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SEPARATIONS Absorption Rate into a MDEA Aqueous Solution Blended with Piperazine under a High CO2 Partial Pressure Xu Zhang,† Jun Wang,§ Cheng-fang Zhang,*,‡ Yan-hua Yang,† and Ji-jun Xu† Department of Nuclear Power Engineering, Shanghai Jiaotong University, 1954 Huashan Road, Shanghai 200230, P.R. China, and Research Institute of Chemical Technology, East China University of Science and Technology, P.O. Box 274, 130 Meilong Road, Shanghai 200237, P.R. China, Environmental Engineering Department, Donghua University, 1882 Yanan Road West, Shanghai 200051, P. R. China
The absorption rate of carbon dioxide into a methyldiethanolamine (MDEA) aqueous solution blended with piperazine (PZ) under a high carbon dioxide partial pressure was studied by an experimental packed column. Also, the results indicated that the absorption rate of carbon dioxide at a high carbon dioxide partial pressure was larger than that of the calculation value from the kinetics model of atmospheric pressure because the Marangoni effect was strengthened by the elevated carbon dioxide partial pressure. The kinetics model modified by a carbon dioxide partial pressure was obtained as follows: NCO2 ) fHCO2[DCO2(kMDEACMDEA + kPZCPZ)]1/2(P - P*), where f ) 0.83[ln(P/100)]0.8 + 1.0 and 100 kPa e P e 625 kPa. The simulation of a commercial absorption column has verified this modified model successfully. 1. Introduction The piperazine (PZ)-activated methyldiethanolamine (MDEA) technology has been extensively applied to the CO2 removal from the gas stream. Many research works of the kinetics were carried out recently. Xu et al.1 studied the CO2 absorption kinetics of a PZ-activated MDEA aqueous solution at atmospheric pressure. Also, they proposed that a homogeneous activation mechanism could be used to explain this process successfully. Bishnoi and Rochelle2 found that the PZ could directly react with carbon dioxide to form the piperazine carbamate. Zhang et al.3 suggested that PZ can not only react with carbon dioxide to form the stable carbamate but also act as a homogeneous activator during the carbon dioxide absorption into a MDEA aqueous solution blended with PZ. Xu studied the carbon dioxide absorption4 into a PZ-activated MDEA aqueous solution and its desorption5 process by experimental packed columns at atmospheric pressure, with the results demonstrating that the kinetics model of absorption which was obtained from the disk column can be applied to describe the absorption and desorption processes in the experimental packed column. As discussed above, the previous researches were carried out under the condition of atmospheric pressure. However, the partial pressure of carbon dioxide in the bottom of the commercial absorption column is usually more than 0.5 MPa. Up to now, there is no public report of the kinetics of carbon dioxide absorption into a MDEA
aqueous solution blended with PZ in a packed column under high CO2 partial pressure. Benadda et al.6 studied hydrodynamics and mass-transfer phenomena of carbon dioxide absorption in a countercurrent packed column at elevated pressure. Also, they found that there is a significant effect of pressure on mass-transfer parameters. For example, the interfacial area increases with increasing pressure up to 1.3 MPa. Therefore, the information obtained at atmospheric pressure could not simply be extrapolated to describe the same phenomena at elevated pressure. Wu et al.7 studied the process of absorbing water vapor into a triethylene glycol solution by an experimental packed column. Also, their study focused on the effects of the surface tension gradient on the masstransfer performance of the absorption process. The phenomenon of fluid flow in the surface thin film due to the surface tension gradient is termed the Marangoni effect. The conventional mass-transfer correlation was modified by the term of the Ma index to explain the Marangoni effect. This improves the average error between predicted values and experimental data significantly. In this paper, the effects of CO2 partial pressure on the absorption rate of carbon dioxide into a MDEA aqueous solution blended with PZ were investigated by a experimental absorption packed column,8 and then the commercial absorption column was simulated. 2. Theory
* To whom correspondence should be addressed. Telephone: 0086-21-64252386. Fax: 0086-21-64250884. E-mail:
[email protected]. † Shanghai Jiaotong University. ‡ East China University of Science and Technology. § Donghua University.
It is known that everything has the automatic tendency to be at a steady state with a lower energy stage. Also, the fluid flow at the surface will become stable with a lower surface tension. The surface disturbance phenomenon resulting from the temperature difference
10.1021/ie020223t CCC: $25.00 © 2003 American Chemical Society Published on Web 12/10/2002
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and concentration difference between the surface and the bulk liquid is termed the Marangoni effect during the chemical absorption process. Nobuyuki and Katsuhiko9 reported that the absorption rate of gas and liquid could be enhanced by surface disturbance. In this paper, we thought that the hydraulic and mass-transfer phenomena at elevated CO2 partial pressure were different from those of the atmospheric pressure because of the intensified effect of the concentration of reactant in the interface on the surface disturbance phenomenon. Therefore, the absorption rate of carbon dioxide into a MDEA aqueous solution blended with PZ would be more enhanced at elevated pressure than the value of model, which was obtained through a disk column at the atmospheric pressure of CO2. To study the effect of pressure on the absorption process, the complexity of the effect of pressure on the surface disturbance was lumped into an index f. Therefore, this work is mainly to investigate the relationship between f and the CO2 partial pressure by experiments. It should be noted that the profiles of the temperature, liquid conversion, and absorption rate were varied along the height of the adiabatic column during the experimental process. The solution conversions of the inlet and outlet were determined by the chemical analysis method. These data are shown in Table 2. Several thermometers were installed8 along the column to ensure the insulation of the whole column. Therefore, to precisely calculate the amount of carbon dioxide absorption in the column, it is necessary to divide the whole column into many elements and then integrate them. Considering an element of a packed column, the amount of CO2 absorption can be represented by the change of conversion:
dG ) LC0am dy
(1)
The amount of CO2 absorption in an element represented by the absorption rate and f is
dG ) fHCO2xDCO2(kamCam + kpCp)(P - P*) da
(2)
where da represents the effective interfacial area of an element. When eq 2 is substituted by eq 1 and integrated, the modified index f can be defined as
f) LC0am a
∫yy
2
1
dy
HCO2xDCO2(kMDEACMDEA + kPZCPZ)(P - P*)
(3)
The index f represents the CO2 partial pressure modification coefficient for a kinetics model at atmospheric CO2 pressure. All of the detail models used in this integration equation could be seen in the literature.3 3. Experimental Setup Carbon dioxide absorption experiments for high CO2 pressure, 200-625 kPa, were conducted in an adiabatic column. At first, the effective area of the experimental packed column was measured by the chemical absorption method. Then the changes of conversion of CO2 in the MDEA and PZ blends were measured.
Figure 1. Flowsheet of the experimental apparatus: (1) carbon dioxide cylinder; (2) absorption column; (3) bump; (4) solution tank; (5) valve; (6) pressure gauge.
MDEA with 99% purity was supplied by the Wujin Fifth Chemical Factory (Jiangsu province, China), and PZ was of chemical grade with 98.8% purity. The other chemicals used in this experiment are of analysis grade. The main experimental device was a stainless steel packed adiabatic column with an inside diameter of 20 mm and packed with 4 mm ceramic Rasching rings. The packed height was 700 mm. To make the liquid flow in a continuous phase and avoid channeling effects in the packed column, the liquid flow rate was operated beyond the minimum liquid flow rate. The gas and liquid distributors with special structures were also installed at the top and bottom of the column, respectively. According to the experimental data, gas and liquid could be distributed evenly in the packed column. The sketch of the experimental setup is given in Figure 1. 4. Determination of the Effective Interfacial Area Danckwerts10 straight-line method was used to determine the effective interfacial area of this packed column. The whole experimental process was shown as follows: Carbon dioxide with a purity of 99.9% from a gas cylinder is sent to a packed column from the bottom to react with the amine solution. In the packed column, the carbon dioxide is transferred from the gas phase to the liquid phase. The absorption solution was pumped to the packed column from the top to contact countercurrently with carbon dioxide. After the flow process reached a steady state, the absorption rate of CO2 was calculated through measurement of the CO2 flow rate of the inlet and outlet by a soap film meter. The 0.5 M Na2CO3-0.5 M NaHCO3 buffer solution was chosen as the absorption solvent and NaClO as the catalyst with a concentration range from 0.005 to 0.02 M. The ionic strength of the solution would change when different concentrations of catalyst are added. Therefore, the ionic strength of the absorption solution should be adjusted at 2.1 kmol/m3 by adding a small amount of Na2SO4 solution. In this work, the absorption rate of CO2 was measured at 25 °C and atmospheric pressure. The apparent velocity of the liquid was fixed at UL ) 1.52 × 10-3 m/s, and the velocity of CO2 was Ug ) 6.369 × 10-4 m/s.
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Table 1. Measurement of the Experimental Column Interfacial Area CClO-, kmol/m3
RCO2, ×104 kmol/(m3 s)
0.005 0.015 0.020
3.314 4.298 4.458
/ DCO2(aCCO )2 2
/ kL)2 (aCCO 2
0.00497
0.0654
a, m2 0.0243
Table 2. Values of the Experimental Data at Various CO2 Partial Pressures P, kPa
y1, kmol/kmol
y2, kmol/kmol
Nexp, ×106 kmol/(m2 s)
f
faver
200
0.0096 0.2108 0.2681 0.1741 0.1014 0.3167 0.1493 0.3337 0.2816 0.2141 0.4792 0.1187 0.3457 0.5057 0.2992
0.3001 0.3201 0.3207 0.3167 0.3100 0.4448 0.4447 0.4450 0.4449 0.4449 0.5748 0.5720 0.5727 0.5733 0.5727
17.11 6.438 3.098 8.399 1.229 7.545 17.40 6.556 9.618 1.359 5.631 26.699 13.370 3.982 16.11
1.597 1.613 1.570 1.652 1.591 1.808 2.156 1.778 1.932 2.075 2.519 2.459 2.436 2.376 2.503
1.605
375
625
1.950 Figure 2. Pressure-modified index f versus CO2 partial pressures.
2.459
According to the model of Danckwerts, the rate of absorption can be demonstrated as follows: / RCO2 ) aCCO DCO2k1 + kL2 2
x
(4)
/ / )2DCO2k1 + (aCCO k )2 RCO22 ) (aCCO 2 2 L
(5)
k1 ) 0.90 + 1582CClO-
(6)
The representation of the square of the absorbed flux versus catalyst concentration [ClO-] is a straight line whose slope and original ordinate give access to the effective interfacial area a. According to the experimental data, the effective interfacial area of this packed column under these conditions was 0.024 32 m2. The experimental data and the results can be seen in Table 1. The reproducibility error of the experimental data is less than 5.0%. 5. Absorption Experiment at Elevated CO2 Pressure The absorption rate of CO2 into a MDEA aqueous solution blended with PZ at elevated CO2 partial pressure was studied on the same adiabatic column as that of the interfacial area measurement. As discussed above, the amine solution contacted countercurrently with carbon dioxide in the packed column. Samples from liquid phases could not yet be taken until the absorption reached a steady state. The apparatus and experimental method to determine the experimental data for this work have been reported in detail by Zhang et al.8 The temperature of the absorption solution in the entry of the absorption column was fixed at 60 °C. This experiment was undertaking at a CO2 pressure range of 200625 kPa and a weight ratio of MDEA-PZ of 50:5 at a total amine concentration of 3.0 M. The pressurization of the installation is controlled by means of both the inlet and outlet pressure regulators. The flow rate of gas and liquid is the same as that of interfacial area measurement. The Simpson method was used to integrate eq 3 to get the modified index of the absorption rate, f, at
various pressures. Table 2 shows the experimental data about f at various CO2 partial pressures as well as the absorption rate. Seen from Figure 2, f increases with the CO2 partial pressure and the reproducibility error is less than 10%. This is the main reason that the interfacial disturbance was strengthened by elevated CO2 partial pressure. The increase of the interfacial tension resulting from the reaction resultant concentration increase in the interface at high CO2 partial pressure would increase the instability of the gas/liquid-phase interface and lead to the interfacial disturbance, which is called the Marangoni effect. Therefore, the absorption rate would increase when the bulk liquid replaced the interfacial liquid through interfacial disturbance. A relationship of the CO2 partial pressure and f was deduced as follows:
f ) 0.83[ln(P/100)]0.8 + 1.0 100 kPa e P e 625 kPa (7) P represents the carbon dioxide partial pressure in the gas phase. The model value is in good agreement with the experimental value, with the largest error of less than 5%. 6. Simulation of a Commercial Absorption Column A commercial absorption column consisted of two sections, i.e., the upper section and the lower section, and both of them were packed. The lean solution and semilean solution were used as the solvents of carbon dioxide removal for these two sections, respectively. The lean solution was introduced from the top of the column, and the semilean solution was introduced from the middle of the column. A large amount of carbon dioxide was removed by the semilean solution in the lower section of column. Then the remaining carbon dioxide was removed by the lean solution in the upper section of the column. 6.1. Simulation Method of the Height of the Absorption Column. It is too difficult to describe the commercial absorption process in a packed column by strict mathematics equations because of its complex mechanism, which simultaneously included the mass/ heat transfer and chemical reaction process. In general, the chemical absorption process of the multicomponent system was simplified as follows:
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The flow of gas and liquid phases along the axis direction was hypothesized as piston flow. Also, parameters such as the component, temperature, and pressure were consistent in each cross section. Based on these, the differential equations that could describe the absorption process were obtained from the balance calculation of the elemental packed layer. Through solving of these differential equations, the concentration profile, temperature profile, pressure profile, and flow rate of the gas and liquid in the commercial column could be obtained. At first, the amount of carbon dioxide absorption in the whole commercial column per unit time was obtained through carbon dioxide mass balance based on the CO2 content in the gas stream at the inlet and outlet of the column. Based on the dissolve enthalpy of carbon dioxide, the amount of heat from the exothermic reaction in this whole commercial column per unit time was also obtained. Also, the adiabatic rising of temperature would be calculated through the energy balance for every section when the carbon dioxide content is definite. In this paper, the total pressure was hypothesized to be constant in the commercial absorption column. The whole column was evenly divided into n elements. The amounts of CO2 absorption and adiabatic rising of the temperature were equal in every element. The carbon dioxide content in the solution and partial pressure of carbon dioxide in the gas phase could be deduced by the amount of absorption in each element. The rate of absorption of carbon dioxide into the solution was also calculated based on the data discussed above. The height of each element could be calculated through combination of the rate of carbon dioxide absorption of each element, the interfacial area of packing, and the diameter of the absorption column. Finally, the total height of the packed layer was obtained by the sum of all of these elemental heights. Because of the conditions of the absorption process in the upper and lower sections are different, the upper and lower sections of the commercial column are calculated separately. 6.2. Mathematics Model. The mathematics model for this commercial column simulation was listed as follows: (1) Physical Property Relationship. (a) Carbon Dioxide Equilibrium Partial Pressure. To calculate the driving force of the carbon dioxide absorption, it is necessary to determine the equilibrium data of carbon dioxide in the MDEA aqueous solution blended with PZ. In this paper, a simplified gas-liquid equilibrium model11 was adopted. (b) Model of Saturated Vapor Pressure of Water.
Pw ) 3161.224 - 19.486T + 0.03016T 2 (c) Diffusivity of Carbon
(8)
Dioxide.12
Dµ0.54/T ) 6.109 × 10-8
aB/
aA/
+ φBµBeφA
kMDEA ) 5.86 × 106e-3984/T (see ref 15)
(13)
kPZ ) 4 × 1010e-4059.4/T (see ref 3)
(14)
(3) Height of Each Element.
dh )
dG πd2 aN 4
(15)
(4) Effective Interfacial Area of the Commercial Column.14
[ () ( )( ) ( ) ]
δc a ) 1 - exp -1.45 at δL
0.75
GL atµL
0.1
GL2 FLδLat
atGL2 FL2g
0.2
-0.05
(16)
The commercial column of carbon dioxide absorption into the MDEA aqueous solution blended with PZ could be simulated according to the industrial data. In this paper, an absorption column in a large chemical fertilizer factory was simulated by this method. The actual packing height of the lower column is 11 m. When f is set to 1.0, the model predicts a packing height of 19.6 m. While using the f correction, the packing height was predicted to be 10.3 m. The column height of the simulation value is close to the industrial value with a mean error of less than 10%. However, the packing height of the lower column is higher than that of the industrial value when the kinetics model without pressure modifying factor f was used. 7. Conclusion The elevated CO2 partial pressure would strengthen the surface disturbance phenomenon and, thus, enhance the absorption rate. An effects of CO2 partial pressure on the carbon dioxide absorption rates into the MDEA aqueous solution blended with PZ were studied by an experimental packed column. The CO2 pressure modification to the kinetics model was obtained and listed as follows:
NCO2 ) fHCO2xDCO2(kMDEACMDEA + kPZCPZ)(P - P*)
(9)
kPZ ) 4 × 1010e-4059.4/T
(10)
f ) 0.83[ln(P/100)]0.8 + 1.0 100 kPa e P e 625 kPa
(e) Solubility Coefficient of Carbon Dioxide.13
ln HCO2 ) a1 + a2/T + a3/T 2
NCO2 ) fHCO2xDCO2(kMDEACMDEA + kPZCPZ)(P - P*) (12)
kMDEA ) 5.86 × 106e-3984/T
(d) Viscosity of the Solution.12
µ ) φAµAeφB
(2) Absorption Kinetics Model. An absorption kinetics model combined with the pressure modified factor f was adopted in this simulation process.
(11)
The results indicated that the absorption rate of carbon dioxide at elevated CO2 pressure was larger than that of the kinetics model because of the Marangoni effect.
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The simulation of the commercial column of carbon dioxide absorption was carried out successfully by this modified kinetics model. Nomenclature a ) effective interfacial area, m2 at ) total interfacial area, m2 C ) concentration in the liquid phase, kmol/m3 D ) diffusion coefficient in the liquid phase, m2/s D ) diameter, m f ) pressure-modified factor to the kinetics model of atmospheric pressure h ) height of the element, m G ) amount of CO2 absorption in the column per hour, kmol/h H ) solubility coefficient, kmol/(m3 MPa) kl ) first-order rate constant, m3/s kL) mass-transfer coefficient of the liquid film, L/s L ) flow rate of the fluid, m3/h N, R ) absorption rate of CO2 per area, kmol/(m2 s) P ) pressure, kPa T ) temperature, K U ) velocity, m/s y ) conversion of a solution, kmol of CO2/kmol of amine y1 ) conversion of a rich solution, kmol of CO2/kmol of amine y2 ) conversion of a lean solution, kmol of CO2/kmol of amine Superscripts * ) equilibrium at the interface 0 ) initial state Subscripts am ) MDEA aver ) average value exp ) experimental value g ) gas L ) liquid PZ ) piperazine p ) pressure t ) total w ) water Greek Symbols F ) density of liquid, kg/m3 µ ) viscosity, Pa‚s δ ) surface tension, N/m Abbreviations MDEA ) methyldiethanolamine PZ ) piperazine
Literature Cited (1) Xu, G. W.; Zhang, C. F.; Qin, S. J.; Wang, Y. W. Kinetics Study on Absorption of Carbon Dioxide into Solution of Activated Methyldiethanolamine. Ind. Eng. Chem. Res. 1992, 31, 921. (2) Bishnoi, S.; Rochelle, G. T. Absorption of Carbon Dioxide into Aqueous Piperazine Kinetics, Mass Transfer and Solubility. Chem. Eng. Sci. 2000, 55, 5531. (3) Zhang, X.; Zhang, C. F.; Qin, S. J.; Zheng, Z. S. A Kinetics Study on Absorption of Carbon Dioxide into Mixed Aqueous Solution of Methyldiethanolamine and Piperazine. Ind. Eng. Chem. Res. 2001, 40, 3785. (4) Xu, G. W. The fundamental research of CO2 removal technology by activated MDEA solution. Ph.D. Dissertation, East China University of Science and Technology, Shanghai, China, 1993. (5) Xu, G. W.; Zhang, C. F.; Qin, S. J.; Zhu, B. C. Desorption of CO2 from MDEA and Activated MDEA Solution. Ind. Eng. Chem. Res. 1995, 34, 874. (6) Benadda, B.; Kafoufi, K.; Mondam, P.; Otterbein, M. Hydrodynamics and mass transfer phenomenon counter-current packed column at elevated pressure. Chem. Eng. Sci. 2000, 55, 6251. (7) Wu, H. D.; Chung, T. W.; Lai, M. H. Effect of Maragoni Convection on the mass transfer performance in a packed- bed absorber. Ind. Eng. Chem. Res. 2001, 40, 885. (8) Zhang, X.; Zhang, C. F.; Xu, G. W.; Gao, W. H.; Wu, Y. Q. An Experimental Apparatus for Mimicking Carbon Dioxide Removal and Optimum Concentration. Ind. Eng. Chem. Res. 2001, 40, 898. (9) Nobuyuki, I.; Katsuhiko, F. Marangoni Instability to Chemical Absorption. Kagaku Kogaku Ronbunshu 1978, 4, 490; 1980, 6, 431. (10) Danckwerts, P. V. Gas-liquid reactions; McGraw-Hill: New York, 1970. (11) Liu, H. B.; Zhang, C. F.; Xu, G. W. A Study on Equilibrium Solubility for Carbon Dioxide in Methyldiethanolamine-Piperazine-Water Solution. Ind. Eng Chem. Res. 1999, 38, 4032. (12) Haimour, N.; Bidarian, A.; Sandall, O. C. Kinetics of the reaction between carbon dioxide and methyldiethanolamine. Chem. Eng. Sci. 1987, 42, 1393. (13) Al-Ghawas, H. A.; Hagewiesche, D. P.; Ruizlbanez, G.; Sandall, O. C. Physicochemical Properties Important for Carbon Dioxide Absorption in Aqueous Methyldiethanolamine. J. Chem. Eng. Data 1989, 34, 385. (14) Kakusaburo, O.; Hiroshi, T.; Yoshiaki, K. Effect of Packing Material on the Wetted Surface Area. Kagaku Kogaku (Chem. Eng. Jpn.) 1967, 31, 126. (15) Wang, Y. W.; Zhang, C. F.; Qin, S. J. Kinetics Study on Absorption Carbon Dioxide in Aqueous MDEA. J. Chem. Ind. Eng. (China) 1991, 4, 466.
Received for review March 25, 2002 Revised manuscript received October 1, 2002 Accepted October 1, 2002 IE020223T
