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There are three parts to the absorbed duty in a Coker heater. The first is sensible heating to the outlet temperature. The second is the heat of vaporization of the lower boiling fractions. The third is the heat of reaction that occurs within the heater for the thermal decomposition of some of the feed. This reaction separates Coker heaters from Crude unit or Vacuum unit heaters in the calculation of the absorbed duty. To accurately estimate the absorbed duty the percent vaporization and the percent converted as well as the heat of vaporization and the heat of reaction will need to be properly estimated. Typical values for % vaporized range between 15% and 20%. The range for % converted is from a low of 5% to a high of 25% depending on feed type primarily. As you know, operating conditions (heater outlet temperature, outlet pressure and recycle ratio) all impact the absorbed duty.
Heater analysis software like FRNC5 from PFR Engineering has to be adjusted to account for the reactions that occur within the Coker heater. It can be done, but it takes some manual iteration to achieve a solution.