1. Hydrocracking

3. 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

4. Hydrocracking
Hydrocracking is catalytic cracking in the presence of hydrogen. The extra hydrogen saturates, or hydrogenates, the chemical bonds of the cracked hydrocarbons and creates isomers with the desired characteristics.
Hydrocracking
Hydrocracking is also a treating process, because the hydrogen combines with contaminants such as sulphur and nitrogen, allowing them to be removed

5. Feed
The straight run
Coker gas oil
FCC cycle oil
Hydrocracking
Uses high pressure and large amount of hydrogen
Hydrogenation reaction suppress coke formation
Products high in liquid yield and low in sulfur and olefins

6. Process: Hydrocracking
The two reactions that occur are
Cracking (endothermic) and
Hydrogenation (exothermic).
Heat of cracking is supplied by the hydrogenation reaction  autothermal reaction.

7. 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

8. 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

9. 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

14. 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

15. 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

16. 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

17. 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

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

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

20. 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

21. 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

22. 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

23. 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

24. 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

25. 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

26. 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

27. Catalysts
Hydrocracking catalysts are dual functional, (having metallic and acidic sites) promoting cracking and hydrogenation reactions.
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

28. Catalysts Cracking is promoted by metallic sites of the catalyst.
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)

29. 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,…)

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

31. Catalysts
Zeolite-based catalyst is one of the most commonly 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

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

33. End of Chapter Six