EthyleneHeinz Zimmermann, Linde AG, Hoellriegelskreuth, Federal Republic of Germany

Roland Walzl, Linde AG, Hoellriegelskreuth, Federal Republic of Germany

1. Introduction

Ethylene [74-85-1], ethene, H2C=CH2, Mr

28.52, is the largest-volume petrochemical pro-duced worldwide. Ethylene, however, has no di-rect end uses, being used almost exclusively as achemical building block. It has been recoveredfrom coke-oven gas and other sources in Europesince 1930 [1]. Ethylene emerged as a large-vol-ume intermediate in the 1940s when U.S. oil andchemical companies began separating it from re-finery waste gas and producing it from ethaneobtained from refinery byproduct streams andfrom natural gas. Since then, ethylene has almostcompletely replaced acetylene for many syntheses.Ethylene is produced mainly by thermal cracking

of hydrocarbons in the presence of steam, and byrecovery from refinery cracked gas.

In 1996 total worldwide ethylene productioncapacity was 79.3 3 106 t, with an actual demandof ca. 71 3 106 t/a [2], which has growth projec-tions of 4.5% per year worldwide for the period of1996 to 2005 [3] – [5].

2. Physical Properties

Ethylene is a colorless flammable gas with asweet odor. The physical properties of ethylene areas follows :

1. Introduction . . . . . . . . . . . . . . . . . 1

2. Physical Properties . . . . . . . . . . . . 1

3. Chemical Properties . . . . . . . . . . . 2

4. Raw Materials . . . . . . . . . . . . . . . 2

5. Production . . . . . . . . . . . . . . . . . . 4

5.1. Ethylene from Pyrolysis ofHydrocarbons . . . . . . . . . . . . . . . 4

5.1.1. Cracking Conditions . . . . . . . . . . . 45.1.2. Heat Requirements for Hydrocarbon

Pyrolysis . . . . . . . . . . . . . . . . . . . 85.1.3. Commercial Cracking Yields . . . . . 105.1.4. Commercial Cracking Furnaces . . . 155.1.5. Tube Metallurgy . . . . . . . . . . . . . . 215.1.6. Thermal Efficiency of Ethylene

Furnaces . . . . . . . . . . . . . . . . . . . 225.1.7. Coking and Decoking of Furnaces

and Quench Coolers . . . . . . . . . . . 23

5.2. Quenching of Hot Cracked Gas . . . 25

5.3. Recovery Section . . . . . . . . . . . . . 305.3.1. Products . . . . . . . . . . . . . . . . . . . 315.3.2. Cracked Gas Processing. . . . . . . . . 325.3.2.1. Front-End Section . . . . . . . . . . . . . 325.3.2.2. Hydrocarbon Fractionation Section . 355.3.3. Utilities . . . . . . . . . . . . . . . . . . . . 425.3.4. Process Advances . . . . . . . . . . . . . 44

5.4. Other Processes and Feedstocks . . . 44

6. Environmental Protection . . . . . . . 45

7. Quality Specifications . . . . . . . . . . 46

8. Chemical Analysis . . . . . . . . . . . . 47

9. Storage and Transportation . . . . . . 47

10. Uses and Economic Aspects . . . . . . 48

11. Toxicology and Occupational Health 49

12. References . . . . . . . . . . . . . . . . . . 49

Ethylene 1

mp –169.15 @Cbp –103.71 @CCritical temperature, Tc 9.90 @CCritical pressure, Pc 5.117 MPaCritical density 0.21 g/cm3

Densityat bp 0.57 g/cm3

at 0 @C 0.34 g/cm3

Gas density at STP 1.2603 g/LDensity relative to air 0.9686Molar volume at STP 22.258 LSurface tensionat bp 16.5 mN/mat 0 @C 1.1 mN/m

Heat of fusion 119.5 kJ/kgHeat of combustion 47.183 MJ/kgHeat of vaporizationat bp 488 kJ/kgat 0 @C 191 kJ/kg

Specific heatof liquid at bp 2.63 kJ kg–1 K–1

of gas at Tc 1.55 kJ kg–1 K–1

Enthalpy of formation 52.32 kJ/molEntropy 0.220 kJ mol–1 K–1

Thermal conductivityat 0 @C 177310–4 W m–1 K–1

at 100 @C 294310–4 W m–1 K–1

at 400 @C 805310–4 W m–1 K–1

Viscosity of liquidat mp 0.73 mPa · sat bp 0.17 mPa · sat 0 @C 0.07 mPa · s

of gasat mp 36310–4 mPa · sat 0 @C 93310–4 mPa · sat 150 @C 143310–4 mPa · s

Vapor pressureat –150 @C 0.002 MPaat bp 0.102 MPaat –50 @C 1.10 MPaat 0@ 4.27 MPa

Explosive limits in air at0.1 MPa and 20 @Clower (LEL) 2.75 vol% or 34.6 g/cm3

upper (UEL) 28.6 vol% or 360.1 g/cm3

Ignition temperature 425 – 527 @C

3. Chemical Properties

The chemical properties of ethylene resultfrom the carbon – carbon double bond, with a bondlength of 0.134 nm and a planar structure. Ethyl-ene is a very reactive intermediate, which canundergo all typical reactions of a short-chain ole-fin. Due to its reactivity ethylene gained impor-tance as a chemical building block. The complexproduct mixtures that have to be separated duringthe production of ethylene are also due to thereactivity of ethylene.

Ethylene can be converted to saturated hydro-carbons, oligomers, polymers, and derivativesthereof. Chemical reactions of ethylene with com-

mercial importance are: addition, alkylation, halo-genation, hydroformylation, hydration, oligomeri-zation, oxidation, and polymerization.

The following industrial processes are listed inorder of their 1993 worldwide ethylene consump-tion [6]:

1) Polymerization to low-density polyethylene(LDPE) and linear low-density polyethylene(LLDPE)

2) Polymerization to high-density polyethylene(HDPE)

3) Addition of chlorine to form 1,2-dichlor-oethane

4) Oxidation to oxirane [75-21-8] (ethyleneoxide) over a silver catalyst

5) Reaction with benzene to form ethylbenzene[100-41-4], which is dehydrogenated to styrene[100-42-5]

6) Oxidation to acetaldehyde7) Hydration to ethanol8) Reaction with acetic acid and oxygen to form

vinyl acetate9) Other uses, including production of linear al-

cohols, linear olefins, and ethyIchloride [75-00-3], and copolymerization with propene tomake ethylene – propylene (EP) and ethylene –propylene – diene (EPDM) rubber

4. Raw Materials

Table 1 lists the percentage of ethylene pro-duced worldwide from various feedstocks for 1981and 1992 [7]. In Western Europe and Japan, over80% of ethylene is produced from naphthas— theprincipal ethylene raw materials.

A shift in feedstocks occurred for the periodfrom 1980 to 1991. In the United States and Eur-ope larger amounts of light feedstocks (LPG:propane + butane) and NGL (ethane, propane, bu-tane) are used for ethylene production, whereas inJapan more naphtha was used in 1991 compared to1981. The use of gas oils for ethylene productiondecreased slightly during the 1980s.

Ethane [74-84-0] is obtained from wet naturalgases and refinery waste gases. It may be crackedalone or as a mixture with propane. Propane [74-98-6] is obtained from wet natural gases, naturalgasolines, and refinery waste gases. Butanes areobtained from natural gasolines and refinery wastegases. A mixture of light hydrocarbons such aspropane, isobutane [75-28-5], and n-butane [106-

2 Ethylene

97-81], commonly called liquefied petroleum gas(LPG) and obtained from natural gasolines andrefinery gases, is also used as a feedstock.

Naphthas, which are the most important feed-stocks for ethylene production, are mixtures ofhydrocarbons in the boiling range of 30 – 200 @C.Processing of light naphthas (boiling range30 – 90 @C, full range naphthas (30 – 200 @C) andspecial cuts (C6 –C8 raffinates) as feedstocks istypical for naphtha crackers.

A natural-cut full-range naphtha contains morethan 100 individual components, which can bedetected individually by gas chromatography(GC). Depending on the origin naphtha qualitycan vary over a wide range, which necessitatesquality control of the complex feed mixtures.Characterization is typically based on boilingrange; density; and content of paraffins (n-al-kanes), isoalkanes, olefins, naphthenes, and aro-matics (PIONA analysis) by carbon number. Thischaracterization can be carried out by GC analysisor by a newly developed infrared method [8]. Fullcharacterization of feedstocks is even more impor-tant when production is based on varying feed-stocks, e.g. feedstocks of different origins pur-chased on spot markets.

The quality of a feedstock is depending on thepotential to produce the target products (ethyleneand propylene). Simple yield correlations for theseproducts can be used to express the quality of afeedstock in a simple figure, the quality factor,which indicates wether yields of the target pro-ducts are high or low, with aromatic feedstocksbeing poor and saturated feedstocks being goodfeedstocks.

Quality characterization factors for naphthashave been developed, which indicate the aromaticscontent by empirical correlation. Since aromaticscontribute little to ethylene yields in naphthacracking, a rough quality estimate can be madefor naphthas with a typical weight ratio of n- toisoparaffins of 1 – 1.1. The K factor is defined as[9]:

K ¼ 1:8Tkð Þ1=3

d

where Tk is the molal average boiling point in K.Naphthas with a K factor of 12 or higher areconsidered saturated; those below 12 are consid-ered naphthenic or aromatic. The K factor does notdifferentiate between iso- and n-alkanes. The U.S.Bureau of Mines Correlation Index (BMCI) [10]can also be used as a rough quality measure ofnaphthas:

BMCI ¼ 48 640=T þ 473:7d � 456:8

where 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 oils are feedstocks that are gaining impor-tance in several areas of the world. Gas oils usedfor ethylene production are crude oil fractions inthe boiling range of 180 – 350 @C (atmospheric gasoils, AGO) and 350 – 600 @C (vacuum gas oils,VGO). In contrast to naphtha and lighter gas feeds,these feedstocks can not be characterized by in-dividual components.

Gas chromatography coupled with mass spec-trometry (GC –MS) or high performance liquidchromatography (HPLC) allow the analysis ofstructural groups, i.e., the percentage of paraffins,naphthenes, olefins, monoaromatics, and polyaro-matics in the gas oil, and can be used to determinethe quality of the hydrocarbon fraction. If thisinformation is used together with data such ashydrogen content, boiling range, refractive index,etc., the quality can be determined quite accu-rately. A rough estimate of feed quality can bemade by using the BMCI or the calculated cetanenumber of a gas oil. The cetane number, normallyused to calculate the performance of diesel fuels,is an excellent quality measure, since it is verysensitive to the n-paraffin content, which is one

Table 1. Raw materials for ethylene production (as a percentage of total ethylene produced)

Raw materials USA W. Europe Japan World

1979 1991 1981 1991 1981 1991 1981 1991

Refinery gas 1 3 2 17LPG, NGL 65 73 4* 14 10* 2* 31* 27Naphtha 14 18 80 72 90 98 58 48Gas oil 20 6 16 12 0 0 11 8

*Including refinery gas

Ethylene 3

of the key parameters for the ethylene yield. Thecetane number CN is calculated as follows [0]:

CN = 12.822 + 0.1164CI + 0.012976 CI2

where CI = 0.9187 (T50 /10)1.26687 (nD

20/100)1.44227,where T50 is the volume average boiling point in@C and nD

20 the refractive index at 20 @C.

5. Production

5.1. Ethylene from Pyrolysis ofHydrocarbons

The bulk of the worldwide annual commercialproduction of ethylene is based on thermal crack-ing of petroleum hydrocarbons with steam; theprocess is commonly called pyrolysis or steamcracking. The principal arrangement of such acracking reactor is shown in Figure 1.

A hydrocarbon stream is heated by heat ex-change against flue gas in the convection section,mixed with steam, and further heated to incipientcracking temperature (500 – 680 @C, depending onthe feedstock). The stream then enters a fired tub-ular reactor (radiant tube or radiant coil) where,under controlled residence time, temperature pro-file, and partial pressure, it is heated from

500 – 650 to 750 – 875 @C for 0.1 – 0.5 s. Duringthis short reaction time hydrocarbons in the feed-stock are cracked into smaller molecules; ethyl-ene, other olefins, and diolefins are the major pro-ducts. Since the conversion of saturated hydrocar-bons to olefins in the radiant tube is highly en-dothermic, high energy input rates are needed. Thereaction products leaving the radiant tube at800 – 850 @C are cooled to 550 – 650 @C within0.02 – 0.1 s to prevent degradation of the highlyreactive products by secondary reactions.

The resulting product mixtures, which can varywidely, depending on feedstock and severity of thecracking operation, are then separated into thedesired products by using a complex sequence ofseparation and chemical-treatment steps.

The cooling of the cracked gas in the transfer-line exchanger is carried out by vaporization ofhigh-pressure boiler feed water (BFW,p = 6 – 12 MPa), which is separated in the steamdrum and subsequently superheated in the convec-tion section to high-pressure superheated steam(HPSS, 6 – 12 MPa).

5.1.1. Cracking Conditions

Commercial pyrolysis of hydrocarbons to eth-ylene is performed almost exclusively in firedtubular reactors, as shown schematically in Fig-ure 1. These furnaces can be used for all feed-

Figure 1. Principal arrangement ofa cracking furnace

4 Ethylene

stocks from ethane to gas oil, with a limitation inthe end point of the feedstock of 600 @C. Higherboiling materials can not be vaporized under theoperating condition of a cracking furnace.

Increasing availability of heavy gas oil frac-tions, due to a shift in demand to lighter fractions,offers cost advantages for processing heavy feed-stocks 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 feed-stocks cannot be cracked in conventional tubularreactors. Various techniques employing fluidizedbeds, molten salts, recuperators, and high-tem-perature steam have been investigated, but noneof these have attained commercial significance[12].

Pyrolysis of hydrocarbons has been studied foryears. Much effort has been devoted to mathema-tical models of pyrolysis reactions for use in de-signing furnaces and predicting the products ob-tained from various feedstocks under different fur-nace conditions. Three major types of model areused: empirical or regression, molecular, and me-chanistic models [13].

Today, mechanistic computer models, whichare available from various companies, are usedfor design, optimization and operation of modernolefin plants. Sophisticated regression models arealso used, mainly by operators, and offer the ad-vantage of a much lower computer performancerequirements 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 base theempirical regression models can be of sufficientaccuracy for most operating problems, within therange of the data field. These models can be run onsmall computers and are well suited for processcomputer control and optimization.

Molecular kinetic models that use only appar-ent global molecular reactions and thus describethe main products as a function of feedstock con-sumption have been applied with some success tothe pyrolysis of simple compounds such as ethane,propane, and butanes.

For example, cracking of propane can be de-scribed as

C3H8 �! a H2 + b CH4 + c C2H4 + d C3H6 + e C4H8 +f C5+

where a, b, c, d, e, f are empirical factorsdepending 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.

In recent years, advances have been made inmechanistic modeling of pyrolysis, facilitated bythe availability of more accurate thermochemicalkinetic and pyrolysis data and of high-speed com-puters. The major breakthrough in this area, how-ever, has been the development of methods tointegrate large systems of differential equations[14] – [16].

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 the cracked-gas composition for ultrashort residence times.The main application for mechanistic models isthe design of cracking furnaces and complete eth-ylene plants. The accuracy of the models has beenimproved, driven by the competition between thecontractors for ethylene plants. A number of me-chanistic models are used today in the ethyleneindustry, describing the very complex kineticswith hundreds of kinetic equations [17] – [19].

To demonstrate the complexity of the chemicalreactions, the cracking of ethane to ethylene isdiscussed here in detail. A simple reaction equa-tion for ethane cracking is:

C2H6 �! C2H4 + H2 (1)

If this were the only reaction, the product at100% conversion would consist solely of ethyleneand hydrogen; at lower conversion, ethylene, hy-drogen and ethane would be present. In fact, thecracked gas also contains methane, acetylene, pro-pene, propane, butanes, butenes, benzene, toluene,and heavier components. This reaction (Eq. 1) isclearly not the only reaction occurring.

In the 1930s, the free-radical mechanism forthe decomposition of hydrocarbons was estab-lished [20]. Although the free-radical treatmentdoes not explain the complete product distribution,even for a compound as simple as ethane, it hasbeen extremely useful. Ethane cracking representsthe simplest application of the free-radical me-chanism. Ethane is split into two methyl radicalsin the chain initiation step (Eq. 2). The methylradical reacts with an ethane molecule to producean ethyl radical (Eq. 3), which decomposes to eth-ylene and a hydrogen atom (Eq. 4). The hydrogenatom reacts with another ethane molecule to give a

Ethylene 5

molecule of hydrogen and a new ethyl radical(Eq. 5).

Initiation

C2H6 �! CH3 · + CH3 · (2)

Propagation

CH3 · + C2H6 �! CH4 + C2H5 · (3)

C2H5 · �! C2H4 + H · (4)

H · + C2H6 �! H2 + C2H5 · (5)

If 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) and equi-molar quantities of ethylene and hydrogen withunreacted ethane. This is not observed.

Reactions (3) and (4) terminate if either anethyl radical or a hydrogen atom reacts with an-other radical or atom by reactions such as:

Termination

H · + H · �! H2 (6)

CH3 · + H · �! CH4 (7)

H · + C2H5 · �! C2H6 (8)

C2H5 · + CH3 · �! C3H8 (9)

C2H5 · + C2H5 · �! C4H10 (10)

On termination of chain propagation, newmethyl or ethyl radicals or a new hydrogen atommust be generated (Eqs. 2 – 4) to start a new chain.Thus, every time a new chain is initiated, a mole-cule of methane is formed (Eq. 3) and a moleculeof ethylene is produced (Eq. 4). Other normal andbranched-chain alkanes decompose by a similar,but more complex, free-radical mechanism [21].The number of possible free radicals and reactionsincreases rapidly as chain length increases.

The free-radical mechanism is generally ac-cepted to explain hydrocarbon pyrolysis at lowconversion [20]. As conversion and concentrationsof olefins and other products increase, secondaryreactions become more significant. Partial pres-sures of olefins and diolefins increase, favoringcondensation reactions to produce cyclodiolefinsand aromatics. The cracking of heavy feed, such asnaphthas or gas oils, often proceeds far enough to

exhaust most of the crackable material in the feed-stock.

The reaction scheme with heavier feeds ismuch more complex than with gaseous feedstocks,due to the fact the hundreds of reactants (feedcomponents) react in parallel and some of thosecomponents are formed as reaction products dur-ing the reaction. Since the radicals involved arerelatively short lived, their concentration in thereaction products is rather low.

Radical decomposition is one of the most im-portant types of reaction and it directly producesethylene according to the following scheme:

Radical decomposition

RCH2CH2CH2· �! RCH2 · + C2H4 (11)

This b-scission reaction produces a shorter ra-dical (RCH2 ·) and ethylene. Radicals normallydecompose in the b-position, where the C –C bondis weaker due to electronic effects. Large radicalsare more stable than smaller ones and can there-fore undergo isomerization.

Radical isomerization

RCH2CH2CH2 · �! RCH2CHCH3 (12)

The free-radical decomposition of n-butane(Eqs. 12 – 14) results in the molecular equation(Eq. 15):

n-C4H10 + H · �! n-C4H9 · + H2 (14)

n-C4H9 · �! C2H4 + C2H5 · (15)

C2H5 · �! C2H4 + H · (16)

n-C4H10 �! 2 C2H4 + H2 (17)

Reactions like (1) and (15) are highly en-dothermic. Reported values of DH at 827 @C are+ 144.53 kJ/mol for Equation (1) and+ 232.244 kJ/mol for Equation (15).

The mathematical description of these com-plex systems requires special integration algo-rithms [22]. Based on the pseudo steady stateapproximation, the chemical reactions can be in-tegrated and the concentration of all componentsat each location of the reactor (cracking coil) canbe computed [23], [24].

In a generalized and very simplified form thecomplex kinetics of cracking of hydrocarbons(ethane to gas oil) in steam crackers can be sum-marized as follows:

6 Ethylene