Revamping FCC Units: Debutanizer Reboiler Fouling
SUMMARY
FCC debutanizer reboiler shell-side fouling causes unscheduled outages and can make bundle removal difficult or impossible. Expensive solutions such as the addition of a parallel reboiler system only treat the symptoms, not the causes, of the problem. FCC debutanizer fouling is caused by di-olefin polymerization and poor exchanger design. Improved exchanger design has eliminated unscheduled outages in several FCC’s.
TEXT
FCC Debutanizer reboiler shell-side fouling is becoming more common. Existing exchanger designs that have operated without major problems are experiencing severe fouling and are causing unscheduled shutdowns. Severe shell-side fouling is characterized by dramatically lower heat transfer coefficients, which result in very poor fractionation. If the exchanger is not taken out of service to clean the shell-side, the fouling material eventually turns into hard coke, which can deform the exchanger bundle and shell. Once this occurs, the tube bundle is difficult or impossible to remove from the shell, and the exchanger must be replaced.
Fouling reduces fractionation due to lower reboiler heat input and lower column reflux. This increases FCC gasoline RVP, increases C5s in the overhead product, reduces the amount of butane and C5s that can be blended into refinery gasoline, and/or reduces FCC unit capacity. All these lower refinery profitability.
Reboiler system revamps should address the fouling root-cause and not treat the symptoms. Expensive solutions such as adding a parallel reboiler system treat the symptom. One or a combination of the following items may be required to reduce or eliminate shell-side fouling...