Andrzej WILK, Lucyna WIĘCŁAW-SOLNY, Aleksander KRÓTKI, Dariusz ŚPIEWAK – Centre for Process
Research, Institute for Chemical Processing of Coal (IChPW), Zabrze, Poland
Please cite as: CHEMIK 2013, 67, 5, 399–406
Introduction
The amine absorption processes are widely used in the industry
to purify refinery gases, process gases or natural gas. Recently, amine
absorption has also been considered for application in CO2 capture
from exhaust fumes created as by-product of electric power generation
from fossil fuels.
Amine absorption is based on a reversible reaction between weak
bases, e.g. ethanolamine, and acidic gases, including i.a. carbon dioxide.
CO2-containing gas (e.g. exhaust fumes) is fed into the absorption
column, where it comes in counter flow contact with unsaturated
sorption solution. Carbon dioxide is absorbed by the solution, while the
gas leaving the column is purified. The solution with absorbed CO2 is
pre-heated with a solution regenerated in the heat exchanger, and then
directed to the regenerator. In the regenerator, in the supplied heat,
the amine-CO2 compound decomposes, eliminating carbon dioxide
from the solution. Thus, a stream of virtually pure CO2 is obtained.
The regenerated sorbent solution is redirected, via heat exchanger,
back to the absorption unit. A simplified flow sheet of the process is
provided in Figure 1.
but at the same time by high desorption heat levels. Furthermore,
primary amines are less resistant to thermal and oxidative
degradation than tertiary amines or amines with steric hindrance
[10, 12]. Secondary amines in most cases are characterised by slightly
lower absorption kinetics, but are more resilient to degradation and
produce lower desorption heat. Tertiary amines are the slowest
to absorb CO2, produce the lowest desorption heat and feature high
resilience to degradation [15]. The final group, i.e. amines with steric
hindrance, combine the advantages of primary and tertiary amines.
Due to their structure, they are characterised with relatively high
absorption rate, with lower susceptibility to degradation and lower
desorption heat that primary amines [8].
Individual amine groups can be combined to obtain solutions
with the best characteristics, both in terms of kinetics and absorption
capacity. Aside from mixing amines, good results can be obtained
also by using amine blends with physical CO2 sorbents, e.g. with
propylene carbonate [16]. These are some of the advantages of
using such compounds:
• the composed solution has a lower vapour pressure than both pure
physical sorbent and aqueous amine solution
• requires less energy to regenerate the sorbent than amine/H2O
compounds
• increases CO2 solubility in the absorbing solution as compared
to pure physical sorbent and aqueous amine solution.
During the CO2 absorption process in alkanolamine solutions
a number of chemical reaction occurs. We can distinguish 5 main
reactions [4]:
• amine protonation reaction:
RR’NH+H3O+ ⇔ RR’NH2++H2O
(1)
• carbamate formation reaction:
Fig. 1. Simplified flow sheet of process of CO2 absorption
in amine solutions [1]
One major disadvantage of amine absorption processes is the
high energy consumption, arising from high energy levels required
to regenerate the sorbent. There is a number of ways to reduce the
energy consumption: by appropriate configuration of the technological
process line [1], integration of CO2 capturing unit with the power
plant system [2], choice of optimal process parameters or optimal
sorbent [12] with the best possible kinetic and equilibrium parameters,
and the lowest absorption heat and specific heat.
Commercially available technologies usually use amines such
as: monoethanolamine (MEA) – Econamine FG or Kerr-McGee/
ABB Lummus Crest process [3], diethanolamine (DEA) – SNEA
DEA process [1], N-methyldiethanolamine (MDEA) BASF aMDEA
[11, 14], and amines with steric hindrance, e.g. 2-amino-2methylpropan-1-ol (AMP) – KM-CDR Process [10]. Among those
amines, each has a different, individual set of properties. Primary
amines, such as MEA, are characterised by high absorption rate,
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RR’NH+HCO3– ⇔ RR’NCOO–+H2O
(2)
• CO2 dissociation reaction:
CO2+2H2O ⇔ H3O++HCO3–
(3)
• bicarbonate dissociation reaction:
HCO3– +H2O ⇔ H3O++CO32–
(4)
• water dissociation reaction:
2H2O ⇔ H3O++OH–
(5)
Those reactions can be reduced to two fundamental reactions
that can be used to describe the CO2 absorption process in any
amine type:
• reaction of CO2 and amine, resulting in carbamate formation:
CO2+2RR’NH ⇔ RR’NCOO–+RR1NH2+
(6)
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Impact of the composition of absorption blend
on the efficiency of CO2 removal
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• reaction of CO2 and amine resulting in bicarbonate formation:
CO2+H2O+RR’R1’H ⇔ NCO3–+RR1R1’NH+
(7)
The first of the above reactions is characteristic for primary and
secondary amines. The second reaction occurs for all types of amines
– in primary and secondary amines only to a minor extent, while in
tertiary amines and amines with steric hindrance, where the structure
of the molecule prevents the formation of stable carbamate, it is the
main reaction [7, 13].
Equations (6) and (7) also provide way of explaining the
phenomenon of CO2 absorption process activation in tertiary amine
solutions by using small additions of quick-reacting amines, such as
monoethylamine or piperazine.
In the case of tertiary amine solution, the reaction limiting the
absorption rate is the slow reaction of dissociation of dissolved
carbon dioxide (3). However, if the solution contains both tertiary
and primary amines, then it is highly probable that one molecule of
the tertiary amine and one of the primary amine will participate in
the reaction with the carbon dioxide molecule. Due to its structure,
tertiary amines undergo protonation, while primary amines form
carbamate anions. The formed carbamate may undergo hydrolysis,
thus creating a bicarbonate anion and a primary amine molecule is
released, which can then again react with carbon dioxide. Thus, in the
process of bonding carbon dioxide by tertiary amines, the free stage
of CO2 dissociation is omitted. The diagram of reactions occurring in
the solutions of activated tertiary amines is provided in Figure 2.
Fig. 2. Simplified diagram of reactions occurring during carbon dioxide
absorption in solutions containing primary and tertiary amines
Experimental Stage
Reagents used
The following reagents were used for laboratory analysis:
ethanolamine for synthesis, ≥99%, MERCK; N-methyldiethanolamine,
99+%, ACROS ORGANICS; piperazine, ≥99%, MERCK; 2-amine2-methyl-1-propanol, ~95%, MERCK; N-methyl-2-pyrrolidone,
≥99.5%, MERCK and carbon dioxide 4,5, Linde Gaz Polska.
Testing apparatus and experiment procedure
The analysis involved testing the impact of the activator quantity
on the absorption capacity and CO2 absorption rate in tertiary amine
solutions. Also tested was the impact of the amount of organic liquid
in the primary amine / amine with steric hindrance / activator / organic
liquid / water compound (as specified in previous tests) [5, 6] on the
equilibrium and kinetic parameters of the solution.
All tests were conducted in specially designed laboratory station
for analysing equilibriums and kinetics of CO2 absorption in amine
blends (Photo 1).
404 •
Photo 1. Apparatus for studying the absorption kinetics and
equilibriums in amine blends: 1 – cryostat; 2 – magnetic stirrer;
3 – vacuum pump; 4 – thermostated reactor; 5 – cylindrical
separatory funnel; 6 – vacuum gauge; 7 – valve station;
8 – automatic pressure recording system
The laboratory station included a 2.5 dm3 thermostated glass
reactor, equipped with cryostat to maintain temperature during the
test, a cylindrical separatory funnel for liquid sample of the sorbent, and
the system for measuring and automatic recording of pressure in the
apparatus. The vacuum gauge enabled filling the apparatus with carbon
dioxide or removing gas from the apparatus using a vacuum pump.
The tests performed with the laboratory station included
establishing the isotherms of CO2 absorption in amine solutions in
order to obtain data on the equilibrium absorption capacities, as well
as testing the CO2 absorption rate.
For the purpose of equilibrium tests, samples of the absorbing
solution were introduced into the apparatus several times. After
instilling each sample the researchers waited for the equilibrium
pressure in the system to stabilise. On the basis of the obtained points,
the curve of correlation between the CO2 equilibrium pressure and
the amount of carbon dioxide absorbed in 1 dm3 of solution was
drawn, or per mol of amine groups. The tests of absorption kinetics
included a single instillation of a solution sample into the system and
recording the pressure fluctuations for approx. 10 minutes. Thus, it
was possible to determine the correlation between the amount of
absorbed CO2 and the process duration. All tests were performed in
temperatures of 20–60°C and for partial pressures of CO2 in the range
of 0–90 kPa for each test. The stirrer rotated at approx. 750 rpm. For
the set rotation speed the general absorption rate was conditioned
by the chemical reaction rate, and the absorption process ran in the
kinetic area instead of the diffusion course of the reaction.
The correctness of obtained results was verified
by comparing the data from previous tests for aqueous solutions
of N-methyldiethanolamine with other literature data and CO2
absorption models in tertiary amines [9].
Analysis of results
As part of the analysis of the tertiary amine/activator/H2O
compounds, tests were performed on the following solutions: 30%
MDEA; 50% MDEA; 30% MDEA + 2 (4, 6, 8, 10, 12 and 20%) PZ.
The comparison of the obtained data indicated that, as anticipated,
the use of activator in the carbon dioxide absorption in tertiary amine
solutions (MDEA) has a beneficial effect both on the process kinetics
and the absorbing equilibrium.
In the case of absorbing equilibriums, a close correlation
is visible between the amount of activator and the solution’s
absorption capacity. The larger the activator addition, the higher the
equilibrium absorption capacity for the given carbon dioxide partial
pressure. As indicated by the data (Figs. 3 and 4), the correlation
becomes much more prominent in higher process temperatures.
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In the carbon dioxide absorption rate the impact of the amount
of activator does not appear to be as prominent as in absorption
equilibriums. The increase of CO2 absorption rate is visible for a 2%
addition of activator: increasing the concentration to 4% results in
a considerable increase of the absorption rate, however, further
rise of activator concentration does not significantly increase the
values of kinetic parameters. Both the solutions containing 4% and
12% w/w of activator have similar absorption rates. Furthermore,
due to limited solubility of piperazin in water-amine blends, it is
beneficial to apply as low activator amounts as possible, since
with piperazin concentration in the sorbing solution of approx.
20% w/w, the activator undergoes partial crystallisation already
at 30°C. The results of the absorption kinetics tests in MDEA/PZ/
H2O are presented on the charts below (Figs. 5 and 6).
Fig. 6. Comparison of carbon dioxide amount absorbed during
the process at 50°C
The analysis of the impact of the volume of organic liquid and water
in the solution was performed on the compounds: primary amine/amine
with steric hindrance/activator/organic liquid/water. The performed
analysis included tests of equilibriums and kinetics of absorption for
solutions with the following mass concentrations of water/organic
liquid: 63/0, 53/10, 43/20, 33/30, 23/40, 13/50.
As shown by the equilibrium data provided in Figure 7, the volume
of organic liquid has a minor impact on the degree of carbonisation of
the solution and thus on the absorption capacity. Increasing the amount
of the organic liquid improves the absorption capacity of the solution
to a small extent.
Fig. 3. Correlation between partial pressure over solution and
the amount of CO2 absorbed in solution for different activator
concentrations at 30°C
Fig. 7. Correlation between CO2 partial pressures and the equilibrium
absorption capacity for solutions with different concentrations
of organic liquid and water at 30oC
Fig. 4. Correlation between partial pressure over solution and
the amount of CO2 absorbed in solution for different activator
concentrations at 50°C
Fig. 8. Comparison of the carbon dioxide amount absorbed in time
for solutions with different water/organic liquid ratio at 30°C
Fig. 5. Comparison of carbon dioxide amount absorbed during
the process at 30°C
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Unlike the equilibrium data, the data obtained from the tests of
absorption kinetics indicate a major impact of the volume of organic
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liquid and water on carbon dioxide absorption rate. Using a minimum
amount of water (ok. 13%) considerably increases the CO2 absorption
rate, which is higher in this case even than with the widely used 30%
monoethylamine solution. This arises from the fact that the absorption
reaction is directed at carbamate formation due to limited amount
of water and thus inability to form a bicarbonate. The correlation
between the carbon dioxide absorption rate and the concentration of
the organic liquid in the solution is provided in Figure 8.
Summary
The obtained results indicate a considerable impact of the
appropriate solution composition on the equilibrium parameters and,
more importantly, on CO2 absorption rate. Based on the solution
developed in previous analyses, it was possible to obtain, by appropriate
selection of concentration of organic liquid and water, a sorbent
characterised by kinetic parameters that are comparable or – for some
concentrations – even better than the ones in one of the most widely
used solution, i.e. 30% MEA. Furthermore, using an organic liquid
reduces the vapour pressure of the solution and thus its losses, and
reduces the amount of energy consumed in the regeneration process
due to its specific heat, which is several times lower than water’s.
The analysis of the impact of the activator amount indicated that for
MDEA/PZ compounds a considerable increase of the absorption rate
can be obtained already with small activator amounts, i.e. 4–6% w/w.
The kinetic data obtained from the analysis are only a set of comparative
data used to choose the best among the tested CO2 sorbents. To apply
the kinetic data e.g. in the design of CO2 capture systems, it is necessary
to perform further study to comprehensively assess the kinetics of the
absorption which, aside from the chemical reaction, includes also other
partial phenomena, e.g. diffusion [17, 18].
The results presented in this paper were obtained through analysis
co-financed by the National Centre for Research and Development
under agreement SP/E/1/67484/10- Strategic Research Programme
– Advanced Technologies for Energy Generation: Developing a technology
for high efficient zero emission coal blocks integrated with CO2 capture
from exhaust gases, and under agreement SP/E/2/66420/10 – Strategic
Research Programme – Advanced Technologies for Energy Generation:
Developing a technology of oxyfuel combustion for pulverized fuel and
fluidized-bed furnaces integrated with CO2 capture system.
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Andrzej WILK – M.Sc., graduated from the Faculty of Chemistry of the
Silesian University of Technology in Gliwice, majoring in chemical technology
(2010). He is also a graduate of postgraduate studies at the Faculty of Power
and Environmental Engineering of the Silesian University of Technology in
Gliwice (2012), majoring in waste management. He is currently employed
as engineer in the Institute for Chemical Processing of Coal in Zabrze.
Specialisation: chemical technology and environmental protection.
e-mail: [email protected]; phone: +48 517 178 510
Lucyna WIĘCŁAW-SOLNY – Ph.D., Eng. graduated from the Faculty of
Chemistry of the Silesian University of Technology (1998). She defended her
doctoral thesis “Obtaining catalytic coatings on metallic surfaces” in 2004. She
specialises in chemical and process engineering. She is currently employed in
the Institute for Chemical Processing of Coal in Zabrze as the deputy head of
the Centre for Process Research.
Aleksander KRÓTKI – M.Sc., graduated from the Faculty of Chemistry
of the Silesian University of Technology in Gliwice (2010). She is currently
employed in the Institute for Chemical Processing of Coal in Zabrze.
Specialisation – chemical industry and environmental protection apparatus.
Dariusz ŚPIEWAK – M.Sc., graduated from the Faculty of Chemistry of the
Silesian University of Technology in Gliwice (2011). He is currently employed
in the Institute for Chemical Processing of Coal in Zabrze. Specialisation
– chemical and process engineering.
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