Kinetics of Absorption of CO2 into Aqueous Solution of MDEA Blended

Absorption rates of CO2 into aqueous solution of N-methyldiethanolamine (MDEA) blended with diethanolamine (DEA) were investigated by a laboratory dis...
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Ind. Eng. Chem. Res. 2002, 41, 1135-1141

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Kinetics of Absorption of CO2 into Aqueous Solution of MDEA Blended with DEA Xu Zhang,* Cheng-Fang Zhang, and Yi Liu Research Institute of Chemical Technology, East China University of Science and Technology, P.O. Box 274, 130 Meilong Road, Shanghai 200237, People’s Republic of China

Absorption rates of CO2 into aqueous solution of N-methyldiethanolamine (MDEA) blended with diethanolamine (DEA) were investigated by a laboratory disk column, in the temperature range from 313 to 343 K and a weight ratio of MDEA to DEA of from 50/3 to 50/10 at a total amine concentration of 3.0 M. According to the suggested homogeneous activation mechanism, kinetics of model of CO2 absorption into aqueous solution of MDEA blended with DEA was established and the absorption rate can be expressed as NCO2 ) HCO2xDCO2(kMDEACMDEA+kDEACDEA) (pCO2 / pCO ). An equation of second-order reaction rate constant between CO2 with DEA, ln kDEA ) 2 24.515 - 5411.3/T, has been obtained, and it is in good agreement with that of literature reported on CO2 absorption into single DEA aqueous solutions. 1. Introduction Methyldiethanolamine (MDEA) solution is an important solvent in the CO2 removal process, but it reacts relatively slowly with CO2 for its tertiary amine characteristics. A recent researcher1,2 indicated that adding small amounts of monoethanolamine (MEA), diethanolamine (DEA), and a nitrogenous heterocyclic compound, piperazine (PZ), into aqueous MDEA solutions would enhance the absorption and desorption rate of CO2. Up to now, activated MDEA solution technologies have already been applied to commercial processes extensively; the aqueous solution of MDEA blended with DEA is one of the most important one. Many investigations have been carried out for CO2 absorption into aqueous solution of MDEA blended with DEA recently. Adams et al.3 measured the absorption rates of gases CO2 into aqueous solutions of MDEA and DEA blends in a quiescent, inverted-tube diffusionmeter by monitoring the rate of pressure drop. A numeral model for absorption, diffusion, and reaction of CO2 in blends of MDEA, DEA, and water was developed. The results indicated that CO2 absorption in the blends was diffusion controlled by that of the CO2 reaction products, but their experiments are static and cannot reflect the dynamic state of commercial processes truthfully. Glasscock et al.4 studied the CO2 absorption/desorption in MDEA/MEA and MDEA/DEA aqueous solutions over a temperature range from 288 to 313 K with a gas-stirred cell reactor. They concluded that the combined mass transfer/equilibrium model based on the zwitterion mechanism suggested by Blauwhoff et al.5 can effectively represent CO2 mass transfer rate for the mixture of MEA/MDEA and DEA/MDEA aqueous solu* To whom correspondence should be addressed.Telephone: 0086-21-64252386. Fax: 0086-21-64250884. E-mail: [email protected].

tions under wide range of conditions. For the mixed amine DEA/MDEA system, only one additional constant need to be determined the interaction rate constant for MDEA in the DEA zwitterion mechanism. And it is concluded that should be interpreted as MDEA deprotonation of zwitterim in the second step of mechanism. Rinker et al. 19 measured the rate of absorption of CO2 into aqueous solution of DEA and MDEA in a laminar liquid jet absorber and a stirred cell absorber. They developed a model for it based on penetration theory, which incorporates an extensive set of important reversible reactions and takes into account the coupling between chemical equilibrium, mass transfer, and chemical kinetics. The reaction between CO2 and the second amine DEA was described by zwitterion mechanism. Under their experimental conditions, it seems that the tertiary amine, MDEA, does not contribute to the depronation of the zwitterion. Several researchers investigated the kinetics of CO2 absorption into aqueous DEA solution. According to the different mechanism, however, the order of reaction rate is also varied from 1 to 2 with respect to amine concentration.20,21,22 In this paper, the kinetics of absorption CO2 into aqueous solutions of MDEA blended with DEA was experimentally studied with a disk column under the conditions approaching those of industrial process. According to the suggested homogeneous activation mechanism, a kinetics model of CO2 absorption into aqueous solutions of MDEA blended with DEA has been established. 2. Theory 2.1. Equilibrium Reaction in Solution. When CO2 is absorbed into an aqueous solution of MDEA blended with DEA, there are several equilibrium reactions in solution. MDEA and DEA are both easily protonated

10.1021/ie010605j CCC: $22.00 © 2002 American Chemical Society Published on Web 02/01/2002

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Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002

and the corresponding equilibrium reactions exist in solution as shown by eqs 1 and 3. Equation 2 shows the HCO3- equilibrium reaction with DEA to form the carbamate ion.7 Several equilibrium reactions of various spices in aqueous solution of MDEA blended with DEA can be expressed as

MDEA + H 798 MDEAH -

K2

+

-

DEA + HCO3 798 DEACOO + H2O K3

+

DEA + H 798 DEAH

+

K4

HCO3- 798 H+ + CO32K5

H2O + CO/2 798 H+ + HCO3K6

H2O 798 H+ + OH-

(1) (2)

(4) (5)

k2

MDEA + CO2 98 MDEA‚CO2 K8

MDEA‚CO2 + H2O798 MDEAH+ + HCO3kDEA

DEA + CO2 98 DEA‚CO2 K9

DEA‚CO2 + H2O 798DEAH+ + HCO3DEA‚CO2 + MDEA 798 MDEA‚CO2 + DEA

(12)

Overall DEA balance:

(13)

Overall CO2 balance, with definition of total CO2 conversion, y:

CHCO3- + C0DEAy1 ) (C0MDEA + C0DEA)y

(3)

(14)

Charge balance:

CDEAH+ + CMDEAH+ ) CHCO3- + CDEACOO-

(15)

C0DEAy2 + C0MDEAyMDEA ) CHCO3- + C0DEAy1 (16)

(6) Equilibrium constants of eqs 1-3 are given by

2.2. Reaction Mechanism. When a small amount of DEA was added into a MDEA aqueous solution, the absorption and desorption rate of CO2 was enhanced greatly. According to the base catalysis mechanism,6 MDEA cannot react with CO2 directly, but CO2 hydrolysis reaction is catalyzed by a base substance, such as DEA. However, being a diamine, DEA can form the stable DEA carbamate and protonation easily in solution.7 Therefore, the concentration of free DEA during the reaction process, which is much lower than its initial concentration, would be depleted quickly by the CO2 reaction, and the absorption rate is unable to obtain such obvious activated effects if DEA could not transfer CO2 to MDEA. In this paper, we assumed that there is a rapid pseudo-first-order reversible reaction between CO2 and free DEA in parallel with that of CO2 and MDEA, based on the fact that free DEA can transfer the CO2 to MDEA and regenerate by itself simultaneously. And thus, the concentrations of MDEA and DEA in liquid phase should depend on the equilibrium reaction 11. This is the essence of the homogeneous activation mechanism.8,9

K10

CMDEA ) C0MDEA(1 - yMDEA)

CDEA ) C0DEA(1 - y1 - y2)

K1

+

Overall MDEA balance:

(7) (8) (9) (10) (11)

2.3. Mass and Charge Balance. In a state of liquid equilibrium, ions in solution must satisfy mass and charge balance. It is reasonable to neglect their effects on the mass and charge balance equations since the concentrations of H+, OH-, and CO32- are small. Defining yMDEA as the conversion of MDEA to MDEAH+, y1 as that of DEA to DEACOO-, and y2 as that of DEA to DEAH+, then the balance relationship can be deduced as follows.

K1 ) K2 )

CMDEAH+ C0MDEAyMDEA ) CMDEACH+ C 0 (1 - y )C + MDEA am H C0DEAy1

CDEACOO) CDEACHCO3- C0

DEA(1

CDEAH+ ) K3 ) CDEACH+ C0

- y1 - y2)CHCO3-

DEA(1

C0DEAy2 - y1 - y2)CH+

(17)

(18)

(19)

Equation 17 divided into equation 19 gives

yMDEA(1-y1-y2) K1 ) K3 (1 - yMDEA)y2

(20)

By solving these equations including eqs 14, 16, 18, and 20 based on the initial concentration of solution, y, at various temperatures, then the values of the concentrations of various species can be obtained. 2.4. Absorption Rate Coefficient, k. The CO2 absorption reaction into an aqueous solution of MDEA blended with DEA can be regarded as a rapid pseudofirst-order reversible reaction between CO2 and DEA in parallel with that of CO2 and MDEA. And the free concentrations of MDEA and DEA in liquid phase depend on each other based on the homogeneous activation mechanism. The absorption rate can be expressed as / )) NCO2 ) k(pCO2 - pCO 2 / HCO2xDCO2(kMDEACMDEA + kDEACDEA)(pCO2 - pCO ) 2

(21)

The absorption rate coefficient, k, in which treats CO2 partial pressure as driving force, can be indicated as

k ) HCO2xDCO2(kMDEACMDEA + kDEACDEA) )

HCO2xDCO2(kMDEA,app + kDEA,app) (22)

where kMDEA,app and kDEA,app are the apparent first-order rate constants of MDEA and DEA, respectively.

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1137

kMDEA,app ) kMDEACMDEA ) kMDEAC0MDEA(1 - yMDEA) (23) kDEA,app ) kDEACDEA ) kDEAC0DEA(1 - y1 - y2)

(24)

3. Experimental Section A disk column developed by Stephens and Morris was used for the measurements of absorption rate. The experimental apparatus and method have been reported in detail by Xu et al.9 The MDEA with 99% purity and DEA in chemical grade with 98.6% purity were supplied by the Fifth Chemical Factory in Wujin, Jiangsu Province, China. In this work, the absorption rates of CO2 into aqueous solution of MDEA blended with DEA were measured from 313 to 343 K, with the weight ratio of MDEA to DEA running from 50/3 to 50/10 at a total amine concentration of 3.0 M. A flow rate of absorption solution was fixed at 10 L/h for all the experiments. Experimental results are listed in Table 2. The reproducibility error of the experimental data is less than 5%. We measured the liquid film mass transfer coefficient of this disk column using a pure carbon dioxide-water system at 293 K. Experimental results were well correlated as follows:

k L ld 4Γ 0.6453 µ 0.5 ) 33.089 D µ FD

( ) ( )

(25)

The deviation of the calculation is less than 3.82%.

Table 1. Parameter Expressions Used in the Kinetics Model param

expression

source

DCO2 HCO2 K1 K2 K3 K4 K5 K6 k2 kDEA

DCO2µ0.54 ) 6.109 × 10-8 ln HCO2 ) a1 + a2/T + a3/T2 ln K1 ) 32.259-0.0424T ln K2 ) - 4.825 + 1885.0/T ln K3 ) 2.551 + 5652.0/T ln K4 ) 220.067 - 12431.7/T - 35.4819 ln T ln K5 ) 235.482 - 12092.1/T - 36.7816 ln T ln K6 ) 140.932 - 13445.9/T - 22.4773 ln T ln k2 ) 15.584 - 3984/T ln kDEA ) 24.515 - 5411.3/T

Haimour11 Al-Ghawas10 Barth16 Kent7 Kent7 Edwards17 Edwards17 Edwards17 Wang et al.14 this study

DEACOO- at different CO2 loading can be seen from Table 2. Reliable Van Krevelen coefficients for MDEAH+ and HCO3- have been experimentally determined by Browning and Weiland,13 and the Van Krevelen coefficient of DEACOO- is assumed to be that of HCO3-, approximately. The Van Krevelen coefficient of DEAH+ is assumed to be equal to that of MDEAH+. The equilibrium constants of eq 18, K2, and eq 19, K3, were both obtained by Kent7 from experimental results in 25 wt % DEA aqueous solution. Therefore, the influences of activity coefficient on the reaction equilibrium have been included in these two equilibrium constants. Barth16 has obtained the equilibrium constant of eq 17, K1, from a dilute MDEA aqueous solution (2 × 10-2 to 2 × 10-1M). Theoretically, the equilibrium constant K1 should be affected by the activity coefficient when it is directly applied to the high concentration solution. Thus, the equilibrium constant K1 can be described as

4. Physical Properties for Data Processing The physicochemical properties that are important to interpret the kinetics model are solubility, diffusivity, and equilibrium constants. Liu15 has measured the solubility of CO2 in the aqueous solutions of MDEA blended with DEA within a wide range of concentrations. On the basis of the experimental data, the Deshmukh-Mather model, Pitzer’s electrolyte solution theory, and the simplified Kent-Eisenbery model were applied to the CO2-MDEA-DEA-H2O systems, respectively. The calculated and experimental CO2 partial pressures are in good agreement with average deviation of around 10%, and the models can be applied for the engineering calculation. Due to the low concentration of DEA in MDEA aqueous solution, the solubility coefficient correlation HCO2 given by Al-Ghawas et al.,10 the viscosity of the solution and the diffusivity DCO2 of CO2 in MDEA aqueous solution given by Haimour et al.11 are extended to the aqueous solutions of MDEA blended with DEA. When chemical reactions of carbon dioxide with these amine blends are carried out, various ions such as MDEAH+, DEAH+, HCO3-, DEACOO-, etc. are formed. An increase in ionic strength in solution leads to a decrease in CO2 solubility. Joosten and Dankwerts12 suggested an extended Van Krevelen model for the mixed electrolyte solution, and thereby, the solubility coefficient of CO2 in the aqueous mixture of MDEA blended with DEA is evaluated as follows 0 )) log (HCO2/HCO 2

∑i hiIi

(26)

where hi and Ii are the Van Krevelen coefficient and ionic strength attributable to the electrolyte i. The concentration of MDEAH+, HCO3-, DEAH+, and

CMDEAH+ K1 ) Kr CMDEACH+

(27)

where Kr is a factor that indicates the influence of the activity coefficient on the equilibrium constant. Actually, however, the experimental data can fit well with the results of simulation when Kr is equal to 1.0 as seen from Table 2. It can be demonstrated that the equilibrium constant K1 is still effective for a concentration of MDEA aqueous solution as high as 3.0 M. It is noted that there appeared a certain function of CO2 loading on kDEA at CMDEA ) 2.695 M, CDEA ) 0.305 M, and T ) 313 K. Maybe the effect of Kr is more evident than that under other conditions, which should be investigated carefully in the future. The partial pressure of water vapor above the aqueous solution has been measured by Wang et al.14 The parameter expressions used in the kinetics model are shown in Table 1. The experimental data and processing results are also listed in Table 2. 5. Results and Discussion 5.1. Absorption Rate Coefficient, k. The data of absorption rate of CO2 was monitored under different conditions shown in Table 2. The changes of the absorption rate coefficient with various temperatures in MDEA/ DEA ) 50/5(weight ratio) at total amine concentration of 3.0 M, are shown in Figure 1. The absorption rates coefficient increase with temperature and decrease with the increasing of conversion. As shown in Figure 2, the absorption rate coefficient decreases with the increasing of weight ratio of MDEA to DEA.

0.305

0.191

0.555

2.695

2.809

2.445

0 0 CMDEA , CDEA , kmol/m3 kmol/m3

333

333

343

333

323

313

T, K

0.107 0.137 0.165 0.189 0.236 0.275 0.319 0.356 0.391 0.135 0.167 0.201 0.223 0.283 0.320 0.359 0.398 0.431 0.128 0.160 0.192 0.218 0.271 0.301 0.334 0.361 0.390 0.427 0.127 0.194 0.235 0.261 0.295 0.317 0.326 0.055 0.099 0.130 0.241 0.280 0.111 0.160 0.231 0.280 0.331 0.370

8.184 7.161 6.470 5.812 5.331 4.401 3.891 3.410 2.950 8.351 7.280 6.451 5.820 4.481 3.482 2.920 2.411 1.950 8.201 7.310 6.328 5.875 4.235 3.420 2.415 1.840 1.321 0.751 7.550 5.450 3.601 2.670 1.811 1.230 0.991 8.956 7.463 6.190 3.501 2.801 12.50 9.630 6.910 5.171 3.602 2.380

9.546 9.546 9.546 9.546 9.546 9.546 9.546 9.546 9.546 9.133 9.133 9.133 9.133 9.133 9.133 9.133 9.133 9.133 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 7.532 7.532 7.532 7.532 7.532 7.532 7.532 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535 8.535

0.481 0.610 0.840 0.961 1.180 1.680 2.140 2.520 2.980 0.901 1.100 1.450 1.710 2.350 3.380 3.910 4.450 4.900 1.790 1.840 2.180 2.280 3.310 4.010 5.120 5.730 6.410 7.210 2.180 2.900 4.080 4.820 5.520 6.100 6.350 1.210 1.520 1.890 3.770 4.280 1.460 1.900 2.740 3.550 4.610 5.680

6.100 5.934 5.784 5.659 5.420 5.230 5.024 4.856 4.703 5.338 5.184 5.030 4.925 4.662 4.507 4.349 4.196 4.075 4.831 4.691 4.556 4.449 4.238 4.123 4.000 3.903 3.801 3.674 4.353 4.094 3.943 3.850 3.732 3.658 3.628 5.165 4.961 4.822 4.360 4.203 4.911 4.691 4.400 4.203 4.015 3.871

9.027 8.012 7.431 6.755 6.332 5.593 5.252 4.850 4.490 10.14 9.062 8.395 7.829 6.604 6.049 5.590 5.146 4.606 12.15 10.91 9.958 9.392 8.089 7.541 7.071 6.559 6.182 5.660 14.10 11.76 10.42 9.845 8.996 8.589 8.375 12.22 10.63 9.315 7.345 6.580 17.67 14.51 11.92 10.37 9.183 8.344

9.251 8.326 7.630 7.107 6.246 5.656 5.087 4.665 4.309 10.02 9.106 8.269 7.764 6.662 6.094 5.564 5.088 4.733 11.99 10.87 9.948 9.259 8.080 7.512 6.942 6.523 6.102 5.612 14.16 11.69 10.47 9.810 9.022 8.561 8.382 12.45 10.69 9.717 7.254 6.596 17.26 14.69 11.93 10.41 9.169 8.274

2.444 2.364 2.287 2.220 2.089 1.979 1.854 1.749 1.649 2.366 2.279 2.188 2.124 1.955 1.850 1.740 1.630 1.539 2.383 2.297 2.209 2.137 1.989 1.905 1.812 1.735 1.654 1.549 2.385 2.204 2.090 2.018 1.923 1.861 1.836 2.670 2.551 2.465 2.151 2.036 2.230 2.113 1.936 1.806 1.674 1.568

0.136 0.113 0.097 0.086 0.071 0.059 0.051 0.044 0.040 0.125 0.107 0.091 0.083 0.065 0.057 0.050 0.045 0.040 0.139 0.119 0.104 0.093 0.076 0.068 0.061 0.056 0.051 0.046 0.148 0.111 0.094 0.086 0.076 0.071 0.069 0.127 0.097 0.082 0.051 0.044 0.304 0.238 0.175 0.144 0.121 0.105

0.251 0.331 0.408 0.475 0.606 0.716 0.841 0.946 1.046 0.329 0.416 0.507 0.571 0.740 0.845 0.955 1.065 1.156 0.312 0.398 0.486 0.558 0.706 0.790 0.883 0.960 1.041 1.146 0.310 0.491 0.605 0.677 0.772 0.834 0.859 0.139 0.258 0.344 0.658 0.773 0.215 0.332 0.509 0.639 0.771 0.877

0.070 0.079 0.089 0.094 0.102 0.109 0.116 0.122 0.127 0.076 0.085 0.092 0.097 0.109 0.115 0.122 0.129 0.134 0.072 0.081 0.090 0.096 0.107 0.113 0.119 0.123 0.129 0.135 0.070 0.090 0.100 0.106 0.113 0.117 0.119 0.026 0.038 0.045 0.062 0.067 0.115 0.148 0.181 0.201 0.219 0.233

0.100 0.112 0.121 0.126 0.133 0.137 0.138 0.138 0.137 0.104 0.113 0.121 0.124 0.130 0.132 0.132 0.131 0.130 0.094 0.104 0.111 0.116 0.122 0.124 0.125 0.125 0.125 0.124 0.086 0.104 0.110 0.113 0.115 0.116 0.117 0.038 0.055 0.063 0.078 0.079 0.136 0.170 0.198 0.210 0.215 0.217

0.221 0.299 0.374 0.440 0.575 0.688 0.819 0.930 1.035 0.301 0.388 0.479 0.545 0.719 0.828 0.945 1.063 1.063 0.291 0.376 0.465 0.538 0.691 0.779 0.877 0.958 1.045 0.157 0.295 0.478 0.595 0.670 0.770 0.835 0.861 0.128 0.242 0.327 0.643 0.761 0.194 0.310 0.492 0.630 0.775 0.893

37.33

37.33

52.91

37.33

25.77

17.37

1297 1249 1286 1202 1431 1335 1511 1545 1558 2400 2307 2425 2385 2281 2287 2365 2409 2160 3915 3831 3800 3926 3802 3829 3975 3846 3923 3877 6243 6390 6232 6354 6257 6351 6288 3626 3741 3395 3928 3766 3988 3691 3780 3766 3807 3868

3817

3817

6302

3817

2335

1379

Y, kmol kMDEA, kDEA,exp, kDEA,mod, of CO2/ N × 106, HxD × 105, kexp × 105, kmod × 105, kmol of kmol/ P* × 102, P × 102, kmol/m2 kmol/m2 kmol/m2 CDEA, CMDEAH+, CDEAH+, CDEACOO-, CHCO3-, m3/kmol m3/kmol m3/kmol CMDEA, amine m2 s MPa MPa kmol/m3 kmol/m3 kmol/m3 kmol/m3 kmol/m3 kmol/m3 s s s s0.5 MPa s MPa s MPa

Table 2. Experimental Data and Calculation Results

1138 Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002

Ind. Eng. Chem. Res., Vol. 41, No. 5, 2002 1139

Figure 1. Absorption rate coefficient, k, vs temperature in a solution of MDEA/DEA ) 50/5(weight ratio).

Figure 2. Absorption rate coefficient, k, vs various weight ratio of MDEA to DEA at 333 K.

Figure 4. Relationship between free DEA concentration and conversion at various temperatures.

Figure 5. Relationship between free DEA concentration and conversion at different weight ratios of MDEA to DEA

Figure 3. Arrhenius plot for kDEA.

Figure 6. Comparison of absorption rate coefficient between values of experiment and model.

5.2. Second-Order Reaction Rate Constant, kDEA. The second-order reaction rate constant between DEA and CO2, kDEA, can be obtained by substituting the corresponding values of k, HCO2, DCO2, kMDEA, CMDEA, and CDEA into eq 22. It is indicated that kDEA is only a function of temperature, which can be seen from Table 2. An Arrhenius plot of the values of the second-order reaction rate constant, kDEA, shown in Figure 3, can be represented by

crease, and the concentrations of free DEA decrease for the formation of DEA carbamate and protonated DEA. It is demonstrated that concentration of the free DEA decrease with temperature at weight ratio of MDEA to DEA, 50/5 at a total amine concentration of 3.0 M as shown in Figure 4. The free DEA ratio increases with the decrease of weight ratio of MDEA to DEA at 333 K shown in Figure 5. 5.4. Comparison between Experimental Values and Model Values. A comparison of experimental values, kexp, and model values, kmod, on absorption rate constant of CO2 into aqueous solution of MDEA and DEA blends is shown in Figure 6. The mean error between model and experimental values is less than 10%. The suggested homogeneous activation mechanism can express the process of CO2 absorption into aqueous MDEA blended with DEA aqueous solution effectively.

ln kDEA ) 24.515 - 5411.3/T

(28)

These values of kDEA are in good agreement with that of Blanc and Demarais18 obtained from single DEA solution absorption experiments. The comparison of the results of kDEA with others’ results can also be seen in Table 3. 5.3. Free Concentration of DEA. Table 2 shows the change of concentrations of various species in the aqueous solution of MDEA blended with DEA. With the increase in solution conversion, the concentrations of DEAH+, DEACOO-, MDEAH+, and HCO3- also in-

6. Conclusions In this paper, the absorption rates of CO2 into an aqueous solution of MDEA blended with DEA were

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Table 3. Fitted Values of Kinetics Constants kDEA T, K

this work

293 298 303 313 323 333 343

Nguyen23

Sada24

300

1200

Blanc18 485

1379 2335 3817 6302

900 1520 2620 4225

measured with a disk column, with a temperature range from 313 to 343 K and a weight ratio of MDEA to DEA in the range of 50/3 to 51/10 at a total amine concentration of 3.0 M. The CO2 absorption reaction can be regarded as a rapid-pseudo-first-order reversible reaction between CO2 and DEA based on the fact that DEA can transfer CO2 to MDEA and regenerate by itself simultaneously. Therefore, the CO2 absorption rate into an aqueous solution of MDEA blended with DEA can be expressed as / NCO2 ) k(pCO2 - pCO )) 2 / HCO2xDCO2(kMDEACMDEA + kDEACDEA)(pCO2 - pCO ) 2

A second-order rate constant between CO2 and DEA can be obtained under this experimental condition, which can be expressed as

ln kDEA ) 24.515 - 5411.3/T It is in good agreement with that of the literature reported18 in a single DEA aqueous solution. The mean error between values of experiment and model on absorption rate coefficient is less than 10%. Nomenclature C ) concentration in liquid phase, kmol/m3 DCO2 ) CO2 diffusivity in liquid phase, m2/s HCO2 ) solubility coefficient of CO2 in MDEA solution, kmol/ (m3.MPa) K ) equilibrium constant k ) absorption rate coefficient, kmol/(m2 s MPa) kMDEA ) second-order reaction rate constant between MDEA and CO2 m3/(kmol s) kDEA ) second-order reaction rate constant between DEA and CO2, m3/(kmol s) kMDEA,app ) apparent first-order constant of reaction between MDEA and CO2, s-1 kDEA,app ) apparent first-order constant of reaction between DEA and CO2, s-1 NCO2 ) absorption rate, kmol/(m2 s) P ) partial pressure, MPa T ) temperature, K y ) CO2 loading, kmol of CO2/kmol of MDEA + DEA yMDEA ) conversion of MDEA to MDEAH+ y1 ) conversion of DEA to DEACOOy2 ) conversion of DEA to DEAH+ Superscript 0 ) initial state / ) equilibrium state Subscript exp ) experimental value mod ) model calculation value Greek Symbols Γ ) liquid flow rate per unit width of surface, kg/(m s)

F ) density of liquid, kg/m3 µ ) viscosity, cp Abbreviations DEA ) diethanolamine MDEA ) methyldiethanomine MEA ) monoethanolamine PZ ) piperazine

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Received for review July 16, 2001 Revised manuscript received November 15, 2001 Accepted November 21, 2001 IE010605J