Vacuum Unit Fired Heater Coking - Avoid Unscheduled Shutdowns
SUMMARY
Poor heater design leads to unscheduled Vacuum Unit fired heater shutdowns due to coke formation. Oil film temperature and residence time are the variables that determine the rate of coking for a given crude oil blend. Proper heater design and operation controls oil residence time and oil film temperature, allowing refiners to maximize heater run length.
TEXT
Several refiners are targeting crude unit run lengths of 6-8 years to maximize profits through increased onstream factor and lower maintenance costs. Moderate to rapid fired heater coking causes unscheduled shutdowns. Coke forms because the oil in the tubes is no longer thermally stable at the operating conditions. Oil temperature and oil residence time control the rate of coking for a given feedstock. Oil temperature alone is not a good predictor of coke formation rate. Crude oil feedstock stability varies, but it cannot be controlled. However, temperature and oil residence time can be controlled through heater design and operation. Fundamental design principles influence coke formation rates and correct design parameters must be used to build a more reliable heater or understand why an existing heater is coking so it can be fixed. For many refiners, the vacuum heater performance will determine crude unit run-length. Two case histories will highlight some of the problems that cause rapid coking.
Meeting 6-8 year run-length and product yield targets requires very low coking rates. Coke forms an insulating layer inside the tube, which increases the outside tube metal temperature (TMT). Once the maximum TMTs are reached, the coke must be removed. This requires a shutdown, otherwise, tube life is reduced or tube failure can occur. Minimizing oil residence time and oil film temperature are the keys to meeting runlength.
Minimizing oil residence time at high film temperature is essential to limit coke formation. Very high oil film and bulk temperatures can be maintained at an acceptable coke formation rate...