Thermodynamics and kinetics of aqueous piperazine with potassium carbonate for carbon dioxide absorption
Abstract
This work proposes an innovative blend of potassium carbonate (K2CO3) and
piperazine (PZ) as a solvent for CO2 removal from combustion flue gas in an
absorber/stripper. The equilibrium partial pressure and the rate of absorption of CO2
were measured in a wetted-wall column in 0.0 to 6.2 m K+
and 0.6 to 3.6 m PZ at 25 to
110o
C. The equilibrium speciation of the solution was determined by 1
H NMR under
similar conditions. A rigorous thermodynamic model, based on electrolyte non-random
two-liquid (ENRTL) theory, was developed to represent equilibrium behavior. A rate
model was developed to describe the absorption rate by integration of eddy diffusivity
theory with complex kinetics. Both models were used to explain behavior in terms of
equilibrium constants, activity coefficients, and rate constants.
The addition of potassium to the amine increases the concentration of CO3
2-
/HCO3
-
in solution. The buffer reduces protonation of the amine, but increases the
amount of carbamate species, yielding a maximum reactive species concentration at a
K+
:PZ ratio of 2:1. The carbamate stability of piperazine carbamate and dicarbamate
resembles that of primary amines and has approximately equal values for the heats of
reaction, ∆Hrxn (18.3 and 16.5 kJ/mol). The heat of CO2 absorption is lowered by K+
from -75 to -40 kJ/mol. The capacity increases as total solute concentration increases,
comparing favorably with 5 M monoethanolamine (MEA).
The rate approaches second-order behavior with PZ and is highly dependent on
other strong bases. In 1 M PZ, the overall rate constant is 102,000 s-1, 20 times higher
than in MEA. The activation energy is 35 kJ/kmol. In K+
/PZ, the most significant
reactions are PZ and piperazine carbamate with CO2 catalyzed by carbonate. Neutral
salts in aqueous PZ increase the apparent rate constant, by a factor of 8 at 3 M ionic
strength. The absorption rate in 5 m K+
/2.5 m PZ is 3 times faster than 30 wt% MEA.
A pseudo-first order approximation represents the absorption rate under limited
conditions. At high loadings, the reaction approaches instantaneous behavior. Under
industrial conditions, gas film resistance may account for >80% of the total mass
transfer resistance at low loadings.
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