The adoption of membranes for gas dehumidification was already proposed in the past by using dense membranes (based on the sorption diffusion mechanism for vapor permeation). Nevertheless, the scaling up of this technology, mostly due to the high pressure required for promoting the water vapor permeation through the membrane, limited their wider application at industrial scale. Porous hydrophilic polymer membranes were hence adopted for gas dehumidification as a membrane contactor, in which the membrane favors the contact between the humid gas phase and water. On the contrary, a membrane condenser used for recovering evaporated water from industrial gases is constituted of a microporous hydrophobic or a composite hydrophobic/hydrophilic membrane as selective barrier favoring the water condensation.
The saturated water vapor gaseous stream enters the membrane condenser, water is condensed due to the hydrophobic nature of the membrane that prevents its penetration into the pores, and it is recovered in the retentate side. On the other hand, dehydrated gases permeate through the membrane and are collected in the permeate stream. Typically, the pore dimension of the microporous hydrophobic membranes ranges between 0.1 and 0.2 μm, abundantly above the molecular kinetic diameter of any gas potentially transported in the feed stream. Therefore, the membrane doesn't act any separation on the gases present in the feed and a proper design of the membrane condenser module avoids the formation of liquid water film on the membrane surface.
By varying opportunely, the contact time between the water vapor saturated gaseous stream and the membrane condenser, the fraction of contaminants (SOx, NOx, HCl, NxOy, perfluorocarbons, VOCs, Freon, Halon etc.) retained in the condensed water (retentate stream) may be controlled, even though it is not sufficient as a one-stage process to ensure the contaminants abatement below the regulation limits because depending on the temperature difference imposed to the system. The fundamental characteristics required to a membrane used in membrane condenser modules are: 1) high hydrophobicity useful to avoid the membranes pores wetting; 2) high chemical stability (resistance to acid gases and corrosive agents), which is directly connected to its long-term stability; 3) high thermal stability, particularly requested up to 90 °C, which is the typical temperature in flue gas application, (fluorinated polymers are excellent candidates due to their high hydrophobicity and chemical stability); 4) membrane fouling resistant (in case of gaseous streams containing suspended particles); 5) long-term stability, key variable greatly affecting the economic evaluations (as high the membrane lifetime as low the maintenance costs); 6) porosity between 70 and 80%; 7) reduced thickness (20–100 μm) and high thermal conductivity, needed to favor the heat transfer through the membrane. Table summarizes the main materials used for membrane condensers. Commercial microporous hydrophobic membranes are generally based on PP or PVDF, showing high porosity (70–80%), a membrane thickness ranging from 10 to 300 μm and pore sizes ~0.2 μm, under capillary or flat-sheet forms. PP shows good characteristics as membrane for membrane condensers because it is thermally stable, chemically and mechanically resistant and low-cost material.
PVDF possesses a strong inherent hydrophobicity and shows high flexibility as membrane material because it may be produced in hollow fiber, flat sheet, nanofibers and tubular configurations. Amorphous perfluoro-polymers such as Hyflon and ECTFE result to be excellent alternatives to the aforementioned polymers due to their high thermal and chemical stability and low tendency towards swelling, absence of solubility in organic solvents and strong hydrophobic characteristics. Using microporous hydrophilic or composite membranes, the water vapor condenses from the gaseous stream inside the membrane pores, permeating by means of the direct contact with low-temperature water from the permeate side [235,236]. In the former case, PSF, PVC and mixed cellulose triacetate are the most used hydrophilic polymer materials. In water purification field, the integrated membrane systems are today an established technology adopted widely; similarly, also in gas separation area, the ever more pressing need of pre-treatment stages required for the abatement of the contaminants content prior to a proper separation stage well explained the request of prolonging the membrane lifetime at stable performance and the other downstream separation units as well.
Hence, the recovery of the water contained in waste gaseous streams ensures a longer membrane lifetime and reduces the footprint of the membrane units used for gas separation due to a better exploitation of the membrane area itself. This is the reason why membrane condensers are considered a valid pre-treatment stage prior to a membrane separation stage or other separation technologies such as absorption, PSA and cryogenic. For example, membrane condenser may be useful for flue gas treatment, or cooling tower plume, or biogas treatment.
Therefore, till now the potentialities of membrane condenser as pretreatment process resulted to be of great interest in the way towards the development of membrane based integrated systems and in redesigning the whole upgrading process, however needing further research to be implemented at larger scale.
