Predictions for wax deposition in a pipeline carrying paraffinic or ‘waxy’ crude oil from the heat-transfer approach

Abstract

The heat-transfer mechanism for solid deposition from ‘waxy’ or paraffinic oils and mixtures has been developed and validated through several experimental and modeling investigations over the past three decades. This modeling approach considers the transient, unsteady-state wax deposition process to involve (partial) freezing with liquid-to-solid phase transformation, which has been modeled via the Stefan moving (or free) boundary problem formulation. The steady-state deposit thickness is predicted from a model that equates the heat-transfer rate in the radial direction across as many as five thermal resistances in series, including the flowing oil (convective), the deposit layer (conductive), the pipe wall (conductive), an insulation layer (conductive), and the coolant or surroundings (convective). Of these, the two predominant thermal resistances are due to convection in the flowing oil and conduction across the deposit layer. Calculation results are presented to systematically demonstrate the effect of key parameters on the steady-state deposit thickness in a pipeline carrying a paraffinic or ‘waxy’ crude oil in the hot flow regime. Numerical predictions for the deposit thickness, in the radial direction, highlight the effects of flowing oil temperature, the surrounding or coolant temperature, the heat transfer coefficient for the flowing oil, the inner radius of pipeline, the deposit average thermal conductivity, pipe-wall thermal conductivity, insulation thermal conductivity, and the wax appearance temperature of waxy oil. Also included are the predictions that demonstrate the deposit thickness does not depend directly on the overall thermal driving force or temperature difference. All parameters in the heat-transfer calculations are either measured directly or can be estimated from established predictive techniques; that is, the model does not involve any adjusted parameter.