(410j) Numerical Simulation of Fouling in Crude-Oil Heat Exchangers: The Interaction Between Different Fouling Routes

Fouling in crude oil pre-heat trains involves complex chemical and physical mechanisms, including chemical reactions, super saturation, particulate deposition and corrosion. Once the foulant precursors have been produced, they react, aggregate and eventually attach to the wall. The deposited foulants undergo rheological changes due to various thermal and physico-chemical factors, causing them to harden. Once the heat exchangers are fouled, they need to be cleaned (either mechanically or chemically) generating increased costs, fuel consumption and safety concerns.  

In recent decades, intense efforts have been made to address the fouling process in industrial crude oil heat exchangers, involving experimental investigations aimed at determining correlations between fouling rates and physical factors under varying operating conditions. A series of studies based on the use of thermodynamics, statistical mechanics, and molecular modelling have been performed for asphaltene-oil system in order to provide insight into the fouling formation mechanisms. Different fouling routes may be mutually reinforcing or destructive. If the mechanism behind the interaction between different fouling formation routes can be elucidated, the insights can be exploited for the development of fouling mitigation strategies.

The aim of present work is two-fold: to improve fundamental understanding of the phase behaviour of crude oil mixture using the advanced SAFT-γ model; and, to incorporate output from this model together with models for chemical-driven fouling, and deposit ageing into a continuum-based Computational Fluid Dynamics (CFD) numerical tool to simulate crude oil fouling in industrial heat exchangers.  To validate the CFD and heat transfer models, the flow and temperature field in the test section of the High Pressure Crude Oil Rig (HIPOR) design at Imperial College London is simulated. The predicted pressure drops, wall temperatures from the CFD are compared with the experimental data obtained from HIPOR.

To obtain in-depth insights into the interplay between different fouling mechanisms, the chemical fouling and precipitation routes are activated simultaneously or individually during the CFD simulations. The changes in the overall fouling deposition rate resulting from their interaction are analysed. This work suggests that process modifications that slightly enhance precipitation could substantially suppress chemical fouling, which accounts for more than 90% of overall fouling, and thus reduce the total fouling rate. Three possible driving forces (interphase heat transfer, inhibition of heat transfer in the fouling layer and turbulence) were identified that may either strengthen or weaken the processes. Furthermore, by strengthening or weakening these driving forces appropriately, the chemical fouling rate could be substantially reduced with the precipitation rate increasing only slightly.

ACKNOWLEDGEMENTS

This research was performed under the UNIHEAT project. The authors wish to acknowledge the Skolkovo Foundation and BP for financial support. Zulhafiz Tajudin wishes to also acknowledge MARA and UniKL for their support and funding.