(719g) The Research of Coordinative and Competitive Relationship of CO2 Absorption into MEA and DEA in Blended Aqueous Amines

Authors: 
Li, M., Hunan University
Liu, H., Hunan University
Liang, Z., Hunan University
Idem, R., Hunan University
Tontiwachwuthikul, P., Hunan University

American Institute of Chemical Engineers (AIChE)
Annual Meeting

The research of coordinative and competitive
relationship of CO2 absorption into MEA and DEA in blended aqueous
amines

Moxia Li, Helei Liu, Zhiwu Liang *, Raphael
Idem, Paitoon
Tontiwachwuthikul

Joint International Center
for CO2 Capture and Storage (iCCS), Hunan Provincial Key Laboratory
for Cost-effective Utilization of Fossil Fuel Aimed at Reducing CO2
Emissions, College of Chemistry and Chemical Engineering, Hunan University,
Changsha 410082, PR China

Keywords:
Blended amines; CO2 absorption; NMR;
Coordinative and competitive

*
Author for correspondence: Dr. Zhiwu Liang - Email: zwliang@hnu.edu.cn

 Tel: +86-13618481627 and +86-73188573033

ABSTRACT

The extensive usage of fossil fuels by mankind is
resulting in increased levels of CO2 in the atmosphere, which causes
not only the greenhouse effect, but also leads to global warming. In order to
decrease the emission of CO2 by human activities, it is urgent that CO2
capture technology and resource utilization technology be rapidly developed. Meanwhile,
CO2 that is captured can also be used to provide economic benefit to
many processes in industrial production (e.g. Enhanced Oil Recovery and Carbonated
Beverage).1

The technology of chemical absorption with amine
solvents, proposed in the 1930s, is widely studied and has become one of the most
important industrial methods for CO2 capture due to its fast
absorption rate, easy desorption, and low corrosion.2 Monoethanolamine (MEA)
and diethanolamine (DEA) are the
most common commercial solvents for CO2 capture. However, there are
some disadvantages with these amine solvents. Even
though MEA has a very fast CO2 absorption rate, the cost of handling,
the corrosivity,
and the degradation
rate of MEA are high.
MEA, a primary amine, is sometimes substituted by diethanolamine (DEA), a secondary
amine, which has a lower reaction enthalpy and is less corrosive.
Some researchers have proposed mixed solvents containing the advantages of
different amines as the solvent for CO2 absorption.3

However, the process of the mixed amine reacting
with CO2 is very complex due to a coordinative and competitive
relationship between the different types of amines as they react with CO2.4,5 A better understanding of the reaction mechanisms
for the mixed amine reacting with CO2 plays a vital role in the
design of processes for mixed amine absorption of CO2. In this work,
the Nuclear Magnetic Resonance (NMR) technology was employed to present the
process of the reaction of a mixed solvent (MEA and DEA) with CO2.
The process of the reaction with 2M MEA:DEA (1:1) was represented in terms of
concentrations of MEACOO-, DEACOO-, HCO3-,
CO32- in the CO2 loading range of 0-0.7 mol CO2/mol
amine at the temperature of 298K. The concentrations of MEACOO-,
DEACOO-, HCO3-, CO32- are
calculated6 as follows:

Where
Ri (i=0, 1, 2) represents the ratio of the integral of the peak area
for different carbon, [ ] is concentration of species in this system, [CO2]0
represents initial CO2 concentration of the mixed amine solution, ¦Ä represents the chemical shift of HCO3-/CO32-
in the mixed amine solution,
168.84 and 161.23 are the chemical shifts of pure CO32-
in Na2CO3 solution, and HCO3- in
NaHCO3 solution, respectively.

From Figure 1, it can be seen that MEA/MEAH+ have
two different carbons, which show different chemical shifts. However, MEACOO-
would show three different chemical shifts from MEA due to the existence
of the -COO- group. From Figure 2, DEA/DEAH+ has four
carbons of two different bonds, which are different from carbons of MEA. Also, DEACOO-
has three different carbons because of -COO- group. In this present
work, the
aqueous blend of MEA and DEA reacting with CO2 has been studied with CO2 loading from 0 to 0.7 mol
CO2/mol amine using NMR technology at 298K. From Figure 3 and
Figure 4, it can be seen that MEACOO- occurred as soon as CO2
was absorbed by the solution. Meanwhile, DEACOO-, and HCO3-
and CO32- have not yet appeared. The concentration
of MEACOO- increases with CO2 loading. This means that CO2
reacts only with MEA at the low CO2 loading because MEA is a
stronger base compared to DEA and has a stronger ability to react with CO2
The chemical shift of DEACOO- can be seen at the CO2
loading of 0.17 mol CO2/mol amine, which means that DEA has now begun
to react with CO2. In order to develop the ion speciation plots, the
concentrations of MEACOO-, DEACOO-, HCO3-,
CO32- are calculated using equations 1-8, as shown in
Figure 4. In Figure 4, it can be found that both MEA and DEA reacted with CO2
indicating that MEA and DEA have a coordinative relationship in reacting with CO2.
It can be clearly seen that MEA reacts with CO2 first. When the CO2
loading reaches a certain level (about 0.17 mol CO2/mol amine in
this work), DEA begins to react with CO2. This shows that MEA and
DEA in the mixed solvent system react with CO2 in sequence. Above 0.17
mol CO2/mol amine, MEA and DEA compete with each other in the
process of reacting with CO2.

To conclude, MEA and DEA have both coordinative and competitive
relationships when mixed as CO2 absorbents. This investigation provides
some useful information to understand the reaction mechanism of CO2 being
absorbed into mixed amines.

Figure 1. The structure of MEA
/MEAH+ and MEACOO-

Figure 2. The structure of DEA
/DEAH+ and DEACOO-

Figure 3. 13CNMR spectrum of
MEA-DEA-CO2-H2O, as a function of the CO2
loading at the temperature of 298K.

Figure 4. Ion speciation plots of MEA-DEA-CO2-H2O
system at the temperature of 298K.

Acknowledgment: The financial supports from
the National Natural Science Foundation of China (Nos. 21476064, 21376067 and
U1362112), National Key Technology R&D Program (Nos. 2012BAC26B01
and2014BAC18B04), Innovative Research Team Development Plan-Ministry of
Education of China (No. IRT1238), Shanxi Yanchang Petroleum (Group) Co., LTD,
Specialized Research Fund for the Doctoral Program of Higher Education (No.
20130161110025),and China's State "Project 985" in Hunan University-Novel Technology
Research and Development for CO2 Capture as well as Hunan University
to the Joint International Center for CO2 Capture and Storage (iCCS)
is gratefully acknowledged.

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