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Reply To: Coking rate vs. Heat flux

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#28093

Mike Kimbrell
Participant

To estimate the velocity in each tube of the heater requires knowing the flow rate and the tube cross sectional area as a start. It also requires knowledge of the amount of velocity media (steam or condensate), the amount of vaporization of the hydrocarbons and the amount of resid conversion on a tube by tube basis. This requires knowledge of the temperature profile of the hydrocarbons on a tube by tube basis. Once this information is available, the relative coking value can be calculated on a tube by tube basis. Comparing the relative coking value between cases then will show the relative impact of the change in feed rate. The constant I mentioned is any value you want to use to get the results into a range that is easy to compare. The same constant should be used for each case on a particular heater.

It is common for higher flow rates to cause the pressure profile to shift to higher pressures in each tube. This will reduce vaporization and velocity which will increase the residence time. The higher flow rate requires higher duty to meet the same heater outlet temperatures, so the film temperatures will be higher with longer residence times. The tube with the maximum relative coking value sometimes shifts from the outlet tube to one four or five tubes from the outlet at the higher flow rates and heat fluxes. That is why the relative coking value needs to be calculated for each tube for each case.

Conventional fired heater software like FRNC5 by PFR or FH by HTRI will provide tube by tube temperature, pressure and velocity; however, they do not take the decomposition reactions that occur in a Coker heater into consideration. I know that by adjusting the property tables to account for the cracking that occurs can be done with FRNC5 and I understand it only takes a few manual iterations to get the values to converge.

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