Membrane Wetting in Membrane Distillation

CHAPTER 6

Membrane Wetting in Membrane Distillation Mohammad Reza Rahimpour and Mohammad Amin Esmaeilbeig Department of Chemical Engineering, Shiraz University, Shiraz, Iran

1 Introduction It seems that oceans are unlimited sources of water. But, the demand for fresh water for human life and industry leads to developing supportable technologies for producing it. So, some desalination processes for seawater and salty water with different mechanisms [1], such as distillation, reverse osmosis (RO), and electrodialysis are taken into consideration. More than 60% of desalinated water in the world is produced by thermal methods. Some methods such as RO, electrodialysis, and membrane distillation (MD) are membrane-based methods [2,3]. High energy is required for some methods, such as RO and thermal evaporation [4]. In MD, a hydrophobic membrane with temperature gradient is used to produce highly pure water [5
8]. Water vapor molecules are allowed to pass through the membrane, but the salts and liquid water are not allowed. This technology has gained further global attention since the 1980s. High purification of salty water is the principal application of the MD process [5]. Also, aqueous solutions with nonvolatile solutes, such as sugar, blood, and fruit juice are concentrated by MD operation [9]. MD has a potential to separate ethanol from aqueous solution [10,11]. The separation of isotopic water and concentration of numerous acids can be done by MD [12
14]. MD operation has several advantages [15,16]. It has high selectivity for nonvolatiles such as ions, colloids, macromolecules, etc. [17]. The waste heat can be utilized for MD because its operating temperature and pressure are low [18]. Also, some renewable energy sources such as solar and geothermal are possible to utilize for MD operation [15]. Fundamental and applied viewpoints have a great interest in the wetting behavior of materials. This phenomenon is the intersection of physics, chemistry, and engineering. The surface chemistry of solids is an important key in the specification of wetting behavior. So, several researchers are working on the surface modification. Wettability plays an important

Current Trends and Future Developments on (Bio-) Membranes. DOI: https://doi.org/10.1016/B978-0-12-813551-8.00006-1 © 2019 Elsevier Inc. All rights reserved.

143

144 Chapter 6 role in several industries, such as oil recovery, water desalination, etc. [19
22]. Wetting behavior can be quantified by contact angle measurement. If the contact angle of a liquid droplet on a solid surface is larger than 90o, the surface is called hydrophobic but if it is % less than 90o the surface is called hydrophilic. % The purpose of this chapter is to investigate the wetting in MD operation. So, the effective parameters on the wetting of membranes are introduced and their effects are shown. The prediction of the wet mode of membranes is expressed by some mathematical models and their results are compared with experimental ones. Also, some practical methods to eliminate the wetting of membranes are presented.

2 Wetting Behavior of Materials 2.1 Contact Angle Wetting behavior of any material describes the deposition and spreading out of a liquid droplet on it [23]. For example, a uniform layer of water can be created on the glass surface but this phenomenon for liquid mercury is impossible. The difference between these two phenomena is related to the wetting behavior of glass. In any wetting system, there are three phases: (1) solid surface, (2) adhesive liquid, and (3) surrounding fluid [24]. An example of these phases is shown in Fig. 1. To study the wetting behavior of any surface, the concept of contact angle should be defined. Assume that there is a sessile droplet on a surface. The contact angle is the angle between the liquid
solid interface and liquid
vapor interface (the angle θ in Fig. 1). The lines of contact angle are tangent to interfaces and pass through the point that three phases intersect. According to the interactions between molecules of liquid, solid, and surrounding phase, the contact angle is determined. Based on the magnitude of contact angle, the wetting of surfaces is classified into nonwetting, partial wetting, and complete wetting [25]. When the surface is nonwetting the contact angle is larger than 90 and the solid
liquid interface area is low. In the case of complete wetting, the contact angle is approximately equal to zero. This shows that the liquid spreads out completely on the surface. Finally, the

Surr. Phase θ

Liquid Solid

Fig. 1 Three phases in any wetting system.

Membrane Wetting in Membrane Distillation 145 partially wetted surfaces have a contact angle in the range of 0
90 . These three types are shown in Fig. 2.

2.2 Surface Tension In a pure liquid, the net force on the molecules that are in the bulk of fluid is zero [26] because they are impacted by neighboring molecules in all directions and the magnitudes of the forces exerted from every neighboring molecule are equal. The neighbors of molecules that are in the surface of the liquid are not present in all directions. So, the net force on each surface molecule is not zero (Fig. 3). These molecules are attracted by neighboring molecules. On the other hand, the surface molecules experience a repulsive force due to collision with interior molecules of the liquid. If the attraction forces are balanced with repulsive ones, the surface of liquid contracts as far as possible [28]. The intermolecular force that causes the liquid surface to be small is surface tension. Standing of some insects on the free surface of the water and spherical shape of small droplets are the results of this phenomenon. This spherical shape has the lowest surface area for a specified volume of liquid.

(A)

(B) θ

(C) θ

Fig. 2 Three types of surface wetting behavior: (A) nonwetting, (B) partial wetting, and (C) complete wetting.

Fig. 3 The net force on the bulk molecules is zero and on the surface molecules is not zero [27].

146 Chapter 6 γ γsv

γsl

Fig. 4 γ is surface tension. γ SV and γ SL are solid
vapor and solid
liquid interfacial tension, respectively.

In addition to surface tension, some external forces such as interfacial tensions can change the shape of sessile droplets. So, based on these forces, the contact angle of the droplet can be determined. For example, if there is a sessile droplet on an ideal surface, in addition to surface tension, there are two types of interfacial tension [29]. First, between the solid surface and liquid droplet and second, between the solid surface and surrounding phase. Here the surrounding phase is the vapor phase (Fig. 4). The mechanical equilibrium of these forces results in the familiar Young’s equation [30]. θY is the Young contact angle. γLV cosθY 5 γ SV 2 γ SL Young’s equation is not derived to calculate the contact angle of rough surfaces. The interfacial area between the solid surface and liquid droplet for the rough surface is larger than ideal smooth ones. So, in 1936 the Wenzel equation for rough surfaces was released [31]. γ LV cosθW 5 rðγ SV 2 γSL Þ In the above equation, θw is the Wenzel contact angle and r is surface roughness ratio, which is defined below. For smooth surfaces r 5 1 and for rough ones r . 1. r5

Actual surface area Projected surface area

The Wenzel equation assumes that the liquid droplet has a complete contact with the rough surface. For rough and porous surfaces, another equation named Cassie
Baxter was introduced. It assumes that the liquid droplet does not completely penetrate to rough surface and there are some air pockets between the liquid and solid substrate (Fig. 5). So, the wetted area in Cassie
Baxter state is much smaller than Wenzel one. The Cassie
Baxter equation is given below [32]. cosθCB 5 ϕS ðcosθY 1 1Þ 2 1 In this equation, ϕS is the fraction of rough solid surface area that is wetted by sessile droplet. To calculate the contact angle of sessile droplet by Wenzel or Cassie
Baxter equation, the surface pattern should be determined by AFM. According to the equations, the Cassie
Baxter equation calculates a higher contact angle than Wenzel one. Similar to models, such as Wenzel, another model is proposed by Tro¨ger et al. [33]. This model concentrates on porous structures, not on rough and chemically heterogeneous ones.

Membrane Wetting in Membrane Distillation 147 (A)

Liquid Solid (B)

Liquid

Solid

Fig. 5 Sessile droplet on a rough surface. (A) Wenzel state and (B) Cassie
Baxter state.

It relates the surface porosity to the contact angle. To calculate the surface porosity, the shape, size, and size distribution of the pores are neglected by this model. The surface porosity is defined as follows: P AP P ϕ5 P AP 1 As In the equation above, AP and As are pore and solid areas, respectively. So, the surface porosity is the portion of total area of surface that occupied by pores. These areas are schematically shown in Fig. 6A. Also, this model assumes that the contact angle of droplet on all three-phase lines is equal (Fig. 6B). The relation between Young’s contact angle and this model is shown in the following. θ and θ0 represent the Young and modified contact angles, respectively. cosθ 5 cosθ0 2

4ϕ cosθ0 1 1 1 2 ϕ cosθ0 2 1

Determination of surface free energy and surface tension from measured contact angle has been done in several studies [34]. The surface tension can be determined from Van Oss
Chaudhury
Good theory [35]. According to this theory, Lifshitz
van der Waals (LW) and Lewis acid
base (AB) forces take part for determining surface tension. It shows in the following: 1 γAB γ i 5 γ LW i i

148 Chapter 6 AP

(A)

AP

AP

AS

AS

AS

AS

(B)

θ

θ

Fig. 6 (A) Definition of areas of solids and pores in a porous structure. (B) Equality of all three-phase contact angle of sessile droplet on a porous surface.

Interaction between induced dipoles leads to van der Waals forces. Lewis acid
base  1 forces are composed of hydrogen bonding, π-bonding, etc. They are composed of acid γ i and   terms. The Lewis term of surface tension is usually small, because, both acid and base γ2 i base sites are not present in many molecules. The relation between acid, base, and Lewis terms is shown in the following: qffiffiffiffiffiffiffiffiffiffiffiffi AB 2 γ i 5 2 γ1 i γi The final form of this model is presented in the following. To solve this equation for calculating surface free energy, there are three unknown parameters. So, at least the surface tension data of three liquids must be used. qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi qffiffiffiffiffiffiffiffiffiffiffiffi LW LW 1 2 1 γL ð1 1 cosθÞ 5 2 γL γ S 1 2 γL γS 1 2 γ2 L γS

3 Wetting in Membrane Distillation MD is a nonisothermal separation process [36]. The hot liquid feed (aqueous solution) is in direct contact with one side of the membrane. Another side of the membrane contains permeate with low temperature (Fig. 7). In any MD process, there is a thermal gradient across the membrane [37]. In this process, heat and mass transfer occur simultaneously.

Membrane Wetting in Membrane Distillation 149 Porous Membrane Vapor

Hot Feed

Vapor

Cold Permeate

Vapor Vapor

Fig. 7 Membrane distillation scheme. Porous Membrane

Liquid Bulk

(A)

Porous Membrane

(B)

Liquid Bulk

Porous Membrane

Liquid Bulk

(C)

Fig. 8 Three types of membrane wetting. (A) Fully wetted, (B) nonwetted, and (C) partially wetted.

3.1 Membrane Structure The porous membrane should be hydrophobic [38]. So, liquid phase cannot penetrate the membrane and only vapor phase can pass through it [39]. But, if the liquid pressure is higher than a critical value, the liquid penetrates into membrane pores and it is said that the membrane is wetted. This critical value is called liquid entry pressure (LEP) [40]. In MD operation, there are three types of membrane wetting behavior: (1) fully wetted, (2) nonwetted, and (3) partially wetted [41]. They are shown in Fig. 8. The effective parameters on the wetting behavior of membrane determine the wetting type of membrane. It is discussed in Section 5. If the pores of the membrane are wetted in the MD operation, the interface for evaporation decreases and as a result, a small amount of vapor is produced. Also, the saline water can permeate inside the membrane and the distillate is contaminated by it and finally, the performance of MD operation reduces [42]. Thus, monitoring the electrical conductivity of the permeate stream can help to determine if the membrane is wetted or not. If the electrical conductivity of the permeate stream increases during operation of MD, the membrane is wetted.

150 Chapter 6 One important property of the membrane in the MD process is the thickness of the membrane. As the membrane thickness increases, the permeate flux is reduced and the heat loss increases. The optimum value of membrane thickness is in the range of 30
60 μm [15]. Membrane porosity and tortuosity are also important. High permeate flux and low conductive heat loss are the results of a membrane with high porosity. The porosities of membranes in MD are in the range of 30%
85% [8]. The membrane used in MD should have some specific features. Mass transfer resistance across the membrane must be low. To prevent heat loss, the thermal conductivity of the membrane should be low. The membrane should be thermally stable in extreme temperature and its resistance to some chemicals such as acids should be high [15]. Usually, the membranes of MD operation are made of polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), and polypropylene (PP). Four types of membrane modules are used in MD operation. They are plate and frame, hollow fiber, tubular, and spiral wound [15].

4 Wetting Prediction by Mathematical Models LEP is an important characteristic of MD. If the transmembrane pressure is higher than LEP, the membrane is wetted. For membranes with low LEP, pore wetting occurs easily. The pore wetting is not desired in the MD operation. So, the study of LEP and the effective parameters on it can help to design a better MD operation and wetting prevention. Membrane wetting causes the pollution of permeate, reduction in vapor production, and finally, reduction the performance of the membrane. So, the hydrophobicity maintenance of membrane is important in MD operation. The reduction in membrane hydrophobicity leads to gradual wetting of membrane. The prediction of LEP with mathematical models helps to control the applied pressure on the feed stream to prevent the liquid entrance. In this section, some important mathematical models are expressed. One of the mathematical models that estimate the LEP is Young
Laplace [30]. It assumes that the pores are cylindrical (Fig. 9A). In reality, only a few membranes have cylindrical pores. It is given below. ΔP 5

2 2γl cosθ r

In the equation above, γ l is the surface tension of liquid, θ is contact angle, and r is the radius of pores in membrane (Fig. 9B). Also, any change in each parameter will affect the LEP and finally change the membrane wetting. According to the Young
Laplace equation, if the contact angle is larger than 90 , the LEP will be greater than zero and for θ , 90 the LEP has negative value. Previous studies [44] show positive values for LEP when θ . 90 . They are in contrast to the Young
Laplace

Membrane Wetting in Membrane Distillation 151 (A)

(B)

ΔP θ

2r

Fig. 9 (A) cylindrical pore and (B) Young
Laplace model for cylindrical pore [43].

Pore

Fiber

Fig. 10 SEM image of nylon membrane [43].

model. This is due to the noncylindrical shape of pores. So, a more general Franken et al. model [45] was introduced the following: ΔP 5

2 2Bγ l cosθ rmax

In this equation, a new B parameter is added. This parameter is related to the geometry of pores and its value for cylindrical pores is equal to 1 and 0 , B , 1 for noncylindrical pores [46]. This value can be related to the curvature radius of pores. Stretched membranes, such as PTFE have small curvature radius and their B value is in the range of 0.4
0.6 [40]. Also, rmax is the maximum size of pores. The B value only considers the radial variation of pores in membranes. But, in real space, the irregularity of pores can be in radial and axial directions. Two variables, hydraulic radius (rh ), and structural angle (α), can describe the deviation of a pore from cylindrical shape. The noncircular shape of pore cross-section and axial deviation of pore from verticality are reflected by hydraulic radius and structural angle, respectively [40]. In many membranes, the pores involve spaces between discrete fibers (Fig. 10). It is better to consider these pores as tori [43]. For toroidal pores, the Purcell model was introduced

152 Chapter 6 (A)

Pore

(B)

ΔP 2r

2R

α θ

Fig. 11 (A) The shape of pore is tori and (B) Purcell model for toroidal pore [43].

(Fig. 11A). This model was developed to calculate the pressure difference through the water
oil interface [40]. It neglects the radial variation of pores and assumes that the crosssection of pores is a circle. As the liquid moves through the pore, the position of pinning point of liquid changes. The pinning point is the intersection of the liquid surface with pore wall (Fig. 11B). This model is given below [43].   2 2γ cosðθ 1 αÞ 2γ cosθeff 5 ΔP 5 r r 1 1 Rr ð1 2 cosαÞ In the above equation, r is the mean pore radius, R shows the fiber radius, and α is the structural angle. The angle is shown in Fig. 11B. Effective contact angle (θeff ) is defined by Kim and Harriot [44]. The LEP is calculated as cosθeff approaches the maximum value. To do this, the magnitude of α should be calculated at breakthrough. So, by differentiating this equation with respect to α and equating to zero, the following expression appears. sinðθ 1 αÞ 5

sinθ 1 1 r=R

Fig. 12 shows a comparison between experimental data, Purcell model, and Young
Laplace model for nylon
pPFDA and nylon
pDVB composite membranes. pDVB and pPFDA are the abbreviations of polydivinylbenzene and poly-1H,1H,2H,2Hperfluorodecyl acrylate, respectively [43]. In this comparison, LEPw is plotted versus contact angle. The LEP of any solution can be normalized by the following expression: LEPw 5

LEP 3 γ w γ

In the above equation, γ and γ w are the surface tensions of the desired fluid and water, respectively. γw is equal to 0.072 N/m. Fig. 12 shows that the Young
Laplace model is not suitable for these types of membrane. As the contact angle decreased, the deviation of the Young
Laplace model from experimental data increases. The predicted LEPs by the Purcell model are better than the Young
Laplace one. For contact angles larger than 90 , the Purcell results are very good.

Membrane Wetting in Membrane Distillation 153 600

400

LEPw (kPa)

200

0

–200

–400

Young-Laplace Purcell nylon-pPFDA nylon-pDVB

–600 0

20

40

60 80 100 120 Contact angle, θ (deg)

140

160

180

Fig. 12 A comparison between experimental data, Young
Laplace, and Purcell models [43].

All LEPs calculated by the Purcell model have a positive value. But some experimental studies showed that if the value of the contact angle is very low, a lot of membranes wet [43]. Also, in Fig. 12, the experimental value of LEP for θ 5 63 is equal to zero. The positive values of LEP for a whole range of contact angles shows that even superhydrophilic membranes will not wet. But in reality, spontaneous wetting occurs for many hydrophilic membranes. The Purcell model assumes that a membrane is one plane of pores with toroidal shape [43]. So, for a hydrophilic membrane with this configuration, as soon as the water enters the membrane, it adheres to the pores. Thus, to discharge the membrane from the water, a positive applied pressure is required. According to this, the Purcell model predicts the positive value of LEP. But, real membranes are composed of multiple planes of pores. The pores and fibers in two planes can overlap each other. So, when the water enters the pores of the real membrane, it faces the fibers inside the membrane and it can interact with them. If the membrane is hydrophilic, these interactions are attractive and the water can be drawn into the membrane. Servi et al. developed a model based on the Purcell one [43]. They considered a “floor” under each pore. This “floor” plays the role of fibers, with any form that interacts with liquid (Fig. 13A). In this model, the pressure that releases water from the pore is defined as LEP. The new parameter “h” is introduced as the distance between the floor and the bottom of the pore (Fig. 13B). Also, in this model, the relation between pressure difference and the structural angle is similar to the Purcell model.

154 Chapter 6 (A)

(B)

Pore

Pore

ΔP

2r Fiber

Floor Fiber

2R

α θ

h Floor

Fig. 13 (A) Three dimension and (B) side view of the new model [43].

ΔP 5

2 2γcosðθ 1 αÞ r 1 Rð1 2 cosαÞ

But to calculate the structural angle, a different strategy is adopted. Because in this model the floor is introduced, the liquid may come into contact with the floor with a lower value of α than the Purcell model. The Servi et al. model suggests that if the liquid contacts with the floor, the liquid entry occurs. The value of α that corresponds to this situation is used to calculate the LEP. So, the previous equation with the following expression is used to calculate the LEP. r 1 Rð1 2 cosαÞ Rð1 2 sinαÞ 5 1h 2 cosðθ 1 αÞ 1 2 sinðα 1 θÞ A comparison is carried out between experimental data of LEPw and the predicted values of it from the new model. It is shown in Fig. 14. According to this comparison, in contrast to the Purcell model, the new model predicts both negative and positive values of LEPw. This prediction is more consistent with experimental data. The understanding of LEP was improved by this model. According to the introduced models, the modified Purcell model is the best because its assumptions are more consistent with the real structure of membranes.

5 Effective Parameters on Wetting of MD By investigating the mathematical models for prediction of MD wetting in the previous section, it is revealed that any variable that changes the LEP of the membrane can affect the membrane wetting behavior. Some variables change directly the membrane wetting such as surface tension, pore radius, and pore shape. These are the main effective parameters.

Membrane Wetting in Membrane Distillation 155 600

400

LEPw (kPa)

200

0

–200

–400

–600 0

New model nylon-pPFDA nylon-pDVB

20

40

60

80 100 120 Contact angle, θ(deg)

140

160

180

Fig. 14 A comparison between experimental data and the new model [43].

Other variables, such as temperature, brine concentration, and presence of a surfactant, affect the main parameters and change the membrane wetting indirectly. In the following, the parameters and the effects of them on membrane wetting are expressed.

5.1 Membrane Structure In an MD process, the membrane itself is the core of the operation. There are many types of membranes based on their transport and structural properties. The microstructure of a membrane is very important and it is determined by preparation method. The membranes are classified to dense, porous, and composite based on their morphology. The structure of each is shown in Fig. 15. Most membranes are porous (Fig. 15B) or composite (Fig. 15C). In the composite case, the porous layer is under the dense one [47]. The pores of symmetric membranes are straight or sponge-like. Also, the dense membranes (Fig. 15A) are in the category of symmetric ones. These symmetric membranes are useful for the MD operation [48]. The structural properties of the porous membranes include thickness, porosity, surface structure, tortuosity, roughness, etc. Also, the membrane material is important. There are a lot of synthetic membranes for MD operation [36, 49
67].

156 Chapter 6 (A)

(B)

(C)

Fig. 15 The structures of each type of membrane: (A) dense, (B) porous, and (C) composite. (A)

(B) F F F F F

F F F F

F F F F F

Si O

F F F F F F F F

F F F F F

O

F F F F

F

(C)

F F F F F F F F

Si O

F

O

F F F F

(D) F F F F F F F F

Si O

F F F F F F F F F

O

F F F F F F F F

F F F F F F F F F

F F F F F F F F

Si O

F F F F F F F F F

F F F F F F F F

O

Si

Si

Si

Si

O

O

O

O

Fig. 16 Modified glass fiber surfaces with (A) 9-FAS, (B) Si nanoparticles and 9-FAS, (C) 17-FAS, and (D) Si nanoparticles and 17-FAS.

Fig. 16 shows Si nanoparticles and fluoroalkyl silane (FAS) with two different lengths are used to modify the membrane surface [53]. The modifications divide into two groups. First, the surfaces are modified with both Si nanoparticle and FAS (Figs. 16A and C). The modification of the second group was done by FAS only (Figs. 16B and D). Fig. 17 shows the contact angles of a water droplet on modified membranes. The chain length of 17-FAS is longer than 9-FAS. So, the water repellency of surfaces modified with 17-FAS is more than the other. Because, as the length of fluoroalkyl chain increases, the surface energy decreases [53]. The contact angles of two modified surfaces that contain Si nanoparticles are high value. The roughness of surfaces that are coated by Si nanoparticles increases the contact angle. The performance of modified membranes is investigated by DCMD

Membrane Wetting in Membrane Distillation 157 200

Contact angle (deg)

150

100

50

0

9-FAS

9-FAS & Si 17-FAS Modified Surfaces

17-FAS & Si

Fig. 17 The contact angle of water droplet on modified surfaces.

operations. The experiments are done using feed solutions with various surface tensions. The surface tension of feed solutions is changed by sodium dodecyl sulfate (SDS). It was used as the surface active agent. The SDS reduces the surface tension of liquid feed. The results of membrane performance are shown in Fig. 18. As shown in Fig. 18A, if the concentration of SDS is less than 0.1 mM, the water flux and salt rejection are stable and the membrane works properly. But, for concentrations above the 0.1 mM, the water flux increases and the salt rejection decreases. This indicates that the feed solution wets the membrane. In the case of Fig. 18B, the membrane wetting occurs at a higher concentration of SDS. This is due to the presence of nanoparticles. As mentioned before, the existence of nanoparticles increases the roughness of the surfaces. This leads to an increase in the hydrophobicity of the membrane. Lower surface energy in Fig. 18C causes the membrane wetting occurring at 0.3 mM. Finally, in the case of Fig. 18D, the wetting resistance of membrane is at the highest level. This is due to both lower surface energy and roughness of the surface. According to this study, the surface energy of membrane and the roughness of surface affect on the wettability and eventually the performance of MD operation. The effect of structural properties on the hydrophobicity of five different coated membranes are investigated [40]. The membranes are PTFE-1, PTFE-2, PVDF, nylon, and polycarbonate (PC). PTFE-1 and PTFE-2 are membranes with different support; PTFE-1 on PP scrim support and PTFE-2 on PP nonwoven support. All membranes were coated with 1H,1H,2H,2H-perfluorooctyl acrylate. The structural variables of coated membranes are

(A) 104

30

Salt Rejection (%)

20 96 15 92 10 88 Salt Rejection Water Flux

84

0

0.04 0.08 0.12 0.16 SDS Concentration in Feed (mM)

(B) 104

Water Flux (Lm–2 h–1)

25

100

5 0 0.2 40

Salt Rejection (%)

30 96 92

20

88

Water Flux (Lm–2 h–1)

100

10 84

Salt Rejection Water Flux

80 0

0.04

0.08 0.12 0.16 0.2 SDS Concentration in Feed (mM)

(C) 120

40

80 30 60 20 40 10

20 0

Water Flux (Lm–2 h–1)

50

100 Salt Rejection (%)

0 0.24

Salt Rejection Water Flux

0

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32 0.36 SDS Concentration in Feed (mM)

Fig. 18 The salt rejection and water flux of modified membranes versus SDS concentration. (A) 9-FAS, (B) 9-FAS and Si nanoparticles, (C) 17-FAS, and (D) 17-FAS and Si nanoparticles.

Membrane Wetting in Membrane Distillation 159

100 Salt Rejection (%)

30 80 20 60

Water Flux (Lm–2 h–1)

40

(D) 120

10 40 Salt Rejection Water Flux

20

0

0 0.04 0.08 0.12 0.16 0.2 0.24 0.28 0.32 0.36 0.4 0.44 SDS Concentration in Feed (mM)

Fig. 18 (Continued). Table 1 The structural properties of coated membranes [40]

PTFE-1 PTFE-2 PVDF PC Nylon

Roughness (nm)

rave (nm)

rmax (nm)

R/r (AFM)

Thickness (n)

Surface Porosity (%)

157.7 176.5 258.2 63.7 152.7

106 72 101 81 107

227 156 252 88 229

1.4 2.5 5 7.5 3.5

0.28 0.18 0.2 0.01 0.12

33.1 36.5 23.1 7.1 19.3

shown in Table 1. In this table, rave is the average pore size, rmax is the maximum pore size, and R/r ratio is the relative curvature of pore opening. Round edge pores have larger values of R/r ratio [68]. The hydrophobicity of coated membranes is determined by the contact angle. The contact angles of membranes are in Fig. 19. Since all membranes are coated with the same material, the difference in the contact angles is related to structural properties of membranes. The experimental values of LEP for each coated membrane is shown in Fig. 20. To investigate the effect of structural variables on LEP, the variable B is introduced. B is the experimental LEP-to-predicted LEP ratio. This is the same B variable in the Franken model. The predicted value of LEP is calculated by the Young
Laplace equation. It discussed in Section 4. The variation of this coefficient versus each structural variable shows the effect of that variable on the membrane wetting. These variations are shown in Fig. 21. According to Fig. 21, the correlation between B and structural properties of the membrane is positive for all properties (Fig. 21A
F) except R/r ratio (Fig. 21D). In the case of

160 Chapter 6 150

Contact Angle (deg.)

145

130

115

100

PTFE-1

PTFE-2

PVDF

PC

Nylon

Fig. 19 The contact angles of coated membranes. 6

5

LEP (bar)

4

3

2

1

0 PTFE-1

PTFE-2

PVDF

PC

NYLON

Fig. 20 The experimental values of LEP for each coated membrane [40].

roughness, the effect of this property on the membrane wetting strongly depends on the sample size [40]. In this study, the size of the sample is 100 μm2. If values of R/r ratio are high and surface porosity is low, the membrane prevents liquid from entering. The R/r ratio and surface porosity are related to each other. The reduction of R/r ratio leads to growth in the void space and as a result, the porosity of the membrane increases.

Membrane Wetting in Membrane Distillation 161 (B) 3

PTFE-1

2

B′ Factor

B′ Factor

(A) 3

PTFE-2

PVDF

Nylon

1

PTFE-1

2

PTFE-2 PVDF Nylon

1

PC 0 100

PC 0

160

120 140 Contact Angle (deg.)

(C) 3

0

300

(D) 3

2 PVDF

PTFE-2

Nylon

1

B′ Factor

PTFE-1 B′ Factor

100 200 Roughness (nm)

PTFE-1

2

PVDF

PTFE-2 Nylon

1

PC PC 0

0

10

20 Porosity (%)

30

0

40

(E) 3

(F) 3

2

2

0

2

4 R/r ratio

6

8

PVDF

PTFE-1

PTFE-2 Nylon

1

B′ Factor

B′ Factor

PTFE-1

Nylon

1 PC

PC 0

0

PVDF

PTFE-2

0.1 0.2 Thickness (mm)

0.3

0

0

100

200

300

rmax (nm)

Fig. 21 The variation of B’ factor versus structural parameters for various membranes. Each figure belongs to one structural parameter. (A) Contact angle, (B) roughness, (C) porosity, (D) R/r ratio, (E) thickness, and (F) rmax.

162 Chapter 6

5.2 Operational Parameters In any MD operation, heat and mass transfer occur simultaneously. These two phenomena control the system and both occur in a similar direction. Their direction is from the feed side to permeate side. The operational conditions, such as temperature, flow rate, and salt concentration can affect the membrane wetting [41]. There are some studies that reported the different salt rejection factor in MD operation for different feed temperature and flow rates [69]. The temperature of feed and distillate solutions affect the wetting behavior of the membrane both directly [5,70
72] and indirectly [72,73]. The effect of temperature on the contact angle, surface tension, and viscosity of the fluid is a direct effect. The indirect effect is related to the change in the pore structure of membrane by temperature effect. As the feed temperature increases or the distillate one decreases, the driving force of MD operation increases. So, the flux of vapor inside the membrane increases and the wetting of the membrane occurs later [71]. For three PTFE membranes with different pore sizes, the influence of temperature on the wetting behavior of the membrane is investigated by He et al. [74]. The variation of flux and conductivity of permeate versus temperature is shown in Fig. 22. According to Fig. 22A, as the temperature of the hot side of membranes increases, the flux of the permeate increases. This is due to the growth of the driving force. Fig. 22B shows that the conductivity of each membrane approximately did not change with variation of hot side temperature. This shows that the membrane did not wet. The surface tension of a liquid is related to the cohesive forces between molecules. Generally, it decreases as the temperature of liquid rises. As the temperature of liquid increases, the activity of liquid molecules rises and thus the cohesive forces between molecules decreases. Reduction in surface tension of feed liquid leads to decrease the LEPw [72]. So, the membrane is wetted easily. The surface tension of a saline water feed at a specific temperature can be predicted by the following expression [75]: γl 1 γ0 5

Δγ CSolution ΔCSolution

In the above equation, γ l and γ0 are surface tensions of saline water and pure water at specific temperature, respectively. Δγ=ΔCSolution for NaCl solution is equal to 1.467 6 0.05 3 1023 N.L/mol.m. The concentration of saline water is represented by CSolution in mol/L. The effect of temperature on the contact angle is different. For PTFE and PVDF membranes, the contact angles of droplets decrease as the temperature increases [72,73]. The dependences of contact angle in PTFE and PVDF membranes on temperature are shown in Figs. 23 and 24.

Membrane Wetting in Membrane Distillation 163

Fig. 22 The influence of temperature on (A) flux and (B) conductivity of different PTFE membranes. The pore sizes of samples I, II, and III are 0.22, 0.45, and 1.00 μm, respectively.

The pore structure of membrane may be changed by temperature effect. The thermal stability of the membrane plays an important role in this situation. Pore size distribution and pore geometry are factors that can be investigated to study the effect of temperature on the pore structure. For PTFE membranes the pore diameters for various temperatures are investigated [72]. This is shown in Fig. 25. This figure generally shows that the pore diameter increases with increasing temperature. The variation of LEP versus temperature determines the overall effect of temperature on the wetting behavior of the system. Usually, the variation of temperature affects more than one of the variables related to the wetting behavior of the membrane. So, the effect of temperature on the wetting behavior of each MD operation may be unique. It can be predicted by the models that are presented in Section 4. For thermally stable membranes, at a high temperature of feed solutions, the wetting of the membrane becomes important.

Fig. 23 Variation of the contact angle with temperature for PTFE membrane.

Fig. 24 Variation of the contact angle with temperature for PVDF membrane.

Fig. 25 The effect of temperature on the pore diameter of PTFE membranes.

Membrane Wetting in Membrane Distillation 165 As the feed rate increases, its Reynolds number increases. So, the thickness of liquid boundary layer on the feed side of the membrane decreases. In this situation, the heat transfer occurs faster and thus, the temperature difference between membrane surface and bulk of the liquid at feed side decreases. Also, increasing the Reynolds number leads to increase the mass transfer inside the membrane [70]. The high flux of vapor in the membrane leads to avoiding getting wet. But, in some cases [71,74], when the permeate flux increases, the membrane is wetted. This is due to the fluctuation of pressure at high flow rates [74]. It is important that the effect of temperature on the wetting behavior of membrane is stronger than the effect of flow rates [71]. Fig. 26 shows the effect of feed flow rate on the permeate flux and conductivity of the membrane for different PTFE membranes [74].

(A) 14 I Flux (L/m2hr)

13

II

12

11

10 120

40

200 280 Hot Side Flowrate (ml/min)

360

440

Conductivity (μS/cm)

(B) 160 I 120

II

80

40

0 40

140

240 340 Hot Side Flowrate (ml/min)

440

Fig. 26 The effect of hot side flow rate on the (A) permeate flux and (B) conductivity. The pore sizes of samples I and II are 0.45 μm and 1.00 μm, respectively.

166 Chapter 6

Fig. 27 Salt concentration effect on (A) permeate flux and (B) conductivity. The pore sizes of samples I and II are 0.45 and 1.00 μm, respectively.

When the concentration of NaCl in the feed solution increases, the boiling point of the solution increases and its vapor pressure decreases. It leads to a reduction in feed vaporization at the hot side of the membrane. Thus, the flux of permeate inside the membrane reduces. The LEP reduction is the result of high concentration of salt. The effect of NaCl salt on the permeate flux and conductivity is shown in Fig. 27 [74]. According to this figure, membrane wetting improves as the concentration of NaCl increases.

5.3 Membrane Degradation and Fouling For any MD process, vapor phase maintenance inside the pores of the membrane during the MD operation is very important. But, in previous studies [76,77], the membranes are partially wetted during the MD processes. So, the variations of membrane hydrophobicity in a MD operation is essential. The partial wetting phenomenon occurred for polymer membranes made of PP, PTFE, and PVDF [78]. The surface chemistry of the membrane

Membrane Wetting in Membrane Distillation 167

Flux (dm3 m–2 d–1)

1000

960

920

880

840 0

20

40 Time (h)

60

80

Fig. 28 The reduction in permeate flux during MD operation for PP membranes.

characterizes its hydrophobic property. If some active groups such as CQO, OH, and COOH are present on the surface of the membrane, the tendency of water to bond to the surface via hydrogen bonding increases [78,79]. They are hydrophilic groups. As the size of pores increases, the higher level of partial wetting occurs [71]. The inorganic fouling is introduced as the main reason of partial wetting of the membrane by some studies [76,78,80,81]. At the hot side of the membrane, the salt concentration has the highest amount [42]. Thus, in this region, the probability of salt precipitation is very high. If the salt deposits on the surface of this region, first, the membrane wets in the place of deposition and second, the liquid penetrates into the adjacent pores [78,82]. This penetration leads to the development of deposits inside the pores [83,84]. Thus, the partial wetting of the pores speeds up. As a result, the fouling and wetting are interdependent. The fouling increases as the wetting rises and vice versa. So, the performance of the membrane reduces rapidly [85]. The permeate flux of PP membrane in a MD operation is decreased during the time of operations [86]. This shown in Fig. 28.

6 Membrane Dewetting The MD operation is ruined by two main factors. First, reduction in permeate flux due to the blocking of pores [5,87] and second, the wetting of the membrane. In the case of wetted membrane, the salt water diffuses to the membrane and thus, the permeate becomes polluted [88,89]. The second reason is more important because the quality of permeate decreases extremely.

168 Chapter 6

Nonaqueous Solution

Fig. 29 Membrane dewetting with nonaqueous solution.

Air

Fig. 30 Membrane backwashing with air.

Some experimental studies were done to restore the wetted membranes. The main method is drying out the membrane for several hours [90]. In the traditional drying process, the salt particulates remain on the membrane surface during the vaporization of water. This leads to blocking the pores and decreasing the permeate flux [78,84]. After this drying process the possibility of wetting the membrane increases. Also, using water and solvent to clean the surface of the membrane from precipitated salts is a poor technique [89]. The membranes were restored moderately by backwashing them with a nonaqueous solution (Fig. 29). This solution has low surface energy. First, the membranes were wetted by this solution completely and second, the solution evaporated from the membrane by drying in an oven [91]. The costs of heating and nonaqueous solution have made this method more

Membrane Wetting in Membrane Distillation 169 expensive. Also, if the nonaqueous solution doesn’t remove completely, the membrane may become polluted. An alternative dewetting method is backwashing the membrane by pressurized air [85]. This method is done in only several seconds. The foulant and water in the pores come out by pressurizing air stream (Fig. 30). The required pressure is higher than LEP. Unlike evaporating methods, the salt remains less in this method.

7 Conclusion and Future Trends MD is one of membrane-based methods for desalination of salty water. In this operation, highly pure water is produced by a hydrophobic membrane, because only water vapor can pass through the membrane. So, the wetting behavior of the membrane is an important parameter in any MD operation because it is directly related to the performance of the MD operation and quality of the permeate. Predicting the wetting behavior leads to better membrane design and preventing membrane wetting. If the transmembrane pressure is higher than LEP, the membrane becomes wetted. There are some mathematical models to predict the LEP. The selection of a suitable model should be done based on the structure of pores in the membrane. There are some effective parameters on the performance of the operation. These parameters are categorized to membrane structure parameters, operational parameters, etc. The effect of each parameter on the MD operation should be studied and suitable adjustment of each should be done. Finally, if the membrane is wetted during the MD operation, an appropriate dewetting method should be applied.

List of Acronyms MD LEP RO Fig. LW AB PTFE PVDF PP FAS SDS PC AFM

Membrane distillation Liquid entry pressure Reverse osmosis Figure Lifshitz
van der waals Lewis acid
base Polytetrafluoroethylene Polyvinylidene fluoride Polypropylene Fluoroalkyl silane Sodium dodecyl sulfate Polycarbonate Atomic force microscopy

List of Symbols γ LV θY

Liquid surface tension Young’s contact angle

170 Chapter 6 γ SV γ SL θW θCB AP AS rh α C

Solid
vapor interfacial tension Solid
liquid interfacial tension Wenzel’s contact angle Cassie
baxter’s contact angle Pore area Solid area Hydraulic radius Structural angle Concentration

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