Enhanced filtration of organic viscous liquids by supercritical CO2 addition and fluidification. Application to used oil regeneration

Cross-flow filtration of viscous liquids is a difficult challenge because i t demands high-energy consumption for poor permeate flow rates. Moreover, t he final concentration is clearly limited by the pressure drop through pipe s and membrane, Usually, apparent viscosity is decreased by means of temper ature, addition of organic fluidiser (organic solvent) or by means of incre asing shear when a non-Newtonian fluid is concerned. Another possibility ha s been developed by CEA since 1998 and we propose to use a supercritical ag ent as a fluidiser. This process is very well adapted for the regeneration of used motor oils, as well as natural oils or polymers filtration. The exp erimental pressurised cross-flow filtration sets up is designed with inorga nic membranes, using model compounds or real oil effluents. Model compounds (polymers, standard oils) are then tested in order to study hydrodynamic b ehaviours with ultrafiltration membranes whose pore sizes are much greater than compound sizes. Finally, two different used motor oils are ultrafiltre d. One is a standard type used oil, while the other is a residue of a vacuu m distillation column. The hydrodynamic results clearly show an increasing flow resistance for a low value of the retentate velocity. For high velocit ies, no polarisation is observed. Used oils ultrafiltration leads to a comp lete purification while the addition of supercritical CO2 and a decrease of viscosity ranging from 5 to 500 is observed. A complete metal separation a nd an almost dry concentrate residue are achieved. (C) 2001 Elsevier Scienc e B.V. All rights reserved.