Mitigation of crude oil fouling by the use of HiTRAN inserts

Fouling of heat exchange equipment can be defined as the accumulation of material on the heat transfer surface in sufficient quantities to reduce the effectiveness of the exchanger. This study was initiated to quantify the benefits of HiTRAN® inserts with regard to fouling prevention and to establish a generic method for their confident implementation. The effect on fouling rates of the key process parameters such as initial surface temperature, flow-rate and insert presence and geometry have been studied using Maya crude oil flowing inside round tubes. Experimental studies with bare tubes have shown that increases in velocity could either increase or decrease the initial fouling rate depending on whether the overall mechanism was controlled by mass transfer or kinetics. Over the range of velocities studied (0.5 m s'1 to 4.0 m s'1) the apparent activation energy for the overall fouling process was found to vary between 26 kJ mol'1 and 149 kJ mol'1, the value increasing with increasing velocity, indicating a strong influence by mass transfer phenomena. Experimental studies with inserts have shown that under nominally identical operating conditions to bare tubes, fouling rates can either be increased or decreased by their presence, depending on whether the mechanism is mass transfer or kinetically controlled. At relatively low velocities (~ 0.5 m s'1), where the fouling reaction was typically mass transfer controlled, the presence of inserts increased the fouling rate. At relatively high velocities (> 1.0 m s'1), where the fouling reaction was typically kinetically controlled, the presence of inserts reduced fouling rates. Studies with inserts have also revealed evidence of a critical insert density of 290 loops m '1. Below this value variations in both density and loop wire diameter significantly affected the fouling rate. Conversely, above the critical loop density, increasing both density and wire loop diameter appeared to have little effect. Epstein's mathematical model has been adapted in an attempt to account for both bare tube and insert fouling rates with Maya crude. The model was evaluated using three different apparent activation energies, namely 26 kJ mol"1, 77 kJ mol'1 and 149 kJ mol*1. Using the lowest apparent activation energy (26 kJ mol'1) the model shows good agreement with the bare tube experimental data, predicting with reasonable accuracy the velocity at which the fouling process becomes kinetically controlled. To predict the effect of a HiTRAN® insert on fouling the appropriate friction factor for the insert was substituted in to the model. However, agreement between predicted fouling rates and experimental results was poor. Replacing the average apparent activation energy with 77 kJ mol'1 the model did not predict the trends seen with the experimental data for bare tubes. Instead the model predicted a decrease in the fouling rate with increasing velocity. However, the trends predicted by the model for tubes containing inserts were in general agreement with those found experimentally. The predicted values of fouling resistance were found to be generally larger, with agreement improving as the velocity was increased. Increasing the apparent activation energy still further to 149 kJ mol'1 resulted in the prediction of fouling rate trends that were contrary to those observed experimentally.