Diffusivity in Novel Diamine-Based Water-Lean Absorbent Systems for CO₂ Capture Applications

Abstract

Significant reduction of the water content of traditional absorbents, increasing organic character of absorbent molecules, and substitution of water with a non-aqueous diluent are increasingly attracting interest as means to improve absorbent performance. From our previous work, the novel diamine absorbents N,N-dimethyl-1,3-propanediamine (DMPDA) and N,N-dimethyl-1,2-ethanediamine (DMEDA), also utilizing N-methyl-2-pyrrolidone (NMP) as a non-aqueous diluent to reduce the water content of the absorbent, were demonstrated to produce an absorbent blend with a significantly lower overall energy consumption (for CO2 regeneration). Alongside the thermodynamic performance, CO2 absorption mass transfer plays an equally critical role in the overall performance of an absorbent for CO2 capture processes. Gaining an understanding of the fundamental factors influencing mass transfer behavior has long been the focus of research efforts, and the diffusivity of the absorbent molecules is a critical factor for amines to be able to rapidly react with CO2. Expanding on the initial investigation of these promising blends, we evaluate herein the diffusivity of the component molecules of a number of blends as a function of temperature, CO2 loading, absorbent composition, and absorbent viscosity. A powerful technique based on nuclear magnetic resonance (NMR) spectroscopy was used to provide direct measurement of the diffusion coefficients of individual chemical species in the blends. Diffusivity and viscosity were found to behave very differently in water-lean and aqueous blends, with water-lean blends being particularly sensitive to CO2 loading and water content. The hydrodynamic radii of species in the water-lean blends were particularly sensitive to temperature relative to the aqueous blends, significantly decreasing as the temperature was increased with associated potential mass transfer benefits. This can be put down to the introduction of NMP, which weakens the intermolecular interactions (forming a hydrogen bond) between molecules and water, and this impact increased through increasing temperature. This highlights that the optimal operating conditions for water-lean blends are likely quite different to those used traditionally for aqueous blends.