Assessment of air-gap membrane distillation with hydrophobic porous membranes utilized for damaged paintings humidification

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.

Fabrication of Polyamide-6 Membranes—The Effect of Gelation Time towards Their Morphological, Physical, and Transport Properties

Porous polyamide-6 membranes were fabricated via a non-solvent induced phase inversion method, and the influence of gelation time on the properties of the membranes was investigated. Membrane samples with various gelation times were prepared. The evaluation of the membranes’ properties was carried out by various analyses and tests, such as scanning electron microscopy, atomic force microscopy, contact angle, wet and dry thickness, mean pore size measurements, porosity, water uptake, mechanical resistance, hydrodynamic water fluxes, membrane hydrodynamic permeability, and retention testing. The scanning electron microscopy images (both surface and cross-section) demonstrated that the increase in gelation time from 0 (M₀) to 10 (M₁₀) min led to the morphological change of membranes from isotropic (M₀) to anisotropic (M₁₀). The wet and dry thickness of the membranes showed a downward tendency with increasing gelation time. The M₀ membrane exhibited the lowest bubble contact angle of 60 ± 4° and the lowest average surface roughness of 124 ± 22 nm. The highest values of mean pore size and porosity were observed for the M₀ sample (0.710 ± 0.06 µm and 72 ± 2%, respectively), whereas the M₁₀ membrane demonstrated the highest tensile strength of 4.1 MPa. The membrane water uptake was diminished from 62 to 39% by increasing the gelation time from 0 to 10 min. The M₀ membrane also showed the highest hydrodynamic water flux among the prepared membranes, equal to 28.6 L m⁻² h⁻¹ (at Δp = 2 bar).

A Critical Review of Membrane Wettability in Membrane Distillation from the Perspective of Interfacial Interactions

Hydrophobic membranes used in membrane distillation (MD) systems are often subject to wetting during long-term operation. Thus, it is of great importance to fully understand factors that influence the wettability of hydrophobic membranes and their impact on the overall separation efficiency that can be achieved in MD systems. This Critical Review summarizes both fundamental and applied aspects of membrane wetting with particular emphasis on interfacial interaction between the membrane and solutes in the feed solution. First, the theoretical background of surface wetting, including the relationship between wettability and interfacial interaction, definition and measurement of contact angle, surface tension, surface free energy, adhesion force, and liquid entry pressure, is described. Second, the nature of wettability, membrane wetting mechanisms, influence of membrane properties, feed characteristics and operating conditions on membrane wetting, and evolution of membrane wetting are reviewed in the context of an MD process. Third, specific membrane features that increase resistance to wetting (e.g., superhydrophobic, omniphobic, and Janus membranes) are discussed briefly followed by the comparison of various cleaning approaches to restore membrane hydrophobicity. Finally, challenges with the prevention of membrane wetting are summarized, and future work is proposed to improve the use of MD technology in a variety of applications.

Direct contact membrane distillation for effective concentration of perfluoroalkyl substances - Impact of surface fouling and material stability

Polyfluoroalkyl and perfluoroalkyl substances (PFAS) are ecotoxic amphiphilic compounds containing alkyl-fluorinated chains terminated with weak acid moieties, and hence difficult to be degraded or removed from water sources. Direct contact membrane distillation (DCMD) was used for concentrating and removing of perfluoropentanoic acid (PFPeA) compounds from model contaminated water using commercially available poly (tetrafluoroethylene) (PTFE) membranes. The membranes were characterised for surface morphology, roughness, contact angle and pore size distribution before and after the DCMD test to investigate and evaluate membrane fouling. During the DCMD test performed for 6 h using 10 ppm PFPeA solution, the membrane exhibited progressive increased flux (from 17 to 43 kg m^-2 h^-1) and decreased PFPeA rejection (from 85 to 58%), as the feed temperature was increased from 50 to 70 °C. Further, the feed/retentate side showed a 1.8, 2.1 and 2.8-fold increase in PFPeA concentration tested at feed temperatures 50, 60, and 70 °C, respectively. The permeate side contained less than 1 ppm of PFPeA revealing that the PFPeA moved across the PTFE membrane during DCMD, which is attributed to progressive surface diffusion over time. This study opens a new route to concentrate and remove amphiphilic molecules, such as PFAS, from source points, relevant to landfill leachates or surface waters. The study also points at gaps in materials science and surface engineering to be tackled to deal with PFAS compounds efficiently.

A New Type of Composite Membrane PVA-NaY/PA-6 for Separation of Industrially Valuable Mixture Ethanol/Ethyl Tert-Butyl Ether by Pervaporation

Pervaporation is a membrane technique used to separate azeotropic and close boiling solvents. Heterogenous PVA composite membranes with NaY zeolite supported on polyamide-6 were fabricated and utilized in organic-organic pervaporation. The efficiency of prepared membranes was evaluated in the separation of ethanol/ethyl tert-butyl ether (EtOH/ETBE) using separation factor (β) and the thickness normalized pervaporation separation index (PSIN). Implementation of the fringe projection phase-shifting method allowed to the determined contact angle corrected by roughness. The influence of the presence of water traces in the feed on the overall separation efficiency was also discussed using the enrichment factor for water (EFwater). The incorporation of NaY into PVA matrix increases surface roughness and hydrophilicity of the composite membrane. It was found that membranes selectively transport ethanol from the binary EtOH/ETBE mixture. The values of β (2.3) and PSIN (288 μm g m-2 h-1) for PVA-NaY/PA6 membrane were improved by 143% and 160% in comparison to the values for the pristine PVA/PA6 membrane. It was found that membranes showed EFwater > 1, thus revealing the preferential transport of water molecules across membranes. These results are also significant for the design of membranes for the removal of water excess from the mixtures of organic solvents.

Incorporation of PVDF Nanofibre Multilayers into Functional Structure for Filtration Applications

Membranes are considered as a promising technology for separation and filtration processes. Here, novel polyvinylidene fluoride (PVDF) nanofibrous multilayer membranes were fabricated by wire-based industrial electrospinning equipment following by a lamination process. The lamination process was optimised under various applied temperature, force of lamination, and lamination time. Air permeability and burst-pressure tests were run to determine the optimum membranes for filtration application. The structures of the prepared membranes were characterised by scanning electron microscopy and pore-size analysis. The hydrophilic properties of the membranes were evaluated using water contact angle measurement, and the mechanical strength of the membranes was analysed. Air and water filtration tests were run to find the possible application of prepared membranes. The air filtration results showed that membranes had high filtration efficiencies: Over 99.00% for PM2.5, and PM0.1. The water filtration results indicated that permeability of the membranes changed from 288 to 3275 L/m²hbar. The successful preparation of such an interesting material may provide a new approach for the design and development of electrospun filter membranes.

Development and Characterization of Polyamide-Supported Chitosan Nanocomposite Membranes for Hydrophilic Pervaporation

An experimental protocol of preparation of homogeneous and nanocomposite chitosan (Ch) based membranes supported on polyamide-6 (PA6) films was developed and described in detail. Montmorillonite (MMT) and Cloisite 30B (C30B) nanoclays were used as nanofillers to improve mechanical properties of chitosan films. The surface, mechanical, and transport properties of PA6 supported Ch, Ch/MMT and Ch/C30B membranes were studied and compared with a pristine, non-supported chitosan membrane. Implementation of advanced analytical techniques e.g., SEM reveal the clays nanoparticles are well dispersed in the chitosan matrix. According to AFM images, composite chitosan/nanoclay membranes possess higher roughness compared with unfilled ones. On the other hand, an incorporation of clay particles insignificantly changed the mechanical and thermal properties of the membranes. It was also found that all membranes are hydrophilic and water is preferentially removed from EtOH/H₂O and iPrOH/H₂O mixtures by pervaporation. Supporting of chitosan and chitosan/nanoclay thin films onto PA6 porous substrate enhanced permeate flux and pervaporation separation index, in comparison to the pristine Ch membrane. Concerning separation factor (β), the highest value equal to 4500 has been found for a chitosan composite membrane containing Cloisite 30B contacting 85/15 wt % iPrOH/H₂O mixture. The mentioned membrane was characterized by the normalized flux of 0.5 μm·kg·m^-2·h^-1. Based on the established data, it was possible to conclude that chitosan membranes are meaningful material in dehydration of azeotropic mixtures. Nevertheless, to boost up the membrane efficiency, the further modification process is required.