Comprehensive condensation flow regimes in air gap membrane distillation: Visualization and energy efficiency

Abstract

The thermal performance of air gap membrane distillation (AGMD) desalination is dominated by heat and mass transfer across the air gap between the membrane and the condensing surface. However, little is known about the impact of condensate flow patterns in some design variations of the air gap. In this study, air gap membrane distillation experiments were performed at various inlet temperatures, varying module inclination angle, condensing surface hydrophobicity, and gap spacer design to identify the effect of each on the permeate production rate and thermal efficiency of the system. Energy efficiency modeling was performed as well. Additionally, this study is one of the first with enhanced visualization of flow patterns within the air gap itself, by using a transparent, high thermal conductivity sapphire plate as the condenser surface. System-level numerical modeling is used to further understand the impact of these flow regimes on overall energy efficiency, including flux and GOR. A brief review of membrane distillation condensation regimes is provided as well. For tilting the AGMD flat-plate module, permeate flux was barely influenced except at extreme positive angles (>80ᵒ), and moderate negative angles (<-30ᵒ), where condensate fell onto the membrane surface. The surface with the hydrophobic coating (for dropwise condensation) was shown to have better droplet shedding (with very small nearly spherical droplets) and fewer droplets bridging the gap. Superhydrophobic surfaces (for jumping droplet condensation) were similar, with much smaller droplet sizes. Meanwhile, the hydrophilic surface for small gap sizes (< 3 mm) often had pinned regions of water around the hydrophilic surface and plastic spacer. Overall, the various results imply that the common assumption of a laminar condensate film poorly describes the flow patterns in real systems for all tilt angles and most spacer designs. Real system performance is likely to be between that of pure AGMD and permeate gap membrane distillation (PGMD) variants, and modeling shows that better condensing in air gaps may improve system energy efficiency significantly, with strong relative advantages at high salinity.

Long-Running Comparison of Feed-Water Scaling in Membrane Distillation

“…On the other hand, membrane distillation utilizes thermal energy to provide the driving pressure force, which does not deteriorate significantly for high salinity feed [9][10][11][12][13][14][15]. Additionally, supplementing the membrane distillation (MD) module with a vacuum pump on the distillate side, commonly known as vacuum membrane distillation (VMD), further improves the performance for extremely high salinity feed [16][17][18][19].…”

Co-axial hollow fiber module for air gap membrane distillation

“…Similarly, Warsinger et al [42] investigation on condensation flow regimes in AGMD suggested that flooding and trapping of droplets would be minimized in case of lower AGMD fluxes. These results are well correlated with our observations when a significant increase in vapor flux achieved at high feed temperatures at n = 3 was decreaded when the number of condenser fibers n was further increased to 4.…”

“…As such, there would exist an optimal number of condenser fibers which would facilitate high condensation area while maintaining small mass transfer resistance. The lower condensate removal rate compared to permeate production rate would cause permeate flooding inside the AGMD module as it has been recently addressed in a number of studies [42][43][44][45]. Despite the improved permeate flux, the thermal efficiency of the process is decreased due to the air replacing water which has poorer insulating properties [42].…”

“…The research reported in this paper was supported by King Abdullah University of Science and Technology [42], Saudi Arabia. The authors acknowledge help, assistance and support from the Water Desalination and Reuse Center (WDRC) staff.…”

Mass Transfer Analysis of Air-Cooled Membrane Distillation Configuration for Desalination

“…The main factors that limit its applications in industrial fields are scale and stability, similar to many emerging technologies. The majority of the studies on MD are focused on the development of novel porous membranes for MD applications [8][9][10][11][12], the transmembrane transport analysis in microscale [13][14][15][16][17], the configurations and their related performance [18][19][20]. The MD performance criteria usually include the permeate flux, salt rejection, specific energy consumption, scaling resistance and operating stability.…”