Why Vacuum Unit Fired Heaters Coke
SUMMARY
A description of the internal workings of vacuum heaters and the causes of coke formation within them. Case studies show how to avoid localised hot spots in order to maintain yield targets and increase run lengths.
TEXT
Some refinery vacuum heaters have chronic problems with coking and short run-lengths. Several of these heaters operate at coil outlet temperatures of only 750-760°F and average radiant section heat flux of 8000Btu/hr-ft2°F or less. Why do these seemingly mild operations have run-lengths less than two years between decokings, while others operature for four years at coil outlet temperatures of 790°F and average flux of 11,000Btu/hr-ft2°F? Common heater monitoring parameters such as coil oulet temperature, average heat flux, and fired duty are generally of little value in determining why a heater develops hot spots. Hot spots are typically localised phenomena. Often, they are a consequence of decisions made to reduce the heater initial investment.
When revamping, the designer should apply fundamental design principles to meet short term product yield targets and long term run-length objectives. Common heater design considerations that affect the rate of coke lay-down are radiant section tube layout, process coil design, and burner performance. This article reviews how heater design influences localised conditions that promote rapid coke formation. Two case studies show how fundamental principles can be applied to eliminate hot spots and increase run-length...