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International Heat Transfer Conference 15

ISSN: 2377-424X (online)
ISSN: 2377-4371 (flashdrive)

Modelling of Fundamental Transfer Processes in Crude-Oil Fouling

Junfeng Yang
Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom

Omar K. Matar
Department of Chemical Engineering, Imperial College London, Prince Consort Road, London SW7 2AZf

Geoffrey F. Hewitt
Department of Chemical Engineering, Imperial College London, London SW7 2AZ, UK

Wentian Zheng
Department of Chemical Engineering, University College London, London WC1E7JE, United Kingdom

Parth Manchanda
Imperial College London, Prince Consort Road, London SW7 2AZ, United Kingdom

DOI: 10.1615/IHTC15.tpn.009595
pages 8729-8744


KEY WORDS: Two-phase/Multiphase flow, Numerical simulation and super-computing, crude oil fouling, modeling

Abstract

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.

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