15.3 Membrane separation processes and applications

Membrane separation processes are crucial in various industries, from water treatment to food processing. These techniques use different pore sizes and driving forces to separate particles, molecules, and ions from solutions, gases, and mixtures.

Advanced membrane development faces challenges in carbon capture and biomedical applications. Researchers are exploring innovative materials and designs to improve performance, stability, and biocompatibility, opening up new possibilities in these critical fields.

Membrane Separation Processes

Types of membrane separation processes

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• Microfiltration (MF) has pore sizes ranging from 0.1-10 μm and separates suspended particles, bacteria (E. coli), and colloids from liquids
• Ultrafiltration (UF) has pore sizes ranging from 0.01-0.1 μm and separates macromolecules, proteins (albumin), and viruses from solutions
• Nanofiltration (NF) has pore sizes ranging from 0.001-0.01 μm and separates divalent ions (Ca2+, Mg2+), sugars (glucose, sucrose), and pesticides from water
• Reverse Osmosis (RO) has pore sizes less than 0.001 μm and separates monovalent ions (Na+, Cl-), small organic molecules (urea), and water from saline solutions
• Gas Separation (GS) separates gas mixtures (CO2/CH4, O2/N2) based on differences in permeability and solubility of the gas components through the membrane

Operating principles of membrane separation

• Pressure-driven processes (MF, UF, NF, RO) use transmembrane pressure (TMP) as the driving force, with higher pressure leading to higher flux and separation efficiency
  • Typical operating pressures: MF (0.1-2 bar), UF (1-10 bar), NF (5-20 bar), RO (10-100 bar)
• Concentration-driven processes (GS) use a concentration gradient as the driving force, with the permeate side having a lower concentration of the more permeable component
  • Separation is achieved by differences in solubility and diffusivity of gas molecules in the membrane material
• Feed composition affects membrane fouling and separation efficiency, with pretreatment (filtration, pH adjustment) often necessary to remove contaminants (suspended solids, oils)
• Membrane properties, such as pore size and distribution, surface charge and hydrophilicity, chemical and thermal stability, and permeability and selectivity, influence the separation performance and durability of the membrane

Applications and Challenges

Performance of polymeric membranes

• Water treatment applications use MF and UF for pre-treatment and clarification, removing suspended solids and colloids, while NF and RO are used for desalination and removal of dissolved contaminants (ions, organics)
• Food processing applications use MF for clarification of juices (apple, orange) and wines, UF for concentration of proteins (whey, soy) and recovery of valuable components (lactose, polyphenols), and NF for demineralization and sugar fractionation (glucose/fructose separation)
• Gas purification applications involve removal of CO2, H2S, and water vapor from natural gas, separation of hydrogen from syngas and refinery streams, and recovery of volatile organic compounds (VOCs) like benzene and toluene from air

Challenges in advanced membrane development

• Carbon capture requires development of high-performance polymeric membranes for CO2/N2 separation, with challenges including achieving high CO2 permeability and selectivity and maintaining stability under harsh conditions (high temperature, pressure)
  • Opportunities lie in incorporation of functional groups (amines), mixed matrix membranes (zeolites, MOFs), and facilitated transport mechanisms (carbonic anhydrase)
• Biomedical devices, such as hemodialysis membranes for blood purification and controlled drug delivery systems, face challenges in biocompatibility, fouling resistance, and long-term stability
  • Opportunities include surface modification (grafting, coating), stimuli-responsive membranes (pH, temperature), and bio-inspired designs (aquaporins, ion channels)