MEA Using Hybrid

May 13, 2015 - The correlation by Onda et al.24 has been widely used for predicting mass-transfer coefficients. The ability of the existing correlatio...
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Characteristics of Absorption/Regeneration of AMP/MEA Using Hybrid Packing for the Improvement of CO2 Absorption Performance Min-Kyoung Kang,† In-Deuk Kim,‡ Bong-Jun Kim,§ Ji-Soon Kang,∥ and Kwang-Joong Oh*,† †

Department of Environmental Engineering, Pusan National University, Busan 609-735, South Korea Busan Metropolitan City Suyeong-Gu Environment & Sanitation Division, 100 Namcheondong, Suyeong-gu, Busan 613-702, South Korea § Winner of Technology, Gamno-ri, Sangdong-myeon, Gimhae-si, Gyeongnam 621-811, South Korea ∥ Department of Environment Management, Environment Analysis Team, Korea Environment Corporation HQ of Gyeongnam Region, Busan 616-101, South Korea ‡

ABSTRACT: The development of new packing materials and absorbents for use in CO2 recovery is urgently needed as a means of reducing greenhouse gases. In this study, we evaluated the absorbents 2-amino-2-methyl-1-propanol (AMP) and monoethanolamine (MEA) in combination with random packing materials (ceramic Raschig rings and Berl saddles) and a structured gauze packing material. To improve the CO2 absorption efficiency, we developed hybrid packing materials comprising Raschig rings and the structured packing material in various ratios. The effects of the ratio on the gas flow rate, absorbent liquid flow rate, and CO2 absorption/regeneration efficiencies over time were evaluated. The overall mass-transfer coefficients were also calculated. The height of the tower corresponding to one theoretical plate in the absorptive tower design and the available gas− liquid contact area were calculated. This fundamental research will help support efforts to improve the efficiency of CO2 absorption from various industrial exhaust streams.

1. INTRODUCTION Since the start of the industrial revolution, technological development and the use of fossil fuels have increased significantly, resulting in higher atmospheric greenhouse gas concentrations, which, in turn, have contributed to global warming. Global warming is expected to become more serious as a result of continued energy consumption and to significantly affect our future.1,2 Greenhouse gases comprise carbon dioxide (CO2), methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (CFCs). Carbon dioxide is the major contributor to global warming; great quantities are generated worldwide in the natural gas, oil refining, ironworks, and petrochemical industries.3 Presently, several technologies are used to recover CO2 from the exhaust gases of industrial facilities, including absorption, low-temperature separation, and membrane separation. The absorption method, which typically uses an alkanolamine aqueous solution, has advantages over the other methods in terms of its reliability and CO2-processing capacity.4 Monoethanolamine (MEA) and 2-amino-2-methyl-1-propanol (AMP) are frequently used singly as absorbents. Research on the physical and chemical properties of the MEA and AMP single absorbents for use in packed columns is progressing, with the goal of maximizing the absorption efficiency of the chemical process. The devices in which the gas−liquid contact reaction occurs have two general forms. Spray towers have a relatively low efficiency. Therefore, most often, a pretreatment device is used. Studies of the exhaust gases from thermal power plants that contain high concentrations of CO2 have shown that the amounts of CO2 trapped are relatively small. On the other hand, in the present study, a packed column improves the © 2015 American Chemical Society

absorption efficiency and facilitates mass transfer with a low pressure loss. In addition, because of the high separation efficiency, the use of packing in the adsorption tower currently seems to produce the most stable performance characteristics. These characteristics are the reasons for the wide use of packed columns in industrial processes.5 Generally, when a gas containing CO2 comes into contact with a fluid absorption solution in a packed column, the CO2 absorbent reacts. The absorption process depends on the extent of gas−liquid contact provided in the packed column. A packing material can increase the contact area of the gas and fluid in the column, facilitating mass transfer. Packing materials can be classified as either regular or irregular (random) packing materials.6−10 On the other hand, the pressure loss is lower and higher efficiency is obtained from a high packing load. Regular packing materials such as gauze are widely used industrially as compared to irregular packing materials, in part because the pressure losses are lower.11 In this study, commercially available AMP and MEA absorbents and single packing materials including both random (Raschig rings, Berl saddles) and structured (SP 1000G) packings were used. We compared the properties of these single packing materials in terms of CO2 absorption/regeneration efficiency and amount of CO2 absorbed. To improve the efficiency and cost-effectiveness of CO2 absorption, we also investigated the characteristics of hybrid packings, which consisted of various ratios of Raschig rings (R) Received: Revised: Accepted: Published: 5853

January 20, 2015 May 13, 2015 May 13, 2015 May 13, 2015 DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

Article

Industrial & Engineering Chemistry Research

film is uniform, and the continuous replenishment of the film implies that there are no long-range diffusion processes taking place in the liquid phase between the interface and the bulk liquid.19,20 Mass transfer occurs when component A in the gas phase transfers across a gas−liquid interface into the liquid phase. The mass-transfer rate of component A is influenced by the concentration gradient of that component in the direction of mass transfer within each phase. The mass flux of component A (NA) across the gas−liquid interface in the steady state can be represented in terms of the gas-side mass-transfer coefficient (kG) and the driving force (yA − yA,i) as follows7,21

and SP 1000G structured packing (S). The effects of the mixing ratio in the hybrid packing, gas flow rate, and liquid flow rate on the CO2 absorption/regeneration efficiency as a function of reaction time were investigated. The overall mass-transfer coefficients for the single and hybrid packing materials were determined to assess the improvements in CO2 absorption performance. In the absorption process, the height of the packing equivalent to a theoretical plate (HETP) for the absorptive tower design was calculated. The gas−liquid contact area as a function of the absorbent and packing material was calculated to investigate the absorption in the packed column and the gas−liquid behavior during continuous CO 2 absorption. The goal of the current study was to improve the CO2 absorption performance in the packed column as a function of the absorbent and type of packing material.

NA = k GP(yA − yA,i )

where P, yA, and yA,i represent the total pressure, mole fraction of component A in the bulk gas, and mole fraction of component A on the gas side of the gas−liquid interface, respectively. Because the mass-transfer driving force occurs over extremely small distances, determining the concentration of component A at the gas−liquid interface is particularly difficult. Therefore, the mass flux is expressed in terms of the overall mass-transfer coefficient (KG) and the mole fraction of component A in the gas phase (y*A ) that is in equilibrium with the concentration of A in the bulk liquid

2. MECHANISM OF CO2 ABSORPTION IN AQUEOUS AMP/MEA SOLUTION 2.1. Reactions of CO2 with AMP/MEA. The following chemical reactions take place with CO2 in aqueous solutions of primary alkanolamines12 CO2 + 2RR′NH ↔ RR′NCOO− + RR′NH 2+

(1)

where R represents an alkyl group and R′ represents H for primary amines or an alkyl group for secondary amines. The zwitterion mechanism originally proposed by Caplow13 and reintroduced by Danckwerts12 is generally accepted for reaction 1 CO2 + RR′NH ↔ RR′NH+COO−

(2)

RR′NH+COO− + B′ ↔ RR′NCOO− + BH+

(3)

NA = K GP(yA − yA*)

(6)

Then, KG can be expressed as KG =

NA P(yA − yA*)

(7)

In a gas-absorption apparatus (such as a packed column), the effective gas−liquid interfacial area per unit volume of the column (av) is considered to be an important parameter in the mass-transfer process, in addition to the mass-transfer coefficients. Therefore, it is more suitable to present the mass-transfer coefficient based on the unit volume of the absorption column rather than the interfacial unit area, as follows

The preceding absorption reaction mechanism mainly consists of two steps: the formation of the CO2−amine zwitterion (reaction 2) and the base-catalyzed deprotonation of the zwitterion (reaction 3). B′ denotes a base, which could be an amine, OH−, or H2O.14 For primary and secondary amines such as MEA and diethanolamine (DEA), the carbamates formed in reaction 3 are quite stable. In the case of sterically hindered amines such as AMP, the carbamate form is unstable and undergoes a carbamatereversible reaction as follows15 RR′NCOO− + H 2O′ ↔ RR′NH + HCO3−

(5)

K Ga v =

(4)

NAa v [P(yA − yA*)]

(8)

To evaluate KGav, the term NAav can be simply determined by performing absorption experiments in packed columns, where the concentration profile of the absorbed component A in the gas phase can be measured along the column height. The concentration of A (in this case, CO2) in the gas phase can be measured along the height of the packed column using an infrared CO2 gas analyzer. The measured CO2 concentration, in terms of the mole fraction of A in the gas phase (yA,i), is converted to the mole ratio of A in the gas phase (YA) and is then plotted against the height of the column (Z) to obtain the solute molar ratio concentration gradient. Considering an element of a packed column with a height dZ, the mass balance of the element is expressed as

According to reaction 4, for hindered amines, 1 mol of CO2 is absorbed per mole of amine. Steric effects influence the stability of the carbamate formed by the reaction of an amine with CO2. It has been suggested that, owing to its instability, the carbamate readily undergoes hydrolysis to form bicarbonate, freeing the amine to react again with CO2.16,17 However, a recent study18 reported the presence of the carbamate in AMP−CO2−H2O absorption systems throughout the run, although the quantity was small. These results imply that the carbamate formation reaction might play some role in the absorption of CO2. This is a significant observation, as earlier researchers overlooked the importance of sterically hindered amines in the carbamate formation reaction. 2.2. Determination of the Overall Mass-Transfer Coefficient and HETP. The chemical absorption of CO2 into a thin film can be described as a combination of diffusion and chemical reaction processes. CO2 diffuses from the gas phase to the gas−liquid interface and into the liquid phase, where it undergoes a chemical reaction. It is assumed that the

⎡ yA ⎤ ⎥ NAa v dZ = G I d⎢ ⎢⎣ (1 − yA ) ⎥⎦

(9)

where GI represents the inert gas molar flux per cross-sectional area of the column and av is the gas−liquid interfacial area. Based on eqs 8 and 9, KGav can be expressed as 5854

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

Figure 1. Schematic diagram of the absorption and regeneration experimental system: (1) air compressor, (2) filter, (3) CO2 cylinder, (4) mass flow controller, (5) analyzer, (6) mixing chamber, (7) absorber, (8) thermocouple, (9) liquid pump, (10) heat exchanger and storage tank, (11) regenerator, (12) condenser, (13) gas meter.

⎡ ⎤ dY GI ⎥ A K Ga v dZ = ⎢ ⎢⎣ P(yA − yA*) ⎥⎦ dZ

The liquid-phase mass-transfer coefficient is then given by27,28 ⎛ ρ ⎞1/3 ⎛ L ⎞2/3⎛ μ ⎞−1/2 L ⎟⎟ = 0.0051⎜⎜ ⎟⎟ ⎜ L ⎟ kL⎜⎜ (a tDP)0.4 ⎝ μL g ⎠ ⎝ a w μ L ⎠ ⎝ ρD L ⎠

(10)

where KGav is the overall mass-transfer coefficient per unit volume of packing, yA* is the equilibrium mole fraction of component A in the gas phase, and YA is the mole ratio of component A in the gas phase. In several mass-transfer studies,8,22,23 y*A has been assumed to be zero and neglected from consideration. The same assumption was also applied in the present study. Thus, the value of KGav can be calculated for any YA value of interest. In this work, the comparison criteria for the KGav values were obtained at a CO2 molar ratio of YA = 0.12. Additionally, HETP values can be calculated from masstransfer coefficients as follows ⎛ G ⎞ ln(λ) HETP = ⎜ Gm ⎟ ⎝ K Gam′ P ⎠ λ − 1

(13)

To obtain the total mass-transfer area, we used the equations24 −0.05 0.1 ⎡ aw ⎛ σc ⎞0.75⎛ 1 ⎞ ⎛ L2a t ⎞ ⎢ ⎜ ⎟ ⎜ ⎟ ⎜ ⎟ = 1 − exp − 1.45 ⎢ ⎝ σ ⎠ ⎜⎝ a tμ ⎟⎠ ⎜⎝ ρ 2 g ⎟⎠ at L L ⎣

⎛ L2 ⎞0.2 ⎤ ⎜⎜ ⎟⎟ ⎥ ⎝ ρL σa t ⎠ ⎥⎦

aw =

(11)

where λ is an absorption factor defined as m(G/L), GGm is the molar flux of the gas phase, and GLm is the molar flux of the liquid phase. 2.3. Determination of the Effective Interfacial Area of a Packed Column. Three structurally different correlations have been proposed to predict the effective interfacial ares a (aw) of packing materials. The correlation by Onda et al.24 has been widely used for predicting mass-transfer coefficients. The ability of the existing correlations to predict aw for various pollution abatement contexts was discussed by Staudinger and Dvorakl.25,26 The effective gas−liquid contact area can be estimated using the mass-transfer coefficients of the gas and liquid sides. First, the gas-side mass-transfer coefficient is found using the equation27,28 ⎛ G ⎞0.7 ⎛ μ ⎞1/3 k GRT ⎟⎟ ⎜⎜ G ⎟⎟ (a tDP)−2.0 = 2.0⎜⎜ μ a tDG a ⎝ t G ⎠ ⎝ ρG DG ⎠

(14)

K GAρG,molar K GP

(15)

3. EXPERIMENTAL SECTION 3.1. Materials. Analytical-grade AMP (95%) and MEA (99%) were supplied by Sigma-Aldrich (St. Louis, MO). Aqueous solutions were prepared using distilled water. The CO2 and N2 gases were commercial grade with purities of 99.99%. The packing materials were either random (6-mm Raschig rings, 6-mm Berl saddles) or structured (SP 1000G). 3.2. Experimental Apparatus and Procedure. A schematic diagram of the experimental apparatus for studying CO2 absorption/regeneration is shown in Figure 1. The apparatus consisted of a gas injector, absorber, regenerator, and CO2 analyzer. The absorber and regenerator were made of glass and had an internal diameter of 50 mm and a height of 600 mm. The packing materials that were used are shown in Figure 2. The temperatures of the absorber and regenerator were measured by a thermocouple with an accuracy of ±0.1 K. Compressed air was used to remove particulates and was then

(12) 5855

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

introduced into the reaction flask using a graduated titration buret. As CO2 vapor evolved from the reaction, the fluid in the reservoir was displaced, allowing for an evolved gas measurement. The amine solution concentration was determined from the titration with the relationship

C1V1 = C2V2

(16)

where C1 is the amine solution concentration (M), V1 is the amine solution sample volume (mL), C 2 is the acid concentration (M), and V2 is the acid volume from the titration (mL). First, 100 mL of distilled water was injected into a 250 mL beaker and stirred with a magnetic stirrer. The pH of the distilled water was increased to 11.4−11.6 by addition of 0.5 M NaOH solution. The amine solutions were added to the beaker using a 10 mL pipet for the rich amine solution and a 25 mL pipet for the lean amine solution. Then, 0.5 M NaOH was added to the amine solution by the titrimetric method until the solution pH reached 11.5. The amount of CO2 absorbed by the amine solution was obtained in conjunction with the concentration derivation. The captured CO2 evolved through the titration was analyzed with the resulting equation, where the CO2 loading is defined in moles of CO2 per mole of amine. Calculate L (R)

Figure 2. Images and schematic of packing materials: (a) Raschig rings, (b) Berl saddles, (c) structured packing SP 1000G, (d) hybrid packing (Raschig rings mixed with SP 1000G).

dehumidified by passage through a silica gel air dryer. The clean air served as the diluent gas and was mixed with pure CO2 gas. The flow rates of the gases were controlled by mass flow controllers (5850E, Brooks Instruments, Hatfield, PA). CO2 gas analyzers (ZRF model, 0−20 vol %, Fuji Electric, Tokyo, Japan) were used to measure CO2 outlet gas concentrations in the absorber and regenerator. A pump (AX1-12-PEC-Z, Cheonsei Industrial Co., Ltd., Ansan, Korea) was used to circulate the aqueous amine solution, which reacted with the CO2 gas. The condenser was connected to the top of the regenerator to prevent loss of absorbent. The CO2 removal efficiencies and absorption amounts were calculated using breakthrough curves, and the CO2 absorption amounts, expressed in moles of CO2 per mole of amine, were determined. 3.2.1. Continuous Absorption/Regeneration Process. Experiments to study the absorption characteristics were carried out by operating both the absorber and the regenerator. The aqueous amine solution (500 g) was injected into a storage tank and circulated through the regenerator. The temperatures in the absorber and regenerator were kept constant to prevent regeneration. The outlet CO2 concentration from the absorber was measured continuously by a CO2 gas analyzer. The prepared solvent was then pumped at a given flow rate to the top of the column. A needle valve was installed on the top of the column to control the liquid flow rate so as to create countercurrent contact between the gas and liquid. After absorbing CO2 and traveling through the column, the CO2-rich solution was collected continuously in the liquid receiving tank. This operation was continued for at least 30 min to allow the system to reach steady-state conditions. At the same time, liquid samples were withdrawn from the bottom of the column and analyzed for their concentrations and CO2 loadings. 3.2.2. Analysis of Lean/Rich Amine Solution Samples. Rich and lean amines were extracted from the absorber and regenerator, respectively. To determine the characteristics of CO2 absorption and regeneration, the samples were analyzed using the titrimetric method: A liquid amine sample of known volume was placed in the reaction flask. Acid titrant was

L (R ) (equiv/L) = 0.02 (0.05) × mL of NaOH

(17)

Calculate the CO2 loading in mol/mol ⎛ mol ⎞ ⎟ CO2 loading ⎜ ⎝ mol ⎠ =

L (R ) × molecular weight of absorbent/10 amine solution (wt %)

(18)

where L is the lean amine solution, and R is the rich amine solution.

4. RESULTS AND DISCUSSION 4.1. CO2 Absorption/Regeneration with a Single Packing Material. The CO2 removal efficiencies and CO2 absorption amounts were investigated for single packing materials (Raschig rings, Berl saddles, and structured packing) in the continuous absorption/regeneration process. The following conditions were used: carbon dioxide concentration, 15%; absorber temperature, 40 °C; regenerator temperature, 100 °C; absorbent concentration, 30 wt %; gas flow rate, 500 L/h; liquid flow rate, 40 mL/min; absorbent, AMP or MEA. As shown in Figure 3 for the AMP absorbent, the removal efficiencies of the Raschig rings, Berl saddles, and structured packing materials were 70.0%, 49.0%, and 71.4%, respectively. For the MEA absorbent, the corresponding removal efficiencies were 93.6%, 86.7%, and 99.8%, respectively. Random packing materials exhibit many points of contact among their components, resulting in a relatively small surface area. The absorption efficiency of the Raschig rings was higher than that of the Berl saddles, and the Raschig rings showed a lower absorption efficiency with the AMP absorbent (21.0%) than with the MEA absorbent (6.9%). As a result of the increased surface area and larger porosity, mass transfer is facilitated, and pressure loss is decreased. The Raschig ring and Berl saddle space efficiencies were similar at 0.64. Therefore, in terms of absorption efficiency, which is high in a packed column, the difference in the packing 5856

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

Figure 3. CO2 removal efficiencies and absorption amounts for different packing materials.

Figure 4. CO2 removal efficiencies for 1:1, 2:1, and 1:2 packing material ratios with AMP or MEA absorbent as a function of gas flow rate.

surface area, rather than the space efficiency, is the reason for the difference in CO2 absorption efficiencies. On the other hand, in the case of the structured packing, it was thought that the use of a gauze-type structured packing material would result, overall, in a smoother flow of gas. The gas−liquid contact would also improve because of the improved flow between the fine mesh, resulting in improved absorption efficiency. The CO2 absorption amount reflects the number of moles of CO2 absorbed per mole of absorbent. The absorbent and CO2 gas were analyzed after 90 min, when the reaction was finished. For the AMP absorbent, the CO2 absorption amounts for the Raschig rings, Berl saddles, and structured packing materials were 0.47, 0.43, and 0.52 mol of CO2/mol of absorbent, respectively, whereas for the MEA absorbent, the corresponding values were 0.67, 0.64, and 0.71 mol of CO2/mol of absorbent, respectively. These results were expected because the amount of CO2 absorbed can be very high owing to the outstanding absorption capacity of the structured packing compared with random packing materials. The increased absorption also increases the loading, which is rich in amine. Thus, a random packing results in a greater CO2 absorption capacity than can be absorbed by a highly structured packing. Furthermore, in a continuous CO2 absorption/regeneration process, a structured packing shows a greater difference in the load capacity than does a random packing. Thus, it was decided to examine whether a structured packing could efficiently remove CO2. 4.2. Effect of Gas Flow Rate with Hybrid Packing. In terms of equipment operation, the combustible waste gas inflow must be controlled. The interaction between the CO2 gas and absorbent selectively increases the chemical reaction, and it can have many effects. The greater the amount of CO2 removed, the more economical it is to run the equipment.29 Therefore, the CO2 removal efficiency was investigated as a function of the gas flow rate (200−600 L/h). In these experiments, 1:1, 2:1, and 1:2 mixtures of the Raschig rings and structured packing, a liquid flow rate of 40 mL/min, and an AMP or MEA absorbent concentration of 30 wt % were used. Figure 4 shows the CO2 removal efficiency as a function of the gas flow rate for AMP and MEA absorbents and hybrid packing mixtures. With the AMP absorbent and mixing ratios of 1:1, 2:1, and 1:2, the CO2 absorption efficiencies were

approximately 57.6−94.8%, 75.2−100%, and 53.1−89.8%, respectively. For the MEA absorbent and mixing ratios of 1:1, 2:1, and 1:2, the absorption efficiencies were approximately 79−100%, 88.9−100%, and 69.6−100%, respectively. As shown in the figure, significant amounts of CO2 were absorbed as the gas flow rate was increased. Then, the column attained a higher CO2 loading, and the reactivity and absorption efficiency decreased. The absorption efficiency as a function of packing material mixture ratio follows the order 1:2 < 1:1 < 2:1. As the gas flow rate increased, the efficiency processing per unit of CO2 decreased because of the pressure load. This affected the efficiency of the flow rates of gas and liquid passing per unit area. As the gas flow rate increased due to the pressure load, the efficiency per unit of CO2 dropped. The absorption efficiency was expected to decrease for higher mixing ratios of Raschig rings. At low gas flow rates, the CO2 removal efficiency was high, but the increased reservoir increased the corrosion resistance of the device. Because a removal ratio of about 90% would be ideal, either AMP or MEA absorbent with a gas flow rate in the range of 300−400 L/h for a 2:1 packing material mixture ratio would be satisfactory. 4.3. Effect of Liquid Flow Rate with Hybrid Packing. Significantly more CO2 is eliminated as the absorption liquid flow rate increases. However, the latter condition requires a higher power ratio and a higher cost of materials for the absorbent, making the process uneconomical. Therefore, the selection of an appropriate absorbent flux that is suitable for the column scale is important. The CO2 removal efficiency was investigated as a function of the absorption liquid flow rate in the range of 20−50 mL/min for the hybrid packing mixtures and the two adsorbents. The results are shown in Figure 5. With increasing liquid flow rate, the removal efficiencies for the AMP absorbent were 60−72%, 70.8−81.5%, and 55.5−65.3% for packing material mixture ratios of 1:1, 2:1, and 1:2, respectively. The MEA absorbent showed corresponding efficiencies of 78−91.1%, 85.4−100%, and 63.8−80.2% for the same ratios. An increase in the absorbent liquid flow rate increased the amount of liquid passing through the system. This increased the interface per unit volume and the efficiency of CO2 5857

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

63.9−80.2%. With the 2:1 ratio of packing materials, the CO2 outlet concentration was 2.31−4.58 vol %, and the CO2 removal efficiency was 69.5−84.6%, and at a 1:2 ratio, these values were 2.92−6.50 vol % and 56.7−80.5%, respectively. The 2:1 mixture ratio showed 4.4−5.6% and 4.1−12.8% higher absorption efficiencies in comparison with the 1:1 and 1:2 mixture ratios, respectively. For the MEA absorbent and hybrid packing, the CO2 outlet concentration was 0−1.89 vol % for the 1:1 mixture ratio, with a CO2 absorption efficiency of 87.4−100%. For the 2:1 mixture ratio, the CO2 outlet concentration was 0−1.46 vol %, and the absorption efficiency was 90.3−100%. For the 1:2 mixture ratio, the values changed to 0−3.06 vol % and 79.6−100%, respectively. The 2:1 mixture ratio showed 2.9% and 10.7% higher absorption efficiencies than the 1:1 and 1:2 ratios, respectively. The absorption/regeneration efficiency of the 1:2 mixture ratio was lower because of the narrower spaces through which the gas passed. The gas accumulated in the packed column, creating resistance. Losses due to the pressure increased, and longer residence times when the fluid was not evenly diffused before it reached the structured packing increased. Because the contact between the packing materials was uneven, the efficiency decreased. However, for the 2:1 mixture, the gas transverse section was enlarged, and the fluid bed that passed the Raschig ring junctions flowed into the wider spaces of the structured packing. The number of points of contact between the fluid and gas also increased. The absorption efficiency was thus high because smooth diffusion was facilitated. The pressure loss was small because the porosity of the packing material was high. Thus, mass transfer occurred efficiently when the CO2 throughput in the packed column was increased. With a packing material mixture ratio of 1:1, the gas load flowed from the lower side to the upper side. Initially, the gas load was large as a result of the structured packing. When meeting the Raschig ring material, the gas load decreased in comparison with the liquid fluid flow when the liquid flowed to the lower part from the upper part, somewhat decreasing the CO2 absorption efficiency. That is, the absorbed gas reached the absorption liquid interface by molecular diffusion. The residence time when the fluid reached the interface in comparison with the residence time when the gas reached the interface was delayed, and thus, the gas−liquid contact was not smooth. Therefore, the 2:1 ratio appears to be optimal for maximizing the CO2 removal efficiency.

Figure 5. CO2 removal efficiencies for 1:1, 2:1, and 1:2 packing material ratios with AMP or MEA absorbent as a function of liquid flow rate.

removal. In addition, the absorbent was more uniformly distributed through the pores in the packing material when the liquid flow rate was increased. Accordingly, mass transport was promoted as the gas−liquid contact became more homogeneous throughout the system. The absorption capacity of the absorbent increased simultaneously. Thus, the CO2 removal efficiency increased because the CO2 loading of the absorbent decreased. 4.4. CO2 Absorption/Regeneration over Time for the Hybrid Packing System. These experiments examined the effects of the hybrid packing mixture ratio on the CO2 outlet concentration using AMP and MEA absorbents. The experimental conditions were as follows: carbon dioxide concentration, 15%; absorber temperature, 40 °C; regenerator temperature, 100 °C; absorbent concentration, 30 wt %; gas flow rate, 500 L/h; and liquid flow rate, 40 mL/min. As shown in Figure 6 for the AMP absorbent, the CO2 outlet concentration was initially 2.97−5.41 vol % over 20 min when the hybrid ratio was 1:1, and the CO2 removal efficiency was

5. OVERALL MASS-TRANSFER COEFFICIENT ACCORDING TO PACKING MATERIAL The overall mass-transfer coefficients of the AMP and MEA absorbents and packing materials were calculated. The results are shown in Figure 7. The AMP absorbent with the Raschig rings, Berl saddles, and structured packing showed overall masstransfer coefficients of (0.61−1.75) × 106, (0.60−1.74) × 106, and (0.64−1.80) × 106 kmol·h−1·m−3·kPa, respectively. Among the hybrid packing materials, the 2:1 mixture had a value of (0.64−1.85) × 106 kmol·h−1·m−3·kPa. Thus, the hybrid packing showed an increase of (0.027−0.101) × 106 kmol·h−1·m−3·kPa, the Berl saddles showed an increase of (0.033−0.110) × 106 kmol·h−1·m−3·kPa, and the structured packing showed an increase of (0.0003−0.051) × 106 kmol·h−1·m−3·kPa. The MEA absorbent with the Raschig rings exhibited an overall mass-transfer coefficient of (0.62−1.79) × 106 kmol·h−1·

Figure 6. CO2 outlet concentrations for 1:1, 2:1, and 1:2 packing material ratios with AMP or MEA absorbent as a function of reaction time. 5858

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

Figure 8. HETP values for different packing materials as a function of L/G ratio.

Figure 7. Values of KGav for different packing materials as a function of L/G ratio. The liquid circulation rate is the result of the fixed and the gas circulation rate change.

the Raschig rings, Berl saddles, and structured packing were 0.005−0.023, 0.005−0.024, and 0−0.007 m, respectively. Low HETP values were obtained with the hybrid packing. The HETP values were expected to decrease despite increases in the gas−liquid ratio. As the gas−liquid ratio increases, the CO2 adsorption efficiency decreases for a given packed column height and type of packing material. That is, there is a close relationship between the HETP value and the CO2 absorption efficiency. As has been reported, there is a direct link between the HETP value and the mass-transfer coefficient.30 The size of the packed column used for the experiments in this study was 0.6 m. Depending on the packing material, the amount required to produce an HETP in the range of 0.15− 0.41 m is low. As shown by these results, the HETP value is related to the efficiency of the packed column. It is evaluated by a factor determining the height of a tower corresponding to one theoretical plate whose efficiency increases as this value decreases. Therefore, in terms of cost, there is a close relationship between the size of the packed column and the packing material in the absorptive tower design. The HETP value, which can be predicted from the CO2 absorption efficiency, is necessary for determining the economics of the tower design. 5.2. Effective Interfacial Area Estimate According to the Liquid Flow Rate. We used the method presented by Onda et al.24 to calculate the interfacial contact area as a function of liquid flow rate for the different packing materials and absorbents, and the results are shown in Figures 9 and 10. The CO2 removal efficiencies for different contact areas were then compared. The Raschig rings, Berl saddles, structured packing, and hybrid packing afforded effective interfacial contact areas of 125.39−174.16, 109.04−150.46, 148.46−207.62, and 192.94− 272.18 m2/m3, respectively. Thus, the gas−liquid contact areas increased in the order Berl saddles < Raschig rings < structured packing < hybrid packing. The gas−liquid contact area of the hybrid packing was 67.54−121.72 m2/m3 higher than those of the random packings, as well as 44.48−64.55 m2/m3 higher than that of the structured packing. From these results, it is apparent that the spacing efficiencies of the packing materials were similar. However, the spacing efficiency differences are dependent on at, the apparent area of

m−3·kPa, whereas the values for the Berl saddles and structured packing were (0.61−1.78) × 106 and (0.64−1.86) × 106 kmol· h−1·m−3·kPa, respectively. In addition, the 2:1 hybrid packing exhibited a coefficient of (0.64−1.90) × 106 kmol·h−1·m−3·kPa. The value for the hybrid packing was (0.017−0.105) × 106 kmol·h−1·m−3·kPa higher than that for the Raschig rings, (0.019−0.111) × 106 kmol·h−1·m−3·kPa higher than that for the Berl saddles, and (0−0.035) × 106 kmol·h−1·m−3·kPa higher than that for the structured packing. An increase in the gas−liquid ratio allows greater movement of the CO2 molecules past the air−liquid interface from the gas state, resulting in high material-transfer performance. Not only the mass transfer in the gas state but also the amount of absorbent fluid acts as a significant factor in controlling the absorption process by mass transfer. Therefore, increasing the absorption efficiency increases the mass-transfer coefficient. That is, the absorption efficiency is known to be directly proportional to the overall mass-transfer coefficient as the absorbent flow rate increases. The absorbing molecules react many times per unit of CO2. In addition, because molecular diffusion of the absorbed CO2 occurs at the absorbent interface, the increased diffusion of the solvent molecules in the fluid flowing at the interface at higher flow rates contributes to a higher absorption efficiency. 5.1. Calculation of HETP According to Packing Material. We calculated the HETP values using the exit concentrations and mass-transfer interface values from experiments with Raschig rings, Berl saddles, structured packing, and 2:1 hybrid packing and examined the effects as a function of L/ G ratio. As shown in Figure 8, the KGav values for the AMP absorbent were 0.154−0.352 m for the Raschig rings, 0.155−0.353 m for the Berl saddles, 0.147−0.342 m for the structured packing, and 0.147−0.332 m for the 2:1 hybrid packing. The differences between the hybrid packing and the Raschig rings, Berl saddles, and the structured packing were 0.007−0.019, 0.008−0.021, and 0−0.009 m, respectively. On the other hand, the KGav value for the MEA absorbent with the Raschig rings was 0.183−0.406 m, whereas values of 0.184−0.41, 0.178−0.393, and 0.178−0.386 m were obtained for the Berl saddles, structured packing, and 2:1 hybrid packing, respectively. The differences between the hybrid packing and 5859

DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research

liquid behavior in the packed column. The following conclusions can be made: First, the CO2 removal efficiency and amount of CO2 absorbed for single packing materials were investigated in a continuous absorption/regeneration process. For the AMP absorbent, the removal efficiencies of Raschig rings, Berl saddles, and structured packing materials were 70.0%, 49.0%, and 71.4%, respectively. The MEA absorbent performed better, with removal efficiencies of 93.6%, 86.7%, and 99.8%, respectively. The performance of the single random packing materials and structured packing was improved by mixing. The absorption efficiency for the 1:1, 2:1, and 1:2 hybrid packing mixtures reached an optimum value when the mixing ratio was 2:1 (S/R, where S corresponds to the structured packing and R corresponds to Raschig rings). To investigate the characteristics of the packing materials, the overall mass-transfer coefficients, HETP values, and available gas−liquid contact areas were calculated. The overall mass-transfer coefficients increased in the order Berl saddles < Raschig rings < structured packing < 2:1 hybrid packing. The HETP was actually produced by the 0.6-m size of the packed column used in this work. Depending on the packing material, the values obtained were low, at 0.15− 0.41 m. Actually, the HETP value can be predicted for an absorption process by considering the characteristics of the absorbent and the packing material. In this study, use of a hybrid packing improved the absorption performance with advantages and disadvantages that were complementary to those of the single packing materials. Thus, the hybrid packing in the present study was shown to have higher absorption/regeneration capabilities when applied as an optimal packing material.

Figure 9. Effective areas and CO2 removal efficiencies of different packing materials in AMP absorbent as a function of liquid flow rate.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

Figure 10. Effective areas and CO2 removal efficiencies of different packing materials in MEA absorbent as a function of liquid flow rate.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Brain Korea 21 Plus Project in the Division of Creative Low Impact Development and Management for Ocean Port City Infrastructures. This work was also supported by a two-year Research Grant of Pusan National University.

each packing material. In the case of the Berl saddles, the available gas−liquid contact area was the lowest, because their surface area was the smallest. As a result, the pressure difference was apparent, and contact between the gas and fluid was smoothly made. In addition, the material-transfer constant of the liquid phase increased as the liquid flow rate increased, and the available gas−liquid contact area also increased. The effective area in contact with the bulk liquid and the interface increased as the fluid amount increased. The CO2 absorption efficiency was expected to increase as the available gas−liquid contact area increased because the gas−liquid contact would be simultaneously improved. At the gas−liquid interface, the diffusion of the fluid increased, and the CO2 absorption efficiency increased linearly.

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6. CONCLUSIONS We examined the characteristics of several packing materials and two absorbents (AMP and MEA) to improve the CO2 absorption performance from a simulated exhaust stream. The overall mass-transfer coefficients, HETP values, and available gas−liquid contact areas were investigated to evaluate the gas− 5860

NOMENCLATURE a′m = specific interfacial mass-transfer area per volume of column at = specific surface area of a packing material (m2·m−3) av = specific surface area of the contactor (m2·m−3) aw = effective interfacial area of a packing material DG = gas diffusion coefficient DP = nominal diameter of the packing element g = constant of gravitational acceleration (m·s−2) G = molar flow rate of solute-free gas GGm = molar flux of the vapor phase GI = inert gas flux (kg·mol·m−2·h−1) GLm = molar flux of the liquid phase HETP = height of packing equivalent to a theoretical plate kG = gas-side mass-transfer coefficient (kg·mol·m−2·s−1· kPa−1) DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861

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Industrial & Engineering Chemistry Research KG = overall mass-transfer coefficient (kg·mol·m−2·s−1· kPa−1) KGav = volumetric overall mass-transfer coefficient (kg·mol· m−2·s−1·kPa−1) L = molar flow rate of solute-free absorbent NA = mass flux P = pressure (kPa) R = universal gas law constant (0.082 L·atm·mol−1·K−1) T = temperature (K) YA = mole ratio of component A in the gas phase (mol· mol−1) yA* = mole fraction of component A in the bulk gas phase at equilibrium yA,i = mole fraction of component A on the gas side of the gas−liquid interface (kg·mol·kg−1 mol−1) Z = height of packed column (m) μG = gas viscosity (kg·m−1·h−1) μL = liquid viscosity (cP) ρG = density of gas (g·cm−3 or kg·m−3) ρL = density of liquid (g·cm−3 or kg·m−3) σ = surface tension (mN·m−1) σc = critical surface tension (mN·m−1)

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DOI: 10.1021/acs.iecr.5b01378 Ind. Eng. Chem. Res. 2015, 54, 5853−5861