Application of CFD in the study of supercritical fluid extraction with structured packing: Dry pressure drop calculations

Liquid phase SFE has been carried out in extraction columns, with structured packings particularly of the gauze type. Structured packing performs very well during extractions under these conditions, mainly due to their relatively large surface area and free volume. Nevertheless, there are also important disadvantages such as high cost, low capacities at high flow rates, and premature flooding. The assessment of the real efficiencies of these packings poses extreme difficulties related to the moderately high pressures involved in these processes. Computational fluid dynamics (CFD) can be used to characterize the complex single and multiphase flow inside the packed bed and evaluate the influence of the shape and geometry of the packing on the hydrodynamics and mass and heat transfer rates in the SFE packed column. In this first study, and with a very detailed modelling of the packing geometry, the dry pressure drop is calculated for both laminar and turbulent flow conditions. Results are seen to agree well with the experimental data in the literature.

Liquid phase SFE has been carried out in extraction columns, with structured packings particularly of the gauze type. Structured packing performs very well during extractions under these conditions, mainly due to their relatively large surface area and free volume. Nevertheless, there are also important disadvantages such as high cost, low capacities at high flow rates, and premature flooding. The assessment of the real efficiencies of these packings poses extreme difficulties related to the moderately high pressures involved in these processes. Computational fluid dynamics (CFD) can be used to characterize the complex single and multiphase flow inside the packed bed and evaluate the influence of the shape and geometry of the packing on the hydrodynamics and mass and heat transfer rates in the SFE packed column. Two computational models are presented in this work for the estimation of the pressure drop caused by Sulzer EX packing. The first one being a simple representation of two packing sheets and assuming periodicity perpendicularly to the main flow direction and the second one a partial packing module, this one with periodicity in the flow direction. Simulations were carried out to determine the pressure drops of Sulzer EX at various gas mass flowrates and various conditions of pressure (0.1–30 MPa) and temperature (298–393 K) and good agreement was found between predicted and experimental pressure drops.Even though the dry pressure drop is a very important factor in the design of SFE columns and is used in many correlations for the determination of wet pressure drop, alone it is not enough to determine their hydrodynamics. The next step will be to modify and test these models against real extraction conditions on two-phase counter-current flow. This implies a study of the heat and mass transfer in the column.Figure optionsDownload as PowerPoint slide