Wax Deposition in the Presence of Suspended Crystals

Wax deposition in pipelines is one of the most relevant flow assurance problems faced by the petroleum industry. Molecular diffusion of dissolved paraffin has been considered the dominant deposition mechanism in simulation models available in the literature. In case the pipeline operational conditions are such that the fluid temperature is below the wax appearance temperature (WAT), wax crystals could be present in the bulk of the flowing solution and particle deposition mechanism may play a relevant role. In the present research, bench-scale, well-controlled deposition experiments were conducted for laminar channel flow of a solution with an inlet temperature below the WAT, so that wax crystals were available for deposition. In the experiments, visualizations of the deposition process were sought for three distinct heat flux conditions at the channel boundary: negative, zero, and positive heat flux. Detailed information on the temporal and spatial distributions of the wax deposited along the channel walls was obtained for three values of the laminar channel Reynolds number. It was verified that a channel wall cooler than the flowing fluid (the negative heat flux condition) is a necessary condition to produce deposition. For all of the experimental conditions tested, no deposition was verified under zero or positive heat flux boundary conditions. In all cases studied, the deposits measured were significantly thicker than those obtained for similar flow conditions and fluid-to-wall temperature differences but for inlet fluid temperatures above the WAT. The visualization experiments revealed that wax crystals and crystal agglomerates presented trajectories nearly parallel to the channel wall. These crystals and agglomerates were seen to be decelerated and stopped, being incorporated on a thin wax deposit formed at the initial cooling stages.