Chapter 6: Catalytic Cracking. Catalytic cracking is the most important and widely used reﬁnery process for converting heavy oils into more valuable gasoline and lighter products
Catalytic cracking is the most important and widely used reﬁnery process for converting heavy oils into more valuable gasoline and lighter products
The cracking process produces carbon (coke) which remains on the catalyst particle and rapidly lowers its activity.
Average riser reactor temperatures are in the range 900 to 1000°F (480– 540°C), with oil feed temperatures from 500 to 800°F (260–425°C) and regenera- tor exit temperatures for catalyst from 1200 to 1500°F (650–815°C).
The spent catalyst ﬂows into the regenerator and is reactivated by burning off the coke deposits with air.
The products formed in catalytic cracking are the result of both primary and secondary reactions
Primary reactions are designed as those involving the initial carbon–carbon bond scission and the immediate neutralization of the carbonium ion
The advantages of the zeolite catalysts over the natural and synthetic amorphous catalysts are:
3. Production of gasolines containing a larger percentage of parafﬁnic and aromatic hydrocarbons
4. Lower coke yield (and therefore usually a larger throughput at a given conversion level)
5. Increased isobutane production
6. Ability to go to higher conversions per pass without overcracking
The catalytic effects of zeolitic catalysts can be achieved with only 10 to 25% of the circulating catalyst as zeolites and the remainder amorphous silica-alumina cracking catalyst.
Space velocity: Space velocity may be deﬁned on either a volume (LHSV) or a weight (WHSV) basis.
1. Reaction temperature
2. Catalyst/oil ratio
3. Catalyst activity
4. Contact time
In ﬂuidized-bed units, the reactor pressure is generally limited to 15 to 20 psig by the design of the unit and is therefore not widely used as an operating variable.
Assuming a regenerator ﬂue gas discharge pressure of 20 psig (138 kPa) and 1000°F (538°C), the available horsepower per pound per second of gas ﬂow for the various schemes are