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Information Derivation from Vapour-liquid Equilibria Data: A Simple Shortcut to Evaluate the Energy Performance in an Amine-based Post-combustion CO2 Capture Kaiqi Jiang, Kangkang Li, Graeme Puxty, Hai Yu, and Paul H. M. Feron Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.8b03512 • Publication Date (Web): 27 Aug 2018 Downloaded from http://pubs.acs.org on August 30, 2018
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Environmental Science & Technology
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Information Derivation from Vapour-liquid Equilibria Data: A Simple Shortcut to
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Evaluate the Energy Performance in an Amine-based Post-combustion CO2 Capture
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Kaiqi Jiang, Kangkang Li*, Graeme Puxty, Hai Yu, Paul H. M. Feron
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CSIRO Energy, 10 Murray Dwyer Circuit, Mayfield West, New South Wales, Australia 2304
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*
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Tel: +61-2-49606199
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Email address:
[email protected] Corresponding author:
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Abstract Art
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Abstract
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Evaluation of amine absorbents is crucial for the development of a technically and
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economically feasible CO2 capture process. However, the capture performance estimation
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usually requires a load of experiments, which is time-consuming and labour-intensive. The
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present study proposed a simple but effective shortcut that employs the fewest experimental
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data, i.e. vapour-liquid equilibria (VLE) data only, to estimate the CO2 capture performance
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by developing a validated chemical VLE model and a simple shortcut approach. The
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reliability of the proposed method was validated by the excellent agreement with the results
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from the laboratory and pilot plant experiments, and rigorous rate-based MEA model in
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Aspen Plus. We demonstrated that this approach can reliably predict the important capture
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performance indicators, such as CO2 solubility, heat of CO2 reaction, lean/rich CO2 loadings
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and heat requirement of absorbent regeneration. Moreover, this shortcut approach can
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provide the guidance for process modification to achieve the minimum regeneration energy.
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The extended application of this approach to other amines, i.e. piperazine (PZ), 2-amino-2-
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methyl-1-propanol (AMP), and blended PZ and AMP (PZ/AMP), also showed the good
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consistency with the published experimental and simulation results, further indicating the
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reliability of the shortcut approach to estimate the energy performance of amine processes.
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It is anticipated that the proposed method would simplify the evaluation of CO2 capture
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performance by using VLE data only, providing an efficient and effective shortcut for
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screening and evaluating amine-based CO2 capture.
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Key words: CO2 capture, VLE model, shortcut approach, capture performance
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1. Introduction 1
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The increasing atmospheric level of carbon dioxide
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contributor to the greenhouse effect and climate change, leading to a worldwide interest in
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the development of a low-CO2 emission technology. CO2 capture and storage is considered
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to be a critical strategy to mitigate the progress of global warming, which is anticipated to
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contribute ~13% of the cumulative reductions in emissions by 2050 to limit the global
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temperature increase to 2 ºC or below1, 2. Chemical absorption-based CO2 capture
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processes, i.e. amine scrubbing (Process flow sheet in Supplementary Figure S1), are
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widely regarded the dominant technology to commercially capture CO2 from the fossil fuel-
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fired power stations in near or middle term, owing to their relative rich experience in
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industrial applications3. However the commercial application of amine technologies is largely
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restricted by the high capital investment and operating costs. It is estimated that the
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integration of a state-of-the-art amine scrubbing process with a coal-fired power plant would
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lead to a decrease in overall thermal efficiency by 18-30% and an increase in the cost of
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electricity of 45-70%1, 4-6.
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To minimise the capital investment and the operating costs, current research efforts mainly
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focus on two aspects: (i) advancement of amine absorbent to have desirable CO2 capture
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performance and low cost7, 8, and (ii) process design and optimisation to improve process
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efficiency9-11. The latter is based on an existing process to reduce the energy requirement for
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absorbent regeneration through modifying the process configurations, whilst the new
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absorbent formulation is of critical importance for the development of a low-cost and energy-
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efficient CO2 capture process. Extensive research has been conducted to find out a suitable
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amine absorbent through laboratory solvent screening12-15, with a particular focus on the CO2
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absorption performance, such as reaction rate, absorption capacity and equilibrium
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constants. These absorption characteristics directly determine the CO2 absorber size,
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solvent flow rate, solvent pumping, etc. The CO2 stripping performance, specifically the
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energy performance, is also particularly concerned, as the intensive energy consumption is
has been identified as a major
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one of the most critical challenges for commercial application of amine process. The
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laboratory investigation of CO2 desorption is mainly focused on the measurement of the CO2
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reaction heat using calorimeters16, 17, or the calculation of reaction enthalpy by developing a
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chemical model18. When evaluating the energy consumption of absorbent regeneration, one
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should take the caution that the heat of CO2 reaction is only a part of its regeneration duty
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and cannot entirely represent the heat requirement of the CO2 stripping process. Heat of
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vaporisation and sensible heat are the other two heat components of regeneration duty19,
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which are usually determined either by process experiments or by process modelling.
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Process experiments, such as pilot-scale trials, are the most direct and accurate approach to
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practically evaluate the CO2 capture performance. However, it is usually labour- and cost-
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intensive to set up and operate the pilot plant, rendering the inconvenience to obtain the
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energy performances. While process modelling is an easier option, the development and
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validation of process models also requires a series of experimentally measured
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thermodynamic and/or kinetic data in order to achieve reliable estimations. A variety of
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experimental activities should be involved to acquire the experimental data such as the CO2
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solubility, cyclic CO2 loading, heat of CO2 reaction, equilibrium constants, regeneration duty,
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etc. Hence the model-based evaluation is also labour- and time-consuming, particularly
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when multiple new amine absorbents are being screened. If these important solvent
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properties can be obtained by one experimental activity, it would greatly facilitate the
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screening and evaluation of amine absorbent which is beneficial for the development of
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amine-based processes.
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The integration of vapour–liquid-equilibria (VLE) data with a valid VLE model offers such an
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opportunity to develop a simple and effective method to evaluate the performance of amine-
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based capture processes. The VLE model consists of a series of chemical reactions
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associated with their equilibrium constants and an activity model, and its validity can be
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verified by experimental VLE data. A critical challenge in the development of such a valid
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VLE model is the use of an effective activity model that can reasonably describe the non5 ACS Paragon Plus Environment
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ideal behaviours of chemical species, particularly at high ionic strength. Previous
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investigations usually employed complex activity models to describe the VLE behaviour,
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such as Pitzer20, UNIQUAC21, eNRTL22. These activity models are based on the
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assumptions of some physical definitions, which are anticipated to accurately represent the
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interactions between molecules, electrolytes and ion species. However, these complex
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models have the drawback that many tens of unknown parameters need to be determined
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before modelling. If limited experimental datasets are used to regress these unknown
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parameters, it would lead to over-fitting, with the resulting parameter values having little
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physical relevance. Puxty et al.
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interaction theory (SIT), which required fewer parameters. They found that the SIT-based
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VLE model can reasonably represent the VLE behaviour and greatly simplifies the model.
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However, it is unsure of its comparability with other complex models to accurately represent
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VLE data, and its ability to suitably evaluate CO2 capture performance has not been tested.
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The present study therefore explored the validity of a SIT-based VLE model by comparing it
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with the rigorous rate-based e-NRTL model developed in Aspen Plus. A chemical VLE
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model was developed and validated against the experimentally measured data, and then
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employed to extract more information to obtain a detailed performance evaluation of amine-
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based process using a simple shortcut approach. These derived performance information
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included not only the equilibrium constants, cyclic CO2 capacity, CO2 reaction enthalpy, and
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the regeneration energy requirement, but also the implication of process improvement to
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reduce regeneration duty. The reliability of the shortcut approach was first verified by the
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MEA process, and then validated by the piperazine (PZ), 2-Amino-2-methyl-1-propanol
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(AMP) and PZ/AMP blended solvent. The major objective of the present study was to
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develop a simple but effective approach to evaluate the CO2 capture performance of amine
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process using fewest parameters - specifically just the VLE data. The resulting information is
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anticipated to provide a clear screening shortcut to determine the desirable amine absorbent,
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and a simple evaluation of energy performance for amine-based PCC processes.
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developed a simplified activity model based on specific ion
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2. VLE model development and validation
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2.1 Model development
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The dissolution and absorption of CO2 into aqueous amine solutions can be described by a
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series of physical and chemical reactions, as illustrated in Figure 1. The gaseous CO2 is
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physically dissolved into the amine solution, and the concentration of free aqueous CO2 is
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governed by the Henry’s law (R0). In the liquid phase, CO2 hydration (R1-R3) occurs to form
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the bicarbonate/carbonate species, whilst the interactions between the amine and the
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hydrated CO2 are via amine protonation (R4), carbamate protonation (R5) and carbamate
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formation (R6). While carbamate protonation is not usually included in the chemistry model,
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carbamate formation is typically taken as the dominant reaction pathway of the amine-HCO3-
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interaction for primary and secondary amines, and its equilibrium constant is often
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designated as the carbamate stability constant24. Hence, this reaction pathway is of
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particular importance in amine- CO2-H2O chemistry, as it reflects the equilibrium of the two
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most important species, i.e. amineCOO- and HCO3-. For the sterical and tertiary amines, the
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reaction pathway undergoes bicarbonate formation only as they are unable to form stable
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carbamates.
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,2 , ∆0
(R0)
1 ,∆1
2 + + − 2 ,∆2
2 () + 2
CO2 hydration:
3 ,∆3 3−
Amine protonation: Carbamate protonation: Carbamate formation:
3−
32− + ,∆4 + 4
+
+
(R1)
+
(R2) (R3)
Amine + + 5 ,∆5
+ −
+
3−
+
6 ,∆6
(R4) (R5)
+ 2 (R6) −
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Figure 1 Chemical reactions and vapour-liquid phase equilibrium in an amine–CO2–H2O
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system
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Table 1 shows the equilibrium constants of these reactions and their temperature-dependent
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constants which were determined by
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" = ∏ %&,'(&,' = "* ∏ (&,'
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where " and "* are the activity-based and concentration-based equilibrium constants for
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reaction ; +,," and +-," are the concentrations of product species and reactant species
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involved in the reaction ; .,," and .-," represent the activity coefficients of product species
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and reactant species, respectively, which can be calculated using the SIT-based activity
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model.
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/0* ." =
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where >" is the charge of species ; ? is the ionic strength (mol/L); @ is the density of water
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(kg/dm-3); A = (1.8248 × 106)/(eT)3/2 is the Debye–Hückel law slope. In Puxty et al.
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found that no improvement in the model was found by inclusion of any additional specific ion
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interaction parameters for aqueous MEA, PZ, AMP and PZ/AMP. The chemical VLE model
∏%
(
),' ),'
∏(
(E1)
),'
12'3 4√6 070.9:;
