Everything about Hydrocracking totally explained
In
petroleum geology and
chemistry,
cracking is the process whereby complex
organic molecules such as
kerogens or heavy
hydrocarbons are broken down into simpler molecules (for example light hydrocarbons) by the breaking of
carbon-carbon
bonds in the precursors. The
rate of cracking and the end products are strongly dependent on the
temperature and presence of any
catalysts. Cracking, also referred to as
pyrolysis, is the breakdown of a large
alkane into smaller, more useful
alkenes and an
alkane. Simply put, cracking hydrocarbons is when you separate long chain hydrocarbons into short ones.
History
In 1855, petroleum cracking methods were pioneered by
American chemistry professor,
Benjamin Silliman, Jr., of
Sheffield Scientific School (SSS) at
Yale University.
The first thermal cracking method, the
Shukhov cracking process, was invented by
Russian engineer
Vladimir Shukhov, in the Russian empire, Patent No. 12926,
November 27,
1891.
Eugene Houdry, a French mechanical engineer, pioneered catalytic cracking and developed the first commercially successful process after emigrating to the United States. The first commercial plant was built in 1936. His process doubled the amount of gasoline that could be produced from a barrel of crude oil.
Applications
Oil refinery cracking processes allow the production of "light" products such as
LPG and
gasoline from heavier
crude oil distillation fractions such as
gas oils and residues. Fluid Catalytic Cracking (FCC) produces a high yield of
gasoline and
LPG while hydrocracking is a major source of
jet fuel, diesel, naphtha and LPG.
Thermal cracking is currently used to "upgrade" very heavy fractions ("
upgrading", "
visbreaking"), or to produce light fractions or distillates, burner fuel and/or
petroleum coke. Two extremes of the thermal cracking in terms of product range are represented by the high-temperature process called "steam cracking" or
pyrolysis (ca. 750 to 900 °C or more) which produces valuable
ethylene and other feedstocks for the petrochemical industry, and the milder-temperature
delayed coking (ca. 500 °C) which can produce, under the right conditions, valuable
needle coke, a highly crystalline petroleum coke used in the production of
electrodes for the
steel and
aluminium industries.
Fluid catalytic cracking
Fluid catalytic cracking is a commonly used process and a modern oil refinery will typically include a
cat cracker, particularly at refineries in the
USA due to the high demand for
gasoline. The process was first used in around
1942 and employs a powdered
catalyst. During the Second World War, it provided Allied Forces with plentiful supplies of gasoline and artificial rubber that contrasted with the penury suffered by the Axis Forces. Initial process implementations were based on a low activity
alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a
fluidized bed.
Alumina-catalyzed cracking systems are still in use in
high school and
university laboratories in experiments concerning alkanes and alkenes. The catalyst is usually obtained by crushing
pumice stones, which contain mainly
aluminium oxide and
silicon(IV) oxide into small, porous pieces. In the laboratory, aluminium oxide (or porous pot) must be heated.
In newer designs, cracking takes place using a very active
zeolite-based catalyst in a short-contact time vertical or upward sloped pipe called the "riser". Pre-heated feed is sprayed into the base of the riser via feed nozzles where it contacts extremely hot fluidized catalyst at 1230 to 1400 °
F (665 to 760 °
C). The hot catalyst vaporizes the feed and catalyzes the cracking reactions that break down the high molecular weight oil into lighter components including LPG, gasoline, and diesel. The catalyst-hydrocarbon mixture flows upward through the riser for just a few seconds and then the mixture is separated via
cyclones. The catalyst-free hydrocarbons are routed to a main
fractionator for separation into fuel gas, LPG, gasoline,
naphtha, light cycle oils used in diesel and jet fuel, and heavy fuel oil.
During the trip up the riser, the cracking catalyst is "spent" by reactions which deposit coke on the catalyst and greatly reduce activity and selectivity. The "spent" catalyst is disengaged from the cracked hydrocarbon vapors and sent to a stripper where it's contacted with steam to remove hydrocarbons remaining in the catalyst pores. The "spent" catalyst then flows into a fluidized-bed regenerator where air (or in some cases air plus
oxygen) is used to burn off the coke to restore catalyst activity and also provide the necessary heat for the next reaction cycle, cracking being an
endothermic reaction. The "regenerated" catalyst then flows to the base of the riser, repeating the cycle.
The gasoline produced in the FCC unit has an elevated octane rating but is less chemically stable compared to other gasoline components due to its
olefinic profile. Olefins in gasoline are responsible for the formation of
polymeric deposits in storage
tanks, fuel ducts and
injectors. The FCC LPG is an important source of
C3-C
4 olefins and
isobutane that are essential feeds for the
alkylation process and the production of polymers such as
polypropylene.
Hydrocracking
Hydrocracking is a catalytic cracking process assisted by the presence of an elevated
partial pressure of
hydrogen gas. Similar to the
hydrotreater, the function of hydrogen is the purification of the carbon stream from sulfur and nitrogen hetero-atoms.
The products of this process are
saturated hydrocarbons; depending on the reaction conditions (temperature, pressure, catalyst activity) these products range from
ethane, LPG to heavier hydrocarbons comprising mostly of
isoparaffins. Hydrocracking is normally facilitated by a bi functional catalyst that's capable of rearranging and breaking
hydrocarbon chains as well as adding hydrogen to
aromatics and
olefins to produce
naphthenes and
alkanes. Increased product yields can be obtained using titanium oxide in the fermentation process.
Major products from hydrocracking are
jet fuel,
diesel, relatively high octane rating gasoline fractions and LPG. All these products have a very low content of
sulfur and
contaminants.
It is very common in
India because of that country's high demand for diesel and
kerosene.
Steam cracking
Steam cracking is a
petrochemical process in which saturated
hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter
alkenes (or commonly
olefins), including
ethene (or
ethylene) and
propene (or
propylene).
In steam cracking, a gaseous or liquid hydrocarbon feed like
Naphtha,
LPG or
Ethane is diluted with steam and then briefly heated in a furnace (obviously without the presence of oxygen). Typically, the reaction temperature is very hot —around 850°C—but the reaction is only allowed to take place very briefly. In modern cracking furnaces, the residence time is even reduced to milliseconds (resulting in gas velocities reaching speeds beyond the
speed of sound) in order to improve the yield of desired products. After the cracking temperature has been reached, the gas is quickly quenched to stop the reaction in a transfer line
heat exchanger.
The products produced in the reaction depend on the composition of the feed, the hydrocarbon to steam ratio and on the cracking temperature and furnace residence time.
Light hydrocarbon feeds (such as
ethane, LPGs or light
naphthas) give product streams rich in the lighter alkenes, including ethylene, propylene, and
butadiene. Heavier hydrocarbon (full range and heavy naphthas as well as other refinery products) feeds give some of these, but also give products rich in
aromatic hydrocarbons and hydrocarbons suitable for inclusion in
gasoline or
fuel oil.
The higher cracking temperature (also referred to as severity) favours the production of ethene and benzene, whereas lower severity produces relatively higher amounts of propene, C4-hydrocarbons and liquid products.
The process also results in the slow deposition of
coke, a form of
carbon, on the reactor walls. This degrades the efficiency of the reactor, so reaction conditions are designed to minimize this. Nonetheless, a steam cracking furnace can usually only run for a few months at a time between de-cokings. Decokes require the furnace to be isolated from the process and then a flow of steam or a steam/air mixture is passed through the furnace coils at C . This converts the hard solid carbon layer to carbon monoxide and carbon dioxide. Once this reaction is complete, the furnace can be returned to service.
Chemistry
"Cracking" breaks larger molecules into smaller ones. This can be done with a thermic or catalytic method.
The thermal cracking process follows a homolytic mechanism, that is, bonds break symmetrically and thus pairs of
free radicals are formed.
The catalytic cracking process involves the presence of
acid catalysts (usually solid acids such as
silica-alumina and
zeolites) which promote a heterolytic (asymmetric) breakage of bonds yielding pairs of
ions of opposite charges, usually a
carbocation and the very unstable
hydride anion. Carbon-localized free radicals and cations are both highly unstable and undergo processes of chain rearrangement, C-C scission in position
beta (for example, cracking) and
intra- and
intermolecular hydrogen transfer or
hydride transfer. In both types of processes, the corresponding reactive intermediates (radicals, ions) are permanently regenerated, and thus they proceed by a self-propagating chain mechanism. The chain of reactions is eventually terminated by radical or ion recombination.
Catalytic cracking
Catalytic cracking uses a zeolite
catalyst and moderately-high temperatures (400-500 °C) to aid the process of breaking down large hydrocarbon molecules into smaller ones. During this process, less reactive, and therefore more stable and longer lived intermediate cations accumulate on the catalysts'
active sites generating deposits of
carbonaceous products generally known as
coke. Such deposits need to be removed (usually by controlled burning) in order to restore catalyst activity.
Thermal cracking
In thermal cracking elevated temperatures (~800
oC) and pressures (~7000kPa) are used, a process first developed by
William Merriam Burton. An overall process of disproportionation can be observed, where "light", hydrogen-rich products are formed at the expense of heavier molecules which condense and are depleted of hydrogen. The actual reaction is known as
homolytic fission and produces
alkenes, which are the basis for the economically important production of
polymers.
A large number of
chemical reactions take place during steam cracking, most of them based on
free radicals.
Computer simulations aimed at modeling what takes place during steam cracking have included hundreds or even thousands of reactions in their models. The main reactions that take place include:
Initiation reactions, where a single molecule breaks apart into two free radicals. Only a small fraction of the feed molecules actually undergo initiation, but these reactions are necessary to produce the free radicals that drive the rest of the reactions. In steam cracking, initiation usually involves breaking a
chemical bond between two carbon atoms, rather than the bond between a carbon and a
hydrogen atom.
» CH
3CH
3 → 2 CH
3•
Hydrogen abstraction, where a free radical removes a hydrogen atom from another molecule, turning the second molecule into a free radical.
» CH
3• + CH
3CH
3 → CH
4 + CH
3CH
2•
Radical decomposition, where a free radical breaks apart into two molecules, one an alkene, the other a free radical. This is the process that results in the alkene products of steam cracking.
» CH
3CH
2• → CH
2=CH
2 + H•
Radical addition, the reverse of radical decomposition, in which a radical reacts with an alkene to form a single, larger free radical. These processes are involved in forming the aromatic products that result when heavier feedstocks are used.
» CH
3CH
2• + CH
2=CH
2 → CH
3CH
2CH
2CH
2•
Termination reactions, which happen when two free radicals react with each other to produce products that are not free radicals. Two common forms of termination are
recombination, where the two radicals combine to form one larger molecule, and
disproportionation, where one radical transfers a hydrogen atom to the other, giving an alkene and an alkane.
» CH
3• + CH
3CH
2• → CH
3CH
2CH
3
CH
3CH
2• + CH
3CH
2• → CH
2=CH
2 + CH
3CH
3
Thermal cracking is an example of a reaction whose energetics are dominated by entropy (∆S°) rather than by enthalpy (∆H°) in the Gibbs Free Energy equation ∆G°=∆H°-T∆S°. Although the bond dissociation energy D for a carbon-carbon single bond is relatively high (about 375 kJ/mol) and cracking is highly endothermic, the large positive entropy change resulting from the fragmentation of one large molecule into several smaller pieces, together with the extremely high temperature, makes T∆S° term larger than the ∆H° term, thereby favoring the cracking reaction.
Here is an example of cracking with butane CH
3-CH
2-CH
2-CH
3
- 1st possibility (48%): breaking is done on the CH3-CH2 bond.
CH
3* / *CH
2-CH
2-CH
3
after a certain number of steps, we'll obtain an alkane and an
alkene:
CH
4 + CH
2=CH-CH
3
2nd possibility (38%): breaking is done on the CH2-CH2 bond.
CH3-CH2* / *CH2-CH3
after a certain number of steps, we'll obtain an alkane and an alkene
from different types: CH3-CH3 + CH2=CH2
3rd possibility (14%): breaking of a C-H bond
after a certain number of steps, we'll obtain an alkene and hydrogen gas: CH2=CH-CH2-CH3 + H2 this is very useful since the catalyst can be recycled.
Further Information
Get more info on 'Hydrocracking'.
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