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 SERVIA^a, Nicolas LALOUE^a, Julien GRANDJEAN^a, Sabine RODE^b, Christine ROIZARD^b

 

^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

1. Introduction

 

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 P_CO2 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.

 

	Equation 1
	Equation 2
	Equation 3
	Equation 4
	Equation 5

As CO₂ is absorbed, P_CO2 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.

4. Results

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.

a)	b)
c)	d)

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.

4.2. CO₂ absorption on loaded solutions

 

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 mol_CO2/mol_amine, at two different P_CO2. The experimental results and model predictions were presented in Figure 2 .The model systematically overestimates CO₂ flux at low P_CO2, 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 P_CO2. 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.

 

a)	b)
c)	d)

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, y_CO2^inlet = 7000 ppm; b – T = 319 K, y_CO2^inlet = 7000 ppm; c – T = 328 K, y_CO2^inlet = 7000 ppm; d – Parity chart at y_CO2^inlet = 50000 ppm.

4.3. Sensitivity analysis

 

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.

a)	b)
c)	d)

Figure 3 – Sensitivity analysis of the kinetics parameters and liquid phase species diffusion coefficient. a – T = 298 K, y_CO2^inlet = 7000 ppm; b – T = 298 K, y_CO2^inlet = 50000 ppm; c – T = 330 K, y_CO2^inlet = 7000 ppm; d – T = 330 K, y_CO2^inlet = 50000.

5. Conclusion

 

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.

 

References

  Bishnoi, S., Rochelle, G.T., (2002). Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine. Aiche Journal, 48, 2788-2799.

  Edali, M. et al.  (2010). 1D and 2D absorption-rate/kinetic modeling and simulation of carbon dioxide absorption into mixed aqueous solutions of MDEA and PZ in a laminar jet apparatus. International Journal of Greenhouse Gas Control, 4, 143-151.   Samanta, A., Bandyopadhyay, S. S., (2011). Absorption of carbon dioxide into piperazine activated aqueous N-methyldiethanolamine . Chemical Engineering Journal, 171, 3, 734-741.   Servia, A., Laloue, N., Grandjean, J., Rode, S. and Roizard, C. (2013). Modeling of the CO₂ absorption in a wetted wall column by piperazine solutions. Oil & Gas Science and Technology. DOI: 10.2516/ogst/2013136   Zhang, X. et al.  (2001). A kinetics study on the absorption of carbon dioxide into a mixed aqueous solution of methyldiethanolamine and piperazine. Industrial & Engineering Chemistry Research, 40, 3785-3791.