(397d) Validation of a Process Model of CO2 Capture in an Aqueous Solvent, Using an Implicit Molecular Based Treatment of the Reactions

Validation of a process model
of CO₂ capture in an aqueous solvent, using an implicit treatment of
the reactions

C. V. Brand, J. Rodriguez-Perez, A. Galindo, G.
Jackson, C .S. Adjiman

Departement of Chemical Engineering, Centre for
Process Systems Engineering,

Imperial College London, London SW7 2AZ, United
Kingdom

Carbon dioxide
(CO₂) emissions are considered by the majority of the scientific
community to play a major role in climate change and particularly in global
warming. In this context, the development of carbon capture systems is a
necessity that must be addressed in the short term. The most promising early
stage technology, both in terms of performance and applicability, is currently
thought to be post-combustion CO₂ absorption using amine solvents. There
are, however, a number of concerns with the large scale deployment of this
technology, including energy requirements, solvent degradation and the
environmental and health impact resulting from loss of solvent and solvent
degradation products. Modelling studies can play a useful role in addressing
some of these issues and identifying the choice of solvent and operating
conditions that yield the best performance. A key challenge is to develop
models that can predict accurately the behaviour of the process, in the
presence of limited experimental data.

We address this
challenge by developing a complete CO₂ absorber-desorber model that
incorporates state-of-the-art thermodynamics integrated into a rate-based
process model. A characteristic of the model is that all reactions are treated
within the thermodynamic model, based on the assumption that the reaction
kinetics are not rate-limiting. This greatly reduces the amount of experimental
data required to model the interactions between the solvent and CO₂
and therefore makes the approach ideally suited to study new solvents.
Furthermore, no enhancement factor is used in our process model. Due to the
transferability of the thermodynamic description, the same model is used for
the absorber and the desorber. This approach is applied to CO₂
capture using an aqueous monoethanolamine (MEA) solution. The extent to which
pilot plant data can be modelled is examined in detail, allowing the validity
of the assumptions to be verified

The
thermodynamic model used to determine the vapour-liquid equilibrium and the
chemical equilibrium is based on the statistical fluid association theory for
potential of variable range (SAFT-VR) [1]. In the SAFT-VR thermodynamic
description, the reactions are treated implicitly within a physical
perspective, with the products of the chemical reaction treated as associated
aggregates of the reactant molecules, so that there is no need to incorporate
explicit rates of reaction, temperature-dependent equilibrium constants, or
mass balances on the reaction products. This greatly simplifies the model. It
has been shown that this approach yields a good representation of the
vapour-liquid and chemical equilibria of mixtures of CO₂, water and
MEA under a wide range of conditions [2]. Different correlations for the
rate-based equations governing the heat and mass transfer in the absorber are
considered [3, 4].

The
absorber-desorber process model is implemented in the gPROMS software and is
validated using published pilot plant experimental data. The predictive
capabilities of the mass transfer correlations used in this model are assessed
through a sensitivity analysis and a scaling of the liquid-phase mass transfer
coefficient is proposed. This scaling of the mass transfer is transferable to
different operating conditions for both the absorber and the desorber and good
predictions are obtained for the temperature and composition profiles in the
gas and liquid phases.

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Gil-Villegas, A. Galindo, P. J. Whitehead, S. J. Mills, G. Jackson, and A. N.
Burgess. J. Chem. Phys.,
106(10):4168?4186, 1997.

[2] N. Mac
Dowell, F. Llovell, C. S. Adjiman, G. Jackson, and A. Galindo. Ind. Eng. Chem. Res., 49, 1883-1899,
2003.

[3] K. Onda, E.
Sada, and H. Takeuchi. J. Chem. Eng.
Japan, 1(1):62-66, 1968.

[4] J. A. Rocha,
J. L. Bravo and J. R Fair. Ind. Eng.
Chem. Res., 32(4):641-651, 1993.