Fouling in a hydrocarbon vaporizer

The initial fouling rate of air-saturated odourless kerosene was studied in a small scale (0.5 ins. O.D.) radiant heated horizontal tubular furnace. Two runs were carried out with the feedstock in the all liquid phase flow, fifteen during vaporization and one with all vapour phase flow. The effect of oxygen was also studied by repeating one of the vaporization runs with the relative oxygen content reduced to 15% of the saturation value. For all the runs the mass flow rate of the feedstock was held constant and corresponded to a Reynolds number of 1170 with kerosene in the liquid state at 14.7 psia and 60 °F. A complex plot of the logarithm of the initial fouling rate versus the reciprocal absolute temperature for all the air saturated runs was obtained. With increasing temperature the initial fouling rate increased in the all liquid phase flow region, decreased during the vaporization region and increased again in the all vapour phase flow region. The experimental results showed clearly that a relatively low initial fouling rate occurred close to the dryout point in vaporization. Pressure was found to have a complex effect on the fouling rate. In general however increasing the pressure caused an increase in the initial fouling rate during heating in the subcooled liquid region and during vaporization. Decreasing the oxygen content from 100% saturation to 15% saturation caused a decrease in the initial fouling rate by between three and sixty times. The critical dependence of the initial fouling rate on air or oxygen content during subcooled heating and vaporization is suggested to be due to the formation of bubbles of oxygen rich gas on the heat transfer surface. The formation of such bubbles requires the transport of oxygen to the heat transfer surface thus leading to an increase in the rate of the autoxidation fouling process. During vaporization the initial fouling rate varied circumferentially around the tube. The highest rate was obtained at the bottom of the tube where the liquid tended to flow. The lowest rate was obtained at the top of the tube where the vapour tended to flow.