CRUDE OIL FOULING ON HEAT TRANSFER SURFACES

Hydrocarbon fouling in a petroleum refinery crude preheat train has been identified as a critical issue affecting the economy of the plant very badly. Fouling undergoes different mechanisms at different stages of heating the crude oil in the preheat train. Understanding the fouling mechanisms is essential in formulating appropriate fouling mitigation strategies.
The present research focuses on the study of fouling characteristics of four different Malaysian crude oils through experiments in a pilot-scale, high-pressure and high-temperature recirculation flow loop fitted with two identical fouling probes. The procedures reported in the open literature employ very high surface temperatures. It has been identified in this study that there is a maximum surface temperature/heat flux beyond which the forced convective heat transfer regime changes to boiling regime. As the industrial preheat exchangers operate at forced convective heat transfer regime, it is therefore, necessary to carry out the experiments in the same heat transfer regime. In this study, an improved method has been developed for calibrating the surface temperature using the heater temperature measurement by the Wilson plot technique. This method enables identification of the heat transfer regimes more accurately. Maximum heat flux under the forced convective heat transfer regime was determined for each crude oil at the corresponding operating conditions. A model to determine the maximum heat flux has also been proposed in terms of the crude oil true boiling point data.
A series of experiments were planned and carried out to study the fouling characteristics of different crude oils at different initial surface temperatures, bulk temperatures and flow velocities at a pressure of 50 bar. Data from each experiment were collected, processed and the resistance due to fouling was determined. The induction periods and the initial fouling rates were estimated from the fouling resistance profiles. It was observed that the induction period decreased with an increase in initial surface temperature; increased with an increase in the bulk temperature and flow velocity. It was also observed that the initial fouling rates increased with increase in initial surface temperature; decreased with increase in bulk temperature and flow velocity for all the crude oils.
The experimental data were analyzed using the existing threshold fouling model. This model assumes the rate of fouling is the net effect of fouling precursor formation through chemical reaction and deposition, and removal by the wall shear. The apparent activation energy values were estimated for the crude oils at different bulk temperatures and flow velocities. It was observed that the variations in the apparent activation energy values for flow velocities of 0.4 and 0.5 m/s are insignificant and that it increased linearly with increase in the bulk temperature. The existing threshold fouling models predict an increase in the initial fouling rates with an increase in the film temperatures. Although the existing models predict the fouling rates well for increase in film temperature due to increased surface temperature at constant bulk temperature, they fail to predict the fouling rates for an increase in film temperature due to the increase in bulk temperature at constant surface temperature. A new threshold fouling model was developed to account for the effect of bulk temperature on fouling by considering the apparent activation energy as a function of bulk temperature. The new threshold fouling models for the crude oils tested were proposed. The proposed threshold fouling model has been found to be in good agreement with the experimental data.