Things to consider in valve selection for catalytic cracking
Reliable and accurate control, on-off and ESD-valve performance is important for total process efficiency, productivity and safety. Here’s presented some of the key-applications and things to consider in selecting valves for a typical FCCU.
Air flow control
The cracking process produces carbon (coke) which remains on the catalyst particle and rapidly lowers its activity. To maintain the catalyst activity at a useful level, it’s necessary to regenerate the catalyst by burning off this coke with air. Regeneration is a key part of the FCC process. It’s critical to control the regenerator temperature carefully to prevent catalyst deactivation by overheating and to provide the desired amount of coke burn-off. This is done by controlling the air flow. A typical air temperature is around 600 – 700°C. Here the valve selection should consider:
Good control combined with long lasting tightness
- Reliable metal seated, bi-directional tightness for long life cycle
- Modular metal seated design and material selection for ease of maintenance
- Economical control valve for low differential pressures
Stripping steam flow control
Remaining oil on the catalyst is removed by steam stripping before catalyst enters the regenerator. The steam supply to the reactor takes place at a temperature at dry saturated steam. In the valve selection for this application the following should be considered:
- Reliability to ensure effective oil stripping
- Noise reduction capabilities
- Wide rangeability with single valve solution for changing steaming needs
- Control accuracy to optimize steam consumption
High temperature catalyst valves
Catalyst handling valves play an important role in ensuring proper FCC performance. Reliable and accurate control, on-off and ESD-valve performance is important for total process efficiency.
The feedstock is vaporized by the hot regenerated catalyst, the cracking begins, and the resultant vapor carries the catalyst upward through the riser. The heat of combustion raises the catalyst temperature to (620 – 845 °C), and most of this heat is transferred by the catalyst to the oil feed in the feed riser. The regenerator/reactor cycle continues until catalyst is spent and removed from process through extraction valve. A typical temperature here is 760 °C.
Catalyst handling valves have typically following requirements:
- Medium build-up resistant seat design and sealing surface materials to avoid torque increase
- Suitable design and materials to resist highly erosive medium in high temperature
- Long lasting tightness and cycle life
- Emission proof packing
- Field proven performance
Catalyst addition valves
Each day fresh catalyst is added to replace losses through the cyclones and to maintain the activity of the unit’s inventory at an acceptable level. A typical temperature here is ambient. Things to consider in suitable valve selection:
- Tight shutoff
- High performance soft seat design to optimize torque actuator size
Fractionator bottom slurry valves
The remaining heavy residual oil, together with any catalyst carryover, collects at the bottom of the fractionator and recycles back to the reactor for the catalyst to be used in the reactor. Bottom recycle is used to recover heat for feed preheat through kettle boilers and exchangers. The fluid is known as catalyst oil slurry and its control and isolation, due to its highly abrasive nature and temperature sets high demands for valves.
Things to consider for control valve selection:
- Eccentric plug valve with stellited seat construction and flow to close direction to prevent seat and trim erosion
- Full ceramic valve for extreme erosive cases
- Rotary designs to avoid medium build-up for packing and avoid emissions
- Long cycle life designs
- Erosion resistant design for both medium and high erosive applications
Things to consider for on-off valve selection:
- Metal seated ball valves
- Rotary designs for low emission packing
- Long cycle life
- Erosion resistant design
Flue gas expander valves
The flue gas from the FCC process exiting the regenerator has significant pressure, temperature and volume, and it is a source of useful energy that represents an energy cost-saving opportunity to a refinery. Using an expander could maximize recovery of available energy from the flue gas. This energy can then be used to drive the compressor that provides air to the regenerator (the main air blower) or an electric generator.
The expander valves should be able to work under very high temperature (up to 760 °C). The flue gas may entrain some abrasive catalyst particles. Fast closing / opening (0.6 – 1 second), in a case of expander trip, suitable shut-off capabilities and reliability over long periods are required. Defined small leakage might be required in case the pipe cool too much potentially below the dew point. Huge thermal shock and inconstant temperature during the emergency case may lead to destructive high pressure loaded on seat by disc.
Consider these for flue gas expander valve selection:
- Special aperture brings seat element in good operation condition
- Changeable sealing element
- Special designed flexible seat against thermal shock and avoid overstress
- Customized engineering design ability
- Long cycle life
- Erosion resistant design
Written by Sari Aronen
This blog post has been up-dated in July 2020, due to company name change to Neles.