Experimental study on carbamate formation in the AMP–CO2–H2O system at different temperatures
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
- D1
delay time between two transitions, s
- I
ionic strength
- m
molality, mol/kg
- NS
number of scans
total mol of
Symbols
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.
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