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The reaction kinetics of CO2 with mono-ethanolamine in n-propanol – Part I: Reaction kinetic data and comparison with existing rate law expressions Louis Jacobus du Preez, JP Barnard, Linda H. Callanan, and Johannes H. Knoetze Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b01482 • Publication Date (Web): 14 Jun 2018 Downloaded from http://pubs.acs.org on June 14, 2018
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Industrial & Engineering Chemistry Research
The reaction kinetics of CO2 with mono-ethanolamine in n-propanol – Part I: Reaction kinetic data and comparison with existing rate law expressions Louis J. Du Preeza, Jakobus P. Barnarda, Linda H. Callanana, Johannes H. Knoetzea,* a
Department of Process Engineering, Stellenbosch University, Stellenbosch 7602, South Africa
*Corresponding author. Tel: +27 21 808 4421. e-mail address:
[email protected] Abstract The reaction kinetics of CO2 with mono-ethanolamine (MEA) is of industrial significance with respect to both CO2 sequestration applications and characterising the effective interfacial mass transfer area of packed separation columns. Reaction kinetic data were previously, by necessity, only measured under pseudo first order conditions with respect to CO2. Furthermore, mass transfer limitations encountered by the heterogeneous techniques restricted the validity range of the reaction kinetic models developed from the data. New reaction kinetic data, independent of mass transfer limitations and outside pseudo first order conditions, are presented. The data, collected via a previously developed novel, in-situ FTIR technique, were subsequently compared with the predictions of two widely accepted rate expressions, the power rate law and pseudo steady state hypothesis (PSSH) rate law. The expressions were modelled on the data using a novel multi-objective goal attainment algorithm also developed in this study. The PSSH rate law predictions were in closer agreement with the data than the power rate law, but both rate expressions were found to be unable to accurately describe the reaction kinetics of CO2 proving that they should be used with caution outside of the pseudo first order conditions of their derivation. It was, therefore, concluded that a rate law able to describe the reaction kinetics for all reaction conditions should include the zwitterion reaction intermediate concentration in its fundamentally derived rate expression(s).
Keywords FTIR, power rate law, pseudo steady state hypothesis (PSSH), carbon dioxide (CO2), mono-ethanolamine (MEA), zwitterion
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Introduction
The reactive absorption of CO2 into both aqueous and non-aqueous solutions of mono-ethanolamine (MEA) has industrial application in both CO2 sequestration technologies1-4 and determining the effective interfacial mass transfer area of separation column internals5-8 such as random and structured packing material. In order for both these applications to be optimally applied over a wide range of MEA and CO2 concentrations, accurate reaction kinetic data and subsequent reaction rate expressions are required. Various rate laws, most notably the power rate law and pseudo steady state hypothesis (PSSH) rate law are widely accepted as the best rate laws currently available for describing the reaction kinetics9-16.
Previous experimental techniques used to develop these rate laws were limited to studying the reaction under pseudo first order conditions with respect to CO2. The power and PSSH rate laws are, therefore, strictly only valid under conditions where the liquid phase concentration of MEA is in such great excess that it is assumed to remain constant. To the authors’ knowledge, no reaction kinetic data outside the pseudo first order regime were available for comparison with the predictions of these rate laws. Such a comparison will shed light on the validity of these rate laws to completely describe the reaction kinetics of CO2 with MEA over a broader range of concentrations and operating conditions.
A novel, in-situ method capable of measuring reaction kinetic data in alcoholic solvents outside pseudo first order conditions was previously developed17. This study firstly aims to report and discuss the reaction kinetic data collected for the homogeneous liquid phase reaction of CO2 with MEA in n-propanol as representative alcoholic solvent. Secondly, the study aims to perform and evaluate a comparison of the predictions of the power rate law and PSSH rate law expressions with the data. A new modelling algorithm based on multi-objective goal optimisation was developed to facilitate the intended comparison.
Ultimately, this reaction kinetic study aims to provide a reaction rate expression derived from fundamental principles with the validated zwitterion reaction mechanism17 as basis for the derivation. This rate expression will be presented in Part II and will be valid over a wide range of CO2 and MEA concentrations including concentrations outside the pseudo first order regime. It will, furthermore, enable more accurate predictions of separation column performance over a wider range of operating conditions and could improve the accuracy of effective mass transfer area characterisation of packed column internals.
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Theory
The reaction of CO2 with amines have been described by either the zwitterion mechanism, initially proposed by Caplow18 and later re-introduced by Danckwerts19, or the termolecular mechanism first proposed by Crooks and Donnelan20. Both mechanisms describe the formation of carbamate and a protonated base at reaction equilibrium, but propose two different reaction pathways. The former involves the formation of a zwitterion reactive intermediate which either decomposes back to CO2 and MEA or is readily deprotonated by a nearby base in a consecutive reaction step. The latter proposes a single step reaction of CO2 and MEA with a nearby base to form carbamate via a loosely bound complex. Basically identical rate expressions were derived from either mechanism14,21 to model reaction kinetic data measured under pseudo first order conditions, leading to strong arguments in support of both the zwitterion9-17 and termolecular21,22 mechanisms in literature. The zwitterion reaction mechanism is, however, most widely used to describe the reaction of CO2 with the primary amine, MEA, in both aqueous11,12,14 and non-aqueous solvents9-13,15,16. In non-aqueous solvents with no stronger base present, MEA (R = -CH2CH2OH) is responsible for deprotonating the zwitterion9-11, as shown in equation 1.
k1 → RNH 2+ COO − a) CO2 + RNH 2 ← k2
k3 → RNHCOO − + RNH 3+ b) RNH COO + RNH 2 ← k4
+ 2
−
(1)
The homogeneous (rapid mixing23, stopped flow16) and heterogeneous (wetted wall7,9,10,24, laminar jet25,26, stirred vessel6,15 etc.) methods previously used were, as mentioned above, analytically limited to studying the reaction under pseudo first conditions. The resulting data required the use of a pseudo steady state hypothesis (PSSH) in order to derive suitable reaction rate expressions, since these methods were unable to quantify the concentration of the zwitterion intermediate. The PSSH assumes that the nett reaction rate of the zwitterion is zero11,12,14,27, which eliminates the zwitterion concentration from the rate expression (equation 2). In alcoholic solvents the reaction mechanism (equation 1) shows that equal amounts of the ionic salts form during the reaction, leading to the simplified form of the PSSH rate law (equation 3).
k ′′′ [CO2 ][ RNH 2 ] − k4 RNH 3+ RNHCOO − 2
− rCO2 =
where k ′′ =
1 + k ′′ [ RNH 2 ]
k3 k2
(2)
and k ′′′ = k1k ′′ .
3
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k ′′′ [CO2 ][ RNH 2 ] − k4 RNH 3+ 2
−rCO2 =
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2
(3)
1 + k ′′ [ RNH 2 ]
The limitations of previous experimental techniques also meant that the concentrations of the ionic salt products of equation 1(b) were unknown, forcing the assumption of the deprotonation reaction of the zwitterion to be irreversible9,11,13,18. This assumption simplified the rate expression (equation 4) considerably. The pseudo first order conditions allowed further simplifications of the rate expression, resulting in two possible forms (equations 5 and 6) of the power rate law.
− rCO2 =
[CO2 ][ RNH 2 ] 1 1 + k1 k1k ′′ [ RNH 2 ]
k k ′′ [CO2 ][ RNH 2 ] = 1 1 + k ′′ [ RNH 2 ]
2
( when
−rCO2 = k1 [CO2 ][ RNH 2 ]
)
k ′′ [ RNH 2 ] >> 1
−rCO2 = k1k ′′ [CO2 ][ RNH 2 ] = k [CO2 ][ RNH 2 ] 2
(4)
2
( when
(5)
)
k ′′ [ RNH 2 ]
