Table 1 Comparison between membrane technologies for water recovery
Process | Gas separation with dense membrane | Membrane condenser | Transport membrane condenser |
|---|---|---|---|
Concept |
|
|
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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. |
