Comprehensive Study of the Kinetics of the Oxidative Degradation of

Ind. Eng. Chem. Res. 2006, 45, 2569-2579

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Comprehensive Study of the Kinetics of the Oxidative Degradation of CO2 Loaded and Concentrated Aqueous Monoethanolamine (MEA) with and without Sodium Metavanadate during CO2 Absorption from Flue Gases Adeola Bello and Raphael O. Idem* Process Systems Engineering Laboratory, Faculty of Engineering, UniVersity of Regina, 3737 Wascana Parkway, Regina, SK, Canada S4S OA2

A comprehensive mechanistic-based kinetic study of the oxidative degradation of CO2 loaded MEA, with and without a corrosion inhibitor (NaVO3), was performed in a stainless steel rotary-type autoclave with MEA concentrations of 11.4 and 17.9 mol %, NaVO3 concentration of 0.1 mol %, O2 pressures of 250 and 350 kPa, and CO2 loading ranging from 0 to 0.44 (mol of CO2)/(mol of MEA) at temperatures of 328-393 K (typical absorber and stripper temperatures). The results showed that the presence of NaVO3 and increases in MEA concentration, temperature, or O2 pressure resulted in an increase in the MEA degradation rate. In contrast, an increase in CO2 loading led to a decrease in the degradation rate. The general mechanistic rate model obtained to represent all the systems investigated was of the following form: - rMEA ) {k11[MEA]a[O2]b}/{k2 + k3[O2]c + k5[CO2]e}. This rate model shows that, in a CO2 loaded system, the loaded CO2 acts as a degradation inhibitor. In addition, the order of reaction with respect to MEA (a) for all the systems investigated was ∼1. 1. Introduction Several standard processes can be used for carbon dioxide (CO2) removal from flue gas streams. However, the most economical method for the separation of CO2 from dilute, lowpressure flue gas streams, such as from coal-fired power plants and other industrial point sources, appears to be gas absorption using aqueous alkanolamines. Although there are different types of industrially utilized alkanolamines, monoethanolamine (MEA) is the most widely used for CO2 absorption.1 In an ideal MEA-CO2 absorption system, the solvent is recycled and reused. However, because of the high propensity for MEA to degrade, some of the byproducts of MEA degradation can decrease the efficiency of CO2 capture and have also been implicated in the corrosion of machinery and toxicity to the environment. Also, the degraded MEA has to be disposed of in an environmentally sound manner,2,3 which can lead to increased material and waste disposal costs. Also, degradation and corrosivity have forced the use of low concentrations of MEA, leading to a larger overall equipment size, higher solvent circulation rates, and, therefore, an increased energy requirement for CO2 regeneration.4,5 To effectively prevent MEA degradation, a degradation prevention strategy needs to be formulated, and this requires knowledge of the products, stoichiometry, mechanism, and kinetics of the degradation process5,6 as a function of the various operating variables. Most of the earlier studies performed on the degradation of single alkanolamines have focused on understanding the natural gas sweetening processes where O2 is absent. However, degradation involving flue gases is more complicated due to the presence of O2 and other components including CO2, CO, SOx, NOx, and fly ash.5,6,7 Also, severe corrosion problems plague alkanolamine systems for CO2 absorption; hence, corrosion inhibitors are added. These inhibitors are based primarily on heavy metals, the most common of which are copper salts8,9 * Corresponding author. [email protected].

Fax:

(306)

585-485.

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and vanadium salts, usually in the form of the vanadate salt (e.g., sodium metavanadate).10 Thus, to carry out a proper study of MEA degradation during CO2 absorption from power plant flue gases, the effects of all possible components in the system must be considered in terms of identifying the products, determining the degradation stoichiometry, elucidating the mechanism for products formation,10 and evaluating the kinetics of the degradation process in a systematic manner. In an earlier work,10 it was identified based on literature review that there has been no pathway proposed for the reaction of CO2 loaded MEA with O2, though kinetic data have been provided. It is also evident from the literature that mechanisms have been proposed for the formation of only some of the products of the reaction of MEA with O2 alone and CO2 alone.3,11,12,13 Other possible products were not reported probably because of difficulty in their identification. Also, only one group has proposed a mechanism for the formation of only some of the products observed for the combined CO2 and O2 induced degradation of MEA based on the analysis of the reclaimer waste from the IMC Chemicals Facility.5 This mechanistic work did not involve all of the possible products of a reaction involving both O2 and CO2 exclusively with MEA. Also, the identified products cannot be considered to be representative of oxidative degradation products under absorber or stripper conditions because in MEA reclaiming, sodium carbonate or sodium hydroxide (known to react with heat stable salts) is added in order to liberate the amine from the heat stable acid salts. Hence, other products, apart from the true degradation products, are also formed during this process. In an earlier work,10,14 we identified almost all the possible degradation products, determined the degradation stoichiometry, and elucidated a possible mechanism for the oxidative degradation of CO2 loaded, aqueous MEA in the absence of NaVO3. The present study is aimed at using kinetic studies to validate the degradation mechanism proposed in our earlier studies.10,14 This work is also a study comparing both the kinetics and mechanism of degradation of an MEA system (i.e., MEAH2O-CO2-O2) without any corrosion inhibitor with those of

10.1021/ie050562x CCC: $33.50 © 2006 American Chemical Society Published on Web 12/21/2005

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Ind. Eng. Chem. Res., Vol. 45, No. 8, 2006

Figure 1. Schematic representation of experimental setup.

a system with NaVO3 (i.e., MEA-H2O-CO2-O2-NaVO3). This comprehensive kinetic model, which is applicable to all the systems discussed in this paper, was derived based on a generalized mechanism10 proposed based on our earlier work. These results are presented and discussed in this paper. 2. Experiment 2.1. Equipment. The reactor used to obtain kinetic data was a stainless steel rotary-type autoclave (model BC0030SS05AH, obtained from Autoclave Engineers, Erie, PA) which consisted of a baffle bar, an impeller, cooling coils, a gas feed port, an extra feed port, a gas product port, a liquid sampling valve, a thermowell, and a pressure gauge. The pressure gauge was fitted to the extra feed port and used to measure the total pressure in the reactor. A K-type thermocouple placed in the thermowell was used to measure the temperature of the reaction mixture, while an electric furnace, controlled using a proportional-integral-derivative (PID) temperature controller (model MTCPKS00, also from Autoclave Engineers, Erie, PA) and a K-type thermocouple, was used to supply the heat. The accuracy of the temperature control was within (2 K. A schematic of the experimental setup is given in Figure 1. 2.2. Chemicals. Concentrated MEA (research grade, 99% purity) and NaVO3 (90% purity) were obtained from Fischer Scientific, Whitby, Ontario, Canada. For each experimental run not involving NaVO3, MEA was diluted to the desired concentration and then standardized using 1M hydrochloric acid (also obtained from Fischer Scientific). For each experimental run involving NaVO3, the MEA was diluted to the desired concentration and standardized using 1M hydrochloric acid; then, a desired amount of NaVO3 to make 0.1 mol % NaVO3 was weighed and added to the aqueous MEA solution. 2.3. Typical Experimental Runs. 2.3.1. MEA-H2O-O2 and MEA-NaVO3-H2O-O2 Systems. Details of the description of a typical experimental run are given in our earlier work.10,14 Briefly, the reaction was conducted using aqueous solutions of 11.4 and 17.9 mol % MEA at temperatures of 328, 373, and 393 K as well as O2 pressures of 250 and 350 kPa. About 230 mL of aqueous MEA was loaded into the reactor

and stirred at the rate of 300 rpm while being heated to the desired temperature and simultaneously cooling the stirrer. When the reaction mixture reached the desired temperature, the pressure on the reactor pressure gauge was noted and added to the desired O2 pressure to obtain a final desired reactor pressure. The reactor O2 gas inlet valve was then opened, and O2 was fed to the reactor by opening the O2 cylinder and regulating it to the desired reactor pressure. The gas inlet and the O2 cylinder were left open, and the first sample (2.5 mL) was taken immediately. Because of the solubility of O2 in MEA, there was a pressure depletion in the reactor. The reactor was boosted with extra O2 to maintain the desired pressure. This was also repeated each time a sample was taken. About 2.5 mL of other samples were taken at predetermined intervals. The reaction in each sample was quenched by running ice-cold water over the sample vial for ∼1 min. In the case of the MEA-NaVO3H2O-O2 system, the reaction was conducted using aqueous solutions of 11.4 mol % MEA containing 0.01 mol % NaVO3 at temperatures of 328 and 393 K as well as an O2 pressure of 250 kPa. Apart from using 11.4 mol % MEA containing 0.1 mol % NaVO3 in place of 11.4 mol % MEA as in the earlier case, the procedure was the same as in the case of the MEAH2O-O2 system. 2.3.2. MEA-H2O-CO2 and MEA-NaVO3-H2O-CO2 Systems. In the case of the MEA-H2O-CO2 system, the reaction was conducted using aqueous solutions of 11.4 and 17.9 mol % MEA at temperatures of 393 K as well as CO2 pressures of 250 kPa. Also, apart from using CO2 in place of O2, the procedure was the same as in the MEA-H2O-O2 system. In the case of the MEA-NaVO3-H2O-CO2 system, the reaction was conducted using aqueous solutions of 11.4 mol % MEA containing 0.1 mol % NaVO3 at a temperature of 393 K as well as an O2 pressure of 250 kPa. Apart from using 11.4 mol % MEA containing 0.1 mol % NaVO3 in place of 11.4 and 17.9 mol % MEA, the procedure was the same as in the MEA-H2O-CO2 system. 2.3.3. MEA-H2O-O2-CO2 and MEA-NaVO3-H2OO2-CO2 Systems. In the case of the MEA-H2O-O2-CO2 system, the reactor was also loaded with 230 mL of 11.4 or

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17.9 mol % MEA as desired. CO2 was preloaded into each solution by allowing 250 kPa of CO2 to contact the MEA in the reactor without heating while stirring at 300 rpm. After 24 h, a sample was taken and the CO2 loading was determined. The CO2 loaded MEA was then heated to the desired temperature, and the CO2 loading was again determined. Also, the pressure on the reactor pressure gauge was noted and added to the desired O2 pressure to obtain the reactor pressure. In this case, too, apart from the preloading of the MEA solution with CO2, the procedure was similar to the case for the MEAH2O-O2 system. In the case of the MEA-NaVO3-H2OO2-CO2 system, the reaction was conducted using aqueous solutions of 11.4 mol % MEA containing 0.1 mol % NaVO3 at a temperature of 393 K as well as an O2 pressure of 350 kPa. Apart from using 11.4 mol % MEA containing 0.1 mol % NaVO3 in place of one without NaVO3, the procedure was the same as in the case of the MEA-H2O-O2-CO2 system. It is pertinent to note that the conventional way of operating the CO2 capture plant may not operate with high CO2 loading in the stripping section of the plant, where temperatures can reach 100-140 °C. However, in a previous study16 in which we attempted to minimize the heat duty for stripping, we have established that the CO2 loading in the lean MEA solution will still be high. Consequently, to improve the capture efficiencies at low energy for stripping, both the absorption side and the stripping side will have to operate at CO2 loadings above the conventional levels. In the present work, we have reflected these possibilities and have, thus, used high CO2 loadings at 100 and 120 °C. 2.4. Analysis of Products. The samples were analyzed using a high performance liquid chromatograph (HPLC). A HPLC mobile phase consisting of 5 mMol/L tartaric acid, 1 mMol/L dipicolinic acid, and 1.5 g/L boric acid was used, and this was prepared with Nanopure water. It was then degassed using an ultrasonic vibrator prior to being filtered with a membrane filter. The mobile phase was then used to dilute the samples 100× before analyzing with the HPLC (model Agilent 1100 series, supplied by Agilent Technologies Canada Incorporated, Mississauga, ON L4W 5M2, Canada). The HPLC was equipped with a column (model Shodex IC YK-421) as well as a guard column (model Shodex IC YK-G), both of which were supplied by JM Science Incorporated, Grand Island, NY. The HPLC conditions used are summarized as follows. The quaternary pump column flow was 1 mL/min, the pump stop time was 15 min, and the minimum pressure limit was 0 bar, while the maximum pressure limit was 400 bar. The eluent used was 100% of the mobile phase. An Agilent 1100 refractive index detector (model G1362A) with an optical unit temperature of 318 K, positive polarity, and peak width > 0.2 min was used. Sample injection was done by an Agilent 1100 auto sampler, using a standard injection mode to inject 1 µL of sample. An Agilent 1100 column thermostat was used to maintain the column temperature at 318 K. Each sample analysis took 15 min. MEA concentrations were based on calibrations using standardized MEA. The error of the HPLC was less than (2%. 2.5. Quantitative Analysis of MEA via HPLC. An MEA calibration curve was obtained by preparing MEA samples to cover the MEA concentration range of 0.6-18.4 mol %. This was then followed by a 1:100 dilution (to prevent column overload), and then filtration was performed using a 0.4 µm poly(tetrafluoroethylene) (PTFE) membrane filter prior to loading into vials and running the samples on the HPLC. Each sample was injected three times, and an average area was obtained. A plot of MEA concentration versus the area was then

Figure 2. (a) Typical concentration-time curve (MEA-H2O-O2 system) at 373 K. (b) Typical concentration-time curve (MEA-H2O-O2 system) at 393 K.

made and then fitted using Microsoft Excel 2000. The equation of the line of fit was then used to obtain the actual concentration of MEA based on the area obtained from the HPLC. 3. Results and Discussion 3.1. Kinetic Data. 3.1.1. MEA-H2O-O2 System. MEA concentration-time curves for this system are reported for temperatures of 328, 373, and 393 K and initial MEA concentrations of 11.4 and 17.9 mol % MEA. Typical concentration versus time curves are given in parts a and b of Figure 2 for 11.4 mol % MEA with 250 and 350 kPa O2 at 373 and 393 K, respectively. Experimental degradation rates in terms of instantaneous rates were evaluated as slopes of the concentration-time curves. The rates were obtained as follows. Initially, a concentration-time curve was plotted for all the systems investigated. This was followed by curve fitting of the data by using an exponential function available on Microsoft Excel. The equation of the line was obtained and differentiated to obtain the rate at each instant of time. The rate data obtained was then plotted versus the time to obtain the rate-time plots. The effect

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Figure 3. Rate versus time curves for all conditions considered in the MEA-H2O-O2 system.

of operating variables such as O2 pressure, temperature, and initial MEA concentration on instantaneous rates were evaluated in terms of rate-versus-time curves. These are given in Figure 3 for all the operating conditions investigated. 3.1.1.1. Effect of O2 Pressure. The effect of O2 pressure was evaluated in experimental runs with 11.4 mol % MEA at both 373 and 393 K by comparing the results obtained using 250 kPa O2 pressure with those of 350 kPa O2 pressure. The plot of MEA degradation rate versus time at both temperatures is given in Figure 3. The result shows that the MEA degradation rate as a function of time was higher with 350 kPa O2 than with 250 kPa O2 pressure at both 373 and 393 K. As expected, this indicates that the rate of degradation increases with an increase in O2 pressure. After 135 h, the overall rate of MEA degradation (calculated as δC/δt, where δC is the change in MEA concentration while δt is the change in reaction time from time 0 to time t) with 250 kPa of O2 pressure was 0.014 6 mol/(L‚h), while with 350 kPa of O2 pressure, it was 0.022 2 mol/(L‚h); both were evaluated at 393 K. On the other hand, after 525 h, the overall rate of MEA degradation with 250 kPa O2 was 0.003 151 mol/L, while with 350 kPa O2, after 496 h, the overall rate of MEA degradation was 0.004 7 mol/(L‚h); all were evaluated at 373 K. This further confirms that the overall rate of degradation at 350 kPa O2 is higher than that at 250 kPa O2. 3.1.1.2. Effect of Temperature. The effect of temperature was verified using experimental runs with 11.4 mol % MEA and O2 pressures of 250 and 350 kPa, at 328, 373, and 393 K. The plot of MEA degradation rate versus time at all three temperatures is given in Figure 3. The result shows that MEA degradation rate as a function of time was higher at 393 K than at 373 K, which was, in turn, higher than at 328 K. This indicates that the rate of degradation increases with an increase in temperature, as expected. After 135 h, the overall rate (δC/δt) of MEA degradation was 0.014 6 mol/(L‚h) at 393 K. At 373 K, the overall rate of MEA degradation was 0.004 1 mol/(L‚h) after 190 h, while at 328 K, the overall rate of MEA degradation was 0.000 2 mol/(L‚h) after 168 h. This is further evidence that MEA degradation is more rapid at higher temperatures than at lower temperatures, as expected. 3.1.1.3. Effect of MEA Concentration. The effect of MEA concentration was evaluated with 250 and 350 kPa O2 pressures at 393 K by comparing the results of 11.4 mol % MEA with those of 17.9 mol % MEA. The plot of MEA degradation rate versus time at both MEA concentrations is given in Figure 3. The result shows that, when 250 kPa O2 was used, MEA degradation rate as a function of time was higher with 17.9 mol % MEA than with 11.4 mol % MEA.

With 250 kPa O2 and 11.4 mol % MEA, the overall rate of MEA degradation (δC/δt) was 0.014 6 mol/(L‚h), while with 17.9 mol % MEA, the overall rate of MEA degradation was 0.026 0 mol/(L‚h), both after 135 h. With 350 kPa O2 and 11.4 mol % MEA, the overall rate of MEA degradation was 0.022 2 mol/(L‚h), while with 17.9 mol % MEA, the overall rate of MEA degradation was 0.028 0 mol/(L‚h), both after 135 h. These results illustrate the detrimental effects of higher MEA and O2 concentrations on the overall rate of degradation. 3.1.2. MEA-NaVO3-H2O-O2 System. MEA concentration-time curves for this system are reported as a function of reaction time for temperatures of 328 and 393 K, as well as an initial MEA concentration of 11.4 mol %. Experimental degradation rates in terms instantaneous rates were evaluated as slopes of the concentration-time curves. The effects of operating variables such as O2 pressure, temperature, and initial MEA concentration on instantaneous rates were evaluated in terms of rates versus time curves. These are given in Figure 5 for all the operating conditions investigated. 3.1.2.1. Effect of Temperature. The effect of temperature was directly verified using experimental runs with 11.4 mol % MEA + 0.1 mol % NaVO3 and O2 pressures of 250 at 328 and 393 K. The plot of MEA degradation rate versus time at both temperatures is given in Figure 5. The result shows that the degradation rate with degradation time is higher at 393 K than at 328 K. This also shows that the rate of degradation increases with an increase in temperature, just as in the case without NaVO3. After 135 h at 393 K, the overall rate of MEA degradation was 0.028 3 mol/(L‚h), while at 373 K, the overall rate of MEA degradation was 0.009 9 mol/(L‚h), illustrating that the overall rate of degradation increases with an increase in temperature, just as in the case without NaVO3. 3.1.3. MEA-H2O-O2 System Versus MEA-NaVO3H2O-O2 System. The effect of NaVO3 on MEA degradation was evaluated by using 11.4 mol % MEA and 11.4 mol % MEA + 0.1 mol % NaVO3 with 250 kPa O2 at both 328 and 393 K. The runs were conducted for the same time duration to enable the comparison of the results. 3.1.3.1. Effect of NaVO3 on the rate of MEA Oxidative Degradation. Figure 5 shows the rate-time curve of the results obtained for 11.4 mol % MEA and 11.4 mol % MEA + 0.1 mol % NaVO3 with 250 kPa O2 at both 328 and 393 K. The figure shows that the degradation rate is higher at both temperatures in the presence of NaVO3 than in its absence. Also, after 135 h at 393 K, the overall rate of MEA degradation for the MEA-H2O-O2 system was 0.014 6 mol/(L‚h), while for the MEA-NaVO3-H2O-O2 system, it was 0.026 8 mol/L. Furthermore, after 341 h at 328 K, the overall rate of MEA degradation for the MEA-H2O-O2 system was 0.000 292 mol/(L‚h), while for the MEA-NaVO3-H2O-O2 system, the overall rate was 0.003 68 mol/L. Thus, MEA degradation was more rapid in the MEA-NaVO3-H2O-O2 system than in the MEA-H2O-O2 system, as is widely known in the literature.3 3.1.4. MEA-H2O-CO2 System. 3.1.4.1. Effect of Concentration. The effect of MEA concentration was evaluated by comparing the results of 11.4 mol % MEA with 17.9 mol % MEA for 250 kPa CO2 pressure at 393 K. The plot of MEA degradation rate versus time at both concentrations is given in Figure 6. The result shows that the degradation rate is higher with 17.9 mol % MEA than with 11.4 mol % MEA. This indicates that the rate of degradation increases with an increase in MEA concentration, just as in the MEA-H2O-O2 system. This is further confirmed from results with 11.4 mol % MEA, for which, after 490 h, the overall rate of MEA degradation


Figure 4. (a) Plot of (-ra) predicted from mechanism-based rate model versus (-ra) obtained from experiment for the MEA-H2O-O2 system. (b) Plot of (-ra) predicted from mechanism-based rate model versus (-ra) obtained from experiment for the MEA-H2O-NaVO3-O2 system. (c) Plot of (-ra) predicted from mechanism-based rate model versus (-ra) obtained from experiment for the MEA-H2O-O2 system. (d) Plot of (-ra) predicted from mechanismbased rate model versus (-ra) obtained from experiment for the MEA-H2O-NaVO3-O2 system.

Figure 5. Rate versus time curves for all conditions considered in the MEA-NaVO3-H2O-O2 system with the corresponding rate versus time curve for the MEA-H2O-O2.

was 0.000 436 mol/(L‚h), while with 17.9 mol % MEA, the overall rate was 0.001 9 mol/(L‚h) for the same time duration. 3.1.5. MEA-NaVO3-H2O-CO2 System. Results from the experiments for this system are reported in term of MEA concentrations as a function of reaction time for 393 K, initial MEA concentration of 11.4 mol % MEA + 0.1 mol % NaVO3, and CO2 pressure of 250 kPa. This system was investigated to evaluate the effect of NaVO3 on the potential of CO2 induced degradation of MEA. The plot of MEA degradation rate versus time is also given in Figure 6. The result shows that the rate of

MEA degradation even in the presence of NaVO3 is very low, just as in the MEA-H2O-CO2 system. 3.1.6. MEA-H2O-CO2 System Versus MEA-NaVO3H2O-CO2 System. The effect of NaVO3 on MEA degradation was evaluated by using 11.4 mol % MEA and 11.4 mol % MEA + 0.1 mol % NaVO3 with 250 kPa CO2, both at 393 K, for runs conducted for the same time period. Figure 6 shows the degradation rate-time curve of the results obtained for 11.4 mol % MEA and 11.4 mol % MEA + NaVO3, both with 250 kPa CO2 at 393 K. The figure shows that the degradation rate

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Figure 6. Effect of MEA concentration and the presence of NaVO3 on MEA degradation for CO2 induced MEA degradation, as shown by ratetime curve.

is just a little higher in the MEA-NaVO3-H2O-CO2 system than in the MEA-H2O-CO2 system This shows that not only is the rate of CO2 induced MEA degradation low but also the addition of NaVO3 to the solution does not significantly increase CO2 induced MEA degradation. This is important, because MEA degradation will not be expected in the stripping column where the temperature is usually high (>393 K) and the aqueous MEA contains a corrosion inhibitor to mitigate the problems of corrosion with carbon steel columns. The insignificant degradation caused by CO2 as well as the negligible effect of the presence of NaVO3 in the absence of O2 was further verified at 393 K by evaluating the overall rate of degradation after 490 h of degradation time, for which the overall rate of MEA degradation for the MEA-H2O-CO2 system was 0.000 436 mol/(L‚h), while for the MEA-NaVO3-H2O-CO2 system, the overall rate was 0.000 454 mol/(L‚h). 3.1.7. Comparison of MEA-H2O-CO2 System with the Corresponding MEA-H2O-O2 System. A comparison was made between the MEA-H2O-CO2 system and the corresponding MEA-H2O-O2 system with 11.4 and 17.9 mol % MEA at 393 K using 250 kPa pressure of either O2 or CO2. The degradation rate-time curves for both scenarios are shown in Figure 7. Figure 7 shows that the degradation rate in the MEA-H2O-O2 system is very much higher than in the MEAH2O-CO2 system. This result is very consistent with the literature15 that shows that MEA is more prone to degradation in the presence of O2 as compared to the presence of CO2. 3.1.8. MEA-H2O-O2-CO2 System. Test runs were conducted for this system using 11.4 and 17.9 mol % aqueous MEA solutions with O2 pressures of 250 and 350 kPa at 373 and 393 K. 3.1.8.1. Effect of Temperature. The effect of temperature on the MEA degradation rate for this system was evaluated with 11.4 mol % MEA for 350 kPa O2 pressure at 373 (0.41 (mol of CO2)/(mol of MEA)) and 393 K (0.44 (mol of CO2)/(mol of MEA)). The plot of MEA degradation rate versus time at both temperatures is given in Figure 8. The result shows that the degradation rate with reaction time was more rapid at 393 K than at 373 K, despite the presence of a higher CO2 loading at the higher temperature. This indicates that the rate of degradation increases with an increase in temperature. After 111 h, the overall rate of MEA degradation was 0.000 634 mol/(L‚h) at 373 K, while at 393 K, the overall rate of MEA degradation was 0.003 573 mol/(L‚h) after 105 h. This result is consistent with a higher MEA degradation rate at higher temperatures than at lower temperatures, as expected. 3.1.8.2. Effect of CO2 Loading. As stated earlier, results obtained in this study showed that an increase in the O2 pressure

led to an increase in the rate of MEA oxidative degradation. However, in the CO2 loaded MEA, an increase in the O2 pressure produced results that were the exact opposite. This was followed by an evaluation of the effect of CO2 loading on the rate of reaction of this system by comparing the run obtained using 350 kPa O2 pressure and 0.33 mol CO2/mol MEA with that obtained using 250 kPa O2 with 0.27 mol CO2/mol MEA, both for 17.9 mol % MEA at 393 K. The plot of MEA concentration versus time at both temperatures is given in Figure 8. The results show that the MEA degradation rate was higher at the lower CO2 pressure than at the higher CO2 pressure. In contrast, our previous result showed the opposite for the MEAH2O-O2 system. The present result is attributed to the fact that a lower CO2 loading was used for the lower O2 pressure run, whereas a higher CO2 loading was used for the higher O2 pressure run. Thus, the higher CO2 loading caused a reduction in the solubility of O2 in the amine (MEA) even at a high O2 pressure, thereby reducing the MEA degradation rate. This is consistent with the literature,12,15,17 which shows that the rate of the oxidative degradation of MEA decreases with an increase in the CO2 loading. 3.2. Degradation Kinetics. The general mechanistic rate model obtained to represent all the systems investigated was of the following form:

- rMEA )

k11[MEA]a[O2]b k2 + k3[O2]c + k5[CO2]e

This mechanistic rate model was obtained by carrying out an analysis based on our previous studies on the mechanism of MEA degradation.11,14 The analysis showed that a simplified general scheme could be developed to account for the oxidative degradation of MEA with or without CO2 loading. This scheme could be categorized into the following: (1) reaction of MEA with O2, (2) further conversion of intermediate products to form stable products, (3) reaction of intermediate products with O2 to form stable products and other intermediates, (4) reaction of intermediate products with O2 to form other products, (5) reaction of intermediate products with CO2 to form other products, (6) reaction of MEA with CO2 to form an intermediate, and (7) further conversion of another intermediate product to form other products This simplified scheme is represented by the reactions in eqs 1-7: k1

aMEA + bO2 98 I*

(1)

(reaction of MEA with O2) k2

I* 98 products

(2)

(further conversion of intermediate product to form stable products) k3

I* + cO2 98 products + I**

(3)

(reaction of intermediate products with O2 to form stable products and other intermediates) k4

I** + dO2 98 products

(4)

(reaction of intermediate products with O2 to form other stable products)


Figure 7. Comparison between MEA-H2O-O2 and MEA-H2O-CO2.

[I ] )

k1[MEA]a[O2]b

*

(10)

k2 + k3[O2]c + k5[CO2]e

The same procedure was repeated for the other intermediates (I** and I***) to yield eqs 11 and 12 for I*** and I**, respectively.

[I***] )

k6[MEA]f[CO2]g ) k61[MEA]f[CO2]g k7 [I**] )

Figure 8. Plot of MEA degradation rate versus time for various operating conditions for the MEA-H2O-CO2-O2 system. k5

I* + eCO2 98 products

(5)

k3[I*][O2]c

f[MEA] + gCO2 98 I

***

(6)

Also, the rate of disappearance of MEA was expressed as shown in eq 13.

-rMEA ) k2[I*] + k3[O2]c[I*] + k4[O2]d[I**] + k5[CO2]e[I*] + k7[I***] (13)

k7

Equations 11 and 12 were then substituted into eq 13 to yield eq 14.

-rMEA ) [I*](k2 + 2k3[O2]c + k5[CO2]e) + k6[MEA]f[CO2]g

(reaction of MEA with CO2 to form an intermediate) I*** 98 products

(12)

k4[O2]d

(reaction of CO2 with intermediate to form stable products) k6

(11)

(14) (7) Substituting eq 10 into eq 14 yields

(further conversion of intermediate products to form stable products) where I*, I**, and I*** represent intermediates and a, b, c, d, e, f, and g represent the stoichiometric coefficients of the reacting species. A kinetic model was developed based on eqs 1-7 as follows. Using the pseudo-steady-state hypothesis for which the rate of formation of an active intermediate is equal to its rate of disappearance, the rates of formation of the active intermediates were calculated as shown below:

rI* ) k1[MEA]a[O2]b

(8)

rI* ) k2[I*] + k1[O2]c[I*] + k5[CO2]e[I*]

(9)

Equating eqs 8 and 9 and simplifying for I* yields eq 10.

-rMEA )

k2k1[MEA]a[O2]b k2 + k3[O2]c + k5[CO2]e

2k3k1[MEA]a[O2]b k2 + k3[O2] + k5[CO2] c

e

+

+ k5[CO2]e[I*] + k6[MEA]f[CO2]g (15)

Equation 15 can then be condensed as shown in eq 16:

-rMEA )

k2k1[MEA]a[O2]b + 2k3k1[MEA]a[O2]b k2 + k3[O2]c + k5[CO2]e

+

k5[CO2]e[I*] + k6[MEA]f[CO2]g (16) Also, it was observed from results with 11.4 mol % MEA that, after 490 h, the overall rate of MEA degradation was

2576


0.000 436 mol/(L‚h), while with 17.9 mol % MEA, the overall rate was 0.001 9 mol/(L‚h) for the same time duration. This shows that the rate of reaction of MEA with CO2 is negligible. With this experimental observation, eq 16 can be approximated to

-rMEA )

k2k1[MEA]a[O2]b + 2k3k1[MEA]a[O2]b

-rMEA )

k2 + k3[O2]c + k5[CO2]e k1[MEA]a[O2]b(k2 + 2k3[O2]c) k2 + k3[O2]c + k5[CO2]e

(17)

(18)

The O2 pressure for each condition is kept constant; therefore, k2 + 2k3[O2]c can be represented by a constant, k/. Equation 18 then becomes

-rMEA )

k1k/[MEA]a[O2]b k2 + k3[O2] + k5[CO2] c

e

(18b)

Also, k1k/ can be represented as k1/. Then, eq 18b becomes

-rMEA )

k1/[MEA]a[O2]b k2 + k3[O2]c + k5[CO2]e

(19)

Equation 19, therefore, represents the general rate expression for all the systems investigated. This rate expression shows that the rate of MEA degradation increases with O2 concentration. However, for very high O2 concentrations, the presence of O2 may become detrimental to the rate, since this is also involved in the denominator of the rate expression. The magnitude of this effect is not quite clear as yet until the stoichiometric coefficients are evaluated. In any case, this effect occurs because, at higher O2 concentrations, part of the O2 will react with MEA degradation products instead of with MEA, as was shown in our earlier study.10 Also, the presence of CO2 in the denominator shows that CO2 behaves as a degradation inhibitor. This is because the presence of CO2 leads to a reduction in the solubility of O2 in MEA. Thus, for a liquidphase reaction typical of MEA degradation, dissolved CO2 will limit the amount of O2 in the liquid phase to react with MEA. 3.2.1. MEA-H2O-O2 System. As has been illustrated, the generalized comprehensive rate model presented in eq 19 was derived from a generalized simplified mechanism based on our earlier work.10 Equation 19 is general enough and can be modified to suit any set of operating conditions. For example, in the MEA-H2O-O2 system, the concentration of CO2 ([CO2]) is zero. Therefore, substituting [CO2] ) 0 into eq 19 yields eq 20.

-rMEA )

k01 e-Ea/RT[MEA]a[O2]b k2 + k3[O2]c

(20)

3.2.1.1. Estimation of Parameters of Rate Expression. From eq 20, it is clear that the parameters that need to be estimated are k10, Ea, k2, k3, a, b, and c. The procedures used to obtain these parameters are explained below. The degradation of MEA by O2 under typical flue-gas-treating conditions is an example of a heterogeneous gas-liquid reaction in which O2 in the gas phase must contact MEA in the liquid phase before any reaction can occur. Therefore, both mass transfer rates and

the chemical reaction rates could be involved in the overall rate expression. However, the high stirring rates used,6 as well as the fact that MEA is not very volatile and can be assumed to remain in the liquid phase for the temperatures used, enabled us to eliminate any mass transfer effects and to assume that the reaction occurred only in the liquid phase between MEA and the O2 dissolved in the reaction mixture. Therefore, the kinetic evaluation of the MEA-H2O-O2 system was performed as a homogeneous liquid-phase reaction in terms of instantaneous rates for various MEA concentrations, O2 pressures, and temperatures. A power-law-type equation that also includes the temperature dependency of the reaction was formulated to describe the kinetics of the oxidative degradation of MEA in order to obtain the activation energy (Ea) and the frequency factor (k0) and to simplify the procedure for the estimation of the parameters of eq 20. This model is given in eq 21,

-rMEA ) k0 e-(Ea/RT)[MEA]m[O2]n

(21)

where -rMEA is the rate of oxidative degradation of MEA [mol/(L‚h)], k0 is the preexponential constant (unit depends on the values of m and n), Ea is the activation energy (J/mol) for the oxidative degradation of MEA, R is the universal gas constant [8.314 J/(mol‚K)], m and n are the reaction orders with respect to MEA and O2, respectively, and T is the degradation temperature (K). It is important to note that the concentration of O2 used in this calculation was obtained by converting the O2 pressures to actual concentrations by using the equation formulated by Rooney and Daniels17 as given in eq 22.

[O2] (mol/m3) ) -2.545 + (0.807 × 10-2)T (2.322 × 104)p + T (3.911 × 102)p2 (22) 1.027p2 T

84.14p + (2.096 × 10-4)pT2 +

Equation 21 was linearized to eq 23 so as to estimate the parameters Ea and k0 using the multiple linear least-squares regression package available in Microsoft Excel 2000.

ln(-rMEA) ) ln k0 -

Ea + m ln[MEA] + n ln[O2] (23) RT

Estimates of the values of the parameters were obtained at a 95% confidence level. The activation energy (Ea) for the oxidative degradation of MEA was 70 642 J/(mol‚K), while the k0 obtained was 1.21 × 1010 L2/(mol2‚h). The values of Ea and k0 were then introduced into eq 20. Thus, the only parameters left to be estimated were a, b, c, k2, and k3. By trial and error combined with regression using Microsoft Excel, the values of a, b, and c that resulted in reasonable values for k2 and k3 were 1.08, 2, and 0.3, respectively. These were substituted into eq 22 to obtain eq 23.

-rMEA )

k0 e-Ea/RT[MEA]1[O2]2 k2 + k3[O2]0.3

(24)

The coefficients k2 and k3 were then found using the multiple linear least-squares regression package available in Microsoft Excel 2000 for eq 25, which was obtained by linearizing eq


obtained was 6 576 892. Also, the order of reaction with respect to MEA was 1.0. Hence, the kinetic model can be given as

Table 1. Estimates of the Values of Parameters of Mechanism-Based Rate Models for All Systems Investigated parameter

estimate

parameter

estimate

MEA-H2O-CO2-O2 system k0 8.8 × 1014 K8 0.04 ( 0.0 K9 9.7 × 10-6 ( 5.3 × 10-6 a 1 b -3.5 ( 0.0

MEA-H2O-O2 system Ea 70 642.0 J/(mol‚K) k0 1.2 × 1010 K2 0.03 K3 0.3 ( 0.03 a 1.08 b 2 c 0.3 MEA-NaVO3-H2O-O2 system Ea 66 182.7 J/(mol‚K) 6 576 892 k0 a 1 ( 0.0

MEA-NaVO3-H2O-CO2-O2 system K 0.000 6 a 1

24. Estimates of the values of k2 and k3 obtained are given in Table 1.

k2 k3[O2]0.5b 1 ) + -rMEA k e-Ea/RT[MEA]1[O ]2 k e-Ea/RT[MEA]1[O ]2 0 2 0 2 (25) Also, a parity chart of the predicted rate from the mechanismbased rate model versus the experimental rate was made. The plot (Figure 4a) showed that the rates predicted with the model fitted the experimental data. In addition, the absolute average deviation obtained between the experimental rates and the predicted rates was 18.07%. All these show that the generalized mechanism represents the degradation process. 3.2.2. MEA-NaVO3-H2O-O2. For this system, the concentration of CO2 is also zero, while that of O2 [O2] is constant. Therefore, [CO2] ) 0 is substituted into eq 19 to yield eq 20 (the rate expression for the MEA-H2O-O2 system). Since the concentration of O2 is constant, this equation becomes

k0 e-Ea/RT[MEA]a[O2]b ) k01 e-Ea/RT[MEA]a

(26)

and k2 + k3[O2]b ) k111

(27)

By substituting eqs 26 and 27 into eq 22 and letting k01 e-Ea/RT/k111 ) k011 e-Ea/RT, eq 22 reduces to eq 28.

-rMEA ) k011 e-Ea/RT[MEA]a

(28)

where -rMEA is the rate of oxidative degradation of MEA [mol/(L‚h)], k0 is the preexponential constant (unit depends on the value of x), Ea is the activation energy (J/mol) for the oxidative degradation of MEA, R is the universal gas constant [8.314 J/(mol‚K)], a is the reaction order with respect to MEA, and T is the degradation temperature (K). 3.2.2.1. Estimation of the Values of the Parameters of the Rate Expression. Equation 27 was linearized to eq 28 so as to estimate the parameters Ea, k0, and a of the kinetic model using the multiple linear least-squares regression package available in Microsoft Excel 2000.

ln(-rMEA) ) ln k//0 -

Ea + a ln[MEA] RT

(29)

Estimates of the values of the parameters were obtained at a 95% confidence level. The activation energy for the oxidative degradation of MEA was 66 182.7 J/(mol‚K), while the k0

-rMEA ) k011 e-Ea/RT[MEA]1

(30)

A parity chart of the predicted rate from the mechanism-based rate model versus the experimental rate was made. The plot (Figure 4b) showed that the predicted rates fit the experimental data. In addition, the absolute average deviation between the experimental rates and the predicted rates was approximately 0%. This shows that the generalized mechanism represents the degradation process, just as in the MEA-H2O-O2 system. 3.2.3. MEA-H2O-CO2 and MEA-NaVO3-H2O-CO2 Systems. It is pertinent to note that, after 490 h at 393 K, the overall rate of MEA degradation for the MEA-H2O-CO2 system was 0.000 436 mol/(L‚h), while that for the MEANaVO3-H2O-CO2 system was 0.000 454 mol/(L‚h). This shows that the effect of CO2 on MEA degradation even in the presence of NaVO3 is very minimal. Hence, there was no need to obtain a degradation rate model. 3.2.4. MEA-H2O-CO2-O2 System. The rate expression for this system was obtained by simplifying the generalized rate expression (eq 19). The MEA-H2O-CO2 system shows that the rate of reaction with CO2 is very low; therefore, k5[CO2] , k11[MEA]a[O2]b. This implies that

k11[MEA]a[O2]b + k5[CO2]e ≈ k11[MEA]a[O2]b (31) For constant O2, k2 + k3[O2]c ≈ k2 + k3/. Thus, the eq reduces to

-rMEA )

k7 e-Ea/RT[MEA]a k8 + k9[CO2]e

(32)

3.2.4.1. Estimation of Parameters of Rate Model. The parameters to be estimated are k7, Ea, k8, k9, a, and e. The procedure used for parameter estimation was similar to the one used in the case of the O2 alone system, in which a power law model was used to estimate Ea and k0 in order to simplify the estimation of the other parameters. The model in this case was of the form given in eq 33.

-rMEA ) k0/ e-(Ea/RT)[MEA]m[CO2]o

(33)

The linearized form of the equation is given in eq 34. Ea and k0 of the kinetic model were estimated using the multiple linear least-squares regression package available in Microsoft Excel 2000.

ln(-rMEA) ) ln k0/ -

Ea + m ln[MEA] + o ln[CO2] (34) RT

Estimates of the values of the parameters obtained at a 95% confidence level were an activation energy for the oxidative degradation of MEA of 145 658.7 J/(mol) and k0 of 8.804 × 1014 h-1. In the same way as in the O2 alone system, the values of a and b found to work with the model were 1.0 and -3.5, respectively. These were substituted into eq 32 to obtain eq 35, which was then linearized to eq 36. The coefficients k2 and k3 were then found using the multiple linear least-squares regression package available in Microsoft Excel 2000. The values of k8 and k9 obtained are given in Table 1.

2578


-rMEA )

k7 e-Ea/RT[MEA]1 k8 + k9[CO2]3.5

k8 k9[CO2]3.5 1 ) + -rMEA k e-Ea/RT[MEA]1 k e-Ea/RT[MEA]1[O ]2 7 7 2

(35)

(36)

The negative reaction order for CO2 loading provides a clear indication that, the higher the CO2 loading, the lower is the rate of oxidative degradation of MEA. This implies that oxidative degradation could be controlled by the CO2 loading of the system. Also, a parity chart (Figure 4c) shows that the predicted rates from the mechanism-based rate model fit the experimental data well with an absolute average deviation of 0.07%. This shows that the generalized mechanism represents the degradation process just as in the MEA-H2O-O2 and the MEA-NaVO3H2O-O2 systems. 3.2.5. MEA-NaVO3-H2O-CO2-O2 System. For this system, the concentrations of CO2 and O2 were constant and the experiment was carried out at one temperature. Therefore, the generalized rate expression eq 19 was simplified as follows:

k0 e-Ea/RT[MEA]a[O2]b ) K[MEA]a

(37)

and k2 + k3[O2]b + k5[CO2]e ) k111

(38)

By substituting eqs 35 and 36 into eq 19 and letting K/k111 ) K*, eq 19 reduces to eq 39.

-rMEA ) K*[MEA]a

(39)

where -rMEA is the rate of oxidative degradation of MEA [mol/(L‚h)] and K is the reaction rate constant (unit depends on the value of a). 3.2.5.1. Estimation of the Parameters of the Rate Expression. Equation 40 in a linearized form was regressed using THE multiple linear least-squares regression package available in Microsoft Excel 2000 ,and the value of 0.000 6 was obtained as the reaction rate constant while a was unity (Table 1). The rate expression obtained is given in eq 40.

-rMEA ) K*[MEA]1

(40)

Also, a parity chart (Figure 4d) and an average absolute deviation of 0% shows that the predicted rates from the mechanism-based rate model fit the experimental rates. 3.3. Comparison of Rate Model for MEA-H2O-O2 and MEA-NaVO3-H2O-O2 Systems. The kinetic model for the MEA-H2O-O2 system is given in eq 24, while that for the MEA-NaVO3-H2O-O2 system is given in eq 30. Also, the value of the activation energy for the MEA-H2O-O2 system is 70 642 J/(mol), while that for the MEA-NaVO3-H2O-O2 system is 66 182 J/(mol). Hence, the activation energy in the MEA-H2O-O2 system is higher than that of the MEANaVO3-H2O-O2 system, implying that a larger energy barrier needs to be overcome in order to degrade MEA in the absence of NaVO3 than in its presence. The implication is that NaVO3 acted as a catalyst by lowering the activation energy required for the reactions to take place in the MEA-NaVO3-H2O-O2 system. Also, in a calculation involving the rates of reaction of both systems using the respective rate models obtained, it was

observed that, for the same MEA concentration (11.4 mol %), O2 pressure (250 kPa), and temperature (393 K), the rate of MEA degradation for the MEA-H2O-O2 system was 0.003 mol/(L‚h) while that for the MEA-NaVO3-H2O-O2 system was 0.05 mol/(L‚h). This further confirms that the rate of MEA degradation increases with the addition of NaVO3. 3.3.1. Comparison of the Rate Models for MEA-H2OCO2-O2 and MEA-NaVO3-H2O-CO2-O2 Systems. The kinetic model for the MEA-H2O-CO2-O2 system is given in eq 35, while that for the MEA-NaVO3-H2O-CO2-O2 system is given in eq 40. The activation energy in the two systems could not be used in this case, because the experiments involving the MEA-NaVO3-H2O-CO2-O2 system were conducted for only one constant temperature. However, the rate expression obtained for each system was used to calculate the rates for the same set of conditions. The results showed that, for MEA concentration of 11.4 mol %, O2 pressure of 250 kPa, and temperature of 393 K, the rate of MEA degradation for the MEA-H2O-O2 system was 0.001 mol/(L‚h) while that for the MEA-NaVO3-H2O-O2 system was 0.003 mol/(L‚h). This shows that the addition of NaVO3 is detrimental to the rate of degradation of MEA. 3.4. Interpretation of Results. The general mechanistic rate model obtained to represent all the systems investigated is given in eq 19. This rate model shows that, in a CO2 loaded system, the loaded CO2 acts as a degradation inhibitor. The results also show that an increase in MEA concentration, temperature, or O2 pressure results in an increase in the MEA degradation rate. In contrast, an increase in CO2 loading leads to a decrease in the degradation rate. In addition, an increase in MEA degradation rate is observed in the presence of NaVO3 for all the systems investigated. This is further evidenced by the fact that the activation energy in the MEA-H2O-NaVO3-O2 system is 66 182 J/(mol‚K), while that for the MEA-H2O-O2 system is 70 642 J/(mol‚K). Despite the general low rate of degradation observed in the MEA-NaVO3-H2O-CO2 and MEA-NaVO3H2O-CO2 systems, after 490 h at 393 K, the overall rate of MEA degradation for the MEA-H2O-CO2 system was 0.000 436 mol/(L‚h) while that for the MEA-NaVO3-H2OCO2 system was 0.000 454 mol/(L‚h). Thus, considering the activation energies, the results imply that less energy is required to overcome the energy barrier required for MEA to react in the presence of NaVO3 than in its absence in the O2 system. Hence, it can be deduced that NaVO3 acts as a catalyst in the degradation process. Also, it appears that the limiting reactant in all the cases is the concentration of O2 in the solution. We attribute the large difference between the 350 and 250 kPa O2 pressure for the 11.4 mol % case as compared to the 17.9 mol % case to the relatively very low ratio of O2 to MEA in the latter case. This makes the O2 in this case all the more limiting. This implies that even the increase in O2 pressure does not satisfy the excess MEA present in the 17.9 mol % case as compared to the 11.4 mol % case. 3.5. Recommendation for Future Work. An expression for the concentration of O2 in solution derived by Rooney and Daniels17 was used for calculating the concentration of O2 in solution, even though it does not involve the concentration of MEA. There is bound to be a different solubility in an amine solution than in water. It is recommended that work be done which establishes a more accurate concentration of O2 as a function of temperature and pressure as well as the concentration of MEA.


4. Conclusions For all the systems investigated, an increase in O2 and initial MEA concentrations, and an increase in temperature as well as the presence of NaVO3, led to an increase in the MEA degradation rate. 1. For the MEA-H2O-CO2-O2 system, the rate of MEA degradation decreases with an increase in CO2 loading. 2. A simplified generalized mechanism has been used to develop a rate model for the oxidative degradation of MEA with and without CO2 and with and without NaVO3. 3. The generalized rate model and the experimental results showed that CO2 in a CO2 loaded MEA system (MEA-H2OCO2-O2) acted as a degradation inhibitor. Acknowledgment The financial support provided by the Natural Sciences and Engineering Research Council Of Canada (NSERC) is greatly appreciated. Literature Cited (1) Kohl, A. L.; Nielson, R. B. Gas Purification; Gulf Publishing: Houston, TX, 1997. (2) Shrikar, C.; Guptha, A.; Humek, B. Advanced technology for the capture of CO2 from flue gases. In Proceedings of First National Conference on Carbon Sequestration, Washington, DC, May 15-17, 2001; National Energy Technology Laboratory, USDGE: Pittsburgh, PA, 2001; pp 1-11. (3) Rochelle, G. T.; Chi, S. Oxidative degradation of monoethanolamine. Ind. Eng. Chem. Res. 2002, 41, 4178-4186. (4) Maddox, R. N. Gas Conditioning and Liquid Sweetening; Campbell Petroleum Series: Norman, OK, 1974. (5) Straizisar, B. R.; Anderson, R. R.; White, C. M. Degradation of MEA used in CO2 capture from flue gas of a coal-fired electric power generating station; Energy Fuel 2003, 17, 1034-1039.

(6) Supap, T.; Idem, R.; Veawab, A.; Aroonwilas, A.; Tontiwachwuthikul, P.; Chakma, A.; Kybett, B. D. Kinetics of the oxidative degradation of aqueous MEA in a flue gas treating unit. Ind. Eng. Chem. Res. 2001, 40, 3445-3450. (7) Howard, H. An Introduction into CO2 Separation and Capture Technologies; MIT Energy Laboratory: Cambridge, MA, 1999. (8) Pearce, R. L. Removal of carbon dioxide from industrial gases. U.S. Patent 4440731, 1984. (9) Pearce, R. L.; Pauley, C. R.; Wolcott, R. A. Recovery of carbon dioxide from gases. U.S. Patent 2135900, 1984. (10) Bello, A.; Idem, R. O. Pathways for the formation of the oxidative degradation products of CO2-loaded concentrated aqueous monoethanolamine solutions during CO2 capture from flue gases. Ind. Eng. Chem. Res. 2005, 44, 945-969. (11) Ranney, M. W. Corrosion inhibitorssManufacture and technology; Noyes Data Corporations: Park Ridge, NJ, 1976. (12) Rooney, P. C.; Dupart, M. S.; Bacon, T. R. Oxygen’s role in alkanolamine degradation. Hydrocarbon Process., Int. Ed. 1998, 109-113. (13) Kim, C. J. Degradation of alkanolamines in gas-treating solutions: Kinetics of di-2-propanolamine degradation in aqueous solutions containing carbon dioxide. Ind. Eng. Chem. Res. 1988, 27 (1), 1-3. (14) Lawal, O.; Bello, A.; Idem, R. O. The role of methyl diethanolamine (MDEA) in preventing the oxidative degradation of CO2 loaded and concentrated aqueous monoethanolamine (MEA)-MDEA blends during CO2 absorption from flue gases. Ind. Eng. Chem. Res. 2005, 44, 18741896. (15) Dawodu, O. F.; Meisen A. Degradation of alkanolamine blends by CO2. Can. J. Chem. Eng. 1996, 74, 960. (16) Wilson, M.; Tontiwachuthikul, P.; Chakma, A.; Idem, R.; Veawab, A.; Aroonwilas, A.; Gelowitz, D. Test results from a CO2 extraction plant at Boundary Dam coal-fired power station. Energy 2004, 298 (9-10), 1259-1267. (17) Rooney, P. C.; Daniels, D. D. Oxygen’s solubility in various alkanolamine/water systems. J. Pet. Technol. 1998, 3 (1), 97.

ReceiVed for reView May 13, 2005 ReVised manuscript receiVed October 3, 2005 Accepted November 22, 2005 IE050562X

Comprehensive Study of the Kinetics of the Oxidative Degradation of

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