(364d) Multi-Scale Modelling of Refinery Pre-Heat Trains: Effects of Crude Fouling On Energy Recovery, Carbon Emissions and Economics

Pre-heat trains (PHTs) in crude oil distillation units are complex networks of shell and tube heat exchangers used to recovery up to 60-70% of all the energy necessary for primary fractionation (Yeap 2003). Efficiency of the train is severely impaired by fouling, the deposition of unwanted material on the thermal surfaces. Economic losses - estimated on the order of US$ 1 billion/year in the US alone (ESDU 2000) - maintenance issues as well as health and safety hazards are risen by this problem that oil refineries have been facing for decades.

Experimental evidence shows that fouling rates are function of crude composition and process conditions, namely temperature and velocity. It has also been demonstrated that, for a given crude composition, threshold conditions of temperatures and velocities exist below which no fouling occurs. However, the complex chemical and physical mechanisms underlying fouling deposition are poorly understood.

Thermal fouling models (such as the Ebert-Panchal one) seem capable of capturing its dependency on process conditions and time within a satisfactory degree of accuracy. Based on this approach, a multi-scale first principles model for a shell and tube heat exchanger has been proposed and validated against real refinery data. The model is dynamic, distributed and accounts for local variation of physical properties. Several scales of investigation are considered simultaneously. At the tube level, the interactions between operating conditions and fouling are captured through the Ebert-Panchal model; this allowes calculating the thickness of the fouling layer along the tubes in each exchanger and its interactions with thermal exchanges and fluid-dynamics. At unit level, it is possible to identify critical zones where deposition is particularly severe. Complex interactions affecting the overall performance of the pre-heat train are unveiled by interconnecting sevreal units in a network.

In this work, simulation of the hot end of a pre-heat train showed how it is possible to accurately quantify energy and production losses. Appropriate actions to mitigate fouling are then suggested from both an operating and a retrofitting point of view.

REFERENCES

ESDU (2000). Heat exchanger fouling in the pre-heat train of a crude oil distillation unit, Data Item 00016. London, ESDU International plc.

Yeap, B. L. (2003). Designing heat Exchanger Networks to Mitigate Fouling. PhD Thesis, Department of Chemical Engineering. Cambridge (UK), Cambridge University.