ISSN Online: 2377-424X
ISBN Print: 978-1-56700-421-2
International Heat Transfer Conference 15
Modelling of Fundamental Transfer Processes in Crude-Oil Fouling
Sinopsis
Crude-oil fouling commonly occurs at the heat transfer surfaces of oil refinery pre-heat trains, reducing the heat
transfer and hydrodynamic efficiency of heat exchangers. The underlying mechanisms of this process must be
understood to design effective fouling mitigation strategies. Fouling is believed to proceed via several individual
steps: initiation, transportation, attachment, removal and ageing. However, the mechanisms of initiation, removal
and ageing of the fouling layer are not yet fully known. Current models for crude-oil fouling are mostly based on
relatively simple empirical or semi-empirical correlations that are only accurate for specific crude oils under
certain conditions and have no solid chemical or physical basis. It is therefore essential to develop an accurate
comprehensive numerical fouling model and to understand the underlying physical and chemical processes. In
this work, a Crude Oil Surrogate consisting of gasoline-, diesel-, and residual-range organic compounds in
volumetric proportions of 24:34:42 is proposed as a practical standard crude oil model. The liquid thermalphysical
properties of its constituent organic compounds were predicted using various empirical methods. The
chemical reactions of the fouling process were modeled as one-step multi-phase heterogeneous reactions
whereby sparingly soluble precursors in the crude oil form insoluble foulants. The asphaltene precipitation
process was described using a chemical equilibrium model based on the Gibbs free energy. A rheological model
and a first order kinetic model for deposition were used to describe the effects of ageing on liquid viscosity and
thermal conductivity, respectively. Deposit removal by interfacial shear stress was modeled using the Large
Eddy Simulation method. The resulting comprehensive model was implemented in a CFD tool to investigate the
crude oil fouling process under a typical industrial heat exchanger. Based on these studies, we conclude that the
rates of the chemical reactions involved in fouling are an order of magnitude faster than that of asphaltene
precipitation and increase with the surface temperature. High bulk flow velocities generate a strong interface
shear stress that eventually strengthens the interfacial wave rupture and foulant droplet entrainment, thereby
increasing the removal rate. Ageing has negligible effects on the removal rate due to competition between
structuration and destructuration terms.