Field Data, New Design Correct Faulty FCC Tower Revamp
SUMMARY
A large US Gulf Coast refinery had poor fractionation in their FCC main column after replacing trays with structured packing. Large amounts of gasoline were being downgraded to LCO because low cost liquid distributors were used. Packed main fractionators offer theoretical improvements, but are less forgiving than trayed columns so proper design and inspection are paramount. Once the design errors were corrected the fractionating problems were eliminated and unit goals were achieved.
TEXT
In 1987, Lyondell Petrochemical Co. revamped a fluid catalytic cracking unit (FCCU) main fractionator by replacing trays with structured packing. This revamp did not achieve its design objectives; therefore, a second revamp was performed in 1992. Packing large diameter main fractionators can increase unit capacity and decrease pressure drop while meeting fractionation objectives. However, a packed main fractionator revamp is less forgiving than a trayed column revamp and must take into consideration proper design and inspection procedures. Lyondell’s experience illustrates the approach needed to have a successful structured packing revamp.
The 79,000 b/d FCCU main fractionator at Lyondell Petrochemical Co.’s Houston refinery was first revamped from trays to structured packing in 1987. The justification for the revamp was a capacity increase to 92,000 b/d. An ultimate capacity of 100,000 b/d was anticipated, at the same gasoline cutpoint as the trayed column, and at reduced column pressure drop. The revamp design gasoline D86 endpoint was 445ºF.
Reactor-regenerator pressure balance is affected by main column pressure drop. Before the revamp, unit capacity and conversion had been limited by low cat-to-oil ratio. Reducing pressure drop would allow lower reactor operating pressure, which permits higher catalyst circulation and...