Principles of Membrane Separation and Adsorption Processes
Membrane Separation Processes
Gel Polarisation Model in Ultrafiltration
During ultrafiltration, retained solutes accumulate at the membrane surface, forming a concentration polarisation layer. When solute concentration reaches a limiting value (gel concentration, Cg), a gel layer forms, acting as a secondary resistance. The permeate flux is given by: J = k · ln(Cg / Cb), where k is the mass transfer coefficient. Beyond a critical pressure, flux becomes pressure-independent. Flux can be improved by increasing turbulence (raising k) or reducing bulk concentration (Cb).
Advantages and Disadvantages of Membrane Processes
- Advantages: Operates at ambient temperatures (ideal for heat-sensitive materials), no phase change (energy efficient), continuous and scalable, compact modular design, and high selectivity.
- Disadvantages: Membrane fouling, concentration polarisation, high capital/replacement costs, limited chemical/thermal resistance, and difficulty separating similar-sized molecules.
Dead-End vs. Cross-Flow Filtration
- Dead-End Filtration: Feed is perpendicular to the membrane; no retentate stream; forms a filter cake; flux declines continuously; batch operation; simple and inexpensive.
- Cross-Flow Filtration: Feed flows parallel to the membrane; retentate is continuously withdrawn; high velocity prevents thick cake buildup; pseudo-steady state flux; continuous operation; higher energy consumption.
Reverse Osmosis (RO) and Nanofiltration (NF)
RO uses pressure to overcome osmotic pressure, forcing pure solvent through dense, non-porous membranes (e.g., Thin-Film Composite). NF removes divalent ions and larger molecules at lower pressures. Osmotic pressure is negligible in MF and UF because they remove particles/colloids, allowing small solutes to pass freely.
Advanced Separation Techniques
Reactive Distillation
A process intensification technique combining reaction and distillation in a single unit. It shifts equilibrium toward products (Le Chatelier’s principle), increasing conversion. Ideal for reversible reactions like esterification and etherification.
Supercritical Fluid Extraction (SFE)
Uses fluids (typically CO₂) above their critical temperature and pressure. It combines liquid-like density (solvating power) with gas-like diffusivity (fast mass transfer). It is ideal for heat-sensitive materials and leaves a solvent-free extract.
Pervaporation
Separates liquid mixtures (e.g., azeotropes) using a dense, non-porous membrane. The driving force is the chemical potential difference. It involves selective sorption, diffusion, and desorption as vapour.
Adsorption and Biofiltration
Pressure Swing Adsorption (PSA)
Operates on the principle that gases adsorb at high pressure and desorb at low pressure. Two beds are typically used alternately to ensure continuous operation.
Biofiltration
A treatment process where microorganisms in a porous medium (e.g., soil, sand) degrade pollutants. Processes include adsorption, biodegradation, oxidation, nitrification, and denitrification. It is used for wastewater treatment and VOC removal.
Regeneration of Adsorbents
Techniques include thermal regeneration, pressure swing, vacuum regeneration, steam stripping, and chemical regeneration.
Chromatography
Classified by mobile phase:
- Gas Chromatography (GC): Uses gas; for volatile, thermally stable compounds.
- Liquid Chromatography (LC): Uses liquid; includes adsorption and partition techniques.