Catalytic Cracking Units: Best Practices for Valve Application

Overview

Catalytic cracking is one of the most crucial secondary processing methods in the petroleum refining industry. It plays a vital role in lightening heavy oils, increasing the yield of light oils, and is essential for producing gasoline and diesel. As a core technology in modern refineries, catalytic cracking converts heavy gas oil and residual oil into lighter products through a process that involves breaking down large hydrocarbon molecules into smaller ones. This occurs at approximately 500°C and pressures of 0.1 to 0.3 MPa. The process generates light oils, gases, and coke through decomposition and condensation reactions. Modern refineries typically use fluid catalytic cracking units (FCC), where catalysts flow and mix with gas, simulating a fluid-like state.

Reaction and Regeneration System

Material and Pressure Considerations

• Temperature and Pressure: The feedstock pipelines of the catalytic cracking unit operate at relatively low temperatures (not exceeding 290°C) and pressures (0.1–2.0 MPa).
• Valve Pressure Ratings and Diameters: Valves typically have pressure ratings below CLASS300 and nominal diameters up to DN1000.
• Valve Materials:
  • Below 240°C: Valves are often made from carbon steel.
  • Above 240°C: Materials may include carbon steel and alloy steels (15CrMo, 1Cr5Mo) depending on the corrosive nature of the medium.

Valve Types

• Shut-off Valves:
  • DN500 and below: Gate valves.
  • DN600 to DN1500: Butterfly valves.
• Check Valves:
  • DN40 and below: Lift check valves.
  • DN50 and above: Swing check valves.
  • DN200 and above: Tilting disc check valves, which help mitigate impact during valve operations.
• Valve Structure: Low-pressure conditions require bolted bonnet connections.
• Packing Material: Stainless steel and flexible graphite packing are recommended.

Reactant Gas and Oil Pipelines

• Valve Materials: Similar to the main system, materials include carbon steel and alloy steels.
• Valve Types:
  • Ball valves are ideal due to solid catalyst particles, but gate valves can also be used with precautions against particle accumulation.
  • Y-shaped globe valves are recommended for bypass lines to reduce flow resistance and erosion from solid particles.

High-Temperature Flue Gas Pipelines

• Valve Types: Due to the presence of solid particles, special sliding valves are used, often integrated with actuation mechanisms and control parameters.

Shut-off Valves

• Material Considerations: For temperatures exceeding the limits of chromium-molybdenum alloy steels, high-carbon austenitic stainless steel (H-class 300 series) is used.
• Valve Types:
  • High-temperature, large-diameter applications: Triple eccentric hard-seal butterfly valves.
  • Catalyst transport pipelines: Full-bore ball valves (for non-high temperature) or gate valves with purge holes (for high and non-high temperature).

Fractionation System

Fractionation Tower

• Valve Application: Follow similar guidelines as the reaction system.
• Bottom Oil Slurry Pipelines:
  • Valve Materials: Similar to the reaction system, including carbon steel and alloy steels.
  • Valve Types:
    • Eccentric half-ball valves are preferred for their self-cleaning and tight-sealing properties.
    • Triple eccentric metal-sealed butterfly valves are also acceptable.
• Check Valves: Use Y-type lift check valves for DN15-40 and swing check valves for DN50 and above.

Side Line Pipelines and Top Cooling System

• Valve Material Selection: Based on medium temperature and sulfur content.
• Low Emission Structures: Required for handling hydrogen sulfide and hydrogen cyanide, adhering to NACEMR0103 standards.

Absorption and Stabilization System

Sulfur and Hydrogen Cyanide Containment

• Valve Structures: Low emission, meeting NACEMR0103 standards.
• Liquefied Gas Pipelines: Nominal pressure ratings must be at least equal to the liquefied gas's vapor pressure.
• Shut-off Valves: Globe valves are preferred, while gate and ball valves require cavity pressure relief systems.

Energy Recovery System

High-Temperature Flue Gas

• Valve Requirements: Similar to the reaction and regeneration system, focused on handling high temperatures and potential solid particles.

Conclusion

Valves in catalytic cracking units must be carefully selected based on operational conditions such as temperature, pressure, and the presence of corrosive or erosive substances. Proper material selection and valve types ensure reliability and efficiency in the harsh environments typical of these systems. Adherence to industry standards like NACEMR0103 for low emission structures is crucial, especially in handling toxic gases. Overall, understanding these specifics helps maintain optimal performance and safety in catalytic cracking processes.

FAQs

1. Why is carbon steel preferred for valves below 240°C in catalytic cracking units?
   • Carbon steel is cost-effective and sufficient to handle the operational temperatures and pressures below 240°C in catalytic cracking units.
2. What are the benefits of using triple eccentric hard-seal butterfly valves in high-temperature applications?
   • Triple eccentric hard-seal butterfly valves provide tight sealing, durability, and resistance to high temperatures and pressures, making them ideal for high-temperature applications.
3. Why are Y-shaped globe valves recommended for bypass lines?
   • Y-shaped globe valves reduce flow resistance and minimize the erosion caused by solid particles, making them suitable for bypass lines in catalytic cracking units.
4. How do tilting disc check valves help in large diameter valve applications?
   • Tilting disc check valves have a buffering effect that reduces the impact force during valve opening and closing, preventing damage and extending valve life.
5. What measures are taken to prevent valve cavity pressure buildup in liquefied gas pipelines?
   • In liquefied gas pipelines, gate and ball valves must have cavity pressure relief systems to prevent pressure buildup and ensure safe operation.