Chemical and Physical Ionic Liquids in CO2 Capture System Using Membrane Vacuum Regeneration

Carbon Capture Utilization and Storage technologies are essential mitigation options to reach net-zero CO2 emissions. However, this challenge requires the development of sustainable and economic separation technologies. This work presents a novel CO2 capture technology strategy based on non-dispersive CO2 absorption and membrane vacuum regeneration (MVR) technology, and employs two imidazolium ionic liquids (ILs), [emim][Ac] and [emim][MS], with different behavior to absorb CO2. Continuous absorption–desorption experiments were carried out using polypropylene hollow fiber membrane contactors. The results show the highest desorption behavior in the case of [emim][Ac], with a MVR performance efficiency of 92% at 313 K and vacuum pressure of 0.04 bar. On the other hand, the IL [emim][MS] reached an efficiency of 83% under the same conditions. The MVR technology could increase the overall CO2 capture performance by up to 61% for [emim][Ac] and 21% for [emim][MS], which represents an increase of 26% and 9%, respectively. Moreover, adding 30%vol. demonstrates that the process was only favorable by using the physical IL. The results presented here indicate the interest in membrane vacuum regeneration technology based on chemical ILs, but further techno-economic evaluation is needed to ensure the competitiveness of this novel CO2 desorption approach for large-scale application.


Introduction
Great efforts to reduce carbon dioxide (CO 2 ) emissions from the industrial and energy sectors (decarbonization) are crucial to reach the commitment to have net-zero greenhouse gas emissions by 2050 [1]. In this context, post-combustion Carbon Capture, Utilization, and Sequestration (CCUS) technologies are currently gaining interest due to their potential to significantly capture CO 2 from large emission sources, including, on the one hand, thermal and power generation plants, which contribute greatly to the increase of CO 2 in the atmosphere, and on the other hand, non-energy industrial sectors, such as the cement, chemical, and steel industries, where there is currently no real alternative to reach net-zero CO 2 . The different CCUS approaches, depending on the application of the captured carbon dioxide technology, involve the capture of CO 2 from the output gas of industrial processes for permanent storage in geological cavities (CCS) [2] or use as a resource for carbon-based products (CCU) [3].
Solvent-based absorption-desorption technology, which typically separates CO 2 from flue gas in packing columns, is presented as one of the most mature technologies. Nevertheless, the main challenge is to achieve a reduction in energy consumption for CO 2 -rich solvent regeneration carried out in the desorption column, which is estimated to constitute 80% of the total carbon capture system energy required [4]. Focusing on this, Membrane Vacuum Regeneration (MVR) technology has been proposed as a promising CO 2 desorption process due to its potential for reducing the energy needed for regeneration with respect to conventional packed columns [5][6][7][8]. Using the MVR system, CO 2 is desorbed in a hollow Table 1. Abbreviation, molecular formula, and chemical structure of the two ILs studied.

Abbreviation
Molecular Formula Chemical Structure dispersive absorption-desorption process using membrane contactors and vacuum desorption as a promising carbon capture technology.

Materials and Characterization
The feed gas of the CO2 capture system was composed of 15% carbon dioxide (99.7, Air Liquide TM , Madrid, Spain and 85% nitrogen (99.9%, Air Liquide TM , Madrid, Spain), which was in the range of typical electro-intensive industries (10-25%) [25].
The IL [emim][MS] (≥95%) was supplied by Sigma Aldrich TM and was selected due to its high values of surface tension and contact angle, moderate values of viscosity, and the presence of physical absorption, which potentially decreases the energy consumption during the solvent regeneration process [26]. The process performance was compared with previous works using the IL [emim][Ac] (≥90%) provided by Sigma Aldrich TM Darmstadt, Germany; which presents chemical absorption (also called chemisorption) [10]. Table 1 shows the identification of the studied solvents. To ensure the stability of the ILs for further CO2 capture experiments, the ILs' decomposition temperatures were calculated by thermogravimetric analysis using a TGA-60H Shimazdu Thermobalance (Izasa, Japan). The viscosity was measured using a rotational viscometer at room temperature. In contrast, in order to evaluate the hydrophobicity/hydrophilicity and wetting behavior of the membranes, the static contact angles between the membrane and different ILs were estimated by the sessile liquid drop method using the contact angle quantification system (DSA25, Krüss,Hamburg Germany). The contact angles were calculated at room temperature and atmospheric pressure. Adjustment of the picolitre dispenser (0.5 mm syringe) and camera image were also done before each component measurement. Then, a 2.0 µ L drop with the desired component was deposited on the membrane's surface at various sites (at least 5 points). Each value was obtained using the software provided through image recognition, and the average contact angle value was then considered.

Membrane Contactor
A hydrophobic polypropylene HFMC (1 × 5.5 MiniModule TM ) supplied by Liqui-Cel TM (3M Madrid, Spain) was used in parallel flow for the continuous absorption-desorption process. The module consists of mesoporous polypropylene hollow fibers with 40% porosity and a mean pore diameter of 0.04 μm. Figure 1 describes the flow configuration and Table 2 shows the specifications of the commercial membrane module used in this work. dispersive absorption-desorption process using membrane contactors and vacuum desorption as a promising carbon capture technology.

Materials and Characterization
The feed gas of the CO2 capture system was composed of 15% carbon dioxide (99.7, Air Liquide TM , Madrid, Spain and 85% nitrogen (99.9%, Air Liquide TM , Madrid, Spain), which was in the range of typical electro-intensive industries (10-25%) [25].
The IL [emim][MS] (≥95%) was supplied by Sigma Aldrich TM and was selected due to its high values of surface tension and contact angle, moderate values of viscosity, and the presence of physical absorption, which potentially decreases the energy consumption during the solvent regeneration process [26]. The process performance was compared with previous works using the IL [emim][Ac] (≥90%) provided by Sigma Aldrich TM Darmstadt, Germany; which presents chemical absorption (also called chemisorption) [10]. Table 1 shows the identification of the studied solvents. To ensure the stability of the ILs for further CO2 capture experiments, the ILs' decomposition temperatures were calculated by thermogravimetric analysis using a TGA-60H Shimazdu Thermobalance (Izasa, Japan). The viscosity was measured using a rotational viscometer at room temperature. In contrast, in order to evaluate the hydrophobicity/hydrophilicity and wetting behavior of the membranes, the static contact angles between the membrane and different ILs were estimated by the sessile liquid drop method using the contact angle quantification system (DSA25, Krüss,Hamburg Germany). The contact angles were calculated at room temperature and atmospheric pressure. Adjustment of the picolitre dispenser (0.5 mm syringe) and camera image were also done before each component measurement. Then, a 2.0 µ L drop with the desired component was deposited on the membrane's surface at various sites (at least 5 points). Each value was obtained using the software provided through image recognition, and the average contact angle value was then considered.

Membrane Contactor
A hydrophobic polypropylene HFMC (1 × 5.5 MiniModule TM ) supplied by Liqui-Cel TM (3M Madrid, Spain) was used in parallel flow for the continuous absorption-desorption process. The module consists of mesoporous polypropylene hollow fibers with 40% porosity and a mean pore diameter of 0.04 μm. Figure 1 describes the flow configuration and Table 2 shows the specifications of the commercial membrane module used in this work.
To ensure the stability of the ILs for further CO 2 capture experiments, the ILs' decomposition temperatures were calculated by thermogravimetric analysis using a TGA-60H Shimazdu Thermobalance (Izasa, Japan). The viscosity was measured using a rotational viscometer at room temperature. In contrast, in order to evaluate the hydrophobicity/hydrophilicity and wetting behavior of the membranes, the static contact angles between the membrane and different ILs were estimated by the sessile liquid drop method using the contact angle quantification system (DSA25, Krüss, Hamburg, Germany). The contact angles were calculated at room temperature and atmospheric pressure. Adjustment of the picolitre dispenser (0.5 mm syringe) and camera image were also done before each component measurement. Then, a 2.0 µL drop with the desired component was deposited on the membrane's surface at various sites (at least 5 points). Each value was obtained using the software provided through image recognition, and the average contact angle value was then considered.

Membrane Contactor
A hydrophobic polypropylene HFMC (1 × 5.5 MiniModule TM ) supplied by Liqui-Cel TM (3M Madrid, Spain) was used in parallel flow for the continuous absorptiondesorption process. The module consists of mesoporous polypropylene hollow fibers with 40% porosity and a mean pore diameter of 0.04 µm. Figure 1 describes the flow configuration and Table 2 shows the specifications of the commercial membrane module used in this work.

Experimental Set-Up
In this section, the process units used for the CO2 capture experimental plant based on non-dispersive absorption using ionic liquids and MVR technology was described. (1) Two hollow fiber membrane contactors that can operate interconnected, for both the non-dispersive absorption of the CO2 from the feed gas and the CO2 desorption by applying vacuum. (2) Two mass flow controllers (Alicat Scientific TM , Duiven, The Netherlands MC-gas mass flow controller USA) to control the flows coming from the pure gas cylinders(CO2 and N2) to set the concentration and flow of the feed gas. rate and CO2 concentration of the gas streams (feed gas, clean gas, and desorbed CO2 output). The analyzer is based on non-dispersive infrared spectroscopy (NDIR). The CO2 concentration in the output gas stream was monitored using the NGA Win-Control software.

Experimental Set-Up
In this section, the process units used for the CO 2 capture experimental plant based on non-dispersive absorption using ionic liquids and MVR technology was described. Figure 2 shows a picture of the experimental system, consisting of: (1) Two hollow fiber membrane contactors that can operate interconnected, for both the non-dispersive absorption of the CO 2 from the feed gas and the CO 2 desorption by applying vacuum. The CO 2 concentration in the output gas stream was monitored using the NGA Win-Control software. (6) A vacuum pump, with condenser included (Vacuubrand TM , PC 3001 VARIO PRO, Wertheim, Germany), to set the gas phase of the membrane contactor (used for CO 2 desorption) at the desired vacuum pressure.
Membranes 2022, 12, x FOR PEER REVIEW 5 of 16 (6) A vacuum pump, with condenser included (Vacuubrand TM , PC 3001 VARIO PRO, Wertheim, Germany), to set the gas phase of the membrane contactor (used for CO2 desorption) at the desired vacuum pressure. In addition, to visualize the system more clearly, Figure 3 shows the flow diagram of the continuous CO2 non-dispersive absorption-desorption plant using membrane contactors with the HFMC configuration.  In addition, to visualize the system more clearly, Figure 3 shows the flow diagram of the continuous CO 2 non-dispersive absorption-desorption plant using membrane contactors with the HFMC configuration.
The IL was recirculated through the lumen side of both HFMCs (absorber and desorber) in a closed loop. The solvent flow rate was kept constant by a digital gear pump. A heater was used in order to provide isothermal conditions during the continuous absorptiondesorption process. The feed gas mixture was introduced through a counter-current at nearly atmospheric pressure across the HFMC absorber shell side in open-loop conditions with a constant flow rate, while the IL passes through the lumen side of the module absorbing the CO 2 . The CO 2 -rich IL was pumped into the HFMC desorber lumen side where the CO 2 was transferred through the gas-filled membrane pores because of the shell side's reduced pressure that was generated by the vacuum pump. The experiments of the continuous absorption-desorption process were running until the CO 2 concentration on both gas outputs (clean gas and CO 2 desorbed) were constant. Moreover, the experiments were carried out three times in order to ensure the results' reproducibility. The data presented in the manuscript were the average values for the set of three experiments, with an experimental error within ±5%. Table 3 shows the process operating conditions. In addition, to visualize the system more clearly, Figure 3 shows the flow diagram of the continuous CO2 non-dispersive absorption-desorption plant using membrane contactors with the HFMC configuration.

Data Analysis
To analyze the performance of the CO 2 desorption process based on MVR technology and its effect on the overall CO 2 capture process, three parameters were mainly studied throughout the work.
(1) The CO 2 desorption efficiency is calculated by Equation (1), where α rich and α lean are the CO 2 loading in the IL (mol CO2·mol_IL −1 ) before and after one pass of IL through the HFMC desorber, respectively.
(2) The overall CO 2 capture efficiency, which is defined as the concentration difference in the HFMC absorber between the feed gas and the clean gas, is obtained by Equation (2), where C (CO2,g) in (mol CO 2 ·L −1 gas) is the CO 2 concentration in the feed gas and C (CO2,g) out (mol CO 2 ·L −1 gas) is the CO 2 concentration at the outlet of the module. The overall CO 2 capture efficiency is important in order to study the influence of the MVR technology on the continuous absorption-desorption process.
Overall CO 2 capture eff. (3) The CO 2 desorbed flux (G V , mol·h −1 m −2 ) is estimated by Equation (3), where F V is the CO 2 flow rate desorbed from the HFMC desorber measured on the vacuum pump output (L·h −1 ), v m is the molar volume of CO 2 in ideal gas conditions (L·mol CO2-1 ), and A is the specific membrane area (m 2 ).

Results and Discussion
The imidazolium ILs, [emim][Ac] and [emim][MS], were studied in this work as chemical and physical CO 2 absorbents, respectively, for non-dispersive absorption technology using ionic liquid and MVR. In this section, the CO 2 absorption capacity for each IL was discussed according to the equilibrium isotherms reported in the literature. Moreover, the experimental results of the absorption-desorption process operating at different MVR operating conditions (desorption vacuum pressures and solvent temperatures) were analyzed.

Absorption Properties
The equilibrium isotherms of the [emim][Ac]-CO 2 and [emim][MS]-CO 2 interactions were experimentally studied by Shifflet et al. [27] and Yim et al. [28]. Figure 4 shows the P-X diagram of CO 2 solubility at different temperatures in both ILs used in this work.

Results and Discussion
The imidazolium ILs, [emim][Ac] and [emim][MS], were studied in this work as chemical and physical CO2 absorbents, respectively, for non-dispersive absorption technology using ionic liquid and MVR. In this section, the CO2 absorption capacity for each IL was discussed according to the equilibrium isotherms reported in the literature. Moreover, the experimental results of the absorption-desorption process operating at different MVR operating conditions (desorption vacuum pressures and solvent temperatures) were analyzed.

Absorption Properties
The equilibrium isotherms of the [emim][Ac]-CO2 and [emim][MS]-CO2 interactions were experimentally studied by Shifflet et al. [27] and Yim et al. [28]. Figure 4 shows the P-X diagram of CO2 solubility at different temperatures in both ILs used in this work.

IL [emim]
[MS] is considered a suitable absorbent for this system at higher partial pressures based not only on its good solvent properties (low volatility and very low viscosity), but also based on its behavior as a physical absorbent of CO2. IL [emim][Ac] is a better overall absorbent when attending to its isotherms and is specifically better at low CO2 partial pressures (where post-combustion CO2 capture processes work) due to the more thermodynamically favorable chemical reaction. Although the chemical IL, [emim][Ac], seems to be a better solvent for CO2 capture applications, the continuous absorption-desorption process not only depends on the CO2 absorption capacity of the solvent, but also on the MVR operation conditions (mainly vacuum pressure and liquid temperature), the CO2-IL chemical and/or physical interactions, and the contactor characteristics, such as membrane geometry and fluid dynamics [29]. In order to investigate the regeneration operation conditions and the IL nature influence on the CO2 desorption performance, a parametric study was considered in the next section.

ILs Characterization
Firstly, TGA analysis shows that the ILs remain without reducing more than 5% of their weight up to temperatures of 450 K and 470 K for [emim][MS] and [emim][Ac], respectively, as depicted in Figure 5. These issues confirm the ionic liquids' stability and their capacity to carry out the absorption-desorption processes under real conditions without damage and degradation.

IL [emim][MS]
is considered a suitable absorbent for this system at higher partial pressures based not only on its good solvent properties (low volatility and very low viscosity), but also based on its behavior as a physical absorbent of CO 2 . IL [emim][Ac] is a better overall absorbent when attending to its isotherms and is specifically better at low CO 2 partial pressures (where post-combustion CO 2 capture processes work) due to the more thermodynamically favorable chemical reaction. Although the chemical IL, [emim][Ac], seems to be a better solvent for CO 2 capture applications, the continuous absorptiondesorption process not only depends on the CO 2 absorption capacity of the solvent, but also on the MVR operation conditions (mainly vacuum pressure and liquid temperature), the CO 2 -IL chemical and/or physical interactions, and the contactor characteristics, such as membrane geometry and fluid dynamics [29]. In order to investigate the regeneration operation conditions and the IL nature influence on the CO 2 desorption performance, a parametric study was considered in the next section.

ILs Characterization
Firstly, TGA analysis shows that the ILs remain without reducing more than 5% of their weight up to temperatures of 450 K and 470 K for [emim][MS] and [emim][Ac], respectively, as depicted in Figure 5. These issues confirm the ionic liquids' stability and their capacity to carry out the absorption-desorption processes under real conditions without damage and degradation.
Moreover, the viscosity of both ILs was measured to ensure the good quality and purity of the samples. In addition, these values were compared with the previous data reported in the literature [30,31]. Finally, the contact angle demonstrates the hydrophilic or hydrophobic characteristics of the membranes with the ILs. Hydrophilic surfaces show low water contact angle (<90 • ) and hydrophobic surfaces show high water contact angle values. fiber membranes used in this work present a hydrophobic character with the ionic liquids. Therefore, membrane wetting is highly avoided. Moreover, the viscosity of both ILs was measured to ensure the good quality and purity of the samples. In addition, these values were compared with the previous data reported in the literature [30,31]. Finally, the contact angle demonstrates the hydrophilic or hydrophobic characteristics of the membranes with the ILs. Hydrophilic surfaces show low water contact angle (<90°) and hydrophobic surfaces show high water contact angle values. Table 4

summarizes the viscosity and measured contact angle values of both the [emim][Ac] and [emim][MS]
ILs. Thus, it is possible to say that the polypropylene hollow fiber membranes used in this work present a hydrophobic character with the ionic liquids. Therefore, membrane wetting is highly avoided.

Parametric Study of Desorption Process
The following part of this study discusses the experimental results (CO2 desorption performance and CO2 desorbed flux) with both the chemical and physical ILs at different operating conditions of the vacuum regeneration system. The process efficiency and the CO2 desorbed flux results, which vary with the operating vacuum pressure and liquid temperature, were analyzed in Figure 6 and Figure 7, respectively.

Parametric Study of Desorption Process
The following part of this study discusses the experimental results (CO 2 desorption performance and CO 2 desorbed flux) with both the chemical and physical ILs at different operating conditions of the vacuum regeneration system. The process efficiency and the CO 2 desorbed flux results, which vary with the operating vacuum pressure and liquid temperature, were analyzed in Figures 6 and 7, respectively.
On the one hand, a lower CO 2 partial pressure (higher vacuum applied) on the permeate side of the HFMC desorber promotes the CO 2 mass transfer driving force through the membrane, which increases the desorption performance as a result of the higher CO 2 desorbed flux. On the other hand, the CO 2 desorption process efficiency and the CO 2 desorbed flux increase with a higher solution temperature. This behavior in both ILs could be explained by the lower viscosity (µ) at higher temperatures, which increases the diffusivity of CO 2 in the absorbent since the mass transfer coefficient is controlled by the liquid-phase mass transfer resistance. Moreover, the CO 2 partial pressure increases because of the higher concentration gradient at higher temperatures, leading to an increase of the CO 2 desorbed flux.
From these experiments, it is clear that the vacuum level and liquid temperature should be as high as possible. However, two process limitations have to be taken into account in order to avoid HFMC operational problems: (i) the pressure applied by the vacuum pump in the permeate side was recommended to be greater than 0.035 bar in order to avoid wetting phenomena and (ii) temperatures higher than 310-320 K may require more resistant membrane materials due to thermal and chemical constraints of the commercial polypropylene HFMC used in our work. Physical IL [emim][MS] showed lower CO 2 desorption performance and CO 2 desorbed flux at the experimentally-tested temperatures (289-313 K) than the corresponding IL [emim][Ac]. This could be explained due to the lower CO 2 loading capacity of the physical IL that leads to a lower CO 2 driving force through the membrane.  On the one hand, a lower CO2 partial pressure (higher vacuum applied) on the permeate side of the HFMC desorber promotes the CO2 mass transfer driving force through the membrane, which increases the desorption performance as a result of the higher CO2 desorbed flux. On the other hand, the CO2 desorption process efficiency and the CO2 desorbed flux increase with a higher solution temperature. This behavior in both ILs could be explained by the lower viscosity (µ ) at higher temperatures, which increases the diffusivity of CO2 in the absorbent since the mass transfer coefficient is controlled by the liquidphase mass transfer resistance. Moreover, the CO2 partial pressure increases because of the higher concentration gradient at higher temperatures, leading to an increase of the CO2 desorbed flux.  On the one hand, a lower CO2 partial pressure (higher vacuum applied) on the permeate side of the HFMC desorber promotes the CO2 mass transfer driving force through the membrane, which increases the desorption performance as a result of the higher CO2 desorbed flux. On the other hand, the CO2 desorption process efficiency and the CO2 desorbed flux increase with a higher solution temperature. This behavior in both ILs could be explained by the lower viscosity (µ ) at higher temperatures, which increases the diffusivity of CO2 in the absorbent since the mass transfer coefficient is controlled by the liquidphase mass transfer resistance. Moreover, the CO2 partial pressure increases because of the higher concentration gradient at higher temperatures, leading to an increase of the CO2 desorbed flux. Furthermore, only chemical [emim][Ac] was able to reach the target of 90% desorption efficiency for vacuum regeneration using the commercial PP-HFMC at the operating conditions (module characteristics given in Table 1). Two main points, which are related to the vacuum pressure effect, could be concluded from the results shown in Figures 6 and 7: (i) Chemical IL [emim][Ac] was less sensitive to vacuum pressure conditions, which could be explained by the fact that the chemical IL requires more energy to break the CO 2 -IL chemical bond, and (ii) physical IL [emim][MS] required the application of higher vacuum than the corresponding IL, [emim][Ac], to reach the same desorption efficiency, which increased the energy consumption for both the vacuum pump and compressor unit operation, resulting in a higher overall cost of the CO 2 vacuum desorption process [32].
In summary, the chemical IL, [emim][Ac], shows a better CO 2 desorption performance and higher CO 2 desorbed flux at different vacuum pressures and liquid temperatures. This could mainly be explained due to the larger capacity of CO 2 to be absorbed into chemical IL by chemisorption, which increases the CO 2 driving force through the membrane in the CO 2 vacuum desorption process. However, the total energy consumption of the solvent regeneration with chemical ILs, such as [emim][Ac], may be expected to be higher than physical ILs, such as [emim] [MS], due to the extra energy required to reverse the CO 2 -IL chemical reaction as previously reported [33,34].

ILs Comparison in the Overall CO 2 Capture
The desorption process efficiency and the CO 2 desorbed flux were evaluated in the previous section at different operation conditions in a continuous steady-state absorptiondesorption CO 2  In this section, the influence of the CO 2 desorption stage (based on MVR technology) in the overall CO 2 capture system was evaluated by Equation (3), which was described in Section 2.4 (Data Analysis). The membrane contactor specifications and the operational conditions were described in Tables 2 and 3. Figure 8 shows the experimental results of the steady state absorption-desorption system at two representative operating conditions in order to evaluate the influence of both solvent temperature (289 K and 313 K) and CO 2 desorption vacuum pressure (0.2 bar and 0.04 bar) on the overall CO 2 capture efficiency for each IL analyzed in this work. , higher vacuum applied in the MVR process increased the overall CO2 capture efficiency from 12% to 21%. However, at the same operating vacuum pressure, the solvent temperature effect on the overall CO2 capture performance was not significant. This could be explained because of both: (i) the decrease in viscosity at the studied temperatures (289 and 313 K, respectively) was less considerable in the CO2 absorption capacity and (ii) the absence of CO2-IL chemical interactions. For chemical IL [emim][Ac], higher solvent temperatures and CO 2 desorption vacuum pressures enhanced the overall CO 2 capture efficiency from 29% to 61%. Moreover, the temperature contribution on the CO 2 capture performance was more significant than the CO 2 desorption vacuum pressure (Pv) since the lower viscosity of the [emim][Ac] at higher temperatures and the chemisorption effect increases the CO 2 absorption capacity. For physical IL [emim][MS], higher vacuum applied in the MVR process increased the overall CO 2 capture efficiency from 12% to 21%. However, at the same operating vacuum pressure, the solvent temperature effect on the overall CO 2 capture performance was not significant. This could be explained because of both: (i) the decrease in viscosity at the studied temperatures (289 and 313 K, respectively) was less considerable in the CO 2 absorption capacity and (ii) the absence of CO 2 -IL chemical interactions.
Previous studies of the single absorption process (without desorption technology) have reported efficiencies of 20-35% with chemical IL [emim][Ac] [35] and 10-12% with physical IL [emim][MS] [31]. Furthermore, in this work, the non-dispersive absorption using ionic liquids and vacuum regeneration, at the most favorable operating conditions studied (0.04 bar and 313 K), increase the CO 2 capture performance up to 61% and 21%, respectively, which represents a percentage increase compared with the only-absorption process of 26% for [emim][Ac] and 9% for [emim] [MS].
Although better absorption-desorption performance is obtained operating at the highest possible temperature due to the lower viscosity of the ILs, the operating temperature was kept below 315 K according to the specifications of the commercial membrane module used in order to avoid chemical or thermal degradation. Therefore, the option of using aqueous [emim][Ac] and [emim][MS] was checked to decrease the solvent viscosities, thereby keeping the temperature in the operational range.

Influence of IL Water Content in the Process Performance
In this section, experiments were carried out to investigate the effect on the continuous steady-state absorption-desorption process of 30%-vol. water content in the ILs. The overall CO 2 capture efficiency was evaluated in Figure 9 at a liquid temperature of 313 K and different vacuum pressures (0.2 and 0.04 bar) by Equation (3)

Conclusions
The main objective has been to study the behavior of ILs with different natures as CO 2 absorbents in terms of process performance. The chemical IL, [emim][Ac], and the physical, [emim][MS], were chosen as representative ILs for this study.
The influence on the CO 2 desorption efficiency and CO 2 desorbed fluxes for different operating conditions has been studied for both ILs. In general, higher temperature and vacuum applied are beneficial to the overall process performance and CO 2 desorbed flux. In particular, the chemical IL, [emim][Ac], showed better CO 2 desorption performance and higher CO 2 desorbed flux than physical IL [emim][MS] at the operating conditions studied. The maximum CO 2 desorption efficiencies obtained in this work were 92% and 83% for chemical IL [emim][Ac] and physical IL [emim][MS], respectively. Both performances were at 0.04 bar vacuum pressure, 313 K temperature, and a 60 mL·min −1 liquid flow rate.
Moreover, the overall CO 2 capture efficiency was evaluated in order to analyze the influence of vacuum regeneration on the continuous absorption-desorption process at different liquid temperatures and vacuum pressures. Higher CO 2 capture efficiency was reached for both ILs at the lowest desorption pressure (0.04 bar) and the highest liquid temperature (313 K) studied in this work. The CO 2 capture efficiencies were 61% and 21% for chemical IL [emim][Ac] and physical IL [emim][MS], respectively. Moreover, the addition of water to reduce the ILs viscosity was evaluated in terms of CO 2 absorptiondesorption system performance. The results indicated that aqueous physical IL [emim] [MS] increases the CO 2 capture while aqueous chemical IL [emim][Ac] decreases the process performance due to the loss of chemical reaction potential by adding water. However, the study of the counterbalance effect of water content into ILs requires further study since water content could significantly affect the CO 2 mass transfer through the membrane, as concluded in this work.
As a whole, the process performances of the chemical IL seem to be better than that of the physical IL in the continuous absorption-desorption CO 2 capture system, while physical ILs could be considered as promising energy-saving absorbents for CO 2 capture by designing more advanced physical ILs capable of absorbing more CO 2 .
From the viewpoint of scale-up, coupled membrane contactors, IL-based processes were addressed as a process intensification for CO 2 capture. However, more studies of the continuous absorption-desorption systems are needed to drive the industrial implementation and commercial viability of non-dispersive absorption technology using ionic liquids and vacuum regeneration.