(654d) Modeling the Absorption of CO2 Into Aqueous Blends of Deea and Mapa

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 25ºC to 60ºC 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 (K_OV) 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
(k_L and k_G), as well as initial reaction rate constant k_obs.

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:

-r_CO2
= r₁+r₂+r₃ = k_obsa_CO2                                                                                                           eq.1

The initial rate constant can
then be modeled in terms of the rate constants of the 3 reactions defined above
(k_OH and k_DEEA and k_MAPA):

kobs = k_OH*a_OH-+k_DEEA*a_DEEA+k_MAPA*a_MAPA                                                                              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, k_MAPA is described as:

k_MAPA*a_MAPA
= k'_DEEA*a_DEEA + k'_MAPA*a_MAPA +k_H2O*a_H2O
                                                         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

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