Thermal Effects in the Absorption of Pure CO2 into Aqueous Solutions

Article pubs.acs.org/IECR

Thermal Effects in the Absorption of Pure CO2 into Aqueous Solutions of 2-Methyl-amino-ethanol Rafael Pacheco,† Antonio Sánchez,† María Dolores La Rubia,† Ana B. López,† Sebastián Sánchez,†,* and Fernando Camacho‡ †

Department of Chemical, Environmental and Materials Engineering, University of Jaén, 23071 Jaén, Spain Department of Chemical Engineering, Faculty of Science, University of Granada, 18071 Granada, Spain

‡

ABSTRACT: The process of pure CO2 absorption by aqueous solutions of 2-methyl-amino-ethanol (MAE) has been studied in relation to the thermal effects of the operation. This study evaluates the consequences of heat effects during absorption and chemical reaction. The experiments were performed in a stirred tank reactor operated in batches with respect to the gas−liquid phases, having a plane interfacial area. The working variables considered were the alkanolamine concentration within the interval 0.1−2.0 kmol/m3 and the temperature in the range 288−313 K. From the results, it is deduced that the CO2 absorption at high pressures in aqueous MAE solutions, occur in the instantaneous reaction regime with high interface temperature. An expression is proposed to relate the experimental results to the initial concentration of alkanolamine and at the same time enable the determination of the interfacial temperature (Ts). In relation to temperature in the bulk liquid phase (TB), increases (Ts − TB) close to 43 K were determined in the experiments performed at high concentrations and in the highest temperature series.

1. INTRODUCTION Processes to separate acidic gases (CO2, H2S, ...) from gaseous currents by mass transfer are common in the chemical industry and have been undertaken for more than 75 years, by absorption using aqueous or organic solutions of alkanolamines as the absorbent agent, these being frequently used in the production of natural gas, hydrogen purification, treatment of refinery gas, production of synthesis gas, etc. In recent years, the need to eliminate the gas currents from CO2 in certain industrial processes and from the combustion of fossil fuels has become necessary in order to avoid the increase in atmospheric greenhouse gases and to abide by the Kyoto Accords for the next years. In addition, as indicated by Li et al.,1 energy demand is predicted to grow by 60% by the year 2030, and CO2 emissions will augment by 63%, implying a 90% rise over levels recorded in 1990. This is not only moving governments to apply emission taxes but also prompting industry and research to place more emphasis on halting the rise in CO2 which carry gaseous effluents. Therefore, for their absorbent characteristics, alkanolamines are of great importance. The most commonly used alkanolamines include monoethanolamine, isopropanolamine, diethanolamine, diisopropanolamine, triethanolamine, and N-methyldiethanolamine. Also, tests have been made with sterically impeded alkanolamines such as the primary 2-amino-2-methyl-1-propanol, or the secondary 2-isopropylaminoethanol, all of which present good CO2-absorption capacity, quick reaction, and low corrosion even at high concentrations, at the same time as requiring lower regeneration than do conventional alkanolamines.2,3 Also, searching for better advantages, some research groups have tested alkanolamine mixtures.4,5 Ample information is available in the literature concerning absorption kinetics under isothermal conditions of CO2 at low partial pressures, in classical alkanolamines (MEA, DEA, or TEA), but less information is available on the process at high © 2012 American Chemical Society

partial pressures of carbon dioxide. In any case, the reactions between CO2 and the different alkanolamines is complex and not entirely known,6,7 since these are highly complex systems that at times involve up to 30 different reactions.8 Therefore, it is necessary to know exactly the kinetic behavior of each alkanolamine, which would differ both in its structure as well as in terms of the organic or aqueous solvent used in the process. Also, the speed with which the process would take place would depend on the design of the industrial equipment used for the absorption. Furthermore, both in the physical absorption of a highly soluble gas as well as in the absorption with a chemical reaction, the temperature of the liquid phase can rise, mainly near the gas−liquid interface, due to the solution and reaction heat.9−11 In some systems, the increases in temperatures are negligible but in others the rise in temperature is important and influences the process.12 Indirectly-detected increases in interface temperature of up to 53 K in chlorination reactions and 58 K in sulphonation reactions13 have been seen, and in direct form, by an infrared technique, temperature increases of 20 K have been found in the separation interface.14 Depending on the type of contactor used, there are differences found in the temperature rises. For a column with wet walls, the increases are lower than for a flat-area interfacial reactor, due to the greater heat dissipation in the column. Therefore in absorption systems with chemical reactions, it is necessary to know these thermal effects in order to make an appropriate design of the contactor to be used. In the case of the thermal effects produced in absorption with the chemical reaction of CO2 in aqueous alkanolamine Received: Revised: Accepted: Published: 4809

May 14, 2011 March 3, 2012 March 4, 2012 March 4, 2012 dx.doi.org/10.1021/ie201035y | Ind. Eng. Chem. Res. 2012, 51, 4809−4818

Industrial & Engineering Chemistry Research

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The diffusion coefficients of CO2 in the aqueous solution were measured using the relationships of Sada et al.,23 Versteeg et al.,24 and Versteeg and Swaaij:25

solutions, the partial pressure of the gas should be taken into account. In this sense, all the research groups that have worked at low partial pressures agree in considering the thermal effects negligible. However, when the CO2 absorption occurs at high partial pressures, elevated temperatures can result, since the heat generated cannot be evacuated quickly enough. This fact was noted by Clarke15 on using CO2 in aqueous solutions of monoethanolamine at near atmospheric pressures. This researcher found that the influence of the heat of reaction on the absorption rate was notable, while the influence was negligible when the partial pressure of the CO2 was on the order of 80 mmHg.15 Similarly, these thermal effects have been detected by our research group in the absorption of CO2 by aqueous solutions of 1-amino-2-propanol,16 monoethanolamine,17 3-amino-1-propanol,18 and 2-amino-2-methyl-1-propanol.12 The aim of the present work is to study the absorption process of carbon dioxide at high partial pressures in aqueous solutions of 2-methyl-amino-ethanol (MAE) in relation to the thermal effects of the process.

DCO2 DCO2,w

D N2O D N2O,w

(5)

DCO2,w = 2.35 × 10‐6e−2119/ T

(6)

D N2O,w = 5.07 × 10−6e−2371/ T

(7)

γ D N2OμB = D N2O,w μ γw

(8)

where

where γ = 0.8.25 The diffusion coefficients of MAE in aqueous solution were calculated by means of the relationship of Wilke and Chang:26

2. MATERIALS AND METHODS

DB = 3.03 × 10−15

The carbon dioxide used in this work was CO2 N-38 (99.8% SEO), with oxygen, water steam, and hydrocarbons as impurities; 2-methyl-amino-ethanol was a Fluka product with nominal purity of 98.0%. Aqueous solutions of alkanolamine were prepared with distilled−deionized water (resistivity 18.2 MΩ-cm). The experiments were made in a stirred tank reactor at a stirring rate of 80 ± 1 rpm, a batch with respect to the gas− liquid phases, and with a flat and known interface area of 35.26 cm2. The experimental device used is described in previous research.19 Volumes of 100 cm3 of the aqueous solutions of MAE were used in the range 0.1−2.0 mol/dm3, and the temperature was set within the interval 288−313 K. 2.1. Physical and Transport Properties. Under these experimental conditions, we measured the viscosity of the amine solutions and their densities. The calculation of the initial partial pressure of the CO2 is given by pA = P − pV − pI

=

T μB

(9)

The values of μw were determined in a previous work at the assayed temperatures.27 2.2. Analytical Methods. The initial amine concentration was determined by titration with HCl solutions using methyl orange as the indicator. The CO2 concentration in liquid was determined by the method described in previous work.28

(1)

The CO2 solubility in the aqueous MAE solutions was determined by the Danckwerts20 relationship, according to the expression: ⎛ ⎞ He ⎟ log⎜⎜ ⎟ = hI ⎝ He H2O ⎠

(2)

Considering the temperature effect21,22 on the value of h, eq 2 for the MAE solutions results: He = 10(5.3 + 0.035CBo− 1140/ T )

(3)

He = 10(5.3 + 0.026CBo− 1140/ T )

(4) Figure 1. CO2 absorbed per unit of surface and time in the experiments with MAE at 298 K.

where eq 3 is valid for 288−298 K, and eq 4 for the interval 303−313 K. 4810

dx.doi.org/10.1021/ie201035y | Ind. Eng. Chem. Res. 2012, 51, 4809−4818


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temperature and CBo = 1.71 kmol/m3, the value was 7.89 × 10−6 kmol/(m2 s).17

Table 1. Flow Densities in the Absorption of Carbon Dioxide in 2-Methyl-amino-ethanol Solutions T (K) 288

293

298

303

308

313

pA (kPa)

CBo (kmol/m3)

η (mol CO2/mol MAE)

NA 106 (kmol/m2 s)

93.0 92.8 92.9 93.5 92.8 92.3 91.8 91.9 92.2 90.0 91.7 91.1 90.3 90.4 90.3 90.1 89.9 89.0 89.0 89.0 89.7 89.7 89.3 88.5 87.9 87.8 87.5 87.5 87.0 86.0 86.8 85.7 86.3 85.3 84.8 84.0

0.0374 0.106 0.229 0.412 0.818 1.78 0.0589 0.151 0.233 0.412 0.849 1.72 0.0594 0.163 0.270 0.411 0.822 1.59 0.0607 0.154 0.245 0.423 0.859 1.84 0.0632 0.146 0.251 0.434 0.891 1.63 0.0827 0.193 0.264 0.460 0.898 1.80

1.11 0.39 0.18 0.099 0.048 0.020 0.60 0.23 0.15 0.083 0.039 0.018 0.50 0.18 0.11 0.070 0.034 0.016 0.42 0.16 0.10 0.059 0.028 0.012 0.34 0.15 0.086 0.049 0.023 0.012 0.23 0.095 0.070 0.039 0.019 0.0091

1.29 1.44 1.75 2.16 3.17 4.40 1.29 1.55 1.82 2.35 3.44 4.92 1.37 1.70 2.15 2.55 3.94 5.88 1.38 1.76 2.13 2.75 4.34 6.69 1.41 1.95 2.32 3.06 4.81 7.28 1.79 2.33 2.61 3.63 5.48 8.40

Figure 2. (a) Variation in pH of MAE solution with time, over the course of the absorption process at 293 K at the initial concentrations. (b) Variation in pH of the MAE initial concentration at 293 K.

3. RESULTS AND DISCUSSION The flow densities, NA, were calculated assuming that the gas follows ideal behavior, using the expression: NA =

PQ ′ n′ = A RTA

Figure 3. Reaction regime for MAE at 293 K.

3.1. Absorption in MAE Solutions. MAE, CH3−NH− CH2−CH2OH, is a secondary alkanolamine in which nitrogen is bonded to a methyl group and to an ethanol group. It is a compound that has been studied in the search of different absorbents that offer good characteristics for absorbing CO2 but at the same time need small quantities of energy for their regeneration, as the conventional alkanolamines used to date require high thermal energy to recover the absorbent.2,3,29 Little information is available on this alkanolamine, although there is unanimous consensus on the reactions that take place in an aqueous medium for the CO2−MAE system,29−31 in which both the amine group (>NH) as well as the alcohol group (−OH) can react with the CO2 in the form

(10)

The value of the volumetric flow (Q′) coincided with the value of the slope of the straight lines on representing the volume of the CO2 absorbed against time (t) for each experiment. The determinations were made using the linear regression method of the experimental results (Figure 1). In the absorption experiments with pure CO2 in aqueous MAE solutions, the influence of the initial alkanolamine concentration, and the operating temperature were analyzed. The flow densities, determined with MAE, were lower than those obtained when a standard primary alkanolamine was used, such as monoethanolamine (MEA) in the same experimental conditions. Thereby using MAE, at 293 K and a concentration of CBo = 1.72 kmol/m3, the value of NA was 4.92 × 10−6 kmol/(m2 s) (Table 1), while with MEA, at the same 4811



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following any of the mechanisms proposed for the reaction of the carbon dioxide with the secondary alkanolamines that are involved with the amine in the formation of carbamic acid and its ionization,32 the formation of a carbamate and subsequent ionization33 or the formation of an intermediate zwitterion compound that afterward reacts with the amine to form the carbamate34 and which for secondary alkanolamines leads to the general stoichiometric expression +

CO2 + 2R2NH → R2NCOO− + R2NH2

(13)

The quantity of carbonate ion (eq 12), depends on the pH of the solution and therefore on its dissociation constant. Since the dissociation is possible only at pH ≥ 11,35 the formation of the derivative of the carbonic acid and its contribution to the elimination of the CO2 at lower pH values may be considered practically negligible with respect to eq 13. For the process of CO2 absorption by aqueous MAE solutions, Ohno et al.,36 after spectroscopic studies completed with the results of RMN, proposed the reactions

Figure 4. Variation in NA with MAE concentration at 293 K.

2CH3NHCH2CH2OH + CO2 ⥂CH3N(COO−)CH2CH2OH +

+ CH3NH2CH2CH2OH

(14)

CH3N(COO−)CH2CH2OH + H2O ⥃ CH3NHCH2CH2OH + HCO− 3

(15)

CH3NHCH2CH2OH + CO2 + H2O +

⥂CH3NH2CH2CH2OH + HCO− 3

These authors indicate that the reaction 14 is dominant in the first part of the process, while reactions 15 and 16, would occur later, and reaction 15 would be displaced toward the formation of HCO3−. It can be appreciated that the mechanism proposed by Ohno et al.36 is an interpretation of what is generally accepted for secondary alkanolamines. In relation to the pH measurements in the MAE solutions used, it should be indicated that if the series 293 K (Figure 2a) is considered as an example, at the beginning of the experiments the values are between 10.5 for the lowest concentrations and 11.5 for the highest ones. It can be stated that the formation of the carbamic acid derivative for the all the series tested takes priority over the formation of the carbonic acid derivative and only at the highest concentrations during the first instants of the reaction could the reaction take place, eq 12. Moreover, due to the basic character of MAE, its reaction with CO2 causes the alkanolamine concentration to diminish over time and with it the pH of the solution, as reflected in Figure 2a. The initial pH values of the solutions were correlated with the initial concentrations of the alkanolamine, giving acceptable linear relations for each temperature, as shown in Figure 2b for the experiments at 293 K; for this, the fit is determined by eq 17, and similar fits for the rest of the temperatures assayed were made.

Figure 5. (a) Variation of NA with MAE concentration at 288−298 K in the experiments with instantaneous reaction regime, at CBo < 1.0 kmol/m3. (b) Variation of NA with MAE concentration at 303−313 K in the experiments with instantaneous reaction regime, at CBo < 2.0 kmol/m3.

Table 2. Data Corresponding to the NA vs CBo Linear Regression T (K)

(kL/2 )106(m/s)

S 106(m/s)

(0.0) 106(kmol/(m2 s))

r2

288 293 298 303 308 313

1.20 1.45 1.60 1.67 1.78 2.04

2.40 2.71 3.36 3.67 3.69 3.85

1.13 1.16 1.18 1.19 1.37 1.67

0.999 0.997 0.999 0.999 0.997 0.992

(16)

pH = 0.55 log CBo + 11.2

r 2 = 0.960

(17)

In addition, apart from reactions 11 and 12, the characteristics of the solution in which the CO2 is absorbed must be 4812



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Table 3. Interfacial Temperature in the Experiments with 2-Methyl-amino-ethanol TB (K)

CBo (kmol/m3)

PAs (kPa)

HeS (kPa m3/kmol)

DAs 109 (m2/s)

DB 109 (m2/s)

TS (K)

ΔT (K)

288

0.106 0.229 0.412 0.818 0.0589 0.151 0.233 0.412 0.849 0.0594 0.163 0.270 0.411 0.822 1.59 0.0607 0.154 0.245 0.423 0.859 1.84 0.0632 0.146 0.251 0.434 0.891 1.63 0.0827 0.193 0.264 0.460 0.898 1.80

92.3 91.2 90.4 88.1 91.1 89.5 88.6 86.4 82.0 89.9 87.3 85.8 82.9 79.3 71.8 88.9 86.4 84.9 81.0 76.2 61.5 88.4 86.3 82.1 77.4 69.9 61.2 86.4 80.1 76.5 69.6 58.3 38.1

2569.0 3151.4 3856.3 4579.3 2976.5 3763.3 4192.4 4935.5 6315.8 3235.9 4289.5 4793.1 5656.4 6754.3 8759.0 3534.5 4489.4 5183.0 6273.9 7474.8 10702.8 3997.5 4566.7 5743.0 6938.5 8590.3 10436.2 4493.3 6192.5 7183.5 8439.7 10562.6 14090.8

1.583 1.796 1.992 1.989 1.808 2.126 2.263 2.441 2.576 1.941 2.364 2.505 2.744 2.777 2.590 2.112 2.499 2.737 3.037 3.071 2.895 2.305 2.512 2.940 3.258 3.392 3.111 2.389 3.010 3.332 3.604 3.821 3.604

0.726 0.692 0.644 0.548 0.856 0.827 0.802 0.750 0.637 0.981 0.946 0.911 0.866 0.749 0.572 1.125 1.090 1.057 0.994 0.856 0.612 1.270 1.236 1.195 1.124 0.967 0.759 1.437 1.386 1.354 1.269 1.098 0.817

292.3 298.7 305.3 310.3 297.3 305.5 308.8 314.3 322.5 300.1 309.8 313.6 319.5 325.2 333.4 303.3 311.7 316.9 323.9 330.0 342.9 307.6 312.3 320.8 327.9 335.7 342.3 311.9 323.9 329.8 336.1 344.8 355.8

4.3 10.7 17.3 22.3 4.3 12.2 15.8 21.3 29.5 2.1 11.8 15.6 21.5 27.2 35.4 0.3 8.7 13.9 20.9 27.0 39.9 −0.4 (0) 4.3 12.8 19.9 27.7 34.3 −1.1 (0) 10.9 16.8 23.1 31.8 42.8

293

298

303

308

313

However, as indicated above, the separation of CO2 for the formation of bicarbonate is considered negligible compared to the formation of carbamic acid. The extent to which any of the above-mentioned reactions can take place depends on their kinetic constants and concentrations in alkanolamines and OH− ions. Therefore, the relative quantities of carbon dioxide and MAE are determinant in the formation of the carbamic acid derivative. Thus, when the carbonation ratio (η)the quotient between the concentration of dissolved CO2 in equilibrium and the amine concentrationis lower than 0.5 mol CO2/mol MAE, the main product is the carbamic acid derivative (eq 13), but if η ≥ 0.5, in addition to eq 13, carbamate can be turned back into bicarbonate (eq 20).35 +

R2NCOO− + 2H2O + CO2 → R2NH2 + 2HCO− 3

Figure 6. Variation of the interfacial temperature increase with the concentration in the experiments at TB values considered.

(20)

In the majority of the experiments made with MAE, the carbonation relation has been lower than 0.5 mol CO2/mol MAE (Table 1), and hence it can be stated that the reaction product is the derivative of the carbamic acid formed by the most accepted mechanism, according to

taken into account. As it is an aqueous solution, following reaction would take place: CO2 + H2O ⇄ H2CO3

(18)

and due to the basic character of MAE, the following reaction would occur: CO2 + OH− ⇄ HCO− 3

(19) 4813

CO2 + R2NH → R2NCOOH

(21)

R2NCOOH → R2NCOO− + H+

(22)



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Table 4. Interfacial Temperature, Kinetics Parameters, Enhancement Factors and Hatta Number in the Experiments of MAE CBo (kmol/m3)

TS (K)

k2s (m3/(kmol s))

Eis

Ha

E

0.106 0.229 0.412 0.818 0.0589 0.151 0.233 0.412 0.849 0.0594 0.163 0.270 0.411 0.822 1.59 0.0607 0.154 0.245 0.423 0.859 1.84 0.0632 0.146 0.251 0.434 0.891 1.63 0.0827 0.193 0.264 0.460 0.898 1.80

292.3 298.7 305.3 310.3 297.3 305.2 308.8 314.3 322.5 300.1 309.8 313.6 319.5 325.2 333.5 303.2 311.7 316.9 323.9 330.0 342.9 307.6 312.3 320.8 327.9 335.7 342.4 311.9 323.9 329.8 336.1 344.8 355.8

5430.2 7055.5 9076.4 10946.0 6654.7 9035.2 1035.9 12653.6 16818.9 7446.9 10762.5 12353.2 15205.4 18464.7 24114.2 8399.5 11503.8 13857.0 17664.8 21591.4 32345.3 9912.0 11778.6 15902.5 20212.9 25971.3 31804.5 11583.6 17682.6 21472.5 26257.5 34286.3 47005.9

1.67 2.52 3.84 6.86 1.45 2.23 2.95 4.61 9.09 1.54 2.60 3.74 5.42 10.44 22.36 1.64 2.74 3.88 6.36 12.74 34.83 1.78 2.90 4.56 7.71 16.62 34.88 2.29 4.43 6.03 10.81 24.39 76.29

39.77 70.94 113.75 175.89 28.93 58.55 80.27 122.60 208.47 28.86 63.46 90.08 128.96 202.26 310.17 31.08 62.99 91.21 142.71 225.94 393.10 33.76 58.40 96.12 150.17 248.90 356.63 37.17 78.71 106.70 162.04 266.46 428.77

1.68 2.52 3.84 6.86 1.45 2.23 2.95 4.61 9.08 1.54 2.60 3.74 5.42 10.42 22.26 1.64 2.74 3.88 6.36 12.70 34.57 1.79 2.90 4.55 7.69 16.55 34.57 2.28 4.42 6.02 10.77 24.20 74.05

Figure 7. Variation of the enhancement factor (E) with the Hatta number (Ha) in the experiments of MAE at the temperatures indicated.

and 12044 m3/kmol s reported by Sotelo et al.30 at 293, 303, and 313 K, respectively, using a bubble column absorber. These differences, according to these latter authors may be due to the possible deficiencies observed in the work of Sharma.37 Meanwhile, Mimura et al.29 determined a kinetic constant of 7940 m3/kmol s, a value that presents a certain agreement with those calculated by Sotelo et al.30 for an overall second-order kinetic, one for the CO2 and one for the amine. However, Bavbek and Alper31 gave a value, for the order of reaction with respect to the amine of 1.56, using MAE concentrations in the range 0.02−0.05 kmol/m3. Given that the reaction regime appears to be directly linked to the carbonation relation and that, as indicated by Astarita et al.,35 when η < 0.5 mol CO2/mol amine the reaction continues to be a second-order mechanism, in order to recognize the regime in which the reaction took place between the pure carbon dioxide and the aqueous solutions of 2-methyl-aminoethanol, the criteria of Astarita et al.35 will be extended to the case of secondary alkanolamines. Afterward it will be ruled out that the process occurs in the hydrodynamic or physicalabsorption regime; it has been assumed that the process could take place in the fast-reaction regime, with the reaction order m with respect to the CO2 and n with respect to MAE. In this case, the flow density, NA, could be determined by eq 24:38

+

R2NH + H+ → R2NH2

(23)

Table 1 indicates that only in some experiments and under certain concentrations is the value of η greater than 0.5 mol CO2/mol MAE; nevertheless, when the results are interpreted, these values are not taken into account. The pH results (Figure 2a) indicate that the derivative of carbamic acid is formed throughout the CO2-absorption process by aqueous solutions of 2-methyl-amino-ethanol and that, at high concentrations during the first instants, carbonic acid forms, though in insignificant quantities. This is confirmed by the values found for the carbonation relation, η, which, except in the experiments made at concentrations of 0.0374 and 0.0589 kmol/m3 and at temperatures of 288 and 293 K, respectively, were lower than 0.5 mol CO2/mol MAE, in agreement with Astarita et al.35 This implies that the product of the reaction would be derived from carbamic acid and the mechanism would be indicated by eqs 21−23. 3.2. Reaction Regime. The kinetic study of the absorption process of pure carbon dioxide by aqueous MAE solutions can prove complex on the basis of the existing literature. That is, the experimental data for the kinetic constant present discrepancies inasmuch as Sharma37 determined 30000 m3/ kmol s for a temperature of 298 K, as opposed to 5573, 8355,

NA =

2 m+1 n DA k m , nCAo CBo m+1

(24)

If the reaction order for the CO2 is considered 1 in its reactions with secondary alkanolamines, eq 24 reduces to n NA = CAo DA k1, nCBo

(25)

If it is considered that CAo coincides with the concentration of the carbon dioxide in equilibrium with the gaseous phase, CA*, and this can be evaluated by Henry’s law (pA = HeCA*), eq 25 can be linearized as ⎛ 2 2⎞ N He ⎟ log⎜⎜ A 2 ⎟ = log k1, n + n log CBo ⎝ pA DA ⎠

(26)

The absence of a linear relationship in the graphic representations of the first member of eq 26 against log CBo for the temperatures tested rules out that the absorption of 4814



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phase, converting the operation, probably, into a nonisothermal process. At the end of each absorption experiment, by high-sensitivity digital thermometer, temperature in the bulk liquid phase was measured and compared with the pre-established temperature, this revealed increments up to 0.6 K. If it is considered that the reaction takes place in a thin film very close to the gas−liquid interface, the rise in temperature is sufficiently high to be able to transmit heat energy to the entire liquid mass in order to achieve the above-mentioned temperature increases. This implies that despite using a thermostatic reactor, the absorption happened in a nonisothermal way and the interfacial temperature was higher than that in the bulk liquid phase of the solution. If it is accepted that the absorption process happens in the nonisothermal instantaneous regime and if it is considered that Ts is the temperature in the interfacial film and TB is the temperature in the bulk liquid phase, the difference (Ts − TB) is the temperature increase of the film with respect to the working temperature TB Given that the calculation of NA by eq 28 generates smaller values than those found experimentally, probably because the absorption occurs with a temperature increase, different models formulated for the nonisothermal regime were applied to the experimental results. The best fit for NA and CBo was obtained with a modification of eq 28 by which the temperature Ts could be evaluated, resulting in eq 30

pure CO2 by the aqueous 2-methyl-amino-ethanol solutions is a fast-reaction regime, as shown for the 293 K series (Figure 3). However, the graphic representation of the flow density by total transport, NA against the initial MAE concentration, CBo, for the same 293 K series (Figure 4), indicates the possibility that the process takes place by an instantaneous reaction regime, at least for concentrations lower than 1.0 kmol/m3, according to deductions that can be made from the linear relationship. It could also take place at higher concentrations through a transition regime. Similar behavior can be appreciated for the rest of the temperature series, although for those between 298 and 313 K the linear relationship can be extended to higher concentrations. Consequently, for this amine (MAE) the kinetics of the reaction cannot be studied in the range of temperatures tested using a stirred tank reactor with a plane interfacial area, and only the thermic effects of the reaction process have been considered. 3.3. Influence of the MAE Concentration and Partial Pressure of CO2. If it is accepted that the regime is instantaneous, at least in a concentration interval (Figure 5), an effort will be made to relate the values of flow density, NA, to the MAE concentrations in the bulk liquid phase and the partial pressures of carbon dioxide. In the absorption process with a chemical reaction, the flow density can be expressed as * NA = EkLCA

(27)

When the reaction regime is instantaneous, the enhancement factor (E) coincides with the instantaneous enhancement factor (Ei), and considering its definition by the film theory, the above expression becomes: ⎛ D C ⎞ * NA = ⎜1 + B Bo ⎟kLCA * ⎠ zDA CA ⎝

* + NA = kLCAs

(30)

This equation considers that the alkanolamine diffuses from the bulk liquid phase to the interface at temperature TB, since the diffusion process of MAE occurs practically at this temperature, and that the individual mass transfer coefficient for the liquid phase is also given at TB. However, the diffusion coefficient of the gas in the liquid phase, DAs, and the CO2 concentration in equilibrium, CAs * , are given at the interfacial temperature Ts, adjusting well to the results obtained in this study. In the equilibrium CAs * = pAs/Hes, and eq 30 can be expressed as

(28)

Given that the stoichiometric coefficient z in the reaction between CO2 and MAE is 2, when it can be considered that DB ≈ DA and CBo/(2C*A) ≫ 1, eq 28 reduces to C NA = kL Bo 2

DBkL CBo zDAs

(29)

If an analysis is made from Figure 5, which presents only the experiments concerning the instantaneous reaction regime, it can be appreciated that, although there is concordance with the foregoing equation with respect to the linear relationship between NA and CBo, two questions cannot be disregarded, one being the existence of a non-negligible ordinate at the origin (0.0), with a value close to 1.2 × 106 kmol/(m2 s), and another being that, according to eq 29, the slope of this straight line(s) should be equal to half of the value of the individual mass transfer coefficient (kL), found in the experiments on physical absorption in previous research made with the same experimental equipment and under the same conditions.16,39 However, for each temperature the slope considerably differs from the value kL/2 and simultaneously augments, as shown in Table 2. It can be concluded that the fit of the experimental data to eq 30 is not entirely appropriate, and a nonisothermal instantaneous regime could exist. 3.4. Temperature Rise. When the carbon dioxide makes contact with the liquid phase, one part diffuses and another part reacts with the alkanolamine. In both cases, energy is given off as a consequence of the heat of dissolution and reaction. This flow of heat can appreciably raise the temperature of the liquid

p D k NA = kL As + B L CBo Hes 2DAs

(31)

In the above expression, the values pAs, Hes, and DAs at Ts can be determined by eqs 1, 4, 5, and 6, respectively, so that by these equations, eq 31, and the use of an iterative procedure Ts can be calculated. The sequence of calculations is explained in a previous work. 12 The T s values and the increases in temperature (Ts − TB) are presented in Table 3. In general, it is observed that the values of ΔT are higher when TB and CBo increase. Temperature increases of approximately 43 K were determined in the experiments made at high concentrations and in the highest temperature series, as can be seen in Figure 6 for the experiments corresponding to three of the temperatures assayed. At high temperatures and at the lowest amine concentration, the calculation model gives values for ΔT, next to zero; this occurs only in the experiments in the interval 303−313 K. In these experiments, there was no appreciable rise in interfacial temperature, and the values obtained are due probably to adjustments in the NA evaluation. 4815



■

3.5. Enhancement Factor and the Hatta Number. If it is considered that the necessary conditions of the instantaneous reaction regime are E = Ei

and

Ha > 10E i

DBCBoHes 2DAspAs

■

(32)

Meanwhile, the Hatta number, can be determined from eq 33, Ha =

k2sDAsCBo kL

(33)

where the value of the kinetic constant for the interfacial temperature, k2s, can be evaluated by eq 3430 for the interval of temperatures assayed: log k2 = 20.69 −

3532 T

(34)

Once Eis and Ha are known, the enhancement factor E can be determined by the expression of DeCoursey,40 E=

Ha2 + 2(E is − 1)

E isHa2 + +1 E is − 1 4(E is − 1)2 Ha4

(35)

The values obtained for Eis, Ha, and E are presented in Table 4, where the results are consistent with the nonisothermal instantaneous regime proposed, fulfilling the condition that E is ≈ E

and

Ha ≫ 10E is

The representation, in logarithmic coordinates, of E vs Ha (Figure 7), confirms the instantaneous reaction regime with a rise in temperatures and shows that the points adjust acceptably to the same curve. The points are far from the bisector of the first quadrant, since, if they were found in this bisector, this situation would indicate a fast-reaction regime.

4. CONCLUSIONS A process of gas absorption with nonisothermal reaction has been analyzed, and a procedure for the calculation of interfacial temperature rise and enhancement factors has been discussed. During the absorption of pure CO2 into aqueous solutions of 2methyl-amino-ethanol (MAE), the control of the temperature in the bulk liquid phase indicates that the process occurs under nonisothermal conditions, following an instantaneous reaction regime with a rise in the interfacial temperature. Finally, an expression is proposed to relate the experimental results with the initial alkanolamine concentration, which at the same time enables the determination of the interfacial temperature.

■

ACKNOWLEDGMENTS

The authors are grateful to the Andalusia Regional Government (Spain) for its financial support of this work carried out by the Research Group ‘Bioprocesses TEP-138’.

The instantaneous enhancement factor, Ei, according to film theory, can be evaluated for the temperature Ts, according to E is = 1 +

Article

AUTHOR INFORMATION

Corresponding Author

*Tel. +0034-953-212219. Fax: +0034-953-212141. E-mail: [email protected]. Notes

The authors declare no competing financial interest. 4816

NOMENCLATURE A = interfacial area (m2) CA = concentration of component A (CO2) (kmol/m3) CAo = initial concentration of component A (CO2) (kmol/ m3) CA* = A (CO2) concentration in equilibrium with the gaseous phase (kmol/m3) CAs * = A (CO2) concentration in equilibrium with the gaseous phase at temperature Ts (kmol/m3) CB = concentration of component B (alkanolamine) (kmol/ m3) CBo = initial concentration of amine in the aqueous phase (kmol/m3) Ci = concentration of ion i (kg ion/m3) DA (DCO2) = diffusion coefficient of component A (CO2) in the aqueous alkanolamine solution (m2/s) DAs = diffusion coefficient of component A (CO2) in the liquid phase at temperature Ts (m2/s) DB (DMAE ) = diffusion coefficient of alkanolamine in the liquid phase (m2 /s) DCO2,W = diffusion coefficient of CO2 in water (m2/s) DN2O= diffusion coefficient of N2O in the alkanolamine solution (m2/s). DN2O,W= diffusion coefficient of N2O in water (m2/s) E = enhancement factor, dimensionless Ei = instantaneous-enhancement factor, dimensionless Eis′ = modified instantaneous-enhancement factor at the temperature Ts, dimensionless h = constant defined in eq 2. h = h+ + hG + h− (m3/kg ion) Ha = Hatta number, dimensionless He = Henry’s law constant (kPa m3/kmol) HeH2O = Henry’s law constant in the pure water (kPa m3/ kmol) Hes = Henry’s law constant at the temperature Ts (kPa m3/ kmol) I = ionic extension I = 1/2∑Cizi2 (kg ion/m3) k = reaction rate constant kL = liquid-phase mass-transfer coefficient (m/s) km,n = reaction rate constant between a reactant A (order n) and another component B (order m) k2 = second-order reaction-rate constant (m3/(kmol s)) k2s = second-order reaction-rate constant at the temperature Ts (m3/(kmol s)) m = order of reaction with respect to CO2 n = order of reaction with respect to amine n′ = rate of absorption of CO2 (kmol/s) NA = rate of absorption per unit interfacial area of component A (CO2) (kmol/(m2 s)) 0.0 = ordinates at the origin in Figure 5 pA = partial pressure of component A (CO2) (kPa) pAs = partial pressure of component A (CO2) at the temperature Ts (kPa) pI = partial pressure of impurities (kPa) P = total pressure (kPa) pv = vapor pressure of the water (kPa) Q′ = volumetric flow rate of absorbed CO2 (m3/s) dx.doi.org/10.1021/ie201035y | Ind. Eng. Chem. Res. 2012, 51, 4809−4818


Article

(17) Camacho, F.; Sánchez, S.; Pacheco, R. Absorción de Dióxido de Carbono Puro en Disoluciones Acuosas de Monoetanolamina. Afinidad. 2000, 57 (485), 17. (18) Camacho, F.; Sánchez, S.; Pacheco, R. Thermal Effects during the Absorption of CO2 in Aqueous Solution of 3-Amino-1-propanol. Chem. Eng. Technol. 2000, 23, 1073. (19) Camacho, F.; Sánchez, S.; Pacheco, R.; La Rubia, M. D.; Sánchez, A. Kinetics of the Reaction of Pure CO2 with NMethyldiethanolamine in Aqueous Solutions. Int. J. Chem. Kin. 2009, 41, 204. (20) Danckwerts, P. V. Gas-Liquid Reactions; McGraw-Hill: London, 1970. (21) Van Krevelen, D. W.; Hoftijzer, P. J. Kinetics of Simultaneous Absorption and Chemical Reaction. Chem. Eng. Prog. 1948, 44, 529. (22) Pohorecki, R.; Moniuk, W. Kinetics of Reaction between Carbon Dioxide and Hydroxyl Ions in Aqueous Electrolyte Solutions. Chem. Eng. Sci. 1988, 43, 1677. (23) Sada, E.; Kumazawa, H.; Butt, M. A. Solubility and Diffusivity of Gases in Aqueous Solutions of Amines. J. Chem. Eng. Data 1978, 23, 161. (24) Versteeg, G. F.; Blauwhoff, P. M. M.; van Swaaij, W. P. M. The Effect of Diffusivity on Gas−Liquid Mass Transfer in Stirred Vessels. Experiments at atmospheric and elevated pressures. Chem. Eng. Sci. 1987, 42 (5), 1103. (25) Versteeg, G. F.; Van Swaaij, W. P. M. Solubility and Diffusivity of Acid Gases (CO2, N2O) in Aqueous Alkanolamine Solutions. J. Chem. Eng. Data 1988, 33, 29. (26) Wilke, C. R.; Chang, P. Correlation of Diffusion Coefficients in Dilute Solutions. AIChE J. 1955, 1, 264. (27) Sánchez, A. Estudio Comparativo de la Absorción de Dióxido de Carbono en Disoluciones de Alcanolaminas Secundarias. Ph.D. Dissertation, University of Jaén, Spain, 2001. (28) Camacho, F.; Sánchez, S.; Pacheco, R.; Sánchez, A.; La Rubia, M. D. Absorption of Carbon Dioxide at High Partial Pressures in Aqueous Solutions of Di-isopropanolamine. Ind. Eng. Chem. Res. 2005, 44, 7451. (29) Mimura, T.; Suda, T.; Iwaki, I.; Honda, A.; Kumazawa, H. Kinetic of Reaction between Carbon Dioxide and Sterically Hindered Amines for Carbon Dioxide Recovery from Power Plant Flue Gases. Chem. Eng. Commun. 1998, 170, 245. (30) Sotelo, J. L.; Benítez, F. J.; Beltrán-Heredia, J.; Rodríguez, C. Cinética de la Reacción de Dióxido de Carbono con Aminas. I. Amina Secundaria: Metil-aminoetanol. An. Quim. 1991, 87, 206. (31) Bavbek, O.; Alper, E. Reaction Mechanism and Kinetics of Aqueous Solutions of Primary and Secondary Alkanolamines and Carbon Dioxide. Turk. J. Chem. 1999, 23, 293. (32) Blanc, C.; Demarais, G. The Reaction Rate of CO2 with Diethanolamine. Int. Chem. Eng. 1984, 24, 43. (33) Hikita, H.; Asai, S.; Ishikawa, H.; Honda, M. The Kinetics of Reactions of Carbon Dioxide with Monoethanolamine, Diethanolamine and Triethanolamine by a Rapid Mixing Method. Chem. Eng. J. 1977, 13, 7. (34) Danckwerts, P. V. The Reaction of CO2 with Ethanolamines. Chem. Eng. Sci. 1979, 34, 443. (35) Astarita, G.; Marrucci, G.; Gioia, F. The Influence of Carbonation Ratio and Total Amine Concentration on Carbon Dioxide Absorption in Aqueous Monoethanolamine Solutions. Chem. Eng. Sci. 1964, 19, 95. (36) Ohno, K.; Inoue, Y.; Yoshida, H.; Matsuura, H. Reaction of Aqueous 2-(N-methylamino)ethanol Solutions with Carbon Dioxide. Chemical Species and their Conformations Studied by Vibrational Spectroscopy and ab Initio Theories. J. Phys. Chem. A. 1999, 103, 4283. (37) Sharma, M. M. Kinetics of Reactions of Carbonyl Sulphide and Carbon Dioxide with Amines and Catalysis by Brönsted Bases of the Hydrolysis of COS. Trans. Faraday Soc. 1965, 61, 681. (38) Charpentier, J. C. Mass Transfer Rates in Gas−Liquid Absorbers and Reactors. In Advances in Chemical Engineering; Drew,

r = reaction rate r2 = lineal regression coefficient R = gas constant, 8.314 (kPa m3/(K kmol)) s = slope of straight line in Figure 5 t = time (s) T = temperature (K) TB = temperature in the bulk liquid phase (K) Ts = temperature in the interfacial film (K) zi = ion i electric charge z = stoichiometric coefficient of alkanolamine in the reaction with CO2 Greek Letters

γ = constant defined in eq 9 η = carbonation ratio, η = CA*/SB (mol CO2/mol amine) μB = viscosity of the solution amine (Pa s) μw = viscosity of the pure water (Pa s)

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Thermal Effects in the Absorption of Pure CO2 into Aqueous Solutions

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