368144 Kinetic Study of Carbon Dioxide Absorption By Aqueous Solutions of N-Methyldiethanolamine and Piperazine
Kinetic study of carbon dioxide absorption by aqueous solutions of N-methyldiethanolamine and piperazine
Alberto SERVIAa, Nicolas LALOUEa, Julien GRANDJEANa, Sabine RODEb, Christine ROIZARDb
a IFP Energies nouvelles, Rond-point de l'échangeur de Solaize BP3, 69360, Solaize, France
b LRGP-CNRS Université de Lorraine, 1 rue Grandville - BP 20451 - 54001 Nancy, France
The kinetics of CO2 absorption with aqueous blends of N-MethylDiEthanolAmine (MDEA) and PiperaZine (PZ) has been widely studied (Zhang et al., 2001, Bishnoi and Rochelle, 2002, Edali et al., 2010, Samanta and Bandyopadhyay, 2011). Nevertheless, some discrepancies remain concerning the reaction mechanisms proposed in the literature. For instance, the synergy between both amines is still not completely understood and proposed kinetics models cannot fully explain the experimental data at very high loadings. Moreover, the models used in the literature to deduce the kinetics of CO2 absorption by aqueous blends of PZ and MDEA from experiments present some limitations. They all assume a constant CO2 partial pressure in the gas phase. This hypothesis can lead to noticeable deviations if high CO2 quantities are absorbed.
The objective of this work is to investigate the kinetics of CO2 absorption by aqueous blends of MDEA and PZ. Experiments are performed in a wetted wall column (WWC) under various operating conditions and simulated using a rigorous reactor model.
2. Experimental setup
A WWC was used to obtain experimental data on the kinetics of CO2 absorption by aqueous blends of MDEA and PZ. Within the reactor, the gas phase flows counter-currently with the liquid that overflows from the inside of a cylinder to form a thin liquid film. The gas phase, composed of CO2 and Nitrogen (N2), is water-saturated before being in contact with the liquid in the reactor to prevent from water mass transfer in the reaction zone. The experimental flux is calculated using the variation of the CO2 gas concentration between the inlet and the outlet of the reactor, measured by an in-line infra-red spectrometer.
The WWC validation and hydrodynamic characterization were presented in a previous work (Servia et al., 2013).
3. Model description
A 2D stationary model has been developed using COMSOL software to predict the absorption flux of CO2 in aqueous blends of PZ and MDEA in the WWC. This model couples hydrodynamics, gas-liquid equilibrium, mass transfer and chemical reactions (Servia et al., 2013).
The liquid phase velocity profile is determined by the Navier-Stokes equation for an incompressible fluid associated with specific boundary conditions. Since CO2 was transferred from the gas into the liquid phase, the gas velocity varied within the reactor. This evolution was modeled through a mass balance on the inert compounds (N2 and water).
The concentration of each species at equilibrium conditions were provided by a thermodynamic model accounting for the deviations from the ideal behavior through an electrolyte NRTL approach provided by ASPEN. In this model, the CO2 Henry constant is determined by the ratio between PCO2 and the molecular CO2 concentration at equilibrium provided by the thermodynamic model, which assures the consistency between the mass transfer and the thermodynamic models.
Amines, water and bicarbonate dissociation were supposed to be instantaneously equilibrated, while CO2 chemical reactions with amines and OH- were assumed to be kinetically controlled and reversible.
As CO2 is absorbed, PCO2 presents a decreasing profile within the reactor. Consequently, the proposed model takes into account the evolution of the CO2 partial pressure through a plug-flow model combined with the film theory.
This model has been validated by comparing its predictions with experimental data obtained on unloaded aqueous solutions of PZ ranging from 0,2 to 1 M and at temperatures between 24 and 60 °C (Servia et al., 2013). The reaction mechanism proposed by Bishnoi and Rochelle, 2002 and their kinetic constants associated to equations 2 and 4 were used to model the kinetics of the CO2/PZ system.
4.1. CO2 absorption on unloaded solutions
A first set of experiments were performed
on unloaded solutions presenting MDEA concentrations of 2 and 3 M, PZ
concentrations ranging from 0.2 to 1 M and temperatures from 297 to 328 K. The
experimental results and model predictions were presented in Figure 1. The
increase of MDEA concentrations has a negative effect on the overall CO2
mass transfer, especially at low temperature. Indeed, the solution viscosity
rises involving a decrease of the CO2 diffusion coefficient within
the liquid phase. The experimental results were used to regress the kinetic
constant of the synergy chemical reaction (Equation 3), which plays a
significant role at the tested conditions. Indeed, the model systematically
underestimates CO2 flux with an AAD of 10 %, if this chemical
reaction is not considered in the reaction mechanism.
Figure 1 – CO2 absorption flux evolution as a function of PZ concentration for different conditions of MDEA concentrations and temperatures. Filled symbols: [MDEA] = 2 M; empty symbols: [MDEA] = 3 M; continuous curves: [MDEA] = 2 M; discontinuous curves: [MDEA] = 3M. Curves represent model predictions whereas symbols represent experimental data. a – T = 297 K; b – T = 319 K; c – T = 328 K; d – Comparison between model predictions without considering the synergy chemical reaction between MDEA and PZ and experimental data as a function of temperature.
A second set of experiments were carried
out on loaded solutions presenting MDEA concentrations of 2 and 3 M with a fixed
PZ concentration of 1 M, temperatures ranging from 297 to 328 K and CO2
loadings up to 0.25 molCO2/molamine, at two different PCO2.
The experimental results and model predictions were presented in Figure 2 .The
model systematically overestimates CO2 flux at low PCO2,
even if the deviation between experimental and model data is lower than the
determined experimental error. A methodology based on the experimental data
acquisition at operating conditions allowing the experimental error and the
model limitations impact to be reduced was then proposed and validated. This
methodology is based on the experimental data acquisition at higher PCO2.
Model predictions are in good agreement with the experimental data at high
solutions loadings indicating that the reaction between MDEA and PZCOO-
can be neglected at these conditions. Moreover, the model accurately reproduces
the overall CO2 flux, even in presence of mass transfer limitations.
Figure 2 – CO2 absorption flux evolution as a function of solution loading for different conditions of MDEA concentrations and temperatures. Filled symbols: [MDEA] = 2 M; empty symbols: [MDEA] = 3 M; continuous curves: [MDEA] = 2 M; discontinuous curves: [MDEA] = 3M. Curves represent model predictions whereas symbols represent experimental data. a – T = 297 K, yCO2inlet = 7000 ppm; b – T = 319 K, yCO2inlet = 7000 ppm; c – T = 328 K, yCO2inlet = 7000 ppm; d – Parity chart at yCO2inlet = 50000 ppm.
A sensitivity analysis (see Figure 3) was
carried out involving all kinetics and diffusion parameters. At low loadings,
the kinetic constant associated to the interaction between PZ, CO2
and water as a significant impact. An accurate prediction of the reaction
between PZCOO-, CO2 and water, as well as the liquid
phase species diffusion are required at higher CO2 loadings. The
enhancement factor is sensitive to the synergy reaction (Equation 3) at low
loadings, high temperatures, and high MDEA concentrations.
Figure 3 – Sensitivity analysis of the kinetics parameters and liquid phase species diffusion coefficient. a – T = 298 K, yCO2inlet = 7000 ppm; b – T = 298 K, yCO2inlet = 50000 ppm; c – T = 330 K, yCO2inlet = 7000 ppm; d – T = 330 K, yCO2inlet = 50000.
This paper presents an experimental and numerical investigation on the CO2 absorption by aqueous blends of MDEA and PZ. A synergy exists between both amines to enhance CO2 capture, whereas the chemical reaction between the PZCOO- and MDEA to accelerate CO2 capture can be neglected at high loadings.
A sensitivity analysis on the kinetic and diffusion parameters indicates that an accurate description of the PZ interaction with CO2 is required at low loadings, whereas diffusion phenomenon and PZCOO- interactions with CO2 become essential at higher loadings.