Also known as: fluid catalytic cracker, cat cracker, cat unit, FCCU, CCU, Houdry
The FCC is the most common unit used to upgrade heavier distillation cuts to light products. The FCC takes VGO and similar intermediate streams and cracks them using heat in the presence of a catalyst. The primary product is FCC gasoline, which is used in gasoline product blending. However, the FCC also produces lighter products that feed the alkylation unit and heavier products that can be blended into diesel and residual fuel oil.
The FCC is particularly valuable in a refinery that is trying to maximize gasoline production over residual fuel oil. The FCC yields a high volume of gasoline of pretty good quality (high octane and low vapor pressure). However, its volume yield of diesel is low and of low quality (low cetane), since it is comprised of cracked material which tends to have a low cetane. In a market where diesel is preferred over gasoline, an FCC is generally less valuable than a hydrocracker.
The RCC (resid catalytic cracker) is a variant on the FCC. It is a similar unit yielding a similar range and quality of products, but is designed to handle heavier atmospheric resid as a feed.
How it works
Heated feed is mixed with a heated catalyst and injected into a reactor, where the catalyst freely mixes with the feed as a fluid. As the feed is cracked, coke deposits on the catalyst causing it to gradually deactivate. Cracked product is drawn off at the top of the reactor and is sent to a fractionator. Deactivated catalyst is drawn off the bottom of the reactor and is sent to a regenerator where the coke is burned off by injecting heat and air. The cleaned (regenerated) catalyst is then sent back to the reactor and the cycle repeats.
The catalyst moves around the reactor and regenerator circuits in seconds at very high velocities so many internal surfaces on the catalyst circuit have to be protected against erosion by having ceramic coatings. The heat generated in the regenerator from burning the coke off the catalyst provides the majority of the heat required for the separation reactions taking place in the reactor and the unit has to be heat balanced between the reactor and regenerator. Coke burned off the catalyst in the regenerator creates a mix of carbon monoxide and carbon dioxide plus some SOx. This gas stream is passed through a CO boiler and recovery gas compressor to recover some energy, then cleaned of catalyst fines and evacuated to the atmosphere, so the FCC is a major emitter of CO2 from refineries.
The feed to the FCC is a variety of heavy gasoils that would otherwise be blended into fuel oil. Typical feeds include:
- VGO - This heavy cut from the vacuum distillation unit is the major feed to the FCC. Often it will be processed through a gasoil hydrotreater to reduce its sulfur content before being fed to the FCC
- Coker gasoil - For refineries with a coker, this VGO range material from that unit is also a major feedstock for the FCC
The FCC produces a range of mostly lighter products, with the most significant being FCC gasoline. Typical products are:
- FCC gasoline - This is a naphtha range material with octane and vapor pressure close to the quality specifications for finished gasoline. This is typically the largest product at around 50% of FCC output
- Cycle oils - The FCC produces a diesel range product called cycle oil. This is highly aromatic and often high in sulfur, which makes it a poor diesel blendstock. It is typically blended into lower quality diesel, used as a cutter stock in fuel oil blending, or sent to the hydrocracker for upgrading
- FCC slurry - The heaviest product from the the FCC is a highly aromatic resid stream, typically with high sulfur content. This is typically sent to fuel oil blending, used as feed for the coker, or used in to make specialty products such as carbon black or needle coke
- FCC gas - The light ends from the FCC include both saturated and unsaturated C3s and C4s. This is typically sent to an unsaturated gas plant where the components are separated. The butylenes, and sometimes the propylenes, are fed to the alkylation unit if there is one. Lighter gases (ethane and methane) are sent to the fuel system
Technology licensors include:
- KBR (Kellogg) - Orthoflow FCC technology
- UOP - FCC and RFCC technologies
- Axens/IFP - R2R RFCC technology
- Shell Global Solutions - FCC technology
- Foster Wheeler - FCC technology