Development of an improved falling ball viscometer for high-pressure measurements with supercritical CO₂

Brice Calvignac (1) , Élisabeth Rodier (1) , Jean‐jacques Letourneau (1) , Pauline Vitoux (2) , Cyril Aymonier (2) , Jacques Fages (1)

1 RAPSODEE - Centre de recherche d'Albi en génie des procédés des solides divisés, de l'énergie et de l'environnement
2 ICMCB - Institut de Chimie de la Matière Condensée de Bordeaux

This study presents the development of an improved technique for viscosity measurements under high pressure. The apparatus is based on the principle of the falling ball viscometer, implemented in a high-pressure autoclave fitted with visualisation windows. The originality here is that the balls fall through a tube open at both ends with a diameter slightly greater than that of the balls, allowing a simplified modelling and numerical simulation. A numerical approach has been used for viscosity determination. Calculations have been made with COMSOL Multiphysics^® with the laminar Navier–Stokes model for Newtonian mixtures. It includes the specific hydrodynamic effects without the need for a calibration fluid. However, validation experiments were carried out at atmospheric pressure with dimethylsulfoxide (DMSO) at 298, 308 and 318 K and with cocoa butter at 313 and 353 K, with values of viscosity in the range from 1.4 to 45.4 mPa s. Comparative measurements with literature data have been conducted with cocoa butter saturated with carbon dioxide at 313 and 353 K and for pressures ranging from 0.1 to 25 MPa. At 313 K, viscosity varies from 45.4 mPa s to 3.1 mPa s while at 353 K it varies from 12.4 to 1.9 mPa s. For both isotherms tested, within the range 0–15 MPa, the higher the CO₂ dissolution in the cocoa butter, the lower the viscosity. However, this decrease in viscosity is more pronounced at the lowest temperature. Above 15 MPa the CO₂ dissolution effect on viscosity becomes insignificant, i.e. within the experimental error, due to a counter effect linked with the high hydrostatic pressure. Furthermore, the limits of use of this method have been determined. This technique is revealed as reliable and can therefore be used with other binary systems.

Mots clés: Cocoa butter; Supercritical CO2; Viscosity; CFD; Falling ball viscometer; Density

Domaines: Génie des procédés; Matériaux

Brice Calvignac, Élisabeth Rodier, Jean‐jacques Letourneau, Pauline Vitoux, Cyril Aymonier, et al.. Development of an improved falling ball viscometer for high-pressure measurements with supercritical CO₂. Journal of Supercritical Fluids, 2010, 55 (1), pp.96-106. ⟨10.1016/j.supflu.2010.07.012⟩. ⟨hal-00545053⟩