A Review on Hollow Fiber Membrane Contactors for Carbon Capture: Recent Advances and Future Challenges

: Energy need is predicted to increase by 48% in the next 30 years. Global warming resulting from the continuously increasing atmospheric CO 2 concentration is becoming a serious and pressing issue that needs to be controlled. CO 2 capture and storage/use (CCS/CCU) provide a promising route to mitigate the environmental consequences of CO 2 emission from fossil fuel combustion. In recent years, hollow ﬁber membrane contactors are regarded as an advanced technique with several competitive advantages over conventional technologies such as easy scale-up, independent control of ﬂow rates, more operational ﬂexibility, absence of ﬂooding and foaming as well as high interfacial area per unit volume. However, many factors such as the membrane material selection, proper choice of solvent, and membrane module design are critical to success. In this regard, this paper aims at covering all areas related to hollow ﬁber membranes, including membrane material, membrane modiﬁcation, membrane surface modiﬁcation, shape, solvent characterization, operating parameters and costs, hybrid process, membrane lifetime, and energy consumption as well as commercially available systems. Current progress, future potential, and development of pilot-scale applications of this strategy are also assessed carefully. Furthermore, pore wetting as the main technical challenge in membrane contactor industrial implementation for post- and pre-combustion CO 2 capture processes is investigated in detail.


Introduction
Economic development and global population growth have enormously contributed to excessive energy consumption worldwide [1]. Fossil fuels are recognized as a major energy source for the near-to-middle term, although their combustion undoubtedly generates large quantities of carbon dioxide into the atmosphere [2]. The greenhouse gas emissions depending on energy conversion are estimated to increase by approximately 30% by 2040 [3]. This massive release of CO 2 due to human activities has caused severe environmental consequences, such as rising sea levels and global warming, and its impact on climate change has become increasingly apparent in recent years [4]. Therefore, it is crucial to implement suitable and reliable processes to manage CO 2 emissions from various industrial gas streams. Post-combustion capture, pre-combustion capture, and oxyfuel combustion are three main strategies to decrease CO 2 emissions from the use of fossil fuels [5]. Currently, the technologies for CO 2 capture from gas mixtures mainly include chemical absorption, physical adsorption, and cryogenic, membrane-based, and calcium looping processes [6,7]. Of all carbon capture approaches, absorption using amine solutions in a packed column is the most common deployed separation technique available due to its high separation purity and fast absorption rate [8]. However, operating costs and energy consumption are still high [9]. Aside from the cost, amine scrubbing suffers from several operational A liquid absorbent is fed to one side of the membrane while a gas mixture is supplied to the opposite side. Due to its modularity, CO 2 from the gas mixture can diffuse from the bulk gas phase to the surface of the membrane, transport through the membrane pores, and reach the liquid phase. Compared with the membrane-based gas separation technique, the materials that are incorporated in membrane contactors generally have no separation selectivity [18]. Instead, a porous membrane is employed as a nondispersive barrier that provides the separation between the gas and the liquid phases while significantly enhancing the effective contact surface between two phases without dispersing one phase into the other [19]. Different phases on each side of the membrane can be regulated independently, which increases the process's flexibility. Existing resistance to the mass transfer is offered by the existing phase(s), the membrane, or a combination of both. To ensure a high gas flux across the membrane, the membrane needs to be highly porous. Additionally, one main requirement for a successful gas separation membrane is the compatibility of the absorption liquid and the membrane material [20]. Polyimide (PI), polyethersulfone (PES), polysulfone (PS), polytetrafluoroethylene (PTFE), polyvinylide-nefluoride (PVDF), and polypropylene (PP) are the most widely applied membrane materials while 2-amino-2methyl-1-propanol (AMP), methyldiethanolamine (MDEA), monoethanolamine (MEA) and diethanolamine (DEA) have been widely considered as liquid absorbents for CO 2 absorption in membrane contactors.
According to Falk-Pedersen et al. [21], the weight and dimensions of the membrane contactor can be reduced by 66% and 72%, respectively, compared to conventional absorption columns. Other unique advantages of membrane hollow fiber contactors include reduced capital costs, operational flexibility, easy scale-up, and lower energy consumption [22]. Thanks to the advantages of hollow fiber membranes, this technique is a viable strategy for different applications including gas separation, food processing, azeotropic mixture separation, medicine, and water treatment, as shown in Figure 2 [23,24]. Ref. [24]. 2020, Elsevier.
Membrane crystallizers, gas-liquid membrane contactors, membrane distillation, membrane extractors, and membrane emulsifiers are the main membrane-based processes that might be applied for advanced separation. Gas-liquid membrane contactors have been occasionally mentioned in the literature for the separation of acid gases from different sources such as natural gas, industrial gas, and effluent gas from domestic processes. From this perspective, this review is focused on the development of various types of hollow fiber membrane contactors for CO 2 removal from the point of view of the membrane material, membrane modification, composite membrane, surface modification, membrane module shape, solvent development, operating parameters, hybrid process, and membrane lifetime performance.
To the best of our knowledge, detailed and comprehensive reviews that cover all relevant properties of hollow fiber membranes have not been reported. This review aims at providing recent research developments in the field of membrane contactors-based carbon capture, as well as a reference for researchers to understand the technological progress further. First, the most important membrane materials employed for the application of gas-liquid membrane contactors, as well as their properties, synthesis procedures, and modification are introduced. Then, a special section features the membrane structure, including the shape of developed membranes, additives implemented in the membrane matrix, etc. Next, a section is devoted to the solvent type that is used as an absorption liquid. Understanding the operating parameters and their influence on the performance of the membrane process is discussed separately. Gas-liquid membrane contactors as a hybrid process as well as their merits are listed in a different section with in-depth discussion. The final section is assigned to the cost of operation as well as the challenges and lifetime of gas-liquid membrane contactors to provide valuable insights for large-scale practical operations.

Membrane Materials
From the point of view of material types, membranes are commonly classified into polymeric and inorganic membranes [25]. Up to now, polymeric materials have been favored for CO 2 capture due to their low manufacturing costs and good performance as well as physicochemical properties, which can be regulated much easier than inorganic membranes. Table 1 shows the chemical structures of common polymers used in membrane contactors. The membranes employed for gas-liquid membrane contactors are typically microporous with symmetric or asymmetric structures. To ensure a successful gas absorption process in hollow fiber membrane contactors for long-term operation, the properties of membrane materials should be well studied. The materials need to be chemically and thermally compatible with the liquid absorbents and operating conditions.  Generally, a membrane material with a high contact angle with the solvent is preferable as it decreases the potential for pore wetting, resulting in high mass transfer rates and avoiding mixing between liquid and gas.
Thermal extrusion and phase inversion are the most widely employed techniques for membrane fabrication. Depending on the membrane fabrication method and specific conditions, the contact angle, porosity, and pore size of the membrane can be controlled, and lead to an improved CO 2 absorption flux and removal efficiency. Polymers such as polyetherimide (PEI), polysulfone (PSF), PVDF, PTFE, polyethylene (PE), and PP have been the most popular choices in the literature for CO 2 capture applications. Membranes that have been recently used in the gas separation process are listed in Table 2 along with their preparation method and their characteristics. PEI and PSF membranes have good chemical and thermal stability and dissolve well in common organic solvents. Owing to their great hydrophobicity and superior thermal and chemical stability, PTFE membranes have become very common in the field of gas separation [26]. PVDF membranes show high chemical resistance in contact with corrosive chemicals and have a moderate melting temperature. Low price, excellent processability, high melting temperature, widely adjustable mechanical properties, and high resistance to water and chemical environments have made PP membranes one of the most widely used membranes in the literature [27].
A performance comparison between PP and PTFE membranes for CO 2 absorption was performed by deMontigny et al. [28]. Two amine solutions including MEA and AMP were also utilized as liquid absorbents. The authors concluded that PTFE exhibits a better resistance than PP when wetting is a matter of importance. Their long-term studies showed that PP membrane performance reduces over time when tested with MEA and AMP solutions, while the PTFE membrane maintains its initial level of performance. This poor performance of the PP membranes compared to PTFE can be explained by the fact that PTFE had a porosity of 50%, while the PP membranes had a porosity of 35%. PEEK membranes have the advantages of high resistance to chemicals and superior mechanical properties as well as a good performance at capturing CO 2 with low CO 2 concentration. Li et al. [29] scaled up PEEK hollow fibers for the CO 2 absorption process. Their measurements revealed that a CO 2 removal rate greater than 90% can be obtained with a large PEEK membrane module. However, the synthesis of porous PEEK membranes is challenging owing to the limited solubility of PEEK in common solvents and the necessity of extreme operating conditions in the melt-spinning process. Furthermore, the mass transfer coefficient in the absorption step was found to be an order of magnitude greater than that of conventional column contactors.
Similarly, the effect of the pore size of PTFE membranes on CO 2 removal was studied by Chen et al. [30]. It was concluded that CO 2 recovery increased with a decrease in the pore size of the PTFE membrane from 1.3 µm to 0.1 µm. The smaller pore size means a decrease in the wetting of the membrane pore. Therefore, it is expected that a pore with a smaller diameter would reveal less wetting with chemical absorbents, which leads to higher CO 2 recovery. Furthermore, an asymmetric PTFE membrane indicated a better CO 2 recovery than a symmetric membrane.
To improve the PP membrane materials that resist wetting, Franco et al. [31] prepared a plasma-treated PP membrane and examined its wetting performance in contact with an MEA solution. They found that untreated PP becomes degraded while the plasma-treated PP membranes are less susceptible to wetting and degradation when coupled with MEA and showed a 30% higher CO 2 mass transfer rate than untreated PP membranes. Since PP materials have a cheaper price than PTFE, treated PP material will remain competitive with PTFE. Li et al. [26] studied the effect of porosity and pore size on the performance of PTFE hollow fibers, and their results revealed that the increase in membrane porosity and pore size in the non-wetting zone improved CO 2 removal efficiency. A larger pore size is beneficial to reduce the mass transfer resistance of the membrane phase. However, it increases the risk of membrane wetting during the absorption process. According to their results, the CO 2 removal efficiency increased as pore size increased from 0.15 µm to 0.18 µm. With a further increase in pore size from 0.18 µm to 0.27 µm, CO 2 removal efficiency decreased, which could be caused by pore wetting.
Hedayat et al. [32] evaluated PVDF and PSF membrane contactors for simultaneous absorption of H 2 S and CO 2 from a gas mixture similar to natural gas. Based on their results, less membrane wetting in dense layer PVDF membrane by MDEA solution was observed in comparison with micropores of PSF membrane. Fabrication of porous PEI membranes via wet phase inversion technique using different amounts of polymer in a spinning solution was performed by Bakeri et al. [33,34]. Their investigation outcomes revealed that with a proper selection of polymer concentration in PEI, a greater CO 2 absorption rate than commercial PVDF hollow fiber membrane can be obtained. A thermally-induced phase separation method was applied by Ghasem et al. [35] for the fabrication of PVDF. Membrane performance was tested in gas-liquid membrane contactor modules used in the absorption of CO 2 from CO 2 /CH 4 gas mixture in an aqueous 0.5 M NaOH solution. According to their results, CO 2 absorption flux decreases with increasing polymer concentration in the dope solution. The reason for this is that as polymer concentration increases, the outer skin layer of the membranes became denser and thicker. PP and PVDF membranes were suggested by Lin et al. [36] for CO 2 capture in MDEA blended with PZ solution. The authors claimed that the effect of the viscosity of solvent on the wetting ratio is more significant than that of the contact angle between the membrane and absorbent. Moreover, PVDF showed a greater performance than PP membrane from the point of view of lower pore wetting. Fashandi et al. [37] evaluated the potential of poly(vinyl chloride) (PVC) membranes for the first time for CO 2 absorption in a membrane contactor unit. Low cost, good processability, and acceptable chemical resistance are advantages of PVS as a hydrophobic polymer. Figure 3 shows SEM images of the cross-section and surface of the PVC membrane. The reduction in hollow fiber membrane wall thickness, optimum pore size, and optimum orientation of PVC chains resulted in a good CO 2 absorption performance. According to their result, hollow fiber membranes prepared from PVC polymer enjoy advantages including the hydrophobic nature of PVC, the presence of macrovoids within the PVC structure, the optimum orientation of PVC chains, optimum wall thickness, and the corrugated inner surface of PVC membranes. A few researchers studied CO 2 absorption using dual-layer membrane contactors due to their advantages such as higher packing density, self-supporting structure, and self-containing vacuum channel. Figure 4 shows the conceptual schematic of dual-layer flat and hollow membranes. Chen et al. [38] tested dual-layer PVDF in membrane contactors with DEA as an absorbent for CO 2 absorption. It was found that the outer layer of the membrane shows a large finger-like porous structure, while the inner layer is composed of PVDF and hydrophobic modified SiO 2 . Their CO 2 absorption measurements showed that dual-layer membrane exhibits higher CO 2 flux and better long-term stability compared to single-layer. The stability of the membrane process is critical in the corrosive environment of amine solvents. Although polymeric membranes presented good performance for CO 2 capture, they still suffer from low thermal and chemical stability in contact with aggressive solvents in high-temperature environments, which limits their application for long-time processes in industrial scales where harsh conditions are commonly employed [40]. To overcome such limitations of polymeric membranes, ceramic hollow fiber membranes that showed better mechanical and thermal stability as well as excellent porosity and permeability by controlling the sintering temperature were developed [41]. However, like other types of membranes, these ceramic membranes are not free of shortcomings. They are fragile and expensive, and their preparation is time-consuming and difficult [42]. Lee et al. [43] fabricated hydrophobic modified ceramic hollow fiber membranes by the phase inversion technique and observed that CO 2 removal efficiency can be obtained up to 90% with 30% MEA solution. In another work [44], the same authors examined the effect of temperature on the performance of ceramic membrane contactors. According to their observation, the solvent temperature had no significant effect on CO 2 removal efficiency, while gas temperature showed a greater impact on CO 2 removal efficiency. As the gas temperature increases from 20 to 35 • C at a gas flow rate of 0.75 Nm 3 /h, CO 2 removal efficiency increases and reaches a maximum value of 71%. With further increases in gas temperature to 50 • C, CO 2 removal efficiency decreases. Koonaphapdeelert et al. [45], for the first time, conducted CO 2 desorption experiments from MEA solution at a temperature of 363 K in a ceramic hollow fiber membrane contactor. The finding of the study proved that even in the region of an ordinary column indicating loading or flooding, the membrane contactors could operate very well. Magnone et al. [46] implemented a ceramic membrane contactor for CO 2 absorption with several single and blended amines. They fabricated ceramic hollow fiber membranes by a phase inversion technique. It was found that the highest absorption flux can be determined by aqueous MEA and DEA solutions. CO 2 separation from biogas using a ceramic membrane contactor was carried out by Lee et al. [47]. The authors reported that their developed ceramic membranes with 96% CO 2 removal efficiency can replace the polymeric membranes or packed columns. Kong et al. [48] assessed SO 2 absorption in a hydrophilic ceramic membrane contactor and concluded that the multichannel ceramic membranes revealed a more compact configuration and better application potential for SO 2 absorption compared to a single-tube membrane contactor.  The summary of this section is as follows: • The success of membrane gas absorption over other processes will largely depend on the type of membrane materials and fabrication method used. • At high temperatures, ceramic hollow fiber membranes have better mechanical and thermal stability than polymeric membranes. • PTFE offers better resistance than PP when wetting is a matter of importance.

•
The plasma-treated membranes exhibited a lower degradation rate and a higher mass transfer rate compared to untreated membranes. • Pore wetting significantly reduces the CO 2 mass transfer rate, as the mass transfer coefficient through liquid-filled pores is orders of magnitude lower than that if the pores are gas-filled.

•
Membrane materials with a high contact angle with the solvent could be resistant to pore wetting. • A membrane with small pores will resist wetting, but there is a trade-off as smaller pores will also reduce the rate of CO 2 mass transfer. • From a wetting point of view, hydrophobic membrane materials are preferred due to their relatively high contact angle. • PP and PE membranes can be chemically degraded during long-term operation in contact with an amine-based solvent.

Membrane Modification
The low mass transfer resistance, small pore size, good wetting resistance, and high surface porosity are the essential requirements of membranes, resulting in a high absorption performance through membrane contactors in a long-term stable operation. Using thermal or stretching techniques to prepare PTFE, PE, and PP membranes leads to a symmetrical structure with low porosity and large pore size. On the other hand, PVDF also has the limitation of difficulty in fabrication and small critical surface tension, which restricts its application. As a result, modification of the membranes should be considered to improve their properties and morphology. Modification of polymers through the introduction of organic and inorganic additives, surface treatment, surface coating, and blending with other polymers has been evaluated to develop new membranes which offer higher performance. The opportunity to prepare better membranes for a specific application such as gas-liquid membrane contactors has been recently studied by several researchers (See Table 3), and thus some remarkable results have been reported regarding the preparation and development of membranes.

Role of Additives
One of the attractive methods to enhance the wettability resistance of membranes, as well as their properties, is using salt additives in the casting solutions. Generally, additives are water soluble, such as water-soluble polymers, e.g., polyvinyl pirrolidone (PVP), polyethylene glycol (PEG), inorganic salts such as LiCl, or non-solvents such as glycerol, ethanol, acetone, methanol, phosphoric acid, and acetic acid.
To modify the membrane morphology and structure, several researchers have used additives in the casting solution. For instance, Naim et al. [59] compared the performance of modified PEI and PVDF membranes in terms of morphology in aqueous DEA solution for the CO 2 stripping process at 353 K. They added non-solvent additive PEG into the polymer dope. A higher contact angle, effective surface porosity, and gas permeation were observed for PVDF compared to PEI. Moreover, it was reported that the addition of PEG to PVDF and PEI has a significant effect on stripping flux. Based on their results, at the liquid velocity of 0.7 m/s, stripping flux increased up to 90% and 110% with the addition of PEG to PVDF and PEI, respectively, which can be explained due to the enhancement of effective surface porosity that allowed more surface contact for gas and liquid. In another work, the same authors [60] assessed the effect of the addition of various non-solvent additives on the viscosity, pore size, and surface porosity of PVDF membrane as depicted in Figure 5. Phosphoric acid (PA), methanol, PEG-400, glycerol, and lithium chloride were selected as additives. It is observed that the effect of additives on pore size and surface porosity is significant. For instance, the surface porosity of PVDF increases from 2 to 1690 (1/m) with the addition of PEG400. PVDF-LiCl and PVDF-PA presented the highest viscosity, while PVDF-PEG400 indicated the lowest. In summary, the pore size of evaluated membranes follows the order: PVDF > PVDF-Glycerol > PVDF-methanol > PVDF-PA > PVDF-PEG400 > PVDF-LiCl. Furthermore, their experimental results revealed that the PVDF-PEG400 membrane has the highest stripping flux among other additives. Figure 6 shows the crosssectional morphology of the inner and outer structure of the non-additive PVDF as well as PVDF membranes with additives.  The formation of small drop-shaped cavities was observed near the inner surface of PVDF-LiCl due to its high viscosity. Moreover, the longer finger-like structure was formed for polymer dope solutions with low viscosity such as PVDF-PEG400, PVDF-methanol, and PVDF-glycerol due to rapid liquid-liquid demixing.
Rahbari-Sisakht et al. [62] selected ethanol, glycerol, and acetone as additives in the spinning dope during the fabrication of PEI hollow fiber membranes. Based on gas permeation results, the largest and smallest pore size was obtained with additives of ethanol and glycerol, respectively. Additionally, PEI-glycerol indicated the highest CO 2 absorption flux when compared to other additives. In another work, Rahbari-Sisakht et al. [63,64] fabricated PSF hollow fiber membranes via a wet-spinning process, whereas 4 wt% glycerol was used as the additive. As a result, a thick sponge-like layer and a thin finger-like layer were obtained in the membrane structure, which resulted in a faster CO 2 absorption rate. Mansourizadeh et al. [66] prepared porous hydrophobic PVDF membranes with a developed structure for CO 2 absorption. They have found that the addition of 4 wt% LiCl resulted in a very small mean pore size (3.9 nm) with high surface porosity. PEG-400, ethanol, phosphoric acid, and glycerol were suggested by Mansourizadeh et al. [67] as non-solvent additives in the polymer dope for PVDF membrane fabrication. According to their observation, the membranes developed by using phosphoric acid, glycerol, and PEG-400 exhibited spongelike structures, which provided high wetting resistance.
Mansourizadeh et al., in another work [70], conducted CO 2 absorption experiments using modified PEI and PVDF membranes. An amount of 6 wt% glycerol was used as an additive. As a result, developed PEI and PVDF membranes exhibited much higher CO 2 flux than pure membranes due to improved structure with good hydrophobicity and low resistance. Furthermore, the developed PVDF membrane showed 35% higher CO 2 flux than PEI. It was also found that the compatibility of glycerol with the PVDF solution is better than the PEI solution.
An effort was made by DashtArzhandi et al. [71] to improve the hydrophobicity, wetting resistance, and structure of PEI hollow fiber membrane by incorporating hydrophobic clay particles into the polymer matrix. They investigated the effects of incorporating hy-drophobic MMT clay particles as an additive into the polymer matrix in different loadings on the membrane properties. It was shown that modified PEI membranes possess better mechanical stability, smaller pore size, and higher porosity than PEI membranes without MMT. Bakeri et al. [34] successfully applied five compounds including acetic acid, glycerol, ethanol, methanol, and water in their study for structure modification of the PEI membrane. Their characterization results on the structure of PEI revealed that the PEI modified with water shows a sublayer with a sponge-like structure, while the addition of other additives to PEI exhibits a sublayer with finger-like macrovoids due to their lower viscosity. According to their viscosity results, the values of viscosity of PEI with water, methanol, ethanol, glycerol, and acetic acid are 2095, 826, 1014, 1341, and 1049 cP, respectively. To further improve the performance of the PVDF membrane, Pang et al. [79] proposed the idea of using a mixture of two additives including LiCl and phosphoric acid with different concentrations. According to their results, the highest CO 2 flux was achieved by the addition of 8 wt% phosphoric acid, while the PVDF membrane without additives offered the lowest CO 2 flux. This positive effect of the additive on the CO 2 flux can be explained by the fact that the wetting resistance of the membrane decreases with the increase in non-solvent additive in the polymer dope. Although the addition of a mixture of phosphoric acid + LiCl to PVDF showed a positive effect on CO 2 flux, its influence was found to be lower than single phosphoric acid.
In flat sheet membranes, the incorporation of higher loading of ZIF-8 is possible and beneficial for enhanced gas separation performance. In this regard, the flat sheet mixed matrix membrane with 0.5 wt% loading of ZIF-8 was used by Nordin et al. [93], and a 19% enhancement in CO 2 /CH 4 selectivity was observed compared to the neat PSF membrane. Nabian et al. [80] employed a PSF flat membrane contactor with polyvinyl pyrrolidone (PVP) as an additive in CO 2 absorption. It was reported that the porosity and hydrophobicity of the PSF membrane increased with the addition of PVP while a reduction in mass transfer resistance was observed. As a result, the CO 2 absorption flux of the PSF + PVP flat membrane was shown to be 169% greater than the flat PSF membrane without PVP.

Surface Modifying Macromolecules
High hydrophobicity is one of the fundamental properties of membranes utilized in membrane contactors to ensure that the membrane pores remain dry for effective gas transport. Various surface modification methods have been used to enhance surface hydrophobicity, such as UV-assisted grafting, low-temperature plasma treatment, redox-initiated graft polymerization, ion beam irradiation, and photochemical grafting. Membrane surface modification using surface modifying macromolecules (SMM) in the casting or spinning dope has been applied by many researchers as a promising choice to increase the hydrophobicity of membranes. For example, Rahbari-Sisakht et al. [86,87] studied the effects of the SMM concentration on the morphology of SMM-blended PVDF hollow fibers and the performance of the membranes fabricated in membrane contactor applications for CO 2 capture. Surface modifying macromolecules with 0, 2, 4, and 6 wt% were tested as additives in the spinning dope. The surface-modified PVDF membranes showed a larger pore size, higher gas permeance, effective surface porosity, contact angle, and overall porosity but lower critical water entry pressure compared to the PVDF hollow fiber membrane without SMM. Furthermore, by increasing SMM concentration in the spinning dope, the CO 2 absorption flux increased significantly.
In another work, Rahbari-Sisakht et al. [62] fabricated a novel surface-modified polysulfone hollow fiber membrane via a dry-wet phase inversion process using surface-modifying macromolecules. Their results showed that surface-modified membranes offer higher performance compared to plain polysulfone membranes. Moreover, they found that with the membrane prepared from SMM in the spinning dope, a maximum CO 2 flux was achieved, which was almost 76% more than the membrane without SMM. Bakeri et al. [90,91] used surface-modifying macromolecules to enhance the surface hydrophobicity of the PEI mem-brane. They compared the performance of the SMM-modified membrane with PEI hollow fiber without SMM modification. According to their results, the surface-modified membrane exhibited a large pore size and higher effective surface porosity compared to polyetherimide without SMM. Additionally, the surface-modified PEI membrane indicated a higher contact angle than the PEI membrane without SMM due to the higher surface hydrophobicity of the former membrane. It was also concluded that the surface-modified PEI membrane has a higher CO 2 absorption flux than the PEI membrane without SMM due to the larger pore size and higher surface porosity of the former membrane. Mansourizadeh et al. [92] used surface-modifying macromolecules to improve the hydrophobic PVDF membrane properties for CO 2 absorption in gas-liquid membrane contactors. They investigated the effect of SMM concentration in the polymer dope, air gap distance, and bore fluid composition on the structure and performance of the membranes. The authors observed that by increasing SMM in the polymer dope, the membranes present smaller mean pore sizes, higher permeability, and surface hydrophobicity.

Composite Membranes
A major issue in gas-liquid membrane contactors is polymeric membrane wetting by liquid absorbent which will result in significant resistance to mass transfer as shown in Figure 7 [94]. Furthermore, polymeric membranes are limited to short-term and mild operating conditions such as low alkalinity, acidity, and low-temperature applications [95]. Therefore, the chemical and mechanical stabilities as well as non-wetting characteristics of polymeric membranes need to be improved. In this regard, many researchers proposed selecting composite membranes with dense skin layers as they combine the advantages of both polymeric and inorganic membranes [96]. Several criteria must be fulfilled for an ideal dense skin layer, including great chemical and thermal stabilities, high CO 2 permeability, and negligible mass transfer resistance [97]. Dibrov et al. [98] developed thin film composite PTMSP membranes for desorption by a novel two-layer coating technique. The authors revealed that composite membranes with a thin PTFE layer deposited on a cermet substrate ensure a stable CO 2 flux for 100 h during regeneration from a 50 wt% MDEA solution at 100 • C and 30 bar. A comparison between the performance of porous PTFE membrane and nonporous Teflon-PP composite membrane was conducted by Dai et al. [99]. According to their investigation, the Teflon-PP composite membrane offers better stability while the PTFE membrane exhibited higher CO 2 flux. However, both membranes showed good thermal stability at 100 • C for 6 h. Lin et al. [100] employed a high-temperature steam-induced hydrolysis technique to prepare a fluorinated OH/SiO 2 -TiO 2 /PES composite hollow fiber membrane. It was found that SiO 2 and TiO 2 can significantly affect the surface hydrophobicity of composite membranes. The contact angle and hydrophobicity of the composite membrane increased significantly compared with the original PES membrane. Amirabedi et al. [96] used techniques of blending the inorganic particles with the dope solution before membrane fabricating and coating the inorganic particles on the surface of the prepared membrane for the fabrication of a PP composite hollow fiber membrane. The resulting composite membranes indicated a higher contact angle (145-168 • ) than the PP membrane (124 • ). This trend can be attributed to the incorporation of methyl functional groups with lower surface free energy and an increase in surface roughness. The mechanical stability results showed that the incorporation of the inorganic particles on the surface and in the cross-section of the PP composite membrane improved stress compared to the PP membrane. Furthermore, an effective improvement in the non-wetting property of the membranes was observed. Wu et al. [101] tested a superhydrophobic PVDF composite flat-sheet membrane which was prepared via a non-solvent-induced phase inversion method. Based on their results, PVDF composite membrane exhibited strong anti-wettability to MEA solution and long-term stability even after 20 days. Xu et al. [102] prepared composite dense hollow fiber membranes by dispersing aminosilane-modified ZIF_8 nanocrystals into a PDMS matrix deposited on a porous PVDF substrate as shown in Figure 8. Their fabricated composite membrane presented a significant hydrophobicity with a high contact angle of 130 • . Furthermore, CO 2 absorption flux in MEA solution using fabricated composite membrane was obtained equal to 3.8 × 10 −3 (mol/(m 2 ·s)) which is higher than other dense membranes such as PVDF (0.2 × 10 −3 mol/(m 2 ·s)), PEI (1.3 × 10 −3 mol/m 2 ·s), and PTFE (1.1 × 10 −3 mol/(m 2 ·s)). CO 2 absorption flux indicated an approximate 80% reduction in PVDF while composite PVDF exhibited only showed an 11% drop after 10 days of testing, indicating its good stability. Similarly, Fosi-Kofal et al. [103] evaluated a PVDF composite hollow fiber membrane prepared by embedding CaCO 3 nanoparticles in the polymer matrix. They detected better wettability resistance and higher surface roughness with more abundant and narrower finger-like pores, permeation rate, and porosity for composite membranes compared to the neat membrane. The key messages of the membrane modification section can be summarized as follows: • Permeability, wetting resistance, and pore size are important factors, which can seriously affect the performance of the membranes. • By controlling the phase inversion rate of the spinning dope using the appropriate amount of non-solvent additives, an improved membrane structure can be achieved.

•
Membranes that are composed of microporous support and an ultrathin dense layer could prevent membrane wetting, due to the dense thin film on the liquid side of the membrane, which prevents the liquid breakthrough. Reprinted with permission from Ref. [102]. 2019, Elsevier.

Module Shape
The gas-liquid membrane contactors have been tested for CO 2 absorption in different module shapes and configurations. Depending on the mode of operation, contactors can be classified into two modes including parallel and cross-flow, as shown in Figure 9. Compared to parallel flow, cross-flow enjoys various advantages including lower pressure drop on the shell side, higher mass transfer coefficients, well-defined flow conditions on both phase fluid sides, and less mass transfer resistance due to the remixing effect [104]. Yang et al. [105] proved that cross-flow membrane contactors offer better absorption performance than parallel flow. Constantinou et al. [106] studied CO 2 absorption in a single-channel PTFE membrane contactor, a nickel mesh contactor, and an eight-channel PTFE membrane reactor. The PTFE membrane used in the experiments consisted of 20 µm thick pure PTFE supported on an 80 µm thick PP layer. It was found that higher CO 2 removal can be obtained when the PP supporting layer was on the gas side rather than on the liquid side, due to less mass transfer resistance of the gas phase. Furthermore, the eight-channel PTFE contactor with 71% removal efficiency showed higher performance compared to the nickel mesh contactor with about 30% efficiency. DeMontigny et al. [28] revealed that the mass transfer rate in the counter-current mode is 20% higher compared to the co-current mode. From a geometry point of view, gas-liquid membrane contactors can be classified into two classes: flat sheet and cylindrical membrane modules. A cylindrical membrane module is divided into three types of hollow fibers, capillaries, and tubes [8]. Large gas-liquid contact areas, high surface-to-volume ratio, self-supporting characteristics, and ease of scale-up are the main advantages of hollow fiber membranes compared to flat-sheet membranes [97]. An ideal membrane module design should enable facile process integration and operation, reduce hydrodynamic pressure loss, maximizes effective membrane area, minimizes concentration polarization, and have a cheap price [107].
Although most of the current studies focus on CO 2 capture by hollow fiber membrane contactors, a number of researchers studied CO 2 absorption and desorption using flat-sheet membrane contactors [108][109][110][111][112][113]. The main advantage of flat-sheet membranes is the ease of membrane fabrication and module assembly. Constantinou et al. [114] studied the feasibility of a flat membrane microstructured contactor (see Figure 10) for CO 2 absorption, and 76% CO 2 removal efficiency was reported by using the eight-channel PTFE contactor.

Absorbents for Membrane Contactors
The selection of an absorbent is a key factor in hollow fiber membrane contactors, as the absorbent's properties directly affect the performance of the process from the capture efficiency and economic aspect point of view [115]. Furthermore, the wetting phenomena in membranes depend on the interactions between the solvent and the membrane [116]. To make the hollow fiber membrane approach more competitive on an industrial scale, liquid solvents as one of the main parts of this technology should ideally possess the following criteria: (a) high CO 2 absorption capacity leading to a smaller plant footprint and less solvent flow; (b) easily regenerable, resulting in reduced energy consumption for regeneration, thereby reducing process cost; (c) fast reactivity with CO 2 to increase absorption efficiency and reduce liquid mass transfer resistance; (d) good physical and chemical compatibility with membrane materials to improve the long-term stability of the membrane module; (e) non-toxic, which is favorable to environmental aspects; (f) low viscosity to minimize the mass-transfer resistance; (g) high surface tension results in a lower potential to wet membrane pores [117]. Among these criteria, viscosity along with surface tension has the greatest effect on the pore wettability in a hollow fiber membrane contactor [20]. To date, chemical absorbents are the most widely utilized absorbents in CO 2 separation processes. The molecular structures of some common solvents for CO 2 absorption are given in Table 4.
For the initial screening of membrane-solvent combinations, solvents that are used in traditional absorption columns and contactors are in favor since they have proven performance for CO 2 absorption processes, and therefore thermodynamic data are readily available for the absorption phenomena. Various absorbents such as cyclic amines, primary, secondary, and tertiary amines, amino acids, ionic liquids, and inorganic solvents were presented in the literature for gas absorption in hollow fiber membrane contactors. A summary of recent publications using different types of solvent for CO 2 separation is provided in Table 5.

Alkanolamines
Well-known amines such as primary amines (MEA and DGA), secondary amines (DEA and DIPA), tertiary amines (MDEA and TEA), diamines (PZ and AEEA), and sterically hindered amines (AMP), have been widely tested for the CO 2 absorption process from flue gas. Each type of amine exhibits its pros and cons in the treatment of acidic gas. For instance, MEA has the advantages of high reactivity with CO 2 and high absorption efficiency at low CO 2 partial pressure, while MDEA takes advantage of low energy consumption in the regeneration process and large absorption capacity [162].
A PTFE membrane was selected by Khaisri et al. [141] to study membrane stability in CO 2 desorption membrane contactors using a CO 2 -loaded MEA solution at a temperature of 373 K. They found that a high membrane porosity has a significant effect on solvent desorption performance. It was observed that an increase in the membrane porosity from 23% to 40% resulted in an increase in the CO 2 desorption flux by roughly 260%. Yang et al. [140] evaluated the effect of SO 2 on the CO 2 capture in a PP membrane contactor using MEA solution and they observed an increase in MEA loss per ton of captured CO 2 with an increasing concentration of SO 2 in the absorption-desorption loop system. It was reported that enhancing MEA loss from 1.34 kg to 6.67 kg led to a decrease in the CO 2 mass transfer rate from 0.45 × 10 −4 to 0.15 × 10 −4 (mol/(m 2 ·s)) and CO 2 removal efficiency from 95% to 30%. This trend can be explained by the promotional effect of SO 2 on the degradation of MEA by the formation of sulfate. Membrane-wetting studies were conducted by Rongwong et al. [139] in a PVDF membrane with three different amines to gain a better understanding of the role of solvents on membrane wetting. According to their obtained data, the CO 2 flux of MEA, AMP, and DEA continuously decreased by roughly 26%, 39%, and 78%, respectively, after 12 days of operation. The results can be explained due to the enhancement in the membrane mass transfer resistance due to membrane wetting. Furthermore, it was pointed out that MEA solution mixed with sodium glycinate resulted in stable CO 2 absorption flux, indicating that the membrane wetting was negligible.
Until now, no single amine has been found to exhibit satisfactory performance in both absorption and desorption. The mixed solvents are an attempt to capitalize on the advantages of each single amine, and thus represent an attractive method for solvent performance improvement [163]. A mixture of MDEA and PZEA solution as a CO 2 absorbent in the membrane contactor was suggested by Zhang et al. [132]. In another work by Zhang et al. [143], CO 2 absorption experiments using a combination of MEA and DMEA solution were carried out in a PTFE membrane contactor with different heights. It was concluded that both desorption rate and absorption capacity can be improved by mixing MEA and DMEA solutions, indicating a synergistic impact in CO 2 absorption into MEA + DMEA. Furthermore, it was claimed that an optimum membrane height should be determined to enhance the efficiency of the process. They concluded that membrane contactor height has a negative effect on CO 2 flux, which could be explained by the fact that the free concentration of absorbent decreases along with the contactor. Nakhjiri et al. [133] investigated the separation process of CO 2 from CO 2 /CH 4 gaseous stream using two amines including ethylenediamine (EDA) and 2-(1-piperazinyl)-ethylamine (PZEA) in a PP membrane. Based on their observation, PZEA separated about 92% of CO 2 that existed in the feed gas while the removal efficiency using EDA was about 86%. DEAB as a novel tertiary amine was utilized by Cao et al. [134] for CO 2 absorption in a PTFE membrane contactor system and the results showed a greater CO 2 absorption rate for DEAB compared to AMP and MDEA solutions, which makes DEAB an attractive choice for CO 2 absorption. Additionally, it was reported that the overall mass transfer coefficient and CO 2 absorption performance decrease by 80% and 49% when membrane wetting ratios are 50% and 5%, respectively.
Wang et al. [129] used three blend solutions including MDEA + MEA, MDEA + PZ, and AMP + DEA for CO 2 absorption from CO 2 /N 2 mixture in a polypropylene membrane contactor. Paul et al. [130] studied capturing CO 2 using different aqueous single and blended alkanolamine solutions in a hollow-fiber membrane contactor. They used the aqueous solutions of MEA, DEA, MDEA, and AMP, as well as aqueous blends of MEA + MDEA, MEA + AMP, DEA + MDEA, and DEA + AMP as absorbents. Among the single solutions, the aqueous solution of MEA showed the highest CO 2 absorption flux. Their results revealed that except for DEA + AMP, other blend solutions indicated a higher absorption rate as the concentration of MEA or DEA increased. Gao et al. [144] proposed a blend solution of PZ and DEEA as a new chemical solvent for CO 2 absorption and examined its performance using PTFE hollow fiber membrane contactor at 313 and 353 K. Their experimental outcomes demonstrated that PZ + DEEA solution exhibits a cyclic capacity of 0.8 mol CO 2 /L and an absorption rate of about 32 × 10 −5 (mol/(L·s)).

Amino Acids
Although amines have several advantages, they suffer from some drawbacks such as corrosion of process equipment, high solvent regeneration energy requirement, thermal and oxidative degradation, and high vapor pressure [164]. To overcome these limitations, alternative solvents based on amino acid salts have been studied, and so far, these have presented potential benefits over conventional chemical absorbents such as no environmental or toxic issues, less volatility, high absorption capacity, and fast reactivity with CO 2 [165]. Additionally, they are more stable toward oxidative degradation than most amines and have better resistance to thermal degradation [166]. Glycine, arginine, alanine, glutamine, proline, sarcosine, lysine, serine, leucine, valine, threonine, methionine, asparagine, taurine, and histidine are some of the important amino acids used for CO 2 absorption in the literature [167]. One of the distinct properties of amino acid salts is their high surface tension, resulting in fewer pore-wetting problems in hollow fiber membrane contactors [167]. Therefore, the combination of amino acid salts with membrane contactors is expected to be an effective technique to address the issues associated with using alkanolamines.
He et al. [118] assessed the CO 2 absorption performance of several amino acid salts in terms of surface tension and CO 2 absorption flux in a polypropylene membrane contactor. Figure 11 shows a comparison between the surface tension of MEA solution and amino acid salts at different temperatures. It can be seen from this figure that amino acid salts have greater surface tension than MEA solution at all temperatures, which makes them attractive candidates in membrane absorption modules from a lower membrane wetting point of view. Additionally, potassium salts of taurine and glycine showed higher surface tension values than other amino acids leading them to be considered proper choices in membrane contactors. Furthermore, their comparison demonstrated that among studied amino acid salts, potassium argininate exhibited the highest CO 2 absorption flux in the PP membrane contactor, while potassium threoninate presented the lowest absorption flux. Their observation is consistent with the result from Gusnawan et al. [119] that potassium argininate indicated the maximum CO 2 absorption flux compared to potassium salts of threonine, alanine, and serine in a polyvinylidene fluoride membrane contactor.
Lu et al. [120] proposed glycine salt mixed with MEA solution for CO 2 absorption in a PP hollow fiber membrane contactor. The authors concluded that the addition of MEA to amino acid salt of glycine has a positive effect on the overall mass transfer coefficient, and reaches a capture efficiency of more than 90%. The CO 2 removal from a gaseous mixture containing 80% CH 4 and 20% CO 2 in a PVDF hollow fiber was explored by Nakhjiri et al. [121]. The authors selected potassium salts of arginine and glycine as liquid absorbents. It was found that potassium glycinate and potassium argininate can remove CO 2 by up to 62% and 95%, respectively. In another study by Nakhjiri et al. [122], the authors evaluated CO 2 absorption using potassium salt of lysine and they found that potassium lysinate has a removal efficiency of CO 2 of about 91% and can be considered an effective absorbent for natural gas sweetening. Masoumi et al. [123] employed potassium salt of sarcosine for CO 2 capture at high CO 2 partial pressure using a membrane contactor for the first time and observed that potassium sarcosinate has a better absorption flux than DEA and MDEA solutions. Yan et al. [124] conducted CO 2 absorption experiments from flue gas using three absorbents including MEA, MDEA, and potassium glycinate in pilot-scale hollow fiber membrane contactors. To study the wettability of the PP membrane by aqueous potassium glycinate solution, the authors also measured the surface tension of absorbents. Their investigation results revealed that potassium glycinate has a lower potential for membrane wetting after a continuously steady operation for 40 h due to its higher surface tension than MDEA and MEA. Zheng et al. [125] and Lu et al. [168] prepared a novel CO 2 absorbent by mixing glycine and piperazine and applied it to a PP membrane contactor to absorb CO 2 from a CO 2 /N 2 mixture. The authors claimed that the addition of piperazine to glycine increases the overall mass transfer coefficient by up to 30%, and the efficiency can be determined by more than 90%.
The separation of CO 2 from CH 4 using potassium sarcosinate was carried out by Simon et al. [126], and its performance was compared to MEA solution. Based on their results, potassium sarcosinate offers a higher CO 2 /CH 4 selectivity (120) compared to MEA with a value of 60 under the same conditions. Likewise, Rahim et al. [127] studied the feasibility of using potassium and sodium salts of glycine and sarcosine in a PVDF hollow fiber gas-liquid membrane contactor. The finding of the study showed that potassium sarcosinate has a better absorption performance compared to potassium glycinate due to its high CO 2 loading capacity, while potassium glycinate exhibited better stripping efficiency as glycine is more acidic than potassium sarcosinate. Their long-term experiments indicated that potassium glycinate is more stable than potassium sarcosinate. Furthermore, it was concluded that potassium salts of amino acids show better desorption performance while higher performance in absorption was determined by sodium salt of amino acids.

Ionic Liquids
Due to their special characteristics such as negligible vapor pressures, which essentially minimize the opportunity for solvent release into the atmosphere, ionic liquids are considered green solvents [169]. These types of solvents carry the advantage of being easy to be recycled, having a low melting point, non-flammability, remarkable thermal stability, and ease of regeneration [170]. Qazi et al. [155] evaluated the membrane-wetting condition in a PP membrane contactor with ionic liquid 1-ethyl-3-methylimidazolium ethylsulfate ([emim][EtSO4]). By considering their results, a drop of 40% in CO 2 removal efficiency was observed when the membrane condition changes from non-wetted to 5% membrane wetting, which means membrane wetting has a noteworthy effect on CO 2 separation efficiency due to an enhancement in the mass transfer resistance. Moreover, for both wetted and non-wetted membranes, CO 2 removal efficiency is favored by an increase in membrane porosity. The absorption performance and energy-consuming of [bmim][BF4] blended with MEA solution in a membrane contactor and a stripping column were studied by Yang et al. [157]. Their experimental outcomes showed that the CO 2 removal efficiency of about 96% can be obtained using 30

Deep Eutectic Solvents (DES)
Despite the advantages of ionic liquids discussed in the previous section, their limitations in CO 2 capture processes, such as high viscosity during CO 2 absorption, low CO 2 capacity, and their high price, unfortunately impair the process efficiency in a large-scale application [171]. Furthermore, there are growing concerns regarding the toxicity of some conventional ionic liquids. Therefore, it is critically important to develop novel absorbents, which are more advantageous than ionic liquids. In this regard, deep eutectic solvents as a new class of ionic liquids have attracted lots of attention and were proposed by several researchers due to their ease of synthesis, cost-effectiveness compared to ionic liquids, and eco-friendly nature as well as their lack of by-product generation [172]. However, like other categories of absorbents, low CO 2 uptakes of deep eutectic solvents need to be resolved. This section provides a brief description of works that have been undertaken on using deep eutectic solvents in CO 2 capture units. Deep eutectic solvents are prepared by mixing two components including hydrogen bond acceptors (HBAs) and hydrogen bond donors (HBDs) [173].
Trivedi et al. [174] tested a combination of EDA and [MEA-Cl] as a deep eutectic solvent and reported that their synthesized material exhibited gravimetric CO 2 uptake of around 33.7 wt%, which is higher than all ionic liquids, which have a maximum uptake of 18.5 wt% CO 2 . The authors explained that this excellent performance of [MEA-Cl][EDA] can be attributed to different polarities and basicities of the deep eutectic solvents originating from the HBA functional groups. Gu et al. [175] synthesized a series of hydrophobic functional deep eutectic solvents based on polyamine hydrochloride as an HBA and thymol as an HBD. According to their finding, [TEPA]Cl-thymol could efficiently capture CO 2 even at low partial pressures. The CO 2 capacity of [TEPA]Cl-thymol was high, up to 1.355 mol CO 2 /mol DES at 40 • C and 101.3 kPa.
Supported DES membranes have been evaluated in recent years; they combine highly processable and flexible polymeric support impregnated with DES for CO 2 capture [176]. Vatanpour et al. [177] proposed the hydrophilic DES ethaline, which is a mixture of choline chloride and ethylene glycol in a PES/dimethyl acetamide polymer solution. Their results showed that DES can be used as hydrophilic additives to improve membranes' antifouling properties. Craveiro et al. [178] showed that PTFE membranes containing deep eutectic solvents based on choline chloride offer high selectivity for CO 2 /CH 4 , with values above the Robeson upper bound indicating their good potential for CO 2 /CH 4 separations. In another work, Jiang et al. [179] found that the addition of deep eutectic solvents to casting solution in PES membrane fabrication could improve membrane porous structure, which contributed to a significantly increased permeability and a high selectivity.

Inorganic Solvents
Since the absorption of CO 2 using inorganic solvents is effective at high temperatures and pressure, this process has gained widespread acceptance for CO 2 removal from natural gas [180]. The low environmental impacts, high chemical stability, high CO 2 absorption capacity, negligible volatility, better resistance to the presence of sulfur oxides and nitrogen oxides, and slow degradation are the main benefits of inorganic absorbents [181]. However, this category of absorbents suffers from slow reaction kinetics with CO 2 at prevailing low temperatures resulting in larger equipment to achieve the same removal [182]. The addition of amine promoters such as amines, amino acids, and cyclic amines to potassium carbonate (K 2 CO 3 ) solution was suggested as an efficient technique to enhance its absorption rate. An experimental investigation of the simultaneous separation of H 2 S and CO 2 from biogas using K 2 CO 3 mixed with potassium sarcosinate was conducted by Jin et al. [149]. It was concluded that the addition of potassium sarcosinate increases both CO 2 and H 2 S absorption flux. They also observed that CO 2 absorption flux reduces by approximately 30% after 90 min of operation due to partial wetting of PVDF membrane, while it showed little effect on H 2 S absorption flux. Niknam et al. [147] used proline as a promoter for K 2 CO 3 solution in a PP membrane contactor and compared its performance with K 2 CO 3 promoted with MEA solution. According to their observation, proline and MEA have the same positive effect on CO 2 removals. Furthermore, their results revealed that membrane wetting up to 10% significantly decreases CO 2 recovery in K 2 CO 3 + proline solution from 90% to 45%. In another work, Izaddoust et al. [150] added piperazine to a K 2 CO 3 solution for CO 2 separation in a PP membrane and discovered that 90% CO 2 recovery can be determined by the addition of the 0.03 M PZ. Additionally, their results revealed that aqueous K 2 CO 3 + PZ solution has better performance than single K 2 CO 3 in terms of membrane wetting. This can be explained by the fact that at low wetting conditions, a fast reaction rate in the stagnant phase can prevent a sharp reduction in recovery. The potentialities of potassium salts of arginine, lysine, and glycine as promoters in K 2 CO 3 solution were explored by Li et al. [154] in a PP membrane contactor. Based on their results, K 2 CO 3 promoted by potassium glycinate exhibits a more efficient performance than the other solvents with the highest removal efficiency and CO 2 absorption flux.
Ammonia is another attractive solvent due to its low regeneration energy demands compared to MEA solution, low thermal and oxidative degradation, cheap price, and high CO 2 absorption capacity. Therefore, the feasibility of ammonia-based CO 2 absorption processes using hollow fiber membrane contactors was assessed by several researchers in the literature. For example, the membrane contactor approach was tested for CO 2 capture using 1 to 5 wt% ammonia solution by Makhloufi et al. [146]. The authors performed CO 2 absorption experiments using two different types of hollow fiber membrane contactors including a composite membrane contactor and a microporous membrane contactor.
According to their results, the composite membrane contactor showed no performance loss after more than 6 months of use, while the CO 2 removal efficiency in ammonia using microporous membrane contactor decreases over time due to ammonium salt precipitation within the porous structure as well as on the lumen side of the microporous fibers.
The important conclusions of this section are as follows: • The choice of proper absorbent plays a critical role in membrane gas absorption as it can strongly influence mass transfer, efficiency, and operating cost.

•
High surface tension and good compatibility with the membrane are the most important criteria for selecting an absorbent. • Proline is the most promising amino acid for bulk absorption of CO 2 with faster reaction kinetics than other amino acids while glycine exhibits the highest surface tension.

•
Better absorption and desorption performance can be expected when a mixture of two solvents is used. • Potassium salt of amino acids has greater absorption properties than sodium salt of amino acids.

•
Amino acid salts are a better choice than amines for use in membrane contactors from the surface tension point of view. • Absorbent properties and their interaction with membrane material are essential factors in determining the extent of wetting.

•
The interaction between chemical solvent and membrane pores leads to a change in the internal pore structure, which enhances membrane wetting.

Operating Parameters
In addition to the type of membrane and absorption liquid, also the process parameters such as the temperature of absorption and desorption, liquid flow rate, and feed pressure, play an important role and contribute to the overall process performance. The operating conditions must be carefully adjusted to optimize the membrane performance while membrane wetting is avoided. The most important parameters which influence the performance of gas-liquid membrane contactors are listed in Table 6. Table 6. Operational parameters affecting the performance of a membrane contactor. Here, recent publications which studied the influence of operating parameters on the performance of membrane contactors are discussed briefly. Mohammadi et al. [183] showed that when half of the membrane thickness is filled with glycerol, CO 2 removal decreases by about 40%. They also investigated the effect of the membrane porosity-to-tortuosity ratio on CO 2 removal and concluded that CO 2 removal is enhanced from 66% to 71% when the membrane porosity-to-tortuosity ratio increases from 0.1 to 0.9. The porosity has a significant effect on the tortuosity of the membrane and the mass transfer of CO 2 . There is a direct relationship between the effective diffusion coefficient of CO 2 inside the membrane side and membrane porosity. It means that enhancing the porosity of the membrane results in a significant increase in the membrane diffusivity and therefore, the mass transfer of CO 2 inside the membrane side increases, which leads to an increase in CO 2 removal efficiency under non-wetting conditions. It is worth quoting that in comparison to the non-wetting mode of operation, in partial wetting conditions, penetration of liquid into the pores causes a decrease in the effective diffusion coefficient of CO 2 and hence decreases the removal efficiency. In addition, it was observed that increasing the glycerol concentration from 10 wt% to 35 wt% causes a significant enhancement in CO 2 removal while a further increase up to 50 wt% in concentration decreased CO 2 removal due to an increase in the viscosity of the solution, which leads to an increase in the resistance of mass transfer and a reduction in the fluid mobility. Lin et al. [100] tested PES composite hollow fiber membranes for CO 2 capture under different gas and liquid flow rates. According to their observation, higher CO 2 flux and separation efficiency can be achieved as the flow rate of the liquid increases from 2 to 16 L/h. This positive effect can be explained by the fact that more CO 2 is absorbed per unit volume of absorbent at a high flow rate, which is sufficient to ensure that the amount of fresh absorbent is sufficient. In other words, a higher solvent flow rate increases the driving force for CO 2 due to the greater concentration difference. Moreover, the authors indicated that too high a gas flow rate results in a lower separation of CO 2 /CH 4 . It was described that a rise in gas flow rate leads to minimal gas residence time in the tube, which allows more CO 2 to pass from the contactor without absorption, resulting in reduced CO 2 removal efficiency. A similar finding has been reported by Mesbah et al. [148].

Membrane Parameters Affecting Membrane Performance
The effect of fiber inner diameter was studied by Qazi et al. [155], and a significant drop in removal efficiency was determined by increasing the fiber inner diameter. The reason is that as the fiber inner diameter increases, gas volume increases, and thus more CO 2 will be available for the same amount of liquid, which decreases removal efficiency. The authors also assessed the effect of module length and showed that a longer membrane length causes a considerable increase in absorbent residence time in the module and mass transfer surface area, which improves the process efficiency. However, an optimized value for membrane module length should be used for optimum separations as pressure drop can be observed when membrane length increases and also CO 2 concentration drops progressively along membrane length, which has a negative impact on the driving force for separation. It was also found that there is an optimized value for the liquid flow rate. Although a high liquid flow rate has a positive effect on removal efficiency, it also increases the potential for pore wetting. This behavior was attributed to the increase in pressure with an increase in liquid flow rate, thus resulting in higher transmembrane pressure and increasing membrane pore wetting. Hence, it is imperative to maintain the operation in a reasonable liquid velocity range to ensure higher CO 2 capture efficiency. In another work, the impact of the number of fibers on CO 2 removal efficiency inside hollow fiber membrane contactors was evaluated by Nakhjiri et al. [122]. Based on their results, a rise in the total number of fibers from 100 to 600 positively encourages gas-liquid mass transfer interface and as a result improves the gas/liquid contact zone inside the hydrophobic membrane contactor, which leads to enhancement in the CO 2 removal from 40% to 92%.
Chen et al. [184] pointed out that CO 2 absorption flux increases with the rising of liquid temperature from 298 K to 318 K. This is mainly attributed to the fact that with an increase in liquid temperature, both the chemical reaction rate and diffusion coefficients of species in the liquid increase. Furthermore, the authors investigated the effect of CO 2 partial pressure on absorption flux and concluded that the CO 2 absorption flux rises up quickly as the CO 2 partial pressure increases. It was reported that as the amount of CO 2 in the inlet gas increases, more CO 2 molecules will travel through the gas bulk and the porous membrane to contact and react with solvent at the gas-liquid interface. As a result, a higher mass transfer driving force and CO 2 absorption flux are achieved. Cao et al. [185] claimed that the membrane wetting decreases slightly with increasing CO 2 partial pressure. This phenomenon can be explained by the fact that a higher concentration of CO 2 can consume more amounts of reactive absorbent molecules. Therefore, more protonated absorbent and bicarbonate can be generated at the higher CO 2 concentration, resulting in higher surface tension with the increased CO 2 loading. As a result, the membrane wetting could be mitigated at the higher CO 2 concentration in the gas mixture. Dai et al. [99] examined the influence of operating pressure on CO 2 flux in a PTFE membrane contactor and showed that CO 2 flux greatly increases from 0.5 to 4 × 10 −4 mol/(m 2 ·s) when operating pressure increases from 1 bar to 20 bar due to the higher CO 2 driving forces. In addition, it was found that increasing operating pressure is more effective in enhancing CO 2 flux in the porous PTFE membrane contactor compared to the nonporous membrane due to less mass transfer resistance in the porous membrane. The effect of module packing density in PTFE membrane contactors was studied by Ghobadi et al. [186]. The authors pointed out that a rise in packing density from 10 to 50% can improve the CO 2 separation efficiency by 20%, as given in Figure 12. This can be explained by the fact that an increase in the packing density results in a higher membrane surface area over the available module volume and also a more ordered fiber arrangement inside the modules. The key messages of this section are as follows: • An increase in operating pressure, liquid temperature, and packing density favor CO 2 flux. • Membrane wetting becomes more serious with the increase in solvent flow rate. • Higher gas flow rates can improve CO 2 absorption flux, but at the cost of reducing CO 2 removal efficiency. • Improving CO 2 absorption performance by increasing the liquid flow rate to a very high level may not be ideal because of the risk of membrane wetting.

•
The module with a smaller diameter hollow fiber membrane can accommodate a much greater interfacial area per unit volume, which makes it a more efficient absorber unit.

•
Increasing module length, number of fibers, and porosity provide better CO 2 separation performance.

Membrane-Based Hybrid Processes
One of the main challenges of existing CO 2 capture approaches is the energy penalty. The energy consumption for regeneration of the MEA-based absorption process varies from 2.5 to 3.5 GJ/ton CO 2 depending on the source of CO 2 , which is believed to be responsible for approximately 80% of the total energy consumption [7]. This high energy needs to be decreased to around 1 GJ/ton CO 2 while keeping CO 2 removal efficiency above 90% [187]. In this regard, the integration of two technologies (see Figure 13) was represented in the literature as an effective way to overcome the issue of the energy penalty [188]. Substitution of the conventional energy-consuming process by an energy-saving membrane process or their combination, which is called the hybrid process, is principally economical. The specific feature of the hybrid process is its synergy outcome from the integration of twounit operations. The combination of the conventional process with membrane separation technology improves the capture efficiency of the process while the energy consumption is minimized accordingly. The membrane-based hybrid process may be classified into two types including the membrane-cryogenic process and the membrane-chemical absorption process. To select a suitable hybrid process, several factors should be taken into account such as pressure and temperature of the gas stream, CO 2 concentration, the availability of the capture equipment, as well as desired product purity, as the energy consumption is significantly affected by chosen hybrid process type. Yeon et al. [189] studied the performance of a hybrid process that includes a membrane contactor and packed tower as an absorber and a thermal stripper, respectively, for CO 2 capture from flue gas. They used a porous PVDF hollow fiber module with MEA and TEA as absorbent solutions. Their investigation results showed that the integration of membrane and stripper as a hybrid process leads to a higher capture efficiency than the conventional absorption column. The CO 2 absorption rate per unit volume of the membrane contactor was obtained in their work to be 2.7 times greater than that of the packed column. The authors also measured operating power consumption and concluded that the electricity consumption in the membrane contactor (0.78 GJ/ton CO 2 ) is lower than the packed column unit (1.16 GJ/ton CO 2 ). Likewise, Kosaraju et al. [57] assessed asymmetric poly (4-methyl-1-pentene) (PMP) hollow fiber membranes with an ultrathin, dense skin layer for an absorption and desorption loop system, and MEA as an absorption liquid for the separation. Their long-term test results revealed a decrease in CO 2 absorption performance after 55 days. A hybrid membrane-liquefaction system was proposed by Anantharaman et al. [190] for CO 2 absorption. A techno-economic analysis was carried out by the authors and the results indicated that the cost of CO 2 avoided (48 €/ton CO 2 ) is about 9% more costefficient than the conventional MEA-based absorption process. Lee et al. [191] evaluated the performance of a ceramic hollow fiber membrane contactor module integrated with a stripping column for CO 2 capture. It was found that the hybrid process can absorb more than 90% of CO 2 with high stability for more than 60 h. Moreover, their results showed the overall gas mass transfer coefficient in a ceramic hollow fiber membrane contactor could be improved by 1.8 times compared to a conventional packed column. It can therefore be concluded that the ceramic hollow fiber membrane contactor is a promising alternative to the polymeric membrane contactor and packed column systems. Wang et al. [192] examined CO 2 desorption from 20 wt% MEA solution in a hollow fiber membrane contactor combined with a packed column. According to their results, the performance of the stripper increases as the membrane's length increases and membrane diameter decreases. Jiang et al. [193] compared CO 2 separation in four different configurations including one-stage membrane, two-stage membrane, chemical absorption, and membrane-chemical absorption. Based on their observation, the CO 2 purity in the two-stage membrane design was found to be 46.2% greater than the one-stage membrane. This promotion of CO 2 removal efficiency can be explained that using two PI membrane modules leads to more separation of the feed gas flow and a rise in the CO 2 concentration of the permeate gas resulting in an efficiency of around 99%. Furthermore, the combination of membrane and chemical absorption showed a reduction in regeneration heat and absorbent flow rate.
Burdyny et al. [194] utilized a hybrid cryogenic and membrane process in oxy-fuel combustion. They reported that hybrid configuration enhanced the efficiency by 0.9% as well as reducing the size of the cryogenic equipment and distillation columns, which will benefit the industry substantially. A hybrid process including PP hollow fiber membrane contactor as an absorber and a packed column as a stripper was used by Yan et al. [124] for CO 2 capture from flue gas. Several amine solutions such as MEA, MDEA, and potassium glycinate were tested as the solvent, and their performance in CO 2 capture was compared. It was found that the process has a capture efficiency of more than 90% with a lower potential of membrane wetting when potassium glycinate is employed as an absorbent. In another work, Yan et al. [195] studied the effect of the combination of the membrane with the stripper column on CO 2 absorption. Aqueous MEA solution and PP were used as absorbent and hollow fiber, respectively. It was concluded that in the case of using a fresh membrane, the membrane separation system showed an overall mass transfer coefficient about 2.9 times higher than that of the packed column while having a lower absorption cost. However, this improvement could reduce when all the membrane pores were completely wetted. Belaissaoui et al. [196] explored the potential of membrane separation combined with a cryogenic process for post-combustion CO 2 capture. Based on their results, the hybrid configuration needs lower energy in comparison to conventional CO 2 absorption for a CO 2 concentration of 15-30 mol%. This improvement was in the range of 12 to 25%, which makes the use of this technology an attractive strategy. Kundu et al. [197] conducted a study on the membrane-absorption system for CO 2 removal and investigated its performance from an energy requirement perspective. It was revealed that the heat duty of the reboiler in a hybrid process can be significantly saved due to an increase in the membrane area. The authors also claimed that the membrane technology exhibits less energy demand and can save up to 35-55% energy compared to the MEA system.
The major conclusions are drawn as follows: • The hybrid process offers a high degree of flexibility, with respect to the capture ratio and/or final CO 2 purity.

•
The improvement to either membrane or cryogenic technologies will improve the hybrid system as well.

•
Combining membranes and absorption technologies could result in significant energy savings by reducing the steam required for amine regeneration.

•
The hybrid process showed a higher CO 2 removal efficiency than the conventional absorption tower.

•
The membrane contactor hybrid process was proved to be economic by evaluation through CO 2 recovery cost and operating power consumption.

•
The membrane-absorption unit can improve the absorption process by generating 400-1500% greater mass transfer area per unit volume leading to smaller equipment sizes.

•
Compared to standalone methods, hybrid processes showed superiority not only in CO 2 recovery and energy penalty but also in installation investment.

Long-Term Stability of Membranes
The membranes' pores in most hollow fiber membrane contactors are gradually wetted over time, which will lead to a sharp increase in the membrane mass transfer resistance and a remarkable reduction in CO 2 absorption fluxes. It is well known that any reaction between the absorbent and membrane material could possibly change the membrane matrix and surface morphology. Therefore, to make gas-liquid membrane contactor technology more competitive on an industrial scale, the long-term stability of membranes should be improved. The long-term stable operation of the gas-liquid membrane contactor requires that the pores of the membrane remain completely gas-filled over the long operational time.
Dai et al. [99] conducted a long-term CO 2 absorption test of PTFE membrane in contact with [Bmim] [TCM] as an absorbent at 20 bar in a 14-day test. It was found that the CO 2 flux through the PTFE membrane decreased by 50% after 7 days of operation due to the morphological rearrangements and pore wetting by the absorbent. The authors observed that the PTFE membrane surface was partially destroyed by absorbent after two weeks of testing at 20 bar. Jin et al. [149] studied the effect of operation time on CO 2 and H 2 S flux in a PVDF membrane contactor and observed that the CO 2 absorption flux decreases significantly after 5 h while the H 2 S flux changes over time were negligible. Similar results were found by Rajabzadeh et al. [198]. They showed that the PVDF membrane contactor presents a performance reduction of more than 80% after 60 h of operation. Furthermore, it was concluded that membranes with lower porosity and pore diameter were stable for a longer time, compared to membranes with larger porosity and pore diameter, which were completely wetted during operation.
Xu et al. [102] fabricated a composite membrane with a dense skin layer by dispersing ZIF-8 nanocrystals into a PDMS matrix deposited on porous PVDF and studied its performance throughout 15 days of operation. The authors revealed that the developed membrane remains constant at its high CO 2 flux even after 15 days of the test, indicating a significant improvement in its long-term stability. Zhang et al. [199] evaluated the longterm stability of a PEI hollow fiber membrane over 60 days. According to the results of the long-term stability test, the membrane was able to maintain a relatively stable performance with a CO 2 absorption flux of about 1.4 × 10 −2 mol/(m 2 ·s), indicating lower wetting issues. Amirabedi et al. [96] compared the chemical stability of neat PP and composite membranes with MEA solution. It was found that the composite membrane exhibited a more stable non-wetting property during the long-term process compared to a neat membrane due to the coating of nanoparticles on the membrane surface. Furthermore, the neat membrane showed a significant reduction in CO 2 flux after 30 days of testing. This trend can be attributed to the penetration of MEA into the membrane pores. The long-term stability of PP, PMP, and PTFE membranes was investigated by Chabanon et al. [200], and it was reported that the PP membrane has the highest sensitivity toward wetting conditions among other membranes. PTFE and PMP showed better resistance to wetting.
The key messages of this section are as follows: • The membrane wetting phenomena increase the mass transfer resistance and limit the long-term process stability. • Membrane material and absorbent compatibility are important to secure long-term absorption performance.

•
The long-term absorption performance of the membrane contactors is predominantly related to the hydrophobicity of the hollow fibers and subsequently the membrane wetting.

Challenges in Membrane Contactors
Membrane fouling in membrane contactors has received less attention than membrane wetting. Nevertheless, membrane fouling should not be neglected as the typical flue gas contains contaminants including fine particulates, aerosols that originate from SO 2, and water vapor, which reduces CO 2 removal efficiency in the membrane contactor process [201]. The fine particles in the flue gas may severely impact the performance of the membrane or even destroy membrane material, which has become an important limitation for the industrial applications of membrane contactors [202]. The availability of suitable membranes with the desired performance appropriate for an intended application is another concern. Although a wide range of membrane products are available in the market, each product is only suited for limited applications. This could be the limiting factor in the application of that technology in practice [203].
The presence of the membrane in the gas-liquid contactors adds additional resistance to mass transfer, which results in a reduction in the overall mass transfer rate. This membrane resistance can be more important if the membranes are wetted by liquid solvents. Membrane wetting has become the main concern for the practical application of membrane contactors, which results in economically unviable operations. The hollow fiber membranes are generally packed randomly in parallel alignment into a shell. The non-uniform fiber distribution can cause severe fluid channeling and bypassing on the shell side of the module which results in the decline of the mass transfer process. These underperformances are often attributed to the disadvantages in the configuration that cause limitations to its application in the industry [116].
Another major challenge with membrane contactors is the selection of an ideal membrane-absorbent combination. The most important requirements of absorbent in membrane gas absorption applications are that the long-term use of absorbent should not damage the membrane either physically or chemically and that the membrane-absorbent combination should have sufficiently high critical entry pressure to avoid wetting. It is difficult to maintain the membrane performance in long-term operation. Most membranes do not have the resilience in practical industry conditions and quickly fail, which is one of the biggest challenges for their potential applications in industrial practice. The regeneration process in membrane contactors has the ability to minimize the temperature required for the solvent regeneration process; however, the additional energy cost to maintain the reduced partial pressure in the gas side by sweeping gas and/or vacuum must be considered. It is important to remark that energy duty contributions will be different depending on the membrane material and their operational limits.

Conclusions
Membrane contactors are considered one of the most promising technologies in gasliquid absorption processes. This review summarized the recent advances in membrane contactor-based gas separation. It was found that the success of a membrane contactor process will strongly depend on the type of membrane material chosen and its fabrication technique. The material must be thermally and chemically compatible with the absorbent and it must be processable to ensure it can be manufactured at scale for a moderate cost. A membrane material with a high contact angle with the solvent is preferable as it decreases the potential for pore wetting. The ceramic hollow fiber membranes showed better mechanical and thermal stability compared to polymeric membranes, which makes them suitable candidates for high-temperature and long operating applications. The addition of non-solvent additives into the polymer dope, surface treatment, surface coating, and the combination of two polymers have been suggested to modify the membrane morphology and structure. The composite membranes with dense skin layers exhibited strong anti-wettability to absorbents and long-term stability even for several days. Compared to flat-sheet membranes, hollow fiber membranes have the advantages of large gas-liquid contact areas, high surface-to-volume ratio, self-supporting characteristics, and ease of scale-up. Chemical absorbents play a critical role in the absorption and desorption processes in membrane contactors. Good chemical compatibility, fast CO 2 reactivity, low regeneration energy, and high surface tension were reported to have the greatest influence on the capture cost and so were usually prioritized. In this regard, the latest developments in absorbents have been discussed, and their advantages and limitations were pointed out. The operating conditions, among other parameters, have a strong influence on the efficiency of the separation and can lead to an economical strategy. Therefore, the effects of important operating parameters such as gas and liquid flow rate, membrane area, module length, membrane porosity, fiber diameter, operating pressure, number of fibers, module packing density, absorbent concentration, and temperature on the CO 2 separation in the HFMC were comprehensively investigated. The integration of two technologies of the conventional process with membrane separation was represented as an effective way to overcome the issue of the energy penalty. Finally, to make gas-liquid membrane contactor technology more competitive on an industrial scale, the long-term stability of the membrane should be improved.

Data Availability Statement:
The data that support the findings of this study are available upon reasonable request from the authors.

Conflicts of Interest:
The authors have declared that no competing interests exist.