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Removal of Carbon Dioxide by Absorption in Microporous Tube-in-Tube Microchannel Reactor Na-Na Gao,† Jie-Xin Wang,*,† Lei Shao,‡ and Jian-Feng Chen*,‡ †
Key Lab for Nanomaterials, Ministry of Education, and ‡Research Center of the Ministry of Education for High Gravity Engineering and Technology, Beijing University of Chemical Technology, Beijing 100029, P.R. China ABSTRACT: In this article, preliminary experimental results are presented on the absorption of carbon dioxide (CO2) in a novel high-throughput microporous tube-in-tube microchannel reactor (MTMCR), with an aqueous solution of monoethanolamine (MEA) and a mixture of CO2/N2 as the working fluids. The effects of design and operating parameters on the CO2 removal efficiency were investigated. The absorbent concentration was given the priority as a key factor for consideration, with the result that the CO2 removal efficiency increased with increasing concentrationan and could reach 90% or even higher at a high throughput of 440 L/h for gas with an MEA concentration of 30 wt %. With a decrease of the superficial gas velocity or an increase of the superficial liquid velocity, the CO2 removal efficiency increased. Increasing the absorbent temperature yielded better absorption performance. Reducing the most important structural parameters of the MTMCR, such as the micropore size and the annular channel width, led to a higher mass-transfer rate and was beneficial for CO2 removal. This work also investigated the characteristics of the pressure drop of two-phase flows through the MTMCR. The results obtained imply a great potential for MTMCRs applied to the separation of the greenhouse gas CO2 from the exhausted gases.
1. INTRODUCTION Carbon dioxide (CO2) is an important greenhouse gas, contributing to the global warming problem, and has received great attention around the world. Many countries have agreed to reduce the emissions of greenhouse gases to the atmosphere or at least to keep them at the current levels, which challenged researchers to mitigate the global warming effect by curtailing the increase of the CO2 concentration in the atmosphere mainly through its emission from the combustion of fossil fuels.13 CO2 emissions from industrial waste gases, particularly flue gases from coal-fired power plants, have become a major target for reduction, and the addition of a CO2 removal system is a possible way for such plants to meet this target.4 The removal of CO2 from gas streams can be achieved by various techniques, including chemical absorption, physical absorption, cryogenic methods, and membrane separation. CO2 absorption by chemical solvents, such as aqueous solutions of alkanolamine, is generally recognized as one of the most effective methods.512 Monoethanolamine (MEA) solution has been the most widely used alkanolamine absorbent owing to its excellent proterties.13,14 However, it also has some disadvantages such as high regeneration energy, corrosion, solvent degradation, and a maximum CO2 absorption capacity limited by stoichiometry to 0.5 mol of CO2/mol of amine. Current efforts to the make absorption process more efficient and reduce the operating costs include the development of alternative solvents to MEA, the implementation of energy integration with other sections of the power plant, and the use of innovative process configurations or contactors. Even for alternative solvents, it is critical to devise gasliquid contactors that allow effective gas absorption to solve the problem of significant mass-transfer limitations existing in conventional gasliquid contactors, so more studies have appeared in which attention has focused on the reactive mass-transfer system of CO2/MEA with the r 2011 American Chemical Society
application of the effective contactors, especially rotating packed beds and membrane contactors.1529 Recently, it has become attractive to investigate the performance of microreactors for CO2 absorption, because microreaction technology has experienced spectacular development in the past decade. The characteristic micrometer dimensions, the extremely large surface-to-volume ratio, and the short transport path in microreactors enhance heat and mass transfer dramatically, which contributes to distict advantages such as effective mixing, easy scale-up, enhancement of chemical product quality, and increased safety,3033 and microreactors have been extended to many valuable applications in the fields of chemistry, chemical engineering, biochemistry, and pharmacy.3442 When gas absorption is performed in a microchannel reactor, the gasliquid interfacial areas are expected to be very large, and the gasliquid mass-transfer rates are greatly increased. Many investigators have performed experiments to confirm that the process intensification of gasliquid mass transfer can be obtained in microreactors. For instance, Tegrotenhuis et al.37 investigated the performance of microchannels in the gas absorption process and found that over 90% of the carbon dioxide was removed by diethanolamine (DEA) solution in less than 10 s from a stream containing 25% CO2. It was suggested that microchannel technology could miniaturize chemical separation equipment by an order of magnitude or more, by decreasing the height equivalent of a theoretical plate (HETP) to a fraction of a centimeter. Recently, Yue et al.43 systematically studied the hydrodynamics and mass-transfer characteristics of cocurrent Received: December 13, 2010 Accepted: March 19, 2011 Revised: March 9, 2011 Published: April 06, 2011 6369
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Industrial & Engineering Chemistry Research gasliquid flow with the absorption of pure CO2 into water, buffer solution, and NaOH solution through a rectangular microchannel. Comparison of the mass-transfer performance among different gasliquid contactors revealed that gasliquid microchannel contactor can provide at least one or 2 orders of magnitude higher liquid-side volumetric mass-transfer coefficients and interfacial areas than traditional gasliquid contactors.43 Advantages offered by microreactors have been cited in many literature reports.3033 However, the maximum throughputs of most reported microreactors are much smaller than those of conventional reactors, making it hard for them to meet the demands of industrial applications. To obtain a high throughput, a so-called numbering-up method for scale-up of microreactors is employed.43,44 However, it should be noted that, until now, this concept has been insufficiently implemented from a technical point of view. The main problem is how to ensure equal flow distribution with minimal pressure loss in each microchannel.45 Another difficulty is that the high expenditure of fabrication in numbering-up.46 Therefore, it is significant to develop an alternative approach that depends on the improvement of the microreactor itself to achieve high throughput, even flow distribution, and easy operation. In our laboratory, a high-throughput microporous tube-intube microchannel reactor (MTMCR) was first designed and fabricated as a novel liquidliquid reactor and gasliquid contactor.47,48 In the MTMCR, two tubes are configured coaxially to form an annular microchannel, and micropores on the annular surface of one end of the inner tube are used for dispersion. The gas flowing through the annular micropore belt is dispersed into many small separate streams, followed by the high-speed impinging crosscurrently with the laminar flow in the chamber between the inner and outer tubes within a very short time. Thus, the MTMCR will exhibit the combined action of many T-type microchannels configured in parallel circumferentially and realize a high-throughput capacity. CO2 absorption processes as model gasliquid contact systems in an MTMCR have been investigated in water, NaHCO3/Na2CO3 buffer solution, and NaOH solution.48 The experimentally measured liquid-side volumetric mass-transfer coefficients in the MTMCR were at least one or 2 orders of magnitude higher than those in conventional gasliquid contactors and close to those of typical microchannel reactors owing to a superhigh interfacial area of 2.2 105 m2/m3, which was 20 times higher than that in microchannel reactors. Furthermore, the gas and liquid throughputs were over 60 times higher than those of T-type microchannels. The objective of this work was to systematically investigate the absorption of CO2 by utilizing MEA solution as the absorbent in an MTMCR. The effects of operating parameters such as gas flow rate, liquid flow rate, liquid temperature, absorbent concentration, mean micropore size, annular channel width, and MTMCR length were explored. The characteristics of the pressure drop of two-phase flows through the MTMCR during the absorption process are first presented in detail.
2. EXPERIMENTAL SECTION A schematic diagram of the experimental apparatus for gas absorption is shown in Figure 1. The MTMCR as the key part of the experimental setup consists of two same-axis tubes, that is, an inner tube and an outer tube. Many micropores are distributed around the wall near the entrance end of the inner tube. Figure 2a,b shows photographs of the MTMCR and the micropore section.
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Figure 1. Schematic diagram of the experimental apparatus: (1) N2 þ CO2 cylinder, (2) gas flow meter, (3) feed reservoir, (4) peristaltic pump, (5) constant-temperature water container, (6) microporous tube-in-tube microreactor, (7) phase separator, (8) infrared gas analyzer, (9) liquid receiver, (10) differential pressure transducer, (A,B) sample points.
Figure 2. Microporous tube-in-tube microchannel reactor: (a) photograph of the MTMCR, (b) Microporous section.
The microporous section is composed of several layers of metal mesh. In this study, inner tubes with mean micropore sizes (dm) of 10, 20, 40, and 80 μm were employed. The outer diameter of the inner tube was 15 mm, and the inner diameters of the outer tubes used were 15.5, 16, 16.5, and 17 mm. Thus, the mixing chamber widths (da), that is, the distances between the inner and outer tubes, were 250, 500, 750, and 1000 μm, respectively. In addition, the length of the micropore section was 17 mm, and the length of the mixing chamber was 156 mm. In addition to the outlet, two sampling points were designated along the axial position of the outer tube. The distances from the micropore section to sampling points A and B were 16 and 71 mm, respectively. More details about MTMCR can be found in our previous publications.47,48 Prior to the experiments, an aqueous solution of MEA was prepared in the feed tank by diluting analytical-reagent-grade MEA (Beijing Reagent Factory of China) with deionized water to a given concentration (6, 12, 18, 24, or 30 wt %). During operation, a N2 gas stream containing 10.0 vol % CO2 (Beijing Ruyuanruquan Technology Co, Ltd.) from the gas cylinder was conveyed by a pressure-regulating valve and introduced into the inner tube of the MTMCR. Simultaneously, the aqueous absorbent was pumped from the storage tank and flowed into the outer tube. After impinging with the liquid flow in the chamber between the inner and outer tube crosscurrently and at high speed, the gasliquid mixture flowed downward into a phase separator through a connection tube. The liquid phase was sewed, and the gas phase was introduced into an infrared gas analyzer (XLZ1090GXH, Beijing Xilinzi Technology Co., Ltd., China). The analyzer could measure CO2 concentrations of up to 10.0 vol % with an accuracy of (1% full scale. The separator was small enough to neglect the absorption in the phase separator, as verified by a blank experiment. In addition, a constant-temperature water bath was used to control 6370
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Figure 3. Effects of absorbent concentration and gas throughput on removal efficiency (T = 303 K, QL = 8.8 L/h, da = 500 μm, dm = 10 μm).
Figure 4. Effects of superficial gas and liquid velocities on removal efficiency (T = 303 K, 12 wt % MEA, da = 500 μm, dm = 10 μm).
the temperatures of both the reactor and the absorbent entering the reactor. All of the absorption experiments were conducted under atmospheric pressure. The pressure drop between the inlet and the outlet of the MTMCR was measured by an intelligent differential pressure transmitter (ZP3051, Beijing Zhongpusanke Sensing Technology, Co., Ltd.). One end of the test section was close to the gas inlet, and the other end was placed near the outlet of the MTMCR. The mixture of two-phase flow from the outlet of the MTMCR was directly exposed to air. The experiments were conducted at 300 K and atmospheric pressure. Each experimental data point was taken as the average of five determinations.
3. RESULTS AND DISCUSSION The experimental results obtained as CO2 concentration profiles reflect the change in CO2 content in gas phase, which was used to evaluate the CO2 absorption performance of MEA solution in an MTMCR according to CO2 removal efficiency. The removal efficiency was simply determined from the difference between the amounts of CO2 entering and leaving the MTMCR, expressed as " ! !# yCO2 , out 1 yCO2 , in removal efficiency ¼ 1 1 yCO2 , out yCO2 , in 100% ð1Þ where yCO2,in and yCO2,out denote the mole fractions of gas-phase CO2 entering and leaving the MTMCR, respectively. 3.1. Effect of Absorbent Concentration and Gas Throughput. Figure 3 shows the CO2 removal efficiency as a function of absorbent concentration and gas throughput (QG) at constant liquid throughput (QL). It can be clearly seen that the CO2 removal efficiency increased with an increase in the absorbent concentration from 6 to 30 wt %, whereas it decreased with the an in the gas throughput from 120 to 440 L/h. In particular, the removal efficiency was more sensitive to the gas throughput (larger change trend) at low concentration than that at high concentration, indicating the main role of the absorbent concentration. At a concentration of 30 wt %, the CO2 removal efficiency always remained higher than 90%, and it exhibited only a small change throughout the whole experimental range of gas throughputs because of the availability of amines for CO2.
Figure 5. Effect of the mean micropore size on removal efficiency (T = 303 K, UL = 0.20 m/s, 12 wt % MEA, da = 500 μm).
3.2. Effects of Superficial Gas and Liquid Velocities. Figure 4 presents the effects of the superficial gas velocity, UG, and the superficial liquid velocity, UL, on the CO2 removal efficiency. The removal efficiency decreased significantly with increasing UG at constant UL. Although increasing UG could provide a higher gasliquid interfacial area, which is beneficial to the removal efficiency,48 the reduction in the residence time caused by an increase of UG could decrease the removal efficiency to an even greater extent.49 An increase of the removal efficiency was also observed for an increase in UL from 0.04 to 0.20 m/s at constant UG. This can be explained by an increase of the liquidside mass-transfer coefficient. An increased UL would result in the formation of a thinning liquid boundary layer and the corresponding reduction of mass-transfer resistance, thereby benefiting the increase of the liquid-side mass-transfer coefficient.48 In addition, an increased removal efficiency would also result from chemical reaction because the availability of MEA would become easier when UL was increased. 3.3. Effect of Structural Dimensions. In addition to UG and UL, the structural dimensions of the MTMCR, such as the micropore size and the annular channel width, are also important factors affecting the microchannel absorption system. Figure 5 displays the effect of the micropore size on the CO2 removal efficiency. As the micropore size increased from 10 to 80 μm, the removal efficiency decreased by 510% throughout the whole UG range. A possible reason for this behavior is that the bubble 6371
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Figure 6. Effect of the width of the mixing chamber on removal efficiency (T = 303 K, UL = 0.10 m/s, UG = 2.74 m/s, 12 wt % MEA, dm = 10 μm).
Figure 7. Effect of MTMCR length on removal efficiency (T = 303 K, UL= 0.10 m/s, UG = 2.74 m/s, 12 wt % MEA, da = 500 μm, dm = 10 μm).
size of the gas phase as the dispersed phase is mainly determined by the micropore size. Smaller gas bubbles would be achieved as the micropore size became smaller, contributing to a higher interfacial area and leading to a greater liquid-side mass-transfer coefficient. Figure 6 illustrates the effect of the annular channel width on the CO2 removal efficiency at constant UG and UL. When the annular channel width was increased from 250 to 1000 μm, the removal efficiency decreased from 91% to 66%. This can be attributed to the size effect for mass transfer in the microchannel. Because the flow in a microchannel usually is laminar, a shorter diffusion distance and a high surface-to-volume ratio can be achieved with a decrease in the channel dimensions. This would lead to the occurrence of faster diffusive mixing, thus increasing the mass-transfer rate.49 Figure 7 shows the effect of the mixing distance, that is, the distance from the micropore zone to each sampling point, on the CO2 removal efficiency. The CO2 removal efficiency increased with increasing mixing distance because of the increased contact time. The removal efficiency increased by approximately 10% when the mixing distance was increased from 16 to 71 mm and increased by 1020% when the mixing distance was increased from 71 to 156 mm. Therefore, the length of the MTMCR should be designed according to the practical requirements for CO2 removal efficiency in industry as well as manufacturing costs. 3.4. Effect of Liquid Temperature. The liquid temperature is an important parameter for the MEACO2 system. Figure 8
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Figure 8. Effect of liquid temperature on removal efficiency (UL = 0.20 m/s, 12 wt % MEA, da = 500 μm, dm = 10 μm).
Figure 9. Dependence of pressure drop on the distance between the liquid inlet and the outlet of the MTMCR (T = 300 K, 12 wt % MEA, da = 500 μm, dm = 10 μm).
shows the effect of the liquid temperature on the removal efficiency. An increase in the temperature of the MEA solution from 283 to 323 K caused an increase in the removal efficiency at constant UG and UL. Such behavior can be mainly attributed to the nature of CO2 absorption kinetics.19,50,51 Not only the diffusivity of CO2 in aqueous MEA solution, but also the reaction rate constant of CO2 with MEA would increase with an increase in temperature. However, it should be noted that the removal efficiency had a slight increase when the temperature was increased from 303 to 323 K, probably because of the thermodynamics of the CO2 absorption system in regard to exothermic reversible reactions. As the temperature was increased beyond a certain value, the rate of mass transfer across the gasliquid interface would be limited by the decreased vaporliquid equilibrium of the system.52 The optimum temperature in this work was in the range of 303313 K. 3.5. Characteristics of Pressure Drop in an MTMCR. When both gas and liquid phases are confined in microchannels, the system will take on complex fluidic behaviors. As the channel diameter becomes smaller, the surface forces play a predominant role in causing two-phase flow in microchannels to perform differently from that in conventional large-sized tubes.53 Therefore, the pressure drop of fluid flow in a microchannel contactor is considered to be a significant design parameter. 6372
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The micropore size and the annular channel size obviously exhibited the size effects of microreactors on the mass-transfer rate. A smaller micropore size and annular channel width contributed to a higher CO2 removal efficiency. An increase in the liquid temperature from 283 to 323 K caused the removal efficiency to increase. In addition, the pressure drop of the MTMCR increased with increasing superficial gas and liquid velocities. A decrease of the micropore size or the annular width would lead to a higher pressure drop, which should be designed according to the requirements for CO2 removal. In summary, it can be expected that MTMCRs can provide a promising and cost-effective platform for the separation of CO2 from exhaust gases.
’ AUTHOR INFORMATION Figure 10. Effect of the mean micropore size on pressure drop (T = 300 K, UL = 0.10 m/s, 12 wt % MEA, da = 500 μm).
Corresponding Author
*Tel.: þ86-10-64447274 (J.-X.W.), þ86-10-64446466 (J.-F.C.). Fax: þ86-10-64423474 (J.-X.W.), þ86-10-64434784 (J.-F.C.). E-mail:
[email protected] (J.-X.W.),
[email protected]. edu.cn (J.-F.C.).
’ ACKNOWLEDGMENT This work was financially supported by the National “973” Program of China (No. 2009CB219903), National Natural Science Foundation of China (No. 20821004), and National “863” Program of China (Nos. 2007AA030207 and 2009AA033301). ’ REFERENCES Figure 11. Effect of the width of the mixing chamber on pressure drop (T = 300 K, UG = 2.28 m/s, UL = 0.10 m/s, 12 wt % MEA, dm = 10 μm).
The pressure drop of the gasliquid flow is shown in Figure 9. As could be speculated, the pressure drop increased with increasing UG and UL. In the MTMCR, the gas flowing through the micropore belt as the dispersed phase impinges crosscurrently with the liquid flow. The reduction in the flow area of the two phases and the special contacting state contribute to the pressure drop. It is possible that the micropore size should have a significant effect on the pressure drop in the MTMCR. The experimental results verified this speculation, as shown in Figure 10. The pressure drop decreased by approximately 50% when the micropore size was increased from 10 to 80 μm. In addition, the annular width also played an important role in the investigation of pressure drop. As can be seen in Figure 11, the pressure drop increased by 24% as the annular width decreased from 1000 to 250 μm at fixed UG and UL. It is obvious that the micropore size and the annular width need to be designed in an optimum range to avoid excessive pressure drop for meeting the requirements of CO2 removal in industry.
4. CONCLUSIONS This study was undertaken to explore the performance of MTMCRs for the removal of CO2 from gas streams with MEA as the absorbent. One of the most significant factors that influenced the removal efficiency was the MEA concentration, as the driving force was proportional to this variable. The CO2 removal efficiency was also found to increase with decreasing superficial gas velocity and increasing superficial liquid velocity.
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