Explosion at a catalytic cracking unit of a refinery

SUMMARY
The explosion occurred in a catalyst regenerator of the catalytic cracking unit in a fluidized bed during a technical shutdown. A few seconds later, a second explosion is perceived, it is the pressure wave of the first explosion which reached the site recovery boiler 30 m away and connected to the regenerator by pipe.
It left 24 people injured; 8 were hospitalized, including 2 suffering from severe burns. Material damage is significant, particularly in this boiler.

DETAILED ACCIDENT SEQUENCE
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 as described in the field ‘ pre-event summary’. 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.
Simultaneously a flame front and hot particles exited from the small manhole on the other side of the regenerator platform.
The flame and pressure front passed through the regenerator into the downstream flue ductwork. Where the duct was broken and plant items removed flame fronts and hot catalyst exited.
After a period of a few seconds, there was a louder secondary noise which emanated from the waste heat boiler and associated flues which sustained structural damage.

E-MARS identified 3 causes: Plant/Equipment / Human / Organisational

The INITIATING cause was the ignition of hydrogen, light hydrocarbon gases and carbon monoxide. These gases were generated by contact of not regenerated catalyst with steam in an oxygen deficient atmosphere.
Removal of a manway to allow access for vacuum truck removal of catalyst allowed oxygen re-enrichment of the internal atmosphere and the re-establishment of conditions that permitted ignition.
Lighter-than-air combustible gases were trapped in a reservoir created by the internal configuration of the plant. The opening of the manway caused some gases to be dispersed into the ductwork prior to the ignition.

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.

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.

According to the eMARS report (see references), the following measures were established after the accident:
1- in future, the catalyst will be cooled to 400°C before its unloading from the regenerator prior to entry the vessel;
2- gas composition in the regenerator will be identified before the removal of the two man-way covers;
3- the entry into any equipment on the catalyst circuit will be prohibited if the blower powdered by powder recovery train fails and, therefore, the proper purging of the regenerator has not occurred;
4- if purging would be necessary, it 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 these dangerous by-products. This will also cool the catalyst bed;
5- tighter control on personnel around future openings of man-ways