A three-dimensional model for the heat and mass transfer in air-gap membrane distillation

Membrane distillation (MD) is a process to desalinate sea water. Pilot plants are operated aiming
at increasing the modules’ efficiency for large-scale applications. In air-gap membrane distillation
(AGMD) the state-of-the-art modeling of mass and heat transfer is one-dimensional, combining
evaporation and diffusion through the membrane and the condenser channel in one correlation.
In this work, a numerical model is developed which computes AGMD modules in three dimen-
sions. For evaporation and condensation, energy conservation equations at the interfaces are solved.
Simulation results are compared to experimental data and a good agreement is found. The model
is then employed to compare numerically two air-gap MD module configurations and evaluate
their performance at different feed inlet temperatures, velocities and air-gap thicknesses. In the
upside configuration, the hot feed flows above the membrane, while in the downside configura-
tion it flows below the air-gap and membrane. In the latter, the feed solution is not in contact with
the membrane but separated by the air-gap which is expected to improve the fouling resistance of
the membrane. The three-dimensional computational fluid dynamic computation allows the visu-
alization of the velocity profile in the air-gap due to buoyancy in the downside configuration