1. Kinetics of CO2 Absorption into MEA-AMP Blended Solution
Roongrat Sakwattanapong
Adisorn Aroonwilas
Amornvadee Veawab
Good morning ladies and gentlemen. My name is roongrat sakwattanapong from university of Regina. Today I’d like to present to everyone the kinetic of CO2 absorption into MEA-AMP blended solution. And this work is supervised by Dr.Adisorn Aroonwilas and Dr. Amornvadee Veawab from university of Regina as well.
Faculty of Engineering
University of Regina
Saskatchewan, Canada
Presented at the Annual Research Review Meeting, University of Texas at Austin, Jan 10-11, 2008

2. Outline Introduction & Research Motivation Research Objective
CO2 Absorption Experiments
Experimental Results and Discussion
Kinetic Model for MEA-AMP System
Conclusions
Acknowledgement
The outline for this presentation. The presentation will start from introduction and research motivation.
Then I will talk about research objective, absorption experiments, results and discussion.
And followed by kinetic model for the MEA-AMP system, conclusion, and acknowledgements at the end.

3. Introduction CO2 capture technology  Reduction in GHG emissions
Low pressure flue gas  Chemical absorption into amines
Performance of CO2 absorption
Higher performance  [Smaller unit]  Lower cost
In the past few years, global climate change has received much attention around the world. And the earth will experience increased frequency of extreme weather conditions.
However, the increasing of greenhouse gas emissions is to blame. Since carbon dioxide accounts for the largest portion of the emissions of GHGs, therefore, CO2 capture technology play an important role.
For a low pressure flue gas system, chemical absorption by using amine solution has been widely used.
One of the major concerns in absorption process is the performance of CO2 absorption. The higher the performance, the smaller the absorption unit which will result in a lower cost.
The performance of the CO2 absorption can be developed through two means of performance development. They are either the development of the process design or the development of the new absorption solvents. In this study, the focus will be mainly on the development of the solvent.
Process Design
Absorption solvents

4. Introduction (Solvent Characteristics)
MEA
DEA
MDEA
Absorption efficiency or rate
rCO2 = k2 [CO2][Amine]
k2 ~ 6000 to 7500 m3/kmol-s
k2 ~ 550 to 1600 m3/kmol-s
k2 ~ 5
m3/kmol-s
Heat of reaction (kJ/mol CO2)
85.6
76.3
60.9
Energy requirement for regeneration (kJ/kg CO2)
High
Medium
Low
CO2 solubility
(mol CO2/mol Amine)
0.5
1.0
This slide shows the solvent characteristics of the three popular solvents. We can see that MEA has the best absorption efficiency among the three solvents. However, MDEA has the best feature according to the capacity and the energy requirement for regeneration.
In order to combine the advantages of each of the solution, blended alkanolamine is introduced. And recently, blended-alkanolamines has been receiving a great deal of interest.
Therefore, a search for a solution with low energy requirement with acceptable rate ?????????????.
Blended-alkanolamines
Blended alkanolamines have been receiving a great deal of interest.
Low energy requirement with acceptable absorption rate

5. Research Motivation MDEA-based solvents  Low rate of CO2 absorption.
AMP can absorb CO2 with the similar capacity with MDEA but at a much higher rate.
The knowledge of CO2 absorption kinetics for MEA-AMP is minimum and limited.
The common blended alkanolamines are MDEA-based solvents, which are claimed to have low energy requirement, high absorption capacity, and excellent stability.
However, the use of MDEA-based solvents may be limited by the low rate of CO2 absorption. Moreover, MDEA is banned in some countries because of the increasing concern on the environmental impact.
Compare to MDEA, AMP can absorb CO2 with the similar capacity but at a much higher rate (Yih and Shen, 1988; Crooks and Donnellan, 1990). As we can see in this figure (POINT on the left). The figure shows the comparison of the mass transfer. The mixture of MEA and MDEA is overhere. But the mixture of MEA and AMP is up here. We can see the opportunity to increase the performance of the system by using mixture of AMP instead of MDEA. Moreover, AMP plays the same role as MDEA to lower the energy consumption when mixed with MEA.
Therefore, the MEA-AMP blended solvents appear to be another alternative option for gas processing. An understanding of the absorption characteristics in blended MEA-AMP becomes essential.
Since, the knowledge of CO2 absorption kinetics for MEA-AMP is minimum and limited. So the objective of this study are
Aroonwilas and Veawab, (Ind. Eng. Chem. Res.)

6. Research Objective
To measure kinetic rate of CO2 absorption into aqueous MEA-AMP solution
To investigate the effects of process parameters on the kinetic rate of the blend. (The parameters of interest are temperature, total amine concentration, and MEA-AMP mixing ratio.)
To understand the kinetic rate data using reaction mechanism model
To measure kinetic rate of CO2 absorption into aqueous MEA-AMP solution.
To investigate the effects of process parameters on the kinetic rate of the blend. (The parameters of interest are temperature, total amine concentration, and MEA-AMP mixing ratio.)
And another objective of this research is to understand the kinetic rate data using reaction mechanism model.

7. CO2 Absorption Experiment
Wetted Wall Column
Diameter = 12 mm, OD (stainless steel)
Column height = up to 100 mm.
Temperature measurement at different locations
CO2 absorption experiment
This slide shows the schematic diagram of the apparatus.
The experiments are carried out by using wetted wall column absorber.
The wetted wall column was made from 12 mm (OD) stainless steel tubing with an adjustable length with the maximum height of 10 cm.
(The column was fitted inside a glass cell where a stream of CO2 saturated with water was flowed through.)
Temperature of the cell was controlled by a connected water bath (POINT OUT).
The liquid solvent entered the cell through the inside of the wetted wall column (POINT OUT) and distributed as a thin film covering the outside of the column.
Flow rate of gas entering and leaving the column was measured by a soap-film meter (POINT OUT).
The gas absorption rate was determined by the difference between the inlet and outlet gas flow rates.
(Flow rate of the feed liquid was varied from 1.8 – 2.8 cm3/sec.)

8. CO2 Absorption Experiment (cont’d)
And this slide shows the photograph of the set-up. (POINT OUT) In this glass cell contains the wetted wall cell…the saturation cell…and the heat exchanger. Water bath is here and the bubble flow meter. 8 8

9. System Verification
Measurement of diffusion coefficient for CO2-water system
T = 298 – 325 K
Since the wetted wall column absorber is a very sensitive equipment, in order to get reliable results we need to adjust and calibrate our system with the well-known results available in literatures.
The next three slides will show the system verification.
The first verification system is the measurement of diffusion coefficient for CO2-Water.
This system is based on the physical absorption of CO2 into water. The experiments were carried out from 298 K upto 325 K.
This figure shows the diffusion coefficient and the inverse of the temperature. The result in this study represent by the circle symbol.
From the figure, the results are well fitted with other studies.
In other words, our wetted wall column produce reliable sets of data. 9 9

10. System Verification (cont’d)
Measurement of reaction rate constant for CO2-MEA system
Temperature = 298 – 318 K (at Various liquid flow rates)
MEA concentration = 1 – 4 kmol/m3
The next verification system is the measurement of reaction rate constant for CO2-MEA system.
The chemical absorption of CO2 was tested to further validate the wetted wall column and also used as a comparison based.
The experiments for CO2-MEA system were carried out over the temperature range of 298 up to 318 K. From 1 to 4 kmol/m3 of MEA at various liquid flow rates.
The figure shows the second-order rate of reaction and the inverse of the temperature. As you can see from the figure that the experimental results are lying in between these three groups (POINT OUT) of well-known researchers. 10 10

11. System Verification (cont’d)
Measurement of reaction rate constant for CO2-AMP system
Temperature = 298 – 318 K (at Various liquid flow rates)
AMP concentration = 1 – 4 kmol/m3
And this is the last system verification.
The system was verified by measuring the reaction rate constant of CO2-AMP system. Once again the experiments for CO2-AMP system were performed over the temperature range of 298 up to 318 K. From 1 to 4 kmol/m3 of AMP at various liquid flow rates.
The figure shows the second-order rate of reaction of AMP with the inverse of the temperature. The experimental results are in between reported literatures.
So, from these three verification systems, we can confirm that our wetted-wall column can produce reliable data. Moreover, the accuracy of experimental kinetic data are in the same range with other reported data from literatures. 11 11

12. Test Condition for MEA-AMP Blend
Test Parameters
Condition
Molar mixing ratio
MEA : AMP
1 : 0 (xMEA = 1.0)
4 : 1 (xMEA = 0.8)
1 : 1 (xMEA = 0.5)
1 : 4 (xMEA = 0.2)
0 : 1 (xMEA = 0.0)
Temperature
298, 303, 308, 313, and 318 K
Total amine concentration
1.0 , 1.5, 2.0, 3.0, and 4.0 kmol/m3
Now we will take a look at the test condition for MEA-AMP blend. The experiments were carried out over the range of conditions as listed in this Table.
The molar mixing ratio between MEA and AMP are 1:0, 4:1, 1:1, 1:4, and 0:1 mole MEA to mole AMP.
The experiments were conducted under five temperatures: 298, 303, 308, 313 and 318 K. And the total concentration are at 1, 1.5, 2, 3, and 4 kmol/m3.
(The interface area for wetted-wall columns can be varied from cm2 and diffusion times from sec..really!!!).

13. Experimental Results Overall rate constant (kOV)
Parametric effects on kOV (Temperature, Amine conc., MEA-AMP mixing ratio)
Regression of diffusion coefficient and Henry’s constant for MEA-AMP blend.
Since the reaction between blended MEA-AMP and CO2 is very complicated, so it will be less complicated to present the results by using the overall rate of reaction instead of the second-order rate of reaction.
The overall rate of reaction is in the function of rate of CO2 absorption, Henry’s constant, CO2 partial pressure, and diffusion coefficient.
Since there is no correlation to describe the diffusion coefficient and the Henry’s constant available for MEA-AMP blend, therefore, the regression of diffusion coefficient and Henry’s constant for the blend are necessary. And this slide shows the parity plot between the correlation used in this study against the reported data. 13 13

14. Effect of Temperature General representation MEA : AMP = 1 : 1
Next, Effect of Temperature on the Overall Rate Constant
This Figure shows relationship between overall rate constant (POINT OUT) with the inverse of temperature (POINT OUT). We can see that the temperature has an impact on the rate constant. The rate constant increases as the temperature increases (POINT OUT ALONG THE CURVE).
The effect of temperature follows the Arrhenius’ law.
The rate of a chemical reaction increases exponentially with the absolute temperature.
And in this figure, the results are at mixing ratio of 1:1
MEA : AMP = 1 : 1

15. Effect of Temperature (cont’d)
Individual Mixing Ratio
MEA : AMP ratio
1 : 0 (xMEA = 1.0)
4 : 1 (xMEA = 0.8)
1 : 1 (xMEA = 0.5)
1 : 4 (xMEA = 0.2)
0 : 1 (xMEA = 0.0)
The following figures again showing the effect of temperature but at different ratio
The main reason that the overall rate increases when the temperature increases is ???????????????????????????? Use the concept of reactivity and ahrenius law.

16. Effect of Amine Concentration
General representation
Next set of result is the effect of amine concentration on the overall rate constant.
The figure shows the overall rate constant on the y-axis (POINT OUT) and the total concentration on the X-axis (POINT OUT). Each line in this figure shows result from different mixing ratio at 318 K. At each of the mixing ratio, the overall rate constant is increasing when the total concentration is increasing.
T = 318 K

17. Effect of Amine Concentration (cont’d)
Individual temperatures
These following figures will show the effect of total concentration. At each tested temperatures. (We can see that at each line which represents the mixing ratio, the overall rate constant is increasing when the total concentration is increasing.)
The major reason that higher concentration has higher rate of constant because higher concentration has more available amine to capture CO2 which result in a higher rate of absorption. Therefore, the system has higher overall rate constant. 17 17

18. Effect of Mixing Ratio General Representation MEA : AMP ratio
1 : 0 (xMEA = 1.0)
4 : 1 (xMEA = 0.8)
1 : 1 (xMEA = 0.5)
1 : 4 (xMEA = 0.2)
0 : 1 (xMEA = 0.0)
Without Synergy Effect
Next, is the effect of mixing ratio on the overall rate constant. This figure shows a plot of overall rate constant kov (POINT OUT) with the ratio of MEA concentration to total amine concentration (POINT OUT) at 318 K.
It should be noted that the solution with concentration ratio of 1 is the aqueous solution of single MEA (POINT OUT), and the one with the ratio of 0 is the aqueous solution of single AMP (POINT OUT).
It appears from the figure that the rate constant increases (POINT OUT ALONG CURVE) as the MEA concentration in the blend increases until it reaches a certain ratio of solution. And then the rate constant starts level off.
A nonlinear correlation has been found for all temperatures tested which indicates a synergy effect (CLICK) from the interaction between parent amines, MEA and AMP during the CO2 absorption process.
This means the blended solution does not provide the average rate constant of the two single amines.
[The nonlinear behavior of the blended MEA-AMP solution is consistent with the behavior found in other blended system (such as MEA-MDEA as reported by Liao and Li in 2002).]
AMP
MEA

19. Effect of Mixing Ratio (cont’d)
Individual Temperatures
And now I will show you, still, the effect of the mixing ratio on the overall rate constant but at different temperature. Starting from the lowest temperature 298 (CLICK), 303 (CLICK), 308 (CLICK), 313 (CLICK) and 318 K (CLICK).
REASON FOR THIS????????????????????????????????
Up to this point we can make a brief conclusion on the results.
The highest tested temperature and the most concentrated solution give the highest value of the overall rate of reaction. But the highest overall rate happens when we mixed MEA with AMP, and from our tested ratio it happens at the ratio of 1 mole of MEA to 1 mole of AMP.
Single AMP
Single MEA

20. Kinetic Model for MEA-AMP System
Xiao et al. (2000) proposed a model based on a hybrid reaction rate
Ali (2005) expressed the reaction rates of both AMP and MEA based on the zwitterion mechanism (for low amine concentration)
CO2-MEA System
CO2-AMP System
Next section of the presentation involves the kinetic model for MEA-AMP system.
Chemical absorption of CO2 involves one or more reversible chemical reactions between CO2 and another substance.
So, there are two well-known models available for the prediction of the overall rate constant of blended MEA-AMP system.
Xiao et al. (2000) proposed a model based on a hybrid reaction rate which composed of zwitterion mechanism for AMP and a simplified first-order reaction for MEA
Ali (2005) expressed the reaction rates of both AMP and MEA based on the zwitterion mechanism
The zwitterion mechanism will be favor in this study since the formation of zwitterion intermediate is the rate determining step for both solutions.
And the rate of reaction can be described by this 2 equations at the bottom…..CO2-MEA system (POINT OUT)…..and CO2-AMP system (POINT OUT).
Xiao, J., Li, C.W., and Li, M.H., “Kinetics of absorption of carbon dioxide into aqueous solutions of 2-amino-2-methyl-1-propanol + monoethanolamine,” Chemical Engineering Science, 55(1), (2000).
Ali, S.H., “Kinetics of the Reaction of Carbon Dioxide with Blends of Amines in Aqueous Media Using the Stopped-Flow Technique,” International Journal of Chemical Kinetics, 37(7), , July 2005.

21. Kinetic Model (cont’d)
Overall reaction of CO2-MEA-AMP System
Apparent reaction rate
For the CO2 absorption into the blend MEA-AMP,
the reaction rate can be expressed as the overall rate of reaction.
The overall rate of reaction is a summation of the rate of reaction between CO2 and MEA…CO2 and AMP…, and… CO2 and Hydroxyl Ion.
After applying the zwitterion concept, we have the overall rate of reaction as shown in the second equation here. (POINT OUT)
And the correlations in the this box (POINT OUT), are the values used in this study.

22. Speciation [MEA], [AMP], [H2O], [OH-] CO2 Absorption Reaction
In order to find a model to fit with the experimental results, we need to know the concentration of MEA, AMP, water, and hydroxyl ion. Therefore, these 10 equations are generated and used to calculate the concentration of chemical species needed. (Carbamate from AMP react with CO2 is not accounted here since the carbamate from AMP is not stable and is quickly transformed to bicarbonate or carbonate ions.)

23. Comparison (Model & Experimental data)
Single AMP
Single MEA
Single AMP
Single MEA
Single AMP
Single MEA
Single AMP
Single MEA
Single AMP
Single MEA
Now, these figures show fitted data by using speciation calculation at each temperature.

24. Conclusions
The overall rate constant increases with the absolute temperature.
At the same mixing ratio, the overall rate constant increases when the total concentration increases.
An increase in MEA concentration in the blended solution causes the overall rate constant to change in a nonlinear manner.
Rate constant => 1:1 < 4:1 < 1:0 < 1:4 < 0:1 (MEA:AMP)
Existing model developed for low amine concentration provides reasonable prediction for single amine, but not for the blend.
The kinetics of CO2 absorption by blended MEA-AMP solution was investigated using a laboratory wetted wall column.
The experiments were carried out from 298K up to 318K with the molar MEA:AMP mixing ratio of 1:0, 4:1, 1:1, 1:4, and 0:1.
The kinetic data were presented in terms of the overall rate constant as a function of mixing ratio, temperature, and total concentration.
It was found that an increase in MEA concentration in the blended solution causes the rate constant to change in a nonlinear manner.
And also, the rate constant increases with the absolute temperature, which follows the Arrhenius’ law.
At the same mixing ratio, the overall rate constant increases when the total concentration increases.

25. Further work
Mechanism of CO2 absorption into MEA-AMP blended solution will be further investigated.
CO2-loaded solution will be tested.
Degraded solution will be tested.
Empirical correlation of absorption kinetics will be developed.
For the future work,
Carbonated solution will be tested.
And a mechanism explain the experimental results will be investigated.

26. Acknowledgement
Faculty of Graduate Studies and Research (FGSR), University of Regina
Faculty of Engineering, University of Regina
The Natural Sciences and Engineering Research Council of Canada (NSERC)
Faculty of Graduate Studies and Research (FGSR) and Faculty of Engineering, University of Regina, (are acknowledged for giving me a chance to pursue my study).
The Natural Sciences and Engineering Research Council of Canada (NSERC) is gratefully acknowledged for financial support.

27. Thank You