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

20. Hydrocracking

22. What is Hydrocracking?
Hydrocracking is the conversion process of higher boiling point petroleum fractions to light hydrocarbons (mainly gasoline and jet fuels) in the presence of a catalyst

23. Why Hydrocracking?
The increasing demand for gasoline and jet fuel compared to diesel fuel and home heating oils was a dominant factor in the development of hydrocracking process
Hydrogen as a byproduct of catalytic reforming process was available in large amounts and relatively cheap

24. Benefits of Hydrocracking
Hydrocracking is one of the most versatile process, which facilitate product balance with the market demand
Other advantages are:
Very high gasoline yield
High octane numbers
Production of large amount of isobutane
Supplementing FCC (Fluid Catalytic Cracking) to upgrade heavy stocks, aromatics and coker oils

25. Hydrocracking vs FCC
Fluid Catalytic Cracking (FCC) takes more easily cracked paraffinic atmospheric and vacuum gas oil
Hydrocracking is capable of using aromatics and cycle oils and coker distillates as feed (these compounds resist FCC)
Cycle oils and aromatics formed in catalytic cracking (FCC) are satisfactory feedstock for hydrocracking
Middle distillate and even light crude oil can also be used as feedstock for hydrocracking

26. Hydrocracking processes
The fresh feed is mixed with hydrogen gas and recycle gas (high in hydrogen content) and passed through a heater to the first reactor
If the feed is high in sulfur and nitrogen a guard reactor is employed to convert sulfur to hydrogen sulfide and nitrogen to ammonia to protect precious catalyst in the following reactor.
Jet fuel
Two-Stage Hydrocracking
gasoline

27. Hydrocracking processes
Hydrocracking reactors are operated at high temperatures to produce materials with boiling point below 400 F
The reactor gaseous effluent goes through heat exchangers and a high pressure separator where the hydrogen rich gases are separated and recycled to the first stage.
Jet fuel
Two-Stage Hydrocracking
gasoline

28. Hydrocracking processes
The liquid product from the reactor is sent to a distillation column where C1-C4 and lighter gases are taken off and the gasoline, jet fuel, naphta and/or diesel fuel streams are removed as liquid side streams.
The distillation bottom product is sent to the second hydrocracker
Jet fuel
Two-Stage Hydrocracking
gasoline

29. Hydrocracking Reactions
There are hundreds of simultaneous chemical reactions occurring in hydrocracking
It is assumed that the mechanism of hydrocracking is that of catalytic cracking with hydrogen superimposed
In catalytic cracking the C – C bond is broken, while in hydrogenation, H2 is added to a carbon- carbon double bond
Cracking is an endothermic reaction
Hydrogenation is an exothermic reaction

30. Hydrocracking Reactions
Cracking and hydrogentation are complementary as shown below

31. Hydrocracking reactions
Aromatics which are difficult to process in FCC are converted to useful products in Hydrocrackers.

32. Hydrocracking Reactions
Cracking provides olefins for hydrogenation and Hydrogenation provides heat for cracking
The overall reaction provides an excess of heat as hydrogenation produces much larger heat than the heat required for cracking operation
Therefore the overall process is exothermic and quenching is achieved by injecting cold hydrogen into the reactor and apply other means of heat transfer, e.g. intermediate heat exchanger
Isomerization is another type of reaction, which occurs in hydrocracking

33. Hydrocracking Reactions
As a result of isomerization, the olefinic products formed are rapidly hydrogenated which provides high octane isoparaffins
The volumetric yield can be as high as 125% as the hydrogenated products have a higher API gravity
Hydrocracking reactions are normally carried out at an average catalyst temperature between 550 and 750 F

34. Hydrocracking Reactions
The reactor pressure ranges from 8275 – kPa (1200 to 2000 psig)
Large quantity of hydrogen is circulated in order to prevent excessive catalyst fouling
Catalyst poisons are removed from the feedstock to enhance catalyst life
The feedstock may be hydrotreated to reduce the sulfur and nitrogen levels as well as metals
In recent designs, the first reactor in the reactor train may be used for sulfur and nitrogen removal

35. Reaction Kinetics
Kinetic modeling of reactions in hydrocracking is difficult due to the complexity of feedstock and products
In general, adsorption of hydrogen to the active sites on the catalyst surface can be shown as
H2 + S
H2.S kA
k-A
S = vacant adsorption site on the catalyst surface
kA = adsorption rate constant
k-A = desorption rate constant

36. Reaction Kinetics
The rate of adsorption ra is proportional to the concentration of active sites Ca and the partial pressure of hydrogen
ra = kA pH2 Ca
The desorption rate would be
rd = k-A CH2S
The net rate of adsorption is
ra - rd = kA pH2 Ca - k-A CH2S
The total concentration of active sites is constant
CT = Ca + CH2S

37. Reaction Kinetics At equilibrium, the net rate will be zero
CH2S = (KA PH2 CT )/(1 + KA PH2)
Where KA = kA/k-A
Once the molecules have been adsorbed, they undergo several possible types of surface reactions
The rate constant is also related to activation energy of the reaction
In general, the rate of hydrocracking follows a first order kinetics

38. Catalysts Characteristics of good catalyst:
- ability to produce desirable product and not coke
- selective to valuable products (e.g. high octane gasoline).
- stable so it does not deactivate at the high temperature levels in regenerators.
- resistant to contamination

39. Catalysts
Hydrocracking catalysts are dual functional, (having metallic and acidic sites) promoting cracking and hydrogenation
The main reactions are:
Cracking
Hydrogenation of unsaturated hydrocarbons obtained from cracking
Hydrogenation of aromatic compounds
Hydrogenolysis (breaking C-C bond by the addition of hydrogen) of naphthenic structure

40. Catalysts Cracking is promoted by metallic sites of the catalysts
Acid sites transform the alkenes formed into ions
Hydrogenation reactions are also occur on metallic sites
Both metallic and acidic sites take part in the hydrogenolysis reactions
To minimize coke formation a proper balance must be achieved with the two sites on the catalyst (depending on the conditions of the operation)

41. Catalysts
High temperatures lead to more reactions on acidic sites while increase in hydrogen partial pressure enhances hydrogenation on metallic sites
Conventional catalysts are composed of transition metals deposited on acidic sites.
The metals are those from group VIII (e.g. molybdenum, cobalt, nickel,…)

42. Catalysts Three classes: acid-treated natural aluminosilicates
amorphous synthetic silica-alumina combinations
crystalline synthetic silica-alumina catalyst called zeolite or molecular sieves

43. Catalysts
Zeolite-based catalyst is one of the most common used catalysts in hydrocracking
The use of zeolite catalyst minimizes coke formation and improves catalyst stability
Zeolites have large concentration of Brunsted acid sites which enhances their hydrocracking activity
Zeolites also need lower temperatures to achieve a specified conversion
Amorphous -alumina is also widely applied as a catalyst support due to its mechanical and thermal stability and porous structure

44. Advantages of using Zeolite as Catalyst
Higher activity
Higher gasoline yield
Higher octane number
Lower coke yield
Increased isobutane production
Higher conversion without overcracking

45. End of Chapter Six