Modelling solutions of hydrocarbons in dense CO2 gas

Shaping of advanced ceramic parts generally requires the use of organic add itives which, obviously, have to be removed prior to sintering. In order to obtain free defect green pieces in a short time, an alternative technique to the classical thermal debinding, based on the unique properties of dense gases such as supercritical fluids, has proven to be very efficient. Super critical extraction of organic additives from a green ceramic part involved both solubilisation and diffusion. The debinding treatment has to be condu cted to remove a high amount of the organic phase but in such a way that th e cohesion of the green part was maintained. Then, the capillary migration of a molten organic additive has to be avoided and a low amount of binder h as to remain in the green part after the debinding treatment. This residual organic phase could be easily removed during the sintering treatment becau se the porosity is. at this stage, entirely open and interconnected. In thi s context, extraction was performed using carbon dioxide, (i) at low temper ature (lower than 80 degreesC) for which the organic additives used, i.e. p araffin waxes composed of n-alkanes, remain in a solid state and, (ii) unde r experimental conditions of extraction (pressure, temperature, time) chose n to maintain a non soluble part of the binder. It is then necessary to be able to predict the solubility of organic binders in supercritical CO2 to d efine a suitable organic formulation (appropriate paraffin waxes) and an ad apted debinding treatment. A review of existing models of solubility of org anic molecules in supercritical fluids and an analysis of the principal con cepts of modelisation are exposed. The choice of an equation of slats for s olubility prediction of n-alkanes in supercritical CO2 was made on the basi s of (i) the values of average errors between calculations using models of solubility and experimental values of solubility measured in this study and , (ii) the facility of equation employment. Then, calculated values of solu bility, using this equation of state, were compared to experimental values (FT-IR) for an alkane (n-C-28). (C) 2001 Elsevier Science Ltd. All rights r eserved.