(501b) Impact of Liquid Structure on Thermodynamics and Kinetics of Chemical Reactions: Experimental and Molecular Simulations Demonstration with Acid Gas Absorption. | AIChE

(501b) Impact of Liquid Structure on Thermodynamics and Kinetics of Chemical Reactions: Experimental and Molecular Simulations Demonstration with Acid Gas Absorption.

Authors 

de Meyer, F. - Presenter, Totalenergies S.E.
Cloarec, E., Total S.E.
Rozanska, X., Materials Design sarl
Coquelet, C., Mines ParisTech
The chemical absorption of CO2 and H2S in aqueous tertiary amines are well-known acid-base reactions. Important applications are the purification of fuel gases like natural gas, syngas and biogas, and the removal of carbon dioxide from flue gas.

Here we present new kinetic and vapor-liquid equilibrium experiments that show that the addition of an amide such as HMPA, that is known to be a strong liquid structure maker, significantly inhibits the acid-base reactions. The impact is more pronounced for CO2 than for H2S absorption. Despite the presence of a large excess of water in the solvent, the absorption becomes almost physical. Due to hydrogen bonding and the hydrophobic effect, each amide molecule is involved in a cluster containing several water molecules thus rendering the water molecules less available to participate in the reaction and to solvate HS- and HCO3- ions.

This effect is absent when ethylene glycol, a weak structure maker, is added, even in large quantities.

Subsequently, we investigated whether state-of-the-art molecular dynamics simulations could capture this effect. However, simulations of the water-HMPA system could not even reproduce the strongly negative excess volume of the mixture. This illustrates the need for more accurate force fields to simulate the structuring effect and their impact on chemical reactions.

This study clearly demonstrates the importance of solvent structure in the study of chemical reactions.