Visualization and measurement of filmwise and dropwise air gap membrane distillation at varied module inclination angle and gap spacer Orientation

Abstract

Improving efficiency is the main target for improving rapidly developing water desalination technologies such as air gap membrane distillation (AGMD). While the system is dominated by heat and mass transfer resistances in the air gap specifically, very little is known about the impact of variations in the air gap, including air gap spacers and module tilting. AGMD experiments were performed by varying inlet temperature, module inclination angle, and gap spacer to identify the effect on the permeate production rate of the system. While AGMD is potentially one of the most efficient types of membrane distillation, experimenting with the tilt angle of the module so as to rotate the membrane and condensation plate off the vertical, provides a method to alter the behavior of the film condensation layer and its associated thermal resistance. In this study, we confirm the previous work of Warsinger et al. (2014) through the added enhancement of visualization of condensation in the air gap by use of a clear and conductive sapphire condensation plate. Additional experiments were run observing the effect of changing the orientation of the mesh spacer holding the air gap in the vertical orientation. Numerical modeling is also conducted to understand and potentially verify the experimental results. As predicted, the results maintain that at near vertical and at non-extreme positive angles (<15° from the vertical), there is no significant difference in the permeate production output of the system due to tilting the module. At declined and extremely inclined tilt angles, the air gap in the module behaves as a "permeate gap", or a flooded system, resulting in the air gap saturated with liquid water. At negative tilt angles past 30°, it was found that condensate tends to fall on the membrane, causing thermal bridging, and as a result, significantly increasing the overall production of permeate condensate. It was also found that the orientation of the mesh spacer holding the air gap apart does not have a significant effect on the permeate production rate.