Mass Transfer Correlation and Optimization of Carbon Dioxide Capture in a Microchannel Contactor: A Case of CO 2 -Rich Gas

: This work focused on the application of a microchannel contactor for CO 2 capture using water as absorbent, especially for the application of CO 2 -rich gas. The inﬂuence of operating conditions (temperature, volumetric ﬂow rate of gas and liquid, and CO 2 concentration) on the absorption e ﬃ ciency and the overall liquid-side volumetric mass transfer coe ﬃ cient was presented in terms of the main e ﬀ ects and interactions based on the factorial design of experiments. It was found that 70.9% of CO 2 capture was achieved under the operating conditions as follows; temperature of 50 ◦ C, CO 2 inlet fraction of 53.7%, total gas volumetric ﬂow rate of 150 mL min − 1 , and adsorbent volumetric ﬂow rate of 1 mL min − 1 . Outstanding performance of CO 2 capture was demonstrated with the overall liquid-side volumetric mass transfer coe ﬃ cient of 0.26 s − 1 . Further enhancing the system by using 2.2 M of monoethanolamine in water (1:1 molar ratio of MEA-to-CO 2 ) boosted the absorption e ﬃ ciency up to 88%.


Introduction
High concentration of carbon dioxide in the synthesis gas or biogas product is one of the major issues regarding the environmental pollution (greenhouse effect) and fuel quality, leading to the requirement of post-treatment process(es) for CO 2 removal. For instance, high CO 2 content in biogas degrades the fuel quality such as the calorific value and anti-knock properties of engine [1]. Over the past decades, many CO 2 separation methods have been proposed such as absorption, adsorption, and membrane separation [2]. Among these methods, CO 2 absorption is the most widely used due to the relatively lower operating cost and higher efficiency [3]. However, amid the successes, the use of toxic chemical solvents has been involved, causing serious impact on the environment (corrosion) and economic sustainability.
A wide range of toxic solvents have been reported for the use of CO 2 removal with the high absorption capacity such as monoethanolamine (MEA) and diethanolamine (DEA) [3,4]. Various achievements of highly effective CO 2 capture have been reported. For example, Sahraie et al. [5] investigated the effect of absorption variables on the CO 2 absorption efficiency using MEA concentration in the range of 15-30 wt.%. It was found that the maximum of absorption efficiency (95%) was obtained liquid streams. The microchannel contactor system and experimental set-up for CO2 absorption is shown in Figure 1.

CO2 Capture Process
In this work, nitrogen and CO2 were separately fed to the system via individual mass flow controllers. These two streams combined at a T-mixer to obtain a CO2-N2 gas mixture at a specific ratio or concentration. A stream of water (green solvent) was fed via an HPLC pump to mix with the CO2-N2 gas mixture at another T-mixer located in the front section of CO2 absorber. Note that both streams were separately preheated to the desired temperature prior to entering in the T-micromixer. The absorption process took place in the microtube contactor which was immersed in a hot water bath to control the absorption temperature. The exit-end of the microtube was connected to a separator that has two outlets; one for gas product and another for liquid product. The pressure of the system was controlled by means of a back-pressure regulator connected to the gas product stream. The gas mixture exiting through the back-pressure regulator was analyzed for CO2 content using a CO2 detector (COZIR Wide Range GC-0016). The absorption pressure was kept constant at 1.7 bar. Note that, the effect of pressure was assumed negligible in our system. The valve for liquid product was adjusted to ensure that no liquid was accumulated in the separator where the CO2 mass transfer was neglected due to the very short contact time of CO2 and water. The experimental range of absorption variables are summarized in Table 1.

Mass Transfer Coefficient Calculation
In this work, the physicochemical absorption took place in the CO2-H2O absorption system. The absorption of N2 in water was neglected in this work since the N2 can barely dissolve in water. The

CO 2 Capture Process
In this work, nitrogen and CO 2 were separately fed to the system via individual mass flow controllers. These two streams combined at a T-mixer to obtain a CO 2 -N 2 gas mixture at a specific ratio or concentration. A stream of water (green solvent) was fed via an HPLC pump to mix with the CO 2 -N 2 gas mixture at another T-mixer located in the front section of CO 2 absorber. Note that both streams were separately preheated to the desired temperature prior to entering in the T-micromixer. The absorption process took place in the microtube contactor which was immersed in a hot water bath to control the absorption temperature. The exit-end of the microtube was connected to a separator that has two outlets; one for gas product and another for liquid product. The pressure of the system was controlled by means of a back-pressure regulator connected to the gas product stream. The gas mixture exiting through the back-pressure regulator was analyzed for CO 2 content using a CO 2 detector (COZIR Wide Range GC-0016). The absorption pressure was kept constant at 1.7 bar. Note that, the effect of pressure was assumed negligible in our system. The valve for liquid product was adjusted to ensure that no liquid was accumulated in the separator where the CO 2 mass transfer was neglected due to the very short contact time of CO 2 and water. The experimental range of absorption variables are summarized in Table 1.

Mass Transfer Coefficient Calculation
In this work, the physicochemical absorption took place in the CO 2 -H 2 O absorption system. The absorption of N 2 in water was neglected in this work since the N 2 can barely dissolve in water. The transport phenomena of CO 2 can be described based on the two-film theory. First, CO 2 from the bulk gas phase is transferred into the gas film. Then CO 2 diffuses into the gas-liquid interface and is subsequently transferred across the liquid film out into the bulk of liquid. Hence, the CO 2 mass transfer flux (N CO 2 ) can be calculated via Equations (1) and (2).
where n CO 2 is the mole of CO 2 ; a is the gas-liquid interfacial area per unit reactor volume; V R is the reactor volume; and k G and k L are the individual gas-side and liquid-side mass transfer coefficient, respectively. In our absorption system, the gas-side mass transfer coefficient could be neglected due to the low solubility of CO 2 in water. Consequently, the overall liquid-side mass transfer coefficient (K L ) was approximately equal to the liquid-side mass transfer coefficient (k L ). Since the concentration of CO 2 at the gas-liquid interface is not readily measurable, the CO 2 absorption flux can be expressed in terms of CO 2 concentration (C i CO 2 ) and the overall liquid-side mass transfer coefficient (K L ) (see Equation (3)). Henry's Law was used to describe the linear relationship between the partial pressure of CO 2 in the gas phase and C i CO 2 (Equation (4) [14]).
where H CO 2 is the Henry's coefficient. The average pressure of CO 2 in the bulk gas can be expressed in terms of a logarithmic mean pressure difference based on the inlet and outlet conditions [15] as shown in Equation (5).
The Henry constant and diffusion coefficient of CO 2 in H 2 O as a function of temperature were obtained from the literature by Versteeg and Swaaij [16], and Karlsson and Svensson [17] (Equations (6) and (7)). The absorption efficiency is expressed in Equation (8).

Main Effect of Absorption Variables
The main effect of variables including the amount of CO 2 , volumetric flow rate of water, volumetric flow rate of gas mixture, and absorption temperature on the mean absorption efficiency (%) Energies 2020, 13, 5465 5 of 15 was evaluated based on the 3 k factorial design (see Table 1). The main effect results shown in Figure 2a indicate a strong negative effect on the mean absorption efficiency when either the volumetric flow rate of liquid or gas was increased. This was due to the short absorption time (contact time) between gas phase and liquid phase. A similar trend was observed for the overall liquid-side volumetric mass transfer coefficient (K L a) (see Figure 2b) which decreased by increasing the volumetric flow rate. To further describe this behavior, the effect of volumetric flow rates of gas (150 to 200 mL min −1 ) and liquid (1 to 2 mL min −1 ) on the absorption efficiency was plotted when other variables were held constant (CO 2 inlet fraction of 0.5 and absorption temperature of 40 • C) as shown in Figure 3. The significant decline of absorption efficiency was observed when the volumetric flow rate of liquid was increased. For instance, at the volumetric flow rate of gas of 175 mL min −1 , the absorption efficiency was dramatically reduced from 62.1% to 16.2% when the volumetric flow rate of liquid was increased from 1 mL min −1 to 2 mL min −1 , resulting in the poor mass transfer rate of CO 2 to water (see Figure 3b). Similar behavior was observed when the total flow rate of liquid was increased instead of the volumetric flow rate of liquid ( Figure 3).
increased instead of the volumetric flow rate of liquid ( Figure 3).
For high CO2 concentration level, the adsorption capacity of CO2 in water proceeded via the hydration reaction. The series of reactions involved were as follows: CO2 (aq) +H2O ↔ H2CO3 These reactions could enhance both the solubility of CO2 in water and the absorption efficacy. In order to confirm this, a separate CO2 absorption experiment was carried out while the conductivity of the liquid stream exiting the CO2 absorption apparatus was monitored. The operating conditions were at 40% of CO2 inlet concentration, temperature of 30 °C, 150 mL min −1 of gas flow rate, and 1 mL min −1 of liquid flow rate. The conductivity probe was placed inside a round-bottom glass tube with two ports on the side; one for the incoming liquid stream from the apparatus located near the bottom and another one (located at the position such that the probe was sufficiently immersed) for output stream going to the waste collector. The tube was initially filled with deionized water. It was observed that the conductivity of liquid stream significantly rose from 4.4 μS cm −1 (pure DI water) and reached equilibrium at 37.5 μS cm −1 . The change in conductivity of the water was due to the existence of bicarbonate ion (carbonic acid). Note that, the CO2 (aq) in water cannot increase the conductivity of the water. This observation was also in line with the work of Bhaduri et al. [18] who investigated the improvement of CO2 capture by enhancing the CO2 hydration reaction. The high rate of CO2 hydration was evidently observed when the pure CO2 was used.  The low impact on the mean absorption efficiency was observed for the cases of the absorption temperature and CO2 fraction in the feed ( Figure 2). In case of absorption temperature, the absorption efficiency was approximately unchanged at temperatures below 40 °C and slightly increased at 50 °C. This behavior involved the change of physical properties of CO2 with temperature. For instance, increasing temperature can help increase the CO2 diffusion coefficient [19]; however, the content of CO2 dissolved in water (CO2 solubility) is also decreased [20,21]. This means that the effect of For high CO 2 concentration level, the adsorption capacity of CO 2 in water proceeded via the hydration reaction. The series of reactions involved were as follows: These reactions could enhance both the solubility of CO 2 in water and the absorption efficacy. In order to confirm this, a separate CO 2 absorption experiment was carried out while the conductivity of the liquid stream exiting the CO 2 absorption apparatus was monitored. The operating conditions were at 40% of CO 2 inlet concentration, temperature of 30 • C, 150 mL min −1 of gas flow rate, and 1 mL min −1 of liquid flow rate. The conductivity probe was placed inside a round-bottom glass tube with two ports on the side; one for the incoming liquid stream from the apparatus located near the bottom and another one (located at the position such that the probe was sufficiently immersed) for output stream going to the waste collector. The tube was initially filled with deionized water. It was observed that the conductivity of liquid stream significantly rose from 4.4 µS cm −1 (pure DI water) and reached equilibrium at 37.5 µS cm −1 . The change in conductivity of the water was due to the existence of bicarbonate ion (carbonic acid). Note that, the CO 2 (aq) in water cannot increase the conductivity of the water. This observation was also in line with the work of Bhaduri et al. [18] who investigated the improvement of CO 2 capture by enhancing the CO 2 hydration reaction. The high rate of CO 2 hydration was evidently observed when the pure CO 2 was used.
The low impact on the mean absorption efficiency was observed for the cases of the absorption temperature and CO 2 fraction in the feed (Figure 2). In case of absorption temperature, the absorption efficiency was approximately unchanged at temperatures below 40 • C and slightly increased at 50 • C. This behavior involved the change of physical properties of CO 2 with temperature. For instance, increasing temperature can help increase the CO 2 diffusion coefficient [19]; however, the content of CO 2 dissolved in water (CO 2 solubility) is also decreased [20,21]. This means that the effect of diffusion was dominant over the effect of solubility at high absorption temperature. For the effect of CO 2 inlet fraction, the mean absorption efficiency improved when the CO 2 inlet fraction increased from 40% to 50% due to the large mass transfer flux (N CO 2 ) as a result of high CO 2 feed concentration as a driving force (see Equation (3)). Further increasing in the CO 2 feed concentration (up to 60%) led to a slight decline of CO 2 absorption efficiency, as the system was approaching the limit of CO 2 solubility in water. These findings were confirmed when plotting the CO 2 inlet fraction (40 to 60%) with the absorption temperature (30 to 50 • C) while holding the volumetric flow rate of 175 mL min −1 and 1.5 mL min −1 for gas and liquid streams, respectively (Figure 4). At the same CO 2 inlet fraction, a slight effect of reaction temperature on the absorption efficiency was observed. For the case of CO 2 fraction, the highest absorption efficiency was obtained for the CO 2 inlet fraction of 50%.
Energies 2020, 13, 5465 7 of 15 solubility in water. These findings were confirmed when plotting the CO2 inlet fraction (40 to 60%) with the absorption temperature (30 to 50 °C) while holding the volumetric flow rate of 175 mL min −1 and 1.5 mL min −1 for gas and liquid streams, respectively (Figure 4). At the same CO2 inlet fraction, a slight effect of reaction temperature on the absorption efficiency was observed. For the case of CO2 fraction, the highest absorption efficiency was obtained for the CO2 inlet fraction of 50%.

Interaction Effect of Absorption Variables
The interaction effect of absorption variable pairs can be elaborated through both P-value and contour plot. When the P-value is larger than 0.05 or the slope of contour line is straight and parallel, the interaction effect is not statistically significant. The ANOVA results as shown in Table 2 indicate that the interaction effect between the total gas volumetric flow rate (G) and liquid volumetric flow rate (L) was statistically significant. This behavior was related to the contact time between gas phase and liquid phase which was inversely proportional to the volumetric flow rate. The contact time was relatively shorter at high volumetric flow rate of gas and high volumetric flow rate of liquid, and vice versa. The contour plot of this variable pair is shown in Figure 5a. At high level of the total gas volumetric flow rate, the absorption efficiency increased slightly with decreasing volumetric flow rate of liquid. A similar behavior was observed with higher sensitivity at low level of the total gas volumetric flow rate, signifying the interaction effect of these variables. versa. The contour plot of this variable pair is shown in Figure 5a. At high level of the total gas volumetric flow rate, the absorption efficiency increased slightly with decreasing volumetric flow rate of liquid. A similar behavior was observed with higher sensitivity at low level of the total gas volumetric flow rate, signifying the interaction effect of these variables. On the contrary, the interaction effect of other variable pairs was not statistically significant due to the large p-value (>0.05). This was confirmed by the contour plots in Figure 5b,c.

Flow Pattern of CO2 Capture Process
According to the literature, the superficial velocity of liquid and gas plays an important role on the gas-liquid flow pattern in a microreactor system. Hassan et al. [22] described the flow pattern of gas-liquid inside the microreactor system by analyzing the literature data for gas-liquid systems and On the contrary, the interaction effect of other variable pairs was not statistically significant due to the large p-value (>0.05). This was confirmed by the contour plots in Figure 5b,c.

Flow Pattern of CO 2 Capture Process
According to the literature, the superficial velocity of liquid and gas plays an important role on the gas-liquid flow pattern in a microreactor system. Hassan et al. [22] described the flow pattern of gas-liquid inside the microreactor system by analyzing the literature data for gas-liquid systems and developed a universal map of gas-liquid flow regime (using the superficial velocity of gas and liquid as coordinates) to predict the flow pattern of the system. Note that, similar flow pattern maps were also found in the literature for different hydraulic diameters between 0.1 mm to 1 mm, also with different materials [23,24]. Under the condition of high superficial velocity of gas and low superficial velocity of liquid, the slug-annular flow pattern was observed. This observation was confirmed by the experiment of Yue et al. [25], who studied the flow pattern of CO 2 -water inside a microchannel contractor with the hydraulic diameter of 0.667 mm and observed the slug-annular flow pattern when using high superficial velocity of gas and low superficial velocity of liquid. Our microreactor reactor had a hydraulic diameter of 0.5 mm and was operated with high superficial velocity of gas (10-13 m s −1 ) and high superficial velocity of liquid (0.07-0.13 m s −1 ). Under these conditions, our gas-liquid flow pattern would fall into the churn regime or slug-annular flow pattern, contributing to the large interfacial area between the liquid and gas phases [25].

Correlation Model
The correlation between the response (%absorption efficiency) and the operating variables of our absorption system (CO 2 fraction (F), total gas volumetric flow rate (G), liquid volumetric flow rate (L), and temperature (T)) was be evaluated by regression analysis. The result is shown in Equation (11). The optimization model indicated that the volumetric flow rate of gas and the volumetric flow rate of liquid strongly influenced the absorption efficiency. This was in agreement with the optimization of other absorption systems reported in the literature [7,26]. The precision of this model was verified based on the coefficient of determination (R 2 ) by the parity plot between the experimental and predicted values in terms of the response parameter as shown in Figure 6a. The R 2 value of 0.9 suggested that the accuracy of model prediction was reasonable.

Optimization
The objective function for optimization was set to maximize the absorption efficiency. The optimization was performed based on the response surface methodology and the optimal conditions with the maximum %absorption of 70.9% were found at the temperature of 50 °C, CO2 fraction of 53.7%, the total volumetric flow rate of gas of 150 mL min −1 , and the volumetric flow rate of water of 1 mL min −1 . Under these conditions, KLa of 0.26 s −1 was calculated. This will be compared with other systems of CO2 absorption using water in Section 3.6. It is noted that this technique may be particularly useful for reducing the costs associated with chemical absorption for CO2 removal [32,33]. Although the quality of water in practical applications can also varied drastically such as tap water, underground water, and wastewater, this result can be used as a guideline for choosing the operating conditions.

Comparison of the Liquid-Side Volumetric Mass Transfer Coefficient for Different Systems
The performance of our system for CO2 capture application was compared with that of the other systems as summarized in Table 3. Although the pressure of the system was not the same in each absorption process, the effect of pressure on the KL was not significantly observed when the low pressure was used (1-2 bar). This was in line with several reports [10,34]. Note that, the effect of pressure on the KL would be noticeable when the absorption system is operated at pressures exceeding 10 bar [35].
As compared among different absorption systems, our system offered the largest KLa coefficient for CO2 absorption in water (0.02-0.29 s −1 ), implying superior CO2 capture efficiency. This was due to the unique characteristics of microchannels. For instance, high surface-to-volume ratio of microchannel (3400-9000 m 2 m −3 ), which is considerably larger than the other absorption devices, promotes heat and mass transfer rates [25,34]. This was in line with the KLa results and surface-tovolume ratio of the other absorbers such as packed column (0.0127 s −1 , 10-350 m 2 m −3 ) [36] and stirred tank (0.0056-0.0333 s −1 , 100-2000 m 2 m −3 ) [37]. Our work also presents a superior absorption performance when compared to the physical-chemical absorption in a microchamber reported by Zhu et al. [33] who investigated the CO2 capture efficiency under a broad range of gas volumetric flow rate (up to 300 mL min −1 ). Note that when the extremely short absorption time was applied, the physical absorption was dominant especially in the region of the high volumetric flow rate of gas and liquid. This was owing to the fact that the contact time in our system was longer, allowing for better performance. Lower absorption efficiency of our system compared to that of the hollow fiber The overall liquid-side volumetric mass transfer coefficient of the system (K L a) can be expressed in terms of dimensionless parameters consisting of a group of physical properties and geometry of the system [27,28], i.e., Reynold number (Re), Sherwood number (Sh), Schmidt number (Sc), and capillary number (Ca). The correlation is presented in Equation (12). Note that, the K L a is embedded in the Sherwood number as modified Sherwood number (Sh*) (see Equation (13)). The correlation of our model was in line with those reported in the literature [29,30].
The accuracy of this model was validated as presented in Figure 6b. The mean square errors (MSE) of the efficiency model and overall liquid-side volumetric mass transfer coefficient of the system were 4.1% and 4.9%, respectively. The deviation of the model especially for the cases of low absorption efficiency and small K L a was possibly caused by the unstable pressure inside the system when the high volumetric flow rate of gas and liquid conditions were applied. The change of pressure can affect the hydrodynamic behavior, interfacial area, and mass transfer coefficient [31]. However, the effect of pressure on the absorption efficiency and K L a was little when compared to the other variables, i.e., temperature, and volumetric flow rate of gas and liquid [10].

Optimization
The objective function for optimization was set to maximize the absorption efficiency. The optimization was performed based on the response surface methodology and the optimal conditions with the maximum %absorption of 70.9% were found at the temperature of 50 • C, CO 2 fraction of 53.7%, the total volumetric flow rate of gas of 150 mL min −1 , and the volumetric flow rate of water of 1 mL min −1 . Under these conditions, K L a of 0.26 s −1 was calculated. This will be compared with other systems of CO 2 absorption using water in Section 3.6. It is noted that this technique may be particularly useful for reducing the costs associated with chemical absorption for CO 2 removal [32,33]. Although the quality Energies 2020, 13, 5465 10 of 15 of water in practical applications can also varied drastically such as tap water, underground water, and wastewater, this result can be used as a guideline for choosing the operating conditions.

Comparison of the Liquid-Side Volumetric Mass Transfer Coefficient for Different Systems
The performance of our system for CO 2 capture application was compared with that of the other systems as summarized in Table 3. Although the pressure of the system was not the same in each absorption process, the effect of pressure on the K L was not significantly observed when the low pressure was used (1-2 bar). This was in line with several reports [10,34]. Note that, the effect of pressure on the K L would be noticeable when the absorption system is operated at pressures exceeding 10 bar [35].
As compared among different absorption systems, our system offered the largest K L a coefficient for CO 2 absorption in water (0.02-0.29 s −1 ), implying superior CO 2 capture efficiency. This was due to the unique characteristics of microchannels. For instance, high surface-to-volume ratio of microchannel (3400-9000 m 2 m −3 ), which is considerably larger than the other absorption devices, promotes heat and mass transfer rates [25,34]. This was in line with the K L a results and surface-to-volume ratio of the other absorbers such as packed column (0.0127 s −1 , 10-350 m 2 m −3 ) [36] and stirred tank (0.0056-0.0333 s −1 , 100-2000 m 2 m −3 ) [37]. Our work also presents a superior absorption performance when compared to the physical-chemical absorption in a microchamber reported by Zhu et al. [33] who investigated the CO 2 capture efficiency under a broad range of gas volumetric flow rate (up to 300 mL min −1 ). Note that when the extremely short absorption time was applied, the physical absorption was dominant especially in the region of the high volumetric flow rate of gas and liquid. This was owing to the fact that the contact time in our system was longer, allowing for better performance. Lower absorption efficiency of our system compared to that of the hollow fiber membrane was because of the lower interfacial area of gas and liquid; however, the difficulty related to the flow bypassing and channeling of the liquid is a trade-off.
In terms of water utilization, our system offered much lower flow ratio between liquid and gas. Regarding the footprint, the size of absorber can be substantially reduced using microchannel absorber as compared to the conventional packed bed. For example, based on our experimental results, only a small bundle of approximately 454 microchannels operating in parallel can handle the input gas flow rate of 6.8 × 10 4 mL min −1 . This small footprint also offers the potential for reduced power consumption, investment and maintenance cost. Therefore, microchannel can be effectively applied as an alternative device for CO 2 capture applications.

Physicochemical Absorption
The findings from our experiments indicate that the maximum absorption efficiency of 70.9% (gas purity of 84%) was obtained by using water as a green absorbent. Since the purity of product gas at the maximum absorption efficiency was lower than 90%, we then extended our findings to the physicochemical absorption using MEA, where the required amount of solvent used significantly affects the process viability. Note that, in general chemisorption process, relatively high concentration of solvent is required to achieve high CO 2 absorption efficiency, i.e., 30 wt.% of MEA solution [3,4]. Our assumption was that a slight addition of chemical solvent (much less than what is commonly used) to our CO 2 absorption system would fulfill the requirement in terms of absorption efficiency.
To verify this statement, we adapted our CO 2 absorption system by adding a little amount of MEA into the water. Three sets of experiment were carried out to demonstrate the possibility of using this system to further enhance the absorption efficiency. The experiments were carried out at constant temperature of 40 • C, MEA concentration of 2.2 M (1:1 molar ratio of MEA-to-CO 2 ), liquid flow rate of 1 mL min −1 , and volumetric flow rate of gas of 150 mL min −1 , while the CO 2 fraction (40-60 vol.%) were varied. As shown in Figure 7, the addition of small amount of MEA could significantly enhance the %absorption by more than 37% for various levels of CO 2 feed concentration. For example, for the CO 2 fraction of 40%, using MEA of 1 mol% could enhance the %absorption from 56.2% to 80.2%, suggesting that small amount of solvent was adequate to achieve high absorption efficiency. It is also possible to extend our findings for the application CO 2 absorption using wastewater containing ammonia which can be related to many industries such as fertilizer, rubber processing, leather manufacturing, etc. However, this requires further investigation on the effect of operating conditions and characteristics of absorbent on the efficiency of CO 2 absorption.

Conclusions
The findings of this work indicate that the absorption of CO2 from the gas using water in a microchannel contactor was efficient compared to other absorbers in terms of the overall liquid-side volumetric mass transfer coefficient. The influence of operating conditions including temperature, gas and liquid volumetric flow rate on the %absorption and overall liquid-side volumetric mass transfer coefficient was significant. The maximum %absorption of 70.9% was achieved at temperature of 50 °C, CO2 fraction of 53.7%, total volumetric flow rate of gas of 150 mL min −1 , and volumetric flow rate of water of 1 mL min −1 . It was also demonstrated that this system can be much improved by adding little amount of MEA in the liquid absorbent. The %absorption up to 88% was achieved by using 2.

Conclusions
The findings of this work indicate that the absorption of CO 2 from the gas using water in a microchannel contactor was efficient compared to other absorbers in terms of the overall liquid-side volumetric mass transfer coefficient. The influence of operating conditions including temperature, gas and liquid volumetric flow rate on the %absorption and overall liquid-side volumetric mass transfer coefficient was significant. The maximum %absorption of 70.9% was achieved at temperature of 50 • C, CO 2 fraction of 53.7%, total volumetric flow rate of gas of 150 mL min −1 , and volumetric flow rate of water of 1 mL min −1 . It was also demonstrated that this system can be much improved by adding little amount of MEA in the liquid absorbent. The %absorption up to 88% was achieved by using 2.2 M of MEA (1:1 molar ratio of MEA-to-CO 2 ). Acknowledgments: Financial support from Kasetsart University Research and Development Institute was acknowledged.

Conflicts of Interest:
The authors declare no conflict of interest.