(398a) Investigation of CO2 Desorption Performance in Tri-Solvent Blends (MEA-AMP-PZ) with and without Catalyst

Investigation of CO₂ desorption performance
in tri-solvent blends (MEA-AMP-PZ) with and without catalyst

Xiaowen Zhang ^a, Helei Liu^b*, and Zhiwu
Liang ^a,b*

^a
Joint International Center for CO₂ Capture and Storage (iCCS),
Provincial Hunan 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

^b
Clean Energy Technologies Research Institute (CETRI), Faculty of Engineering
and Applied Science, University of Regina, Regina, Saskatchewan,
S4S 0A2, Canada

Abstract

Carbon dioxide (CO₂) is widely considered
as a predominant contributor of greenhouse gases. Thus, it is an urgent to
mitigate CO₂ from atmosphere. Chemical
absorption technology using amine based solvent for capturing CO₂ has
attracted the most attention because of its cost-effectiveness, maturity and absorption
large volumes of flue gas. However, a large quantity of energy is demanded during CO₂
stripping which accounts for about 70-80% of the total cost of the CO₂
capture system. The high energy penalty accompanying the high running cost is
the biggest barrier for post-combustion CO₂ capture by using chemical
absorption.

Considerable research efforts have been devoted to reducing
the regeneration energy requirement. Recently, some researchers have reported
that the development of amines tri-solvent blends for CO₂ capture is
an effective method. Because it could not only dramatically utilize the potentials
of the individual amine, but also address their individual problems. Nwaoha et
al.¹ studied the tri-solvent blend with two absorption rate promoters (i.e.
piperazine (PZ) and monoethanolamine (MEA)) and a hindered amine 2-amino-2-methyl-1-propanol
(AMP). The results indicated that the tri-solvent blends amines had faster desorption
rate and lower regeneration heat duty in comparison with the commercial amine
MEA. Based on their work, the tri-solvent blends amines could be considered as
one of potential novel absorbents for CO₂ capture in post-combustion
processes. However, the heat duty of tri-solvent blends amines is still high. Idem et al. proposed that the addition of solid acid catalysts such
as H-ZSM-5 into rich amine solution for CO₂
stripping is an efficient method to decrease the heat duty². However, the study of regeneration behavior of the amine tri-solvent
blends with the catalysts is not available. Hence, the investigation on the tri-solvent blends amines with
catalyst is interesting to present in order to further reduce of solvent
regeneration energy requirement. In the present study, the test of MEA-AMP-PZ tri-solvent
blends with four different acid catalysts (e.g. HZSM-5, gamma-Al₂O₃,
SAPO-34, and SO₄^2-/TiO₂) and without catalyst were investigated in
a batch reactor for understand the process of regeneration.

In this work, the experiments regeneration was studied
by using a batch reactor, which was displayed
in Figure 1. 2 L rich amine solutions with the desired CO₂ loading
were introduced into a 3 L four-necked flask, which was immersed in the oil
bath (DF-101T). The rich CO₂ loading of all amines was set at their
equilibrium CO₂ loading at absorption conditions of 313 K and 15.1% CO₂
partial pressure^1,2. 25 g catalyst were put
into the amine solution. The time was not recorded until the amine solvent
temperature reached at 338 K. The CO₂ loadings of solution at
time of at 0.25, 0.5, 0.75, 1, 1.5, 2, 4, 6, and 9 h were measured by using a Chittick
apparatus³. The heat duty for the CO₂ desorption was measured by an
electric energy meter.

Figure 1. Schematic diagram of the batch reactor for CO₂
desorption process

The heat duty (H, kJ/mol) is could be obtained
by equation (1) .

           (1)

The relative heat duty (RH, %) was used in this work. The
heat duty of 5M MEA without catalyst regeneration in first 2 h is considered as
H_baseline(kJ/mol). The heat duty of new systems regeneration in
first 2 h was considered as H_i (kJ/mol). RH was calculated as the
following equation (2).

                     
(2)

The typical CO₂ desorption profile of the
four amine systems (e.g. 5M MEA, 3M MEA-2.5M AMP-0.5M PZ, 3M MEA-2M AMP-1M PZ and 3M MEA-1.5M
AMP-1.5M PZ) at the regeneration temperature of 369 K is shown in Figure 2. It is clear that all the studied
tri-solvent blends show a faster desorption rate in comparison with the 5M MEA.
The 3M MEA-2.5M AMP-0.5M PZ system shows the fastest CO₂
desorption rate. As shown in Figure 2, the CO₂ loading decreased dramatically
in the first 2 h. In this work, the heat duty was calculated by considering the
first 2 h of the regeneration process.

Figure 2. CO₂ desorption in single and
tri-solvent blends amines without catalyst at 369 K.

Figure 3 depicts the relative energy requirement of
amine solvent regeneration. It is noted that all the studied tri-solvent blends
show much lower regeneration heat duty than 5M MEA. The 3M MEA-2.5M
AMP-0.5M PZ with AMP/PZ molar
ratio of 5 requires the lowest regeneration heat duty. It could be found that
the heat duty decreased with the increase of concentration of the AMP. Based on
this result, the system of 3M MEA-2.5M
AMP-0.5M PZ could be considered as the best blend solvent in term of CO₂
desorption performance.

Figure 3. Relative heat duty for MEA-AMP-PZ
tri-solvent blends and 5M MEA without catalyst at 369 K.

To further accelerate the CO₂ desorption
rate and reduce the heat duty, the regeneration behavior of 3M MEA-2.5M
AMP-0.5M PZ system studied by adding four different catalysts (e.g. H-ZSM-5, gamma-Al₂O₃, SAPO-34, and SO₄^2-/TiO₂).
The performance of CO₂ desorption curves of this system with and
without catalyst are shown in Figure 4. It can be seen that the lean loading
value of different systems declined to be the same, which means that the
catalyst can only change the reaction rate but will not alter the chemical
equilibrium. However, the addition of four acid catalysts play an positive
effect on CO₂ desorption rate. The result proved that the catalyst
can only accelerate the reaction rate without the effect on the chemical
equilibrium. Figure 5 displays
the relative regeneration heat duty of different amine-catalyst systems. The CO₂ desorption performance in
terms of the relative heat duty for different catalysts could be ranked as: H-ZSM-5 (38.4%) > gamma-Al₂O₃(40.1%)
> SAPO-34 (41.0%) > SO₄^2-/TiO₂ (42.8%) > the blank (48.1%). The result indicated
that four catalysts further accelerated the CO₂ desorption rate and reduced
the heat duty in comparison with the blank test, and H-ZSM-5 showed the best
catalytic performance of CO₂ desorption. The addition of acid
catalyst into the rich loading amine solution results in increasing proton concentration,
which could react with the carbamate
(i.e. Amine-COO^-). Thereby, the improvement of CO₂
desorption rate and the decrease of the heat duty were observed.

Figure 4. CO₂ desorption in 3M MEA-2.5M AMP-0.5M PZ with and
without catalyst at 369 K.

Figure 5. Relative heat duty for 3M MEA-2.5M AMP-0.5M
PZ with and without catalyst at 369 K.

Acknowledgment: The financial support from the National
Natural Science Foundation of China (Nos. 21476064, 21376067 and U1362112), National
Key Technology R&D Program (Nos. 2012BAC26B01 and 2014BAC18B04), Innovative
Research Team Development Plan-Ministry of Education of China (No. IRT1238), 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) are gratefully
acknowledged.

References

1.            Nwaoha
C, Saiwan C, Tontiwachwuthikul P, et al. Carbon dioxide (CO₂)
capture: Absorption-desorption capabilities of 2-amino-2-methyl-1-propanol
(AMP), piperazine (PZ) and monoethanolamine (MEA) tri-solvent blends. Journal
of Natural Gas Science and Engineering. 2016;33:742-750.

2.            Shi
H, Naami A, Idem R, Tontiwachwuthikul P. Catalytic and non catalytic solvent
regeneration during absorption-based CO₂ capture with single and
blended reactive amine solvents. International Journal of Greenhouse Gas
Control. 2014;26:39-50.

3.            Horwitz
W. Association of official analytical chemists (AOAC) methods. George Banta
Company, Menasha, WI. 1975.