Download Theoretical Calculation of Enthalpy of reactions involved in PZ

2nd Post Combustion Capture Conference (PCCC2)
Theoretical Calculation of Enthalpy of reactions involved in PZCO2-H2O system at infinite dilution
Mayuri Guptaa, Eirik Falck da Silvab, Ardi Hartonoa, Hallvard F. Svendsena,*
a
Department of Chemical Engineering, Norwegian University of Science and Technology, Trondheim, Norway
b
Department of Process Technology, SINTEF Materials and Chemistry,
Trondheim, Norway
Keywords: Enthalpy, Piperazine, CO2 capture, Equilibirum constant, Solvation models
1. Introduction
Heat of absorption is a very important thermodynamic parameter in CO2 removal using amine solvent solutions
in post combustion temperature swing processes. The temperature dependency of the heat of absorption for such
processes can be calculated from the theoretical equilibrium constants [1] for each of the reactions involved in the
absorption sequence.
2. Methods
Reaction energies for various reactions involved in chemistry of PZ reacting with CO2 are studied with density
functional theory. Free energy of solution is anchored to experimental value at room temperature [1]. Temperature
dependency of these reaction energies are studied with the help of SM8T continuum solvation model. Piperazine
undergoes the following reactions in aqueous solution with CO2.
↔
↔
↔
* Corresponding author. Tel.: +47-73-594-100; fax: +47-73-594-080.
E-mail address: [email protected]
.
(1)
(2)
(3)
2
↔
(4)
The temperature dependency of the equilibrium constants and the Henry's law constant can be expressed in the form
[2]
ln
ln
(5)
We know that free energy of reaction can be expressed as
ln
∆
(6)
Combining the well-known Gibbs Helmholtz equation [3] with equations 8 and 9, the enthalpy of the overall
reaction can be expressed as
∆ ⁄
∆
2
(7)
3. Computational Details
In this work, All calculations were initially done at HF/3-21G* level in vacuum. Single point calculations were
done on the optimized gas phase configurations with the SM 5.4A [4] solvation model. These calculations were
done by using Spartan 08. These optimized structures are used for density functional theoretical calculations by
using B3LYP functional at 6-311++G (d, p) basis set level for both gaseous phase and solution phase calculations.
Gaseous phase calculations are done in Gaussian 03 [5]. The solvent effects are studied by using SM8T continuum
solvation model [6] in Gamessplus software.
Figure 1: Thermodynamical cycles employed for deprotonation of piperazine (Reaction 1)
4. Results and Discussions
Temperature dependency of theoretical equilibrium constants for reactions 1-4 calculated in this work are used to
calculate the enthalpies of the corresponding reactions. Enthalpy of each of the reactions is calculated by correlating
ln K to equation 5 and will be given in full paper. Deprotonation of amine is one of the most important temperature
dependent reactions in CO2 capture post combustion solvent chemistry. Piperazine has two ionizable hydrogens
giving rise to first dissociation constant of PZ and second dissociation constant of PZ.
Parameter
K (pKa1)
K (pKa2)
A
B
37.31499
48525.05
C
-6091.49
-983443
D
-8.39402
-9213.91
E
0.033076
28.80816
-1.7E-05
-0.01498
Reference
This work
This work
Author namee / Energy Procediia 00 (2013) 000––000
3
Figurre 2: First and seccond protonation dissociation consstants (1(a) and 2(a)) and Enthalpiies of protonation
n for first and seccond group of
pipera
azine (1(b) and 2(b
b)) as a function of
o temperature. *Literature
*
resou
urces will be given
n in full paper.
Ackn
nowledgmentss
Fiinancial suppo
ort for this wo
ork by Aker Clean
C
Carbon, EON, EnBW
W and the Norw
wegian Researrch Council
C
CLIMIT program through the SOLVit projecct is greatly app
preciated.
Referrences
Gupta, M.; da Silva, E. F.;
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