1. Conversion Processes: Cracking

2. Cracking
Cracking is the breakdown of heavy hydrocarbon molecules into lighter ones.
Cracking can be done Catalytically and/or Thermally.
Catalytic Cracking: catalyzed by the presence of defined catalyst. Examples of catalytic cracking are fluidized-bed catalytic cracking (FCC) and hydrocracking
Thermal Cracking: Catalyzed by heat. Examples of thermal cracking processes are visbreaking and solvent deasphalting
In this Chapter, the focus will be on Catalytic Cracking. 2

3. 3

4. Fluidized Catalytic Cracking Fluidized Catalytic Cracking
Convert heavy hydrocarbon fractions from vacuum distillation into a lighter mixture of more useful products
Feedstock undergoes a chemical breakdown, under controlled heat (450 – 500 oC) and pressure, in the presence of a catalyst
Fluidized Catalytic Cracking
Effective catalyst: small pellets of silica, alumina or magnesia and nowadays zeolite. 4

5. Fluidized Catalytic Cracking: Products
Primary goals:
- To make gasoline & diesel
- To minimize the production of heavy fuel oil
- To produce large amounts of olefins, which are used as
» Feedstocks for petrochemical industry
» Production of butylene, propylene & ethylene
» C5+ olefins can be alkylated to produce high-octane gasoline 5

6. Fluidized Catalytic Cracking
REGENERATOR
REACTOR
RISER
Catalytic cracking is the most important and widely used refinery process for converting heavy oils into more valuable gasoline and lighter products (e.g. LPG).
The cracking process also produces carbon (coke) which remains on the catalyst particle and rapidly lowers its activity. To maintain the catalyst activity at a useful level, it is necessary to regenerate the catalyst by burning off this coke with air.
As a result, the catalyst is continuously moved from reactor to regenerator and back to reactor.
The cracking reaction is endothermic and the regeneration reaction exothermic. 6

7. Chemistry of Catalytic Cracking
The “Cracking” reaction is a composite of many reactions
1. Initiation --- making the carbenium ion
2. Isomerization --- manipulating the carbenium ion
3. ß-scission --- cutting the carbenium ion into two
4. Hydrogen ion transfer --- the engine that keeps things going
5. Termination --- removing the carbenium ion as an olefin

8. Initiation
1. Production of olefin through mild thermal cracking of paraffins, then these olefins attract a proton from the catalyst to form carbenium ions. 2. 8

9. Isomerization - Stabilizing Carbenium Ions9

10. Cracking: cutting C-C bond10

11. Hydrogen Ion Transfer
The carbenium ions propagate the chain reaction by
transferring a hydrogen ion from a n-paraffin to form a
paraffin molecule and a new carbenium ion.
Thus another carbenium ion is formed and the chain
is ready to repeat itself. 11

12. Termination 12

13. Fluidized Catalytic Cracking Catalyst
The FCC process employs a catalyst in the form of very fine particles [average particle size about 70 micrometers (microns)] which behave as a fluid when aerated with a vapor (and this is why it is called fluidized catalytic cracking). The fluidized catalyst is circulated continuously between the reaction zone and the regeneration zone and acts as a vehicle to transfer heat from the regenerator to the oil feed and reactor.
REGENERATOR
REACTOR
RISER 13

14. Fluidized Catalytic Cracking
To Fractionator
The hot oil feed is contacted with the catalyst in he feed riser line and/or the reactor.
As the cracking reaction progresses, the catalyst
is progressively deactivated by
the formation of coke on the surface of the catalyst.
The catalyst and hydrocarbon vapors are separated mechanically, and oil remaining on the catalyst is removed by steam stripping before the catalyst enters the regenerator.
The oil vapors are taken overhead to a fractionation tower for separation into streams having the desired boiling ranges.
REGENERATOR
REACTOR
RISER
Hot oil
air 14

15. Fluidized Catalytic Cracking
To Fractionator
REGENERATOR
REACTOR
RISER
Flue gases
The spent catalyst flows into the regenerator and is reactivated by burning off the coke deposits with air.
Regenerator temperatures are carefully controlled to prevent catalyst deactivation by overheating.
Hot oil
air
This is done by controlling the air flow to give a desired CO2/CO ratio in the exit flue gases or the desired temperature in the regenerator.
The flue gas and catalyst are separated by cyclone separators and electrostatic
precipitators. 15

16. Fluidized Catalytic cracking
Two basic types of FCC units in use today are the ‘‘side-by-side’’ type, where the reactor and regenerator are separate vessels adjacent to each other, and the stacked type, where the reactor is mounted on top of the regenerator.
side-by-side FCC units
stacked type FCC units 16

17. FCC Process FCC feed is preheated to 650oF and is fed into the riser
The ratio of catalyst: oil = 4:1 to 9:1 by weight.
Residence time of the gas < 5 seconds
The vapor generated by the cracking process lifts the catalyst up the riser. The vapor velocity at the base of the riser is about 6 m/s and increases to over 20 m/s at the riser exit.
REACTOR
REGENERATOR
FCC feed is preheated to 650oF and is fed into the riser
Which contains hot catalyst from the regenerator
RISER 17

18. A spent catalyst is withdrawn from the bottom of reactors and stripped with steam to vaporize the hydrocarbons remaining on the surface
Stripping removes most of the hydrocarbon vapors which are entrained between the particles of catalyst
FCC Process
REACTOR
It is desirable to separate the vapor and catalyst as quickly as possible to prevent overcracking of the desired products.
REGENERATOR
RISER 18

19. Fluidized Catalytic Cracking (FCC) Flowsheet19