The basics of membrane technology
Membrane technology is a physical process used to separate material mixtures, in which a thin material layer just a few microns thick functions as a filter. The separated substances are not thermally, chemically, or biologically modified. In recent years, new concepts such as membrane contactors and functionalization have significantly expanded the fields of application and the interest in membrane technology.
A membrane material is a permeable or semi-permeable thin-layer barrier between two phases that restricts the movement of specific components. The phases on either side of the membrane can be in liquid or gas form.
Membranes can selectively transport one component from the upstream phase to the downstream phase more efficiently than others, enabling separation. Additionally, porous thin layers are increasingly used as highly effective tools to bring different phases into contact. In such cases, the selectivity of the membrane material becomes secondary; this is the concept of the "membrane contactor," which finds applications in separation or reaction processes. Membrane contactors are employed in distillation, extraction, distribution, and reaction processes.
Membrane materials are classified based on various features, including material type (organic or inorganic), morphology and structure (symmetric or asymmetric), and manufacturing process. Depending on the manufacturing method, membranes can take different forms, such as tubular, flat, or spiral-wound membranes. These membranes are integrated into an engineered unit called the module, which plays a decisive role in ensuring membrane efficiency. A wide variety of module designs exist to meet specific user requirements.
Membrane processes vary in terms of molecular separation size and the driving force used. Some common processes include:
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Microfiltration (MF): Resembling conventional coarse filtration, MF separates particles between 0.1 and 10 µm, such as suspended solids, bacteria, and large proteins. It is commonly used for clarification, sterilization, cell harvesting, and separating oil-water emulsions.
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Ultrafiltration (UF): This pressure-driven process employs microporous membranes with pore diameters between 1 and 100 nm, allowing the passage of small molecules like water and salts while retaining larger molecules such as polymers and proteins. UF operates at pressures of 1 to 5 bars and is ideal for fractionation, concentration, and purification.
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Reverse Osmosis (RO): RO separates solution components using a pressure-driven process based on the electrochemical potential difference across the membrane. Operating pressures range from 10 to 100 bars, with typical applications including seawater desalination.
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Nanofiltration (NF): NF is a pressure-driven process primarily used for recycling aqueous solutions. Operating pressures range from 5 to 20 bars.
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Electrodialysis (ED): In ED, ions are transported through semi-permeable membranes under an electric potential. Cation- or anion-selective membranes allow only specific ions to pass. Applications include desalination, demineralization, and metal removal.
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Pervaporation (PV): PV is a fractionation process where a liquid mixture at atmospheric pressure is separated, with the permeate removed as a vapor. Transport is induced by vacuum or by cooling the vapor to create a partial vacuum.
Gas separation membranes are widely used in industrial applications such as air separation, hydrogen recovery, natural gas processing, air dehydration, organic vapor recovery, and CO2 emission reduction from coal- or gas-fired power plants.
Significant advancements are anticipated in the functionalization of membranes, such as modifications with ligands, and integration with enzymes or nanoparticles, to further enhance their capabilities and applications.
