Carbon dioxide solubility in mixtures of methyldiethanolamine with monoethylene glycol, monoethylene glycol–water, water and triethylene glycol

Abstract

Carbon dioxide solubility in non-aqueous and aqueous mixtures of methyldiethanolamine (MDEA) with monoethylene glycol (MEG) was studied due to the relevance of these solvents for the combined acid gas removal and hydrate control in natural gas treatment. Vapor-liquid equilibrium (VLE) measurements were conducted at temperatures from 303 K to 393 K and pressures up to 600 kPa. In the aqueous solvents, the effect of water content in carbon dioxide solubility was investigated. The absorption capacity of the aqueous solvents decreased with increasing glycol content and decreasing water content, at constant amine concentration. A comparison of the studied systems with concentrated aqueous MDEA was also performed. The non-aqueous solvents were studied in the whole composition range, from pure MDEA to pure MEG. The solubility of carbon dioxide increased with increasing amine content only up to 30–50 wt% MDEA-MEG, upon which it decreased. Water content determination and Nuclear Magnetic Resonance (NMR) analysis were used for the chemical characterization of the systems and explanation of the results. It was found that in the presence of MDEA, a chemical reaction occurs between carbon dioxide and MEG. A theory based on MEG autoprotolysis is proposed which is further supported by supplementary VLE data obtained in blends of MDEA and triethylene glycol.