FCC
Also known as:
Fluid catalytic cracker, cat cracker, cat unit, FCCU, CCU, Houdry
The FCC is the most common conversion unit used in refining 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.
How it works
The feed to the FCC is usually VGO, coker gasoil and possibly some atmospheric resid or hydtotreated vacuum resid.
Feed is heated to 500-525 F and mixed with hot fluid catalyst. This raises it to a reaction temperature of around 975F. Most of the cracking reaction happens in the short time that the catalyst and feed are in contact in the riser. As cracking occurs, coke deposits on the catalyst, deactivating it.
The mixture of catalyst and hydrocarbon (now cracked and vaporized) flows into the reactor. The hydrocarbon and catalyst are separated using a cyclone.
The hydrocarbon vapors flow out of the top of the reactor and are sent to a fractionator. In the fractionator the product is separated into the major product fractions: FCC gases, naphtha, some of the cycle oil, and slurry. The gases go through a wet gas compressor and then sent to the gas plant for further separation. The slurry and or cycle oil are recycled to the front of the unit and mixed with fresh feed.
The catalyst, once separated in the reactor, is stripped with steam to remove any remaining hydrocarbons. It then flows to the regenerator for removal of coke deposits and reactivation.
In the catalyst regenerator hot air is mixed with the catalyst to burn off coke deposits. Additional oxygen is often added to help achieve full coke burning, control CO/CO2 content in the flue gas, and maintain a sage regenerator bed catalyst temperature (typically 1300 F) to avoid overheating and deactivating the catalyst.
Hot flue gas from the regenerator goes through a cyclone or electrostatic separator to remove any catalyst and then sent to a CO boiler. Heat from the flue gas is recovered before it is released.
Hot, re-activated catalyst from the regenerator has a small amount of fresh catalyst added to it, and is then sent back to the front of the unit to mix with feed in the riser.
Feed
The feed to the FCC is a variety of heavy gasoils. 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 feed stock for the FCC. This is almost always hydrotreated.
Atmospheric resid - typically from lighter crudes and requires hydrotreating
Vacuum resid - typically in very small amount, from crudes with low levels of contaminants, and requires hydrotreating
DAO - Vacuum resid that has had asphaltenes removed in an SDA unit
Products
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 (LCO). This is highly aromatic and often high in sulfur, which makes it a poor diesel blend stock. 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. Propylene may also be separated and sold directly as a petrochemical feedstock. Lighter gases (ethane and methane) are sent to the fuel system.
Economics
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). It also is the primary source of light olefins (propylene and butylene) that feed to alkylation unit making even more high quality gasoline. FCC gasoline typically accounts for over 1/3 of the total gasoline blend pool.
While an FCC yields some diesel range material (LCO), it is 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, which processes the same feed but yields more and better-quality Diesel.
Some refineries have both an FCC and a hydrocracker in order to maximize total yield and quality of the gasoline and diesel combined. With both units there is typically an advantage to putting cracked gasoil into the hydrocracker and sending some of the heavy gas oil from the hydrocracker to the FCC.
The FCC is a good complement to a coker, which takes the heavier vacuum resid fraction that the FCC can not handle, and converts it into the VGO the FCCs process, as well as other lighter streams.
FCC margins are typically highest when gasoline is at a premium to diesel and when light products (gasoline and diesel together) are at a high premium to residual fuel oil.
There is a also a growing trend toward maximizing olefins production from the FCC, both to feed the alkylation unit and to separate and sell propylene as a product.
Technology types and licensors
FCC designs can vary in a few key ways:
Lay out - Originally the reactor and regenerator were separate, side-by-side units. Increasingly these are designed with the reactor above the regenerator, allowing gravity to assist in moving spent catalyst to the regenerator.
Cracking zone - Originally the cracking reaction in an FCC occurred entirely in the reactor. But iwth growing use of fast reaction zeolite catalysts, it is most common now for the cracking to occur primarily in the riser feeding the reactor vessel (riser cracking).
Feed type - There has been a trend over time towards sending heavier feed to an FCC. Traditionally this was limited by the higher level of contaminants in heavier feed (metals and nitrogen) and also higher conradson carbon that resulted in excessively high coke make. The RCC (resid catalytic cracker) is a variant on the FCC that was to perform well with heavier feed. It is a similar unit yielding a similar range and quality of products, but is designed to handle atmospheric resid (both VGO and vacuum resid) as a feed.
There are a number of licensors of FCC process technology:
KBR (Kellogg) - Orthoflow FCC technology
UOP - UOP FCC and UOP Resid FCC technologies
Axens/IFP - VGO FCC gasoil FCC, R2R resid FCC technology, HP-FCC high propylene yield FCC, HS-FCC high severity FCC
Shell Global Solutions - Shell FCC technology and MILOS max propylene FCC
Foster Wheeler - FCC technology