181
6.4.2 Thermal Condition in Membrane Distillation Operation
Temperature is one of the significant factors contributing to fouling and scaling
in membrane distillation. Furthermore, temperature also affects the solubility of
salts. For example, the solubility of NaCl increases with a rise in temperature,
whereas an inverse relation can be observed for CaCO 3 , Mg(OH) 2 , and Ca 3 (PO 4 ) 2 .
Investigations suggest the inverse relation of solubility of alkaline salt with temperature, wherein water dissociates into hydrogen and hydroxide, forming scales;
such dissociation increases at higher temperatures (Morel et al. 1993). The salts
with negative relation (CaSO 4 and CaCO 3 ) with temperature also get saturated in
the salinated feed solutions (CaSO 4 concentration is higher in seawater, while
CaCO 3 concentration is higher in groundwater sources). Altogether, studies highlight a raise in scaling as the operating temperature increases (Martı́ nez-Dı́ ez and
Vazquez-Gonzalez 1999).
6.4.3 Role of Dissolved Gases in Membrane
Distillation Performance
Relatively, all feeds in membrane distillation contain dissolved gases, most of which
contribute to both scaling and fouling. Dissolved gases in feed streams facilitate the
breakdown of bicarbonates and carry them along the vapors into the membrane
pores, thus increasing the diffusive resistance of water vapors. These gases hinder
the permeate flow and reduce concentration polarization and scaling (Liu et al.
1998). The dissolved gas along the feed stream occupies the membrane pores,
thereby increasing the mass transfer resistance to water vapor and also adding to the
mass transfer resistance in the air gaps of membranes. The effect is to reduce the
condensation heat transfer rate, possibly creating the system mass transfer limited
on the air side, thus reducing the overall vapor flux. Interestingly, the lack of dissolved gases can maximize the membrane wetting by removing the air trapped in
the membrane pores; that has been experimentally confirmed by Schofield et al.
(1990). In general, the dissolved gases present in the feed occupy the pores and create a barrier for foulants and scaling agents. Therefore, decreasing the dissolved
gases by deaeration technique or other means may increase fouling propensity.
6.4.4 Superhydrophobic Membranes for Preventing Wetting
Although surface modification can reduce membrane wetting, it changes the wettability of membrane surface affecting the bulk properties. When hydrophobic
membranes are modified to superhydrophobic surface, the surface roughness
increases with higher contact angle; however, it has less effect in maximizing the
6 Fouling and Wetting: A Major Challenge for Membrane Distillation
6.4.2 Thermal Condition in Membrane Distillation Operation
Temperature is one of the significant factors contributing to fouling and scaling
in membrane distillation. Furthermore, temperature also affects the solubility of
salts. For example, the solubility of NaCl increases with a rise in temperature,
whereas an inverse relation can be observed for CaCO 3 , Mg(OH) 2 , and Ca 3 (PO 4 ) 2 .
Investigations suggest the inverse relation of solubility of alkaline salt with temperature, wherein water dissociates into hydrogen and hydroxide, forming scales;
such dissociation increases at higher temperatures (Morel et al. 1993). The salts
with negative relation (CaSO 4 and CaCO 3 ) with temperature also get saturated in
the salinated feed solutions (CaSO 4 concentration is higher in seawater, while
CaCO 3 concentration is higher in groundwater sources). Altogether, studies highlight a raise in scaling as the operating temperature increases (Martı́ nez-Dı́ ez and
Vazquez-Gonzalez 1999).
6.4.3 Role of Dissolved Gases in Membrane
Distillation Performance
Relatively, all feeds in membrane distillation contain dissolved gases, most of which
contribute to both scaling and fouling. Dissolved gases in feed streams facilitate the
breakdown of bicarbonates and carry them along the vapors into the membrane
pores, thus increasing the diffusive resistance of water vapors. These gases hinder
the permeate flow and reduce concentration polarization and scaling (Liu et al.
1998). The dissolved gas along the feed stream occupies the membrane pores,
thereby increasing the mass transfer resistance to water vapor and also adding to the
mass transfer resistance in the air gaps of membranes. The effect is to reduce the
condensation heat transfer rate, possibly creating the system mass transfer limited
on the air side, thus reducing the overall vapor flux. Interestingly, the lack of dissolved gases can maximize the membrane wetting by removing the air trapped in
the membrane pores; that has been experimentally confirmed by Schofield et al.
(1990). In general, the dissolved gases present in the feed occupy the pores and create a barrier for foulants and scaling agents. Therefore, decreasing the dissolved
gases by deaeration technique or other means may increase fouling propensity.
6.4.4 Superhydrophobic Membranes for Preventing Wetting
Although surface modification can reduce membrane wetting, it changes the wettability of membrane surface affecting the bulk properties. When hydrophobic
membranes are modified to superhydrophobic surface, the surface roughness
increases with higher contact angle; however, it has less effect in maximizing the
6 Fouling and Wetting: A Major Challenge for Membrane Distillation