Catalytic Hydrocracking
Why Catalytic Hydrocracking?
Catalytic hydrocracking has emerged due to the
following reasons:
Demand for gasoline and jet fuel is more than diesel.
Availability of hydrogen in large amounts.
Environmental concern of sulfur and high aromatics in
motor fuels.
It is used for feedstocks that are difficult to process by
either catalytic cracking or reforming, since these
feedstocks are characterized usually by a high polycyclic
aromatics content and/or high concentrations of the two
principal catalyst poisons, sulfur and nitrogen
compounds.
Advantages of Catalytic Hydrocracking
Better balance of gasoline and distillate production
Greater gasoline yield
Improved gasoline pool octane quality
Production of relatively high 1-butane in the butane
fraction
Can be used to treat many refinery products such as
cycle gas, aromatics, coker oil , and heavy cracking
stocks to gasoline, jet fuels, and light fuel oil.
Feed to Catalytic Hydrocracking
Catalytic Cracking and Hydrocracking work as a
team.
Feed to Hydrocracker: Aromatic cycle oil from the
catalytic cracking processes. Vacuum and coker gas
oils are also used.
Feed to Catalytic Cracker are paraffinic atmospheric
and vacuum gas oils.
7.1 Hydrocracking Reactions
Catalytic hydrocracking is catalytic cracking (scission of
carbon-carbon single bond) followed by hydrogenation
(addition of hydrogen to a carbon-carbon double bond).
Hundreds of simultaneous chemical reactions take
place.
Cracking: Breaks naphthenes, paraffins and aromatics
to lower molecular weight paraffins and olefins.
(endothermic)
C7 H 16 ⎯Heat
⎯⎯ ⎯⎯→ C 3 H 8 + CH 2 = CH - CH 2 - CH 3
(cracking)
Heptane (paraffin) ⎯Heat
⎯⎯ ⎯⎯→ Propane + Butene (olifin)
(cracking)
Side chains crack off small ring aromatics (SRA) & cyclo-
paraffins (naphthenes).
Hydrocracking Reactions...
Hydrogenation: saturates the olefins to paraffins and
some olefins to iso-paraffins. (exothermic)
CH 2 = CH - CH 2 - CH 3 ⎯H⎯ ⎯ ⎯ ⎯⎯→ C 4 H10 + Heat
2 (Hydrogenation)
Butene (olifin) ⎯H⎯ ⎯ ⎯ ⎯⎯→ Butane (paraffin) + Heat
2 (Hydrogenation)
Hydrogenation reactions provide heat to the cracking
reaction. Extra heat is extracted using cold feed of H2
which controls the reaction temperature.
Hydrogenation reactions create light ends; heavier
distillates make more light components.
Hydrogenation...
The primary role of the hydrogen is to prevent
the formation of polycyclic aromatic compounds
and to reduce tar formation and prevent buildup
of coke on the catalyst.
Upon hydrogenation: Sulfur is converted to H2S,
nitrogen to ammonia NH3, oxygen to water, and
organic chloride to HCl.
Isomerisation
Isomerisation is another reaction type that
occurs in hydrocracking and accompanies the
cracking reactions. The olefinic products formed
are rapidly hydrogenated, thus maintaining a
high concentration of high octane isoparaffins
and preventing the reverse reaction back to
straight-chain molecules.
Table 7.1 Typical Hydrocracker Feedstocks
Feed Products
Kerosine Naphtha
Straight-run diesel Naphtha and/or jet fuel
Atmospheric gas oil Naphtha, jet fuel, and/or diesel
Vacuum gas oil Naphtha, jet fuel, diesel, lube oil
FCC LCO Naphtha
CC HCO Naphtha and/or distillates
Coker LCGO Naphtha and/or distillates
Coker HCGO Naphtha and/or distillates
