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

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

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^-,
CO₃^2- in the CO₂ loading range of 0-0.7 mol CO₂/mol
amine at the temperature of 298K. The concentrations of MEACOO^-,
DEACOO^-, HCO₃^-, CO₃^2- are
calculated⁶ as follows:

Where
R_i (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^-/CO₃^2-
in the mixed amine solution,
168.84 and 161.23 are the chemical shifts of pure CO₃^2-
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 CO₃^2- 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^-,
CO₃^2- 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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