Cold rotating finger: Apparatus to study wax deposition under shear

A bench-top Cold Rotating Finger (CRF) is developed to study the effect of fluid shear on wax deposition. An inner rotating cylinder provides opportunity to study wax deposition on surfaces in the region of high fluid shear stress (and laminar/turbulent flows). A model system of linear n-alkane (C20 to C40) dissolved in dodecane to 9 wt% was used for the deposition tests. The bulk fluid temperature was set to 45°C and the temperature of the inner rotating cylinder to 5°C, with wax deposits showing a mass variation less than ~5% at constant rotational speed. The kinetic of wax deposition is non-linear, with an initial rapid deposition (approximately linear) followed by a slower rate as deposit mass tends to steady-state within 30 min. The wax deposit mass is limited by thermal insulation of the cold surface and not wax depletion. As a function of rotational speed, the wax deposit mass is shown to be maximum under static condition and gradually decrease with increasing rotational speed (25 to 500 rpm). The wax deposit morphology is studied by cryo-scanning electron microscopy with the static deposit showing a layered structure (ridges) which diminished at 200 rpm, confirming a denser, less porous deposit. With its simple design and small sample volume, the CRF provides a route to easily study wax deposition under conditions more relevant to the industry scenario.