(634d) Understanding the Behavior of Degraded Amine Products on CO2 Capture in Aqueous Amines By Molecular Simulations

In the context of sustainable development and clean energy production, one of the most important alternatives to mitigate anthropogenic CO₂ emissions is to capture and separate CO₂ (CCS) from diluted sources, such as gases emitted from fossil fuel combustion and other industrial processes. The most standard process used in industry today is the separation by absorption in aqueous amine solvents. Among them, monoethanolamine (MEA) is the benchmark molecule because of its properties such as high CO₂ cyclic capacity, high kinetic at low CO2 partial pressure, low viscosity, high water solubility, low price, etc. Nevertheless, it has some disadvantages such intensive energy required for desorption and solvent degradation, the last one counting for around 10% of the total cost of CO2 capture. Furthermore, byproducts cause a noteworthy decrease in process efficiency with solvent losses, corrosion, fouling, foaming and an increase in viscosity.

The degradation of MEA may arise thermally in the presence of CO2, which occurs at high temperature and high CO2 partial pressure in the stripper, or through oxidative degradation, mainly due to large amount of O2 in flue gases. Thermal degradation causes dealkylation, dimerization and cyclisation. First, MEA, as primary amine, reacts with CO2 to form carbamate in the absorber and the carbamate can be transformed into 2-oxazolidone (OZD). Subsequently, OZD reacts with the MEA molecule to form HEEDA (N-(2-Hydroxyethyl) ethylenediamine), precursor of HEIA, which is known to thermally degrade quickly. As a result, HEEDA cannot accumulate in the process and HEIA is formed by the reaction between HEEDA with CO2, and followed by intramolecular cyclisation. The three major thermal degradation products of MEA are, hence, HEIA, the dominant one, HEEDA and OZD. HEIA is the largest degradation product found in solution that do not react to form further polymeric products. Its presence increases with time, which is an indication of its relative stability, and it does not affect the solution’s corrosivity.

We will present and discuss results concerning the effect of the presence of degradation amine products into the microscopic behavior of the CO2 capture process with aqueous amines, focused on HEIA. Molecular simulations in the NVT and NPT ensemble have been performed for this purpose. The study has been done in a systematic way. We first studied the pure component (HEIA), validating the force field with experimental density data, followed by the study of the binary (HEIA+H2O, HEIA+MEA, MEA+H2O), ternary (HEIA+H2O+MEA, HEIA+H2O+CO2, MEA+H2O+CO2) and the quaternary system (HEIA+MEA+H2O+CO2). It is observed that the HEIA shows no specific interactions with either the MEA or water, and it forms aggregates to avoid the contact with them. We will discuss the implications of this microscopic behavior into the final absorption performance of the aqueous amine solvent, with and without HEIA. This work shows how molecular simulations can help to elucidate the absorption mechanism and to find optimal absorbents for CO2 capture.

This work has been partially funded by ADNOC Gas Processing and its stakeholders (Total, Shell and Partex) through a Gas Research Center project (GRC2018-003) and Khalifa University of Science and Technology (project RC2-2019-007).