174
6.3 Mechanism of Wetting in Membrane Distillation
Among many of the concerns related to efficient use of membrane distillation systems, the prominent challenge is membrane wetting, wherein product contamination is a consequence of saline water penetration through the membrane. However,
there are numerous fouling mitigation methods, viz., superhydrophobicity, air
backwashing to reverse or prevent wetting, non-fouling operating conditions, and
maintaining air layers on the membrane surface. If a distillation membrane gets
wetted and the micron-sized pores are flooded with liquid, the feed stream can easily pass through the flooded pores in its liquid form, failing the membrane as a
barrier for salts, macromolecules, and other submicron size particles (Franken
et al. 1987; Goh et al. 2013b). The wettability (S) of a material can be defined as
the ability of a liquid to wet a surface. Additionally, the degree of wettability can
also be referred as spreading which is controlled by interfacial forces set up
between a solid (S), liquid (L), and vapor (V) phase at the minimum equilibrium
distance. Typically, the interfacial forces are expressed as surface tensions (γ) and
can be associated to the surface wettability coefficient by given mathematical
eq. (6.3):
S
SV
SL
LV
=
−
+
(
)
γ
γ
γ
(6.3)
There is another way to characterize the membrane wettability in terms of contact angle (θ), expressed mathematically by Young’s equation (eq. (6.4)):
cos θ
γ
γ
γ
( ) =
−
SV
SL
LV
(6.4)
It is considered that (θ)= 90° as a borderline between hydrophobicity and hydrophilicity. Typically, contact angle lesser than 90° denotes hydrophilic surface with a
positive liquid wetting; on the other hand, contact angle higher than 90° is considered as hydrophobic surface with negative wetting (Banat and Simandl 1994; Ge
et al. 2014).
6.3.1 Degree of Wetting
Membrane wetting is a complex physicochemical process categorized into four
phases (degrees) (Table 6.2): non-wetted (dry phase: 1st phase), surface wetted (2nd
phase), partially wetted (3rd phase), and fully wetted (4th phase). Typically, in surface wetting, the interface of vapor/liquid shifts inward, toward the cross section of
the membrane. Consequently, increasing the polarization temperature and decreasing the water flux reduce the temperature of the evaporating interface in the
S. S. Ray et al.
6.3 Mechanism of Wetting in Membrane Distillation
Among many of the concerns related to efficient use of membrane distillation systems, the prominent challenge is membrane wetting, wherein product contamination is a consequence of saline water penetration through the membrane. However,
there are numerous fouling mitigation methods, viz., superhydrophobicity, air
backwashing to reverse or prevent wetting, non-fouling operating conditions, and
maintaining air layers on the membrane surface. If a distillation membrane gets
wetted and the micron-sized pores are flooded with liquid, the feed stream can easily pass through the flooded pores in its liquid form, failing the membrane as a
barrier for salts, macromolecules, and other submicron size particles (Franken
et al. 1987; Goh et al. 2013b). The wettability (S) of a material can be defined as
the ability of a liquid to wet a surface. Additionally, the degree of wettability can
also be referred as spreading which is controlled by interfacial forces set up
between a solid (S), liquid (L), and vapor (V) phase at the minimum equilibrium
distance. Typically, the interfacial forces are expressed as surface tensions (γ) and
can be associated to the surface wettability coefficient by given mathematical
eq. (6.3):
S
SV
SL
LV
=
−
+
(
)
γ
γ
γ
(6.3)
There is another way to characterize the membrane wettability in terms of contact angle (θ), expressed mathematically by Young’s equation (eq. (6.4)):
cos θ
γ
γ
γ
( ) =
−
SV
SL
LV
(6.4)
It is considered that (θ)= 90° as a borderline between hydrophobicity and hydrophilicity. Typically, contact angle lesser than 90° denotes hydrophilic surface with a
positive liquid wetting; on the other hand, contact angle higher than 90° is considered as hydrophobic surface with negative wetting (Banat and Simandl 1994; Ge
et al. 2014).
6.3.1 Degree of Wetting
Membrane wetting is a complex physicochemical process categorized into four
phases (degrees) (Table 6.2): non-wetted (dry phase: 1st phase), surface wetted (2nd
phase), partially wetted (3rd phase), and fully wetted (4th phase). Typically, in surface wetting, the interface of vapor/liquid shifts inward, toward the cross section of
the membrane. Consequently, increasing the polarization temperature and decreasing the water flux reduce the temperature of the evaporating interface in the
S. S. Ray et al.