Characterization and Comparison of the CO2 Absorption Performance

Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2. The performance of carbon dioxide (CO2) absorption into aqueous ...
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Ind. Eng. Chem. Res. 2004, 43, 2228-2237

Characterization and Comparison of the CO2 Absorption Performance into Single and Blended Alkanolamines in a Packed Column Adisorn Aroonwilas* and Amornvadee Veawab Faculty of Engineering, University of Regina, Regina, Saskatchewan, Canada S4S 0A2

The performance of carbon dioxide (CO2) absorption into aqueous solutions of single and blended alkanolamines was evaluated experimentally in a bench-scale absorber packed with highefficiency packings. The absorption experiments were conducted under atmospheric pressure, using a feed gas mixture containing 10% CO2 and 90% nitrogen. Monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), methyldiethanolamine (MDEA), 2-amino2-methyl-1-propanol (AMP), and their mixtures including MEA-MDEA, DEA-MDEA, MEAAMP, and DEA-AMP were tested in this work. The absorption performance was presented in terms of the CO2 removal efficiency, absorber height requirement, effective interfacial area for mass transfer, and overall mass-transfer coefficient (KGae). Comparison of the absorption performance between the tested alkanolamines was made over ranges of operating conditions to establish the correlation between single- and blended-alkanolamine systems. 1. Introduction It is now scientifically evident that human activities have caused concentrations of greenhouse gases (GHGs) to rise significantly over the last 200 years,1 contributing to the global warming problem. The desire to alleviate this problem has resulted in serious environmental concerns deriving from the need to reduce GHG emissions from industrial sources. Global and national emission reduction targets were set, signed, and ratified or acceded by 124 countries under the 1997 Kyoto Protocol. The global target aims at reducing GHG emissions to a level of 5.2% below 1990 levels by the period 2008-2012, while the Canadian target is 6%.2 Because carbon dioxide (CO2) accounts for the largest portion of the world’s annual emissions of GHGs,1 its emissions from industrial waste gases, particularly flue gases from coal-fired power stations, have become a major target for reduction. The removal of CO2 from gas streams can be achieved by a number of separation techniques including absorption into a liquid solvent, adsorption onto a solid, cryogenic separation, permeation through membranes, and chemical conversion. Among these techniques, absorption into a liquid solvent is the most suitable process for removing CO2 from high-volume flue gas streams. The commonly used solvents are aqueous solutions of alkanolamines, such as monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), and methyldiethanolamine (MDEA).3,4 These alkanolamines have been competing with another class of acid gas treating solvents, the sterically hindered amines.5 The most recognized amine of this class is 2-amino-2-methyl-1-propanol (AMP). Among these solvents, MEA is the most widely used because it has a faster rate of reaction with CO2, which allows absorption to take place in a shorter column. However, the operating cost of the absorption process * To whom correspondence should be addressed. Tel.: (306) 337-2469. Fax: (306) 585-4855. E-mail: [email protected].

using MEA is prohibitively high mainly because of (i) the significant amount of energy required for solvent regeneration and (ii) severe operational problems such as corrosion and solvent degradation. DEA, DIPA, and AMP have moderate rates of reaction with CO2 and are prone to a certain extent of corrosion and solvent degradation, whereas MDEA reacts with CO2 at a slow rate and appears to be immune to both operational problems. At the present time, AMP and MDEA are receiving a great deal of attention because they require relatively low energy consumption for solvent regeneration, leading to significant savings in process costs. Today, solvent formulation achieved by blending a variety of single alkanolamines is of interest because it combines the favorable characteristics of different solvents while suppressing their unfavorable characteristics. A number of blended alkanolamines have been developed by several chemical companies such as Dow Chemical Company and UOP.4 The common blends are MDEA-based solvents, which contain MDEA and primary (MEA) or secondary (DEA) alkanolamines. These solvents are claimed to consume less energy during solvent regeneration compared to single alkanolamines and yet provide satisfactory capture efficiency.6 This work focuses on the CO2 absorption performance of both single and blended alkanolamines, information critical for the development of cost-effective capture units. To date, many data sets of equilibrium solubility and the kinetics of single and blended alkanolamines have been generated to provide an understanding of the fundamental behavior of solvents.4,7-16 However, these fundamental data do not represent the overall absorption performance found in the actual operation of gasliquid contactors such as packed columns. This is due to the way in which data were obtained. Customarily, solubility and kinetics are characterized by using classical laboratory reactors, such as stirred cells and laminar jet absorbers where the gas-liquid interfacial area for mass transfer is known and fixed by the dimensions of the cell and jet nozzle orifice.17 The information obtained from these reactors does not

10.1021/ie0306067 CCC: $27.50 © 2004 American Chemical Society Published on Web 04/03/2004

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2229 Table 1. Operating Conditions for CO2 Absorption Tests CO2 concentration in feed gas, % flow rate of the feed gas, kmol/m2‚h concentration of the amine solution, kmol/m3 CO2 loading in the feed amine solution, mol/mol flow rate of the liquid solution, m3/m2‚h

Figure 1. Schematic diagram of the bench-scale absorption unit.

account for variations in the gas-liquid interfacial area during plant operation, which are caused by changes in the process operating conditions as well as the interfacial properties of the gas-liquid pair. From the practical point of view, the mass-transfer behavior obtained from laboratory units such as packed columns best represents the overall absorption performance in gas-liquid contactors because it accounts for not only thermodynamics (solubility) and kinetics contributions but also system hydrodynamics, which captures the actual contacting area provided for absorption. At the present time, knowledge of the mass transfer for CO2-alkanolamine systems, especially for blended alkanolamines, is very limited in the open literature. Therefore, this work was undertaken to evaluate, characterize, and compare the CO2 absorption performance of various alkanolamine systems using a continuous packed column. The tested alkanolamines were MEA, DEA, DIPA, MDEA, AMP, and their mixtures including MEA-MDEA, DEA-MDEA, MEA-AMP, and DEA-AMP. The performance is given in terms of the CO2 removal efficiency, effective interfacial area for mass transfer, overall mass-transfer coefficient, and absorber height requirement. 2. Experimental Section The absorption performance of alkanolamines was evaluated by conducting experiments in a bench-scale absorption unit, of which a simplified flow diagram is given in Figure 1. The unit consisted of an acrylic absorption column (2.0 × 10-2 m in diameter and 2.0 m in height), packed with 36 elements of stainless steel structured packing (Sulzer DX). The column was designed for a countercurrent mode of operation in which a liquid solution was introduced to the column at the top, while a gas mixture entered the column below the packing section. A series of gas sampling points were also installed at regular intervals along the sides of the column to allow measurements of the gas-phase CO2 concentration during experiments. The absorption unit was also composed of (i) two 20-L solution tanks, (ii) two calibrated mass flowmeters, (iii) a variable-speed gear pump, and (iv) an infrared (IR) gas analyzer. The solution tanks served as reservoirs for supplying and receiving the liquid solution used in the experiments. Mass flowmeters from Aalborg Instruments & Controls Inc. (model GFM 17) were used to measure the flow rates of N2 and CO2 gases entering

10 48.2 3.0 0.00, 0.25, 0.40 4.8-10.0

the column. The gear pump (Cole-Parmer) was used to drive the liquid solution to the top of the column. The IR gas analyzer (model 301D, Nova Analytical Systems Inc.) was operated during the experiments to withdraw samples and measure the CO2 concentration of the gas mixture inside the column. This analyzer can measure the CO2 concentration up to 20%. Prior to the experiments, an aqueous solution of alkanolamine (e.g., MEA, DEA, etc.) was prepared in the feed tank by diluting the concentrated alkanolamine with deionized water to a given concentration. The total alkanolamine concentration was determined by titration with a standard 1.0 kmol/m3 hydrochloric acid (HCl) solution using methyl orange indicator. Each experimental run began by introducing N2 and CO2 gases from cylinders through mass flowmeters at desired flow rates to produce a CO2-N2 gas mixture, which was fed to the bottom of the column. The concentration of CO2 in the feed gas was checked by the IR gas analyzer and adjusted until the desired value was obtained. The prepared alkanolamine solution was then pumped at a given flow rate to the column top so as to create countercurrent contact between 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. At this point, gas-phase CO2 concentrations at different positions along the column were measured through a series of sampling points by the IR gas analyzer. At the same time, liquid samples were taken from the bottom of the column and analyzed for their concentrations and CO2 loading. Details on CO2 loading analysis can be found in our previous work.18 All experiments were carried out at room temperature and under atmospheric pressure. Details of the operating conditions are summarized in Table 1. 3. Evaluation of the Mass-Transfer Performance Experimental results in this work were obtained as CO2 concentration profiles, illustrating changes in the gas-phase CO2 content with the height of the absorption column. These profiles were used to evaluate the CO2 absorption performance of alkanolamine solutions in terms of the CO2 removal efficiency, effective area for mass transfer (ae), and volumetric overall mass-transfer coefficient (KGae). 3.1. CO2 Removal Efficiency. This term defines the percentage of CO2 in the gas stream that was removed during absorption operation. The removal efficiency (η) was simply determined from the difference between the amounts of CO2 entering and leaving the column, which can be expressed by the following equation:

[ (

η) 1-

yCO2,out

)(

1 - yCO2,out

)]

1 - yCO2,in yCO2,in

× 100

(1)

where yCO2,in and yCO2,out denote mole fractions of gasphase CO2 entering and leaving the absorption column, respectively.

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3.2. Effective Interfacial Area for Mass Transfer. It is well recognized that the gas-liquid interfacial area is an important factor for mass transfer in the gas absorption process. A higher interfacial area is required for greater mass-transfer efficiency. Determination of the mass-transfer area is considered to be the most formidable task in column design because it involves both experiments and rather complicated calculation steps. The calculations require a large number of fluid dynamic parameters and solvent physical properties, as well as the geometry of column internals that constantly vary from location to location within the column. The calculations can become even more difficult in the case of CO2 absorption into aqueous alkanolamine solutions, where the mass-transfer process involves exothermic chemical reactions, causing a significant variation in the column temperature. In this work, the interfacial area (ae) was determined by using an in-house rigorous mathematical model for column design, which was first developed and used for mass-transfer prediction in our previous work.18,19 The model was built upon the theoretical packed column design procedure for adiabatic gas absorption with chemical reaction, which accounts for heat of absorption, solvent evaporation, and condensation, chemical reaction in the liquid phase, and simultaneous heat- and mass-transfer processes. The model was an integration of three submodels: (i) liquid-flow distribution, (ii) mass transfer with chemical reactions, and (iii) vapor-liquid equilibrium. The liquid distribution submodel provided information regarding the liquid-flow distribution inside the packed column. The mass-transfer submodel applied the penetration theory of Higbie,20 absorption kinetics data, and enhancement factor equation of DeCoursey and Thring21 to determine an individual mass-transfer coefficient (kL). This submodel also employed two-film theory for the determination of an overall mass-transfer coefficient (KG) across the gas-liquid interface. The vapor-liquid equilibrium submodel was used to estimate the concentration of free CO2 in the bulk liquid, the key element for establishing the mass-transfer driving force during absorption operation. The overall model simulation was achieved by dividing the column height into a number of sections. Each section was treated as a nonequilibrium (or rate-based) discrete stage, governed by material and energy equations. The model was modified specifically for this work to allow the determination of ae from experimental results. The modification involved the regression analysis, of which the intent was to fit the simulated CO2 concentration profiles to the profiles obtained from the experiments with ae as an adjustable parameter. The leastsquares method using function F (eq 2) was employed to minimize deviations of simulation results from experimental data

min F )

1m

(xc,i - xe,i)2 ∑ 2i)1

(2)

where xc,i and xe,i represent the ith simulation output and experimental data, respectively. 3.3. Volumetric Overall Mass-Transfer Coefficient. The overall mass-transfer coefficient (KGae) is a lumped parameter that represents the absorption performance per unit volume of packed column. It is a combination of three contributions associated with mass transfer, i.e., thermodynamics, kinetics, and hydrody-

Figure 2. Absorption performance of single-alkanolamine solutions under 0.00 mol/mol CO2 loading and 10.0 m3/m2‚h liquid load: (a) gas-phase CO2 concentration profile; (b) CO2 removal efficiency.

namics of CO2 absorption system. Here, the KGae coefficient was based on both the mass flux and material balance of the transferred component (CO2) across the gas-liquid interface. The coefficient was determined by using the following equation:

KGae )

[

GI

]( )

/ P(yCO2,G - yCO ) 2

dYCO2,G dZ

(3)

The first term on the right-hand side of eq 3 was determined from the operating conditions of the absorption experiments, e.g., gas load GI and total system pressure P. The second term (concentration gradient dYCO2,G/dZ) was obtained from a slope of the measured CO2 concentration profile. Details of the KGae determination can be found in our previous work.18 4. Results and Discussion 4.1. CO2 Absorption Performance of SingleAlkanolamine Solutions. Figure 2a shows the gasphase CO2 concentration profiles that were obtained from absorption experiments using five single-alkanolamine solutions, i.e., MEA, DEA, DIPA, MDEA, and AMP. The concentration of alkanolamines was 3.0 kmol/

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2231 Table 2. Literature Data on the Rate Constants (k2) for the Reaction of Alkanolamine and CO2 alkanolamine

temp (K)

concn (kmol/m3)

k2 (m3/kmol‚s)

MEA MEA DEA DEA DIPA DIPA MDEA MDEA AMP

298 298 298 298 298 293-298 298 298 288-318

1.60-4.80 0.25-1.90 0.25-1.92 0-0.09 0.33-2.16 0.23-3.96 0.50-1.63 0-1.14 0.25-3.50

750022 714023 134024 140025 46726 44327 4.826 7.828 68129

m3, and CO2 loading of feed solutions was nil (0.00 mol/ mol). The corresponding CO2 removal efficiency for each profile is presented in Figure 2b. It is apparent that MDEA gave the lowest CO2 absorption performance of 33%, while DIPA offered a higher performance of 87%. A complete removal (100%) was achieved by MEA, DEA, and AMP. Despite the comparable efficiency, the actual performance of these three alkanolamines can be differentiated by considering the column height required for complete removal. According to Figure 2a, CO2 absorption into a MEA solution took place within only 0.75 m from the column bottom, while DEA and AMP required as high as 1.75 and 2.00 m, respectively, to meet the same removal target. With this information, the CO2 absorption performance can be ranked in the following order: MEA > DEA > AMP > DIPA > MDEA. Note that this performance order is similar to that of the rate constant (k2) shown in Table 2. This is due to the fact that the tested solutions containing no CO2 offer a maximum thermodynamic driving force for the masstransfer process, resulting in a dominant role of reaction kinetics in governing the CO2 absorption performance. The absorption performances of these single-alkanolamine solutions were further evaluated at different CO2 loadings (i.e., 0.25 and 0.40 mol/mol) in order to broaden the performance comparison and also to simulate typical process conditions, where the feed solution contains residual CO2 content resulting from incomplete solvent regeneration. Experimental results in Figure 3 show that CO2 loading of the feed solution had an apparent influence on the shape of CO2 concentration profiles. As CO2 loading increased, the profiles shifted toward the top of the column, resulting in higher amounts of CO2 remaining in the treated gas. This illustrates a reduction in the CO2 removal efficiency. For instance, the efficiency of DEA was reduced from 100% to 93% and 54% when CO2 loading varied from nil to 0.25 and 0.40 mol/mol, respectively. The lower efficiency was simply caused by a reduction in the concentration of reactive alkanolamine. Figure 3 also contains information on the order of the CO2 removal efficiency. It was found that an increase in CO2 loading of the feed solution from nil to 0.25 mol/ mol had no effect on the efficiency order; i.e., it remained similar to that of the fresh solution (MEA > DEA > AMP > DIPA > MDEA). However, once CO2 loading was raised to 0.40 mol/mol (Figure 3b), AMP was more efficient than DEA and became the second best alkanolamine for CO2 absorption. This is because AMP has a greater mass-transfer driving force than DEA. Under the tested conditions, CO2 loading of both solutions leaving the absorption column was approximately 0.49 mol/mol. Because the equilibrium CO2 solubility of AMP and DEA are 1.00 and 0.50 mol/mol, respectively, the AMP solution did not approach its equilibrium before departure from the column, but DEA did. Consequently,

Figure 3. Absorption performance of single-alkanolamine solutions under CO2-loaded conditions: (a) 0.25 mol/mol CO2 loading; (b) 0.40 mol/mol CO2 loading.

the order of the removal efficiency was changed to MEA > AMP > DEA > DIPA > MDEA. In addition to CO2 loading, the performance evaluation was extended to cover a range of liquid load. The results show that liquid load had a great impact on the CO2 absorption performance. Reducing the liquid load caused an increase in the CO2 concentration of the treated gas, demonstrating a lower removal efficiency (Figure 4). Calculations based on our model (section 3.2) indicate that the lower efficiency was mainly due to a reduction in the effective area for the mass-transfer process. The variation in liquid load can also alter the order of the CO2 removal efficiency. According to Figure 4, as the liquid load was reduced from 10.0 to 4.8 m3/m2‚h, AMP became the most efficient solvent, offering superior absorption performance over the rest of alkanolamines, including MEA. This was again due to the greater equilibrium CO2 solubility of AMP. Under this condition, AMP was able to absorb more CO2, while other solvents

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Figure 4. Effect of liquid load on the CO2 absorption performance.

Figure 5. Variation in the mass-transfer area due to alkanolamine type.

(especially MEA and DEA) encountered thermodynamic limitations. It should be noted here that, in addition to kinetics and thermodynamics, the CO2 absorption performance in a packed column is also contributed to by hydrodynamics, which reflects the effective mass-transfer area (ae) provided by packing. Calculation results obtained from our mathematical model (Figure 5) show that the effective mass-transfer area varied significantly with the type of alkanolamine, even when the column was operated under identical conditions. For instance, at 10 m3/m2‚h liquid load, MEA was found to provide the highest mass-transfer area of 570 m2/m3, whereas MDEA offered the lowest area of 200 m2/m3. Masstransfer areas of DEA and DIPA were between those two values. The variation in the mass-transfer area was essentially a result of interaction parameters between physical properties of alkanolamine and characteristics of the packing surface, such as the viscosity, surface tension, and contact angle. The typical relationship between the mass-transfer area and such properties can be found in the literature.30,31 Because of the significant contribution of hydrodynamics to the absorption performance, it is crucial to use an accurate mass-transfer area of the system of interest during column design. At present, there is very limited knowledge on the correlation between the masstransfer area and type of alkanolamine. As such, it is customary to apply the area obtained from a well-known

system to the system of interest. This would, however, lead to an overestimated absorption performance if the assumed area is greater than the actual value. For instance, when the mass-transfer area of the MEA system is used as an assumed value for DIPA and MDEA, the calculated CO2 absorption performance of these systems will be higher than the actual values. As a result, the absorption column will be designed to have a height that is too short for the desired CO2 removal target. 4.2. CO2 Absorption Performance of BlendedAlkanolamine Solutions. This section focuses on the absorption performance of four blended-alkanolamine solutions, i.e., MEA-MDEA, DEA-MDEA, MEA-AMP, and DEA-AMP. Both AMP and MDEA were selected because of their relatively low energy consumption for solvent regeneration, while MEA and DEA were used as promoters to enhance the absorption rate of CO2. The performance of these blended solutions was examined at a total alkanolamine concentration of 3.0 kmol/m3, a mixing ratio of 1:1 mol/mol, and three different CO2 loadings of the feed solution (0.00, 0.25, and 0.40 mol/ mol). The results are shown in Figures 6-9 as CO2 concentration profiles of blended solutions plotted against the profiles of their parent alkanolamines. It is apparent that absorption performance of blended solutions was generally between the performance of their precursors. This also appears to be influenced by variations in CO2 loading in a manner similar to that of the singlealkanolamine systems. That is, as CO2 loading was increased, CO2 concentration profiles of blended solutions shifted upward, indicating a lower CO2 absorption efficiency. Turning to a consideration of the absorption performance of MEA-MDEA and DEA-MDEA (Figures 6 and 7), it was noticed that CO2 concentration profiles of blended solutions approached the profiles of rate promoters (MEA and DEA) at low CO2 loading. However, once CO2 loading was increased, the profiles of blended solutions moved toward the profiles of MDEA. This behavior illustrates a combined kinetic/thermodynamic competition between two reactive species (MEA or DEA and MDEA) in the blended solutions. At low CO2 loading, the rate promoters (MEA and DEA) played a dominant role in controlling the rate of absorption because they reacted with CO2 in a much faster rate than MDEA to form very stable carbamate compounds. As CO2 loading was increased, more CO2 was converted to carbamate of the MEA or DEA, leading to a reduction in the ratio of unreacted promoter to unreacted MDEA. As a result, MDEA gained its role in determining the CO2 absorption rate. This suggests that if these blended alkanolamines are to be put to use, their rate promoters (MEA and DEA) will likely serve as primary reactants absorbing CO2 at the top portion of the column, while MDEA will govern the absorption at the bottom portion. The blended solutions of MEA-AMP and DEA-AMP behaved somewhat differently from the above. As seen in Figure 8, the profiles of MEA-AMP remained close to that of MEA regardless of CO2 loading. This indicates that MEA enhanced the CO2 absorption rate in the MEA-AMP solution more effectively than in the MEAMDEA solution. The results also suggest that, when put to use, MEA-AMP would provide an absorption performance comparable to that of MEA for the entire length of the absorption column. For DEA-AMP, it was revealed in Figure 9 that adding DEA into AMP did not

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2233

Figure 6. Absorption performance of blended MEA-MDEA: (a) 0.00 mol/mol CO2 loading; (b) 0.25 mol/mol CO2 loading; (c) 0.40 mol/mol CO2 loading.

Figure 7. Absorption performance of blended DEA-MDEA: (a) 0.00 mol/mol CO2 loading; (b) 0.25 mol/mol CO2 loading; (c) 0.40 mol/mol CO2 loading.

significantly improve the performance of AMP. This was simply because the performances of DEA and AMP were comparable as indicated by the close position of their CO2 concentration profiles. In addition to the type of blended solution, the mixing ratio also played an essential role in controlling the absorption performance. As seen in Figure 10, the CO2 removal efficiency increased with the mixing ratio of MEA to MDEA. For instance, at a CO2 loading of 0.25 mol/mol, as the ratio increased from 0.0 mol/mol (i.e., single MDEA) to 0.5, 1.0, and 2.0 mol/mol, the efficiency

could improve from 15% to 63, 82, and 99%, respectively. This was a result of the increasing amount of reactive MEA. This suggests that the performance of blended alkanolamines can be customized to fit a specific removal target by varying the mixing ratio. 4.3. Overall Performance Comparison and Industrial Implication. The CO2 absorption performance of all tested alkanolamines was ultimately analyzed in terms of the overall mass-transfer coefficient (KGae) to allow practical comparison and future utilization of data in process design. The KGae values

2234 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004

Figure 8. Absorption performance of blended MEA-AMP: (a) 0.00 mol/mol CO2 loading; (b) 0.25 mol/mol CO2 loading; (c) 0.40 mol/mol CO2 loading.

Figure 9. Absorption performance of blended DEA-AMP: (a) 0.00 mol/mol CO2 loading; (b) 0.25 mol/mol CO2 loading; (c) 0.40 mol/mol CO2 loading.

in Figure 11 show that, among single alkanolamines, MEA offered the greatest absorption performance, followed by DEA or AMP, DIPA, and MDEA regardless of CO2 loading. The relative performance can be quantified as the “mass-transfer index” representing the ratio of the KGae value of a particular alkanolamine to that of MEA (index for MEA ) 1.0). From Figure 12, masstransfer indexes for other single alkanolamines are less than half those of MEA. The indexes for DEA and AMP are 0.39-0.45, while those for DIPA and MDEA are 0.16-0.21 and 0.03, respectively.

The mass-transfer indexes were subsequently converted into relative column heights required for CO2 capture as shown in Figure 13. The relative height simply defines how much higher the height of the absorption column using a particular alkanolamine is than the height of the column using MEA. For a given CO2 removal target, the absorption columns using DEA and AMP would be approximately 2.5 times taller than the column using MEA, while DIPA and MDEA would require as high as 5.9 and 34.4 times taller columns, respectively.

Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 2235

Figure 10. Effect of the mixing ratio on the absorption performance of blended MEA-MDEA. Figure 13. Relative column height requirement for CO2 capture.

Figure 11. Mass-transfer coefficient of single alkanolamine.

Figure 12. Mass-transfer index of single and blended alkanolamines (mixing ratio ) 1:1).

For the sake of solvent formulation, it is necessary to compare the absorption performance of blended alkanolamines with that of single alkanolamines, particularly MEA. According to Figure 12, MEA-AMP is the best blended alkanolamine tested in this work. The second best is DEA-AMP, followed by MEA-MDEA and DEA-MDEA. This trend can also be illustrated in terms of the column height requirement. As shown in Figure 13, the relative heights required for MEA-AMP, DEA-

AMP, MEA-MDEA, and DEA-MDEA are 1.2, 2.3, 3.3, and 5.4, respectively. This suggests that AMP-based solutions are capable of removing CO2 in a more effective manner than MDEA-based solutions because of the intrinsic kinetics of AMP. With its height requirement of 1.2, MEA-AMP is the most promising blended solution that offers a comparable absorption performance to MEA, while requiring low energy consumption during solvent regeneration. It should also be noted that the absorption performance of any blended alkanolamines does not necessarily have a linear correlation with the performance of their parent alkanolamines. This means that, with a mixing ratio of 1:1, the blended solution might not provide the average performance of the two single alkanolamines associated. As seen from Figure 12, the mass-transfer index for MEA-MDEA was in the range of 0.22-0.46, far below the average value of 0.52 (based on 1.00 for MEA and 0.03 for MDEA), while the index of MEA-AMP was between 0.73 and 0.87, more than the average of 0.69 (based on 1.00 for MEA and 0.39 for AMP). Because of such unsystematic behavior, evaluation of the mass-transfer performance by experiments is particularly crucial for the formulation of effective blended solvents. The nonlinear mass-transfer behavior of these blended alkanolamines can perhaps be explained by reaction kinetics and physical properties of solutions. Consider the MEA-MDEA system as an example. We calculated the overall reaction rate constants (kov) for CO2 absorption into a 3.0 kmol/m3 MEA-MDEA aqueous solution as a function of the mixing ratio. The calculation was based on the rate model of Liao and Li,16 which involves a zwitterion mechanism for the CO2-MEA reaction and a pseudo-first-order reaction for CO2-MDEA. The calculation results summarized in Table 3 show the nonlinear relationship between the overall rate constant and mixing ratio. For instance, at 30 °C, kov of 4802.0 s-1 for MEA-MDEA with a mixing ratio of 1:1 is less than the average value calculated from the rate constants of 12 614.0 s-1 for MEA and 23.3 s-1 for MDEA. This suggests that reaction kinetics plays an important role in the nonlinear mass-transfer behavior of MEAMDEA. Physical properties of blended solutions are also responsible for the nonlinearity of the mass-transfer behavior. According to Table 3, the viscosity of the

2236 Ind. Eng. Chem. Res., Vol. 43, No. 9, 2004 Table 3. Calculated Overall Rate Constant and Viscosity of a 3.0 Kmol/m3 MEA-MDEA Aqueous Solution temp (°C)

mixing ratio (mol of MEA/mol of MDEA)

kov (s-1)

viscosity (mPa‚s)

30

0:3 1:2 1:1 2:1 3:0 0:3 1:2 1:1 2:1 3:0

23.3 2771.9 4802.0 7156.3 12614.0 41.5 5334.8 9535.5 14448.1 25831.2

3.53 3.10 2.81 2.45 1.40 2.58 2.28 2.08 1.84 1.11

40

MEA-MDEA solution changes with the mixing ratio; i.e., it increases in a nonlinear manner with the MDEA concentration. The increasing viscosity simply leads to reductions in the diffusion coefficient (DL)32 and effective interfacial area for mass transfer (ae).30 This eventually results in a nonlinear reduction in the mass-transfer performance. 5. Conclusions This work has extended the knowledge of CO2 absorption into aqueous solutions of both single and blended alkanolamines. Regardless of the solvent type, the absorption performance is greatly influenced by two operating parameters, CO2 loading of the solution and liquid load, which demonstrates that the performance can be enhanced by decreasing CO2 loading or increasing liquid load. During the packed column operation, the mass-transfer performance is essentially contributed by kinetics, thermodynamics, and hydrodynamics. In the case that the column is operated under typical service conditions where there is no thermodynamic limitation, kinetics of CO2 absorption would be the primary factor governing the overall removal efficiency. The rank of the CO2 absorption performance is therefore in accordance with the order of the reaction rate constant (k2), i.e., MEA > DEA > AMP > DIPA > MDEA. However, if the absorption column is operated under the conditions where thermodynamic limitation is reached (e.g., high CO2 loading of the feed solution and low liquid load), thermodynamics (solubility) would primarily govern the removal efficiency instead. As a result, the performance order might be altered. System hydrodynamics also contributes significantly to the performance because it offers an effective interfacial area for mass transfer. The interfacial area differs from one solvent to another, even when the column is operated under identical conditions. Hence, it is necessary to determine and use the actual effective interfacial area for process design. Finally, the CO2 absorption performance of blended alkanolamines is generally between but does not necessarily have a linear correlation with the performance of their parent alkanolamines. AMP-based solvents, especially MEA-AMP, are more effective in CO2 absorption than MDEA-based solvents. Acknowledgment Authors gratefully acknowledge the financial support received from the Natural Science and Engineering Research Council of Canada (NSERC) and the structured packing donated by the Sulzer Chemtech, Winterthur, Switzerland.

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Received for review July 18, 2003 Revised manuscript received January 12, 2004 Accepted February 27, 2004 IE0306067