Elsevier

Chemical Engineering Science

Volume 107, 7 April 2014, Pages 317-327
Chemical Engineering Science

Experimental study on carbamate formation in the AMP–CO2–H2O system at different temperatures

https://doi.org/10.1016/j.ces.2013.12.028Get rights and content

Highlights

  • AMP–CO2–H2O system was studied quantitatively by 13C NMR.

  • Carbamate formation was observed even at low loadings.

  • Carbamate formation was almost temperature independent in the range tested.

  • The apparent carbamate stability constant was estimated at 25, 35 and 45 °C.

  • A method of splitting HCO3/CO32− and amine/amineH+ is proposed.

Abstract

Carbamate formation in the 30 wt% of 2-amino-2-methyl-1-propanol (AMP) system at different CO2 loadings and temperatures was studied via nuclear magnetic resonance (NMR) spectroscopy. The results indicate that the main species in this system are AMP/AMPH+, AMPCO2 and HCO3/CO32−. The carbamate was also observed at low loadings and the apparent carbamate stability constant was estimated based on the experimental concentrations of the species from NMR analysis. Carbamate formation was found to have weak temperature dependence in the range tested (25–45 °C). To distinguish within the AMP/AMPH+ and HCO3/CO32− pairs, the amine protonation constant, the dissociation constant of carbonic acid from literature, and pH measurements for different ionic strengths were all employed. All the data were correlated with ionic strength and temperature.

The accuracy of the carbamate stability constant determined from the concentration measurements will depend on pKa, ionic strength (I) used to calculate the speciation and on the uncertainty in the species concentration determinations from NMR, particularly for the carbamate species at low CO2 loadings and high temperature.

Introduction

The removal of acid gases is an important process in many chemical industries and amine based absorption plays a central role in existing and developing processes for post combustion CO2 capture. The most used amines in industry are monoethanolamine (MEA), diethanolamine (DEA), diisopropanolamine (DIPA), N-methyldiethanolamine (MDEA) and 2-amino-2-methyl-1-propanol (AMP) (Astarita et al., 1983, Kohl and Nilsen, 1997).

Primary and secondary amines such as MEA (monoethanolamine) and PZ (piperazine) are among the most frequently used absorbents owing to their high reactivity with CO2, low solvent cost, and ease of regeneration (Mandal et al., 2003). However, the maximum CO2 absorption capacity in MEA is limited by stoichiometry to about 0.5 mol CO2/mol amine. High energy requirements in the stripping stage, rate of degradation over time and corrosivity are important issues for industrial applications and may result in a decreased CO2 capture efficiency and additional costs (Aronu et al., 2009). For this reason, a different class of chemical absorbents, the sterically hindered amines, such as AMP, have been widely studied as potential candidates mainly recommended for their high CO2 loading capacity and ease of regeneration at low temperatures compared with the conventional amines (Chakraborty et al., 1986, Sartori and Savage, 1983). According to Chakraborty et al. (1986) the introduction of substituents at the alpha-carbon creates a carbamate instability which enhances the carbamate hydrolysis, Eq. (5), thus increasing the amount of bicarbonate formation and allowing an increase in CO2 loading. The steric hindrance would be expected to reduce the initial reaction rate with CO2, but as 1 mol of amine is released upon hydrolysis of the carbamate, the concentration level of amine available for reaction with CO2 increases (da Silva and Svendsen, 2006, Sartori and Savage, 1983, Singh et al., 2007).

The carbamate stability constant is difficult to measure but is important for determining and modeling both thermodynamics and absorption kinetics. In hindered amines the carbamate formation is weak resulting in a high loading capacity of up to 1.0. For such compounds, the concentration level of bicarbonate is high, allowing increased CO2 loading. Still, for kinetics, it is important to take the carbamate formation into account. At present, AMP is considered as one of the most important sterically hindered amines for both natural gas treatment processes and post combustion CO2 capture.

Nuclear magnetic resonance (NMR) techniques are widely used for qualitative identification and quantitative determination of concentrations of species (Barzagli et al., 2011, Hartono et al., 2007, Jakobsen et al., 2005, Mani et al., 2006). NMR has been used to determine carbamate stability and liquid phase composition/speciation (Bishnoi and Rochelle, 2002, Ciftja et al., 2011, Hartono et al., 2007, Jakobsen et al., 2005, Ma’mun et al., 2006, Sartori and Savage, 1983, Suda et al., 1996).

The objective of this work is to use NMR techniques, especially 13C-NMR, to study the formation of carbamate and to evaluate the apparent carbamate stability in aqueous solutions of CO2 and 2-amino-2-methyl-1-propanol (AMP) at different temperatures.

Section snippets

Sample preparation

Amine batch solutions were prepared from ≥97% pure AMP, supplied by Acros Organics (Acros Organics BVBA, Geel, Belgium), and distilled water. The resulting solution was 30 wt% AMP. 1,4-Dioxane was added as a reference standard. Pure CO2 (grade 5.0) supplied by AGA Gas (AGA Gas GmbH, Hamburg, Germany) was added by bubbling the gas into the solution. This process took 5–10 min and most of CO2 was directly absorbed and very little released. The loading was estimated from the weight change of the

Liquid – phase speciation obtained directly from NMR

In order to quantitatively determine the species in the system, a method previously used by Jakobsen et al. (2005) and Hartono et al. (2007) was applied. 1,4-Dioxane was chosen as a reference solvent with a chemical shift δ=67.19 ppm (Fulmer et al., 2010). The area under the signal of 1,4-dioxane is proportional to its concentration in the solution and was used as basis for the calculation of the other species. The areas beneath the signals were calculated by the NMR software. Additional

Conclusions

In the present work, carbamate formation for 30 wt% AMP at different CO2 loadings and at three different temperatures was investigated experimentally by quantitative 13C NMR. Spectra at 25, 35 and 45 °C for the AMP–CO2–H2O system at various CO2 loadings were acquired and carbamate was clearly observed and quantified as one of the species existing in the system.

Liquid-phase speciations obtained directly from 13C NMR spectroscopy and full-liquid speciation based on pH measurements, ionic strength

Abbreviations

    AARD

    absolute average relative deviation

    AQ

    acquisition time, s

    AHPD

    2-amino-2-hydroxymethyl-1,3-propanediol

    Am

    amine

    AmH+

    protonated amine

    AMP

    2-amino-2-methyl-1-propanol

    AMPH+

    protonated AMP

    AMPCO2

    AMP carbamate

    CO2

    carbon dioxide

    D2O

    deuterium oxide

    HCl

    hydrochloric acid

    HCO3/CO3

    carbonate/bicarbonate

    MEA

    monoethanolamine

    NaOH

    sodium hydroxide

    NMR

    nuclear magnetic resonance

    NOE

    nuclear overhouser effect

    Obs

    observed

    Symbols

    D1

    delay time between two transitions, s

    I

    ionic strength

    m

    molality, mol/kg

    NS

    number of scans

    ni0

    total mol of

Acknowledgments

This work was financed from DNV (Det Norske Veritas AS) and the CCERT project. The CCERT project is supported by the Research Council of Norway (NFR 182607), Shell Technology Norway AS, Metso Automation, Det Norske Veritas AS, and Statoil AS.

References (32)

  • T.M. Abbott et al.

    13C nuclear magnetic resonance and raman investigations of aqueous carbon dioxide systems

    Can. J. Chem. Eng.

    (1982)
  • G. Astarita et al.

    Gas Treating with Chemical Solvents

    (1983)
  • S. Bishnoi et al.

    Absorption of carbon dioxide in aqueous piperazine/methyldiethanolamine

    AIChE J.

    (2002)
  • W. Böttinger et al.

    Online NMR spectroscopic study of species distribution in MDEA−H2O−CO2 and MDEA–PIP–H2O–CO2

    Ind. Eng. Chem. Res.

    (2008)
  • E.F. da Silva et al.

    Study of the carbamate stability of amines using ab initio methods and free-energy perturbations

    Ind. Eng. Chem. Res.

    (2006)
  • G.R. Fulmer et al.

    NMR chemical shifts of trace impurities: common laboratory solvents, organics, and gases in deuterated solvents relevant to the organometallic chemist

    Organometallics

    (2010)
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