Fouling of crude oils in the heat exchangers of the refinery preheat train (PHT) is a common problem. The varying composition and complex phase behaviour of crude oils presents significant challenges when processing at high temperatures. Precipitation of asphaltenes from the oil and corrosion fouling at high temperatures results in the build-up of deposits on heat exchanger wall surfaces, reducing heat and momentum transfer. This results in significant economic and environmental impact. Studying this phenomenon from a physicochemical perspective remains challenging as crude oil compositions vary widely. In particular, asphaltenes, the heaviest oil fraction, exhibits phase behaviour which is highly dependent on temperature and blend composition.

This work augments existing understanding of fouling behaviour of crude oils at high temperature by focusing on their compositions by blending, fractionation (into asphaltenes and maltenes) and recombination with model solvents. Synthetic asphaltene model-compound solutions were developed to study fouling in a repeatable and controlled manner, and their fouling characteristics were validated against those of naturally occurring crude oils. Fouling experiments for crude oils were conducted in a purpose – built laboratory scale system described in this work. This apparatus simulated the hydrodynamic conditions found in latter stage PHT heat exchangers. These studies formed the basis of comparison for later experiments, testing both fractionated oil solutions and synthetic asphaltene solutions utilising high boiling-point solvents (n-dodecane and 1-methylnaphthalene).

Commissioning experiments revealed that fouling behaviour was influenced by several factors. Not only was the production of deposits highly dependent on the process conditions, blend composition and preparation of the test fluids, but also the cleaning procedure of the system. Traces of residual deposits were believed to act as nucleation sites for further deposit production and activity of the heated wall surface was subject to cycling passivation. When fractionated oil solutions were used, solution stability at process conditions was the primary driver of fouling; high concentrations of asphaltenes were relatively stable in 1-methylnaphthalene, but low concentrations were highly unstable in solutions containing large proportions of n-dodecane, an incompatible solvent. Maltene fouling was similarly dependant on its solution stability. Solutions of synthesised model asphaltenes based on alkylpyrene and perylene diimides, exhibited very different behaviour under process conditions. A long alkyl-chain perylene diimide (PDI-C12) alone exhibited strong fouling behaviour at process conditions. It exhibited marginal solubility in 1-methylnaphthalene and high thermal lability, resulting in significant insoluble deposits which also caused corrosion of the steel wall surface.