Thermal modulation of reaction equilibria controls mass transfer in CO₂-binding organic liquids

Abstract

CO₂-Binding organic liquids (CO₂BOLs) are non-aqueous solvents which may reduce the parasitic energy of carbon capture processes. These solvents exhibit surprising mass transfer behavior: at fixed pressure driving force, the flux of CO₂ into CO₂BOLs decreases exponentially with increased temperature, a phenomenon not observed in aqueous amines. Here, we demonstrate that this phenomenon is primarily driven by a shift in reaction equilibrium, which reduces the degree to which chemical reactions enhance the CO₂ flux. First-principles surface renewal models quantitatively reproduce mass transfer data for CO₂ absorption into 2-EEMPA, IPADM-2-BOL and DBU:Hexanol across a range of temperatures. Density functional theory calculations are used to identify structural modifications likely to improve the CO₂ flux. These findings reveal a fundamental trade-off between CO₂ flux and the energy required for solvent regeneration, and provide a theoretical foundation for rational solvent design and the development of physics-informed mass transfer models.

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Broader context

The capture of CO₂ from dilute gas streams, and its subsequent storage or utilization, is a critical technology for reducing global CO₂ emissions. The best-established carbon capture technologies use aqueous amine solutions, though non-aqueous solvents, such as CO₂-Binding Organic Liquids, have attracted increased attention, as they may allow reductions in process energy requirements. However, the mechanism by which these solvents absorb CO₂ has remained a fundamental mystery since they were first synthesized. When CO₂BOLs are heated, CO₂ absorption rates decrease exponentially – an unintuitive behavior opposite what is observed in aqueous systems. The lack of understanding of basic mass transport mechanisms hampers the development of these solvents, as it rules out the use of physics-based mass transfer models routinely used by process modeling software, and complicates the rational design of new solvent systems. In this work, we explain the surprising mass transport behavior in CO₂BOLs, and develop first-principles mass transport models in excellent agreement with experimental data.

1 Introduction

CO₂-binding organic liquids (CO₂BOLs) are a promising class of non-aqueous solvent for carbon capture and storage (CCS).^1,2 These solvents reversibly react with CO₂, and exhibit CO₂ capacities and uptake rates comparable with state-of-the-art aqueous amines.^3,4 Several modeling studies predict CO₂BOLs may improve the energy efficiency of CCS processes,^5–7 though this is a matter of active debate in the CCS community.^8–12

An enduring mystery in the CO₂BOL literature pertains to the unexpected temperature dependence of the CO₂ absorption rate. The flux of CO₂ into a wide range of CO₂BOLs exponentially decreases with increasing temperature at constant pressure driving force: the opposite trend to what is typically observed in aqueous amines. Formally, the rate of absorption into a chemical solvent may be quantified by the liquid-film mass transfer coefficient, which (in the absence of gas-side resistance) is given by:

where J_i is the gas flux, p_i is the partial pressure in the gas, and p^eq_i is the equilibrium partial pressure in the liquid. As shown in Fig. 1a and b, in a wide range of CO₂BOLs, is experimentally observed to decrease rapidly with temperature, while in more traditional, aqueous solvents is usually a weakly increasing function of temperature. In spite of multiple experimental^1,3,4,13 and molecular modeling studies,^14–19 no adequate explanation of this phenomenon has been discovered. Some authors have speculated that this surprising behaviour may be caused by a decrease in the physical solubility of CO₂, however fundamental studies utilizing non-reactive CO₂ mimics such as N₂O did not show similar thermal effects (Fig. 1c).^4,13 This has led several authors to speculate that novel film theories may be required to explain mass transport phenomena in CO₂BOLs,^3,13 possibly involving mesoscopic structures which can spontaneously form during CO₂ absorption.²⁰ Understanding mass transport mechanisms within CO₂BOLs is essential for molecular design of solvents, for reliable modeling of industrial-scale processes, and is also of basic scientific importance.

Fig. 1 Liquid-film mass transfer coefficients as a function of temperature. (a) Wetted Wall column data for four CO₂BOLs: IPADM-2-BOL,³ DBU:Hexanol,³ GAP-1/TEG,⁴ and EEMPA¹ (b) Wetted wall column data for two aqueous amines: piperazine and monoethanolamine; 80 °C and 100 °C data interpolated.²¹ (c) Wetted wall column data for absorption of CO₂ and N₂O into DBU:Hexanol^3,13 and GAP-1/TEG,⁴ at various CO₂ loadings (mol CO₂/mol amine × 100%). GAP-1/TEG CO₂ data interpolated from data at CO₂ loadings of 4% and 13%.

In this article, we demonstrate that the decrease in the rate of CO₂ absorption into CO₂BOLs at increased temperature is not primarily driven by decreases in the physical solubility of CO₂, but by a thermally-driven shift in reaction equilibrium, which decreases the extent of reaction of CO₂ with the solvent in the vicinity of the gas–liquid interface. This in turn decreases the enhancement in gas flux due to chemical reaction. The CO₂ flux into CO₂BOLs is typically limited by diffusion of reaction products away from the gas–liquid interface, and due to shifts in reaction equilibrium, the rate of this diffusion decreases sharply with increased temperature. Surface renewal models based on this hypothesis are in excellent agreement with available kinetic data for multiple CO₂BOLs. This work demonstrates that novel film theories are not required to explain the surprising thermal effects in CO₂BOLs. The interplay between mass transport and Vapor–Liquid Equilibria (VLE) suggests several non-obvious design criteria for the molecular design of CO₂BOLs, and we use DFT simulations to identify structural modifications which may improve absorption kinetics.

2 CO₂ transport in CO₂BOLs

2.1 Mass transport mechanism

The hypothesized mechanism by which the flux of CO₂ into CO₂BOLs decreases with increased temperature is demonstrated by an idealized example sketched in Fig. 2. Fig. 2a shows VLE data for the CO₂BOL 2-EEMPA (to avoid confusion between molecular names and stoichiometric coefficients, we denote this as EEMPA throughout the manuscript) which reacts with CO₂ as follows:¹

CO₂ + 2EEMPA ⇌ EEMPACOO⁻ + EEMPAH⁺ (2)

Fig. 2 Mechanism for decrease in with temperature. (a) Vapor–Liquid Equilibrium data for CO₂ absorption into EEMPA. Data points taken from Zheng et al.¹ Solid lines regressed to data based on methodology of Gabrielson et al.,²² see ESI,† Section S3. (b and c) Concentration profiles within static EEMPA solution in the vicinity of the gas–liquid interface, 0.1 s and 1 s after it has been exposed to p_CO2 = 10 kPa, at (b) 70 °C, and (c) 30 °C. Calculated using surface renewal model under assumption of instantaneous reaction.

Two points are marked on this plot: both points are at a CO₂ partial pressure of 10 kPa (a pressure typically found in a CCS absorber), but point (1) is at 30 °C, while point (2) is at 70 °C. At point (1), the equilibrium CO₂ loading, α, is around 37% on a mol CO₂:mol EEMPA basis, while at point (2), the equilibrium CO₂ loading is only about 3.5%. This decrease in equilibrium CO₂ loading is responsible for the decrease in solvent kinetics shown in Fig. 1a; the mechanism is illustrated in Fig. 2b and c. These figures show concentration profiles of CO₂, EEMPA and EEMPACOO⁻ within an EEMPA solution with bulk CO₂ loading of α = 0, 0.1 s and 1s after it has been exposed to a gas with p_CO2 = 10 kPa. These profiles were calculated using surface renewal theory models discussed below. In Fig. 2c (which corresponds to point (1), with system temperature of 30 °C) CO₂ reacts strongly with EEMPA, leading to a large concentration of EEMPACOO⁻ at the gas–liquid interface, which carries bound CO₂ into the liquid at a fast rate. On the other hand, at 70 °C (Fig. 2b), due to equilibrium effects, the extent of Rxn (2) is far smaller, being unable to exceed an equilibrium CO₂ loading of ∼3.5%, resulting in a much smaller EEMPACOO⁻ concentration gradient, and slower CO₂ absorption at this higher temperature. In both examples, p_CO2 = 10 kPa and , so by eqn (1) the decrease in CO₂ flux with increased temperature corresponds to a proportional decrease in .

Formally, the liquid-film mass transfer coefficient may be factorized into three terms,

where H is the Henry's constant for the absorbing gas, k⁰_l is the physical liquid-film mass transfer coefficient (which governs mass transport into non-reactive solvents), and E is the enhancement factor, which quantifies the degree to which chemical reactions increase the gas flux, relative to a physical solvent.²³ Based on the mechanism sketched in Fig. 2, we propose that the decrease in with temperature (Fig. 1a) is primarily driven by a decrease in the enhancement factor, E, caused by a thermally-driven shift in reaction equilibrium which decreases the extent of the CO₂–CO₂BOL reaction, and in turn the magnitude of the concentration gradient of CO₂-bound-CO₂BOL adjacent to the gas–liquid interface. Decreases in the physical CO₂ solubility, H, are of secondary importance. The reason this phenomena is observed in CO₂BOLs, but not commonly used aqueous amines, is that for the latter the reaction with CO₂ is more irreversible under absorber conditions, resulting in much smaller shifts in reaction equilibrium. For example, between 30 °C and 60 °C, and at a CO₂ partial pressure of p_CO2 = 10 kPa, the equilibrium CO₂ loading in 30 wt% monoethanol amine decreases from 0.49 mol CO₂/mol amine to 0.46 mol CO₂/mol amine (for reference, the maximum chemical loading based on a 1 : 2 stoichiometry is 0.5 mol CO₂/mol amine).²² Under the same conditions, the equilibrium CO₂ loading into EEMPA decreases from 0.37 mol CO₂/mol amine to 0.07 mol CO₂/mol amine: a much more significant shift in reaction extent (see Fig. S1, ESI†). The effect may also be seen in Fig. S2 (ESI†), which shows results of simulations under identical conditions to those in Fig. 2b and c, for a 30 wt% MEA solution, a 30 wt% K₂CO₃ solution, and an EEMPA solution with finite reaction kinetics. It is clear that the concentration profiles which form adjacent to the gas–liquid interface are far more sensitive to temperature in the CO₂BOL system than in the aqueous systems. Because the equilibrium CO₂ loading is not strongly temperature dependent in the aqueous solvents under these conditions, shifts in reaction equilibrium with T do not significantly affect ; instead, increases in diffusion coefficient and reaction kinetics cause an increase in .

2.2 Model development

The example in the previous section was idealized in several ways. It considered an unloaded solvent () with infinitely fast reaction kinetics, on which a fixed CO₂ pressure was applied. While this example provides useful intuition for why should decrease with T, more sophisticated models are required to capture the interplay between reaction kinetics, VLE and in solvents at arbitrary loading and temperature. For this reason, models for CO₂ absorption into several agitated CO₂BOLs were constructed based on the surface renewal theory of Danckwerts.^23,24 Surface renewal theory has been widely used to simulate mass transfer into agitated chemical solvents,^25,26 and is generally considered more accurate than alternative theories, such as the penetration or film theory.²⁷ In this model, partial differential equations governing reactive absorption into a static liquid are solved, however at random intervals the concentration profiles in the liquid are ‘reset’ or ‘renewed’ to bulk conditions, mimicking a random eddy carrying fresh liquid to the surface. These surface renewals are modelled as a Poisson process,²⁸ so the time between renewals, θ, is governed by an exponential probability distribution with probability density function:

f(θ) = se^−sθ (4)

The actual gas flux is then taken to equal the long-time average gas flux for this stochastic process. The mean surface renewal rate, s, is the only parameter in the model, and it may be calculated via:²³

where D is the diffusivity of the absorbing gas, and k⁰_l is the physical liquid film mass transport coefficient.

In order to use surface renewal theory to estimate CO₂ flux into a CO₂BOL, species diffusivities, reaction rates, and solubilities must be estimated, along with VLE data and k⁰_l in the experimental system. Models were developed for three CO₂BOLs: EEMPA, IPADM-2-BOL, and DBU:Hexanol. Diffusivities were estimated from available literature data or, if no data were available, from the correlation of Wilke and Chang,²⁹ using temperature- and loading-dependent viscosity data from the literature for each CO₂BOL modeled.^1,3 The Henry's constants for CO₂ were estimated from measurements in comparable organic liquids (for reasons discussed in Section S3 of the ESI,† the predicted gas fluxes were only weakly dependent on the choice of Henry's constant.) k⁰_l for wetted-wall column data was estimated at experimental conditions from the correlation of Vivian and Peaceman.³⁰ Experimental VLE data has previously been reported for each CO₂BOL above,^1,3 and this data was fit to models of the kind discussed by Gabrielson²² (see Fig. S3–S5, ESI†). Regarding reaction rate constants: for DBU:Hexanol, the kinetic data of Ozturk et al.³¹ was used. They observed relatively slow kinetics in DBU:Hexanol, with a second order rate constant about 10 times smaller than for MEA.³² On the other hand, several molecular modeling studies^14,16 suggest the reaction of IPADM-2-BOL with CO₂ is instantaneous, though this has not been verified experimentally. Hence, for both IPADM-2-BOL and EEMPA, two reaction rates were considered: an instantaneous reaction, and a reaction with finite kinetics equal to those found in DBU:Hexanol. All reactions were treated as reversible, with equilibrium constants calculated from the VLE data and CO₂ Henry's constant. Further details on the implementation of the surface renewal model are provided in the ESI,† Section S3. The Julia code used to generate all figures is also included in the ESI.†

3 Results and discussion

In Fig. 3, experimentally measured liquid-film mass transfer coefficients in a range of CO₂BOLs and over a range of temperatures are compared with the predictions of the surface renewal theory. We stress that the surface renewal models are not fit to the experimental mass transport data, but are instead physics-based predictions, based on independently measured or estimated VLE, reaction kinetics, diffusivity and solubility data. Not only does the surface renewal theory correctly predict trends in with temperature and between different CO₂BOLs, but the match is quantitatively reasonable.

Fig. 3 Comparison between surface renewal theory models (lines and shared areas) and wetted wall column data (points), for IPADM-2-BOL, EEMPA and DBU:Hexanol. For IPADM-2-BOL and EEMPA, an instantaneous reaction (solid lines) and a finite reaction rate (DBU:Hexanol kinetics, dashed line) were modelled; the range of intermediate values is colored in. Dash-dot lines refer to simulations in which the reaction equilibrium constant is artificially fixed at its value at 30 °C (EEMPA) and 40 °C (IPADM-2-BOL), so that is not influenced by changes in reaction extent. CO₂ loading and sources as follows: EEMPA data from Zheng et al.¹ at 0.01 mol CO₂/mol CO₂BOL; IPADM-2-BOL data from Mathias et al.³ at 0.0106 mol CO₂/mol CO₂BOL; DBU:Hexanol data from Mathias et al.³ at 0.0146 mol CO₂/mol CO₂BOL.

The surface renewal models correctly predict the decrease of with increased temperature. That this decrease is primarily driven by a thermally-driven shift in reaction equilibrium, as discussed above, and not by a decrease in physical solubility of CO₂, is shown by the dash-dotted lines in Fig. 3. These show the results of simulations in which the reaction equilibrium constant is fixed at a constant value, while all other parameters vary with temperature. When the temperature-dependence of the reaction equilibrium constant is ignored, increases with temperature, as it does in aqueous amines (Fig. 1b). In Fig. S6 (ESI†), we factorize the surface renewal model's prediction of for IPADM-2-BOL into contributions from H, E and k⁰_l (see eqn (2)), and plot the relative change in each parameter as T varies from 40 °C to 100 °C. It is clear that it is primarily the thermally-driven decrease in reactive enhancement factor, E (caused by the shift in reaction equilibrium) which drives the decrease in ; the smaller decrease in Henry's constant, H, with T is largely canceled out by the increase in k⁰_l with T, as seen in other chemical solvents.²¹ This may also be seen in Fig. S7 and S8 (ESI†), which show modelled changes in temperature for both and also k_l = Ek⁰_l. As changes in E drive the decrease in mass transfer coefficient, both and k_l exhibit similar trends with temperature.

The relative insignificance of changes in physical CO₂ solubility with temperature is supported by several other lines of evidence. First, when N₂O, a non-reactive CO₂-mimic,^33,34 is absorbed by CO₂BOLs, is much smaller, and does not decrease with temperature, suggesting changes in chemical reaction equilibrium constants, rather than physical solubility, drive this phenomenon (see Fig. 1c). Second, if we make the plausible assumption that the CO₂ flux is at most proportional to the physical CO₂ solubility (i.e. there are no phenomena such as autocatalytic reactions which cause a higher-order dependence of the flux on the CO₂(aq) concentration) then the 10-fold decrease in for IPADM-2-BOL between 40 °C to 100 °C (Fig. 1a) would require at minimum a commensurate decrease in CO₂ solubility. This in turn would imply (via the Gibbs-Helmholtz equation^35–37) that the enthalpy of physical dissolution of CO₂ was on the order of −70 kJ mol⁻¹: much larger than the value of −10 to −15 kJ mol⁻¹ typically observed for physical CO₂ dissolution.³⁸ Finally, we note that several authors have hypothesized that thermal effects in CO₂BOLs may be related to the relatively large equilibrium mole fraction of physically dissolved CO₂ in organic liquids as compared to aqueous solutions.^1,3 However, the small mole fraction of CO₂ in water is predominantly driven by the small molecular weight of water, rather than differences in CO₂ concentration. As shown in Fig. S9 (ESI†), when CO₂ solubility is calculated in reference to volumetric concentration, rather than mole fraction, (the former being the more appropriate driving force for mass flux³⁹) the CO₂ solubility in a range of organic liquids is only modestly larger than in water.

These models explain the trends observed between different solvents. In particular, that EEMPA absorbs CO₂ more slowly than IPADM-2-BOL at low loadings is primarily a consequence of the difference in VLE between the solvents. For example, at 60 °C and p_CO2 = 5 kPa, the CO₂ loading in IPADM-2-BOL and EEMPA are ∼18% (see Fig. S4, ESI†) and ∼3% (see Fig. 2a), respectively. Hence, the reaction of IPADM-2-BOL with CO₂ proceeds much further than the equivalent reaction with EEMPA, resulting in much larger concentration gradients of reaction products and a greater reactive enhancement factor.

The surface renewal model also correctly predicts trends with CO₂ loading, as shown in Fig. S10 (ESI†), which compares the surface renewal model with data for EEMPA collected by Zheng et al.¹ At higher loadings, the mechanism discussed in Section 2.1 for a solvent at zero CO₂ loading must be expanded upon. In general, when temperature changes cause an increase in the gradient of the absorption isotherm, , this causes the loading, α, to be less sensitive to changes in pressure. If diffusion of product species from the surface to the bulk is rate controlling, this will result in a smaller change in α between the interface and the bulk for a fixed pressure driving force, which can cause a reduction in gas flux and also (as the pressure driving force is fixed) a reduction in with increased temperature. Of course, in general such equilibrium phenomena occur alongside and compete with kinetic effects, and it is only through the development of rigorous models that their relative impact on can be assessed.

Finally, we note that reductions in reactive enhancement factor due to shifts in reaction equilibria are not limited to CO₂BOLs. For example, under stripper conditions the equilibrium loading of MEA solutions becomes highly temperature dependent, and this causes a reduction in with increased T (see Fig. S11, ESI†). This effect is not observed in K₂CO₃ solutions, (Fig. S12, ESI†) as the reaction kinetics are too slow for diffusion of product species from the interface to the bulk to be rate limiting. In general, reductions in may be expected in solvents systems with fast reaction kinetics operated in a regime where the VLE is highly temperature-dependent.

3.1 Implications for CO₂BOL development

These results demonstrate that the primary factor influencing CO₂ uptake rates in CO₂BOLs is the extent of the reversible chemical reaction. Hence, in order to maximize the absorption rate, CO₂BOLs should be designed to have large reaction conversion under absorber conditions. Improving the diffusivity of the liquid-phase species (i.e. the CO₂BOL and the CO₂-bound-CO₂BOL) will also improve the CO₂ uptake rate. Increasing reaction kinetics helps to a degree, but the benefit plateaus once diffusion of unreacted CO₂BOL and CO₂-bound-CO₂BOL to and from the bulk becomes rate-limiting.

As CO₂BOL CO₂ uptake kinetics are strongly influenced by the VLE, it is likely they can be tuned via structural changes which shift the reaction equilibrium. To investigate this, quantum chemical simulations were used to explore the enthalpy of CO₂ absorption (ΔH_abs) for selected CO₂BOL species. Initially, CO₂ binding in IPADM-2-BOL and EEMPA was examined to identify key binding modes and inform chemical modifications that may increase the magnitude of ΔH_abs, which in turn would shift the reaction equilibrium towards reaction products, increasing E at higher temperatures.

Molecular dynamic simulations were performed on each compound to identify an initial series of structures corresponding to local energy minima, which were then further optimized via DFT calculations at the M06L/6-31++G(d,p) level. Implicit solvent corrections to energies were applied using the Polarizable Continuum Model using the integral equation formalism variant (IEFPCM). While the data presented was calculated using the dielectric constant of water, calculations using additional solvents (as well as gas phase) were performed to evaluate the impact of dielectric constant on CO₂ binding enthalpy (Table S1, ESI†). Additional computational details are provided in the ESI† (Section S4).

Two different CO₂ bound species emerged as the most thermodynamically favorable for both IPADM-2-BOL and EEMPA: an intramolecularly H-bonded monomolecular species, and intermolecularly H-bonded bimolecular species (Fig. 4a and b). Interestingly, for both CO₂BOLs, the bimolecular species was calculated to have markedly lower ΔH_abs than the monomolecular species, in agreement with the predicted stoichiometry from modeling of the VLE data. Additionally, the bimolecular binding enthalpies of around −52 kJ mol⁻¹ and −77 kJ mol⁻¹ for IPADM-2-BOL and EEMPA, respectively, are in reasonable agreement with those predicted by the VLE data.

Fig. 4 Enthalpies of monomolecular and bimolecular CO₂ absorption for (a) EEMPA, (b) IPADM-2-BOL, and (c) phosphazene based CO₂BOL structures.

Given the prominence of H-bonding in these CO₂ bound structures, we hypothesized that increasing the strength of the basic moiety might be a reasonable approach towards increasing the magnitude of ΔH_abs. Inspired by the use of guanidine in IPADM-2-BOL, we were particularly interested in the effect of incorporating a phosphazene organic superbase, which is a stronger base than guanidine and highly tunable (Fig. 4c). Compared to the structurally similar IPADM-2-BOL, the examined phosphazene-based structure was calculated to have stronger ΔH_abs for both the monomolecular and bimolecular species, by around 15 kJ mol⁻¹ and 25 kJ mol⁻¹, respectively. A shift of this magnitude could improve absorption kinetics by increasing the extent of the CO₂–CO₂BOL reaction under absorber conditions. For example, if the partial molar entropy of CO₂ absorption is not affected by the structural changes, then decreasing ΔH_abs by 20 kJ mol⁻¹ would increase the equilibrium CO₂ loading into EEMPA at 60 °C, p_CO2 = 5 kPa from its actual value of ∼3% to close to the stoichiometric limit of 50%. This would in turn be expected to increase the CO₂ flux into an unloaded solvent by about an order of magnitude. However the impact is likely to be lower at higher solvent loadings. Furthermore, these structural modifications may also affect other properties of the solvent, including the entropy of absorption,⁴⁰ reaction constants, diffusivities, viscosity, volatility, and how easily it may be manufactured. A more complete in silico solvent design study could systematically screen for solvents with favorable physical, thermodynamic and kinetic properties. Classical mass transfer theories such as surface renewal models could also be incorporated into such a framework. As demonstrated in this work, they are a powerful tool for understanding novel solvent systems, and are capable of deconvoluting kinetic, equilibrium and hydrodynamic phenomena in a computationally efficient manner.

System-scale tradeoffs must also be considered. Shifting CO₂BOL VLE closer to the relatively irreversible VLE of aqueous amines could improve absorption kinetics, especially at lower CO₂ loadings, but it will also increase the magnitude of the temperature swing required for solvent regeneration. Aqueous amines must typically be heated to >100 °C during regeneration, and similar thermal swings may be required if CO₂BOLs are designed with the sole aim of maximizing mass transfer rates in the absorber. In practice, there is likely to be a trade-off between enhanced mass flux and ease-of-regeneration, which must be assessed on an application-dependent basis.

4 Conclusion

The mechanism by which the CO₂ flux into CO₂BOLs decreases with temperature has remained a central, unanswered question since CO₂BOLs were first developed. In this study, we have demonstrated that this phenomena is not driven by decreases in physical CO₂ solubility, as has been previously hypothesized, but is instead caused by thermal shifts in chemical equilibrium, which decrease the enhancement factor for gas flux. Surface renewal models based on this hypotheses were in excellent agreement with wetted wall column data for a range of CO₂BOLs. The mechanism directly connects VLE data and the CO₂ absorption rate, and reveals a fundamental tradeoff between maximizing CO₂ flux under absorption conditions, and minimizing the temperature swing required for solvent regeneration, which will inform rational design of CO₂BOLs. This work also provides the necessary theoretical foundation for the development of CFD and process-scale simulations based on fundamental, physics-based models.

Author contributions

Thomas Moore: conceptualization, methodology, software, formal analysis, writing – original draft. Anthony J. Varni: methodology, software, formal analysis, writing – original draft. Simon H. Pang: conceptualization, writing – review & editing. Sneha A. Akhade supervision. Sichi Li writing – review & editing, supervision. Du T. Nguyen: writing – review & editing, supervision. Joshuah K. Stolaroff writing – review & editing, supervision, funding acquisition.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was performed under the auspices of the US Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Funding was provided by the US Department of Energy, Office of Fossil Energy and Carbon Management under FWP-FEW0225. Release number: LLNL-JRNL-840252.

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Footnote

† Electronic supplementary information (ESI) available: SI.pdf contains supplementary figures and tables, numerical methodology. CO₂BOLScripts.zip contains code for generation of plots in Fig. 1–3. See DOI: https://doi.org/10.1039/d2ee03237f

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