addition, PCL-SiO2/CA membrane has a smaller average pore size of 0.23 ± 0.16 μm and an exceptional liquid entry pressure of water (LEPw), which is 3.8 times higher than that of PCL/CA membrane. Moreover, PCL-SiO2/CA membrane achieved a durable permeate flux of 15.6 kg/m2.h, while PCL/CA membrane showed unstable permeate flux decreasing approximately from 25 to 12 kg/m2.h over the DCMD test time. Furthermore, the modified PCL-SiO2/CA membrane achieved a high salt rejection value of 99.97% compared to a low value of 86.2% for the PCL/CA membrane after 24 h continuous DCMD operation. In conclusion, the proposed modified PCL-SiO2/CA dual-layer biopolymeric-based membrane has considerable potential to be used as an environmentally friendly membrane for the MD process.

Study on Low Thermal-Conductivity of PVDF@SiAG/PET Membranes for Direct Contact Membrane Distillation Application

In order to enhance the separation performance and reduce the heat loss of transmembrane for membrane distillation, the thermal efficiency and hydrophobicity of the membrane distillation need to be simultaneously enhanced. In this work, a polyvinylidene difluoride/polyethylene glycol terephthalate (PVDF/PET) hydrophobic/hydrophilic membrane has been prepared by non-solvent phase induction method. Nanosized silica aerogel (SiAG) with high porosity has been added to the composite membranes. The modifying effects and operating conditions on permeate flux and thermal efficiency in direct contact membrane distillation (DCMD) are investigated. Furthermore, the latent heat of vaporization and the heat transfer across the membranes have been compared for SiAG addition, which indicates that the composite PVDF@SiAG/PET membranes demonstrate a great potential for distillation-separation application due to their high heat efficiency.

Structural design of the electrospun nanofibrous membrane for membrane distillation application: a review

Although membrane distillation (MD) is a promising technology for water desalination and industrial wastewater treatment, the MD process is not widely applied in the global water industry due to the lack of a suitable membrane for the MD process. The design and appropriate manufacture are the most important factors for MD membrane optimization. The well-designed porous structure, superhydrophobic surface, and pore-wetting prevention of the membrane are vital properties of the MD membrane. Nowadays, electrospinning that is capable of manufacturing membranes with superhydrophobic or omni phobic properties is considered a promising technology. Electrospun nanofibrous membranes (ENMs) possess the characteristics of cylindrical morphology, re-entrant structure, and easy-shaping for a specific purpose, benefiting the membrane design and modification. Based on that, this review investigates the current state and future progress of the superhydrophobic, multi-layer, and omniphobic ENMs manufactured with various structural designs for seawater desalination and wastewater purification. We expect that this paper will provide some recommendations and guidance for further fabrication research and the configuration design of ENMs in the MD process for seawater desalination and wastewater purification.

Desalination at ambient temperature and pressure by a novel class of biporous anisotropic membrane

Recent scientific advances have made headway in addressing pertinient issues in climate change and the sustainability of our natural environment. This study makes use of a novel approach to desalination that is environment friendly, naturally sustainable and energy efficient, meaning that it is also cost efficient. Evaporation is a key phenomenon in the natural environment and used in many industrial applications including desalination. For a liquid droplet, the vapor pressure changes due to the curved liquid–vapor interface at the droplet surface. The vapor pressure at a convex surface in a pore is, therefore, higher than that at a flat surface due to the capillary effect, and this effect is enhanced as the pore radius decreases. This concept inspired us to design a novel biporous anisotropic membrane for membrane distillation (MD), which enables to desalinate water at ambient temperature and pressure by applying only a small transmembrane temperature gradient. The novel membrane is described as a super-hydrophobic nano-porous/micro-porous composite membrane. A laboratory-made membrane with specifications determined by the theoretical model was prepared for model validation and tested for desalination at different feed inlet temperatures by direct contact MD. A water vapor flux as high as 39.94 ± 8.3 L m⁻² h⁻¹ was achieved by the novel membrane at low feed temperature (25 °C, permeate temperature = 20 °C), while the commercial PTFE membrane, which is widely used in MD research, had zero flux under the same operating conditions. As well, the fluxes of the fabricated membrane were much higher than the commercial membrane at various inlet feed temperatures.

Preparation and Characterization of a Novel Poly(vinylidene fluoride-co-hexafluoropropylene)/Poly(ethersulfone) Blend Membrane Fabricated Using an Innovative Method of Mixing Electrospinning and Phase Inversion

In this work, a novel polymeric membrane was innovated in terms of composition and preparation techniques. A blend of poly(vinylidene fluoride-co-hexafluoropropylene) (PcH) and poly(ethersulfone) (PES) (18 wt.% total polymer concentration) was prepared using a N-methylpyrrolidone (NMP) and N, N-Dimethylformamide (DMF) solvents mixture, while Lithium chloride (0.05–0.5 wt.%) was used as an additive. The electrospinning and phase inversion techniques were used together to obtain a novel membrane structure. The prepared membranes were characterized using scanning electron microscope imaging, energy dispersive X-Ray, differential scanning calorimeter, thermogravimetric analysis, and Fourier transfer infrared spectroscopy-attenuated total reflectance analyses. Moreover, the static water contact angle, membrane thickness, porosity, surface roughness as well as water vapor permeability were determined. ImageJ software was used to estimate the average fiber diameter. Additionally, the effect of the change of PcH concentration and coagulation bath temperature on the properties of the fabricated membrane was studied. The novel developed membrane has shown a good efficiency in terms of properties and features, as a membrane suitable for membrane distillation (MD); a high porosity (84.4% ± 0.6), hydrophobic surface (136.39° ± 3.1 static water contact angle), and a water vapor permeability of around 4.37 × 10⁻⁵ g·m/m²·day·Pa were obtained. The prepared membrane can be compared to the MD membranes commercially available in terms of properties and economic value.

Review of Transport Phenomena and Popular Modelling Approaches in Membrane Distillation

In this paper, the transport phenomena in four common membrane distillation (MD) configurations and three popular modelling approaches are introduced. The mechanism of heat transfer on the feed side of all configurations are the same but are distinctive from each other from the membrane interface to the bulk permeate in each configuration. Based on the features of MD configurations, the mechanisms of mass and heat transfers for four configurations are reviewed together from the bulk feed to the membrane interface on the permeate but reviewed separately from the interface to the bulk permeate. Since the temperature polarisation coefficient cannot be used to quantify the driving force polarisation in Sweeping Gas MD and Vacuum MD, the rate of driving force polarisation is proposed in this paper. The three popular modelling approaches introduced are modelling by conventional methods, computational fluid dynamics (CFD) and response surface methodology (RSM), which are based on classic transport mechanism, computer science and mathematical statistics, respectively. The default assumptions, area for applications, advantages and disadvantages of those modelling approaches are summarised. Assessment and comparison were also conducted based on the review. Since there are only a couple of full-scale plants operating worldwide, the modelling of operational cost of MD was only briefly reviewed. Gaps and future studies were also proposed based on the current research trends, such as the emergence of new membranes, which possess the characteristics of selectivity, anti-wetting, multilayer and incorporation of inorganic particles.