Deposition from “Waxy” Mixtures under Turbulent Flow in Pipelines: Inclusion of a Viscoplastic Deformation Model for Deposit Aging

Abstract

A mathematical model is described and used to predict solids deposition from multicomponent paraffinic “waxy” mixtures under turbulent flow in pipelines. The model is based on the moving boundary problem formulation, in which the deposit formation and growth is modeled primarily as a heat-transfer process with phase change. The effects of shear stress (or Reynolds number) and deposition time (or deposit aging) are incorporated via a viscoplastic model, which is based on one-dimensional deformation of a cubical cage that squeezes out a fraction of the liquid phase from the deposit. Numerical solutions were obtained for the radial and axial growth of the deposit with time at Reynolds numbers (Re) of 10 000−25 000. The predicted trends are in agreement with the experimental results from recent laboratory deposition studies. The steady-state deposit thickness under turbulent flow is predicted to be considerably smaller than that under laminar flow, and it decreased with an increase in Re. The average wax content of the deposit is predicted to also increase with Re and deposition time (or aging), causing the deposit to become enriched in heavier paraffins and depleted in lighter paraffins. The results indicate that, although an increase in Re and deposition time causes wax enrichment in the deposit, the deposit thickness is dependent on heat-transfer and thermodynamic phase equilibrium considerations.