Qtn. Discuss cracking as a refinery process for crude oil.
Introduction.
Cracking is a technique used in oil refineries whereby large and complex hydrocarbon molecules are
broken down into smaller and lighter components that are more useful for commercial or consumer
use.
Crack (break up)
Cracking makes it possible to turn crude oil into a variety of marketable fuels, lubricants and other parts.
There are four types of cracking processes;
a) 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 ethane (or ethylene) and propene (or propylene).
Steam cracker units are facilities in which a feed stock such as naphtha, liquefied petroleum
gas, ethane, propane, or butane is thermally cracked through the use of steam in a bank of
pyrolysis furnaces to produce lighter hydrocarbons.
In steam cracking, a gaseous or liquid hydrocarbon feed like naphtha, LPG, or ethane is diluted
with steam and briefly heated in a furnace without the presence of oxygen.
Typically, the reaction temperature is very high, at around 850OC, but the reaction is only
allowed to take place very briefly.
In modern cracking furnaces, the residence time is reduced to milliseconds to improve yield,
results in gas velocities up to the speed of sound. After the cracking temperature has been
reached, the gas is quickly quenched to stop the reaction in a transfer line heat exchanger or
inside a quenching heater using quench oil.
The products 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 hydrocarbons feeds such as ethane, LPGs or light naphtha give product streams rich in
the lighter alkenes, including ethylene, propylene and butadiene.
Heavier hydrocarbon (full range and heavy naphtha’s 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. Typical product streams include pyrolysis
gasoline(pygas) and BTX.
A higher cracking temperature (also referred to as severity favors the production of ethylene
and benzene, whereas lower severity produces higher amounts of propylene, C4-hydrocarbons
and liquid products.
b) Thermal cracking
Modern high-pressure thermal cracking operates at absolute pressure of about 7000kpa.
An overall process of disproportional 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 hemolytic fission and produces alkenes, which are the basis for the
economically important production of polymers.
Thermal cracking is currently used to ‘upgrade” very heavy 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 0C TO 900OC or higher) which
produces valuable ethylene and other feed stocks for the petrochemical industry, and the milder-
temperature delayed coking (Ca. 500oC) 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.
c) hydrocracking.
Hydrocracking is a catalytic cracking process assisted by the presence of added hydrogen gas.
Unlike a hydro treater, hydrocracking use hydrogen to break c-c bonds (hydro treatment is conducted
prior to hydrocracking to protect the catalysts in a hydrocracking process.
In the year 2010, 256*106 tons of petroleum was processed with this technology. The main feedstock is
vacuum gas oil, a heavy fraction of petroleum.
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 consisting mostly of iso paraffin’s. Hydrocracking is normally facilitated by a bi functional
catalyst that is capable of rearranging and breaking hydrocarbon chains as well as adding hydrogen to
aromatics and olefins to produce naphthenes and alkenes.
The major products from hydrocracking are jet fuel and diesel, but low Sulphur naphtha fractions and
LPG are also produced. All these products have a very low content of Sulphur and other contaminants.
It is very common in Europe and Asia because these regions have high demand for diesel and kerosene.
In the US, fluid catalytic cracking is more common because the demand for gasoline is higher.
The hydrocracking process depends on the nature of the feed stock and the relative rates of the two
competing reactions; hydrogenation and cracking.
Heavy aromatic feed stock is converted into lighter products under a wider range of very high pressure
(1000-2000psi) and fairly high temperature (750 o-1500oF, 400-800oC) in the presence of hydrogen and
special catalysts.
The primary functions of hydrogen are thus
Preventing the formation of polycyclic aromatic compounds. If feed stock has a high paraffinic
content’
Reducing tar formation
Reducing impurities
Preventing buildup of coke on the catalyst.
Converting sulfur and nitrogen compounds present in the feedstock to hydrogen sulfide and
ammonia, and
Achieving high octane number fuel.
d) Fluid catalytic cracking.
The catalytic cracking process involved the presence of solid acid catalysts, usually silica-alumina
and zeolites.
The catalysts promote the formation of carbonations which undergo processes of
rearrangement and scission of c-c bonds. Relative to thermal cracking, catalytic cracking
proceeds at milder temperatures, which saves energy. Furthermore, by operating at lower
temperatures, the yields of alkenes is diminished. Alkenes cause instability of hydrocarbon fuels.
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 123 0 to 1400F (666 to 760oC)
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-hydrocarbons mixture flows upward through the riser for 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 oil.
During the trip up the riser the cracking catalyst and greatly reduce activity and selectively. The
spent catalyst is disengaged from the cracked hydrocarbon vapors and sent to a stripper where
it contacts 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 olefin 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-C4 olefins and iso-butane that as
essential feeds for the alkylation process and the production of polymers such as polypropylene.
