CRUDE OIL FOULING ON HEAT TRANSFER SURFACES

Hydrocarb9n 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 Malay1sian crude oils through experiments in a pilot-scale, high-pressure and
I
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
I
flux beyond wkich 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
I 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 :-vas determined for e:ach crude oil at the corresponding operating
conditions. A mbdel to determine the maximum heat flux has also been proposed in
!
terms of the crude oil true boiling point data.
!
A series of
1
experiments were planned and carried out to study the fouling
characteristics of different crude oils at different initial surface temperatures, bulk
temperatures an<l 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 initi~l 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.