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In the hydrocarbon processing indus-try (HPI), cracking is the processwhereby complex organic molecules(e.g.heavy hydrocarbons) are brokendown into simpler molecules by thebreaking of carbon-carbon bonds inthe base material. The rate of crackingand the nature of end productsstrongly depend on the temperatureand presence of any catalysts. Processautomation equipment including gasanalysis instrumentation contributesessentially to control and optimizecracking processes.
Siemens Sensors and Communica-tion, a leader in process analytics, hasproven worldwide its competence toplan, engineer, manufacture, imple-ment and service analyzer systems foruse in cracking plants.
This case study provides detailedinformation about that.
Crude oil processingCrude oil is a complex mixtureof hydrocarbon compounds andrelatively small quantities of othermaterials such as oxygen, nitrogen,sulphur, salt and water as well asinorganics and metals. It occurs nat-urally in the ground and was formed
millions of years ago.Crude oil varies from source tosource in composition and colour.It is typically mixed with gases,which have to be separated fromthe crude oil befor it can be furtherprocessed. Crude oil itsel is of littleuse. To get the maximum valuefrom crude, it needs to be refinedand blended in a refinery into morevaluable and useful petroleum prod-ucts: fuels, lubricants, waxes,asphalt, plastics, fibres, detergents,fertilizers and many others.
An oil refinery is a well structuredarrangement of manufacturing pro-cesses designed to perform physicaland chemical changes in crude oil toconvert it stepwise into intermedi-ate and end products. Refinery pro-cesses have been and are continu-ously developed and optimized inresponse to changing marketdemands for certain products.Almost all refineries, regardless oftheir special product scope, use thesame few basic processes like sepa-ration (distillation), conversion andtreating. Cracking belongs to themost common conversion methods.
Process analyzers, mainly processgas chromatographs but also con-tinuous gas analyzers, are standardin a refineries process instrumenta-tion. They deliver key data to pro-cess and product quality control andasset management.Siemens Sensors and Communica-tion provides efficient analyzers,expertise and solutions to this tasks.
Use of Process Analytics forCracking Processes in Refineries
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Crude oil refining at a glance
Refining processThe key objective of the refining processis to effect chemical reactions on theraw hydrocarbons.The refining process from crude oil toend-use products comprises severalsub-processes (fig. 1).
Because crude oil is a mixture of hydro-carbons with different boiling tempera-tures, it can be separated by distillation(fractionation) into groups of hydrocar-
bons (called cuts or fractions) that boilbetween two specified boiling points(fig. 2).Two types of distillation are performed:atmospheric and vacuum. Atmosphericdistillation takes place in a distilling col-umn at or near atmospheric pressureand at temperatures of 90 to 650 C.To recover heavy distillates (residues)drawn off from the bottom of this col-umn, the residue is fed to a second dis-tillation column where the process isrepeated under vacuum, called vacuumdistillation. This allows heavy hydrocar-bons with boiling points up to 800 C to
Conversion processes change the sizeand/or structure of hydrocarbon mole-cules by cracking and rearranging mole-cules to add value. The fractions fromthe distillation towers are transformed
into streams (intermediate compo-nents). These processes include:
Decomposition (dividing) bycatalytic and thermal cracking,
Unification (combining) throughalkylation and polymerization and
Alteration (rearranging) with isomer-ization and catalytic reforming.
The most widely used conversionmethod is called cracking because ituses heat and pressure to "crack" heavyhydrocarbon molecules into lighterones to upgrade products and to yield
more desirable hydrocarbon com-pounds.
A cracking unit consists of one or morereactors and a network of furnaces,heat exchangers and other vessels.
Treatment processes (fig. 3) areintended to prepare hydrocarbonstreams for additional processing and toprepare finished products.Treatment includes the removal orseparation of aromatics and naph-thenes as well as impurities and mayinvolve chemical or physical separatione. g. dissolving, absorption or precipita-tion such as desalting, drying, desulfur-izing, sweetening, solvent extraction,etc. Hydrotreating is used to removesulfur and reduce nitrogen and some of
the metals contained in the stream bymeans of hydrogen gas and a catalyst.
Fig. 2 and 3: Crude oil refining, separation by boiling points (left), typical processing steps (right)
Fig. 1: From crude oil to end-use products
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Cracking is, as already explained, a pro-cess used to convert heavy hydrocarbonfractions obtained by vacuum distilla-tion into a mixture of lighter and moreuseful products (fig. 4).
Catalytic CrackingIn catalytic cracking, the feedstockundergoes a chemical breakdown,under controlled heat (450 to 50 C)and pressure, and in the presence of acatalyst, a substance which promotesthe reaction without itself being chemi-
cally changed. The cracking reactionyields petrol, LPG, unsaturated olefincompounds, cracked gas oils, cycle oil,light gases and a solid coke residue.Cycle oil is recycled to cause furtherbreakdown and the coke, which forms alayer on the catalyst, is removed byburning.
Fluid Catalytic Cracking (FCC)
Fluid catalytic cracking (fig. 6) uses acatalyst in the special form of a very finepowder which flows like a liquid whenagitated by steam, air or vapour. Feed-stock entering the process immediately
meets a stream of very hot catalyst inthe riser and vaporises. The resultingvapours keep the catalyst fluidised as itpasses into the reactor, where thecracking takes place and where it is flui-dised by the hydrocarbon vapour. Thecatalyst next passes to a steam strippingsection where most of the volatilehydrocarbons are removed. The spentcatalyst then passes to a regeneratorvessel where it is fluidised by a mixtureof air and the products of combustionwhich are produced as the coke on thecatalyst is burnt off. The regeneratedcatalyst then flows back to the reactor.
The catalyst thus undergoes a continu-
ous circulation between the reactor,stripper and regenerator sections.
The catalyst is usually a mixture of alu-minium oxide and silica. The introduc-tion of synthetic zeolite catalysts hasallowed much shorter reaction timesand improved yields.
Hydrocracking (fig. 8) is catalytic crack-ing in the presence of hydrogen. Theextra hydrogen saturates, or hydroge-nates, the chemical bonds of the
cracked hydrocarbons and creates iso-mers with the desired characteristics.Hydrocracking uses slightly lower tem-peratures and much greater pressure toobtain chemical reactions. Hydrocrack-ing produces no residues, only light oils.
Wax feed is mixed with hydrogen,heated, and sent to a reactor vessel witha fixed bed catalyst, where cracking andhydrogenation take place. Products arethen sent to a fractionator to be sepa-rated. The hydrogen is recycled.
Thermal CrackingThermal cracking uses heat to breakdown the residue from vacuum distilla-tion trough upgrading and visbreakingor to produce light fractions or distil-lates, burner fuel and/or petroleumcoke.
Steam cracking is a high-temperatureprocess at 750 to 900 C or more, whichproduces valuable ethylene and otherfeedstocks for the petrochemicalindustry. In steam cracking, a gaseousor liquid hydrocarbon feed like Naphta,LPG or Ethane is diluted with steam and
then very briefly heated in a furnace toaround 850 C. In modern cracking fur-naces, the residence time is reduced to
milliseconds in order to improve the
yield of desired products.After the cracking temperature hasbeen reached, the gas is quicklyquenched to stop the reaction in atransfer line exchanger. The productsproduced in the reaction depend on thecomposition of the feed, the hydrocar-bon to steam ratio and on the crackingtemperature & furnace residence time.The process also results in the slow dep-osition of coke, a form of carbon, on thereactor walls. This degrades the effec-tiveness of the reactor, so reaction con-ditions are designed to minimize this.
Nonetheless, a steam cracking furnacecan usually only run for a few months ata time between de-cokings.
Some refineries use the most commondelayed coking process, which oper-ates at lower temperatures (ca. 500 C)and moderate pressure to turn residuesinto lighter products and a hard, coallikesubstance (petroleum coke) that is usedas an industrial fuel and in the produc-tion of electrodes for the steel and alu-minum industries.
is a non-catalytic thermal process thatconverts atmospheric or vacuum resi-dues via thermal cracking to gas,naphtha, distillates, and visbroken resi-due. Atmospheric and vacuum residuesare typically charged to a visbreaker toreduce fuel oil viscosity and increasedistillate yield in the refinery. The pro-cess will typically achieve a conversionto gas, gasoline, and distillates of 10 %to 50 %, depending on the severity andfeedstock characteristics.
Fig. 4: Cracking principle: one large is cracked into four smaller molecules
Fig. 5: Cracking plant
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Process Analytics in Cracking Pla