How To Optimize the Membrane Properties for Membrane Distillation

Review pubs.acs.org/IECR

How To Optimize the Membrane Properties for Membrane Distillation: A Review Lies Eykens,*,†,‡ Kristien De Sitter,† Chris Dotremont,† Luc Pinoy,§ and Bart Van der Bruggen‡,∥ †

VITO - Flemish Institute for Technological Research, Boeretang 200, 2400 Mol, Belgium Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium § Department of Chemical Engineering, Cluster Sustainable Chemical Process Technology, KU Leuven, Gebroeders Desmetstraat 1, Ghent B-9000, Belgium ∥ Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa ‡

S Supporting Information *

ABSTRACT: Membrane distillation (MD) uses a microporous hydrophobic membrane to separate dissolved molecules from a liquid stream. Notwithstanding the great potential, membrane distillation is not applied on an industrial level yet, because of the lack of specifically developed membranes, modules, and techno-economic data at full scale. This review gives a comprehensive overview of the optimal membrane properties and can serve as a guideline for the development of new membranes, specifically for membrane distillation. Optimization of the membrane is needed to sufficiently resist wetting. Generally, a pore diameter of 0.3 μm is recommended to balance between a high liquid entry pressure and flux. Since vacuum membrane distillation is more sensitive to wetting, a smaller pore diameter could be appropriate for this configuration to avoid membrane wetting. An optimal membrane thickness is found between 10 and 700 μm, depending on process conditions, balancing between mass transport and energy loss. To improve the mass transfer and energy efficiency, membrane porosity should preferably be as high as possible (>75%), while low tortuosity (1.1−1.2) and thermal conductivity (>0.06 w·m−1·K−1) are recommended as well.

1. INTRODUCTION Membrane distillation is a thermally driven separation technique, using a hydrophobic microporous membrane. The hydrophobicity of the membrane retains the liquid feed solution, whereas the microporous membrane structure allows permeation of vapors. In the most simplified configuration, a hot liquid feed solution and a cold permeate solution sustain a temperature difference and hence a vapor pressure difference over the membrane. The mass transport is initiated by evaporation of the feed liquid at the phase boundary between vapor and liquid at the membrane pores (1). The vapor pressure gradient induces diffusion of the vapor molecules through the membrane (2), after which condensation (3) occurs at the permeate side (Figure 1).1−3 The lower vapor pressure on the permeate side is imposed by a variety of methods, resulting in four basic configurations (Figure 2).4,5 In direct contact membrane distillation (DCMD), a cold permeate with low vapor pressure is in direct contact with the membrane.6 To reduce the heat loss due to conduction, an air gap is introduced between the membrane and a cold surface on the permeate side in air-gap membrane distillation (AGMD).7 A cold sweep gas on the permeate side © 2016 American Chemical Society

provides the driving force in sweeping gas membrane distillation (SGMD), removing the water vapor, which is condensed in an external condenser.8 In vacuum membrane distillation (VMD), a vacuum is applied on the permeate side.9 Recently, research is also increasingly oriented toward permeate gap membrane distillation (PGMD), where the gap in AGMD is completely filled with permeate. Membrane distillation was originally proposed as an alternative for distillation and reverse osmosis.5 Nowadays, the process is explored for a wide variety of separations of nonvolatile dissolved substances from a solvent, most commonly water. The nonvolatile components can include but are not limited to salts, proteins,10 acids,11,12 and minerals.13 Additionally, membrane distillation can also be used to separate components based on a difference in volatility. Examples include the separation of alcohols, 7 volatile aromas,14,15 and ammonia16,17 from an aqueous solution. The Received: Revised: Accepted: Published: 9333

June 8, 2016 August 17, 2016 August 19, 2016 August 19, 2016 DOI: 10.1021/acs.iecr.6b02226 Ind. Eng. Chem. Res. 2016, 55, 9333−9343

Review

Industrial & Engineering Chemistry Research

published with new insights on the important membrane properties for membrane distillation. This work gives an updated state-of-the-art review of membrane requirements and the optimal membrane structure for membrane distillation, as described in the literature, and a critical look at these findings.

2. FUNDAMENTALS Multiple review articles and handbooks are dedicated to the general principles of membrane distillation,1−5,23,24 applications,25−28 modeling and transport mechanisms,8,29 scaling and fouling,30−32 and performance criteria.33 Only the fundamentals required for understanding the effect of a membrane parameter on the performance of the membrane are described in this section. 2.1. Polarization in the Channels. The hydrodynamics of the flow in the channels are determined by the module design, spacer type, fluid properties, and flow velocity.34,35 Due to the difference in temperature on both sides of the membrane, heat is transported through the membrane. As the flow is mostly laminar, the mixing in the channels is far from ideal in the MD process. Therefore, the temperature at the membrane interface differs from the bulk temperature in the channel. At the feed side, the interfacial temperature is lower compared to the bulk temperature, whereas the inverse is true at the permeate side. This effect is called temperature polarization, causing a lower interfacial temperature difference (ΔTi) compared to the bulk temperature difference (ΔTb) and a lower driving force (Δpi). In general, temperature polarization is affected by hydrodynamics and is more pronounced for membranes that are more permeable, are thinner, or have a high thermal conductivity.36,37 Similarly to temperature, the concentration of rejected dissolved substances near the membrane interface is higher compared to the bulk concentration due to the removal of solvent (Figure 3). This effect is called concentration polarization.

Figure 1. Schematic representation of the direct contact membrane distillation process.

relatively low operational temperature compared to distillation and the lower operational pressures compared to pressure driven membrane processes enable the use of waste heat and solar or geothermal energy.18−20 Additionally, the membrane is a contactor between two phases and does not interact with the separation itself. A retention of nonvolatile dissolved substances of 100% is possible, even at high concentrations. Therefore, the process can treat concentrated solutions beyond the operational capability of reverse osmosis.21,22 Despite its great potential, the process has not been widely accepted in industry yet. In order to achieve this acceptance, there is a need for a better performance of the technique. The membranes that are used nowadays are not specifically developed for MD. Moreover, they are expensive and often hardly manageable at large scale, because of the disability to seal them into the modules and their low mechanical strength, if unsupported. Reduction of the production costs of the membranes and modules and improvement of the membrane performance in terms of flux and energy efficiency by the development of new MD membranes are key factors on the road to commercialization. By definition, the membrane must resist wetting by the process liquids and should be porous, as described by Smolders and Franken in 1989.1 The review paper of Lawson and Lloyd in 1997 gives a few considerations regarding pore size, thickness, and porosity.5 The most important review papers defining the optimal properties of a membrane for membrane distillation are published by El-Bourawi et al. in 2006 and by Alkhudhiri et al. in 2012, mainly focusing on seawater desalination.2,4 Since then, numerous publications have been

Figure 3. Concentration and temperature profile in a DCMD system.

Figure 2. Basic membrane distillation configurations. 9334

DOI: 10.1021/acs.iecr.6b02226 Ind. Eng. Chem. Res. 2016, 55, 9333−9343

Review


2.4. Membrane Evaluation. At the membrane level, the most commonly used evaluation criteria are the mass flux (eq 1) and the membrane thermal energy efficiency (eq 7) and the membrane separation efficiency. More details on the evaluation of the membrane distillation process can be found elsewhere.33 Most of the MD applications deal with nonvolatiles. The separation efficiency of dissolved substances from a solvent in membrane distillation is given by the retention coefficient (R):

2.2. Mass Transport through the Membrane. The mass flux (N) in kg·h−1·m−2 represents the amount of water (m) that is collected per unit of membrane surface area (A) and time (t): ⎛ kg ⎞ m ⎟= N⎜ ⎝ h · m2 ⎠ At

(1)

The transport of vapor molecules through the porous membrane structure is driven by a vapor pressure difference over the membrane. The mass flux is directly related to the water interfacial vapor pressure difference over the membrane (Δpi) and the mass transfer coefficient (C).38 N = C Δpi

R=1−

ϵr a τδ

(3)

α=

with a = 0, 1, or 2 for molecular diffusion, Knudsen diffusion, and viscous flow, respectively, ϵ the membrane bulk porosity, τ the membrane tortuosity, δ the membrane thickness, and r the mean pore radius. The mass transfer mechanism is determined by the arbitrary Knudsen number (Kn) expressed as the ratio of the mean free path that a molecule travels (λ) and the pore diameter (d).

Kn = λ /d

QC = EE =

km (Tf,m − Tp,m) δ

(ωa /ω b) in permeate (ωa /ω b) in feed

(9)

3. MEMBRANE PROPERTIES Currently, the membranes used in the commercial MD systems are hydrophobic microporous membranes developed initially for microfiltration. An overview of the available commercial membranes can be found in the literature.8,24,39 These membranes meet the requirements as described by the definition of membrane distillation; that is, the membrane must consist of at least one hydrophobic layer, which is not wetted by the feed liquid and is microporous. However, an optimized membrane specifically designed for membrane distillation could further improve the membrane distillation process. This section describes the effect of the membrane characteristics on the performance in membrane distillation and offers a guideline for the development of improved membranes. 3.1. Material Properties. The membrane material in membrane distillation has specific requirements. The material must be hydrophobic, to retain the liquid phase from entering the membrane. Moreover, the thermal conductivity of the material must be as low as possible to reduce the heat loss due to conduction through the membrane matrix (eq 6). Furthermore, the material must have a minimum mechanical and chemical stability and may not dissolve in the feed solution. The membranes used for membrane distillation are mostly polymeric, due to their versatility and low thermal conductivity (0.1−0.5 W·m−1·K−1). Only a few studies focus on ceramic membranes with improved stability, but also higher thermal conductivity, where 1 W·m−1·K−1 is the lowest reported thermal conductivity value for ceramics.40−44 The majority of the membranes are made from polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), or polypropylene (PP). These polymers provide a sufficient wetting resistance due to the low surface tension. Recently, also PE and modified PES membranes are explored for membrane distillation.39 The properties of these polymers can be found in Table S1 (Supporting Information). 3.2. Wetting Resistance. The resistance to wetting is defined by the liquid entry pressure (LEP), which is the minimum pressure required to wet the membrane. The LEP

(4)

(5)

(6)

QN QN + Q c

(8)

ωa and ωb are the mass fraction (%) of component a and b, respectively.

Kn > 1 indicates that molecules will mainly collide with the pore wall and Knudsen diffusion occurs. For Kn < 0.01, molecule−molecule interactions will mainly occur when air is present in the pores and then molecular diffusion is the prevailing mechanism. For VMD, air is removed from the pores and a hydrostatic pressure difference is applied. In this case, viscous flow occurs instead of molecular diffusion. For 0.01 < Kn < 1, no mechanism prevails and a Knudsen−molecular diffusion transition region or a Knudsen−viscous transition region is assumed. More information on this subject and the full equations can be found elsewhere.8 2.3. Heat Transport through the Membrane. Heat transport through the membrane occurs simultaneously by two mechanisms. The temperature difference on both sides of the membrane induces heat loss due to conduction (QC), whereas the heat required for evaporation of the vapor molecules associated with the flux (QN) is considered as efficient heat transport. The ratio of the efficient heat due to flux to the total heat flux through the membrane is used to evaluate the membrane thermal energy efficiency (EE). Q N = ΔHwN

cf

cp and cf are the concentrations in g·L−1 of the permeate and the feed solution. MD is a contactor application, and hence, volatiles are not retained by the membrane. When volatiles are present in the feed stream, the separation factor is used to evaluate the separation efficiency of the process.

(2)

A simplification of the mass transfer coefficient equations shows the general dependence of the mass transport on the membrane properties.24,33

C∼

cp

(7)

ΔHw is the enthalpy of vaporization of water, Tf,m and Tp,m are the interfacial temperatures at the membrane on the feed and permeate side, and km is the thermal conductivity of the membrane, which is affected by the structure, porosity, and intrinsic thermal conductivity of the polymer. 9335


Review


way.54 More details on the calculations are out of the scope of this review and can be found elsewhere.3,8,29,54 Membranes typically have a pore size distribution rather than a uniform pore size. While some studies indicate that a membrane with narrow pore size distribution is more effective in MD,55 other studies conclude that the effect of pore size distribution on flux is negligible for DCMD for commercial membranes56−58 and that the uncertainties in modeling and experimental design overwhelm any effect of pore size distribution.57 Li et al. found that pore size distribution does not need to be considered in simulations for Knudsen numbers below 1 and for membranes with a sharp distribution, while it might become important when approaching a Knudsen number of 1.38 According to Woods et al., the need for considering the pore size distribution in the calculation and, hence, the effect of it on flux depends on the width of the distribution.57 It is at most 5% for membranes with distributions narrower than the following:

depends both on membrane characteristics and on feed composition and can be calculated by the Laplace equation: LEP =

−2Bγl cos(θ ) rmax

(10)

where γl is the surface tension of the liquid, θ the contact angle, rmax the maximum pore size, and B a geometric factor. To ensure a proper operation under fluctuating pressures and temperatures in the operation plant, a LEP with pure water of minimum 2.5 bar is recommended for aqueous solutions in the absence of surfactants.45 Higher LEP might be required for some applications, considering the presence of surfactants46 or organics (e.g., alcohols47). 3.3. Pore Size and Pore Size Distribution. In general, membranes with a pore size diameter from 0.05 up to 1 μm are used for membrane distillation.23,38,48 However, it is observed that PTFE membranes with a pore diameter of about 1 μm show a reduced salt retention.49,50 Besides the wetting resistance, the pore size also affects the mass transport, as it determines the mass transfer mechanism via the Knudsen number and correlates with flux depending on the mass transfer mechanism (see section 2.2). In Table S2 (Supporting Information), the studies investigating the effect of the pore size on flux and/or energy efficiency are summarized. In general, most studies indicate that the flux increases with increasing pore size for all MD configurations.7,49,51 However, for most of the experimental studies, also porosity and membrane thickness are varied together with pore size, which could lead to incorrect conclusions on the pore size effect. Different studies investigate the effect of pore size using simulations, enabling maintenance of other membrane parameters constant in DCMD and AGMD.52−54 Figure 4

• DCMD: σr < 1.2 for rav > 50 nm; • AGMD: σr < 1.45 for rav > 50 nm; • VMD: σr < 1.07 for all pore sizes. 3.4. Thickness. Most commercial membranes used in MD have thicknesses ranging from 20 to 200 μm.8,24,39,52 Table S2 (Supporting Information) shows that numerous studies indicate that the flux in DCMD is decreasing with increasing membrane thickness. However, it is generally accepted that while mass transfer is improved by reducing the membrane thickness, more heat loss also occurs, reducing the driving force. Therefore, it is suggested that an optimal thickness must exist, balancing between mass transport and driving force. Table S3 summarizes the few in-depth studies on the optimal thickness in DCMD. As shown in Figure 5A), the optimal thickness varies from 0 to 49 μm depending on the salinity, which is in accordance with the range 10−60 μm reported by most publications. However, in most studies, high temperature difference and low salinities are used to calculate the optimal thickness, resulting in values below 60 μm. When using temperature difference down to 5 °C, the optimal membrane thickness can theoretically vary widely (5 up to 700 μm), depending on salinity, process conditions, and membrane structure.59 All studies report that increasing the membrane thickness results in an improved energy efficiency up to asymptotic values, as shown in Figure 5B.38,59−62 For AGMD, it is found that the flux is not affected by membrane thickness or increases with decreasing membrane thickness (Table S2). In general, it can be stated that the effect of the membrane can be neglected if the thickness of the gap is much larger compared to the membrane thickness. In these cases, the gap is the main resistance for mass transport and changing the membrane thickness does not affect the flux.3,49,63−65 It was also found experimentally that the flux decreases with increasing thickness in the VMD configuration.66,67 Simulations for large scale modules minimizing the water production cost by tuning different membrane and module parameters show that the optimal membrane thickness increases with increasing salinity, confirming the results of the lab scale studies in Table S3. For DCMD, optimal thickness ranges from 41 to 146 μm depending on the salt concentration and the availability and cost of the waste heat. The thickest membrane (84−200 μm) is required for permeate gap membrane distillation (PGMD).62 Owing to the additional

Figure 4. Mass transfer coefficient as a function of the pore size in DCMD with fixed membrane structure (ε = 80%, δ = 70 μm, τ = 1.1, T = 60 °C). Reprinted from ref 52 with permission from Elsevier. Copyright 2013.

shows the mass transfer coefficient in a DCMD system for Knudsen (CKn) and molecular diffusion (CD) and the combined mass transfer (C) as a function of the pore size. This figure shows that for molecular diffusion there is no effect of the pore size, while the mass transfer is enhanced at larger pore size for Knudsen diffusion. Since the highest resistance determines the mass transport, molecular diffusion will dominate the mass transport at pore sizes larger than 0.2 μm in this case. When the pore diameter is larger than 0.3 μm, little incentive from the DCMD flux is gained, while the LEP is affected in a negative 9336


Review


Figure 5. Influence of the membrane thickness on the DCMD flux (A, left) and energy efficiency (B, right) for different concentrations of NaCl. Tf = 60 °C, Tp = 45 °C, v = 0.13 m/s. Simulated membrane: PP Accurel 2E HF. Reprinted from ref 59 with permission from Elsevier. Copyright 2016.

loss of driving force in the gap, a thicker membrane is preferred in PGMD to maintain sufficient driving force over the membrane. 3.5. Porosity. Most commercial membranes have porosities ranging from 30 to 85%.4,68 Recently, membranes with porosities above 90% have been produced with electrospinning.69,70 The membrane porosity should be as high as possible, while still maintaining sufficiently high mechanical strength. It is generally considered as the most important membrane parameter in membrane distillation.45,71,72 As indicated by eqs 2 and 3, the flux is linearly proportional with porosity. Additionally, polymers have a thermal conductivity from 0.1 to 0.5 W·m−1.K−1, while the air/water vapor mixture inside the pores has a thermal conductivity of only 0.027 W·m−1·K−1. Therefore, a higher porosity results in less heat loss due to conduction through the membrane and hence a higher driving force, higher flux, and higher energy efficiency. Both experimental results and modeling confirm the higher flux and energy efficiency at higher porosity, regardless of the MD configuration (Table S2, Supporting Information). 3.6. Tortuosity. To our knowledge, no systematic study has been performed concerning the influence of pore tortuosity on MD performance, because of the difficulties in measuring its real value. Different correlations can be used to quickly and roughly estimate the tortuosity of microporous structures.73 The two most commonly used correlations showing relatively good fit with the measured values for membranes are38,74 τ=

1 ε

(11)

τ=

(2 − ε)2 ε

(12)

Table 1. Tortuosity as Measured and Calculated by Different Methods Membrane

ε

τ

eq 11

eq 12

ePTFE: TF200, TF450 (Gelman) ePTFE: M-005, M-010, M-020, M045 (Gore) PVDF: VVHP, GVHP, HVHP (Millipore)

808 7874

1.176 1.1−1.238,62,74

1.3 1.3

1.8 1.9

6639

1.3−1.538 2.1−2.567,74 3.980

1.5

2.7

size, while slightly larger values are being calculated using eqs 11 and 12. Meanwhile, the experimental and calculated results show a relatively large deviation for phase inverted PVDF membranes. The estimation using eq 11 is in accordance with ref 38, while eq 12 fits better with refs 67 and 74. 3.7. Thermal Conductivity. The thermal conductivity of porous membranes is also affected by porosity and ranges from 0.04 to 0.07 W·m −1 ·K−1 for polymeric microfiltration membranes, decreasing with increasing porosity.38,81−83 The polymer thermal conductivity values (κs) and their dependence on temperature for the commonly used polymers for MD membranes are given in Table S4. The thermal conductivity values can be estimated with different models shown in Table 2, of which the isostrain model is most commonly used. However, in a study comparing different models to experimental results, the Maxwell model is found to show the best fit for porous membranes with ε above 60%, whereas the isostrain model overestimates the flux significantly.82 In another study, these mathematical models were compared to the calibrated values in a DCMD model. Based on the calibrated values, the isostrain model was the best model to predict thermal conductivity for phase inverted and electrospun membranes, while the structure of the stretched membranes was predicted better by the Maxwell model.84 Measuring the thermal conductivity of such thin porous membranes is difficult, but also the selection of the most suitable model remains uncertain as well. Therefore, the membrane thermal conductivity is proposed as a calibration parameter when developing MD models.84 As shown in Table S2 (Supporting Information), it is generally accepted that a higher thermal conductivity results in

Often, a value of 2 is assumed for the tortuosity56,75 or the parameter is used as a calibration factor for the prediction of the flux.62,76,77 Additionally, gas permeation tests combined with the measurement of the membrane porosities permit an estimation of this parameter.78 Another method includes the measurement of the gas permeation through prewetted membranes.38,74,79 Membranes with tortuosity of 1.1−3.9 are reported (Table 1). For PTFE membranes, a value between 1.1 and 1.2 is measured independently of the provider or the pore 9337


Review

Industrial & Engineering Chemistry Research Table 2. Overview of Different Models Used for the Thermal Conductivity, and Comparison of the Results for Two Types of Commercial Membranes

Model

Equation 82

κm PVDF GVHP Millipore (W· m−1·K−1)

κm PTFE TF200 Gelman (W·m−1·K−1)

0.041−0.057

0.031−0.039

Isostrain

κm = ϕκs + ϵκg (ϕ = 1 − ϵ)

0.090

0.052

Isostress

−1 ⎛ϕ ϵ ⎞⎟ ⎜ κm = ⎜ + ⎟ κg ⎠ ⎝ κs

0.041

0.031

0.056

0.035

Experimental

κm =

Maxwell

κg(1 + 2βϕ)

1 − βϕ ⎛ κs − κg ⎞ ⎜⎜β = ⎟ κs + 2κg ⎟⎠ ⎝

Figure 6. Effect of compaction due to applied pressure on a compressible membrane on flux and thermal efficiency (Tf = 60 °C; Tp = 45 °C). Reprinted from ref 91 with permission of Elsevier. Copyright 2013.

branes.93 For a supported membrane, the support adds mechanical strength to the membrane, but also imposes an additional resistance in the process. Different authors compare the flux of membranes supported by a nonwoven or scrim support. The study of Zhang et al. finds the highest flux for the scrim supported membranes and indicates that the openness of the support is important.94,95 Winter et al. studied nonsupported and scrim and nonwoven supported membranes with comparable hydrophobic layer thickness, pore size, and porosity.52 In this study, the addition of the support reduced the flux by 22% for the nonwoven support with porosity of 70% and by 35% for a scrim support with 50% porosity. Adnan et al. confirm these statements and found a flux reduction up to 56%.71 Figure 7 shows the temperature for a composite membrane, explaining the decrease in flux when adding a support layer:

a lower flux and energy efficiency. These studies investigate the effect of polymer thermal conductivity on the membrane performance using the parallel model. A flux and thermal efficiency reduction of 26% and 55%, respectively, is calculated by increasing κS from 0.01 to 0.5 W·m−1·K−1 in DCMD.85 As the correct choice of a model to simulate this effect is not clear yet, these simulations remain an educated guess on the effect of thermal conductivity. 3.8. Mechanical Strength. Only a few studies focus on the mechanical properties of membrane distillation membranes.69,86,87 A large variation in values is reported for the elastic modulus (34−491 MPa), tensile strength (3.4−57.9), and elongation at break (41−710%) in these publications. Typically, the stress−strain curves are also reported. No specific minimum requirements are reported for membrane distillation membranes. Few studies focus on the impact of membrane compaction due to the pressure drop over the module. Fernandez-Pineda et al. concludes in their study that, for their experimental conditions, compaction is negligible for their lab scale setup and experimental conditions.80 However, on the industrial scale this pressure drop becomes relevant. Few studies focus on flux measurement at different hydraulic pressures.88−90 Compaction of the membrane can result in a serious reduction of the flux. The higher tortuosity, lower porosity, lower pore size, and lower thickness influence the performance of the membrane. In Figure 6, the simulated and experimental flux as a function of the applied mechanical pressure is presented,88 indicating that thermal efficiency decreases with increasing pressure, while with increasing pressure an increase of flux is being calculated until an optimum is reached; after this optimum, the flux decreases with increasing mechanical pressure. 3.9. Support Layer Properties. Commercial membranes used in MD have thicknesses ranging from 60 to 200 μm.8 Membranes thinner than 60 μm suffer from low mechanical stability, and the risks for defects are high.92 A way to tailor the membrane thickness to the optimum range is the fabrication of supported membranes, for example by using a nonwoven or scrim support.71 Hydrophobic polymers such as PE or PP are commonly used for this purpose, but as the pore size of these nonwoven or scrim support structures is far above 1 μm, this support structure is considered to be wetted during operation. Another way to reduce the membrane thickness is the fabrication of hydrophobic−hydrophilic composite mem-

Figure 7. DCMD transport mechanism for a hydrophobic−hydrophilic composite membrane. Reprinted from ref 96 with permission from Elsevier. Copyright 2005.

• The support layer is filled with water. Therefore, the driving force is the temperature difference over the hydrophobic top layer. The hydrophilic layer is an additional resistance for heat transfer in the channel, and hence, additional temperature polarization for composite membranes occurs.71,96 • Some of the pores are possibly blocked by hydrophilic polymer material, reducing the vapor−liquid interface. Therefore, the support layer possesses an additional mass transfer resistance as well.71,96 9338


Review


Figure 8. Effect of polymer thermal conductivity (A, left) and porosity (B, right) on the membrane thermal conductivity according to the different models.

distribution might become relevant, depending on the width of the distribution for DCMD and AGMD. The same conclusions are expected for PGMD, since the mass transfer mechanism for this configuration is the same. In terms of wetting, the largest pore diameter determines the liquid entry pressure, while, in terms of flux, the average pore diameter is considered. Therefore, it is recommended to use membranes with a small pore size distribution to reduce the risk for wetting as much as possible, while still achieving the highest possible flux. As the mass transport for viscous flow in vacuum membrane distillation is proportional to r2 (eq 3), a larger effect of pore size and pore size distribution is expected. Vacuum membrane distillation is also more sensitive to pore wetting due to the larger hydrostatic transmembrane pressure difference.17 Therefore, a smaller maximum pore diameter of 0.1 to 0.2 μm is recommended for this configuration to ensure stable performance. For the membrane thickness, most authors assume an optimal thickness as small as possible, from 10 to 60 μm, which is perfectly suitable for seawater desalination. However, since membrane distillation is currently also applied for feed streams containing high concentrations of dissolved substances, thicker membranes might be preferred for some applications. More attention should be paid to this parameter when selecting or synthesizing membranes, as it is of major importance for the flux and energy efficiency of the membrane and for the energy consumption of the full system. Tortuosity and thermal conductivity are often not characterized, but estimated by using a correlation including the membrane porosity (eqs 11 and 12 and Table 2). Tortuosity should be as low as possible. However, since the membrane is about 70−90% porous, the tortuosity is already relatively low and the effect of this parameter is not considered as very important for the optimization of the membrane. Figure 8 shows the effect of the thermal conductivity of the polymer and porosity on the membrane thermal conductivity for three different models proposed in the literature. The isostrain model shows a much larger effect on membrane thermal conductivity compared to the isostress and Maxwell model. Nevertheless, the accuracy of each model is uncertain and should be investigated in more detail. Since the isostrain model is most commonly used in MD modeling, the effect of the polymer material and porosity might be largely overestimated. More profound validation of the existing models or improved measurement techniques for the thermal conductivity could increase the accuracy of the simulations on membrane

Only few systematic studies have been performed on the optimal properties of the hydrophobic layer in supported composite membranes. Martinez et al. gives the same conclusions as for a single layer hydrophobic membrane in section 3.4 on the effect of the hydrophobic layer thickness for supported hydrophobic−hydrophilic membranes.60 The hydrophobic layer porosity should be as large as possible for the same reasons indicated in section 3.5.97 According to Qtaishat et al., the influence of the hydrophobic material thermal conductivity can be neglected.97 No systematic studies on the optimization of other membrane properties have been found in the literature. However, the recommendations for the hydrophobic layer, given in the previous sections, are also valid. The properties of the support layer also affect the membrane distillation performance and can be tuned to increase the membrane performance. • Reduction of the support thickness reduces the temperature polarization and hence increases the flux.97 • An increased thermal conductivity results in more heat transfer through the wetted support, less temperature polarization, and a higher driving force and flux through the hydrophobic membrane layer.97,98

4. RECOMMENDATIONS AND FUTURE DIRECTIONS A minimum LEP of 2.5 bar and a maximum pore diameter of 0.3 μm was recommended based on the literature search. Considering pure water, the theoretical contact angle that needs to be achieved can be calculated using eq 10 and equals 105° for a pore diameter of 0.3 μm. Recently, membranes with superhydrophobic behavior (θw above 150°) are prepared using novel coating and grafting techniques. On one hand, a much higher pore size up to 1 μm can then possibly be used, while still achieving the minimum LEP of 2.5 bar. On the other hand, since the pore diameter above 0.3 μm is only slightly improving the MD flux, there is no need to increase the pore size above this value. For example, by using a membrane with a water contact angle of 150 and pore size of 0.3 μm, the LEP equals 8 bar. This higher LEP ensures an additional safety margin, which might be crucial to extend the lifetime of the membrane. Moreover, it enables use of the membrane in more difficult applications with feed streams containing wetting agents. Few studies focus on the effect of the pore size distribution, where different opinions are given. Summarized, the effect of pore size and pore size distribution on flux is expected to be minor in the transitional Knudsen−molecular diffusion regime. Closer to the Knudsen regime, the effect of pore size 9339


Review

Industrial & Engineering Chemistry Research Table 3. Overview of the Membrane Properties Desired for MD Based on a Literature Survey Range used in literature3,8,23,39,99

Parameter

Affects

Contact angle (θ) Liquid entry pressure (LEP) Pore diameter (dav, dmax)

Wetting resistance Wetting resistance Wetting resistance (Minor impact on flux/EE)

80−160° 0.5−4.647,61 0.012−1.2 μm61,97,100

Porosity (ϵ) Thickness (δ)

Flux/EE/strength Flux/EE/strength

38−90% 10−200 μm

Tortuosity (τ) Thermal conductivity (κm)

Flux/EE Flux/EE

Tensile strength

Strength

1.1−3.967,80 0.031−0.057 W·m−1·k−182 2−5 W·m−1·k−140 3.4−57.9 MPa69,86,87

■ ■

NOMENCLATURE A Surface area [m2] AGMD Air-gap membrane distillation B Geometric factor [−] c Concentration [g·L−1] C Mass transfer coefficient [kg·h−1·m−2·.Pa−1] d Pore diameter [μm] DCMD Direct contact membrane distillation EE Energy efficiency [%] LEP Liquid entry pressure [Pa] m Mass [m] MD Membrane distillation N Flux [kg.h‑1.m−2] p vapor pressure [Pa] PE Polyethylene PES Poly(ether sulfone) PP Polypropylene PTFE Polytetrafluoroethylene PVDF Polyvinylidene fluoride Q Heat flux [W·m−1] R Retention [%] r pore radius [μm] RO Reverse osmosis SGMD Sweeping gas membrane distillation t Time [s] T Temperature [°C] Tg Glass transition temperature [°C] Tm Melting point [°C] v Flow velocity [m·s−1] VMD Vacuum membrane distillation γ Surface tension [mN·m−1] δ Thickness [μm] ΔH Enthalpy of vaporization of water [J·kg−1] ε Porosity [%] θ Contact angle [deg] κ Thermal conductivity [W·m−1·K−1] τ Tortuosity [−]

5. CONCLUSIONS Overall, the membrane structure should be optimized specifically for membrane distillation to increase the mass transport and reduce the conductive heat transport. This study shows that the wetting resistance is crucial for applying the membrane in membrane distillation. To achieve high flux and energy efficiency, the membrane porosity is the most important parameter, together with an optimized membrane thickness. Tortuosity is less measured, but is preferably as low as possible. The effect of thermal conductivity can be calculated by using different models. The effect of the material thermal conductivity strongly depends on the choice of the model and is therefore unclear. Additionally, a support can be introduced to increase the mechanical strength of the membrane. The optimal support has a high porosity, a low thickness, and a high thermal conductivity. Despite the potential of membrane distillation, the technology is still not applied on full scale. New and better membranes are required to improve the performance and could contribute to the introduction of membrane distillation at commercial scale. ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.iecr.6b02226. Tables showing the properties of the polymers used for MD membranes, a literature overview on the effect of the membrane structure on MD performance, an overview of the literature regarding the effect of membrane thickness for saline streams in DCMD, and the thermal conductivity of the materials used for MD membranes(PDF)

■

As high as possible

ACKNOWLEDGMENTS L.E. thankfully acknowledges a Ph.D. scholarship provided by VITO.

distillation and indicate the importance of the thermal conductivity of the polymer material and membrane porosity. An overview of the recommended optimal membrane properties is provided in Table 3 together with the range used in the literature.

■

Recommendation >105°, As high as possible >2.5 bar45 0.1−1 μm2,45 0.3 μm52,54 80−90%2 Low salinity: 30−60 μm60,89,101 High salinity: 2−700 μm59 As low as possible As low as possible

AUTHOR INFORMATION

Subscripts

av b c f g

Corresponding Author

*Tel.: +3214/33.56.63, E-mail: [email protected]. Notes

The authors declare no competing financial interest. 9340

Average Bulk Conduction Feed Gas DOI: 10.1021/acs.iecr.6b02226 Ind. Eng. Chem. Res. 2016, 55, 9333−9343

Review

Industrial & Engineering Chemistry Research i l m max N p s

■

(21) Nguyen, Q.-M.; Lee, S. Fouling Analysis and Control in a DCMD Process for SWRO Brine. Desalination 2015, 367, 21. (22) Chen, G.; Lu, Y.; Krantz, W. B.; Wang, R.; Fane, A. G. Optimization of Operating Conditions for a Continuous Membrane Distillation Crystallization Process with Zero Salty Water Discharge. J. Membr. Sci. 2014, 450, 1. (23) Basile, A.; Figoli, A.; Khayet, M. Pervaporation, Vapour Permeation and Membrane Distillation: Principles and Applications, 1st ed.; Woodhead Publishing Ltd: 2015. (24) Drioli, E.; Ali, A.; Macedonio, F. Membrane Distillation: Recent Developments and Perspectives. Desalination 2015, 356, 56. (25) Pangarkar, B. L.; Sane, M. G.; Parjane, S. B.; Guddad, M. Status of Membrane Distillation for Water and Wastewater TreatmentA Review. Desalin. Water Treat. 2014, 52, 28−30. (26) Calabro, V.; Drioli, E.; Matera, F. Membrane Distillation in the Textile Wastewater Treatment. Desalination 1991, 83, 209. (27) Shirazi, M. M. a; Kargari, A. A Review on Application of Membrane Distillation (MD) Process for Wastewater Treatment. J. Membr. Sci. Res. 2015, 98, 101. (28) Ashoor, B. B.; Mansour, S.; Giwa, A.; Dufour, V.; Hasan, S. W. Principles and Applications of Direct Contact Membrane Distillation (DCMD): A Comprehensive Review. Desalination 2016, 398, 222. (29) Hitsov, I.; Maere, T.; De Sitter, K.; Dotremont, C.; Nopens, I.; Sitter, K.; Dotremont, C.; Nopens, I. Modelling Approaches in Membrane Distillation: A Critical Review. Sep. Purif. Technol. 2015, 142, 48. (30) Naidu, G.; Jeong, S.; Vigneswaran, S.; Hwang, T.-M.; Choi, Y.-J.; Kim, S.-H. A Review on Fouling of Membrane Distillation. Desalin. Water Treat. 2015, 57 (22), 10052. (31) Tijing, L. D.; Woo, Y. C.; Choi, J.-S.; Lee, S.; Kim, S.-H.; Shon, H. K.; Fouling. and Its Control in Membrane distillationA Review. J. Membr. Sci. 2015, 475, 215. (32) Warsinger, D. M.; Swaminathan, J.; Guillen-Burrieza, E.; Arafat, H. a.; Lienhard V, J. H. Scaling and Fouling in Membrane Distillation for Desalination Applications: A Review. Desalination 2015, 356, 294. (33) Luo, A.; Lior, N. Critical Review of Membrane Distillation Performance Criteria. Desalin. Water Treat. 2016, 57 (43), 20093. (34) Martinez-Diez, L.; Vazques-Gonzalez, M. I. Temperature and Concentration Polarization in Membrane Distillation of Aqueous Salt Solutions. J. Membr. Sci. 1999, 156, 265. (35) Cath, T. Y.; Adams, V. D.; Childress, A. E. Experimental Study of Desalination Using Direct Contact Membrane Distillation: A New Approach to Flux Enhancement. J. Membr. Sci. 2004, 228 (1), 5. (36) Imdakm, A. O.; Matsuura, T. Simulation of Heat and Mass Transfer in Direct Contact Membrane Distillation (MD): The Effect of Membrane Physical Properties. J. Membr. Sci. 2005, 262, 117. (37) Bahmanyar, A.; Asghari, M.; Khoobi, N. Numerical Simulation and Theoretical Study on Simultaneously Effects of Operating Parameters in Direct Contact Membrane Distillation. Chem. Eng. Process. 2012, 61, 42. (38) Li, L.; Sirkar, K. K. Influence of Microporous Membrane Properties on the Desalination Performance in Direct Contact Membrane Distillation. J. Membr. Sci. 2016, 513, 280. (39) Eykens, L.; De Sitter, K.; Dotremont, C.; Pinoy, L.; Van der Bruggen, B. Characterization and Performance Evaluation of Commercially Available Hydrophobic Membranes for Direct Contact Membrane Distillation. Desalination 2016, 392, 63. (40) Wang, J.-W.; Li, L.; Zhang, J.-W.; Xu, X.; Chen, C.-S. β-Sialon Ceramic Hollow Fiber Membranes with High Strength and Low Thermal Conductivity for Membrane Distillation. J. Eur. Ceram. Soc. 2016, 36 (1), 59. (41) Hendren, Z. D.; Brant, J.; Wiesner, M. R. J. Membr. Sci. 2009, 331, 1. (42) Krajewski, S.; Kujawski, W.; Bukowska, M.; Picard, C.; Larbot, a. Application of Fluoroalkylsilanes (FAS) Grafted Ceramic Membranes in Membrane Distillation Process of NaCl Solutions. J. Membr. Sci. 2006, 281 (1−2), 253.

Interfacial Liquid Membrane Maximum Flux Permeate Solid

REFERENCES

(1) Smolders, K.; Franken, a. C. M. Terminology for Membrane Distillation. Desalination 1989, 72 (3), 249. (2) Alkhudhiri, A.; Darwish, N.; Hilal, N. Membrane Distillation: A Comprehensive Review. Desalination 2012, 287, 2. (3) Khayet, M.; Matsuura, T. Membrane Distillation: Principles and Applications; Elsevier B.V.: 2011. (4) El-Bourawi, M. S.; Ding, Z.; Ma, R.; Khayet, M. A Framework for Better Understanding Membrane Distillation Separation Process. J. Membr. Sci. 2006, 285 (1−2), 4. (5) Lawson, K.; Lloyd, D. Review: Membrane Distillation. J. Membr. Sci. 1997, 124 (1), 1. (6) Phattaranawik, J.; Jiraratananon, R.; Fane, a. Heat Transport and Membrane Distillation Coefficients in Direct Contact Membrane Distillation. J. Membr. Sci. 2003, 212 (1−2), 177. (7) Garcia Payo, M. C.; Izquierdo-Gil, M. A.; Fernández-Pineda, C. Air Gap Membrane Distillation of Aqueous Alcohol Solutions. J. Membr. Sci. 2000, 169, 61. (8) Khayet, M. Membranes and Theoretical Modeling of Membrane Distillation: A Review. Adv. Colloid Interface Sci. 2011, 164 (1−2), 56. (9) Abu-Zeid, M. A. E.-R.; Zhang, Y.; Dong, H.; Zhang, L.; Chen, H.L.; Hou, L. A Comprehensive Review of Vacuum Membrane Distillation Technique. Desalination 2015, 356, 1. (10) Christensen, K.; Andresen, R.; Tandskov, I.; Norddahl, B.; du Preez, J. H. Using Direct Contact Membrane Distillation for Whey Protein Concentration. Desalination 2006, 200 (1−3), 523. (11) Kesieme, U. K. Mine Waste Water Treatment and Acid Recovery Using Membrane Distillation and Solvent Extraction; Victoria University: 2015. (12) Tomaszewska, M.; Gryta, M.; Morawski, A. W. Study on the Concentration of Acids by Membrane Distillation. J. Membr. Sci. 1995, 102 (C), 113. (13) Hickenbottom, K. L.; Cath, T. Y. Sustainable Operation of Membrane Distillation for Enhancement of Mineral Recovery from Hypersaline Solutions. J. Membr. Sci. 2014, 454, 426. (14) Bagger-jørgensen, R.; Jonsson, G. Recovery of Volatile Fruit Juice Aroma Compounds by Membrane Distillation. 2004, http://www.capec. kt.dtu.dk/documents/research/researh-programs/A40_RBJ_ PosterEAM04.pdf. (15) Soni, V.; Abildskov, J.; Jonsson, G.; Gani, R. Modeling and Analysis of Vacuum Membrane Distillation for the Recovery of Volatile Aroma Compounds from Black Currant Juice. J. Membr. Sci. 2008, 320 (1−2), 442. (16) EL-Bourawi, M. S.; Khayet, M.; Ma, R.; Ding, Z.; Li, Z.; Zhang, X. Application of Vacuum Membrane Distillation for Ammonia Removal. J. Membr. Sci. 2007, 301 (1−2), 200. (17) Ding, Z.; Liu, L.; Li, Z.; Ma, R.; Yang, Z. Experimental Study of Ammonia Removal from Water by Membrane Distillation (MD): The Comparison of Three Configurations. J. Membr. Sci. 2006, 286 (1−2), 93. (18) Banat, F.; Jwaied, N.; Rommel, M.; Koschikowski, J.; Wieghaus, M. Desalination by A “compact SMADES” autonomous Solarpowered Membrane Distillation Unit. Desalination 2007, 217 (1−3), 29. (19) Bundschuh, J.; Ghaffour, N.; Mahmoudi, H.; Goosen, M.; Mushtaq, S.; Hoinkis, J. Low-Cost Low-Enthalpy Geothermal Heat for Freshwater Production: Innovative Applications Using Thermal Desalination Processes. Renewable Sustainable Energy Rev. 2015, 43, 196. (20) Schwantes, R.; Cipollina, A.; Gross, F.; Koschikowski, J.; Pfei, D.; Rolletschek, M.; Subiela, V. Desalination 2013, 323, 93. 9341


Review

Industrial & Engineering Chemistry Research (43) Gazagnes, L.; Cerneaux, S.; Persin, M.; Prouzet, E.; Larbot, A. Desalination of Sodium Chloride Solutions and Seawater with Hydrophobic Ceramic Membranes. Desalination 2007, 217, 260. (44) Cerneaux, S.; Strużyńska, I.; Kujawski, W. M.; Persin, M.; Larbot, A. Comparison of Various Membrane Distillation Methods for Desalination Using Hydrophobic Ceramic Membranes. J. Membr. Sci. 2009, 337 (1−2), 55. (45) Schneider, K.; Holz, W.; Wollbeck, R. Membranes and Modules for Transmembrane Distillation. J. Membr. Sci. 1988, 39, 25. (46) Owens, D. K. D. The Dynamic Surface Tension of Sodium Dodecyl Sulfate Solutions. J. Colloid Interface Sci. 1969, 29 (3), 496. (47) García-Payo, M.; Izquierdo-Gil, M.; Fernández-Pineda, C. Wetting Study of Hydrophobic Membranes via Liquid Entry Pressure Measurements with Aqueous Alcohol Solutions. J. Colloid Interface Sci. 2000, 230 (2), 420. (48) Essalhi, M.; Khayet, M. Surface Segregation of Fluorinated Modifying Macromolecule for Hydrophobic/hydrophilic Membrane Preparation and Application in Air Gap and Direct Contact Membrane Distillation. J. Membr. Sci. 2012, 417−418, 163. (49) Xu, J.; Singh, Y. B.; Amy, G. L.; Ghaffour, N. Effect of Operating Parameters and Membrane Characteristics on Air Gap Membrane Distillation Performance for the Treatment of Highly Saline Water. J. Membr. Sci. 2016, 512, 73. (50) Alkhudhiri, A.; Hilal, N. Air Gap Membrane Distillation: A Detailed Study of High Saline Solution. Desalination 2016, DOI: 10.1016/j.desal.2016.07.046. (51) Izquierdo-Gil, M. A.; Garcia Payo, M.; del, C.; FernandezPineda, C. Air Gap Membrane Distillation of Sucrose Aqueous Solutions. J. Membr. Sci. 1999, 155, 291. (52) Winter, D.; Koschikowski, J.; Düver, D.; Hertel, P.; Beuscher, U. Evaluation of MD Process Performance: Effect of Backing Structures and Membrane Properties under Different Operating Conditions. Desalination 2013, 323, 120. (53) Guo, H.; Ali, H. M.; Hassanzadeh, A. Simulation Study of FlatSheet Air Gap Membrane Distillation Modules Coupled with an Evaporative Crystallizer for Zero Liquid Discharge Water Desalination. Appl. Therm. Eng. 2016, 108, 486. (54) Ali, M. I.; Summers, E. K.; Arafat, H. a.; V, J. H. L. Effects of Membrane Properties on Water Production Cost in Small Scale Membrane Distillation Systems. Desalination 2012, 306, 60. (55) Ugrozov, V. V.; Elkinab, I. B. Desalination 2002, 147, 167. (56) Phattaranawik, J.; Jiraratananon, R.; Fane, A. G. Effect of Pore Size Distribution and Air FLux on Mass Transpor. J. Membr. Sci. 2003, 215, 75. (57) Woods, J.; Pellegrino, J.; Burch, J. Generalized Guidance for Considering Pore-Size Distribution in Membrane Distillation. J. Membr. Sci. 2011, 368 (1−2), 124. (58) Khayet, M.; Velazquez, A.; Mengual, J. I. Modelling Mass Transport through a Porous Partition: Effect of Pore Size Distribution. J. Non-Equilib. Thermodyn. 2004, 29 (3), 279. (59) Eykens, L.; Hitsov, I.; De Sitter, K.; Dotremont, C.; Pinoy, L.; Nopens, I.; Van der Bruggen, B. Influence Of Membrane Thickness and Process Conditions on Direct Contact Membrane Distillation at Different Salinities. J. Membr. Sci. 2016, 498, 353. (60) Martínez, L.; Rodríguez-Maroto, J. M. Membrane Thickness Reduction Effects on Direct Contact Membrane Distillation Performance. J. Membr. Sci. 2008, 312 (1−2), 143. (61) Essalhi, M.; Khayet, M. Self-Sustained Webs of Polyvinylidene Fluoride Electrospun Nanofibers at Different Electrospinning Times: 2. Theoretical Analysis, Polarization Effects and Thermal Efficiency. J. Membr. Sci. 2013, 433, 180. (62) Winter, D. Membrane Distillation - a Thermodynamic, Technological and Economic Analysis. Ph.D. Dissertation, University of Kaiserslautern, Germany, 2014. (63) Kimura, S.; Nakao, S.-I.; Shimatani, S. Transport Phenomena in Membrane Distillation. J. Membr. Sci. 1987, 33 (3), 285. (64) Cipollina, A.; Di Sparti, M. G.; Tamburini, A.; Micale, G. Development of a Membrane Distillation Module for Solar Energy Seawater Desalination. Chem. Eng. Res. Des. 2012, 90 (12), 2101.

(65) Jonsson, A. S.; Wimmerstedt, R.; Harrysson, A. C. Membrane Distillation - A Theoretical Study of Evaporation through Microporous Membranes. Desalination 1985, 56, 237. (66) Lawson, K. W.; Lloyd, D. R. Membrane Distillation. I. Module Design and Performance Evaluation Using Vacuum Membrane Distillation. J. Membr. Sci. 1996, 120, 111. (67) Khayet, M.; Khulbe, K.; Matsuura, T. Characterization of Membranes for Membrane Distillation by Atomic Force Microscopy and Estimation of Their Water Vapor Transfer Coefficients in Vacuum Membrane Distillation Process. J. Membr. Sci. 2004, 238 (1−2), 199. (68) Curcio, E.; Drioli, E. Membrane Distillation and Related Operations  A Review. Sep. Purif. Rev. 2005, 34 (1), 35. (69) Essalhi, M.; Khayet, M. Self-Sustained Webs of Polyvinylidene Fluoride Electrospun Nanofibers at Different Electrospinning Times: 1. Desalination by Direct Contact Membrane Distillation. J. Membr. Sci. 2013, 433, 167. (70) Su, C.-I.; Shih, J.-H.; Huang, M.-S.; Wang, C.-M.; Shih, W.-C.; Liu, Y. A Study of Hydrophobic Electrospun Membrane Applied in Seawater Desalination by Membrane Distillation. Fibers Polym. 2012, 13 (6), 698. (71) Adnan, S.; Hoang, M.; Wang, H.; Xie, Z. Commercial PTFE Membranes for Membrane Distillation Application: Effect of Microstructure and Support Material. Desalination 2012, 284, 297. (72) Abu Al-Rub, F. A.; Banat, F.; Beni-Melhim, K. Parametric Sensitivity Analysis of Direct Contact Membrane Distillation. Sep. Sci. Technol. 2002, 37 (14), 3245. (73) Elias-Kohav, T.; Moshe, S.; Avnir, D. Steady-State Diffusion and Reactions in Catalytic Fractal Porous Media. Chem. Eng. Sci. 1991, 46 (11), 2787. (74) Iversen, S. B.; Bhatia, V. K.; Dam-Johansen, K.; Jonsson, G. Characterization of Microporous Membranes for Use in Membrane Contactors. J. Membr. Sci. 1997, 130 (1−2), 205. (75) Schofield, R. W.; Fane, a. G.; Fell, C. J. D. Gas and Vapour Transport through Microporous Membranes. II. Membrane Distillation. J. Membr. Sci. 1990, 53 (1−2), 173. (76) Khayet, M.; Godino, P.; Mengual, J. I. Nature of Flow on Sweeping Gas Membrane Distillation. J. Membr. Sci. 2000, 170, 243. (77) Kurdian, A. R.; Bahreini, M.; Montazeri, G. H.; Sadeghi, S. Modeling of Direct Contact Membrane Distillation Process: Flux Prediction of Sodium Sulfate and Sodium Chloride Solutions. Desalination 2013, 323, 75. (78) Khayet, M.; Matsuura, T. Preparation and Characterization of Polyvinylidene Fluoride Membranes for Membrane Distillation. Ind. Eng. Chem. Res. 2001, 40, 5710. (79) Bhave, R.; Sirkar, K. K. Gas Permeation and Separation by Aqueous Membranes Immobilized across the Whole Thickness or in a Thin Section of the Hydrophobic Microporous Celgard Films. J. Membr. Sci. 1986, 27 (1), 41. (80) Fernandez-Pineda, C.; Izquierdo-Gil, M. A.; Garcia Payo, M. C. Gas Permeation and Direct Contact Membrane Distillation Experiments and Their Analysis Using Different Models. J. Membr. Sci. 2002, 198, 33. (81) Schofield, R. W.; Fane, A. G.; Fell, C. J. D. Heat and Mass Transfer in Membrane Distillation. J. Membr. Sci. 1987, 33 (3), 299. (82) García-Payo, M. C.; Izquierdo-Gil, M. a. Thermal Resistance Technique for Measuring the Thermal Conductivity of Thin Microporous Membranes. J. Phys. D: Appl. Phys. 2004, 37 (21), 3008. (83) Garcia Payo, M. del C. Destilacion En Membranas de Disoluciones Acuosas de Alcoholes; Universidad Complutense de Madrid: 1998. (84) Hitsov, I.; Eykens, L.; De Sitter, K.; Dotremont, C.; Pinoy, L.; Van der Bruggen, B.; Nopens, I. Calibration and Analysis of a Direct Contact Membrane Distillation Model Using Monte Carlo Filtering. J. Membr. Sci. 2016, 515, 63. (85) Alobaidani, S.; Curcio, E.; Macedonio, F.; Diprofio, G.; Alhinai, H.; Drioli, E. Potential of Membrane Distillation in Seawater Desalination: Thermal Efficiency, Sensitivity Study and Cost Estimation. J. Membr. Sci. 2008, 323 (1), 85. 9342


Review

Industrial & Engineering Chemistry Research (86) Feng, C.; Shi, B.; Li, G.; Wu, Y. Preliminary Research on Microporous Membrane from F2.4 for Membrane Distillation. Sep. Purif. Technol. 2004, 39 (3), 221. (87) Liu, F.; Tao, M. M.; Xue, L. X. PVDF Membranes with InterConnected Pores Prepared via a Nat-Ips Process. Desalination 2012, 298, 99. (88) Zhang, J.; Gray, S.; Li, J.-D. Modelling Heat and Mass Transfers in DCMD Using Compressible Membranes. J. Membr. Sci. 2012, 387− 388, 7. (89) Laganà, F.; Barbieri, G.; Drioli, E. Direct Contact Membrane Distillation: Modelling and Concentration Experiments. J. Membr. Sci. 2000, 166 (1), 1. (90) Fernandez-Pineda, C.; Izquierdo-Gil, M. A.; Garcia Payo, M.; del, C.; Garc, M. C.; Fernandez-Pineda, C.; Izquierdo-Gil, M. A.; Garcia-Payo, M. C. Gas Permeation and Direct Contact Membrane Distillation Experiments and Their Analysis Using Different Models. J. Membr. Sci. 2002, 198, 33. (91) Zhang, J.; Gray, S.; Li, J.-D. Predicting the Influence of Operating Conditions on DCMD Flux and Thermal Efficiency for Incompressible and Compressible Membrane Systems. Desalination 2013, 323, 142. (92) Dow, N.; Zhang, J.; Duke, M.; Li, J.; Gray, S. R.; Ostarcevic, E. Membrane Distillation of Brine Wastes; 2008, http://www.xzero.se/ doc/Membrane%20Distillation%20of%20Brine%20WastesWQRA.pdf. (93) Bonyadi, S.; Chung, T. S. Flux Enhancement in Membrane Distillation by Fabrication of Dual Layer Hydrophilic−hydrophobic Hollow Fiber Membranes. J. Membr. Sci. 2007, 306 (1−2), 134. (94) Zhang, J.; Dow, N.; Duke, M.; Ostarcevic, E.; Li, J.-D.; Gray, S. Identification of Material and Physical Features of Membrane Distillation Membranes for High Performance Desalination. J. Membr. Sci. 2010, 349 (1−2), 295. (95) Zhang, J. Theoretical and Experimental Investigation of Membrane Distillation; Victoria University: 2011. (96) Khayet, M.; Matsuura, T.; Mengual, J. Porous Hydrophobic/ hydrophilic Composite Membranes: Estimation of the HydrophobicLayer Thickness. J. Membr. Sci. 2005, 266 (1−2), 68. (97) Qtaishat, M.; Khayet, M.; Matsuura, T. Guidelines for Preparation of Higher Flux Hydrophobic/hydrophilic Composite Membranes for Membrane Distillation. J. Membr. Sci. 2009, 329 (1− 2), 193. (98) Su, M.; Teoh, M. M.; Wang, K. Y.; Su, J.; Chung, T.-S. Effect of Inner-Layer Thermal Conductivity on FLux Enhancement of DualLayer Membrane in M. J. Membr. Sci. 2010, 364, 278. (99) Cui, Z.; Hassankiadeh, N. T.; Zhuang, Y.; Drioli, E.; Lee, Y. M. Crystalline Polymorphism in Poly(vinylidenefluoride) Membranes. Prog. Polym. Sci. 2015, 51, 94. (100) Rao, G.; Hiibel, S. R.; Childress, A. E. Simplified Flux Prediction in Direct-Contact Membrane Distillation Using a Membrane Structural Parameter. Desalination 2014, 351, 151. (101) Wu, H. Y.; Wang, R.; Field, R. W. Direct Contact Membrane Distillation: An Experimental and Analytical Investigation of the Effect of Membrane Thickness upon Transmembrane Flux. J. Membr. Sci. 2014, 470, 257.

9343


How To Optimize the Membrane Properties for Membrane Distillation

Recommend Documents