Influence of Solvent Molecular Structure on Energy Consumption of Post-Combustion CO2 Capture Processes

Influence of Solvent Molecular Structure on Energy Consumption of Post-Combustion CO₂ Capture Processes

Kevin G. Joback¹ J.R. Heberle² and Abhoyjit S. Bhown²

¹Molecular Knowledge Systems, Bedford, NH 03110-0755 USA

²Electric Power Research Institute, Palo Alto, CA 94304, USA

We present an updated physical property-based model for calculating the energy requirement for the post-combustion capture of carbon dioxide by absorption in aqueous amine solutions for any arbitrary amine. The only input the model requires is the amine’s molecular structure.

Equations 1 through 6 show the chemical reactions between carbon dioxide, water and an amine:

CO2g→CO2aq	(1)
2H2Ol→H3O++OH-	(2)

CO2aq+2H2Ol->H3O++HCO3-

(3)

H2Ol+HCO3-=H3O++CO3-2

(4)

RR'R''Naq+H3O+=RR'R''NH++H2Ol

(5)

HCO3-+RR'NH(aq)=H2Ol+RR'NCOO-

(6)

Primary and secondary amines undergo reactions 5 and 6 while tertiary amines undergo only reaction 6, i.e., tertiary amines do not form carbamates.

Our updated model uses:

·         An equation for the Henry’s law constant of carbon dioxide in aqueous solvent mixtures (Equation 1). This equation was developed from experimental data on carbon dioxide solubility in water and nitrous oxide solubility in pure amines.

·         Equilibrium constants regressed from data on the water dissociation reaction (Equation 2) and the carbon dioxide ionization reactions (Equation 3 and Equation 4).

·         A new group contribution technique for estimating a solvent’s pKa as a function of temperature (Equation 5).

·         A new, general equation for the equilibrium constant of the carbamate formation reaction for primary and secondary amines (Equation 6). This equation requires only knowledge of the amine’s molecular structure; no additional experimental data is needed.

·         A group contribution methods to estimate the liquid-phase activity coefficients.

·         A group contribution method to estimate pure component heat capacities.

·         A simple model used to estimate the heat capacity of aqueous amine solutions loaded with carbon dioxide.

Combined, these equations are used to calculate the concentration of chemical species, carbon dioxide partial pressure and the enthalpies of reaction over a range of solvent concentrations, loadings, and system temperatures. Finally, the effective heat capacity is used to calculate the equivalent electrical load (reduction in net electrical output from a power plant) using a simplified process-independent model.

The development of the model was guided by a simplified sensitivity analysis. We varied values for parameters and inputs—e.g., Henry’s law constants, pKa as a function of temperature, equilibrium constants, activity coefficients, and heat capacities—to identify those parameters that had the greatest affect on overall model accuracy. Those parameters with the greatest significance became the focus of improvement. The final model developed has good accuracy when compared with experimental data for species concentrations, partial pressures, and enthalpies of reaction for several common aqueous amine solvents. We also show that our new model has accuracy comparable to several current models that have been regressed directly from experimental data.

While the model uses multiple equations and regressed parameters, we reemphasize that the only input needed to evaluate the performance of any amine solvent is the amine’s molecular structure. We used our model to computationally evaluate hundreds of candidate solvent structures. For the most promising candidates we calculated the species concentrations, partial pressures, enthalpies of reactions and imposed loads. From these calculations, we have drawn some general conclusions on the influence basic physical properties on the energy consumption of post-combustion carbon dioxide capture processes.