CO2 Capture Using Hollow Fiber Membranes: A

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surface tension of different absorbents. The shape of a liquid absorbent droplet in contact with CO2 is determined by its surface tension, which results from.
Review Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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CO2 Capture Using Hollow Fiber Membranes: A Review of Membrane Wetting Mohamed H. Ibrahim,† Muftah H. El-Naas,*,† Zhien Zhang,‡ and Bart Van der Bruggen§,⊥ †

Gas Processing Center, College of Engineering, Qatar University, P.O. Box 2713, Doha, Qatar School of Chemistry and Chemical Engineering, Chongqing University of Technology, Chongqing 400054, China § Department of Chemical Engineering, KU Leuven, Celestijnenlaan 200F, B-3001 Leuven, Belgium ⊥ Faculty of Engineering and the Built Environment, Tshwane University of Technology, Private Bag X680, Pretoria 0001, South Africa ‡

ABSTRACT: Hollow fiber membrane contactors have several advantages that make them a good alternative to conventional absorption processes in the gas industry, and they have attracted the interest of many researchers. However, critical issues such as wetting hinder applications of membranes on a wide scale. Wetting is the penetration of the liquid absorbent through membrane pores, reducing mass transfer and consequently affecting the CO2 absorption efficiency and lowering the effectiveness of the separation process. The availability of membranes that can maintain a high efficiency and remain stable over a long period of operation is the main factor that is required in order to implement membranes in the industry for absorption processes. The wetting phenomenon in hollow fiber membranes is the focus of this review, which offers a critical examination of the literature published on membrane wetting, highlighting the main factors that control the effectiveness of the membrane separation process. These factors include the liquid absorbent, the membrane morphology represented by pore size and porosity, and the mutual interaction between liquid absorbents and the membranes. All of these factors are discussed in detail in view of a better understanding of the wetting phenomenon. Furthermore, methods and approaches to prevent wetting in addition to perspectives for future research in the area are presented.

1. INTRODUCTION Since the start of the Industrial Revolution, fossil fuels were used as the main source of energy, and currently account for 80% of the world energy.1 In spite of the current trend (and necessity) for using renewable energy sources, fossil fuels will remain the dominant energy source for the coming years. This is mainly due to the continuously increasing energy demand created by global economic growth. The International Energy Agency (IEA) reported a total energy demand of 574 exajoules around the globe in 2014.2 Fossil fuel power plants account for approximately 40% of the global CO2 emission and coaloperated power plants have the highest share in this percentage.3 As a result, carbon dioxide concentration increased by 42% in the atmosphere, starting from 280 ppm before the Industrial Revolution era to reach a level of 400 ppm in 2013.4,5 This irresponsible use of nonrenewable resources increased the global temperature by an average of 0.8 °C since 1880.6 The temperature increment is projected to increase to between 1.4 and 5.8 °C by 2100 if no mitigation measures (or insufficient measures) are taken.7,8 This is especially the case for chemical industries since they represent a large fraction of CO 2 emissions, accounting for 40% of the global emissions.9 According to the IEA, reducing CO2 emissions by half by 2050 will require diminishing industry emissions by 21% in 2050 compared to today’s levels.2 Efforts to reduce CO2 emissions revolves around three main approaches: efficient utilization of energy, reducing the carbon footprint, and improving carbon capture and sequestration (CCS), in addition to deploying renewable energy sources such as solar energy and biotechnology. These approaches tackle emissions reduction by © XXXX American Chemical Society

embracing clean sources of energy, using efficient carbon capture technologies, and adopting energy conservation approaches. In general, three key strategies for CO2 capture from industrial combustion processes are utilized:10 postcombustion capture, precombustion capture, and oxyfuel combustion. The simplest technique is postcombustion, since it can be adopted to existing plants by retrofitting. In addition, it is an exceedingly mature technology with well-established applications at fullscale commercial plants. However, it has the inconvenience of relatively low CO2 partial pressure in flue gases. Precombustion is less energy intensive due to the high CO2 pressure and concentration in the fuel. Nonetheless, it still has its pitfalls, such as inadequate commercial availability. Oxyfuel combustion takes place when fuel is combusted with the presence of 95% pure oxygen. There is no full-scale oxyfuel based CO2 capture plants currently in deployment because of unresolved technical uncertainties. In addition, oxyfuel combustion is an energy intensive process due to utilization of the air separation unit.11 There are several conventional CO2 capture technologies based on precombustion or postcombustion approaches. Physical separation based methods such as absorption, adsorption, and membranes are the most common deployed separation technologies available.12 Absorption using amines in a packed column is considered as an industry standard carbon capture technology in the case of postcombustion. It is Received: November 15, 2017 Revised: January 17, 2018 Published: January 18, 2018 A

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The membrane morphology is believed to have a significant effect on the separation performance of hollow fiber membranes, as suggested by Feng et al.;25 a higher membrane porosity enhances the separation because the contact between the two phases occurs in these pores. Hence, porosity and pore size are decisive performance factors. For example, the membrane porosity has a substantial contribution to the separation process, when the absorbent velocity is relatively high.26 Understanding the membrane morphology behavior combined with the interactions between absorbents and the membrane surface is essential to overcome one of the main challenges facing membrane contactors, namely, membrane wetting. The wetting phenomenon is considered one of the major disadvantages and dramatically hinders large-scale industrial deployment of membranes. Wetting increases the mass transfer resistance and decreases the separation efficiency considerably. This review approaches the wetting phenomena in hollow fiber membranes from different perspectives by investigating the parameters that affect wetting attributed to the absorbent and the membrane structure and suggesting ways to prevent wetting. In addition, the emphasis is on wetting aspects that stem from the interactions between the numerous membrane types and different absorbents. 2.1. Wetting of Membranes. To achieve the highest possible mass transfer rate, the gas phase should fill the membrane pores completely. When the membrane pores become wetted (liquid phase is filling membrane pores), the membrane mass transfer resistance starts to build up, making membrane applications unjustified economically.27 Eventually, over a prolonged operating time, a rapid increase in mass transfer resistance takes place,28 causing a decline in mass transfer. Hence, in order to maintain stability and performance on the long run, the membrane pores must be entirely filled with gas for extended operational periods.29 However, having gas-filled pores all the time is not likely to occur. Realistically, membrane pores become partially or in a worst-case scenario fully wetted by the absorbent over a long operation period, as illustrated in Figure 1.30 Karoor and Sirkar31 were the first to introduce the membrane wetting phenomenon in 1993. The consequences of wetting on the mass transfer performance was first discussed by Malek et al.;32 this was followed by several studies investigating the wetting effect on mass transfer parameters. For example, polypropylene (PP) membranes experience a considerable decline in overall mass transfer after operating for a few hours using monoethanolamine (MEA) as the liquid absorption medium.33 In addition, using a hollow fiber polyvinylidene fluoride (PVDF) membrane and diethanolamine (DEA) as the absorbent resulted in a reduction of mass transfer by 26% after 10 h of operation.34 In another study, a PVDF hollow fiber membrane showed a reduction of CO2 absorption by 30% when water is used and a reduction by 23% when NaOH is used as the absorbent.35 Table 1 summarizes the wetting observations in several studies. In order to prevent wetting, several prevention measurements are utilized:16,17,46

implemented on a large scale due to the high amine tendency to absorb CO2.4 A major drawback of amine absorption is the high cost and high energy intensity of regeneration, and the intrinsic energy cost of using amines on a large scale. More than 30% of the plant power is consumed by the amine system in order to capture 90% of the carbon dioxide emissions. As a result, the electricity cost is amplified by 50−90%.13 This led to the development of membrane contactors as a potential alternative to conventional absorption processes in the industry to reduce capital and operating costs.14 This review paper examines CO2 capture using hollow fiber membranes, focusing mainly on membrane wetting. In spite of the importance of membrane wetting in hollow fiber membrane research, there has been only one single review on the topic during the past five years;15 it offers an outlook at the literature prior to 2013, focusing mainly on the causes and prevention of membrane wetting. This current paper provides a comprehensive overview of membrane wetting in hollow fiber membranes, starting from characterization of the wetting phenomenon and focusing on the main factors affecting membrane wetting. It also discusses the role of the interaction of these factors on the membrane-wetting phenomenon. Effective review papers are often expected to offer a deep understanding and valuable critical appraisal of the literature in the area.15,16 This paper, therefore, has been designed to be comprehensive, covering all areas related to membrane wetting, including membrane type, morphology, solvent types, surface tension as well as mathematical modeling. Yet, the review still focuses on membrane wetting as a major challenge to the application of hollow fiber membranes.

2. HOLLOW FIBER MEMBRANE CONTACTORS Membrane contactors can be operated in a flat sheet, spiral wound, and hollow fiber modules.18 Hollow fiber membranes have been investigated in numerous applications, such as partial respiratory support.19 However, they were utilized to absorb CO2 using a solution of sodium hydroxide for the first time by Qi and Cussler.20 Membranes used to absorb acid gases can be nonselective or selective toward a certain acid gas species.21 Gas separation using membranes mostly depends on the membrane selectivity (separation with no liquid absorbent). In contrast, fibers utilized in membrane contactors offer no selectivity. Instead, selectivity is a function of the absorbent liquid. A higher selectivity promotes a lower permeability, which leads to a smaller flux. In contrast, nonselective membranes have the advantage of a higher flux. Membrane gas absorption can achieve a significant reduction in energy demand compared to amine absorption. Thus, it is a potential carbon capture candidate to replace energy intensive processes, which are thermodynamically limited.22 In addition, membrane absorption operates without limitations associated with packed towers such as weeping, flooding, entrainment, and foaming.23 Membrane contactors have unique advantages such as low weight, small volume, modularity, high surface area to volume ratio, and low capital investment, which make them an attractive research area.24 On the downside, membrane contactors have drawbacks such as a higher mass transfer resistance in the membrane fibers. Membranes are subjected to periodic replacement due to their limited lifetime. However, in spite of the disadvantages, hollow fiber membranes have shown a great potential to overcome the intrinsic drawbacks of other CO2 capture technologies.

(1) Hydrophobic surface modification for membranes. (2) Using absorbents with relatively high surface tension and ensuring the compatibility of the used absorbent with the membrane. (3) Optimizing the operation parameters (i.e., temperature, pressure and gas, and liquid flow rate). B

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where km′ is the wetted membrane mass transfer coefficient (m/s). 2.3. Wetting Fraction. Wetting of membrane takes place when the transmembrane pressure (i.e., the difference between liquid and gas pressure) exceeds the breakthrough pressure.39 This may occur by an increasing liquid phase pressure or by a reducing gas phase pressure. The pore with the largest size in the membrane controls the degree of wetting.40 After exceeding the breakthrough pressure, wetting occurs spontaneously from larger pores to smaller ones. However, it is highly unlikely that complete wetting of membrane will take place. Thus, wetting is expressed in terms of the wetting fraction (x):41

x = VW /VP

(3)

where VW and VP represent the volume of the absorbent that penetrated through pores and the total volume of the pores, respectively.

3. WETTING PARAMETERS Membrane wettability is interrelated with absorbent liquid properties and the membrane type, and their mutual interactions. The Laplace−Young equation (eq 4)42 relates significant parameters such as surface tension (γ) of the liquid absorbent, membrane contact angle (θ), and maximum pore diameter (dmax) to breakthrough pressure (ΔP).

Figure 1. Operation modes in hollow fiber membrane in the gas− liquid interface (a) no wetting, (b) complete wetting, (c) partial wetting.

ΔP = −

(4) The main factors that influence membrane wetting are highlighted in Figure 2 and will be discussed in more detail in the following sections. 2.2. Separation Principle. The concentration gradient is the driving force for gas−liquid absorption in hollow fiber membranes. The two phases flow in two divided compartments by a fixed gas−liquid interface created by the membrane. Due to the membrane hydrophobicity, the two phases remain separated. Diffusion of CO2 in the liquid absorbent takes place through membrane pores that are filled with gas.36 2.2.1. Mass Transfer in Series Model. Gas−liquid absorption in hollow fiber membranes can be described according to the film theory, depending on the operating condition: nonwetted or partially wetted.37 For the first case, the overall mass transfer expression based on the gas phase (1/K) consists of three resistances in series (Figure 3) given by d do 1 1 = o + + K k Gd i k mdlm mkL

(4)

ΔP is the pressure at which the absorbent penetrates through the membrane pores. 3.1. Surface Tension and Contact Angle. Studying wettability involves measurement of the contact angle between the two contacting phases as a key indicator of the wetting degree. In the case of contacting between solid and liquid, the contact angle is defined as the angle formed between vapor− liquid and solid−liquid interfaces at their intersection. The contact angle is used as a measure of surface hydrophobicity. A contact angle smaller than 90° (hydrophilic surface) indicates a high degree of wetting due to liquid dispersion on the surface. However, angles larger than 90° (hydrophobic surface) correspond to a low wettability due to liquid droplets being more compact, as shown in Figure 4.43 Hydrophobicity is highly associated with surface energy. Membranes with low surface energy are less susceptible for wetting in comparison to membranes with high surface energy.15 Table 5 shows the surface tension of different absorbents. The shape of a liquid absorbent droplet in contact with CO2 is determined by its surface tension, which results from different intermolecular forces between surface molecules of the two phases (van der Waals and hydrogen bonding). Hence, for a given membrane material and absorbent liquid, the contact angle is an essential wetting characteristic at given operating conditions.43,44 3.2. Membrane Material. The membrane material affects the overall absorption efficiency and contributes greatly to the membrane chemical stability under the applied operating conditions. In addition, the thermal and chemical resistance of a membrane depends on its material. The most used hollow fiber membrane materials are polypropylene (PP), polyethylene (PE), polytetrafluoroethylene (PTFE), PVDF, polysulfone (PS), and polyetherimide (PEI).45 These polymeric materials have relatively high contact angle values with different amine

(1)

where kG, km, and kL are mass transfer coefficient (m/s) in gas phase, nonwetted membrane side and liquid phase, respectively; do and di are the inside and outside diameter of the hollow fiber membrane (m); dlm is the average logarithmic diameter (dimensionless); m is the distribution parameter in the liquid and gas phase (dimensionless). In the case of partial wetting, an additional resistance must be taken into consideration. This resistance is due to partial penetration of the liquid absorbent into the membrane pores; hence, the overall mass transfer coefficient is given by d do 1 1 1 = o + + + ′ K k Gd i k mdlm k mdlm mkL

4γ cos θ dmax

(2) C

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gas mixture

CO2/air (1−10% vol CO2)

CO2/N2 (20% vol CO2)

pure CO2

pure CO2 or CO2/CH4 (20 or 50% vol CO2)

CO2/N2 (20% vol CO2)

CO2/N2 (20% vol CO2)

pure CO2

CO2/N2 (20−100% vol CO2)

CO2/N2 (15% vol CO2)

membrane

hollow fiber PP

hollow fiber PP

hollow fiber PP

hollow fiber PVDF

hollow fiber PP and PVDF

hollow fiber PVDF

hollow fiber PVDF

hollow fiber PTFE

hollow fiber PP PZ; K2CO3; MEA

AHPD; PZ ; MEA

distilled water; NaOH

MEA; DEA; AMP; MEA + SG; MEA + NaCl

DEA

water; NaOH; MEA

water

DEA

water; NaOH

liquid absorbents

porosity (%) = 20 average pore size (μm) = 0.2

porosity (%) = 70.83 ± 2.49 pore diameter (μm) = 0.03−0.08

porosity (%) = 60 mean pore size (μm) = 2.33 ± 0.51

porosity (%) = 40 pore size (urn) = 0.2

porosity (%) = 75 pore size (μm) = 0.04

pore size (μm) = 0.2

pore radius (μm) = 0.015 porosity (%) = 30 pore size (μm) = 0.04 porosity (%) = 40 not available

membrane properties

Table 1. Summary of Observed Wetting Occurrence in Hollow Fiber Membranes Studies wetting phenomena observation

PZ has lower tendency to wet the membrane due to its high contact angle compared to conventional MEA

Using AHPD resulted in an average membrane wetting of 7.5% in comparison with 10% for MEA

Physical absorption by water resulted a 30% decrease of mass transfer flux after 23 h of operation and 20% reduction after 80 h of using NaOH

Wetting was not prevented by using mixed amine solution. Addition of sodium glycinate leads to stable flux and negligible wetting

85% increase in mass transfer resistance was noticed by shifting from non-wetted to wetted operation mode

A 20−50% of the absorption mass transfer resistance is attributed to wetting % of 13% of the pores Over a long period of operation, it was observed that the probability wetting is higher when using MEA solution in comparison to water and NaOH

If membrane pores are wetted by 5%, this will reduce mass transfer coefficient by 20%

less than 2% of pores wetting resulted in an increase in mass transfer resistance up to 60%

ref

103

132

35

131

115

130

129

106

128

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D

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Figure 2. Factors influencing wetting in membrane gas absorption.

Figure 3. A schematic diagram of mass transfer regions and its respective resistance in nonwetted mode (a) and partial wetting mode (b).

ing nanoparticles in the membrane casting solution and by increasing the surface roughness. 3.2.1. Preparation Methods. In addition to the membrane material, the hydrophobicity of polymeric membranes is highly associated with the synthesis method. There are three main fabrication methods: (1) phase inversion, (2) thermally induced phase separation (TIPS), and (3) stretching. 3.2.1.1. Phase Inversion. Phase inversion is used for the fabrication of most commercially available membranes. Membranes are prepared by solidifying a liquid polymer under controlled conditions. The process is applied in several techniques such as precipitation by solvent evaporation, controlled evaporation, and thermal precipitation.48 PVDF fabrication by using phase inversion has been investigated by several researchers due to its relatively low contact angle and the fact that PVDF is an industrial material with acceptable mechanical and thermal properties.49−52 Kuo et al.53 reported a drastic increase in water contact angle from 84° to a maximum value of 144° using ethanol as a coagulant. Similar results were obtained by Ahmed and Ramli54 by preparing PVDF using a two-stage coagulation bath of ethanol and N-methylpyrrolidone (NMP). The membrane exhibited a 33% higher CO 2 absorption efficiency, and a contact angle of 127°. The membrane showed a high porosity (89%), with a narrow

Figure 4. Liquid drop shape at different contact angles.

absorbents, as shown in Table 4. Hydrophobic and hydrophilic polymers are classified according to their surface energy. PTFE has the lowest surface tension, which makes it the most wetting resistant membrane material. Nonetheless, after a certain operational period, almost all membranes suffer from wetting. Lv et al.46 observed a decrease in the contact angle of a PP hollow fiber membrane from 126.1° to 100°, 90.8°, and 92.5° when deionized water, MEDA, and MEA were used as the absorbents, after 60 days of immersion. Wang et al.47 reported a similar behavior for PP hollow fiber membranes when being immersed in 20 wt % DEA for 10 days. Several attempts were made to modify polymeric membranes in order to achieve a high hydrophobicity, and thus a high contact angle. This can be achieved by mixing hydrophobic polymers, by applying a hydrophobic coating on the membrane surface, by incorporatE

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when a PVDF surface was treated by fluoroalkylsilane. A composite coating layer could be obtained by blending several micro-/nanoparticles. In addition, having durable, mechanical properties is one of the main concerns to be taken into consideration when a coating is fabricated. Wang et al.70 fabricated a composite coating consisting of carbon nanofibers and/fluorinated ethylene propylene on a PVDF membrane. The coating was found to have a water contact angle of 164° and demonstrated excellent mechanical properties. This was attributed to the formation of double bond carbon groups in the PVDF structure, resulting from the preparation method. Wang et al.69 prepared a super hydrophobic PVDF membrane after modifying its surface with fluoroalkylsilane by roughening the surface with sand paper. The membrane demonstrated a water contact angle of 160° and maintained its contact angle even after being exposed to a temperature of 150 °C for two consecutive days. Qing et al.71 successfully prepared a super hydrophobic composite surface by using TiO2 mixed in PVDF. The modified surface showed a water contact angle of 162.3°. In another study, Dong et al.72 evaluated blending SiO2 particles in PVDF and then roughening the surface with low surface energy fluoroalkylsilane. An increase in the SiO2 wt % from 0 to 8 resulted in a significant increase in the contact angle, from 130.4° to 160.5°. Increasing the concentration of silica particles in the membrane solution led to a decrease in the pore size and, in turn, an increasing breakthrough pressure from 84 to 195 kPa, consequently decreasing the wettability of the membrane at higher operational pressure. In a similar manner, Zhang et al.73 incorporated SiO2 particles on the surface of PEI. The experimental results showed a significant enhancement of the contact angle from 66.7° to 124.8°. However, the membrane showed a decrease in contact angle to 92.3° after being immersed in an aqueous solution of 2 M sodium taurinate. A new approach that has been barely investigated is the addition of graphene to the membrane solution in the fabrication process. Wu et al.74 incorporated graphene sheets into a PVDF membrane structure to enhance its wetting resistance. Increasing the concentration of graphene increased the contact angle of the bottom of the membrane instead of the top layer, reaching a value of 133° at 7 wt % graphene. More importantly, no loss in mass transfer rate was observed in the prewetted hybrid membranes compared to nonwetted membranes. 3.2.2.1. Solution Casting. Solution casting is used to fabricate super hydrophobic membranes by depositing a rough coating material on the membrane surface. Franco et al.75 fabricated a super hydrophobic PP by depositing a coating of crystalline PP on the surface of commercial membranes. The amount of added PP crystals affected the membrane contact angle; a concentration of 0.2 and 0.6 mg PP/mL solution yielded a contact angle of 138° and 153°, respectively. Similarly, a super hydrophobic PP membrane was fabricated by Lv et al.76 The authors experimented with different solvent coatings, such as cyclohexane and methyl ethyl ketone (MEK). A contact angle of 162°, 149°, and 156° was obtained when the membrane was coated with cyclohexane, MEK, and a cyclohexane−MEK mixture in a 1:1 ratio. 3.2.2.2. Plasma Treatment. The essence of the plasma treatment process is to treat the polymer surface to achieve the desired properties without changing its matrix. This is possible through absorption and polymerization of the ionized gas on the surface of the membrane. The deposition of a thin coating layer by plasma treatment allows changing the surface

pore size distribution. Ooi et al.55 investigated the consequence of using a dual coagulation bath on a PVDF wetting behavior change. At longer coagulation times, the fabricated membrane had a lower porosity and contact angle. This is due to a fundamental change in membrane morphology that adversely decreased the contact angle from 135° to approximately 100°. Subsequently, the membrane became more susceptible to wetting. Adding pore-forming additives during the phase inversion process significantly altered membrane porosity and pore size. Yuliwati et al.56 reported that adding more than 1.95% TiO2 particles to a PVDF membrane resulted in an increase in the contact angle. Moreover, with increasing TiO2 concentration, the overall membrane porosity increased and the mean pore radius decreased. 3.2.1.2. Thermally Induced Phase Separation (TIPS). TIPS is considered one of the simplest and versatile methods to fabricate porous membranes from semicrystalline polymers, such as PP and PE.48,57 A low number of variables has to be controlled when using TIPS. Hence, it is relatively easy to operate.58 A homogeneous polymer mixture is formed by heating the polymeric material and the diluent at high temperature; the mixture is then cooled and phase separation starts.59 Several parameters affect TIPS, such as the polymer type, molecular weight, and concentration, in addition to the cooling rate and diluent type. Altering any of these parameters will change the membrane morphology.60 Membranes with different PVDF concentrations ranging from 25 to 34% were fabricated using TIPS by Ghasem et al.61 Increasing the PVDF concentration led to a decline in overall porosity of the membrane. For example, increasing concentration from 28 to 34% decreased the porosity from 39.2 to 32.3%. In contrast, the pore radius decreased 320 to 50 nm at the same two concentrations. In addition, the authors reported a contact angle of 120° at 34 wt % compared to approximately 88° at 25 wt %. Consequently, this increased the liquid breakthrough pressure to reach a maximum of 2.2 bar. 3.2.1.3. Stretching. Stretching is used to fabricate partially crystalline hydrophobic membranes such as PTEF and PP.62 Stretching contains of three stages: melt-extrusion, annealing, and stretching.60 Stretching is mainly used to adjust the porosity and pore size of the membrane.63 However, the stretching method will not change the contact angle of the membrane. Hence, if the membrane has a low contact angle that causes wetting at moderate pressure, this method will not prevent wetting.64 3.2.2. Super Hydrophobic Membranes. Generally, a super hydrophobic surface is characterized by having a water contact angle higher than 150°. Having a high water contact angle is one of the main criteria to overcome membrane wetting. This contact angle can be achieved by having a polymeric material with low surface energy (i.e., PVDF) and having a rough surface. Several methods have been developed to obtain a rough surface including surface chemical etching,65 particles deposition,66 and layer by layer assembly.67 Several super hydrophobic membranes are reported in the literature, with different fabrication techniques.68−77 Chakradhar et al.68 prepared a stable super hydrophobic coating using PVDF blended with carbon nanotubes at different concentrations. With 33 wt % of carbon nanotubes, the measured contact angle was 150°; this was due to a decrease in membrane porosity (Section 3.4). Increasing the concentration of nanotubes to 66 wt % led to a higher contact angle of 154°. Furthermore, Wang et al.69 observed a contact angle of 164° F

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Energy & Fuels Table 2. Advantages and Disadvantages of Different Fabrication and Surface Modification Methods advantages plasma treatment

solution casting phase inversion

TIPS stretching

disadvantages

• • • • • •

simple method to deposit a coating layer on the membrane good adhesion properties of the coating layer on the polymeric surface can be applied at ambient room temperature uniform coating layer ability to fabricate a wide range of morphologies most of the commercial membranes are fabricated using phase inversion method • low number of variables to be controlled • versatile number of polymers can be used i.e. semi crystalline polymers • no need to add solvents or any other additives (easy to operate)

references

• expensive

15, 77, 133

• not environmentally friendly • availability of different variations of the method

15 48

• solvents can be costly • less environmentally friendly • offers no enhancement for membrane contact angle

134, 135 136, 137

• more suitable for large scale production

characteristics.77 Several studies reported a decrease in membrane wetting after plasma treatment.78−80 Yang et al.79 treated the surface of a flat sheet PVDF membrane with CF4. The authors reported a linear increase in breakthrough pressure as a function of treatment time. After 40 min of treatment, the breakthrough pressure reached a constant value of approximately 3.1 bar. A hollow fiber PP membrane was plasma treated by PTFE.78 The treated membrane showed a 26° and 12° higher contact angle compared to untreated PP and PTFE. In terms of wettability, the treated membrane exhibited a pore wetting fraction of 60% compared to PTFE, which showed a pore wetting fraction of 24% after 25 days of MEA exposure. Table 2 illustrates different advantages and disadvantages of membrane fabrication and modification methods. 3.3. Membrane−Solvent Interactions. Wetting of the membrane is affected by the absorbent concentration.30,80,81 In most cases, increasing the concentration of alkanolamines will reduce its surface tension. Sreedhar et al.82 reviewed several critical absorbent characteristics for different types of absorbents that are usually utilized in hollow fiber membranes. Franken et al.42 explained that a high concentration of an organic absorbent (low surface tension) leads to membrane wetting. The authors devised a penetration test to determine the maximum absorbent concentration after which the liquid will penetrate the membrane pores. In a similar study, GarciaPayo et al.81 tested several types of membranes over a range of isopropanol concentrations and measured the breakthrough pressure. The increase in the absorbent concentration corresponded to a decline in breakthrough pressure, which led to wetting at low operational pressure. Dindore et al.30 reported that a decrease in absorbent surface tension from to 33 to 30 mN/m using a PP membrane corresponded to a decrease in breakthrough pressure form 0.9 to 0.1 bar. Hence, absorbents with high surface tension are preferred to prevent wetting. 3.3.1. Absorbent Type and Breakthrough Pressure. The interactions between the absorbent and the membrane are essential contributors to the wetting phenomenon. The liquid surface tension, viscosity, and membrane surface energy are the most critical parameters that determine pore wettability for a given absorbent-membrane combination. Low surface tension absorbents can penetrate easily through membrane pores. A low surface tension corresponds to a low contact angle with the membrane surface, which means a higher tendency of wetting at low absorbent pressure.15 In addition, absorbents with high viscosity have a low potential to wet membrane pores compared to low viscosity absorbents. Lin et al.83 reported

this relation when different MDEA and AMP viscosities have been used with PVDF membranes. Subsequently, a decline in contact angle with increasing viscosity was observed. A 12% reduction in contact angle occurred when the AMP viscosity increased from 1.1 to 1.33 m Pa s. Throughout the wetting studies, alkanolamines are used, but there are efforts to propose alternative absorbents with high mass transfer efficiency and lower wetting tendency.51−57,85 Sadoogh et al.86 tested the stability of a PVDF membrane made in-house in 1 M MEA and DEA aqueous solutions for 160 h. The membrane wetting resistance increased by approximately 16.8% and 20% when using DEA and MEA, respectively. Subsequently, the CO2 flux experienced a reduction of 43% when MEA was used and 26% when DEA was used. Conducting a scanning electron microscopy (SEM) and atomic force microscopy (AFM) analysis revealed a lower surface roughness for the immersed membranes, which is one of the main reasons to decrease the membrane hydrophobicity. Lu et al.87 experimented with activated alkanolamines by adding pipperazine (PZ) and AMP to MDEA. With small amounts of activators added to MEDA, the authors were able to measure a higher mass transfer and more carbon removal efficiency in comparison to nonactivated MDEA. A combination of PZ and AMP in aqueous solution has been used as an absorbent with a PVDF membrane.83 The absorbent exhibited a wetting ratio of 0.39%, which was found to decrease with increasing PZ concentration (from 0.1 to 0.3 M). Potassium glycinate (PG) aqueous solutions mixed with MEA or MDEA have a higher surface tension than conventional alkanolamines.84 PP hollow fibers and 0.5 M PG maintained an approximately 90% CO2 removal efficiency with no wetting for 40 h of operation. Among the proposed absorbents, amino acid salts are promising absorbents that have been investigated in several studies54−56,85 and shown to have substantially higher surface tension and low volatility.88 Kosaraju et al.90 prepared a novel absorbent by mixing polyamidoamine with MEA. The authors reported no pore wetting after 55 days of operation using a porous PP membrane. Similarly, Mulukuta et al.91 prepared a nonvolatile absorbent by mixing high concentrations of polyamidoamine with ionic liquids. However, the performance with respect to membrane wetting is yet to be determined. Remarks regarding different absorbents used recently in the literature are presented in Table 3. A high surface tension is often translated into a higher breakthrough pressure compared to other alkanolamines. Having liquid penetrating through pores decreases the mass transfer significantly due to the lower gas diffusivity. Hence, to avoid penetration, the pressure is gradually increased until the G

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Review 139 Mixed absorbent removal efficiency was found to be stable over 4000 min of operation compared to conventional MEA. The MEA efficiency dropped dramatically after less than 1000 min of operation.

107 Wetting fraction when using activated MEDA was significantly smaller by approximately 63% at absorbent velocity of 500 m/s. The overall concentration of amines in the activated MEDA is equal to amine concentration in MEDA.

89 The blended salt was able to remove more than 90% of CO2 in the gas feed. The blended amine had a greater average overall mass transfer coefficient by a factor of 1.25.

138

transfer of the first liquid droplet to the gas phase.85 Equation 4 relates the breakthrough pressure (also referred to as liquid entry pressure, LEP) to absorbent properties. Gas and liquid operation conditions have an influence on the LEP. Mainly the absorbent pressure, flow rate, and temperature affect LEP. The difference between the liquid and gas phase pressure provides the necessary driving force to overcome the capillary pressure in membrane pores. However, at lower contact angles wetting occurs spontaneously, because the capillary pressure is preserved and pores are easily wetted. 3.3.2. Absorbent Flow Rate and Pressure. Membrane wetting is highly affected by the absorbent flow rate and pressure as it has been investigated in numerous studies.23,35,38,80,93−97 The wetting fraction of the membrane increases by increasing the liquid flow. This is due to the increase in resistance as a result of liquid penetration into membrane pores. Cui et al.94 confirmed this by measuring the average wetting over a range of velocities for hollow fiber PVDF membranes, using 2 M DEA at a constant gas velocity. Boributh et al.80 also reported that the mass transfer resistance of a wetted membrane increases significantly with increasing the liquid flow rate. At high absorbent flow rate, it accounted for approximately 70% of the overall mass transfer resistance. A similar trend is observed when the liquid pressure is increased. Increasing the absorbent pressure leads to an increased wetting ratio fraction as well.80 Farjami et al.92 showed that increasing the CO2 velocity will lead to less absorption due to the reduction in residence time for the gas. El-Naas et al.23 discussed the effect of the gas to liquid ratio (G/L) on membrane wetting. The authors reported that for low G/L (high solvent and low gas flow rates), large pores in membranes can be born to wetting. Even for nonwetting solvents, such as water, a high solvent flow rate can create “a thin film at the solvent−membrane interface, which acts as a resistance to gas diffusion similar to partial wetting”; they referred to this as “pseudo wetting”. The absorbent pressure must be higher than the gas pressure to avoid any development of bubbles.32 Therefore, the transmembrane pressure across the membrane will increase, and any increase beyond the breakthrough pressure in eq 4 will lead to pore wetting. Mansourizadeh et al.35 fabricated a custom-made PVDF membranes that showed a higher breakthrough pressure in comparison with PP and PTFE membranes that are available commercially. Mainly, this is due to the presence of small pores with an average diameter of 2.33 ± 0.51 nm and porosity of 70.83 ± 2.49%. Hence, the membrane can withstand wetting during the absorption process. Thus, increasing both absorbents’ flow rate from 50 to 200 mL/min led to a higher CO2 flux. However, after 80 h of operation, the CO2 flux decreased gradually most probably due to pore enlargement. Rongwong et al.38 reported a similar wetting behavior with MEA and AMP solution. Authors noted that wetting fraction for PTEF membrane significantly increased at low absorbent velocity. However, at high velocities it tends to be constant. 3.3.3. Effect of Absorbent Temperature. Almost all studies on membrane wetting utilize absorbents at temperatures ranging between 15 and 30 °C.95−97 Garcia-Payo et al.81 varied the temperature with several absorbent types. Increasing the temperature would lead to lower breakthrough pressure, which meant a higher possibility of wetting at high liquid flow rates. The effect of the operating temperature using MDEA as absorbent and a PP membrane has been tested by Lu et al.97 It was found that increasing the absorbent temperature from 288

CO2/ N2

CO2/ N2

hollow fiber PP

hollow fiber PVDF

MEDA and activated MEDA (MEDA + Pz) MEA (5 wt %) and TEA (5 wt %)

CO2/ N2 hollow fiber PP

blended amine salt/ 1 M glycine salt

mean pore size (μm) = 0.05 porosity (%) = 60 mean pore size (μm) = 0.25 porosity (%) = 60 mean pore size (μm) = 0.045 porosity (%) = 60 mean pore size (μm) = 0.03

ref remarks

amino acids (KGlycine, NaGlycine, NaSarcosine, Ksarcosine) show better performance than MEA. At a liquid flow rate of 10 cm3/min, NaSarcosine showed approximately 27% higher absorption flux compared to MEA at the same flow rate The overall removal efficiency of CO2 exceeded 90% when the composite salt is used. Composite salt showed approximately 50% better removal efficiency at an absorbent flow rate of 1.2 × 103 m/s compared to 1 M glycine solution CO2/ CH4 CO2/ N2 hollow fiber PVDF hollow fiber PP

four different amino salts 1 M composite amino acid salt

NA

membrane properties liquid absorbent gas mixture membrane

Table 3. Summary of Several Amines Absorbents Examined in the Literature

88

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H

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Energy & Fuels Table 4. Wetting Observations for Different Absorbents and Membrane Materials absorbent

membrane

contact angle (°)

operation time

PVDF

100 92 109.8 ± 1.20 113 111.3 126.1 104 128 ± 0.7 64 ± 1.3 90 120 ± 1.3 121.6 91.50 91.27 90.12

15 days

water

PTFE PP

PSF PVDF PP PP PVDF PVDF PVDF

2 M NaOH 20 wt % MEA 30 wt % MEA 0.1 M MEA 0.5 M MEA 1 M MEA

immersion time (d)

wettability

ref

30 days

wetted wetted not wetted

130 140 141 142 143 46 142 144 145 130 144 46 131

60 days

15 days

wetted 60 days

12 days

60 h PP PTEF PVDF PP PE PTFE PVDF PP PP PP

2 M MEA 3 M MEA 5 M MEA 1 M DEA 20 wt %% DEA 30 wt % MDEA 0.5 M MDEA

110 60 h 15 days

82 107 84 101 87.92 103.7 121.6 97

6600 h 12 days 10 days 60 days

absorbent

concentration

surface tension (γ) (mN/m)

ref

AMP 25 °C

30 wt %

61.74 63.31 60.17 55.56 52.86 50.42 64.3 64 61.5 72.01 71.2 57.6 46.1 43.3

149

water 25 °C AHPD 25 °C

10 20 wt % 30 wt % 10 wt % 20 wt % 30 wt % 0.25 M 0.5 M 1M pure 23 wt %

MDEA at 50 °C

MEA 20 °C

wetted wetted wetted wetted wetted not wetted not wetted wetted not wetted wetted wetted wetted wetted wetted wetted

146 147 146 130 147 148 131 47 46 97

significant contribution to wetting. These changes occur over prolonged operational times due to the interaction between absorbents and pores. Lu et al.97 reported that the MEDA operating temperature had a considerable effect on the membrane wetting. As the temperature is increased from 288.15 to 308.15 K, the decline in the mass transfer coefficient becomes more significant; the value decreased by approximately 19% and 59% at the same temperature interval. The effect of several operational parameters on wetting is summarized in Table 6. 3.4. Membrane Structure. Membrane contactors often vary in terms of morphology and internal structure, and the membrane morphology has an influence on pore wetting. The pore size and porosity affect the CO2 concentration profile near the surface of the membrane, which in turn affects the overall mass transfer behavior.98 Zhang et al.99 explained that membranes with large porosity or with a small pore size have a negligible effect on the mass transfer. Conversely, a small porosity and large pore size significantly affect the absorption rate, and mass transfer can be increased by decreasing the pore size. Zhang et al.98 verified this observation by studying the effect of porosity on absorption rate of CO2 in flat membranes by having two membranes with different porosities and using 0.1 M NaOH. The authors showed that a membrane with a porosity of 0.89 had a significantly better absorption rate compared to a membrane with a porosity of 0.52. Membranes with large pores are easily wetted compared to membranes with small pores. Atchariyawut et al.100 showed that a PP membrane with an average pore size of 0.04 μm has a lower mass transfer resistance than a similar PP membrane with a mean pore size of 0.05 μm. Since membrane wettability is limited by the number of large pores available, the wetting fraction (eq 4) reaches a constant value after all large pores are wetted with absorbent.

Table 5. Surface Tension of Several Amines at Different Concentrations DEA at 40 °C

wetted wetted

131

150 151 44 150

to 308 K resulted in a 59% reduction of the total mass transfer. Boributh et al.96 reported a similar behavior by simulating physical absorption of CO2. The decline in flux reached 85% by increasing the absorbent temperature from 5 to 85 °C. Considering the exothermic nature of the reaction between CO2 and amines and the flue gas temperature in industrial applications, membranes should be tested at temperatures higher than 40 °C. Only Wang et al.95 studied the immersion of PP hollow fiber membranes up to 40 days in several absorbents at a temperature of 60 °C. Several structural changes took place because of the immersion at high temperature, which will be discussed in the following sections. Membrane characteristics such as membrane morphology, represented by porosity, pore size, and chemical properties (hydrophobicity), have a I

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Review 38

Membrane pores are usually assumed to be cylindrical in shape with a constant radius. In reality, pores are rarely cylindrical and have a noncylindrical geometry. A pore geometry factor (B) (eq 5) was introduced to account for the variation of pore curvature.42

gas flow rate

absorbent velocity MEA and AMP

CO2/air (15% CO2) CO2/air (15% vol CO2) hollow fiber PP, PFE, and PVDF hollow fiber PTFE and PVDF

MEA

absorbent flow rate NaOH, water pure CO2

For PVDF, the wetting fraction increased from 0.005 to 0.065 when the gas velocity changed from approximately 0.02 to 0.08 m/s.

146

35

absorbent temperature water CO2/N2 (20, 40, 60, 80, 100%)

hollow fiber mathematical model hollow fiber PVDF

Custom-made PVDF membranes with average pore diameter of 2.33 ± 0.51 nm and porosity of 70.83 ± 2.49%. Hence, it can withstand high breakthrough pressure for a period of 80 h. CO2 flux remains constant after changing gas flow rate from 200 mL to approximately 510 mL.

96

97 MDEA CO2/N2 (10% CO2) hollow fiber PP

absorbent pressure and concentration absorbent temperature

Increasing gas velocity from a range of 0.07−0.11 m/s has an almost constant wetting fraction. This indicates that wetting fraction is not affected by gas velocity in this study. A decline of the overall mass transfer coefficient (K) is observed when the pressure is varied from 0.02 to 0.08 MPa. 0.05 M MDEA concentration resulted in less wetting compared to higher concentrations (1 M) Increasing MDEA temperature from 288.15 to 308.15 K, decreased the mass transfer coefficient by 19% and 59% at each of the two temperatures. Raising water temperature from 5 to 85 °C resulted in 85% decline in the mass transfer flux. Consequently, pore wetting fraction increased from 0 to 2.8%.

80 Breakthrough pressure is decreased from 2 to 0.5 bar when ethanol concentration is increased from 4 to 15 wt % At high flow rate, wetting fraction becomes independent of concentration of MEA.

absorbent flow rate and concentration gas flow rate MEA CO2/CH4 hollow fiber PVDF

ref effect on wetting

81 absorbent temperature and concentration

parameter liquid absorbent membrane

gas mixture

CH3OH, C2H6O, C3H8O, C4H10O NA flat sheet PVDF, PTFE, and PP

Table 6. Summary of Effect of Liquid Absorbent Operating Parameters on Wetting

Increasing temperature leads to lower breakthrough pressure.

Energy & Fuels

ΔP = −

4Bγ cos θ dmax

(5)

The pore factor only considers the radial deviation of the pore diameter from cylindrical shape. However, the radius deviation may occur both axially and radially. These deviations affect the contact angle, and thus membrane wettability. Kim and Harriot40 developed eq 6 to relate the breakthrough pressure to differences in pore radius deviation and radius of the membrane fiber. ΔP =

2γ cos(θ − α) cos(θ − α) 2γ = cos θeff R r r 1 + r (1 − cos α)

()

(6)

where R is the fiber radius, α is the structural angle accounting for pore deviation in the axial direction, and θeff is the effective contact angle introduced by the authors. Taking the derivative of eq 6 with respect to the structural angle yields a solution for α: sin(θ − α) =

sin θ r 1+ R

(7)

The ratio r/R is used as an indicator of the magnitude of the breakthrough pressure. The larger the ratio, the higher the breakthrough pressure is required for the absorbent to penetrate the membrane pores and vice versa. Guillen-Burrieza et al.101 investigated the effect of structural parameters on different models of breakthrough pressure using five different membranes. The capillary pressure model and B factor models were not very successful in representing the actual contact angle compared to the model proposed by Kim and Harriot. Moreover, the latter model had a better prediction for a stretched PTEF membrane due to its elliptical pore shape. Nonetheless, the same model was accurate in predicting the contact angle for a polycarbonate membrane with round pores. Overall, the Kim and Harriott model estimated a higher breakthrough pressure than the capillary model, and it represented the experimental contact angle with less error. 3.4.1. Changes in Membrane Morphology. Because of the interaction between absorbent and pores, changes in pore structure may occur particularly for prolonged operational periods. These changes mainly cause pore wetting and, consequently, a mass transfer decline. A change in membrane structure was reported for the first time by Kamo et al.102 A hollow fiber PE membrane was immersed in a mixed solvent for 30 min at temperature of 25 °C. It was found that due to the high surface tension of the solvent, the membrane pores were enlarged, the space between microfibers was extended, and the membrane shrunk in longitudinal direction. An enlargement of membrane pores has been reported in several studies.47,78,107−109 Franco et al.78 compared surface morphological changes of PTFE and PP membranes after being immersed in 20 wt % MEA for 25 days. A PP membrane suffered from expanded pores and a 52% reduction in surface porosity. The membrane suffered from a decline in contact angle from 127° to 117°. Although the PTFE membrane had structural changes, J

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Energy & Fuels it suffered less from a reduction in contact angle due to its low surface tension (reduction from 139° to 132°). On the basis of SEM micrographs the authors concluded that the increase in wettability is due to a change in pore size and shape. Barb et al.105 and Wang et al.47 reported a comparable structural change for a PP membrane. Mosadegh-Sedghi et al.104 observed a structural change by immersion of low density polyethylene (LDPE) membranes in several absorbents for 30 days. An immersion in 30 wt % MEA with a surface tension of 64 mN/m resulted in less pore enlargement compared to a mixture of AHPD and PZ, which had a higher surface tension (72 mN/ m). It can be deduced from the studies referenced here that all structural changes include pore enlargement. This causes membrane wetting because larger pores facilitate liquid penetration at low pressure. Hence, according to eq 4, the breakthrough pressure decreases.102 Table 4 illustrates the contact angles for different membrane-absorbent combination and their wettability.

(2) Radial dispersion is negligible in gas and liquid phases; (3) The membrane mass transfer coefficient (km) is constant; (4) Steady state and constant temperature (isothermal) conditions. This approach takes mass transfer coefficients into account locally, which includes the wetted mass transfer coefficient (km′) in the case of wetting. The transfer coefficients can be calculated using different mass transfer correlations. Mass transfer coefficients can be calculated based on membrane morphology parameters as shown in Table 7. Table 7. Mass Transfer Coefficients for Nonwetted, Wetted, and Pores Diffusiona correlation nonwetted membrane mass transfer (km) wetted membrane mass transfer (km′)

4. MODELING APPROACHES FOR WETTING DESCRIPTION In most membrane wetting studies, experimental data are fitted with a theoretical model. There are different approaches to model CO2 absorption in membranes to predict the separation performance. The most suitable mathematical representation combines rigorous modeling and mathematical complexity. The main target of modeling is to predict the experimental results over a different range of operating conditions, membrane modules, and chemical solvents. Every model is based on a set of assumptions that determine its complexity. The assumptions aim to make the model easier to compute by simplifying several chemical and physical characteristics of the system. There are different levels of mathematical complexity associated with different assumptions. For example, membranes can be modeled under nonwetting conditions or under wetting conditions. Several mathematical models have been developed in the literature to describe wetting phenomena.23,38,94,96,107−114 In this section, wetting parameters in different modeling approaches are discussed, and the predictability of each model is compared. Modeling intends to describe mass transfer for the process of CO2 absorption in a given absorbent. Any membrane contactor consists of three sections: shell side, microporous membrane, and tube side. The main model equations are derived from a mass (in some approaches, energy) balance on shell and tube sides of the membrane. Generally, there are two flowing patterns: the liquid absorbent flows through the membrane (tube side) and the gas stream flows on the shell side and vice versa.36 4.1. Constant Overall Mass Transfer Coefficient (K). This model was introduced by Qi and Cussler as the first model dedicated to describe the mass transfer in a membrane contactor.20 This approach is based on the assumption that the mass transfer coefficient is constant throughout the inlet and outlet of the membrane pores. However, in reality, this is not the case as K changes with operating parameters and the wettability of membrane pores. Hence, it is not suitable to describe the wetting phenomenon. 4.2. One-Dimensional (1D) Model. This model is based on the resistance-in-series model that is described in section 2.2.1. The following are main assumptions for the 1D model:111

membrane pores diffusion coefficient

ref

Dε km = O τδ Dε k′m = L τδ 1 1 1 = + Do DM DK

38

a

Tortuosity (τ); porosity (ε); membrane thickness (δ); membrane pores diffusion coefficient (Do); diffusion coefficient of solute in the liquid phase (DL); molecular diffusion (DM); Knudsen diffusion (DK). DM can be calculated using the Fuller equation based on the kinetic theory of gases, while the gas phase temperature, molecular weight, and average pore radius are used to calculate DK.97

Boributh el al. 96 reported that the suggested 1D mathematical model was in agreement with experimental data. The model predicted that the wetting fraction would increase with increasing breakthrough pressure. In addition, Cui et al.94 successfully estimated the effect of absorbent velocity on membrane wetting, with an average relative deviation lower than 5%. Lu et al.107 investigated the effect of wetting on the outlet gas concentration. At nonwetted conditions, the model had an average deviation of 23%. However, at 8% wetting, the model fitted the experimental data. Boributh et al.112 utilized a log-normal and normal distribution function to develop a model that can predict the wetting fraction and the overall mass transfer coefficient. The model is a function of absorbent properties and operating conditions. The authors verified the proposed model by conducting experiments on a PVDF membrane at six different operating conditions. 4.3. Two-Dimensional (2D) Model. Two-dimensional models take axial and radial diffusion into account in the case of chemical absorption.113 Two-dimensional analysis of transport phenomena in porous membrane was first introduced by Keller and Stein.114 This modeling approach leads to a more sophisticated set of equations that takes into account the absorbent and CO2 concentration gradient. A steady state mass balance is made for the system based on fluid flow arrangement (counter current or cocurrent) and the absorbent: ∇Ni ± R i = VZ

∂Ci ∂z

(8)

where Ci, Ni, Ri, Vz, and z are the concentration, flux, reaction rate of species i, velocity, and distance along the length of the membrane, respectively. Several studies used the same modeling strategy with the following main assumptions:23,115−117 (1) Constant temperature (isothermal) conditions;

(1) The film theory is applicable in gas and liquid phases; K

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A few papers reported CO2 removal in hollow fibers using 3D models.126,127 Some studies indicate that 3D models for gas separation in hollow fibers have a higher accuracy compared to 2D models. Bahlake et al.126 proposed a 3D model for simultaneous stripping of CO2 and H2S in multi hollow fibers. It was proven that the simulation results with no adjustable parameter were more accurate and reliable using response surface methodology.

(2) Constant membrane mass transfer coefficient (km); (3) Axial convection and radial diffusion. For example, consider the differential equation the membrane side that is partially wetted:23 For the wetted part of the membrane: ⎛ ∂ 2C 1 ∂CCO2 − wetted CO2 − wetted + DCO2 − wetted ⎜⎜ 2 ∂r r ∂r ⎝ +

∂ 2CCO2 − wetted ⎞ ⎟ + R CO = 0 ⎟ 2 ∂z 2 ⎠

5. SUMMARY AND FUTURE PERSPECTIVES Wetting in membranes occurs due to the penetration of absorbent through membrane pores, which results in a significant increase in mass transfer resistance and consequently a reduction of the overall absorption efficiency. The wetting phenomenon is a result of several interconnected parameters and properties of the absorbent and the membrane. The surface tension of the absorbent combined with the contact angle and the membrane material are essential factors to determine the wettability over a certain operating period. Generally, hydrophobic membrane materials are preferred due to their relatively high contact angle. In addition, the synthesis method of the membrane material greatly contributes to its hydrophobicity. Several fabrication methods are being used and modified to decrease membrane wetting tendency, particularly the phase inversion method. Attempts have been made to fabricate membranes with a contact angle above 150° by modifying the surface of the membrane material, incorporating different particles within the membrane matrix and applying different coatings on the membrane surface. Notably, PVDF membranes have been the most coated membrane material with a significant increase in contact angle. Absorbent properties and their interaction with membrane material are essential factors in determining the extent of wetting, in addition to absorbent operating conditions, such as temperature, pressure, and flow rate. An absorbent pressure higher than the breakthrough pressure will lead to immediate wetting of the membrane. The membrane porosity and pore size are the main parameters that determine the breakthrough pressure of a given absorbent at certain set of operational parameters. Mainly, absorbent concentration has the greatest contribution to the wetting phenomena. The interaction between membrane pores and absorbent leads to a change in the internal pore structure, which accelerates membrane wetting. Hence, several combinations of membrane and absorbent have been tested in the literature. Numerous mathematical models have been proposed in the literature to model membrane wetting with various complexity and assumptions. Generally, 2D models predict wetting with an acceptable accuracy. Membrane contactors have proven to be a great alternative for conventional CO2 capture methods if the wetting problem can be eliminated. This can be achieved if research efforts are devoted toward fabricating membrane materials with a high contact angle or even modifying the existing materials to achieve similar results. Trying to incorporate different particles, such as graphene could result in membranes with high wetting resistance. In addition, examining the durability of these membranes at elevated temperatures is essential to determine their applicability in industry. A full understanding of the wetting mechanism in different membranes will allow for better prevention techniques. For example, the use of wetting inhibitors in the liquid absorbent that can mitigate the causes of wetting before it occurs. Furthermore, investigating

⎛ ∂ 2C ∂ 2Csol − wetted ⎞ 1 ∂Csol − wetted sol − wetted ⎟ + + Dsol − wetted ⎜ ∂r r ∂r 2 ∂z 2 ⎝ ⎠ + R sol = 0

For the nonwetted part of the membrane: ⎛ ∂ 2C 1 ∂CCO2 − nonwet CO2 − nonwet + DCO2 − nonwet ⎜⎜ 2 ∂r r ∂r ⎝ +

∂ 2CCO2 − nonwet ⎞ ⎟ + R CO = 0 ⎟ 2 ∂z 2 ⎠

El-Nass et al.23 simulated the physical and chemical absorption of CO2 using water, MEA and NaOH in a 2D model to explore the membrane wettability. The model took the three wetting modes into account. At full wetting mode using 0.01 M MEA, the model was in good agreement with the experimental results and showed the dependency of CO2 removal on the gas to liquid ratio. A higher ratio corresponds to a higher decline in CO2 removal since less solvent is available for the absorption process. NaOH was used to test the model at partial wetting conditions; the model predicted the wettability of the membrane in accordance with experimental data with increasing gas to liquid ratio. Rongwong et al.38 observed a similar wetting behavior by simulating MEA chemical absorption. Zhang et al.118 also used MDEA/PZEA solutions for the investigation of the effect of membrane wettability on CO2 removal. At complete wetting, the blend of 0.9 M PZEA showed a 80% CO2 removal efficiency. Fazaeli et al.119 developed a mass transfer model for studying CO2 absorption in tetramethylammonium glycinate. The model showed a 20% difference in CO2 removal between a wetted and nonwetted membrane at 15 vol % of the absorbent at 25 °C. DEAB (4diethylamino-2-butanol) is reported to have a higher absorption capacity than DEA, MDEA, and MEA and a low energy is required for regeneration.120 Saidi121 verified that claim through modeling the absorption process and comparing it to several solvents. Masoumi et al.122 developed a similar model for DEAB. It is worth mentioning that both models assumed nonwetting conditions; nonetheless, it would be interesting to understand the wetting behavior of DEAB with different membrane materials and operating conditions. Others have also examined more complex models such as non-isothermal 2D and 3D models. Non-isothermal models are not reported much in the literature due to their complexity; they are mostly utilized to model scale up studies.123−125 Only Zaidiza et al.124 considered the wetting mode in non-isothermal models. The model predicted the wetting ratio at high liquid flow rate that caused a high breakthrough pressure. L

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(19) Kostopoulos, C. H.; Rasidakis, S. M.; Moulopoulos, S. P. Hollow Fiber Hemodialyzers for Partial Respiratory Support. J. Biomater. Appl. 1989, 4 (2), 123−140. (20) Qi, Z.; Cussler, E. L. Microporous Hollow Fibers for Gas Absorption. I. Mass Transfer in the Liquid. J. Membr. Sci. 1985, 23 (3), 321−332. (21) Guha, A. K.; Majumdar, S.; Sirkar, K. K. A Larger-Scale Study of Gas Separation by Hollow-Fiber-Contained Liquid Membrane Permeator. J. Membr. Sci. 1991, 62 (3), 293−307. (22) Koros, W. J. Evolving beyond the Thermal Age of Separation Processes: Membranes Can Lead the Way. AIChE J. 2004, 50 (10), 2326−2334. (23) El-Naas, M. H.; Al-Marzouqi, M.; Marzouk, S. A.; Abdullatif, N. Evaluation of the Removal of CO2 Using Membrane Contactors: Membrane Wettability. J. Membr. Sci. 2010, 350 (1−2), 410−416. (24) Gaeta, S. Membrane Contactors in Industrial Applications. Membr. Oper. Innov. Sep. Transform. 2009, 499−512. (25) Feng, C.; Wang, R.; Zhang, H.; Shi, L. Diverse Morphologies of PVDF Hollow Fiber Membranes and Their Performance Analysis as Gas/liquid Contactors. J. Appl. Polym. Sci. 2011, 119 (3), 1259−1267. (26) Zhang, W.; Li, J.; Chen, G.; You, W.; Jiang, Y.; Sun, W. Experimental Study of Mass Transfer in Membrane Absorption Process Using Membranes with Different Porosities. Ind. Eng. Chem. Res. 2010, 49 (14), 6641−6648. (27) Kreulen, H.; Smolders, C. A.; Versteeg, G. F.; Van Swaaij, W. P. M. Determination of Mass Transfer Rates in Wetted and Non-Wetted Microporous Membranes. Chem. Eng. Sci. 1993, 48 (11), 2093−2102. (28) Mavroudi, M.; Kaldis, S. P.; Sakellaropoulos, G. P. Reduction of CO2 Emissions by a Membrane Contacting Process. Fuel 2003, 82 (15−17), 2153−2159. (29) Kreulen, H.; Smolders, C. A.; Versteeg, G. F.; van Swaaij, W. P. M. Microporous Hollow Fibre Membrane Modules as Gas-Liquid Contactors Part 2. Mass Transfer with Chemical Reaction. J. Membr. Sci. 1993, 78 (3), 217−238. (30) Dindore, V. Y.; Brilman, D. W. F.; Geuzebroek, R. H.; Versteeg, G. F. Membrane-Solvent Selection for CO2 Removal Using Membrane Gas-Liquid Contactors. Sep. Purif. Technol. 2004, 40 (2), 133−145. (31) Karoor, S.; Sirkar, K. K. Gas Absorption Studies in Microporous Hollow Fiber Membrane Modules. Ind. Eng. Chem. Res. 1993, 32 (4), 674−684. (32) Malek, A.; Li, K.; Teo, W. K. Modeling of Microporous Hollow Fiber Membrane Modules Operated under Partially Wetted Conditions. Ind. Eng. Chem. Res. 1997, 36, 784−793. (33) Falk-Pedersen, O.; Dannstrom, H. Separation of Carbon Dioxide from Offshore Gas Turbine Exhaust. Energy Convers. Manage. 1997, 38, S81−S86. (34) Mansourizadeh, A.; Mousavian, S. Structurally Developed Microporous Polyvinylidene Fluoride Hollow-Fiber Membranes for CO2 Absorption with Diethanolamine Solution. J. Polym. Res. 2013, 20 (3), 99. (35) Mansourizadeh, A.; Ismail, A. F.; Matsuura, T. Effect of Operating Conditions on the Physical and Chemical CO2 Absorption through the PVDF Hollow Fiber Membrane Contactor. J. Membr. Sci. 2010, 353 (1−2), 192−200. (36) Zhang, Z. E.; Yan, Y. F.; Zhang, L.; Ju, S. X. Hollow Fiber Membrane Contactor Absorption of CO2 From the Flue Gas: Review and Perspective. Glob. NEST J. 2014, 16 (2), 354−373. (37) Yeon, S.-H.; Sea, B.; Park, Y.-I.; Lee, K.-H. Determination of Mass Transfer Rates in PVDF and PTFE Hollow Fiber Membranes for CO2 Absorption. Sep. Sci. Technol. 2003, 38 (2), 271−293. (38) Rongwong, W.; Fan, C.; Liang, Z.; Rui, Z.; Idem, R. O.; Tontiwachwuthikul, P. Investigation of the Effects of Operating Parameters on the Local Mass Transfer Coefficient and Membrane Wetting in a Membrane Gas Absorption Process. J. Membr. Sci. 2015, 490, 236−246. (39) Gabelman, A.; Hwang, S.-T. Hollow Fiber Membrane Contactors. J. Membr. Sci. 1999, 159 (1−2), 61−106. (40) Kim, B. S.; Harriott, P. Critical Entry Pressure for Liquids in Hydrophobic Membranes. J. Colloid Interface Sci. 1987, 115 (1), 1−8.

membrane wetting on the microscale could provide additional insights into preventing wetting. ̀



AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Muftah H. El-Naas: 0000-0002-6164-1421 Zhien Zhang: 0000-0001-8594-6732 Notes

The authors declare no competing financial interest.



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DOI: 10.1021/acs.energyfuels.7b03493 Energy Fuels XXXX, XXX, XXX−XXX