Amine Based Solvent for CO2 Absorption

225 Pages·2011·6.26 MB·English

Amine Based Solvent for
CO Absorption
2
“From Molecular Structure to Process”
Prachi Singh

Amine Based Solvent for CO
2
Absorption
“From Molecular Structure to Process”

Prachi Singh
Dit proefschrift is goedgekeurd door de promotor:
Prof.dr.ir. W. P. M. van Swaaij

Samenstelling Promotiecommissie:
Voorzitter: Prof.dr. G. van der Steenhoven Universiteit Twente
Secretaris: Prof.dr. G. van der Steenhoven Universiteit Twente
Promotor: Prof.dr.ir. W. P. M. van Swaaij Universiteit Twente
Ass. Promotor: Dr.ir. D. W. F. Brilman Universiteit Twente
Referenten: Dr.ir. J. A. Hogendoorn Universiteit Twente
Dr.ir. E. Falck da Silva SINTEF, Norway
Deskundige: Dr.ir. F. H. Geuzebroek Shell Technology Centre
Amsterdam
Leden: Prof.dr. S. R. A. Kersten Universiteit Twente
Prof.dr.ir. A. Nijmeijer Universiteit Twente
Prof.dr.ir. H. J. Heeres Rijksuniversiteit Groningen

Dit onderzoek werd financieel ondersteund door nationale projecten CATO

Prachi Singh
Amine based solvent for CO absorption “From molecular structure to process”.
2
Thesis, University of Twente, The Netherlands
ISBN 978-90-365-3200-6

Printed by Wöhrmann Print Service
Cover Design: Painting “Green horizon” by Prachi Singh

Copyright © 2011 by Prachi Singh
All rights reserved. NO part of the material protected by this copyright notice may be
reproduced or utilized in any form or by any means, electronic or mechanical, including
photocopy, recording or by any information storage and material system, without written
permission from the publisher.
AMINE BASED SOLVENT FOR CO ABSORPTION
2
“FROM MOLECULAR STRUCTURE TO PROCESS”

PROEFSCHRIFT

ter verkrijging van
de graad van doctor aan de Universiteit Twente,
op gezag van de rector magnificus,
prof.dr. H. Brinksma,
volgens besluit van het College voor Promoties
in het openbaar te verdedigen
op woensdag 1 juni 2011 om 12.45 uur

door

Prachi Singh
geboren op 29 April 1979
te Meerut, Uttar Pradesh, India
Dit proefschrift is goedgekeurd door de promotoren:
Prof.dr.ir. W. P. M. van Swaaij
Dr.ir. D. W. F. Brilman
Dedicated to my
Guru Ji

Summary

Global warming is a well known, worldwide concern, most probably caused by
increasing concentrations of CO and other greenhouse gases in the earth’s
2
atmosphere, due to human activities. Especially the production of electricity is a
major contributor, responsible for 41% of the world CO emissions in 2008.
2
Worldwide, the power sector relies heavily on coal, the most carbon-intensive of
fossil fuels, and hence amplifying its share in global emissions. Carbon Capture
and Storage (CCS) offers the opportunity to reduce the CO emissions associated
2
with the use of fossil fuels. Carbon dioxide capture with a regenerable solvent is
considered to be a mature technology, since it is successfully applied as CO
2
removal technology in industrial applications. In order to make this technology
more economical for post-combustion capture, especially in the power-sector, more
research is required to identify solvents which require less energy and lead to lower
solvent loss- and corrosion rates in this application. In this thesis, the development
of improved, energy efficient amine based solvents is targeted, through
experimental (screening) work and by a further understanding of the role of
molecular structure on the solvent absorption properties for amine based solvents
in a CO absorption process.
2

Primary and secondary amines react with CO forming carbamate-species. The
2
degree of hydrolysis of these carbamate to form bicarbonate depends on several
factors, such as its chemical stability, which is influenced by the temperature.
Tertiary amines groups cannot react with CO directly to form a carbamate,
2
because these amines lack a free proton. Hence, tertiary amines act as a base and
catalyze the hydration of CO , leading to the formation of bicarbonate. Carbamate
2
stability and basicity are the major parameters determining the absorption capacity
and regeneration energy requirement. From literature it was identified that steric
hindrance is an important parameter in reducing the carbamate stability and is also
affecting the basicity of amine based solvents. Steric hindrance is present at
different levels (low to high) depending on the number of functional groups
substituted at α-carbon next to the amine group. Therefore, the effect of molecular
structure like the carbon chain length, functional groups and steric hindrance on the
basicity of various amines was studied. It was noticed that for alkanolamine and
diamine basicity increases with an increase in carbon chain length, whereas alkyl
amine basicity was not affected by an increase in carbon chain length. Difference
between two pKa constants for diamines was reduced with an increase in carbon
chain length. Steric hindrance showed an increase in the basicity for primary
alkanol amines, primary alkylamines and secondary alkylamines, illustrating the
clear relationship between the molecular structure and the basicity of the solvents.

i
The relative carbamate stability for various amine based solvents for CO
2
absorption has been studied by quantum mechanical calculations. Various
molecular structure effects like carbon chain length, steric hindrance, functional
groups and different configurations of cyclic amines have been investigated using
the SM8 solvation model in combination with gas phase reaction energies
calculated with the B3LYP density functional method. The trends observed for
carbamate stability are (qualitatively) compared with experimental data reported in
literature. This work gives theoretical support to the trends observed in earlier
experimental studies Singh et al. (2007, 2009) on the effect of the molecular
structure.

To identify, and to confirm above mentioned-, effects of molecular structure for
amine based solvents on their CO absorption capacity and reactivity, solvent
2
screening experiments were performed for CO absorption and regeneration
2
conditions. The absorption of pure CO was performed at 30°C and atmospheric
2
pressure to assess a preliminary indication on the initial absorption rate and
absorption capacity (or rich loading). The regeneration of CO from saturated
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solvents was performed at 80°C and atmospheric pressure to determine the lean
loading at pseudo equilibrium. Evaluation of desorption capacity (lean loading) at
this relatively low temperature may point out more directly towards a more energy
efficient solvent. Results showed that an increase in chain length between the
amine and different functional groups in the absorbent structure, results in a
decrease of the absorption rate, whereas the absorption capacity was increased in
most of the absorbents. A steric hindrance effect was noticed when an alkyl group
side chain was present at the alpha–carbon next to the amine group in the absorbent
structure. An increase in the number of amine groups, up to 4 amine groups in the
absorbent structure, results in an increase of absorption capacity, but also a higher
lean loading. Aromatic amines substituted with alkyl groups showed slight increase
in an initial absorption rate and absorption capacity. Cyclic saturated diamine
substituted with a hydroxyl group by a side chain at the cyclic ring showed the
lowest lean loading at pseudo equilibrium when compare to that of other functional
group (alkyl or amine) substitution at cyclic ring.

Further study was done to evaluate these amine based solvents for CO absorption
2
capacity and reactivity at low CO partial pressure, more close to flue gas
2
emissions conditions. The CO cyclic capacity ( difference in rich and lean CO
2 2
loading) of various potential aqueous amine-based solvents was determined by
performing CO absorption experiments at 30°C and 10 kPa CO partial pressure
2 2
and regeneration at 90°C and atmospheric pressure. 1,7-Diaminoheptane and 1,6
Hexanediamine, N,N' dimethyl showed under the conditions studied comparatively
high cyclic loadings of 0.81 and 0.85 mole CO /mole amine respectively. Aqueous
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solutions of 1,6 Hexanediamine, N,N' dimethyl of 0.5 and 2.55 mole/L
concentration were selected to study solubility of CO at different CO partial
2 2
pressure ranging from 1 up to 40 kPa, 30°C and at 1 atmosphere. The CO
2
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