'Ethylene,' In: Ullmann's Encyclopedia Of Industrial Chemistry

468EthyleneVol. 13Table 1. Raw materials for ethylene production (as a percentage of total ethylene produced)Raw materialUSA 1979USA 1995Western Europe 1981Western Europe 1995Japan 1981Japan 1993Refinery gasLPG, NGLNaphthaGas oil16514204761194*801631771910*9003*970*Including refinery gas4. Raw MaterialsTable 1 lists the percentage of ethylene producedworldwide from various feedstocks for 1981 and1992 [12]. In Western Europe and Japan, over80 % of ethylene is produced from naphthas —the principal ethylene raw materials.A shift in feedstocks occurred for the periodfrom 1980 to 1991. In the United States andEurope larger amounts of light feedstocks(liquefied petroleum gas LPG: propane þ butanes)and NGL (natural gas liquids: ethane, propane,butane) are used for ethylene production, whereas in Japan more naphtha was used in 1995compared to 1981. The use of gas oils for ethylene production decreased slightly during the1980s. Since 2000 a huge amount of new capacities have been installed in the Middle East basedon ethane as feedstock, increasing the shareof ethane-based production significantly (seeFig. 2 and Table 2). This trend will continue inthe coming years due to favorable productioncosts for ethane-based crackers. As a result theproduction of propene from new crackers willdecrease requiring different ways of productionof this cracker coproduct.Ethane [74-84-0] is obtained from wet naturalgases and refinery waste gases. It may be crackedalone or as a mixture with propane. Propane [7498-6] is obtained from wet natural gases, naturalgasolines, and refinery waste gases. Butanes areobtained from natural gasolines and refinerywaste gases.Naphthas, which are the most important feedstocks for ethylene production, are mixtures ofhydrocarbons in the boiling range of 30–200 C.Processing of light naphthas (boiling range30–90 C, full range (FR) naphthas (30–200 C)and special cuts (C6–C8 raffinates) as feedstocksis typical for naphtha crackers.A natural-cut full-range naphtha containsmore than 100 individual components, whichcan be detected individually by gas chromatography (GC). Depending on the origin naphthaquality can vary over a wide range, which necessitates quality control of the complex feedmixtures. Characterization is typically based onboiling range; density; and content of paraffins(n-alkanes), isoalkanes, olefins, naphthenes, andaromatics ( PIONA analysis) by carbon number.This characterization can be carried out by GCanalysis or by a newly developed infrared method [13]. Full characterization of feedstocks iseven more important when production is basedon varying feedstocks, e.g. feedstocks of different origins purchased on spot markets.The quality of a feedstock is depending on thepotential to produce the target products (ethyleneand propylene). Simple yield correlations forthese products can be used to express the qualityTable 2. World consumption of feedstocks forethylene production in 2002 (as percentage oftotal ethylene produced) [11]Figure 2. Feedstocks used for ethylene productionEthanePropaneButaneNaphthaGas oilOthers29835361

Vol. 13of a feedstock in a simple figure, the qualityfactor, which indicates whether yields of thetarget products are high or low, with aromaticfeedstocks being poor and saturated feedstocksbeing good feedstocks.Quality characterization factors for naphthashave been developed, which indicate the aromatics content by empirical correlation. Sincearomatics contribute little to ethylene yields innaphtha cracking, a rough quality estimate can bemade for naphthas with a typical weight ratio ofn- to isoparaffins of 1–1.1. The K factor is definedas [14]:K¼ð1:8Tk Þ1 3dwhere Tk is the molal average boiling point in K.Naphthas with a K factor of 12 or higher areconsidered saturated; those below 12 are considered naphthenic or aromatic. The K factor doesnot differentiate between iso- and n-alkanes. TheU.S. Bureau of Mines Correlation Index (BMCI)[15] can also be used as a rough quality measureof naphthas:BMCI ¼ 48640 Tþ473:7d 456:8where T is the molal average boiling point in Kand d is the relative density d15:615:6 . A high value ofBMCI indicates a highly aromatic naphtha; a lowvalue, a highly saturated naphtha.Gas condensates are very similar to naphthasin composition with the main difference of having no sharp boiling endpoint. A typical boilingcurve of gas condensates is shown in Figure 3.Gas condensates gain importance as raw materialfor ethylene production because they are cheaperthan naphtha.Gas oils are feedstocks that are gaining importance in several areas of the world. Gas oilsFigure 3. Boiling curves of liquid cracker feedstocksVGO ¼ Vacuum gas oil; AGO ¼ Atmospheric gas oilEthylene469used for ethylene production are crude oil fractions in the boiling range of 180–350 C (atmospheric gas oils, AGO) and 350–600 C (vacuumgas oils, VGO). In contrast to naphtha and lightergas feeds, these feedstocks can not be characterized by individual components.Gas chromatography coupled with mass spectrometry (GC–MS) or high performance liquidchromatography (HPLC) allow the analysis ofstructural groups, i.e., the percentage of paraffins,naphthenes, olefins, monoaromatics, and polyaromatics in the gas oil, and can be used todetermine the quality of the hydrocarbon fraction. If this information is used together with datasuch as hydrogen content, boiling range, refractive index, etc., the quality can be determinedquite accurately. A rough estimate of feed qualitycan be made by using the BMCI or the calculatedcetane number of a gas oil. The cetane number,normally used to calculate the performance ofdiesel fuels, is an excellent quality measure, sinceit is very sensitive to the n-paraffin content,which is one of the key parameters for theethylene yield. The cetane number CN is calculated as follows [16]:CN ¼ 12:822þ0:1164 CIþ0:012976 CI2whereCI ¼ 0.9187(T50/10)1.266871.4422720(nD 100), where T50 is the volume averageboiling point in C and n20D the refractive index at20 C5. Production5.1. Ethylene from Pyrolysisof HydrocarbonsThe bulk of the worldwide annual commercialproduction of ethylene is based on thermal cracking of petroleum hydrocarbons with steam; theprocess is commonly called pyrolysis or steamcracking. The principal arrangement of such acracking reactor is shown in Figure 4, a photograph of the furnace section of a modern ethyleneplant in Figure 5. The technology has beenapplied for more than 50 years with permanentimprovements in details.A hydrocarbon stream is heated by heatexchange against flue gas in the convectionsection, mixed with steam, and further heated toincipient cracking temperature (500–680 C,

470EthyleneVol. 13Figure 4. Principal arrangement of a cracking furnacedepending on the feedstock). The stream thenenters a fired tubular reactor (radiant tube orradiant coil) where, under controlled residencetime, temperature profile, and partial pressure,it is heated from 500–650 to 750–875 C for0.1–0.5 s. During this short reaction timeColorFighydrocarbons in the feedstock are cracked intosmaller molecules; ethylene, other olefins, anddiolefins are the major products. Since the conversion of saturated hydrocarbons to olefins inthe radiant tube is highly endothermic, highenergy input rates are needed. The reaction products leaving the radiant tube at 800–850 C arecooled to 550–650 C within 0.02–0.1 s to prevent degradation of the highly reactive productsby secondary reactions.The resulting product mixtures, which canvary widely, depending on feedstock and severityof the cracking operation, are then separated intothe desired products by using a complex sequenceof separation and chemical-treatment steps.The cooling of the cracked gas in the transferline exchanger is carried out by vaporization ofhigh-pressure boiler feed water (BFW, p ¼ 6–12 MPa), which is separated in the steam drumand subsequently superheated in the convectionsection to high-pressure superheated steam(HPSS, 6–12 MPa).5.1.1. Cracking ConditionsFigure 5. Furnace section of a modern ethylene plantCommercial pyrolysis of hydrocarbons to ethylene is performed almost exclusively in fired

Vol. 13tubular reactors, as shown schematically inFigure 4. These furnaces can be used for allfeedstocks from ethane to gas oil, with a limitation in the end point of the feedstock of 600 C.Higher boiling materials can not be vaporizedunder the operating condition of a crackingfurnace.Increasing availability of heavy gas oil fractions, due to a shift in demand to lighter fractions,offers cost advantages for processing heavy feedstocks in some areas of the world. Furthermore,the availability of large quantities of residual oilhave led some companies to investigate crude oiland residual oils as ethylene sources. Such feedstocks cannot be cracked in conventional tubularreactors. Various techniques employing fluidizedbeds, molten salts, recuperators, and high-temperature steam have been investigated, but noneof these have attained commercial significance[17]. Recently cracking of gas condensates instead of naphthas has gained importance, as somegas condensates offer yield and cost advantagescompared to naphthas. Some gas condensates arecontaminated with Hg or As, which need to beremoved prior to cracking, as these contaminantscould lead to corrosion (Hg) of aluminumheat exchangers used in the separation sectionor to poisoning (As) of hydrogenation catalysts [18].Pyrolysis of hydrocarbons has been studiedfor years. Much effort has been devoted to mathematical models of pyrolysis reactions for use indesigning furnaces and predicting the productsobtained from various feedstocks under differentfurnace conditions. Three major types of modelare used: empirical or regression, molecular, andmechanistic models [19].Today, mechanistic computer models, whichare available from various companies, are usedfor design, optimization and operation of modernolefin plants. Sophisticated regression modelsare also used, mainly by operators, and offerthe advantage of a much lower computer performance requirements than mechanistic models.The regression models are based on a data set,which can consist of historical data or calculateddata. Depending on the quality of the data basethe empirical regression models can be of sufficient accuracy for most operating problems,within the range of the data field. These modelscan be run on small computers and are well suitedfor process computer control and optimization.Ethylene471Molecular kinetic models that use only apparent global molecular reactions and thus describethe main products as a function of feedstockconsumption have been applied with some success to the pyrolysis of simple compounds suchas ethane, propane, and butanes.For example, cracking of propane can bedescribed asC3 H8 !a H2 þb CH4 þc C2 H4 þd C3 H6 þe C4 H8 þf C5þwhere a, b, c, d, e, f are empirical factors depending on the conversion of propane.Gross oversimplification is required if thesemodels are applied to complex mixtures such asnaphthas or gas oils, but some success has beenattained even with these materials.Advances have been made in mechanisticmodeling of pyrolysis, facilitated by the availability of more accurate thermochemical kineticand pyrolysis data and of high-speed computers.The major breakthrough in this area, however,has been the development of methods to integrate large systems of differential equations[20–22].Mechanistic models need less experimentaldata and can be extrapolated. The accuracy ofthese models is very good for most components,but they require permanent tuning of the kineticparameters, especially for computing thecracked-gas composition for ultrashort residencetimes. The main application for mechanisticmodels is the design of cracking furnaces andcomplete ethylene plants. The accuracy of themodels has been improved, driven by the competition between the contractors for ethyleneplants. A number of mechanistic models are usedtoday in the ethylene industry, describing thevery complex kinetics with hundreds of kineticequations [23–25].To demonstrate the complexity of the chemical reactions, the cracking of ethane to ethylene isdiscussed here in detail. A simple reaction equation for ethane cracking is:C2 H6 !C2 H4 þH2ð1ÞIf this were the only reaction, the product at100 % conversion would consist solely of ethylene and hydrogen; at lower conversion, ethylene,hydrogen and ethane would be present. In fact,the cracked gas also contains methane, acetylene,

472EthyleneVol. 13propene, propane, butanes, butenes, benzene,toluene, and heavier components. This reaction(Eq. 1) is clearly not the only reaction occurring.In the 1930s, the free-radical mechanism forthe decomposition of hydrocarbons was established [26]. Although the free-radical treatmentdoes not explain the complete product distribution, even for a compound as simple as ethane, ithas been extremely useful. Ethane cracking represents the simplest application of the freeradical mechanism. Ethane is split into twomethyl radicals in the chain initiation step(Eq. 2). The methyl radical reacts with an ethanemolecule to produce an ethyl radical (Eq. 3),which decomposes to ethylene and a hydrogenatom (Eq. 4). The hydrogen atom reacts withanother ethane molecule to give a molecule ofhydrogen and a new ethyl radical (Eq. 5).Initiation.C2 H6 !CH3 þCH3ð2ÞPropagation.CH3 þC2 H6 !CH4 þC2 H5.C2 H5 !C2 H4 þH.ð3Þð4Þ.H þC2 H6 !H2 þC2 H5ð5ÞIf reactions (4) and (5) proceed uninterrupted,the molecular reaction in Equation (1) results. Ifonly reactions (3)–(5) occurred, the cracked gaswould contain traces of methane (Eq. 3) andequimolar quantities of ethylene and hydrogenwith unreacted ethane. This is not observed.Reactions (3) and (4) terminate if either anethyl radical or a hydrogen atom reacts withanother radical or atom by reactions such as:Termination.H þH !H2.CH3 þH !CH4ð6Þð7Þmust be generated (Eqs. 2–4) to start a new chain.Thus, every time a new chain is initiated, amolecule of methane is formed (Eq. 3) and amolecule of ethylene is produced (Eq. 4). Othernormal and branched-chain alkanes decomposeby a similar, but more complex, free-radicalmechanism [27]. The number of possible freeradicals and reactions increases rapidly as chainlength increases.The free-radical mechanism is generally accepted to explain hydrocarbon pyrolysis at lowconversion [26]. As conversion and concentrations of olefins and other products increase,secondary reactions become more significant.Partial pressures of olefins and diolefins increase,favoring condensation reactions to produce cyclodiolefins and aromatics. The cracking of heavy feed, such as naphthas or gas oils, oftenproceeds far enough to exhaust most of thecrackable material in the feedstock.The reaction scheme with heavier feeds ismuch more complex than with gaseous feedstocks, due to the fact the hundreds of reactants(feed components) react in parallel and some ofthose components are formed as reaction products during the reaction. Since the radicalsinvolved are relatively short lived, their concentration in the reaction products is rather low.Radical decomposition is one of the mostimportant types of reaction and it directly produces ethylene according to the followingscheme:Radical decomposition.RCH2 CH2 CH2 !RCH2 þC2 H4.H þC2 H5 !C2 H6.C2 H5 þCH3 !C3 H8.C2 H5 þC2 H5 !C4 H10ð8Þð9Þð10ÞOn termination of chain propagation, newmethyl or ethyl radicals or a new hydrogen atomð11ÞThis b-scission reaction produces a shorterradical (RCH2 ) and ethylene. Radicals normallydecompose in the b-position, where the C–Cbond is weaker due to electronic effects. Largeradicals are more stable than smaller ones andcan therefore undergo isomerization.Radical isomerizationRCH2 CH2 CH2 !RCH2 C HCH3ð12ÞThe free-radical decomposition of n-butane(Eqs. 12–14) results in the molecular equation(Eq. 15):.n-C4 H10 þH !n-C4 H9 þH2.n-C4 H9 !C2 H4 þC2 H5ð14Þð15Þ

Vol. 13.C2 H5 !C2 H4 þHEthylene.n-C4 H10 !2 C2 H4 þH2ð16ÞPrimary reactionsSecondary reactionsFeedstock ethylene C4 products/steampropeneC5 productsacetylene C6 productshydrogen aromaticsmethaneC7 productsetc.heavier productsThe fundamentals of furnace design and themain influences of the different parameters canbe understood even with this simplifiedmechanism:.cracking coil to avoid long residence times atlow temperatures.ð17ÞReactions like (1) and (15) are highly endothermic. Reported values of DH at 827 C areþ144.53 kJ/mol for Equation (1) and þ232.244kJ/mol for Equation (15).The mathematical description of these complex systems requires special integration algorithms [28]. Based on the pseudo steady stateapproximation, the chemical reactions can beintegrated and the concentration of all components at each location of the reactor (crackingcoil) can be computed [29, 30].In a generalized and very simplified form thecomplex kinetics of cracking of hydrocarbons(ethane to gas oil) in steam crackers can besummarized as follows:.473Residence time: From the above scheme it isclear that a long residence time favors thesecondary reactions, whereas a short residencetime increases the yields of the primary products, such as ethylene and propylene.Partial pressure: Since most of the secondaryproducts result from reactions in which thenumber of molecules decreases, increasingthe pressure favors the secondary products.One function of the steam present in the systemis to reduce the hydrocarbon partial pressureand thus favor the formation of primaryproducts.Temperature and temperature profiles: Theoligomerization reactions involved in the formation of secondary products are favoredby lower temperatures; therefore, specialtemperature profiles are applied along theMost commercial pyrolysis to produce ethylene is carried out in fired tubular reactors inwhich the temperature of the reactant increasescontinuously from the inlet to the outlet. Typicalinlet temperatures are 500–680 C, depending onthe material being processed. Typical outlettemperatures are 775–875 C.Modern cracking furnaces are designed forrapid heating at the radiant coil inlet, wherereaction rate constants are low because of thelow temperature. Most of the heat transferredsimply raises the reactant from the inlet temperature to the necessary reaction temperature. Inthe middle of the coil, the rate of temperature riseis lower, but cracking rates are appreciable. Inthis section,the endothermic reaction absorbsmost of the heat transferred to the mixture. Atthe coil outlet, the rate of temperature rise againincreases but never becomes as rapid as at theinlet.The designers of cracking coils try to optimizethe temperature and pressure profiles along theradiant coils to maximize the yield of valuableproducts yields by special coil design that allowsrapid temperature increase in the inlet section andlow pressure drops in the outlet section of thecracking coils.Typical process gas temperature profilesalong the radiant coil of modern ethylene furnaces are shown in Figure 6 for ethane, propane,butane, and naphtha cracking.The quantity of steam used, generally expressed as steam ratio (kilograms of steam perkilogram of hydrocarbon), varies with feedstock,cracking severity, and design of the cracking coil.Typical steam ratios used at a coil outlet pressureof 165–225 kPa (1.65–2.25 bar) for variousfeedstocks are:EthanePropaneNaphthasGas condensatesAtmospheric gas oils (cut: 180–350 C)Hydrocracker bottoms (cut: 350–600 –0.700.70–0.85Steam dilution lowers the hydrocarbon partialpressure, there

quire ethylene in quantities of 100 103 – 200 103 t/a only and ethylene supply from standalone crackers of this size is not feasible in most cases. 2. Physical Properties Ethylene is a colorless flammable gas with a sweet odor. The physical properties of ethylene are as follows: mp