Development of a falling body viscosimeter for high pressure measurements

This study is a preliminary work of a global project aiming at a new clean recrystallization / fractionation process of cocoa butter (CB) assisted by supercritical CO2 (SC-CO2). The knowledge of properties of the SC-CO2 / CB system such as reciprocal solubility of the binary system, density and viscosity of phases appears to be a pre-requisite for this study. There is a lack of such thermophysical properties in the literature.In this study, we focus on viscosity measurements, which were carried out at 313 and 353 K and pressures of 0.1 up to 25 MPa. The experimental procedures were validated with CB viscosity measurements at atmospheric pressure with a rheometer, for which literature data are available [1]. In a first step, we made density measurements, using the vibrating tube principle [2]. This volumetric property is crucial to determine precisely the viscosity. Lastly, we carried out high pressure viscosity measurements in a new device, specially designed to perform these experiments. The procedure using the Hoppler viscosimeter principle is based on the monitoring of a falling aluminium ball in a glass tube placed in a high pressure vessel equipped with sapphire windows. The determination of the terminal velocity of the ball in the melted media allowed deducing viscosity. This velocity is obtained by the analyses done on the images of a high speed digital video camera, with a software developed with Matlab . CFD calculations by finite elements methods with Comsol Multiphysics were carried out to determine viscosity and corroborate results obtained with a simplified model. Indeed, this model based on the consideration of a falling body in an infinite tube and in a non-compressible Newtonian medium at rest, is in agreement with CFD calculations. The density of CO2-saturated CB increases with pressure whereas the viscosity decreases because of the higher CO2 dissolution in the melted medium, previously measured using a gravimetric procedure in the same operating conditions. Furthermore, the viscosity decreases also with increasing temperature. We compared and discussed our results with those of Venter et al. who performed density and viscosity measurements using a vibrating quartz viscosimeter [1]