Thermal cracking Modern high-pressure thermal cracking operates at absolute pressures of about 7,000 kPa. 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. Thermal cracking is currently used to "upgrade" very heavy fractions or to produce light fractions or distillates, burner fuel and/or
petroleum coke. Two extremes of the thermal cracking in terms of the product range are represented by the high-temperature process called "steam cracking" or
pyrolysis (ca. 750 °C to 900 °C or higher) 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.
William Merriam Burton developed one of the earliest thermal cracking processes in 1912 which operated at and an absolute pressure of and was known as the
Burton process. Shortly thereafter, in 1921,
C.P. Dubbs, an employee of the
Universal Oil Products Company, developed a somewhat more advanced thermal cracking process which operated at and was known as the
Dubbs process. The Dubbs process was used extensively by many
refineries until the early 1940s when catalytic cracking came into use. 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
naphtha 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. 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. The process also results in the slow deposition of
coke, a form of
carbon, on the reactor walls. Since coke degrades the efficiency of the reactor, great care is taken to design reaction conditions to minimize its formation. Nonetheless, a steam cracking furnace can usually only run for a few months 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. This decoking is essentially combustion of the carbons, converting the hard solid carbon layer to carbon monoxide and carbon dioxide.
Fluid catalytic cracking The catalytic cracking process involves the presence of
solid acid catalysts, usually
silica-alumina and
zeolites. The catalysts promote the formation of
carbocations, which undergo processes of rearrangement and scission of C-C bonds. Relative to thermal cracking, cat cracking proceeds at milder temperatures, which saves energy. Furthermore, by operating at lower temperatures, the yield of undesirable alkenes is diminished. Alkenes cause instability of hydrocarbon fuels. Fluid catalytic cracking is a commonly used process, and a modern oil refinery will typically include a
cat cracker, particularly at refineries in the US, due to the high demand for
gasoline. The process was first used around 1942 and employs a powdered
catalyst. During WWII, the Allied Forces had plentiful supplies of the materials in contrast to the Axis Forces, which suffered severe shortages of gasoline and artificial rubber. Initial process implementations were based on low activity
alumina catalyst and a reactor where the catalyst particles were suspended in a rising flow of feed hydrocarbons in a
fluidized bed. 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 . 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 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 (
equilibrium 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
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–C4 olefins and
isobutane that are essential feeds for the
alkylation process and the production of polymers such as
polypropylene.
Typical yields of a UOP Fluid Catalytic Cracker (volume, feed basis, ~23 API feedstock and 74% conversion) Hydrocracking Hydrocracking is a catalytic cracking process assisted by the presence of added
hydrogen gas. Unlike a
hydrotreater, hydrocracking uses hydrogen to break C–C bonds (hydrotreatment is conducted prior to hydrocracking to protect the catalysts in a hydrocracking process). In 2010, 265 million 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 and LPG to heavier hydrocarbons consisting mostly of
isoparaffins. Hydrocracking is normally facilitated by a bifunctional catalyst that is capable of rearranging and breaking
hydrocarbon chains as well as adding hydrogen to
aromatics and
olefins to produce
naphthenes and
alkanes. All these products have a very low content of
sulfur and other
contaminants with a goal of reducing the gasoil and naphtha range material to 10 PPM sulfur or lower. Feedstock: Russian VGO 18.5 API, 2.28% Sulfur by wt, 0.28% Nitrogen by wt, Wax 6.5% by wt. Feedstock Distillation Curve Products from a UOP Hydrocracker Hydrocracking is (mostly) a licensed technology due to its complexity. Typically the licensor is also the catalyst provider. Also, unit internals can often be patented by the process licensors and are designed to support specific functions of the catalyst load. Currently, the major process licensors for hydrocracking are: • UOP • Axens • Chevron Lummus Global • Topsoe • Shell Criterion • Elessent (formerly DuPont) • ExxonMobil (iso-dewaxing for lubricant hydrocracking) ==Fundamentals==