Chapter 7:
Catalytic Hydrocracking
• The interest in the use of hydrocracking has been caused by
several factors, including
(1) The demand for petroleum products has shifted to high
ratios of gasoline and jet fuel compared with the usages of
diesel fuel and home heating oils,
(2) By-product hydrogen at low cost and in large amounts has
become available from catalytic reforming operations, and
(3) Environmental concerns limiting sulfur and aromatic
compound concentrations in motor fuels have increased.
Some of the advantages of hydrocracking are:
1. Better balance of gasoline and distillate production
2.
Greater gasoline yield
3.
Improved gasoline pool octane quality and sensitivity
4.
Production of relatively high amounts of isobutane in the butane
fraction
5.
Supplementing of fluid catalytic cracking to upgrade heavy
cracking stocks, aromatics, cycle oils, and coker oils to gasoline, jet
fuels, and light fuel oils
In a modern refinery catalytic cracking and hydrocracking work as a
team.
The catalytic cracker takes the more easily cracked paraffinic
atmospheric and vacuum gas oils as charge stocks, while the
hydrocracker uses more aromatic cycle oils and coker distillates as
feed
• These streams are very refractory and resist catalytic cracking, while
the higher pressures and hydrogen atmosphere make them
relatively easy to hydrocrack.
• The new zeolite cracking catalysts help improve the gasoline yields
and octanes from catalytic crackers as well as reduce the cycle stock
and gas make.
• However, the cycle oil still represents a difficult fraction to crack
catalytically to extinction.
• One alternative is to use the cycle stock as a component for fuel oil
blending, but this is limited as it is a relatively poor burning stock
and burns with a smoky flame.
• For this reason a limit is placed on the percentage that can be
blended into distillate fuel oils.
• The cycle oils that result from cracking operations with zeolite
catalysts tend to be highly aromatic and therefore make satisfactory
feedstocks for hydrocracking.
• Vacuum and coker gas oils are also used as hydrocracker feed.
• Sometimes diesel boiling range material is included in
hydrocracker feed to make jet and motor gasoline products.
Both straight-run and FCC LCO can be used and in some cases
100% LCO is used.
• In cases where 100% LCO is the feed, there is a follow-up with
a high-pressure hydrotreater to reduce the aromatic content
and increase the smoke point to meet specifications.
• When the feed contains large amounts of LCO, the major
effects are increased heat release and lower smoke point of
the jet fuel produc t
• In addition to middle distillates and cycle oils used as feed for
hydrocrack- ing units, it is also possible to process residual
fuel oils and reduced crude by hydrocracking.
HYDROCRACKING REACTIONS
• Hydrocracking = catalytic cracking + hydrogenation
• Catalytic cracking is the scission of a carbon–carbon single
bond, and hydrogenation is the addition of hydrogen to a
carbon–carbon double bond.
• Cracking and hydrogenation are complementary, for cracking
provides olefins for hydrogenation, while hydrogenation in
turn provides heat for cracking.
• The cracking reaction is endothermic and the hydrogenation
reaction is exothermic.
• The overall reaction provides an excess of heat because the
amount of heat released by the exothermic hydrogenation
reactions is much greater than the amount of heat consumed
by the endothermic cracking reactions
• This surplus of heat causes the reactor temperature to
increase and accelerate the reaction rate. This is controlled by
injecting cold hydrogen as quench into the reactors to absorb
the excess heat of reaction
• Another reaction that occurs and illustrates the
complementary operation of the hydrogenation and cracking
reactions is the initial hydrogenation of a condensed aromatic
compound to a cycloparaffin
• This allows subsequent cracking to proceed to a greater
extent and thus converts a low-value component of catalytic
cycle oils to a useful product
• Isomerization is another reaction type that occurs in
hydrocracking and accompanies the cracking reaction.
• 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.
• An interesting point in connection with the hydrocracking of
these compounds is the relatively small amounts of propane
and lighter materials that are produced as compared with
normal cracking processes.
• Hydrocracking reactions are normally carried out at average
catalyst temperatures between 550 and 750°F (290 to 400°C)
and at reactor pressures between 1200 and 2000 psig (8275
and 13,800 kPa).
• The circulation of large quantities of hydrogen with the
feedstock prevents excessive catalyst fouling and permits long
runs without catalyst regeneration.
• Careful preparation of the feed is also necessary in order to
remove catalyst poisons and to give long catalyst life.
• Frequently the feedstock is hydrotreated to remove sulfur and
nitrogen compounds as well as metals before it is sent to the
first hydrocracking stage or, sometimes, the first reactor in the
reactor train can be used for this purpose.
FEED PREPARATION
• Hydrocracking catalyst is susceptible to poisoning by metallic
salts, oxygen, organic nitrogen compounds, and sulfur in the
feedstocks.
• The feedstock is hydrotreated to saturate the olefins and
remove sulfur, nitrogen, and oxygen compounds.
• Molecules containing metals are cracked and the metals are
retained on the catalyst.
• The nitrogen and sulfur compounds are removed by
conversion to ammonia and hydrogen sulfide.
• Although organic nitrogen compounds are thought to act as
permanent poisons to the catalyst, the ammonia produced by
reaction of the organic nitrogen compounds with hydrogen
does not affect the catalyst permanently
• For some types of hydrocracking catalysts, the presence of
hydrogen sulfide in low concentrations acts as a catalyst to
inhibit the saturation of aromatic rings.
• This is a beneficial effect when maximizing gasoline
production as it conserves hydrogen and produces a higher
octane product.
• In the hydrotreater a number of hydrogenation reactions,
such as olefin saturation and aromatic ring saturation, take
place, but cracking is almost insignificant at the operating
conditions used.
• In addition to the removal of nitrogen and sulfur compounds
and metals, it is also necessary to reduce the water content of
the feed streams to less than 25 ppm because, at the
temperatures required for hydrocracking, steam causes the
crystalline structure of the catalyst to collapse and the
dispersed rare-earth atoms to agglomerate.
• Water removal is accomplished by passing the feed stream
through a silica gel or molecular sieve dryer
• On the average, the hydrogen treating process requires
approximately 150 to 300 ft3 of hydrogen per barrel of feed
(27 to 54 m3 hydrogen per m3 feed).
THE HYDROCRACKING PROCESS
• The hydrocracking process may require either one or two
stages, depending upon the process and the feed stocks used.
• The GOFining process is a fixed-bed regenerative process
employing a molecular-sieve catalyst impregnated with a rareearth metal.
• The process employs either single-stage or two-stage
hydrocracking with typical operating conditions ranging from
660 to 785°F and from 1000 to 2000 psig (350–420°C and
6900–13,800 kPa).
• The temperature and pressure vary with the age of the
catalyst, the product desired, and the properties of the
feedstock.
• The decision to use a single- or two-stage system depends
upon the size of the unit and the product desired.
• For most feedstocks the use of a single stage will permit the
total conversion of the feed material to gasoline and lighter
products by recycling the heavier material back to the reactor.
• The process flow for a two-stage reactor is shown in Figure
7.2. If only one stage is used, the process flow is the same as
that of the first stage of the two-stage plant except the
fractionation tower bottoms is recycled to the reactor feed.
• The fresh feed is mixed with makeup hydrogen and recycle
gas (high in hydrogen content) and passed through a heater to
the first reactor. If the feed has not been hydrotreated, there
is a guard reactor before the first hydrocracking reactor.
• The guard reactor usually has a modified hydrotreating
catalyst such as cobalt-molybdenum on silica-alumina to
convert organic sulfur and nitrogen compounds to hydrogen
sulfide, ammonia, and hydrocarbons to protect the precious
metals catalyst in the following reactors.
• The hydrocracking reactor(s) is operated at a sufficiently high
temperature to convert 40 to 50 vol% of the reactor effluent
to material boiling below 400°F (205°C).
• The reactor effluent goes through heat exchangers to a highpressure separator where the hydrogen-rich gases are
separated and recycled to the first stage for mixing both
makeup hydrogen and fresh feed.
• The liquid product from the separator is sent to a distillation
column where the C4 and lighter gases are taken off overhead,
and the light and heavy naphtha, jet fuel, and diesel fuel
boiling range streams are removed as liquid sidestreams.
• The fractionator bottoms are used as feed to the second-stage
reactor system.
• The unit can be operated to produce all gasoline and lighter
products or to maximize jet fuel or diesel fuel products.
• The bottoms stream from the fractionator is mixed with
recycle hydrogen from the second stage and sent through a
furnace to the second-stage reactor.
• Here the temperature is maintained to bring the total
conversion of the unconverted oil from the first-stage and
second-stage recycle to 50 to 70 vol% per pass.
• The second-stage product is combined with the first-stage
product prior to fractionation.
• Both the first- and second-stage reactors contain several beds
of catalysts.
• The major reason for having separate beds is to provide
locations for injecting old recycled hydrogen into the reactors
for temperature control.
• In addition, redistribution of the feed and hydrogen between
the beds helps to maintain a more uniform utilization of the
catalyst.
HYDROCRACKING CATALYST
• There are a number of hydrocracking catalysts available and
the actual composition is tailored to the process, feed
material, and the products desired.
• Most of the hydrocracking catalysts consist of a crystalline
mixture of silica-alumina with a small uniformly distributed
amount of rare earths contained within the crystalline lattice.
• The silica-alumina portion of the catalyst provides cracking
activity while the rare-earth metals promote hydrogenation.
• Catalyst activity decreases with use, and reactor temperatures
are raised during a run to increase reaction rate and maintain
conversion.
• The catalyst selectivity also changes with age and more gas is
made and less naphtha produced as the catalyst temperature
is raised to maintain conversion.
• With typical feedstocks it will take from two to four years for
catalyst activity to decrease from the accumulation of coke
and other deposits to a level which will require regeneration.
• Regeneration is accomplished by burning off the catalyst
deposits, and catalyst activity is restored to close to its original
level.
• The catalyst can undergo several regenerations before it is
necessary to replace it.
• Almost all hydrocracking catalysts use silica-alumina as the
cracking base but the rare-earth metals vary according to the
manufacturer.
• Those in most common use are platinum, palladium,
tungsten, and nickel.
PROCESS VARIABLES
• The severity of the hydrocracking reaction is measured by the
degree of conversion of the feed to lighter products.
• Conversion is defined as the volume percent of the feed
which disappears to form products boiling below the desired
product end point
• The primary reaction variables are reactor temperature and
pressure, space velocity, hydrogen consumption, nitrogen
content of feed, and hydrogen sulfide content of the gases
• The effects of these are as follows:
 Reactor Temperature
• Reactor temperature is the primary means of conversion
control.
• At normal reactor conditions a 20°F (10°C) increase in
temperature almost doubles the reaction rate, but does not
affect the conversion level as much because a portion of the
reaction involves material that has already been converted to
materials boiling below the desired product end point.
• As the run progresses it is necessary to raise the average
temperature about 0.1 to 0.2°F per day to compensate for the
loss in catalyst activity.
 Reactor Pressure
• The primary effect of reactor pressure is in its effects on the
partial pressures of hydrogen and ammonia.
• An increase in total pressure increases the partial pressures of
both hydrogen and ammonia.
• Conversion increases with increasing hydrogen partial
pressure and decreases with increasing ammonia partial
pressure.
• The hydrogen effect is greater, however, and the net effect of
raising total pressure is to increase conversion.
 Space Velocity
• The volumetric space velocity is the ratio of liquid flow rate, in
barrels per hour, to catalyst volume, in barrels.
• The catalyst volume is constant, therefore the space velocity
varies directly with feed rate.
• As the feed rate increases, the time of catalyst contact for
each barrel of feed is decreased and conversion is lowered.
• In order to maintain conversion at the proper level when the
feed rate is increased, it is necessary to increase the
temperature.
 Nitrogen Content
• The organic nitrogen content of the feed is of great
importance as the hydrocracking catalyst is deactivated by
contact with organic nitrogen compounds.
• An increase in organic nitrogen content of the feed causes a
decrease in conversion.
 Hydrogen Sulfide
• At low concentrations the presence of hydrogen sulfide acts
as a catalyst to inhibit the saturation of aromatic rings.
• This conserves hydrogen and produces a product with a
higher octane number because the aromatic naphtha has a
higher octane than does its naphthenic counterpart.
• However, hydrocracking in the presence of a small amount of
hydrogen sulfide normally produces a very low-smoke-point
jet fuel.
• At high hydrogen sulfide levels corrosion of the equipment
becomes important and the cracking activity of the catalyst is
also affected adversely.
 Heavy Polynuclear Aromatics (HPNA)
• Heavy polynuclear aromatics are formed in small amounts
from hydrocracking reactions and, when the fractionator
bottoms is recycled, can build up to concentrations that cause
fouling of heat exchanger surfaces and equipment.
• Steps such as reducing feed end point or removal of a drag
stream may be necessary to control this problem