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Table 1 Comparison between membrane technologies for water recovery

From: Membrane condenser as emerging technology for water recovery and gas pre-treatment: current status and perspectives

Process Gas separation with dense membrane Membrane condenser Transport membrane condenser
Membrane morphology Dense Porous hydrophobic Porous hydrophilic
Parameter determining process performance High pressure difference between the membrane sides for promoting water vapor permeation. -Cooling of the feed for increasing the amount of liquid water to be recovered, depending on the temperature and relative humidity of the feed gaseous stream.
-Low pressure difference (0.01–0.1 bar) between the membrane sides for promoting the gases permeation.
-High temperature difference between the membrane sides for promoting water vapor condensation within membrane pores.
-Low pressure difference (around 0.3 bar [40]) between the membrane sides for promoting the permeation of condensed species.
Transport mode Solution diffusion mechanism Knudsen-molecular diffusion transition Capillary condensation
Water collection side Permeate Retentate Permeate
Permeating species Mainly water vapor; a small fraction of other gaseous feed species Permanent gases, a small fraction of water vapor Mainly water vapor
Retained species Low permeable species Condensed water and condensable gaseous species (whose amount strongly depends on operating conditions) Non-condensable feed components, a small fraction of water vapor
Advantages High purity of the recovered water ((H2O/N2) and H2O permeability up to 105 barrer [41, 42]). -Possibility to control the liquid water vapor composition by opportunely tuning the operating conditions.
-Possibility to recover condensable components.
-Low energy consumption
-Preferential water permeation, owed to the strong affinity of the hydrophilic materials to the water, which limits the permeation of the other species
-Heat recovery
Drawbacks High energy consumption (because the evaporated water through the cooling tower and stack leaves at near atmospheric pressure, requiring additional vacuum to apply the necessary driving force for separation). - Quality of liquid water eventually effected by the presence of contaminants.
- Limited process performance (in terms of recovered water) in the case of gaseous streams at low temperature and relative humidity.
-High temperature gradient across the membrane.
-Increase of the temperature at the membrane surface and within the membrane pores.
-Membrane pore size influences membrane selectivity [43].
- Heat and water flux must be carefully balanced to maximize the transport membrane condenser performance.