Thermally Driven Separation
Membrane distillation exploits the vapor pressure difference across a hydrophobic microporous membrane. Hot feed water evaporates at the membrane surface, vapor molecules traverse the air-filled pores by diffusion and convection, and condense on the cold permeate side. Because only water vapor can enter the hydrophobic pores — liquid water is repelled by surface tension — MD achieves near-perfect rejection of all non-volatile solutes regardless of feed salinity.
Vapor Pressure & the Antoine Equation
The driving force in MD is the vapor pressure difference between hot and cold sides. Vapor pressure increases exponentially with temperature according to the Antoine equation, which means small increases in hot-side temperature produce large gains in flux. Raising feed temperature from 60°C to 80°C roughly doubles the vapor pressure and the flux. This exponential sensitivity makes MD particularly responsive to operating temperature.
Mass Transfer Through Pores
Vapor transport through membrane pores occurs by three mechanisms depending on pore size and mean free path: Knudsen diffusion (small pores), molecular diffusion (larger pores with trapped air), and Poiseuille flow (very large pores). In typical MD membranes (0.1-0.5 μm pores), a combination of Knudsen and molecular diffusion dominates. Higher porosity and larger pores increase flux but also increase the risk of liquid breakthrough.
Configurations & Applications
Four MD configurations exist: direct contact (DCMD), air gap (AGMD), vacuum (VMD), and sweeping gas (SGMD). DCMD is simplest but has the highest heat losses. AGMD adds an air gap and cooling plate to improve thermal efficiency. MD is being developed for desalination of hypersaline brines, zero-liquid-discharge systems, concentration of fruit juices, and treatment of radioactive wastewater — anywhere complete rejection and low-grade heat availability converge.