Modelling of a modified air gap distillation membrane for the desalination of seawater

Numerical and experimental analyses were performed for a modified air gap distillation membrane that combines the low conductive heat loss of an air gap membrane for distillation with the reduced mass transfer resistance of a direct contact membrane. Instead of a stagnant film of air separating the membrane and the condensing surface, the air is moved by temperature differences between the top and the bottom of the cavity. The air movement within the enclosure modifies the hydrodynamic conditions and the temperature and the mass transfer mechanisms in the air gap. The result shows a slight enhancement of permeate flux. An experimental laboratory apparatus was built, and different operating parameters were investigated such as feed flow rate, temperature difference and air gap width. The results are in good agreement with those found in the numerical study.

Numerical solutions to the Navier–Stokes mass and energy equations for laminar natural convection for the considered cavity were obtained for small Rayleigh numbers using an elliptic procedure and a control volume scheme. This numerical approach allowed us to analyze the complex natural convection flow situations arising in the air gap. The modification of the AGMD increased the velocity level in the air gap and enhanced the permeate flux. V* X* Y* — Dimensionless vertical velocity (V a/") — Dimensionless x coordinate (x/a) — Dimensionless y coordinate (y/a) Greek " $T < R* T* Thermal diffusivity, m2/s Thermal expansion coefficient, K!1 Cinematic viscosity, m2/s Dimensionless stream function (R/") — Dimensionless vorticity (T a2)/" — — — —