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Membrane contactors allow for higher mass transfer per unit volume. Hence, there has been great interest in recent years on its development and applications in separation processes. It offers high interfacial area between liquid and gas phases while preventing direct mixture, and concurrently prevents flooding and foaming, thanks to the independent gas and liquid flow rates. However, wetting of the membrane pores is a serious problem for this technology application, where even partial membrane pores wetting could significantly deteriorate contactor performance. Therefore, it is crucial that the membranes are hydrophobic to maintain membrane nonwetting during operation. Moreover, any membrane surface modification to increase its hydrophobicity must also be highly stable, does not leach out, and can be applied for long-term operation. This chapter looks at the research done on superhydrophobic membranes for gas-liquid membrane contactor application and its recent advances.
Carbon Capture, Utilization & Storage (CCUS) R&D Department, PETRONAS Research Sdn. Bhd, Jalan Ayer Itam, Kawasan Institusi Bangi, Kajang, Malaysia
Athirah Mohd Tamidi
Carbon Capture, Utilization & Storage (CCUS) R&D Department, PETRONAS Research Sdn. Bhd, Jalan Ayer Itam, Kawasan Institusi Bangi, Kajang, Malaysia
Farahdila Kadirkhan
Carbon Capture, Utilization & Storage (CCUS) R&D Department, PETRONAS Research Sdn. Bhd, Jalan Ayer Itam, Kawasan Institusi Bangi, Kajang, Malaysia
Pei Ching Oh
CO2 Research Centre (CO2RES), Chemical Engineering Department, Universiti Teknologi PETRONAS, Bandar Seri Iskandar, Malaysia
*Address all correspondence to: syafiqa_saleh@petronas.com.my
1. Introduction
There is a massive demand in recent years to develop new separation technologies or improve on existing ones, in order to enhance product quality, solve environmental issues, improve energy efficiency, increase process safety and/ or cost reduction. However, to compete with proven conventional separation technologies, critical attention must be given to every detail to achieve the required properties. In one such case, membrane technology has the potential to replace traditional energy-intensive separation techniques and provide a reliable option for sustainable industrial growth [1]. Today, membrane contactor has attained considerable attention due to its wide range of potential applications. The number of publications on the field has risen steadily since the past 20 years, as shown in Figure 1 [2].
Figure 1.
The yearly publications on membrane contactor for the past 20 years shows immense interest in the field. Adopted from Ref. [2].
Since most of chemical separation processes are related to the contact of two different phases (liquid-liquid or gas-liquid), a membrane contactor system is deemed suitable for operations such as gas absorption and stripping, liquid-liquid extraction, distillation, heterogeneous reactions, emulsification, demulsification, humidification, and dehumidification. There have been quite a number of commercialized membrane contactor system reported, such as ammonia recovery, de-gassing of water, and air humidification [3].
2. Membrane contactor: working mechanism and applications
2.1 Introduction to membrane contactor
A membrane contactor is a specialized device that can be used in various industrial processes and applications to transfer materials (such as gases or liquids) across a porous membrane between two phases. The porous membrane acts as a nonselective barrier between the phases, in which the transport of species through the membrane pores occurs by diffusion mechanism. In a membrane contactor system, the selectivity of the separation process is governed by the component’s affinity to the specific phase. The detailed fundamentals and mass transfer transport mechanism of the species through porous membrane contactors can be found in the open literature [4, 5].
The primary purpose of a membrane contactor is to enhance mass transfer, separation, or purification processes. Membrane contactors offer several advantages over traditional contacting devices. They provide an exceptionally high interfacial contact area between the phases, which results in an improved mass transfer coefficient (K) and is a more compact separation system with less capital cost and energy consumption. This is contributed to the use of thousands of hollow membrane fibers in the module. For comparison, based on reported experimental results, the volume of a conventional packed column is 4–10 times bigger than a membrane contactor unit [6, 7].
Another advantage of membrane contactor systems is its operational flexibility. Since the membrane separates the two different phases, independent control of the phase’s flow rates prevents operational problems like flooding, foaming, weeping, entrainment, and loading [4]. In addition, the modularity of membrane contactors leads to easy scale-up and scale-down. The characteristics of membrane contactors compared with conventional contactors are shown in Table 1.
Contactor type
Surface area/ volume (m2/m3)
Mass transfer coefficient (Kl.a) (s−1) × 10−2
Gas/liquid volume flow (%)
Bubble column
50–600
0.5–12
60–98
Packed column
10–350
0.04–7
2–25
Venturi scrubber
150–2500
8–25
5–30
Membrane contactor
1000–10,000
5–50
1–99
Table 1.
Characteristics of different gas-liquid contactors (Ref. [8]).
Membrane contactors, however, do face some challenges. Regular asymmetric membranes have high gas permeability, ultra-thin skin layer, and small thickness; hence any resistance is negligible. In membrane contactor process on the other hand, the membrane phase itself adds extra resistance to mass transfer process. This resistance can be minimized using appropriate materials and optimizing fabrication conditions to improve the membrane structure and properties [8, 9].
Another crucial matter is that wetting of the membrane pores is a serious problem for this technology application. Partial wetting of the membrane can significantly deteriorate the mass transfer coefficient and decrease the separation efficiency [10, 11]. Several attempts have been made to develop and modify the membrane structure to minimize wetting and the mass transfer resistance; these are highlighted in the proceeding sections. In order to successfully implement membrane contactor technology in the industry, current status, and future direction of the technology are discussed critically in this chapter.
2.2 Applications of gas-liquid membrane contactor
The schematic of the mass transfer mechanism of the gas-liquid membrane contactor technology is shown in Figure 2. The overall mass transfer resistance, K is the combination of the gas (g) phase, membrane (m) and liquid (l) phase resistances that is shown in Eq. (1).
Figure 2.
Schematic of nonwetted mode mass transfer profile of component “A” (CA) for different applications of gas-liquid membrane contactor: (a) gas absorption and dehumidification; and (b) stripping, degassing, humidification, distillation.
1Ko=1Km+1Kl+1KgE1
In an ideal case, the membrane pores are considered gas-filled, which minimizes the overall resistance. In fact, the nonwetted mode can be achieved by using highly hydrophobic or superhydrophobic membranes with small pore sizes. Partial wetting of the membrane can seriously increase the overall resistance and deteriorate the separation performance of the membrane [10]. The gas phase resistance can be reduced by increasing velocity and/or the species concentration. Using low-thickness porous hydrophobic membranes with ultra-thin skin layers may minimize the resistance for different applications. So far, several attempts have been reported in the literature to enhance the membrane structure for contactor applications [8, 9, 12]. In addition, introducing chemical reactions for separation purposes and increasing the liquid phase velocity can significantly enhance the liquid mass transfer coefficient and minimize the resistance [11, 13].
Since contact of two phases is possible via the membrane contactor, several applications can be expected in the scope of gas-liquid contacting processes. The use of gas-liquid membrane contactors covers a wide range of separation processes such as absorption/stripping, degassing, humidification/dehumidification and distillation. The varied applications and research on gas-liquid membrane contactors are summarized in Table 2.
Applications of gas/vapor-liquid membrane contactors.
As summarized in Table 2, so far, simultaneous absorption of various types of acid gases and toxic vapors such as VOCs, CO2, H2S, SO2, and NO2 with different liquid absorbents have been conducted using membrane contactors [14, 15, 16, 17, 18]. Many researchers focused on CO2 removal due to its greenhouse effect and impact to global warming. In correspondence, gas-stripping membrane contactors are developed to regenerate the liquid absorbents either as integrated process or standalone system [19, 20, 21]. Degassing to remove O2 and CO2 through membrane contactor modules from boiler feed water was developed to control corrosion [22, 23]. There is also vast interest in its application for membrane distillation of organic solvents [24, 25, 26]. Additionally, membrane contactor is also considered an alternative for air humidity control [27, 28, 29, 30].
3. Current issues and challenges of membrane contactor
Membrane wetting is the main challenge of gas-liquid membrane contactors, as it can deteriorate the separation performance. Although the membrane employed in gas absorption is often hydrophobic and capable of resisting the wetting of absorbents, aqueous absorbent solutions of organic compounds, especially alkanolamines [31], could still penetrate the membrane pores over prolonged operation. The schematic diagram of nonwetting, overall-wetting, and partial wetting of the membrane contactor is shown in Figure 3.
Figure 3.
Operation modes in a hydrophobic microporous hollow fiber membrane and pore-wetting patterns: (a) nonwetting mode, (b) overall-wetting mode, and (c) partial-wetting mode. Adapted from Ref. [32].
The partial-wetting mode will cause the overall mass transfer resistance to increase rapidly and significantly influence the stability of long-term operation [10]. Malek et al. [33] investigated the effects of membrane-wetting pressures on the overall mass transfer. They concluded that partial wetting of the membrane can change the overall mass transfer coefficient of membrane gas absorption. Lu et al. [32] presented the wetting mechanism for hydrophobic hollow fiber membrane-alkanol amine absorbents for CO2 capture. A mathematical model for wetting mechanism was proposed by correlating the resistance-in-series equation, the Laplace equation, and the pore size distribution function of the membrane.
For a given hydrophobic membrane material and structure, the degree of partial wetting of membrane pores depends on the surface tension the contact angle between the absorbents and the membrane surface, and the operating conditions. The minimum or breakthrough pressure (∆P), exerted on the liquid phase to enter the membrane pore can be estimated by the Laplace-Young equation, as shown in Eq. (2) [34]:
∆P=2γcosθrp,maxE2
where γ is the surface tension of the liquid, θ the contact angle between the fluid phase and the membrane, and rp,max is the maximum membrane pore radius.
Thus, based on the Laplace-Young equation, membrane and liquid absorbent properties can influence membrane wetting [34]. Surface hydrophobicity and pore size are two essential membrane properties that can be controlled during fabrication and modification processes.
Among hydrophobic polymers, PP, PTFE, and PVDF have extensively been used for porous membrane fabrication [8, 13, 14]. Microporous PTFE membrane shows good gas absorption performance and long-term stability [35]; however, its application is limited due to its high production cost and unavailability in small diameters [12]. PP and PTFE are however not soluble in common solvents at room temperature and these membranes are normally fabricated using thermal and stretching methods. These methods do not have the flexibility to control pore size and porosity. PVDF, on the other hand, can be dissolved in organic solvents at room temperature, resulting in asymmetric membranes with controlled pore size, porosity, and low mass transfer resistance (favorable factors for membrane contactor) via phase inversion process. There have been extensive studies on improving membrane hydrophobicity, which will be explored further in detail in the next section.
Aside from the membrane itself, the selected liquid absorbent properties are also crucial, where its surface tension is key, affecting membrane wetting. Amine solutions have been used for acid gas capture extensively. These organic solutions possess low surface tension, which can easily penetrate through the membrane pores and result in wetting. Thus, there are studies to develop liquid with high reactivity and surface tension, like ionic liquids and amino acid salt solutions, to minimize wetting and prolong the stability of the operation [17, 36, 37]. The liquid absorbents are tailored to have high affinity to the gas component, high compatibility with the membrane material, as well as low vapor pressure and viscosity. Low vapor pressure is important to minimize liquid loss during the operation, while low viscosity is required to minimize the pressure drop in the membrane contactor module. These tailored properties would increase mass transfer and enhance the membrane contactor separation performance. New and novel absorbents, however, suffer from high cost and low regenerability for a cost-effective operation.
The interaction between membrane material and liquid absorbent can significantly affect the membrane structure and properties, which could result in unviable long-term operation. Other than inert PTFE, materials such as PP and PVDF have shown interaction with alkaline solutions [38, 39], which are usually used as liquid absorbents for acid gas capture. Therefore, compatibility of the absorbent with the membrane needs to be considered in the design of long-term operation.
Aside from polymeric material, the idea of using composite membranes has also been explored due to its long-term wetting stability and high compatibility with liquid absorbents [40, 41, 42]. Composite ceramic membranes have high thermal stability and are proposed for high-temperature processes like gas-stripping membrane contactors. Thin-coated layers are developed to minimize membrane resistance, attain high gas permeability, high hydrophobicity to minimize wetting, high mechanical resistance against erosion of the liquid flow, and high chemical resistance for a prolonged operation. They provide a good alternative for membrane contactor application besides polymeric membranes.
4. Membrane fabrication and modification for superhydrophobicity improvement
A surface is considered superhydrophobic when the static water contact angle exceeds 150o and has low contact angle hysteresis (i.e., less than 10o) [43]. A superhydrophobic surface can be created by reducing the surface free energy of a rough surface or roughening a low surface energy material or a combination of both [44, 45]. The higher the surface roughness, the solid-liquid contact surface would become smaller and hence larger contact angle [46].
One tactic to obtain superhydrophobic membrane is through fabricating a membrane with a rough surface directly, i.e., by having nanoparticles entrapped in the membrane structure by adding them into the polymer matrix itself [47]. The other approach is to modify fabricated membrane surface with low surface energy materials [48]. Figure 4 shows a general summary of the fabrication methods of superhydrophobic polymeric membranes.
Figure 4.
Summary of the fabrication methods of superhydrophobic polymeric membranes.
4.1 Membrane fabrication with nanoparticles to achieve superhydrophobicity
One way of achieving superhydrophobic properties is to have nanoparticles entrapped in the membrane structure by adding them into the polymer matrix itself [47]; this has been intensively studied to improve the membrane properties, such as permeability, selectivity, and physical strength [49]. 2 wt% of nanoparticles is reported to improve significantly the membrane contact angle by 18% [50]. The addition of nanoparticles with longer hydrophobic chains is considered to provide higher membrane hydrophobicity [50, 51]. Blending modification is the most practical way that can be applied to industrial-scale production [52].
Another way to achieve membranes with different structures is to adjust the bath formula and conditions during nonsolvent-induced phase inversion (NIPS). PVDF precipitation is controlled by both liquid-liquid demixing and solid-liquid demixing, which could result in amorphous cellular morphology or interlinked semi-crystalline particle structure, respectively. Hence, the coagulation bath significantly affects the characteristics of the fabricated PVDF membranes [53].
Table 3 shows some of the methods used by researchers to produce superhydrophobic membranes for gas-liquid contact applications. Researchers also reported a significant reduction in surface free energy and increased surface roughness.
Membrane material
Nanoparticles
Fabrication
Contact angle
Surface free energy (mN/m)
Surface roughness (nm)
Application
Reference
PVDF
Hydrophobic modified SiO2 nanoparticles in ethanol bath
The biggest issue is, however, the agglomeration that takes place when blending nanoparticles together with polymer. Agglomeration causes the instability of casting solution and nonuniform distribution of particles in the membranes; this leads to a change in membrane topography, microstructure, and performance [60]. Pang et al. managed to obtain evenly distribution of SiO2 in their membrane by introducing hydroxyl groups into PVDF, which enhanced the affinity between the PVDF chains and the hydrophilic nano-SiO2 particles [54].
4.2 Membrane surface modification for superhydrophobicity
Surface modification methods can be either through physical modification or chemical modification. In physical modification, the modifiers exist on membrane surface via physical interaction but not covalent bonding. Hence, the chemical composition of the membrane remains unchanged. Surface coating/ deposition is a simple yet effective method for membrane modification; however, interaction between the coated layer and the membrane surface is relatively weak, and probably lost during long-term operation [60].
A variety of methods, including the wet-chemical modification [61], plasma treatment [52, 62], sol-gel method [47, 63], and atomic layer deposition [64, 65], can be found in literature and have been used for the modification of membrane to improve its properties, as well as to functionalize the membrane with additional properties [61]. However, surface modification techniques using plasma treatments and particle beam irradiation need expensive and complex equipment, therefore they could not be used widely in industry [66].
For superhydrophobic coating of membranes, nanoparticles are commonly used, such as SiO2, TiO2, Al2O3, ZnO, and CaCO3 [44], to roughen the surface. Polymers that possess hydroxyl groups or carboxyl groups on their molecular chains form bonding under certain conditions with the hydroxyl groups on the surface of inorganic nanoparticles like nano SiO2 and nano TiO2 [67]. Some researchers took the approach to lower surface energy to achieve superhydrophobicity; for example, Xiong et al. [68] used surface silanization and fluorination to lower the surface energy to only one-third of the original PTFE membrane, and Meng et al. [43] used low surface energy of fluorinated silanes with surface roughness of nanoparticles.
Table 4 describes some of the surface modification methods used to attain superhydrophobicity on polymeric membrane.
Membrane material
Modification method
Contact angle
Surface roughness (nm)
Membrane Application
Reference
PP
Coating with granular PP. and the mixtures of Methyl ethyl ketone (MEK) and cyclohexanone
Surface modification of polymeric membrane achieving superhydrophobicity.
4.3 Current separation performance of superhydrophobic membrane
Performance of superhydrophobic membrane for contactor application is compared in this section. For CO2 removal process, the normally reported range of CO2 flux for polymeric membrane contactors is 0.2–6.0 mmol/m2.s [3]. It is worth noting that CO2 removal using membrane contactor performance is much more complex and influenced by a lot of factors, like the choice of absorbent and operating conditions (i.e., flowrate, temperature, and pressure) [75]. However, for ease of comparison, flux is a valid criterion to monitor and evaluate its performance since it is used as a scale-up factor for the technology [35]. Table 5 shows the reported performance of the produced superhydrophobic membrane (listed in column “modified”) and compares it against the flux for membrane in pristine condition.
CO2 removal performance of superhydrophobic membrane in contactor process.
As observed from Table 5, the produced superhydrophobic membrane in general provides better separation performance. The results above describe the performance during short testing, and the effect of superhydrophobicity is actually more substantial for long-term operation. For example, the modified PP/cyclohexanone:MEK membrane, during the initial operating period it, performs poorer than pristine membrane because although the modification treatment enhanced hydrophobicity, it also decreased membrane porosity and increased its thickness. However, after 20 days of operation, modified PP/cyclohexanone:MEK membrane CO2 flux is stable at 1.3 mmol/m2.s; the pristine membrane on the other hand declined by 78% down to 0.5 mmol/ m2.s [12].
Another considerable application of membrane contactor is for membrane distillation. As shown in Table 6, the difference can be seen in the membrane distillation performance between pristine membranes and modified superhydrophobic membranes.
Membrane distillation performance of superhydrophobic membrane.
Similar to membrane contactors for CO2 removal, the significance of superhydrophobic membranes for membrane distillation application is much more impactful after long-duration testing. PVDF-TiO2-FTCS-PEG membrane remained stable at 12 kg/m2.h after 24 hours, while the pristine membrane flux dropped by 44% to 10 kg/m2.h [43]. Therefore, superhydrophobicity is a good strategy to minimize wetting and improve membrane contactors for industrial applications.
4.4 Stability of superhydrophobic membrane
Although superhydrophobic membrane is less prone to wetting, chemical reaction between the surface of the membrane and the absorbent could still occur after prolonged contact, leading to decline in membrane stability and decrease in its mass transfer flux [54]. Hence, it is important to evaluate its performance for long duration. For superhydrophobic membrane as contactor, the decline in performance could be mainly due to chemical degradation of the interface [54] or damage to the coating [73].
Long-term testing of these superhydrophobic membranes for gas-liquid contactor for more than 24 hours is scarcely found in literature. Table 7 lists the reported long-duration performances that are done at least for a day. Despite a slight decline, the results are encouraging and pave the way for potential developments.
Long-duration stability testing of superhydrophobic membrane for gas-liquid contactor application.
Aside from the decline in performance, the membrane hydrophobicity is also observed to have reduced, for example PVDF-SiO2-HDTMS contact angle reduced from 160o to 154o after 20 days of operation. Chemical degradation of the interface by MEA solvent leads to the dehydrogenation of the PVDF surface. This is more destructive for pristine PVDF where the contact angle suffers greatly and dropped to 59o [54].
This chapter has thus far described the methods of superhydrophobic polymeric membrane fabrication, which are either direct fabrication of rough surface membrane, or surface modification of existing membrane. Drawing insights from pertinent literature, these methods demonstrate the capability to achieve an impressive contact angle ranging from 150o to 164o (as described in Section 4.1 and 4.2). Promising membranes for application should have outstanding antiwettability not only on outside surfaces but internal surfaces as well, i.e., the surface of pores and channels, for persistence of the antiwettability in service [53]. For effective scale-up for industrial applications, there would be a trade-off between the best superhydrophobic membrane and the most practical method of membrane fabrication and modification.
Even though research has been conducted to fabricate superhydrophobic membranes successfully, there are still other challenges prior to further scale-up and technology commercialization. As described by Lv et al. [12], the focus on increasing hydrophobicity is good for performance, but this alone would not offset the disadvantages of low porosity and high resistance. Hence, several strategies have been employed to reduce mass transfer resistance. One of the strategies is to braid, weave, or loom together the hollow fiber membranes to reduce liquid boundary layer resistance and improve mass transfer.
To overcome chemical degradation, the right selection and compatibility of the membrane material and solvent are crucial. This chapter has mainly focused on polymeric material; however, ceramic membranes are also possible alternative for membrane contactor application [40].
It is understandable that there is limitation, and only so far that improvement from superhydrophobic membrane can help. Thus, it is crucial for membrane contactor research to also optimize operating parameters to enhance performance. Surface tension of organic solvent decreases at higher temperature, leading to membrane wetting; therefore, operating at lower temperature would be an option. Good transmembrane pressure control during operation is very important to maintain a slight higher pressure at the liquid side, but not high enough to wet the membrane. In addition, it is essential to conduct extended stability testing lasting beyond a few days to confidently move these researches from proof-of-concept to prototype demonstration at the actual site.
Another potential strategy is to consider methods of rejuvenating the used membrane. Restoring the functionality of the membrane is a good alternative that could extend the lifespan and reduce the cost of operation (i.e., instead of replacing it with new membranes). Physical cleaning may be done by simply washing with water and blowing with gas to clear the clogged pores. Other chemical cleaning or repairs could be considered depending on the membrane and application.
The summary of challenges and future directions regarding superhydrophobic membranes for gas-liquid membrane contactor applications are highlighted in Figure 5.
Figure 5.
Summary of challenges and strategies on superhydrophobic membrane for gas-liquid membrane contactor applications.
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Written By
Syafiqa M. Saleh, Athirah Mohd Tamidi, Farahdila Kadirkhan and Pei Ching Oh
Submitted: 15 August 2023Reviewed: 16 August 2023Published: 23 September 2023
Open access peer-reviewed chapter
