333036 Modeling the Absorption of CO2 Into Aqueous Blends of Deea and Mapa

Thursday, November 7, 2013: 1:45 PM
Sutter B (Hilton)
Juliana Garcia Moretz-Sohn Monteiro1, Majeed Hammad1, Saddam Hussain1, Hanna Knuutila2 and Hallvard F. Svendsen2, (1)Chemical Engineering Department, Norwegian University of Science and Technology, Trondheim, Norway, (2)Department of Chemical Engineering, NTNU, Trondheim, Norway

Introduction

The reaction rate of CO2 absorption by amine solutions is a key parameter for designing an absorption column. The faster the mass transfer, the lower the contact area needed and the closer to equilibrium one may get. Tertiary amines promote the reaction of hydration of CO2 leading to bicarbonate formation (Donaldson and Nguyen, 1980), a slow reaction, while primary and secondary amines form carbamates, a relatively fast reaction. Blending different types of amines has been shown to be a promising path in solvent development for CO2 capture by absorption. If there is synergy between the different properties of the amines, this leads to an enhanced process performance.

Aqueous blends of N,N-diethylethanolamine (DEEA), a tertiary alkanolamine, and 3-(methylamino)propylamine (MAPA), a diamine with one primary and one secondary group, are especially interesting for the absorption process because they form two liquid phases upon CO2 loading. The upper phase is lean in CO2 and does not require regeneration, while the lower phase is CO2-rich and is sent to the stripper column. There, DEEA is mainly regenerated (Liebenthal et al., 2012). The process can therefore combine lower energy requirements in the regeneration side and high reaction rate in the absorption side of the process.

The combined reaction rate of CO2 absorption between DEEA and MAPA is presented in this work.

Experiment

The reactive absorption of CO2 into aqueous solutions of DEEA and MAPA was conducted in a string of discs contactor (SDC), previously described by Knuutila et al. (2009) among others. A gas stream, containing N2 and CO2, and the aqueous amine solution were contacted in countercurrent mode.

The inlet flow rates of N2 and CO2 were set using mass flow controllers, while the CO2 composition in the outlet gas was determined by a CO2 analyzer. There were indicators for inlet and outlet temperatures of the liquid and gas streams. As a surplus, the inlet liquid flow rate was controlled. The experiment was set so that the reaction follows the pseudo-first order regime.

The reaction was performed within the temperature range 25ēC to 60ēC for unloaded aqueous blends of DEEA and MAPA with varying composition. In total, 10 blends were tested. Some physical properties of the systems, namely density, viscosity, vapor pressure, Henry's law constant, were measured to enable the modeling.

 

Modeling

The rate of CO2 absorption and the overall mass transfer coefficient (KOV) could be directly calculated from the experimental data. The knowledge of the physical properties of the system (density, viscosity, vapor pressure, Henry's law constant) enabled an evaluation of the liquid and gas-film mass transfer coefficients (kL and kG), as well as initial reaction rate constant kobs.

The reactions involved in the absorption of CO2 by aqueous blends of DEEA and MAPA are described as:

CO2+OH- → HCO3-                                                                                                                    r1

 

DEEA + H2O + CO2 → DEEAH+ + HCO3-                                                                                                         r2

 

MAPA + H2O + CO2 → MAPACOO- + H3O+                                                                         r3

 

Following the approach of Knuutila et al. (2009), the rate equations are written in terms of activities, so that it is consistent with rigorous VLE models in which the equilibrium constants are given in terms of activities. Taking the pseudo-first order assumption into consideration, the forward CO2 reaction rate is given by:

-rCO2 = r1+r2+r3 = kobsaCO2                                                                                                           eq.1

 

The initial rate constant can then be modeled in terms of the rate constants of the 3 reactions defined above (kOH and kDEEA and kMAPA):

kobs = kOH*aOH-+kDEEA*aDEEA+kMAPA*aMAPA                                                                              eq.2

where reactions 1 and 2 are modeled as second-order reactions. If reaction 3 is modeled based on the direct (termolecular) mechanism (da Silva and Svendsen, 2004), considering MAPA, water and DEEA are considered as dominating bases, kMAPA is described as:

kMAPA*aMAPA = k'DEEA*aDEEA + k'MAPA*aMAPA +kH2O*aH2O                                                          eq.3

 

The CO2 activity is given by the experimental Henry's Law constant, while the activities of the other species are calculated using the NRTL model parameters presented by Hartono et al. (2013)

Acknowledgements

Financial support from the EC 7th Framework Programme through the iCap project, Grant Agreement No : iCap-241391, is gratefully acknowledged.

References

da Silva, E.F.; Svendsen, H. F. Ab Initio Study of the Reaction of Carbamate Formation from CO2 and Alkanolamines. Ind. Eng. Chem. Res, 2004, 43, 3413-3418

Donaldson, T.; Nguyen, Y. N. Carbon Dioxide Reaction Kinetics and Transport in Aqueous Amine Membranes. Ind. Eng. Chem. Fundam. 1980, 19, 260-266.

Hartono, A., Saleem, F., Arshad, M.W., Usman, M., Svendsen, H.F., 2013. Binary and ternary VLE of the 2-(diethylamino)-ethanol (DEEA)/ 3-(Methylamino)-propylamine (MAPA)/ Water system. Submitted to the International Journal of Greenhouse Gas Control.

Liebenthal, U.; Pinto, D.D.D.; Monteiro, J. G. M.-S.; Svendsen, H. F.; Kather, A. Overall process analysis and optimization for CO2 capture from coal fired power plants based on phase change solvents forming two liquid phases. 11th International Conference on Greenhouse Gas Control Technologies, 18 - 22 November 2012, Kyoto, Japan

Knuutila, H.; Svendsen, H. F.; Juliussen, O. Kinetics of carbonate based CO2 capture systems. Energy Procedia. 2009, 1, 1011-1018.


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