Reducing heater outlet temperatures risks having unconverted oil in the coke drum that will result in hot spots, eruptions and blow outs. Heater outlet temperatures should be raised high enough to keep the VCM on coke less than 10%. That should prevent tarry drums, hot spots and blow outs.
Time and temperature make coke. Heater fouling has the concentration of coke precursors as well. Coking rates are exponential with temperature and linear with time, so lowering temperatures will have the largest effect. Lowing the temperatures too much will result in significant safety hazards, so coke VCM should be closely monitored to ensure heater outlet temperatures are adequate.
Raising heater oxygen content will reduce the flame temperature in the fire box and shift duty from the radiant section to the convection section. The lower temperatures in the radiant section should reduce the heat flux and film temperatures so that fouling in the lower radiant section should be reduced. This higher oxygen content will increase the convection section heat duty and heat transfer rates. There is an increased risk of coking the convection section of the heater by raising the oxygen content in the firebox. This will also reduce the efficiency of the heater.
A 0.5 deg C to 1.0 deg C temperature increase per day is a reasonable increase in tube metal temperatures. For 9% Cr – 1% Mo heataer tubes, approximately 100 deg C is the start of run to end of run heater tube metal temperature increase. At 0.5 deg C per day that is 200 days or just over 6 months.
A lower feed concarbon content should result in longer heater run lengths, but that may not be an economic alternative.