I agree that the temperature that the various resid molecules start to crack is dependent on their molecular structure. I think the temperature that cracking starts is somewhat lower than the value listed. I think the temperature is approximately 400 C for the beginning of thermal cracking. The rate of cracking is fairly low at this temperature and the rate accelerates exponentially with temperature. The typical coke drum outlet temperature target is 440 C, so the initial cracking temperatures have to be lower.