Explosion at a catalytic cracking unit of a refinery

An explosion occurred in the catalyst regenerator of a catalytic cracking unit.
The unit had been shut down after the failure of the fan supplying oxygen for the regeneration of the catalyst (aluminosilicate). The operator had decided to take advantage of this shutdown to perform maintenance and check the configuration of new equipment recently installed on the regenerator.
To allow for the inspection of the regenerator, the catalyst had to be removed using a steam reduction system under a nitrogen flow. The reactor was first isolated, which involved installing approximately 270 solid flanges. These operations were described in a procedure that has been successfully used during previous shutdowns.
After this preparatory work, the fitters removed the covers from the two manholes. No gas tests were performed inside the reactor because it was considered open to the atmosphere due to the vent connected to the stack.
(1) The smaller manhole cover was removed without incidents.
(2) After the first bolt of the bigger manhole was removed, workers heard a loud rumbling noise inside the regenerator, immediately followed by a red-orange flash from the manhole flange, striking nearby operators.
(3) The flame and pressure front passed through the regenerator into the downstream flue ductwork. The duct was broken and flame fronts and hot catalyst exited.
(4) Simultaneously, a flame front and hot particles exited from the small manhole on the other side of the regenerator platform.
(5) A few seconds later, a second explosion was perceived, due to the pressure wave of the first explosion reaching a recovery boiler 30 m away, connected to the regenerator by pipe.

24 were people injured and the material damage was significant.

The INITIATING cause was the ignition of hydrogen, light hydrocarbon gases and carbon monoxide, after air was allowed to access the reactor through the opening of the two manholes.
The gases were generated by the unregenerated catalyst with steam in an oxygen deficient atmosphere. These conditions were probably due to recent modifications to the unit that prevent proper gas venting to the stack. The injection of steam onto the catalyst to regenerate it and maintain it in the fluidized bed made the things worse.
Finally, the procedure had been designed during a previous maintenance intervention. In this new case, the quantity of unregenerated catalyst in the reactor was much higher than during the previous intervention.
eMARS identified 3 causes: Plant/Equipment / Human / Organisational.

Fluidised Catalytic Cracker Unit

The incident occurred in the regenerator section of a Fluid Catalytic Cracker Unit (FCCU) 50 hours after a unit shutdown. The shutdown was not planned and was caused by mechanical failure of the regenerator air blower.

FCCU regenerators are large vessels containing beds of fluidized catalyst in which air is used to burn off both carbon, referred to as coke, and hydrogen based material trapped in and on aluminium silicate catalyst which has a porous structure. The air flows into the regenerator through a two, tier air grid system from an air blower.
Two days before the incident, the air blower tripped out due to activation of the air blower vibration shutdown monitoring equipment. The vibration was caused by a mechanical failure of one of the air blower rotor discs.

This initiated automatic shutdown of the unit. As a result the regenerator fluidized bed slumped and steam was automatically injected into the catalyst bed.
The air blower rotor assembly was inspected through a small manway inspection door, visually confirming that the rotor was damaged and would have to be repaired. At the same time the decision was taken to enter the regenerator/riser/reactor circuit to undertake other necessary repair work.

Over the subsequent 2 days operations staff prepared the regenerator for manway removal. It was recognized that catalyst temperature would be higher than usual. Previously when the air blower had tripped and the manways to the regenerator, riser/reactor and ductwork, including the waste heat boiler (known as the cat circuit) had been opened, the equipment had been gas tested and entered without incident. During the preparations a large butterfly valve and a critical flow nozzle were removed from the ductwork to the flue. These were normal procedures in preparing the cat circuit for entry. The removal of these items reduced the draught of the flue on the regenerator and would have contributed to an oxygen deficiency in the regenerator.

After all the necessary blinds had been inserted, operational procedures permitted the regenerator manways to be removed to allow the final vacuum truck removal of remaining catalyst.

On the day of the incident, work commenced to remove one of two manways on the regenerator, at the base about 9 m above ground level. A small manway was opened first to ensure that there was not a residual mound of hot catalyst resting against the large manway door that might have slumped onto those on the access platform. This manway was opened as the system was considered to be an air system open to atmosphere by virtue of the flue connection.

Work then proceeded to open the large 1.5 m manway. With one bolt remaining on the large manway, some witnesses reported a rumbling noise inside the regenerator. It was immediately followed by an orange-red flash which came out of the left side of the manway, from where the penultimate bolt had been taken.

From Wikipedia [https://en.wikipedia.org/wiki/Fluid_catalytic_cracking#Flow_diagram_and_process_description, (accessed March 20266)]
A Fluid Catalytic Cracking unit (FCCU) is the process used in petroleum refineries to convert the high-boiling point, high-molecular weight hydrocarbon fractions of petroleum (crude oils) into gasoline, alkene gases, and other petroleum products. The feedstock to the FCC conversion process usually is heavy gas oil (HGO), which is that portion of the petroleum (crude oil) that has an initial boiling-point temperature of 340 °C or higher.

The reactor and regenerator are two main components heart of a FCCU.

(1) The preheated feedstock (at about 315 to 430 °C) is combined with recycle slurry oil from the bottom of the distillation column and injected into the catalyst riser, where it vaporises. In the riser, long-chain hydrocarbons crack into smaller molecules upon contact with the hot powdered catalyst in 2–4 seconds. The hydrocarbon vapours "fluidize" the powdered catalyst and the mixture of hydrocarbon vapours and catalyst flows upward to enter the reactor at a temperature of about 535 °C and a pressure of about 1.72 bar.

(2) The reactor vessel contains the catalyst in which the cracked product vapours are formed by flowing through a set of two-stage cyclones. The spent catalyst flows downward through a steam stripping section to remove any hydrocarbon vapours before the spent catalyst returns to the catalyst regenerator.

(3) The cracking process deposits carbonaceous material on the catalyst and reduces its activity. The catalyst is then regenerated by burning off the deposits with air blown into the regenerator. The regenerator operates at a temperature of about 715 °C and a pressure of about 2.41 bar, hence the regenerator operates at about 0.7 bar higher pressure than the reactor. The combustion of the coke is exothermic.

24 people injured; 8 were hospitalized, including 2 suffering from severe burns.

The explosion caused minor damage around the 60" man-way but extensive damage to the waste heat boiler. No data are available about the cost of the damages.

INTERNAL TO THE ESTABLISHMENT: The full site was alerted of the emergency. Water monitor sprays were trained onto the regenerator after the explosion. Approximately 70 people were involved in dealing with the accident.

EXTERNAL TO THE ESTABLISHMENT: The County Emergency Services were summoned.

According to the eMARS report , the following measures were established after the accident:
1-The catalyst will be cooled to 400°C before its unloading from the regenerator prior to entry the vessel.
2- The gas composition in the regenerator will be measured before the removal of the two man-way covers.
3- The access to the catalyst circuit will be prohibited if a proper purging of the regenerator has not occurred.
4- If purging would be necessary (e.g. due to the failure of the blower), this will be done using large quantities of steam over a long period of time to fluidise the catalyst bed and promote the reactions that caused the accident, removing then these dangerous by-products. This will also cool the catalyst bed.
5- A tighter control on the personnel requested to be around openings of manholes.