Viscosity and rheological behavior of carbon dioxide-expanded fish oil fatty acid ethyl esters: Measurement using a rotational viscometer and modeling

The viscosity of fish oil fatty acid ethyl esters (FAEE) in equilibrium with CO2 was determined at 40, 55, and 70 °C and pressures ranging from 0.1 to about 12 MPa using a rotational rheometer equipped with a high pressure cell. Viscosity of CO2-expanded (CX) FAEE decreased due to the dissolution of CO2 with pressure, which was temperature dependent. The viscosity of CX FAEE was correlated using a new empirical model utilizing saturation pressure and temperature with temperature-dependent parameters, which allows interpolation of viscosity data for any pressure and temperature within the envelope of this study. As well, viscosity was modeled using available data for CO2 solubility in FAEE at specific temperatures of this study utilizing simpler models with only one parameter, such as the Grunberg and Nissan model. The rheological data suggest shear thickening behavior of CX FAEE at elevated pressures. The limitations of the rotational rheometer for measuring CX liquids at viscosities below 1 mPa s are discussed. Understanding of viscosity and rheological behavior of CX FAEE is essential for equipment and process design involving such lipids.