(156f) Molecular and Process Modelling of Selected Novel Solvents for CO2 Capture

Today the debate regarding climate change is serious as CO₂ levels are at record-breaking highs, with 40% of emissions coming from fossil fuels. Without significant action, CO₂ concentrations are projected to reach 570 ppmv by 2100, causing a rise in global surface temperatures and sea levels, highlighting the need for sustainable strategies to reduce emissions and achieve net-zero state. CO₂ capture via absorption using aqueous amine solvents remains the most widely used industrial process for this purpose. However, a significant technical barrier is the high energy demand for solvent regeneration such as the benchmark 30 wt.% aqueous monoethanolamine (MEA), increasing the cost of capture, nowadays capped at new US DOE limits of $30 per ton. To overcome this, it is necessary to search for novel amines with desirable properties that can improve the economics of the capture process. However, evaluating the performance of new amines is challenging due to limited experimental data necessary for detailed techno-economic evaluation, and the absence of a practical framework to determine the most suitable solvent.

Given the recent advancement in molecular modeling tools, molecular-based equations of state provide a suitable platform for the design of modelling screening frameworks. To better enable the implementation of alternative amines for CO₂ capture, a comprehensive understanding of their properties is essential. The main objective of this work is to develop a robust solvent screening tool using the soft-SAFT molecular-based equation of state (EoS) to obtain relevant thermodynamic properties for assessing novel solvents for CO₂ capture. A collection of 40 alternative novel amines were included in this work, belonging to six families: (1) aminoalcohols, (2) alkylethanolamines, (3) dialkylaminoalcohols, (4) cyclic amines, (5) diamines, and (6) multiamines. The soft-SAFT molecular models of these amines were developed to calculate the thermophysical properties of pure amines such as density, vapor pressure, heat capacity and the heat of vaporization. Soft-SAFT also allows the calculation of other thermodynamic properties such as viscosity and surface tension by coupling the equation with appropriate theories, such as free volume theory for viscosity and density gradient theory for surface tension. The capability of soft-SAFT to describe the phase behaviour is a determining factor for the optimization of absorption/desorption process for CO₂ capture. Alongside properties for the aqueous mixtures of amines inclusive of density, vapor-liquid equilibrium and viscosity, we calculated the CO₂ absorption capacities at relevant process conditions.

This work highlights the predictive power of soft-SAFT EoS in cases where no experimental data was available. The modeling was extended to calculate two key performance indicators, namely, cyclic capacity (Δα) which is the amount of CO₂ effectively separated from the capture plant and the regeneration energy (Qregen) which is linked to the reboiler duty of the stripper column and is the energy required to desorb the CO₂ from the rich amine stream, as screening criteria to determine the most optimal solvent. In our search for the most efficient replacement amine to the golden standard MEA, both 2-Diethylaminoethanol (DEEA) and Diethylenetriamine (DETA) showed promising potential in terms of a high cyclic capacity (Δα) and low regeneration energy (Q_regen), shown from Figure-1. Results presented here bolster the significance of using Soft-SAFT molecular equation of state as a practical tool in the search for novel amines for CO₂ capture.

For a holistic analysis of carbon capture, the impact of aqueous viscosity on the mass transfer coefficient and the reaction kinetics on the reaction rate of CO₂ solubility are two important parameters that can be further studied at the target process conditions of 313K for 30 wt% amine. Moreover, the integration of these thermodynamic models with detailed process modeling such as ASPEN HYSYS to examine the techno-economic feasibility can further aid the search for novel solvents by providing a strong base to calculate and predict relevant thermodynamic properties.

Figure-1 Caption: Scatter plot of the Regeneration energy (Q_regen) with cyclic capacity (Δα) of 36 novel amines.