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Review
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 Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b03493 • Publication Date (Web): 18 Jan 2018 Downloaded from http://pubs.acs.org on January 18, 2018
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Energy & Fuels
CO2 Capture using Hollow Fiber Membranes: A review of membrane wetting Mohamed H. Ibrahim a, Muftah H. El-Naasa *, Zhien Zhang b, Bart Van der Bruggen c a
Gas Processing Center, College of Engineering Qatar University, P.O. Box 2713, Doha, Qatar b School of Chemistry and Chemical Engineering Chongqing University of Technology, Chongqing 400054, China c Department of Chemical Engineering, ProcESS, KU Leuven, W. de Croylaan 46, B-3001 Leuven, Belgium 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 long a 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. In addition, methods and approaches to prevent wetting in addition to perspectives for future research in the area are presented.
*
Corresponding author e-mail:
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Table of Contents 1
Introduction ........................................................................................................................................... 3
2
Hollow fiber membrane contactors ....................................................................................................... 5 2.1
Wetting of membranes .................................................................................................................. 6
2.2
Separation principle ...................................................................................................................... 7
2.2.1 2.1 3
Mass transfer in series model ................................................................................................ 7 Wetting fraction ............................................................................................................................ 8
Wetting parameters ............................................................................................................................... 8 3.1
Surface tension and contact angle ................................................................................................. 9
3.2
Membrane material ....................................................................................................................... 9
3.2.1
Preparation methods............................................................................................................ 10
3.2.2
Super hydrophobic membranes ........................................................................................... 12
3.3
Membrane-solvent interactions ................................................................................................... 14
3.3.1
Absorbent type and breakthrough pressure ......................................................................... 15
3.3.2
Absorbent flowrate and pressure......................................................................................... 16
3.3.3
Effect of absorbent temperature .......................................................................................... 17
3.4
Membrane structure .................................................................................................................... 18
3.4.1 4
Changes in membrane morphology .................................................................................... 20
Modeling approaches for wetting description ..................................................................................... 21 4.1
Constant overall mass transfer coefficient (K)............................................................................ 22
4.2
One-dimensional (1D) model...................................................................................................... 22
4.3
Two-dimensional (2D) model ..................................................................................................... 23
5
Summary and future perspectives ....................................................................................................... 25
6
References ........................................................................................................................................... 27
Keywords CO2 Capture; membrane wetting; contact angle; hydrophobic membranes; membrane morphology; surface tension
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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 may remain the dominant energy source for the coming years. This is mainly due to the continuously increasing energy demand created by the 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 non-renewable resources increased the global temperature by an average of 0.8 ℃ since 1880 (6). The temperature increment is projected to
increase to between 1.4 to 5.8 ℃ 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 CO2
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 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): post-combustion capture, pre-combustion capture and oxyfuel combustion. The simplest technique is post-combustion, since it can be adopted to existing plants by retrofitting. In addition, it is an exceedingly mature technology with well-established applications at full-scale commercial plants. However, it has the inconvenience of the relatively low CO2 partial pressure in flue gases. Pre-combustion is less energy
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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 energy intensive process due to utilization of the air separation unit (11). There are several conventional CO2 capture technologies based on pre-combustion or post-combustion 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 post-combustion. It is 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 5090% (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 hallow 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, 4 ACS Paragon Plus Environment
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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 hallow fiber membranes.
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Hollow fiber membrane contactors
Membrane contactors can be operated in 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 non-selective 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, non-selective 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 a low weight, small volume, modularity, a higher 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. M, 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. 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 5 ACS Paragon Plus Environment
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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 Fig. 1 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 Fig. 1 (30). Karoor and Sirkar (31) 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 6 ACS Paragon Plus Environment
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(MEA) as the liquid absorption medium (33). In addition, using a hollow fiber PVDF (polyvinylidene fluoride) membrane and diethanolamine (DEA) as the absorbent resulted in a reduction of mass transfer by 26% after 10 hours 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) (46) : 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). The main factors that influence membrane wetting are highlighted in Fig. 2 and will be discussed in more details 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: non-wetted or partially wetted (37). For the first case, the overall mass transfer expression based on the gas phase (1 ) consists of three resistances in series (Fig. 3) given by:
=
+
+
(1)
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where , and are mass transfer coefficient (m/s) in gas phase, non-wetted membrane side and
liquid phase, respectively; and are the inside and outside diameter of the hollow fiber membrane (m); 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:
=
+
+
+
(2)
where is the wetted membrane mass transfer coefficient (m/s).
2.1
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 ( ) (41): = where
!/ # !
and
(3) #
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 ($) of the liquid absorbent, membrane contact angle (%) and maximum pore diameter (&' ) to breakthrough pressure (∆)).
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∆) = −
+ , -./ 0 12
(4)
∆) 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 Fig. 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 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 9 ACS Paragon Plus Environment
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resistant membrane. 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 incorporating 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)(50-52). 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 Ramli (54) by preparing PVDF using a two-stage coagulation bath of ethanol and Nmethylpyrrolidone (NMP). The membrane exhibited a 33% higher CO2 absorption efficiency, and a contact angle of 127°. The membrane showed a high porosity (89%), with a narrow pore size distribution. Ooi et al. (55) investigated the consequence of using a dual coagulation bath on a PVDF wetting behavior
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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 method to fabricate porous membranes from semi crystalline 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
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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 33wt% of carbon nanotubes, the measured contact angle was 150°, this was due to a decrease in membrane porosity (Sections 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° when a PVDF surface was treated by fluoroalkylsilane. A composite coating layer could be obtained by blending several micro/nano particles. 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 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 12 ACS Paragon Plus Environment
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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 an 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 kPa 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 pre-wetted hybrid membranes compared to non-wetted 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 one to one ratio.
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3.2.2.2
Plasma treatment
The essence of 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 to change the surface 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 minutes 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, Garcia-Payo 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 14 ACS Paragon Plus Environment
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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 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 1M MEA and DEA aqueous solutions for 160 hours. 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 SEM and 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 non-activated 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 15 ACS Paragon Plus Environment
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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 hours of operation. Among the proposed absorbents, amino acid salts are promising absorbents that have been investigated in several studies(54-56) (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 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 transfer of the first liquid droplet to the gas phase (85). Eq. 4 relates the breakthrough pressure (also referred to as liquid entry pressure, LEP) to absorbent properties. Gas and liquid operation conditions have 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 flowrate and pressure
Membrane wetting is highly affected by absorbent flow rate and pressure as it have 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 16 ACS Paragon Plus Environment
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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 non-wetting 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 refereed 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 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 higher CO2 flux. However, after 80 hours 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 15-30°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 higher possibility of wetting at high 17 ACS Paragon Plus Environment
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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 K 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 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 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 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 0.1M NaOH. The authors showed that a membrane 18 ACS Paragon Plus Environment
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with a porosity of 0.89 had 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 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. 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 (4) (Eq. 5) was introduced to account for the variation of pore curvature (42). ∆) = −
+ 5 , -./ 0 12
(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 Harriot (40) developed Eq. 6 to relate the breakthrough pressure to differences in pore radius deviation and radius of the membrane fiber. ∆) =
6 , -./(078) -./(078) ; 9 :( )(7-./ 8)
