Kinetics of the Oxidative Degradation of Aqueous Monoethanolamine

Degradation Kinetics. Results from test runs are reported in terms of MEA concentration as a function of reaction time for various temperatures, initi...
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Ind. Eng. Chem. Res. 2001, 40, 3445-3450

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Kinetics of the Oxidative Degradation of Aqueous Monoethanolamine in a Flue Gas Treating Unit Teeradet Supap,† Raphael Idem,† Amornvadee Veawab,† Adisorn Aroonwilas,† Paitoon Tontiwachwuthikul,*,† Amit Chakma,† and Brian D. Kybett‡ Process Systems Laboratory, Faculty of Engineering, and Energy Research Unit, University of Regina, Regina, Saskatchewan, Canada S4S 0A2

An investigation of the kinetics of the oxidative degradation of aqueous monoethanolamine (MEA) was conducted in a 300-mL autoclave in the temperature range 393-443 K and MEA concentration range 2-11 kmol/m3 at 241-345 kPa O2 pressure (i.e., O2 concentration range). The results show that MEA oxidative degradation depends on both the O2 and the initial MEA concentrations, as well as the temperature, according to the intrinsic kinetic model -rMEA ) 2.5 × 105e-(66 288.9/RT)[MEA][O2]1.5. This work represents the first attempt at both obtaining intrinsic kinetic data for the oxidative degradation of MEA and formulating a kinetic model that fits the data based on the initial rate. The values obtained for the reaction orders for MEA and O2 imply that oxidative degradation of MEA is more sensitive to increases in the O2 concentration than in the MEA concentration. It also shows that, even though our experimental conditions enabled us to model the reaction as a homogeneous liquid-phase reaction, MEA oxidative degradation itself is not an elementary reaction. Introduction The removal of carbon dioxide (CO2) from gas streams (such as natural gas and flue gas) is an essential step in their purification process. The CO2 must be removed to meet the product specifications for these gas streams for industrial applications. Gas-treating solvents [usually alkanolamines, such as monoethanaolamine (MEA) and diethanolamine (DEA)] have been used extensively for the removal of CO2 from natural gas and flue gas via absorption with chemical reaction. These solvents break down as a result of long exposure or repeated use because of side reactions with CO2, oxygen (O2), and other contaminants.1 Although it is wellknown that amines undergo severe degradation in the presence of O2, this gas is usually not present in a typical natural gas stream. As such, there has been no practical incentive to study amine degradation caused by O2. Therefore, although a number of studies have been carried out on the degradation of different solvents due to CO2, very limited information is available on solvent degradation with O2.1 On the other hand, it is well-known that flue gases contain O2. Also, O2 can enter the gas stream through vapor-recovery units and alkanolamines stored in tanks without a gas blanket.2 Because the desire to separate CO2 from flue gas streams is gaining momentum as a result of environmental concerns, there is an urgent need to develop a better understanding of degradation of gas-treating solvents by O2. The main objective of conducting an alkanolamine degradation study is to develop an understanding of the mechanism of degradation and to use this knowledge to formulate a degradation prevention strategy.1 Degradation prevention becomes important because it implies that any suitable * Corresponding author. Tel.: (306) 585-4726. Fax (306) 5854855. E-mail: [email protected]. † Process Systems Laboratory. ‡ Energy Research Unit.

solvent can be used repeatedly without the economic and operational burdens imposed by the loss of CO2 removal capacity that occurs as a result of severe degradation. It is well-known3 that mechanism studies for any reaction require a knowledge of both the stoichiometry and the kinetics of the reaction. In the case of oxidative alkanolamine degradation, limited studies have been reported on the stoichiometry. Early studies on the oxidative degradation of monoethanolamine (MEA), triethanolamine (TEA), and diisoaminopropanol were conducted in the absence of CO24 and were limited to showing the existence of degradation. The results indicated that the O2 degradation resistance of the solvents was in the sequence MEA > TEA > diisoaminopropanol. In a recent study, results have shown that, in the absence of CO2, oxidative degradation resistance increases in the order diethanolimine (DEA) > 50% methyl diethanolimine (MDEA) > 30% MDEA > diglycolamine (DGA) > MEA. However, when CO2-loaded solutions were tested, the O2 degradation resistance order changed to DEA > DGA > MEA > 50% MDEA > 30% MDEA.5 This finding is somewhat similar to that of plant experience where, as a general rule, MDEA seemed to be more sensitive than DEA to heat-stable salts and oxygen contamination.6 Specifically related to stoichiometry, Lloyd and Taylor7 reported that MEA alone or in glycol solution undergoes oxidative deamination to produce a steamdistillable acid mixture. The mixture was later found to contain carboxylic acids, ammonia, water, and amides.8 Also, some workers9 used an ion chromatographic exclusion technique to identify the oxidative degradation products of DEA, MDEA, and MEA as formic acid (major component) and oxalic acid and acetic acid (minor components). Glycolate was also reported to be a product of oxidative degradation, forming in both CO2-loaded and unloaded MEA, DGA, DEA, and MDEA solutions.

10.1021/ie000957a CCC: $20.00 © 2001 American Chemical Society Published on Web 07/11/2001

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Only carbamate was detected as one of the products in CO2-loaded DGA and MEA solutions.4 The consequence of the formation of these carboxylic acids is that they further react with the alkanolamines, resulting in the formation of heat-stable salts.10 These salts are difficult to regenerate, especially under the regeneration conditions typically encountered in gastreating units. The formation of degradation products will eventually result in a loss of capacity and give rise to a number of operating problems including corrosion11-13 and foaming. In the case of severe degradation, the solvent will need to be replaced, and the degraded solvent will need to be disposed of in an environmentally acceptable manner. Because our goal is degradation prevention, we need an understanding of the degradation mechanism, which, in turn, requires knowledge of both the stoichiometry and kinetics. At the present time, most studies focus only on the degradation stoichiometry (identification of the degradation products), whereas others deal with attempting to formulate reaction pathways based only on the reaction stoichiometry. No studies have been reported on the degradation kinetics, to our knowledge. This study was therefore conducted to determine the kinetics of the oxidative degradation of alkanolamines. MEA was selected as a representative alkanolamine solely on the basis of both its popularity and its minimal resistance to degradation. The study was conducted in the absence of CO2 to eliminate its influence and to thus ensure that the kinetics totally represented the reaction of MEA with O2. The effects of operating parameters such as temperature, initial MEA concentration, and O2 pressure were also evaluated. The results are presented and discussed in this paper. Experiment Equipment and Chemicals. The oxidative degradation of MEA was conducted using MEA concentrations ranging from 2 to 11 kmol/m3 at temperatures and O2 pressures ranging from 393 to 443 K and from 241 to 345 kPa, respectively, in a 300-mL vessel. The high O2 pressures used in the present work, as compared to conditions used for typical flue gas-treating applications, enabled us perform accelerated oxidative MEA degradation studies. The vessel used was a stainless steel rotary type autoclave (model BC0030SS05AH obtained from Autoclave Engineers Inc., Erie, PA). It consisted of a liquid sampling tube, a gas feed port, an extra feed port, a gas product port, a thermowell, a baffle bar, an impeller, and cooling coils, as shown in Figure 1. A pressure gauge fitted to the extra feed port was used to measure the total pressure in the reactor (i.e., pressure due to water vapor and O2). The water vapor pressures under various reaction conditions were determined earlier in the absence of O2. O2 pressures were calculated as differences. A K-type thermocouple placed in the thermowell was used to measure the temperature of the reaction mixture, whereas the heat supplied to the reactor was obtained with an electric furnace controlled using a K-type thermocouple. The PID temperature controller (model MTCPKCS00) was also obtained from Autoclave Engineers Inc. The temperature accuracy of the controller was within (0.2%. Analyticalgrade O2 was supplied by Praxair (Regina, SK). Concentrated MEA (reagent grade, >99% purity) was purchased from Fisher Scientific, Nepean, Ontario, Canada. MEA was diluted with deionized water to the

desired concentration. Hydrochloric acid (HCl) used to establish the exact MEA concentration was also obtained from Fischer Scientific. Typical Experimental Run. For a typical test run, 230 mL of aqueous MEA solution of the desired concentration was loaded into the autoclave. The autoclave was flushed with O2, and then the aqueous MEA solution was simultaneously stirred at the speed of 300 rpm and heated to the desired temperature. O2 was then fed into the vessel up to the desired O2 pressure (total pressure - water vapor pressure) by opening the O2 gas inlet valve of the oxygen tank set at a predetermined value. O2 is soluble in aqueous MEA solution. This resulted in an initial decline in the O2 pressure in the reaction chamber. However, to maintain a constant O2 pressure, the reaction chamber was boosted with an extra O2 supply as needed. In all of our test runs, we ensured that the desired O2 pressure was maintained throughout the duration of the reaction. About 2.5-mL samples of the reaction mixture were removed from the vessel through the liquid sampling valve at regular hourly intervals during the run. Extra O2 was also quickly added after each sampling to compensate for the loss of pressure during the sampling process and to maintain the constant pressure of the system. The reaction in each sample was quenched by quickly cooling the sample vial in an ice bath. Also, the reaction between MEA and O2 is exothermic. Thus, cooling water (flowing through the cooling coil) was employed to ensure reactor isothermicity. Analysis of Product. Analysis of the samples was performed using a gas chromatograph-mass spectrometer (GC/MS model HP 6890/5073 supplied by HewlettPackard Canada Ltd., Montreal, Quebec, Canada). An HP-Innowax column packed with cross-linked-poly(ethylene glycol) was used in the GC for the separation of components. These components were identified by their mass spectra. Prior to GC/MS analysis, each sample was diluted with deionized water to five times its original volume to avoid column overload and to improve separation of the components. Sample injection into the GC column was done using an auto-injector (model 7683) supplied by Hewlett-Packard Ltd. The GC/ MS operating conditions are given in Table 1. The exact concentration of fresh aqueous MEA and of each sample was determined by interpolation from a standard MEA calibration curve obtained earlier by titrating known concentrations of MEA (over a wide MEA concentration range) against 1 kmol/m3 of standard HCl using 0.5 wt % methyl orange solution as the indicator. Results and Discussion Degradation Kinetics. Results from test runs are reported in terms of MEA concentration as a function of reaction time for various temperatures, initial MEA concentrations, and O2 pressures. These are given in Figures 2-4, respectively. Degradation rates were evaluated in terms of (i) overall rate measured as ∆[MEA]/∆t, where ∆[MEA] represents the overall change in MEA concentration and ∆t represents the minimum time taken to obtain this change, and (ii) instantaneous rate, which was measured as the slope at any reaction time of the MEA concentration-reaction time curves. A typical degradation rate-reaction time plot is given in Figure 5. Only the initial instantaneous rates were used for further kinetic analysis because these values represent the conditions where the rates are unaffected

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Figure 1. Schematic of the autoclave reactor.

by competition of the reaction products with MEA for the O2 available in the reaction mixture. The initial rates obtained experimentally for various operating conditions are given in Table 2. In this table, the O2 pressures have been converted to actual concentrations of O2 dissolved in the reaction mixture using the equation formulated by Rooney and Daniels.14 This equation is

[O2] (mol/m3) ) -2.545 + 0.807 × 10-2T - 84.14p + 2.096 × 10-4pT 2 +

2.322 × 104p + 1.027p2 T 3.911 × 102p2 (1) T

where p is the O2 pressure (MPa) and T is the reaction temperature (K). Effects of Temperature. Figure 2 shows the variation of the MEA concentration with time for temperatures of 393, 413, and 433 K (initial MEA concentration of 4 kmol/m3 and O2 pressure of 345 kPa). It is seen in the figure that the decrease in MEA concentration with reaction time was more rapid at the higher temperature (433 K) than at the lower temperature (393 K). Also,

the overall degradation rates at 393, 413, and 433 K were calculated as 0.0056, 0.020, and 0.039 kmol/hm3, respectively. Thus, as expected, both the initial rates (given in Table 2) and the overall rates of the oxidative degradation of MEA increased with temperature. Effects of Initial MEA Concentration. Test runs were performed to evaluate the initial MEA concentration dependence of the oxidative degradation of MEA. The initial MEA concentrations ranged from 2 to 11 kmol/m3. Typical results at 433 K are given in Figure 3, which shows that the decrease in MEA concentration with reaction time was more rapid as the initial MEA concentration was increased from 2 to 11 kmol/m3. The overall rates for these initial MEA concentrations were calculated as 0.017, 0.02, 0.039, and 0.072 kmol/hm3 for MEA concentrations of 2, 3, 4, and 11 kmol/m3, respectively. These results show an increase in MEA degradation rate with initial MEA concentration. A similar trend was also observed for tests conducted at 443 K and 345 kPa O2 pressure. The overall rates were 0.030, 0.043, 0.069, and 0.13 kmol/hm3 for MEA concentrations of 2, 4, 6, and 10 kmol/m3, respectively. As in the case of temperature, the instantaneous rates started to decline soon after the products were formed, as con-

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Table 1. GC Program and Operating Conditions for Analysis of Products type temperature pressure split ratio split flow total flow

1. Inlet split/splitless, in split mode 250 °C 71.91 kPa 70:1 69.50 mL/min 73.00 mL/min

2. GC Oven initial temperature 100 °C initial temperature hold time 1 min ramp rate 10 °C/min final temperature 240 °C final temperature hold time 5 min total run time 20 min type

3. GC Column HP-Innowax

type flow rate

4. Carrier Gas UHP-grade helium 1 mL/min

5. MS Parameters interface temperature 250 °C EM voltage 1200 V quad temperature 150 °C source temperature 230 °C sample washes sample pumps injection volume syringe size

Figure 3. MEA concentration-time curves for the oxidative degradation of MEA for various initial MEA concentration (O2 pressure ) 345 kPa, degradation temperature ) 433 K).

6. Autoinjector 4 times 4 times 1 µL 10 µL

Figure 4. MEA concentration-time curves for the oxidative degradation of MEA for various O2 concentrations (initial concentration of MEA ) 3 kmol/m3, degradation temperature ) 443 K).

Figure 2. MEA concentration-time curves for the oxidative degradation of MEA for various temperatures (initial concentration of MEA ) 4 kmol/m3, O2 pressure ) 345 kPa).

firmed by the GC-MS results. The products identified included 3-methyl pyridine, 2-methylamino ethanol, dihomoseriene lactone, and ethylamine. The decline was drastic initially. However, the rates approached steady state after 20 h. This appears to be an indication of competition of some of these products with the resulting much lower amount of MEA for the available O2. Effects of O2 Concentration. In the case of O2, concentration was measured by converting O2 pressures to actual concentrations using eq 1. The O2 pressures ranged from 241 to 345 kPa. This pressure range is much higher than those encountered in typical flue gas applications. As mentioned previously, it was used to enable us to perform accelerated oxidative MEA degradation studies. The MEA concentration-time curves

Figure 5. Instantaneous rate of MEA degradation as a function of degradation time and temperature (initial concentration of MEA ) 4 kmol/m3, O2 pressure ) 345 kPa).

for an initial MEA concentration of 3 kmol/m3 at 443 K are given in Figure 4 for the range of O2 pressures considered. The figure shows that the decline in MEA concentration with time was more rapid for the run with the higher O2 pressure. For example, it was observed

Ind. Eng. Chem. Res., Vol. 40, No. 16, 2001 3449 Table 2. Initial Rates for the Oxidative Degradation of MEA for Various Operating Conditions initial rate [kmol/(h m3)]

temperature (K)

initial MEA concentration (kmol/m3)

O2 concentration (mol/m3)a

0.044 0.065 0.082 0.208 0.104 0.117 0.380 0.431 0.007 0.056 0.070 0.082

433 433 433 433 443 443 443 443 393 413 443 433

2 3 4 11 3 4 8 10 4 4 3 4

3.994 3.994 3.994 3.994 4.293 4.293 4.293 4.293 3.154 3.500 3.305 3.994

a

Calculated using eq 1.

that 16% of the initial MEA content of the reaction mixture in contact with 345 kPa O2 was consumed after 5 h, as compared to only 11% of the mixture in contact with 241 kPa O2 over the same time period. Also, the overall degradation rates were calculated as 0.019 and 0.032 kmol/hm3 for 241 and 345 kPa O2 pressure (i.e., O2 concentration of 3.3 and 4.3 mol/m3), respectively, showing that the overall rates also increased with O2 concentration in the reaction mixture. Again, as in the case of temperature, the rates started to decline soon after products were formed. The decline was drastic initially. However, the rates approached steady state after 20 h. Also, this appears to be an indication of competition of some of these products with the resulting much lower amount of MEA for the available O2. Formulation of Rate Equation. The oxidative degradation of MEA under typical flue-gas-treating conditions is an example of a gas-liquid reaction system in which the O2 in the gas phase must contact MEA in the liquid phase before reaction can occur. In such a situation, both the mass transfer rates and the chemical reaction rate will enter the overall rate expression. However, the experimental apparatus used for the present study, coupled with the negligible MEA vapor pressures, allowed us to assume that the reaction occurred only in the liquid phase between MEA and the O2 dissolved in the reaction mixture. In earlier test runs, we determined whether stirring had an effect on the mass transfer of O2 in the liquid phase and, as such, on degradation rate. Two tests were performed using 4 kmol/m3 initial MEA concentration at 433 K and 345 kPa O2 pressure. In one of the tests, the reaction mixture was stirred at a speed of 300 rpm, whereas in the other test, the reaction was carried out without any stirring. The results are presented in Figure 6, and they show that the initial rates are independent of the stirring speed, whereas the overall rates are indeed affected by stirring. It is seen in Figure 6 that the overall degradation rate obtained after 48 h is greater in the test run conducted with stirring [0.039 kmol/(h m3)] than the one without [0.030 kmol/(h m3)]. This implies that, initially, the dissolved O2 is in intimate contact with MEA so that the reaction can proceed without the need for stirring. Hence, mass transfer limitations can be neglected. After about 20 h, when large amounts of degradation products are formed as a result of significant MEA conversion, there is competition for the available O2 between these products and the much lower amount of MEA. The results show that mass transfer limitations are present under this

Figure 6. Effect of stirring on the rate of the oxidative degradation of MEA (O2 pressure ) 345 kPa, initial concentration of MEA ) 4 kmol/m3, degradation temperature ) 433 K).

scenario beyond 20 h of degradation time. It is in this time region that stirring considerably reduces any mass transfer limitations. The results obtained on the effect of stirring have enabled us to assume that there are no mass transfer limitations as long as the initial rates are used for the kinetic analysis. In this study, therefore, we performed kinetic evaluations of the oxidative degradation of MEA as a homogeneous liquid-phase reaction on the basis of the initial rates for various O2 pressures, MEA concentrations, and temperatures. Thus, the parameters determined represent intrinsic kinetic parameters for the oxidative degradation of MEA. The results from the concentration dependence studies discussed earlier show that the degradation rate is clearly a function of both the MEA and O2 concentrations. Thus, a power law model that accounts for both the MEA and O2 concentration dependences can be formulated to describe the kinetics. For the oxidative degradation of MEA, this model is given by

-rMEA ) k0e-(Ea/RT)[MEA]m[O2]n

(2)

where -rMEA is the rate of oxidative degradation of MEA [mol/(m3 h)], k0 is the preexponential constant (units depend on the values of m and n), Ea is the activation energy (J/mol), R is the gas constant [8.314 J/(mol K)], [MEA] is the initial MEA concentration (kmol/m3), [O2] is the O2 concentration (mol/m3), m is the reaction order with respect to MEA, n is the reaction order with respect to O2, and T is the degradation temperature (K) Estimation of the Parameters of the Model. The kinetic data used for parameter estimation are given in Table 2. Equation 2 was linearized to eq 3 to enable the estimation of the parameters Ea, k0, m, and n of the kinetic model using a multiple linear least-squares regression package available in Microsoft Excel 97.

ln(-rMEA) ) ln(k0) -

Ea + m ln[MEA] + n ln[O2] RT (3)

Table 3 shows estimates of the values of the parameters obtained at a 95% confidence level. The table shows good regression statistics, with a coefficient of correlation (R) of 0.97. Table 4 shows the ln(-rMEA) values obtained experimentally and those predicted by

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Table 3. Estimates of the Values of Parameters of Kinetic Model parameter

estimate

ln(k0) Ea/R (K-1) m n

12.428 ( 4.899 7972 ( 1521 1.05 ( 0.18 1.51 ( 1.27

Table 4. Comparison of Experimental Rates with Those Predicted from the Kinetic Model ln(-rMEA) predicted initial MEA O2 from temperature concentration concentration 3 experimental model (K) (kmol/m ) (mol/m3)a -3.12 -2.73 -2.50 -1.57 -2.26 -2.15 -0.97 -0.84 -4.98 -2.88 -2.65 -2.50 a

-3.19 -2.74 -2.43 -1.37 -2.21 -1.91 -1.18 -0.092 -4.66 -3.53 -2.61 -2.43

433 433 433 433 443 443 443 443 393 413 443 433

2 3 4 11 3 4 8 10 4 4 3 4

3.994 3.994 3.994 3.994 4.293 4.293 4.293 4.293 3.154 3.500 3.305 3.994

Calculated using eq 1.

the model for various operating conditions. Also, a comparison of the experimental results with the model predictions in terms of the actual degradation rate (-rMEA) gave a coefficient of correlation of 0.977. Thus, this coefficient, as well as Tables 3 and 4, shows that the kinetic model adequately fit the kinetic data for the entire initial MEA concentration, O2 concentration, and temperature ranges used for this study. These were a temperature range of 393-443 K and a MEA concentration range of 2-11 kmol/m3 for O2 pressures of 241345 kPa. Within these ranges, the reaction rate constant (k ) k0e-(Ea/RT)) is independent of the initial MEA concentration. Apart from the high O2 pressure, the operating range covered in the present work represents a useful range of conditions usually encountered in CO2 capture from flue gases using chemical solvents. The orders of reaction of unity for MEA and 1.5 for O2 provide a clear indication that oxidative MEA degradation is not an elementary reaction. They also show that the oxidative degradation of MEA is more sensitive to an increase in the O2 concentration than in the MEA concentration. The implication here is that oxidative MEA degradation can be slowed or even eliminated by using chemical inhibitors that can beat MEA in the competition for the available O2 in the system. A further mechanistic study of oxidative degradation should be performed to identify the most appropriate inhibitor(s). Conclusions 1. Intrinsic kinetic data have been obtained for the oxidative degradation of MEA under conditions useful for CO2 extraction from flue gases by chemical absorption. 2. A power-law rate model based on initial rate data has been formulated that adequately describes the oxidative degradation of MEA. This model covers a very

useful range of operating conditions (temperature range of 393-443 K and MEA concentration range of 2-11 kmol/m3) usually encountered in regeneration/thermal reclaimer units during CO2 extraction from flue gases using chemical solvents. 3. The values obtained for the orders of reaction for MEA and O2 (1 and 1.5, respectively) imply that the oxidative degradation of MEA is more sensitive to increases in the O2 concentration than in the MEA concentration. 4. Even though our experimental conditions enabled us to model the reaction as a homogeneous liquid-phase reaction, MEA oxidative degradation itself is not an elementary reaction. Acknowledgment The Natural Sciences and Engineering Research Council of Canada (NSERC) and Saskferco Products Inc. are gratefully acknowledged for their financial support and analytical equipment support, respectively. Literature Cited (1) Rochelle, G. T.; Bishnoi, S.; Chi, S.; Dang, H.; Santos, J. Research Needs for CO2 Capture from Flue Gas by Aqueous Absorption/Stripping; Final Report for U.S. Department of Energy Project DE-AF26-99FT01029; U.S. Department of Energy, U.S. Government Printing Office: Washington, D.C., Sept 2000. (2) Stewart, E. J.; Lanning, R. A. Reduce Amine Plant Losses. Part 1. Hydrocarbon Process. 1994, May, 67. (3) Levenspiel, O. Chemical Reaction Engineering; Wiley: Toronto, Canada, 1999. (4) Gregory, L. B.; Scharmann, W. G. Carbon Dioxide Scrubbing by Amine Solutions. Ind. Eng. Chem. 1937, 29, 514-519. (5) Rooney, P. C.; Dupart, M. S.; Bacon, T. R. Oxygen’s Role in Alkanolamine Degradation. Hydrocarbon Process. 1998, July, 109-113. (6) Asperger, R. G. New Corrosion Issues in Gas Sweetening Plants. In Proceedings of the 73rd Annual Gas Processors Association Convention; New Orleans, LA, Mar 7-9, 1994; Gas Processor Association: Tulsa, OK, 1994; pp 189-192. (7) Lloyd, W. G.; Taylor, F. C. Corrosion by Deterioration of Glycol and Glycol-Amine. Ind. Eng. Chem. 1954, 46, 2407-2416. (8) Hofmeyer, B. G.; Scholten, H. G.; Lloyd, W. G. Contamination and Corrosion in Monoethanolamine Gas Treating Solutions. Internal Report No. 722; The Dow Chemical Company: Midland, MI, 1965. (9) Blanc, C.; Grall, M.; Demarais, G. The Part Played by Degradation Products in the Corrosion of Gas Sweetening Plants Using DEA and MDEA. In Proceedings of the 32nd Annual Laurance Reid Gas Conditioning Conference; Oklahoma University: Norman, OK, 1982. (10) McCullough, J. G.; Nielsen, R. B. Contamination and Purification of Alkaline Gas Treating Solutions. In Corrosion/96; NACE: Houston, TX, 1996; Paper 396. (11) Blake, R. J. How Acid-Gas Treating Process Compare. Oil Gas 1967, Jan, 105-108. (12) Rooney, P. C.; Bacon, T. R.; Dupart, M. S. Effect of Heat Stable Salts on MDEA Solution Corrosivity. Hydrocarbon Process. 1996, March, 95. (13) Rooney, P. C.; Bacon, T. R.; Dupart, M. S. Part 2-Effect of Heat Stable Salts on MDEA Solution Corrosivity. Hydrocarbon Process. 1997, April, 65. (14) Rooney, P. C., Daniels, D. D. Oxygen solubility in various alkanolamine/water mixtures. Pet. Technol. Q. 1998, 3 (1), 97.

Received for review November 8, 2000 Revised manuscript received May 9, 2001 Accepted May 15, 2001 IE000957A