Membrane technology Study Guide

📖 Core Concepts Membrane technology: Uses semi‑permeable membranes to physically separate or reject substances between two phases (gas or liquid). Driving forces: Pressure, concentration gradient, electric potential, or temperature difference push species through the membrane. Transport models Solution‑diffusion: Species dissolve in a dense, non‑porous membrane and diffuse across (key for reverse osmosis, fuel cells). Hydrodynamic (convective): Species are carried through pores; size exclusion dominates (micro‑/ultrafiltration). Membrane shapes & flow Cross‑flow (tangential): Reduces fouling and concentration polarization. Dead‑end: Simpler, but prone to rapid fouling and high polarization. Performance metrics Flux $J$ (volume per area per time). Hydraulic permeability $Lp = \dfrac{J}{\Delta p}$ (inverse of total resistance). Solute sieving coefficient $S = \dfrac{Cp}{Cf}$ (permeate vs feed concentration). Selectivity vs permeability trade‑off: Higher selectivity usually lowers permeability; increasing surface area (e.g., spiral‑wound modules) can compensate. Biomass‑based membranes: Renewable alternatives to traditional polymeric membranes; aim for comparable pore size, strength, and stability. --- 📌 Must Remember Darcy’s Law (dead‑end): $Q = \dfrac{\Delta p\,A}{\mu\,(Rm+R)}$ Sieving coefficient: $S = \dfrac{Cp}{Cf}$ – $S\approx0$ = full rejection, $S\approx1$ = no rejection. Hydraulic permeability: $Lp = \dfrac{J}{\Delta p}$. MWCO rule: Choose a membrane whose MWCO is ≥ 20 % lower than the target molecule’s molecular weight. Concentration polarization → flux loss; mitigated by cross‑flow or cleaning. Main pressure‑driven processes: MF, UF, NF, RO, gas separation. Non‑pressure‑driven: Dialysis, pervaporation, forward osmosis, membrane distillation, electrodialysis (electric). Typical applications MF/UF → colloids, macromolecules, microbes (food, biotech, pharma). NF/RO → water desalination/purification. Dense inorganic membranes → gas separations (CO₂, N₂). --- 🔄 Key Processes Solution‑Diffusion Transport Dissolve → diffuse → desorb. Governs dense membranes (RO, fuel cells). Hydrodynamic (Pore) Transport Convective flow through pores; size exclusion. Governs MF, UF. Cross‑Flow Filtration Feed flows tangentially, creating shear that cracks filter cake → lower fouling. Dead‑End Filtration Feed perpendicular to membrane; particles accumulate → rapid fouling, high polarization. Darcy Flow Calculation (dead‑end) Compute $Q$ using pressure drop, area, viscosity, and resistances. --- 🔍 Key Comparisons Solution‑Diffusion vs. Hydrodynamic Diffusion: Requires solubility in membrane; non‑porous; dominant in RO. Hydrodynamic: Relies on pore size; convective; dominant in MF/UF. Cross‑Flow vs. Dead‑End Cross‑Flow: Tangential flow, lower fouling, higher flux stability. Dead‑End: Simpler, higher fouling, useful for bench‑scale tests. Pressure‑Driven vs. Concentration‑Driven Pressure: Mechanical work (MF‑RO). Concentration: Chemical potential gradient (dialysis, forward osmosis). MWCO vs. Nominal Pore Size MWCO: Molecular weight cut‑off (daltons), relates to retention of globular molecules. Nominal pore size: Physical dimension of largest pores; used for particle size exclusion. --- ⚠️ Common Misunderstandings “All membranes reject everything smaller than the pore” – Dense membranes reject based on solubility/diffusion, not just size. “Higher pressure always increases flux” – Beyond a point, fouling and concentration polarization dominate, reducing net flux. “MWCO = pore diameter” – MWCO is an empirical molecular‑weight metric; actual pore geometry may differ. “Cross‑flow eliminates fouling” – It mitigates but does not completely prevent fouling; cleaning is still required. --- 🧠 Mental Models / Intuition “Sieve vs. Sponge”: Sieve (hydrodynamic) – particles larger than holes are blocked, fluid passes through. Sponge (solution‑diffusion) – everything can enter the material, but only soluble species can migrate across. “Traffic jam on the membrane surface” → concentration polarization; think of cars (solutes) piling up at a toll booth (membrane). Tangential flow is a side road that relieves the jam. --- 🚩 Exceptions & Edge Cases Ultra‑high‑flux membranes: May exhibit slip flow where Darcy’s law under‑predicts flux. Temperature‑gradient processes (membrane distillation) operate without pressure but still require a hydrophobic membrane to prevent liquid penetration. Biomass membranes: May have variable pore distribution; MWCO rule may need tighter safety margins. --- 📍 When to Use Which Reverse Osmosis → Desalination, high‑purity water; need dense, non‑porous membrane, high pressure. Nanofiltration → Softening water, partial organics removal; moderate pressure, semi‑dense membrane. Ultrafiltration → Protein, virus, colloid removal; choose pore size 10–100 nm, cross‑flow preferred. Microfiltration → Bacteria, suspended solids; pore size >0.1 µm, dead‑end feasible for small batches. Electrodialysis → Ion removal from brackish water; apply electric potential, use ion‑exchange membranes. Membrane Distillation → Treat high‑salinity brines where heating is available; use hydrophobic membrane, temperature gradient. --- 👀 Patterns to Recognize Sudden drop in flux + rising pressure → fouling or cake formation. Permeate quality unchanged despite higher pressure → concentration polarization limiting effective driving force. High rejection of small molecules → dense membrane operating under solution‑diffusion regime. Linear relationship between $\Delta p$ and $Q$ → Darcy’s law holds (no severe fouling). --- 🗂️ Exam Traps Choosing “pressure‑driven” for pervaporation – pervaporation is actually driven by a temperature gradient, not pressure. Confusing MWCO with absolute pore diameter – MWCO is a molecular‑weight metric; the actual pore may be larger or smaller. Assuming cross‑flow eliminates all fouling – exam may present a scenario where fouling persists; answer should note mitigation, not elimination. Using Darcy’s law for cross‑flow calculations – Darcy’s law applies to dead‑end; cross‑flow requires additional terms for shear‑induced resistance. ---