On the mass transfer of supercritical fluids, specifically super critical CO2: An overview

In many chemical engineering processes mass transfer is a frequently encountered phenomenon. One of the upcoming processes is extraction with supercritical fluids (SCF), especially the use of supercritical carbon dioxide (scCO2). In order to design extraction processes it is very important to understand and describe the mass transfer steps, and also to be able to determine and estimate the mass transfer coefficients. Dimensionless mass transfer correlations together with mass balance models (process models) are highly recommend to ensure a maximum versatility. However, up till now most mass transfer correlations are related and validated to regular solid–liquid, liquid–liquid or gas–liquid systems and are not automatically and directly applicable for SCF due to the unique properties of supercritical fluids. Moreover, in the case of SCF extraction, natural convection can occur and interfere with forced mass transfer. For forced convection Sherwood depends on Reynolds and Schmidt where for natural convection Sherwood depends on only Grashof and Schmidt. In this paper an overview will be given of the dimensionless mass transfer correlations for scCO2 from experimental data in terms of Sherwood, Reynolds and Schmidt for packed bed, annular duct, liquid film and hollow fiber membranes.