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Petroleum Processing

Exercise 6


Exercise 6 Instructions

Solve a problem on the material balance for the regenerator in Fluid Catalytic Cracking Process.

Material Balance for FCC Regenerator


Burning the coke deposited on the catalyst particles generates all the heat necessary for catalytic cracking. Therefore, the coke burning rate is a critical parameter to control the rate of cracking. The composition of dry flue gas (excluding water) from the regenerator of an FCC unit is given in vol% as follows:

Table 7.5
Gas Volume %
N2 81.6
CO2 15.7
CO 1.5
O2 1.2

The dry air (excluding moisture in the air) flow rate to the regenerator is given as 593 SCMM (standard cubic meters per minute). Considering that a significant portion of coke is carbon, calculate the carbon burning rate in the regenerator in kg/min. Remember: 1 kgmole at STP = 22.4 m3)


In previous offering of this course, I have noted a serious weakness in the students' understanding of how to carry out simple mass balances. Here a few reminders and hints to solve this problem:

  • Gas compositions are always reported as volume % (equivalent to mole %), not in mass, or weight%.
  • Inert compounds (such as N2)go through reactors unchanged. These are referred to as "tie compounds." In this problem, knowing the air flow rate, you can calculate # moles of air, and thus # moles of nitrogen entering the reactor to calculate # moles of the flue gas, knowing the N2 % in the flue gas.
  • After calculating the # moles of flue gas components and knowing their molecular weight, you can calculate the corresponding mass (weight) of the elements making up the flue gas molecules, e.g carbon in kg/min.
  • If you still have questions on how you can use basic mass balances to solve this problem, please post them in the discussion forum.

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