Deposition from Wax-Solvent Mixtures under Turbulent Flow: Effects of Shear Rate and Time on Deposit Properties

A bench-scale flow-loop apparatus, incorporating a double-pipe heat exchanger, was used for investigating the effects of shear rate and deposition time on the deposition of solids, under turbulent flow, from solutions of a multicomponent wax in a paraffinic solvent. The deposit-layer thickness was found to decrease with an increase in the Reynolds number (or shear rate), while it increased asymptotically with the deposition time. Calculations with a steady-state heat-transfer model showed the liquid-deposit interface temperature to be equal to the wax appearance temperature (WAT) and the deposit average thermal conductivity to be 0.35 W m(-1) K-1. Gas chromatography (GC) analyses of deposit samples showed significant changes in the carbon number distribution with shear rate and time. A recently proposed model, involving one-dimensional shear deformation of a cubical cage, was used to express changes in the deposit composition, via the deformation angle (beta), as a function of the Reynolds number and deposition time. The time-dependent variations in deposit composition were expressed with a viscoplastic model, in which Reynolds number (or shear rate) influences the initial deposit properties. The proposed viscoplastic model affords a new explanation for the deposit aging phenomenon. The deposition from waxy mixtures was confirmed to be primarily a thermally driven process, in which the shear rate and deposition time play important roles by influencing the deposit properties.