Coking of heater tubes is a first order reaction that can be approximated using the Arrhenius equation:
k = A exp(-Ea/RT)
The activation energy (Ea) has been reported to be between 40,000 kcal/g-mole and 55,000 kcal/g-mole. T is the absolute temperature in degrees Kelvin. R is 1.987 kcal/g-mole-K. To evaluate relative coking rates, it is common to use a base temperature to compare against. Residence time is represented by the inverse velocity and a constant that makes up all the other items particular to this heater. In this case the relative coking equation becomes:
Rc = C/V exp(-Ea/R(1/T – 1/Tref))
Where the reference temperature is 425 C or 698 K (you could use 700 K).
This would be a reasonable way to estimate the change in heater coking rate at higher film temperatures for the same feed. As feeds are changed, the coke precursor concentration has to be taken into consideration as the base coking rate will be higher for higher coke precursor concentrations.