Self diffusion coefficients of organic solvents and their binary mixtures with CO2 in silica alcogels at pressures up to 6 MPa derived by NMR pulsed gradient spin echo

Abstract

Self diffusion coefficients were determined using NMR (nuclear magnetic resonance) pulsed gradient spin echo (PGSE) in combination with a special high pressure autoclave. The goal was to determine the self diffusion coefficients for different CO2/methanol and CO2/ethanol mixtures within the pore phase of silica alcogels at pressures up to 6MPa. These values are required as input data for the modeling of process steps preceding the supercritical drying of these gels.The self diffusion coefficients derived with the set-up at 15°C for unconfined CO2/methanol and CO2/ethanol mixtures as a function of concentration of the organic solvent as well as of fluid pressure are in line with results from previous publications thus proofing the approach to provide reliable data.Measurements of self diffusion coefficients of methanol at different pressures and methanol/CO2 ratios were performed in tetramethoxysilane based silica alcogels with target densities ranging from 80 to 200kg/m3; the results reveal tortuosities of up to 2.5 depending on the silica fraction in the gel as well as its structure. The experimental self diffusion coefficients are in good agreement with the transport diffusion coefficient determined from time resolved NMR imaging of a fluid exchange within a alcogel of methanol for CO2 at 5.8MPa.