Kinetics of sulfur dioxide- and oxygen-induced degradation of aqueous monoethanolamine solution...

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 3 ( 2 0 0 9 ) 1 3 3 – 1 4 2

Kinetics of sulfur dioxide- and oxygen-induced degradationof aqueous monoethanolamine solution during CO2

absorption from power plant flue gas streams

Teeradet Supap a, Raphael Idem b,*, Paitoon Tontiwachwuthikul b, Chintana Saiwan a

a Petroleum and Petrochemical College, Chulalongkorn University, Pathumwan, Bangkok 10330, Thailandb Process Systems Engineering, Faculty of Engineering, University of Regina, 3737 Wascana Parkway, Regina,

Saskatchewan S4S 0A2, Canada

a r t i c l e i n f o

Article history:

Received 22 February 2008

Received in revised form

26 May 2008

Accepted 12 June 2008

Published on line 21 July 2008

Keywords:

Degradation kinetics

Monoethanolamine

Sulfur dioxide

Oxygen

Carbon dioxide

a b s t r a c t

Studiesof the kineticsof sulfur dioxide (SO2)-andoxygen (O2)-induced degradation of aqueous

monoethanolamine (MEA) during the absorption of carbon dioxide (CO2) from flue gases

derived from coal- or natural gas-fired power plants were conducted as a function of

temperature and the liquid phase concentrations of MEA, O2, SO2 and CO2. The kinetic data

were based on the initial rate which shows the propensity for amine degradation and obtained

under a range of conditions typical of the CO2 absorption process (3–7 kmol/m3 MEA, 6% O2,

0–196 ppm SO2, 0–0.55 CO2 loading, and 328–393 K temperature). The results showed that an

increase in temperature and the concentrations of MEA, O2 and SO2 resulted in a higher MEA

degradation rate. An increase in CO2 concentration gave the opposite effect. A semi-empirical

model based on the initial rate, �rMEA = {6.74 � 109 e�(29,403/RT)[MEA]0.02([O]2.91 + [SO2]3.52)}/

{1 + 1.18[CO2]0.18} was developed to fit the experimental data. With the higher order of reaction,

SO2 has a higher propensity to cause MEA to degrade than O2. Unlike previous models, this

model shows an improvement in that any of the parameters (i.e. O2, SO2, and CO2) can be

removed without affecting the usability of the model.

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1. Introduction

Approximately 85% of the world’s energy supply is provided by

the combustion of fossil fuel which is also the most significant

source of CO2 emissions (Strazisar et al., 2003). CO2 itself

contributes the largest fraction of all the greenhouse gases

emitted into the atmosphere. CO2 capture becomes a very

important tool that can be used to address this environmental

concern. One of the most effective and widely used techniques

to capture CO2 from low-pressure flue gas streams from large

point sources such as power plants is chemical absorption

using alkanolamines solutions (Wilson et al., 2004). The most

widely used alkanolamines for this process include mono-

* Corresponding author. Tel.: +1 306 585 4470; fax: +1 306 585 4855.E-mail address: [email protected] (R. Idem).

1750-5836/$ – see front matter # 2008 Elsevier Ltd. All rights reserveddoi:10.1016/j.ijggc.2008.06.009

ethanolamine (MEA), diethanolamine (DEA) and methyl-

diethanolamine (MDEA) with MEA being the most widely

studied because it is very reactive and thus is able to effect a

high volume of acid gas removal at a fast rate.

One of the major disadvantages of using MEA is its high

energy requirement for CO2 regeneration relative to DEA and

MDEA (Veawab et al., 2003). The other major setback is that

MEA has a limitation that its maximum CO2 loading capacity

based on stoichiometry is about 0.5 mol CO2/mol amine unlike

tertiary amines such as MDEA, which has an equilibrium CO2

loading capacity that approaches 1 mol CO2/mol amine.

Furthermore, MEA undergoes degradation when exposed to

coal-fired power plant flue gas composed of CO2, fly ash, O2, N2,

.

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http://dx.doi.org/10.1016/j.ijggc.2008.06.009


SO2 and NO2 (Bello and Idem, 2005, 2006; Idem et al., 2006). Fly

ash is the fine particulates in flue gas consisting of inorganic

oxides such as SiO2, Al2O3, Fe2O3, CaO, MgO, Na2O, K2O and

P2O5. This amine breakdown deteriorates the performance of

the amine in the absorption process. Not only does it reduce

the CO2 removal capacity, but also, corrosion and foaming are

induced due to the presence of degradation products

(Jouravleva and Davy, 2000; Rooney et al., 1996, 1997). The

prediction of the extent and rate of amine degradation is vital

in the estimation of the exact amine make-up rate needed to

maintain the CO2 absorption capacity of the capture process. It

is also essential to evaluate the kinetics of the degradation

process since this provides the elements for a better under-

standing of the degradation mechanism during the CO2

absorption operation. A kinetic evaluation also helps in the

formulation of a degradation prevention strategy which is

considered to be the overall goal of degradation studies

(Rochelle et al., 2002).

Since O2 is known to be deleterious to most amines (Kohl

and Reisenfeld, 1985), considerable efforts have been focused

on the O2-induced degradation of MEA. Early kinetic studies

were reported in terms of oxidative degradation resistance of

amines either by estimating the percentage of amine

concentration decline (Gregory and Scharmann, 1937) or

concentration build-up of some degradation products such

as formic, acetic, and oxalic acids (Blanc et al., 1982). A more

thorough study done by investigating oxygen’s role in various

degradation systems of amine and air (21% O2) has been

reported in terms of amine concentration loss and concentra-

tion increase of acidic degradation products (e.g. formate,

acetate, glycolate, and oxalate) (Rooney et al., 1998). In the

absence of CO2, oxidation resistance increased in the order of

30% diethanolamine > 50% methyldiethanolamine > 30%

MDEA > 50% diglycolamine (DGA) > 20% monoethanolamine

whereas the resistance order changed to 30% DEA > 50%

DGA > 20% MEA > 50% MDEA > 30% MDEA when CO2 was

present. A mechanism accounting for formation of these

acidic products was also proposed in this study. Our previous

work (Supap et al., 2001) developed a power law based kinetic

model for the oxidative degradation of amine in CO2 removal

unit using aqueous MEA solution. Also, another study of

degradation kinetics using the rate of evolution of ammonia

gaseous (NH3) product to represent MEA degradation has been

reported (Chi and Rochelle, 2002). Both the MEA and O2

concentration terms were absent in this model. Degradation

products and their detection techniques have also been

analyzed and reported for aqueous liquid phase amine

systems (Blanc et al., 1982; Jouravleva and Davy, 2000; Kadnar

and Rieder, 1995; Kaminski et al., 2002; Rooney et al., 1998;

Strazisar et al., 2003). In addition, the kinetics has also been

formulated based on experiments carried out using air even

though this contains 21% O2 considered to be high for a typical

flue gas system. A more recent mechanism-based kinetic

model for the degradation of MEA induced by O2 has also been

reported (Bello and Idem, 2006). The kinetic equation derived

could evaluate the oxidation of MEA in various environments

including one with or without CO2 and with a corrosion

inhibitor such as sodium metavanadate.

Although the kinetic studies based only on the presence of

O2 in the flue gas could provide useful rate information, their

application could be limited. To apply these kinetic models to

coal fired-based application in which an aqueous amine

solution is used to remove CO2 would result in a less than

accurate degradation rate due to the presence of additional

impurities such as SO2 in the flue gas that will also induce

degradation. Other variables such as dissolved iron, NOx,

corrosion inhibitors, flyash, etc. could also be present in the

CO2 capture system of combustion flue gases. Thus, our

overall goal is to determine the effects of all these variables on

amine degradation. We have therefore adopted a step-wise

procedure of adding one variable at a time to determine their

effects on amine degradation. This step-wise manner has also

been used by previous workers (Chi and Rochelle, 2002; Goff

and Rochelle, 2004) in which they investigated the effect of

iron and copper to only O2 (using air)-induced degradation of

MEA systems. The present study incorporates an important

variable, the effects of SO2, to the well-studied effects of O2.

The current study therefore adds to those done previously by

providing a totally different but field applicable degradation

environment containing SO2. When the effects of all the

parameters that affect amine degradation have been eluci-

dated separately, it will be possible to determine whether

there are interactions between these parameters. The adverse

effect of SO2 has been consistently reported in the literature

(Kather and Oexmann, 2007; Rao and Rubin, 2002; Smit et al.,

2002; Strazisar et al., 2003; Idem et al., 2006). Also, its negative

effects in amine degradation have also been reported in terms

of its capability of forming heat stable salts such as

thiosulfates and sulfates (Smit et al., 2002; Veldman, 2000;

Idem et al., 2006) which reduce the CO2 absorption capacity. A

recommendation has been given that in order to avoid

excessive loss of amine solvent, SO2 concentration in flue

gas stream prior to being treated in a CO2 capture unit must

not exceed 10 ppm (Rao and Rubin, 2002). The present study

used concentration of SO2 between 6 and 196 ppm that can be

observed in a typical flue gas stream after the power plant

desulfurization process.

Even though present in small amounts, SO2 can dissolve

and be carried in the amine solution to the regeneration

section of the capture process at which point a high

temperature can trigger serious degradation reactions with

the amine solvent. To take into account the effect of SO2, we

have recently studied MEA kinetics based on power law

analysis by incorporating SO2 into the MEA degradation rate

equation (Uyanga and Idem, 2007). This has made the rate

equation capable of representing a more realistic scenario for

both coal- and natural gas-fired power plant flue gases.

However, where the CO2 concentration in lean amine from the

regeneration section of the CO2 capture plant is close to zero,

for example in very lean regenerated solvent, this model

becomes inapplicable.

The present study was conducted to extend the kinetic

knowledge we have presented in our previous study (Uyanga

and Idem, 2007) by establishing a more practical and versatile

kinetic model of the degradation of MEA for the CO2

absorption from both coal- and natural gas-fired power plant

flue gas streams including very lean aqueous MEA. Our

previous kinetics (Uyanga and Idem, 2007) were also

formulated on the basis of overall degradation rate showing

principally the extent of MEA degradation over a given time

i n t e r n a t i o n a l j o u r n a l o f g r e e n h o u s e g a s c o n t r o l 3 ( 2 0 0 9 ) 1 3 3 – 1 4 2 135

period. In the current study, our objective is to determine the

potential for amine degradation as a function of degradation

parameters such as temperature, and the concentrations of

MEA, O2, SO2 and CO2 based on the initial MEA degradation

rate. This work is one of the few attempts in providing some

insight on the effect of SO2 to amine degradation.

2. Experimental

2.1. Equipment and chemicals

It is possible to use a small but complete CO2 absorption plant

for this experiment. However, we decided to use a closed

vessel since this appeared to be more reasonable to achieve a

better controlled study for a long period of degradation time

(2–3 weeks). As amine degradation is a very slow process, each

experiment requires a long experimentation time. The closed

vessel that was used to carry out the MEA degradation study

was a 600 ml-stainless steel batch reactor (model 5523, Parr

Instrument Co., Moline, IL). The reactor consisted of a

removable head assembly containing a magnetic drive

connected to a stainless steel (T316) stirring shaft and 2

impellers, a 0–300 psi Bourdon-type pressure reading gauge,

gas inlet, gas purge and liquid sampling valves, a preset safety

rupture disc rated at 6,895 kPa at 295 K, a J-type thermocouple,

a dip tube for gas introduction and sample removal, and a

cooling coil regulated by a solenoid valve. An electrical furnace

in which the reactor vessel was housed supplied heat to the

reactor. Heat was regulated by a temperature-speed controller

(Model 4836, Parr Instrument Co., Moline, IL) of �0.1%

accuracy. The temperature of the degradation mixture was

measured by the J-type thermocouple and monitored through

the temperature-speed controller. As well, the temperature-

speed controller was also used to control and monitor the

stirrer speed. The degradation conditions used for kinetic

experiments in this study are summarized in Table 1. These

represent well-defined laboratory conditions.

However, we have attempted to use operating conditions

that are close to what obtain in real life. This implies that most

degradation conditions selected for evaluation in our study

were close to actual operating condition normally encoun-

tered in a CO2 capture unit. Specifically, O2 concentration in

the simulated flue gas stream was 6% typically found in

industrial flue gas streams. For example, the O2 concentration

of 6%, which is within the concentration range normally

encountered in flue gas streams, was used in this study for

various degradation runs in contrast with earlier studies (Chi

and Rochelle, 2002; Goff and Rochelle, 2004; Gregory and

Scharmann, 1937; Rooney et al., 1998) which used air (21% O2)

Table 1 – Degradation conditions for kinetic experiments

Parameter Range

MEA concentration (kmol/m3) 3–7

O2 in feed gas (%) 6–100

SO2 concentration in feed gas (ppm) 0–196

CO2 loading (mol CO2/mol MEA) 0–0.55

Temperature (K) 328–393

in their experiments. To formulate the degradation kinetics, it

was essential to vary concentration of O2 as well as those of

MEA, SO2, and CO2 in order to have adequate data to determine

the kinetic parameters. In this context, 100% O2 was used for

comparison and representation of the worse case scenario for

MEA degradation. This study presents one of the first studies

to determine kinetics by using actual O2 concentration instead

of only air. The same concept applies to the concentration of

MEA (3–7 kmol/m3), SO2 (as high as 196 ppm normally found

after flue gas desulfurization process, Chakma et al., 1995), CO2

loading (0–0.55 within the range of lean and rich loadings), and

temperature (328–393 K; representing typical absorber and

regenerator temperatures, respectively).

In particular, the stripping condition was used to study the

effect of temperature as well as to evaluate the degradation in

the stripping section of the capture plant caused by soluble

SO2 in the MEA solution. Although, little O2 would be carried

over to the stripper, its effect at 373 and 393 K was represented

for the purpose of comparison and representation of the worse

case scenario of MEA degradation. If O2 reacts mostly with

amine in the absorber, its degradation products such as

organic acids as suggested by the literature (Rooney et al.,

1998; Strazisar et al., 2003), are still introduced into the liquid

amine stream. The O2 effect in the form of these O2-derived

products would still be passed into the stripper column where

the high temperatures (e.g. 373 and 393 K) could cause some

degradation reactions. The effect of these O2-derived products

has been consistently reported by industry (Rao and Rubin,

2002; Rooney et al., 1996, 1997; Smit et al., 2002). Therefore, the

degradation data collected at the stripper conditions in the

presence of O2 is absolutely essential to represent this

significant industrial scenario. The only parameter that did

not reflect the actual absorption plant was pressure condition

of the degradation run. Our system was done under 250 kPa

feed gas pressure as opposed to atmospheric condition

(�101 kPa) used in the actual operation. This was only to

enable us to collect the data within a reasonable time period.

Even then, our conditions represent one of the very few

attempts to use operating parameters that are very close to the

real absorption situation.

Research grade 6–100% O2, 100% CO2 and mixtures of 6% O2

(N2 balance) containing SO2 concentration in a range of 6–

196 ppm were used and were all supplied from Praxair (Regina,

Saskatchewan, Canada). Fisher Scientific, Nepean, Ontario,

Canada supplied concentrated MEA of reagent grade having

more than 99% purity. The desired concentration of MEA

solutions were prepared by diluting the concentrated MEA

with deionized water. Volumetric titration with a standard

solution of 1 kmol/m3 hydrochloric acid (HCl) also from Fisher

Scientific was used to establish their exact concentrations

using methyl orange indicator endpoint.

2.2. Typical experimental run

2.2.1. Non-CO2 loaded degradationThe procedure for a typical non-CO2 loaded run is described in

detail in our previous work (Supap et al., 2006). In brief, a

predetermined concentration of MEA solution of 450 ml was

transferred into the reactor vessel. The removable reactor

head was carefully assembled on top of the vessel to prevent


leakage that might occur during the degradation run. The

whole unit was inserted into the furnace while its magnetic

drive-stirrer was connected to the motor. It must be noted that

MEA solution was not degassed prior to degradation experi-

ments. Typically, dissolved O2, if present, would be small and

negligible under atmospheric conditions when compared to

what was dissolved by 6% O2 or mixture of 6%O2/6–196 ppm

SO2 in total feed gas pressure (N2 balance) of 250 kPa. The

solution was heated and stirred at 500 rpm. Once, the

temperature set-point was reached and stabilized, the

pressure inside the reactor was recorded knowing that most

of it resulted from water vapor pressure as previously shown

by our previous studies (Bello and Idem, 2006; Supap et al.,

2001, 2006; Uyanga and Idem, 2007). By opening the appro-

priate gas cylinder set at a predetermined value, additional

250 kPa of O2/N2 or O2–SO2/N2 mixture pressure was intro-

duced into the solution through the gas inlet valve. The system

pressure was now a combination of water vapor pressure and

250 kPa of feed gas pressure (e.g. O2/N2 or O2/SO2/N2). Cooling

water regulated by the solenoid valve was also used at this

stage to maintain isothermality of the degradation by taking

away heat generated during the initial exothermic reaction.

Due to the desire to maintain a constant degradation pressure

(e.g. water vapor pressure and 250 kPa of O2/N2 or O2–SO2/N2)

throughout the experiment, any decrease in the system total

pressure as a result of O2 or O2–SO2 initially dissolved into MEA

solution was quickly compensated by boosting up the reactor

pressure back to the original total reactor pressure through the

gas cylinder regulator. During the initial degradation, the

amount of O2 or O2–SO2 dissolved into MEA solution was small.

This amount was replenished by adding equal pressure lost

during this process. Based on calculation of O2 alone, it has

shown that after the replenishment process, about 90% of the

original partial pressure of O2 still remained in the headspace

of the system throughout the degradation run.

To ensure actual representation of samples collected at

each time interval, the gas introduction and sample removal

dip tube was first rinsed to get rid of the old sample left in the

tube from the last sample collection. Then, a sample of 2.5 ml

was withdrawn from the reactor vessel into a 5 ml glass bottle

by opening the liquid sampling valve. To avoid further

degradation, the sample was quickly quenched by running

cold water over the bottle for several minutes. Pressure

maintenance was also repeated after each sampling process.

Again, the purpose of boosting-up pressure after each

sampling process was to control isobaric condition (i.e. total

pressure) of the degradation system. Added pressure from the

gas cylinder of either mixture of O2/N2 or O2–SO2/N2 (depend-

ing on the condition of the degradation) served to keep the

total pressure of the system constant throughout the run. The

samples were later analyzed for MEA concentration for kinetic

analysis.

2.2.2. CO2 loaded degradationExcept for the CO2 loading step undertaken after the solution

had been loaded into the reactor, the rest of the procedure was

similar to what has been described for non-CO2 loaded

experiments. Prior to heating, CO2 at a pressure of 250 kPa

was passed into the solution by opening the cylinder valve.

The time needed to introduce CO2 into the MEA solution

depended on the desired CO2-loading. This ranged from 0.25 to

2 h for CO2 loading in the range of 0.1–0.5 mol CO2/mol MEA.

After this step, 4 ml of sample was withdrawn through the

liquid sampling valve to determine the CO2 loading using the

aqueous HCl volumetric titration and CO2 displacement

technique described in our previous work (Supap et al.,

2006). The mixture was then heated to the desired tempera-

ture and the CO2 loading was once again determined and

recorded. The pressure inside the reactor at this point was a

combination of pressure of water vapor and non-dissolved

CO2. O2/N2 or O2–SO2/N2 at 250 kPa pressure was introduced

into the system through the gas inlet valve by opening the

appropriate gas cylinder tank. The combination of water vapor

pressure, CO2 vapor pressure, and 250 kPa pressure of O2/N2 or

O2–SO2/N2 was taken as the total reactor pressure. The rest of

the procedure was then carried out following those explained

for non-CO2-loaded experiments.

2.3. Sample analysis

The HPLC (model 1100) was used to determine the MEA

concentration in all degraded samples. It was equipped with

an on-line degasser, a quaternary pump, a thermostatted

column compartment with �0.5 8C temperature accuracy,

and a refractive index detector (RID) (model G1322A/G1311A/

G1316A/G1362A). All were supplied by Agilent Technologies

Canada, Mississauga, Ontario, Canada. Analytical HPLC

column was Nucleosil 100-5 SA packed with a strong cationic

exchanger of sulfonic acid (Machery-Nagel, Duren, Ger-

many). The column dimension was 125 mm � 4.6 mm,

respectively in length and internal diameter (i.d.). The

introduction of liquid sample was achieved by an automatic

liquid sampler (model G1313A) with the repeatability

expressed in terms of RSD at 0.5% of peak area for 5–100 ml

injection range (Agilent Technologies Canada, Mississauga,

Ontario, Canada).

Reagent grade potassium dihydrogen phosphate (KH2PO4,

Sigma–Aldrich Canada, Mississauga, Ontario, Canada) was

used to make up 0.05 kmol/m3 HPLC mobile phase solution.

Also, 85% (w/w) phosphoric acid (H3PO4) also from Sigma–

Aldrich was used to adjust the pH of the mobile phase to 2.6

prior to analysis.

The HPLC technique developed in our previous work

(Supap et al., 2006) was adopted as a method for determination

of MEA concentration. In brief, KH2PO4 mobile phase solution

of 0.05 kmol/m3 adjusted to pH 2.6 by H3PO4 solution was

prepared and first degassed for 3 h to remove dissolved O2 that

might cause interference during the sample analysis. The

mobile phase was subsequently filtered using a 0.20 mm nylon

membrane filter to remove any solid particles to prevent them

from plugging the HPLC column. Degraded sample was diluted

to 1 in 40 using nanopure water and also filtered with 0.20 mm

nylon membrane filter. Sample injection of 8 ml was done

automatically by the automatic liquid sampler as described

previously. The column was controlled isothermally at 303 K.

The isocratic mode of 100% mobile phase at the flow rate of

1 ml/min was used throughout to perform the analysis. The

refractive index detector was used to detect MEA peak and the

optical unit was controlled at 303 K and operated in the

positive mode.


2.3.1. Determination of MEA concentration in degradedsamplesA calibration curve of MEA was used to determine MEA

concentration in all degraded samples using MEA concentra-

tions ranging from 2 to 8 kmol/m3. Each calibration point was

repeated three times to ensure reproducibility. MEA peak

areas in all degraded samples obtained from the same HPLC

technique were calculated and the exact MEA concentrations

extracted from the calibration curve. At the beginning of each

degradation experiment, MEA solution was also measured for

its concentration using HPLC technique with standard

calibration curve. The HPLC determined MEA concentration

was then compared with its exact concentration obtained

from volumetric titration with standard HCl as previously

described. This procedure was used to establish the accuracy

of the HPLC method in this study. The accuracy of HPLC in

measurement of MEA concentration expressed as averaged

absolute deviation percentage (%AAD) for all degradation runs

was in the range of 0.1–6.4%.

3. Results and discussion

3.1. Evaluation of effects of temperature andconcentrations of MEA, O2, SO2 and CO2

Initial rate of MEA degradation was used throughout this study

to evaluate effect of all degradation parameters (e.g. tem-

perature and concentration of MEA, O2, SO2, and CO2) as well

as to determine the degradation kinetics. In addition, although

data was presented on the rates over a long period of time;

however, the intention of using initial rate for evaluation is to

show the tendency to degrade caused by different parameters.

Table 2 – MEA Concentration–time data at various degradation

Degradation conditions

3.18 kmol/m3 MEA, 100% O2, 0.42 CO2 loading, 373 K

6.80 kmol/m3 MEA, 6%O2, 196 ppm SO2, 0.27 CO2 loading, 393 K

5.00 kmol/m3 MEA, 6%O2, 6 ppm SO2, 0.33 CO2 loading, 393 K

To obtain the rate, concentration profile of each run was

constructed by plotting MEA concentration and time. Typical

MEA concentration–time data at various degradation condi-

tions are given in Table 2. Exponential function available from

Microsoft Excel 2003 was used in fitting these concentration–

time data for the purpose of calculating the corresponding

initial degradation rates. Not only did MEA concentration time

data at the initial times go into obtaining the line equation, but

all experimental points were used for this purpose. The initial

degradation rates at time zero were then determined from the

slope of the concentration–time curve at time zero using the

line of best fit equation. Therefore, the initial rates shown

throughout this study involved all the concentration–time

data points. The error between the model and experimental

values in terms of %AAD ranged between 0.2 and 3.7%. The

overall accuracy of each initial rate measured in this study is in

the range of 0.6–6.7%AAD.

The effect of temperature was evaluated by using 5 kmol/

m3 MEA and 100% O2, and the results are shown in Fig. 1. To

evaluate the temperature effect, a bar chart in Fig. 2 is used. It

is clear from Fig. 2 that the initial degradation rate of the run

conducted at 393 K was higher than those carried out at the

lower temperatures of 373 and 328 K. The MEA degradation

rate of the 393 K run measured at 1.84 � 10�2 kmol/m3 h

was about 1.4 and almost 100 times higher than those at

373 and 328 K, respectively measured at 1.29 � 10�2 and

1.86 � 10�4 kmol/m3 h. The initial rate of MEA degradation

was also found to increase if temperature increased for

MEA–O2–SO2–H2O degradation system. Fig. 3 shows that by

using 7 kmol/m3 MEA, 6% O2, and 11 ppm SO2, initial rate was

found to be 4.83 � 10�4 kmol/m3 h at 373 K. The rate then

increased approximately 1.7 times to 8.18 � 10�4 kmol/m3 h

when the temperature was raised to 393 K.

conditions

Time (h) HPLC MEA concentration (kmol/m3)

0 3.15

2 3.15

4 3.12

25 2.98

73 2.79

120 2.64

164 2.57

0 7.23

2 7.15

25 7.17

97 7.10

145 7.06

193 7.05

245 6.93

289 6.95

0 5.04

3 5.04

22.5 4.98

70.5 4.96

118.5 4.98

166.5 4.92

214.5 4.92

262.5 4.92

310.5 4.89

Fig. 1 – Effect of temperature using MEA concentration–time

plot (5 kmol/m3 MEA and 100% O2).

Fig. 2 – Evaluation of temperature effect using initial rate of

MEA degradation (5 kmol/m3 MEA and 100% O2).

Fig. 4 – Evaluation of initial MEA concentration effect using

initial rate of MEA degradation (100% O2 and 393 K).

Fig. 5 – Evaluation of initial gas phase O2 concentration

using initial rate of MEA degradation (5 kmol/m3 MEA and

393 K).


Fig. 4 shows the effect of initial MEA concentration using

the degradation runs with 100% O2 at a temperature of 393 K.

The degradation run conducted with 7 kmol/m3 MEA solution

resulted in 1.89 � 10�2 kmol/m3 h whereas 1.84 � 10�2 kmol/

m3 h initial degradation rate was obtained from the run

carried out with 5 kmol/m3 MEA. Even though the figure

suggests a slight increase in MEA degradation rate with an

increase in MEA concentration from 5 to 7 kmol/m3, but

Fig. 3 – Evaluation of temperature in the presence of SO2

using initial rate of MEA degradation (7 kmol/m3 MEA, 6%

O2, and 11 ppm SO2).

because of uncertainty, we conclude that this increase is

insignificant. The degradation runs involving 6–100% O2

concentration in the simulated flue gas stream, 5 kmol/m3

MEA at 393 K were used to show the effect of O2 concentration

in the feed gas on MEA degradation. Fig. 5 shows that an

increase in initial O2 concentration in the gas stream results in

an increase in the initial MEA degradation rate. Runs

conducted with 6, 21, and 100% initial O2 concentration

respectively resulted in 4.50 � 10�4, 6.46 � 10�4, and

1.84 � 10�2 kmol/m3 h initial degradation rate.

The effect of SO2 concentration in the simulated flue gas

stream on MEA degradation was evaluated using a similar

approach as used for temperature and concentrations of MEA

and O2. The initial rates at various SO2 concentrations in the

feed gas are shown in Figs. 6 and 7. The runs were conducted at

393 K with 5–7 kmol/m3 MEA and feed gas containing 6%O2

with SO2 concentration from 0 to 196 ppm. Fig. 6 compares two

runs conducted using 5 kmol/m3 MEA with 6 ppm SO2 and

without SO2. The results showed that the initial rate of MEA

degradation for the run with 6 ppm SO2 measured as

4.91 � 10�4 kmol/m3 h was higher than that of the run carried

out without SO2 (4.50 � 10�4 kmol/m3 h). In Fig. 7, another

Fig. 6 – Effect of initial gas phase SO2 concentration at

5 kmol/m3 MEA using initial rate of MEA degradation (6%

O2 and 393 K).

Fig. 7 – Effect of initial gas phase SO2 concentration at

7 kmol/m3 MEA with CO2 using initial rate of MEA

degradation (6% O2 and 393 K).

Fig. 8 – Evaluation of initial CO2 concentration effect using

initial rate of MEA degradation (3 kmol/m3 MEA, 100% O2,

and 373 K).


system with a higher MEA concentration of 7 kmol/m3 also

containing CO2 was used to show the SO2 effect. Higher SO2

feed gas concentrations of 11 and 196 were used to evaluate

this effect on MEA degradation. Two runs conducted at 393 K

with 7 kmol/m3 MEA and somewhat similar values of CO2

loading (e.g. 0.24 and 0.27 mol CO2/mol MEA) confirmed the

negative effect of SO2. The run that contained 196 ppm SO2

was conducted in the presence of a higher CO2 loading

(0.27 mol CO2/mol MEA) which is known to retard the

degradation rate (Bello and Idem, 2006; Rooney et al., 1998;

Supap et al., 2006). However, the initial rate measured at

9.11 � 10�4 kmol/m3 h, was still higher than 6.99 � 10�4 kmol/

m3 h of the run with 11 ppm SO2 that contained a lower CO2

loading (0.24 mol CO2/mol MEA). Although, the rate difference

was small, this study has shown that in addition to O2, SO2 also

induces degradation which was similar to what has been

reported in the literature (Idem et al., 2006; Kather and

Oexmann, 2007; Rao and Rubin, 2002; Smit et al., 2002;

Strazisar et al., 2003). This showed the initial tendency of

MEA to degrade if SO2 was present in the flue gas stream. In an

actual CO2 removal unit, this SO2-induced degradation effect

is cumulative which eventually becomes troublesome after

long exposure and repeated use of the amine.

The effect of CO2 was also evaluated in the absence of SO2

by using a higher CO2 loading values between 0.22 and

0.52 mol CO2/mol MEA with 3 kmol/m3 MEA, 100% O2 and

373 K. CO2 was the only degradation component in which an

increase in its concentration resulted in a decrease in the

initial MEA degradation rate. This inhibition effect of CO2 is

confirmed in Fig. 8. CO2 slowed down the degradation rate by

reducing solubility of O2 and SO2 in MEA solution, thus

minimizing intimate contact of O2 and SO2 to MEA that led to

degradation. The CO2 inhibition effect obtained in this study is

consistent with those obtained by other degradation studies

(Rooney et al., 1998). Details of the effects of the degradation

parameters are also summarized in Table 3. These results

imply that MEA has a tendency to degrade if the conditions

used for CO2 removal from fossil-fuel derived flue gas streams

in terms of temperature and the concentrations of MEA, O2

and SO2 were increased. However, the MEA degradation

intensity could be reduced if the MEA prior to being used to

treat the flue gas streams contained some CO2 in the solution.

3.2. Formulation of a rate equation

The rate equation was formulated based on the assumption

that MEA reacted only in the liquid phase with dissolved O2,

SO2 and CO2. Such an assumption is justified based on our

previous experience (Supap et al., 2001) in which under similar

experimental conditions, the vapor pressure of the system

before introduction of the simulated gaseous reactants was

mostly due to water vapor. This enabled us to eliminate MEA

vapor and thus eliminate any gas phase reaction. This allowed

the degradation kinetics to be formulated as a homogeneous

liquid phase system. Although, mass transfer could possibly

control the degradation rate of MEA (Goff and Rochelle, 2004),

our previous experience (Supap et al., 2001) has shown that

mass transfer limitation is insignificant during the initial

degradation time (e.g. 0–20 h of degradation time) if an

appropriate stirring speed is used. With similar operating

conditions used also in the current study, intimate contact of

dissolved O2 as well as SO2 to MEA was achieved. Thus, with

the use of initial degradation rate, mass transfer resistance is

practically neglected. The interference from degradation

Table 3 – Detailed summary of effects of degradation parameters

Temperature (K) Concentration Initial MEA degradationrate (kmol/m3 h)

Error (%)a

MEA(kmol/m3)

O2 (%) SO2

(ppm)CO2 loading

(mol CO2/mol MEA)

328 100 0 0 0.000186 1.4

373 5 0.0129 2.7

393 0.0184 3.1

373 7 6 11 0 0.000483 2.6

393 0.000818 2.1

393 5 100 0 0 0.0184 3.1

7 0.0189 5.4

393 6 0 0 0.00045 1.4

5 21 0.000646 0.6

100 0.0184 3.1

393 5 6 0 0 0.00045 1.4

6 0.00049 2.1

7 11 0.27 0.00069 6.4

196 0.24 0.00091 6.7

373 3 100 0 0.22 0.0051 3.2

0.42 0.0041 2.0

0.52 0.0028 4.2

a % error was calculated from a combination of HPLC error (MEA concentration measurement) and the error from curve fitting of MEA

concentration–time data used to generate initial degradation rates.

Table 4 – Values estimated for parameters of the kineticmodel

Parameter Estimate

k0 (kmol0.99/(m2.96 h [(kmol/m3)2.9

+ (kmol/m3)3.5]))

6.74 � 109

Ea (J/mol) 29,403

a 0.015

b 2.91

c 3.52

d 0.18

k (kmol/m3)�0.18 1.18


products could also be neglected. These assumptions together

with those of our previous work (Uyanga and Idem, 2007) were

used to formulate the kinetic model in this study.

The experimental results have shown that an increase in

the initial concentrations of MEA, O2, and SO2 all lead to an

increase in the initial MEA degradation rate. The kinetic model

was formulated such that these concentration terms appeared

in the numerator of the rate equation. The initial CO2

concentration was the only degradation parameter in this

study that showed an inhibition effect to MEA degradation. In

addition to the inhibition effect of CO2 described in Section 3.1,

the degradation experiments conducted at 393 K using 5 kmol/

m3 MEA, 6% O2, and 6 ppm SO2 with 0.33 mol CO2/mol MEA or

without CO2 were also used to confirm the inhibition effect of

CO2. The initial MEA degradation rate of the run with CO2

measured as 4.32 � 10�4 kmol/m3 h was less than that for run

without CO2 (e.g. 4.50 � 10�4 kmol/m3 h). The same trend was

also observed for 7 kmol/m3 MEA, 6% O2, and 11 ppm SO2

system in which run with 0.24 CO2 loading resulted in a lower

initial rate when compared to the run without CO2. Inhibition

effect of CO2 observed in this study was once again consistent

with the literature (Bello and Idem, 2006; Rooney et al., 1998;

Uyanga and Idem, 2007). Again, the inhibition effect could be

simply explained on basis of the salting out effect whereby CO2

goes into the MEA solution in preference to O2 and SO2. This

led us to add CO2 concentration term in the denominator of

the rate equation. The constant, 1, was introduced into the

denominator as part of the normal procedure in formulating a

mechanistic-based rate equation (Fogler, 1999; Levenspiel,

1999).

The degradation of MEA in the presence of O2, SO2, and CO2

proceeds stoichiometrically. For SO2, although, it is present in

a smaller concentration compared to MEA, its degradation is

not considered catalytic since SO2 was consumed during the

degradation process forming sulfur-containing products with

MEA. Examples of these products are heat stable salts which

have been consistently identified in the literature (Idem et al.,

2006; Rao and Rubin, 2002; Strazisar et al., 2003; Veldman,

2000). Therefore, this allowed us to formulate a rate equation

that also incorporates SO2. The Arrhenius equation was also

introduced into the rate equation to represent the empirical

relationship between temperature and the degradation rate

constant. The resulting rate equation is given in the following

equation

�rMEA ¼k0 e�Ea=RT½MEA�að½O2�b þ ½SO2�cÞ

1þ k½CO2�d(1)

where k0 is the preexponential constant (units depend on

value of a, b, c, and d) representing both the frequency of

collision between species and the orientation of the reacting

species; and Ea is the activation energy (J/mol). R is the gas

constant (8.314 J/mol K) and T is the degradation temperature

(K). [MEA], [O2], [SO2] and [CO2] are respectively the initial MEA,

O2, SO2 and CO2 concentrations (kmol/m3). While k is the

Table 5 – Comparison of experimental rates and those predicted by the rate equation

Temperature (K) Initial concentration Initial MEA degradation rate

MEA(kmol/m3)

O2

(kmol/m3)SO2

(kmol/m3)CO2

(kmol/m3)Experimenta

(kmol/m3 h)Predicted

(kmol/m3 h)

393 5.03 0.00239 0 2.77 0.00840 0.00829

393 5.13 0.00239 0 0 0.0184 0.02000

373 4.95 0.00222 0 0 0.0129 0.00998

328 7.00 0.00231 0 1.54 0.00151 0.00135

373 3.20 0.00222 0 0.70 0.00507 0.00470

373 3.18 0.00222 0 1.34 0.00410 0.00441

393 6.85 0.00239 0 0 0.0189 0.0201

393 5.10 0.00073 0 0 0.000450 0.000636

373 3.25 0.00222 0 1.69 0.00284 0.00431

393 3.15 0.00100 0 0.88 0.000543 0.000733

393 4.95 0.00073 0.00009 0 0.000491 0.000636

373 5.88 0.00057 0.00009 0 0.000360 0.000192

373 6.90 0.00057 0.00017 0 0.000483 0.000192

393 6.95 0.00073 0.00017 0 0.000818 0.000639

373 3.00 0.00057 0.00306 1.41 0.000421 0.000411

393 5.00 0.00073 0.00009 1.65 0.000432 0.000277

393 6.80 0.00073 0.00017 1.84 0.000699 0.000276

393 6.80 0.00073 0.00306 0.66 0.000911 0.000882

393 3.00 0.00073 0.00306 0.66 0.000740 0.000871

a The maximum error of experimental rates is 6.7% AAD.


specific rate constant involving reactions of CO2 (unit

depends on the value of d). Finally, a, b, c, and d are the

respective reaction orders with respect to MEA, O2, SO2 and

CO2. It is well known that dissolved CO2 and SO2 in MEA

solution generate various species (such as HCO3 and N–

COO). In order to eliminate the inability to measure these

species and deciding which carbon/sufur species to use, we

decided to use initial MEA degradation rate in our kinetic

study. In this case, we were able to represent the CO2 and

SO2 species prior to degradation reactions respectively by

their initial CO2 loading and intial SO2 concentration in the

MEA solution. Therefore, [CO2] and [SO2] in Eq. (1) represents

the initial CO2 concentration and initial SO2 concentration,

respectively.

3.3. Estimation of the parameters of the rate equation

The estimation of the values of the parameters of the rate

equation shown in Eq. (1) was done using non-linear

regression using NLREG software, version 6.3. A set of data

obtained with 3–7 kmol/m3 MEA, 6–100% O2, 0–196 ppm SO2,

and loading of 0–0.55 CO2/mol MEA was used. Majority of the

data used for determination of the kinetics were derived from

O2 concentration of 6%. Prior to regression, the values of CO2

loading were converted to their corresponding concentrations

having the same unit as that of MEA (kmol/m3). As well, the O2

concentration in the MEA solution was obtained using the

equation proposed by Rooney and Daniels (1998). SO2 in ppm

unit was also converted to kmol/m3 with the assumption that

all of the SO2 in the gas phase was completely dissolved in the

liquid phase. This was done since data for SO2 solubility in

alkanolamine solutions have not yet been made available, to

our knowledge. The non-linear regression analysis was done

at 95% confidence level and the estimates of k0, Ea, a, b, c, d, and

k obtained with coefficient of correlation (R2) of 0.96 are

summarized in Table 4. The table shows that even though SO2

is present in a smaller amount than O2, its propensity to

degrade the amine is higher than that for O2 as can be seen in

comparing their orders of reaction.

Table 5 shows the initial MEA degradation rate obtained

experimentally and those predicted by the proposed rate

equation for various degradation conditions. The rate equa-

tion fairly predicts the initial MEA degradation rate in the

system containing MEA, O2, SO2 and CO2 with %AAD of 25%. As

this is a semi-empirical model, it appears that all the

parameters needed for a real mechanism may not have been

incorporated in the model. We are currently studying these

extra parameters and results will be released in a future

publication. The coefficient of correlation obtained for the

estimates of the kinetic parameters and data given in Tables 4

and 5 show that the rate model fairly fits the kinetic data for

the entire range of temperature and initial concentrations of

MEA, O2, SO2 and CO2 used in this study. These conditions,

except for high O2 pressure, cover a range of conditions

normally encountered in typical CO2 capture from flue gases

using amine chemical solvents.

In addition, the rate equation proposed in the current study

represents a substantial improvement from our previous

model (Uyanga and Idem, 2007). As such, any of the

parameters (i.e. O2, SO2, and CO2) can be removed without

affecting the usability of the model.

4. Conclusions

1. A
useful kinetic data for O2–SO2-induced MEA degradation
have been obtained under conditions that are typical of a

CO2 absorption process (i.e. 3–7 kmol/m3 MEA, 6% O2, 0–

196 ppm SO2, and 0–0.55 CO2 loading, and 328–393 K

temperature).


2. B
ased on initial rate analysis, MEA showed the tendency to
degrade if temperature and the concentrations of MEA, O2

and SO2 in the system were increased. However, an

increase of CO2 loading in MEA solution led to reduction

in MEA degradation.

3. T
he model could also fairly describe MEA degradation in the
presence of O2, SO2 and CO2.

4. O
ur results confirm that even though SO2 is present in a
smaller than O2, its contribution to the propensity for amine

to degrade is higher than that for O2 as can be seen in

comparing their orders of reaction.

Acknowledgements

The authors would like to acknowledge financial assistance

from the Natural Sciences and Engineering Research Council

of Canada (NSERC), CANMET Energy Research Center/NRCan

Ottawa, Canada, and The Royal Golden Jubilee Ph.D. Program,

Thailand Research Fund (Grant no. PHD/0034/2546).

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