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Indian Journal of Chemical TechnologyVol. 17, November 2010, pp. 431-435
Absorption of carbon dioxide into a mixed aqueous solution of diethanolamine
and piperazine
M K Mondal
Department of Chemical Engineering and Technology, Institute of Technology, Banaras Hindu University,Varanasi 221 005, India
Email: [email protected]
Received 4 August 2009; revised 17 August 2010
The CO2loading in aqueous mixtures of diethanolamine (DEA) and piperazine(PZ), from a mixture of CO2and N2, hasbeen measured for total amine concentrations and mole ratios of PZ to total amine ranging from 2.0 to 3.0 M and 0.01 to
0.20, respectively, at 313.14 K and 15.199 kPa CO2 partial pressure. Measurements were made by a saturation method usinga laboratory scale bubble column. The results of CO2loading are expressed as XCO2(mole CO2/mole of total amine) for allexperimental runs. A model is given to predict the CO2loading in aqueous mixture of DEA and PZ. The model predictionshave been in good agreement with the experimental data of CO 2 loading in aqueous mixture of DEA and PZ with the
average deviation of 8.67.
Keywords: DEA, PZ, CO2Loading, Total amine, Bubble column
The removal of acidic gases (in particular carbon
dioxide and hydrogen sulphide) from a gas stream is
an area of industrial importance. Examples of suchstreams include natural gases, synthesis gases from
the gasification of coal and heavy oils, and tail gases
from sulphur plants and petroleum chemical plants.The removal of acid gases has a goal to increase the
industrial and commercial utility of the hydrocarbonstreams, reducing contaminant emissions to the
environment during the combustion of such streams,
to reduce the corrosion problems in equipment and
pipelines due to the presence of such acid gases, and
additionally to take advantage of these gases forapplications in other industrial processes, such as in
the sulphur production in the case of hydrogen
sulphide1. Due to their chemically active nature, these
acidic gases may be absorbed by a number of
different chemical and physical absorbents2
. Theremoval of acid gases from a gas stream is carried out
mainly by means of a chemical reaction rather than
only physical absorption in which different chemical
reactions occur between the acid gases and the
constituents of the aqueous solution3. Particularly, the
removal of carbon dioxide by using chemical
absorbents has been of great importance, since it was
found that the global warming effect is primarily due
to excessive discharge of carbon dioxide and
methane4. Aqueous alkanolamine solutions have been
extensively used for the removal of CO2 from gas
streams5. A wide variety of alkanolamines, such as
monoethanolamine (MEA), diethanolamine (DEA),di-2-propanolamine (DIPA) and N-
methyldiethanolamine (MDEA), has been used for
industrial gas treating processes6
. Aqueous MEAsolutions are the most frequently used absorbents
because of high reactivity to such chemicals as CO2.However, these solutions can also react with materials
in the reaction vessels, tubing lines, and several
process compartments. For this reason, highly MEA-
concentrated aqueous solutions should be avoided for
the CO2removal process.The use of amine blends may have the potential of
solving this problem. A blended amine solvent, which
is an aqueous blend of a primary or a secondary
amine with a tertiary or a hindered amine, combines
the higher equilibrium capacity of the tertiary orhindered amine, with the higher reaction rate of the
primary or secondary amine. Thus the use of blended
amine solvents, requiring lower circulation rates and
lower regeneration energy, can bring about
considerable improvement and great savings in
individual gas-treating processes7. The low vapour
pressure of DEA makes it suitable for low pressure
operations, as vaporization losses are quite negligible.
Besides DEA solution are in general, less corrosive
than MEA solutions. In view of this, DEA based
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INDIAN J. CHEM. TECHNOL., NOVEMBER 2010432
blends appear to be potential solvents for gas treatingprocesses. Xu et al.
8measured the solubility data for
CO2in 4.28 kmol/m3 MDEA with the Piperazine (PZ)
concentration ranging from 0 to 0.515 kmol/m
3
andCO2partial pressure ranging from 3.83 to 76.77 kPa.
Bishnoi and Rochelle9 showed that PZ has a large
effect on solubility when the ratio of total CO2to PZ
is less than unity.
Piperazine is an effective activator for an industrial
CO2removal process. However the absorption data of
CO2 in the aqueous blends of DEA with PZ is veryscarce. In this study, the absorption data of CO2 in
DEA-PZ-H2O solution was systematically determined
and represented by a simple model.
Experimental ProcedureMaterials
Reagent grade DEA was obtained from Sisco
Research Laboratory Pvt. Ltd., Mumbai with a purity
of 98%. PZ was obtained from SD Fine Chemical
Ltd., Mumbai with a purity of better than 99% and
was used without further purification. All solutions
were prepared with distilled water. The carbon
dioxide and nitrogen gases provided were of a
commercial grade with a purity of 99.5 mole%.
Apparatus and procedure
For the absorption of CO2into an aqueous blend of
DEA and PZ, a borosilicate glass bubble column was
used. The schematic diagram of experimental set-up
is shown in Fig. 1 to determine the experimental datafor this work. Carbon dioxide with a partial pressure
of 20.265 kPa from a gas cylinder is passed through
the gas rotameter at fixed flow rate and sent to thebubble column. The bubble column containing an
aqueous blend of DEA and PZ was submerged in a
constant temperature water bath. The temperature inthe bubble column was controlled within 0.1 K of
the desired level with a thermometer, and allmeasurements were done at atmospheric pressure.
The dimensions of various main units including
material of construction were reported elsewhere10
and the experimental conditions used in the presents
work are shown in Table 1. Before starting theexperiment, a constant temperature was attended
inside the bath to maintain the desired temperature
inside the content of the bubble column. After that the
main gas stream is slowly turned on and maintained
the minimum possible gas flow rate so that it was
bubbled through the liquid. Determination of the CO2
concentration in the gas phase was made for each run
with the help of microprocessor based CO2 analyzer
(UNIPHS 225 PM, United Phosphorous Limited,
Mumbai). The equilibrium state was assumed when
the outlet composition becomes equal to the inlet gas
composition. The equilibrium CO2 loading in the
liquid phase was determined by acidulating known
volume of the loaded liquid sample with 6 M HCl and
Fig. 1Experimental set-up for bubble column: 1- Gas cylinder (CO2and N2); 2- 2-Stage S.S. pressure regulator; 3- S.S. Valve; 4-Gasrotameter; 5-Gas mixing and pressure release chamber; 6- Bubble column; 7- Constant temperature water bath; 8- Glass Tee; 9- Moisture
trap column; 10- CO2analyzer; 11- Wet gas flow meter; 12- Flat bottom flask with absorbing solution
Table 1Experimental conditions used in the present work
CO2Partial pressure in inlet gas stream (kPa) 10.133-20.265Temperature of the liquid bed (K) 303.14-353.14Volume of the liquid (cm3) 500Height of the liquid bed (cm) 28.2
pH of the distilled water used 6.95
Mole ratio of PZ in total amine 0.01-0.20Total amine concentration (M) 1.0-4.0
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MONDAL: ABSORPTION OF CARBON DIOXIDE 433
measuring the volume of the evolved gas by aprecisely graduated gas burette. At a given
temperature and pressure, at least two liquid
equilibrium samples were taken in order tocheck reproducibility and the estimated error in
the measured data is about 0.5%. The temperatureof the liquid inside the bubble column was controlled
within 0.1 K upto 353.15 K. The total pressure
was measured for each run with an uncertainty
of 0.5 kPa upto 20.265 kPa.
Model development
The absorption of CO2into an aqueous blend of PZ
and DEA can be explained by a homogeneous
activation mechanism. The reaction of CO2 with PZ
can be regarded as the rapid pseudo-first order
reaction in parallel with that of CO2 with DEA.Piperazine contains two basic nitrogens and can
theoretically react with 2 mole of CO2. PZ could also
be protonated. Therefore, the effective free PZ,
although its concentration is very low, can transfer
CO2 to DEA as a homogeneous activator andobviously promote the CO2 absorption rate of
activation DEA aqueous solutions. In an aqueous
phase for the CO2-DEA-PZ-H2O system, the
following chemical equilibria are involved11
:
PZH+
PZ + H+
(1)
DEAH+ DEA + H+ (2)
H2O+CO2 H++ HCO-3 (3)
HCO-3 H
++ CO
2-3 (4)
H2O H++ OH- (5)
Experimental work in the present study is limited to
low partial pressure range, the gas phase is considered
to be ideal. In the low range of CO2partial pressure
the fugacity of CO2 becomes equal to its partialpressure and gas-liquid phase equilibria for CO2
may be described by Henrys law. In solution, the
equilibrium constants of the above mentioned
equilibrium reactions are:
K1 = CPZ CH+/ CPZH+ (6)
K2= CDEACH+/CDEAH+ (7)
K3= CH+CHCO3- /CCO2CH2O (8)
K4= CH+CCO32-/CHCO3
- (9)
K5= CH+COH-/CH2O (10)
The concentration of H+and OH-are rather low, so it
is reasonable to neglect their effects on the mass and
charge balance equations. Another assumption is that
all forms of the absorbed carbon dioxide are regardedas bicarbonate since the contents of CO3
2-and CO2are
also very low. Thus only three main dissociation
reactions are taken in consideration and are also
expressed as given by Eqs (6-8).
Mass and charge balances for the reacting speciescan be written as:
CPZ, total= CPZ+ CPZH+ (11)
CDEA, total= CDEA+ CDEAH+ (12)
XCO2= CHCO3_/CPZ, total+ CDEA, total (13)
CPZH++ CDEAH+= CHCO3
_ (14)
Gas-liquid equilibrium of CO2may be expressed as:
pCO2= HCCO2 (15)
The CO2 loading can be expressed as its partial
pressure by solving Eqs (6-8) and Eqs (11-15) and
given below:XCO2= 2K3pCO2/[2K3pCO2 + H {-b + (b
2 4ac)^(1/2)}]
(16)
where
a = (CPZ, total+ CDEA, total) (1 XCO2) (17)
b = (CPZ, totalK2+ CDEA, totalK1) (1 XCO2)
(CPZ, totalK1+ CDEA, totalK2) XCO2 (18)
c = - (CPZ, total+ CDEA, total) K1K2XCO2 (19)
The thermodynamic equilibrium constants and
Henrys law constant are taken from literature and
presented in Table 2.
Table 2Temperature dependence of the equilibrium constants and Henrys constant
Parameter Expression Reference
K1 ln K1= -11.53 4345.5/T Pagano et al.12
K2 ln K2= 0.0099 13.3373/T 4218.7 ln T Austgen et al.13
K3 ln K3= -241.828 + 29.825 104/T 1.485 108/T2 + 0.333 1011/T3 0.282 1013/T4 Kent and Eisenberg14
H lnH= 20.267 1.383 104/T+ 0.069 108/T2 0.016 1011/ T3+ 0.012 1013/ T4 Kent and Eisenberg14
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INDIAN J. CHEM. TECHNOL., NOVEMBER 2010434
Results and Discussion
Comparison of CO2 loading of present study with literature
data
In literature, a lot of amine blends had been workedout for measuring CO2 loading. Among all of
previous works, some of amine blends are comparedwith the experimental data of present study for same
experimental conditions such as partial pressure of
CO2in inlet gas stream, total concentration of amine
blend and mole ratio of an amine constituent in blend.
For comparison purpose, blends consisting either
DEA or PZ as one of the component has been used.
The CO2 loading comparisons of experimental dataof present study with the other blends are taken
at total amine concentrations of 2.0, 2.5 and 3.0 M,
respectively. The CO2loading comparisons are shown
in Figs 2 to 4. From Figs 2-4, it is seen that CO 2
loading is highest for DEA-PZ blend of present study
at all total concentration range 2.0-3.0 M with
experimental conditions of 313.14 K and 15.199 kPa
CO2 partial pressure in comparison with other
blends available in literature. Therefore, DEA-PZblend for present study is assumed to be superior to
other blends available in literature viz. DEA-AMP,
MDEA-PZ, DEA-MDEA, TIPA-PZ regarding CO2loading at experimental conditions of low temperature
and low CO2partial pressure.
Comparison between the experimental and model values
The model has been solved directly without
iterative calculation. In the model, CO2 loading
is expressed interms of partial pressure of CO2in inlet gas stream, Henrys law constant, equilibrium
constants of the reactions, and total concentrations
of DEA and PZ. Henrys constant and equilibrium
constants are function of temperature only, and
Fig. 4Comparison of CO2 loading in 3 M DEA-PZ blend withother 3 M blends available in literature
Fig. 5Comparison of the experimental solubility of CO2(XCO2)
and model values
Fig. 2Comparison of CO2 loading in 2 M DEA-PZ blend withother 2 M blends available in literature
Fig. 3Comparison of CO2loading in 2.5 M DEA-PZ blend with
other 2.5 M blends available in literature
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MONDAL: ABSORPTION OF CARBON DIOXIDE 435
can be calculated using equations as given inTable 2. From Eqs (17)-(19), the terms a, b and c
can be dependent on XCO2 only after knowing the
values of K1, K2 and total concentrations of DEAand PZ. Thereby, finally using Eq. (16) and with
the help of Henrys constant and equilibriumconstant K3 and pCO2, the values of XCO2 can be
obtained. A comparison of the experimental value
XCO2, exp and the model value XCO2, mod for the CO2
loading is shown in Fig. 5. As shown in Fig. 5, the
model value is in good agreement with theexperimental value with a mean deviation of 8.67%.
ConclusionIn this work, the CO2 loading has been measured
into mixed solutions of DEA and PZ within total
concentration range 2.0-3.0 M and with mole
ratios of PZ to total amine of 0.01-0.20 at 313.14 K
and 15.199 kPa CO2 partial pressure. CO2 loading
in the concentration range 2.0-3.0 M of DEA-PZ
blends of present study has been found to be
superior to other blends such as DEA-AMP,
MDEA-PZ, DEA-MDEA, TIPA PZ available
in literature. The data obtained from model is
agreed well with the experimental data of CO2
loading. The average deviation between the
calculated and experimental CO2 loading data
is 8.67%.
AcknowledgementAuthor gratefully acknowledges Banaras Hindu
University for providing the necessary help for
carrying out the present work.
NomenclatureCi = concentration of solute i, kmol/m
3
H = Henrys law constant, kPa.m3/kmol
K1 K5 = equilibrium constants
pCO2 = CO2partial pressure, kPaXCO2 = CO2loading, mole of CO2/mole of total amine
Subscriptsi = species or component
exp = experimental
mod = model
Amine abbreviationsAMP = aminomethylpropanol
DEA = diethanolamine
MDEA = methyldiethanolamine
PZ = piperazine
TIPA = triisopropanolamine
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