Pyrolysis-Furnace-Rev-1.pdf

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PYROLYSIS FURNACE

C. M. [email protected]

ABSTRACT

Pyrolysis or steam cracking is the primary process utilized to manufacture olefins from large hydrocarbon

molecules. This gas-phase reaction takes place in metal alloy tubes within a fired furnace. Pyrolysis

proceeds as a series of free radical reactions and the complexity of the mechanisms increases with the

nature of the feedstock. Both rigorous and empirical solution techniques are available. A mathematical

model for design of pyrolysis coil is presented. The effect of residence time, temperature, pressure and

inert on ethylene yield will be discussed briefly.

Coke formation onto the tubular coils continues to be a challenge faced by engineers. Coke deposition

increases fuel consumption, reduces the furnace throughput and causes non-productive outages for

decoking activities. Various coke formation mechanisms will be discussed and a review of recent

developments to mitigate this phenomenon will be presented.

Keywords:Coking, ethylene, furnace, olefins, pyrolysis, steam cracking

1 INTRODUCTION

Olefins manufacturing is the third largest petrochemical sector after ammonia manufacturing and

petroleum refining. Olefins are the building blocks in manufacturing of polymers and elastomers or

converted into derivatives such as aldehydes, alcohols, glycols, etc. Olefins are primarily produced by

steam cracking of large hydrocarbon molecules. This process is also known as pyrolysis.

Pyrolysis is a gas-phase reaction at very high temperature. As the reaction is highly endothermic, it is

carried out in tubular coils within a fired furnace. Many furnace designs are available today, although thefundamental principles are similar. Commercial technology can be licensed from technology providers

e.g., Kellogg Brown & Root, ABB Lummus, Stone & Webster, Linde, KTI-Technip, etc.

An industrial pyrolysis furnace is a complicated piece of equipment that functions as both a reactor and

high-pressure steam generator. The pyrolysis reactions proceeds in tubular coils made of Cr/Ni alloys.

These coils are hung vertically in a firebox. Depending of furnace design, there may be between 16-128

coils per firebox. Burners are arranged on the walls and on the floor of the firebox for indirect firing. This

section is called the radiant section because the radiant heat is recovered. At the end of the pyrolysis, the

reaction needs to be quenched rapidly to avoid further decomposition of the desired olefins. This is

achieved by either indirect cooling using a quench exchanger or direct cooling by injecting quench oil

into the gas effluent. The heat carried by the flue gas is recovered at the convection section of the furnace.

This section consists of a series of tube banks where the heat is recovered for superheating steam,

preheating the hydrocarbon feed, boiler feed water and dilution steam.

Being the heart of a cracker unit, furnace technology continues to be an active area of research. The large

amount of energy consumed in both the pyrolysis reaction and recovery of the products drives engineers

to continuously improve the energy efficiency of the process. Higher selectivity designs helps to reduce

the size of the recovery section and hence, the capital cost of a steam cracker.

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2 PROCESS CHEMISTRY

The desired products from the pyrolysis reactions are light olefins, i.e., ethylene, propylene and

butadiene. However, depending on the nature of the feedstock, a wide array of by-products can be

produced as result of pyrolysis. This includes hydrogen, methane, acetylene, butene, benzene, toluene,

xylene and other hydrocarbon structures up to C12length. A wide variety of feedstock for pyrolysis can

be employed with ethane and naphtha being the most common. Other feedstocks used are propane,

liquefied petroleum gas (a propane and butane mixture), kerosene, atmospheric gas oil, vacuum gas oil

and wax.

The desired reaction is the decomposition of the hydrocarbon molecule (typically of paraffinic structure)

to its olefinic equivalent. The simplest illustration is decomposition of ethane into an ethylene molecule,

where the overall reaction is:

2 6 2 4 2C H C H H+ (1)

The mechanism of this reaction however involves a series of free radical reactions as proposed by Rice

and Herzfeld (1934). This can be divided into four steps: initiation, hydrogen abstraction, radicaldecomposition and termination.

Initiation1k

2 6 3 3* *C H CH CH+ (2)

Hydrogen abstraction2k

3 2 6 4 2* *CH C H CH C H+ +

5

*

(3)

3k

2 6 2 2 5*H C H H C H+ + (4)

Radical decomposition4k

2 5 2 4* *C H C H H+ (5)

Termination5k

2* *H H H+ (6)

5k

3 4* *H CH CH+ (7)

5k

2 5 2 6* *H C H C H+ (8)

5k

2 5 3 3 8* *C H CH C H+ (9)

5k

2 5 2 5 4 10* *C H C H C H+ (10)

In pyrolysis of larger hydrocarbons, the initiation step of splitting the C-C bond can occur in variety ofways depending on the molecule structure. Addition transient chain reactions can be occurringsimultaneously. In high severity condition (very high reaction temperature), the olefins produced as wellas the species formed in the termination step can proceed for further decomposition.

In pyrolysis of multi-component hydrocarbon mixtures, e.g., naphtha and gas oil, the multiplicity ofreactions that occurred and the largely unknown componential composition, rigorous free radicalmechanism solution is difficult. Techniques of lumping groups of chemical species with similar kineticbehavior, similar to those utilized in catalytic cracking were developed. These techniques lump thehydrocarbon into groups of normal paraffin, iso-paraffin, olefin, naphthene and aromatic. Empiricalsolutions, e.g. Kivlen (1990), correlates the key components in the product to these lumped groups, aswell as the specific gravity, boiling range, hydrogen content, etc. Modern modeling package, e.g.,

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SPYRO offers a rigorous solution that combines technique of lumping components and free radicalmechanism.

Two major undesired reactions that occur simultaneously with the desired cracking of alkanes aredehydrogenation and condensation. Dehydrogenation is the phenomenon where an olefin moleculefurther decomposes into a diolefin or the CC group. Examples are formation of acetylene, methylacetylene and propadiene:

C H 2 4 2 2 2

C H H+ (11)

24363 HHCHC + (12)

Dehydrogenation results in olefin yield loss. Its products are contaminant to the olefin product. In thecase of acetylene, methyl acetylene and propadiene, they had to be removed downstream usingpalladium-based hydrogenation reactors, which are expensive and difficult to operate.

Condensation is a reaction where two or more small molecules combine to form a larger stable structuresuch as cyclo-diolefins and aromatic. This secondary reaction occurs in the latter stage of pyrolysis andthe residence time of the reactor is high. In practice, this can be observed when the aromaticconcentration in the pyrolysis gasoline (a by-product) stream and the residual fuel oil (C9-C12fraction) arehigh.

The extreme of dehydrogenation and condensation is coke formation. Coke forms when hydrogen atomsare removed from the hydrocarbon radicals until the extreme of leaving only a layer of elemental carbonor coke. Although, aromatic are relatively stable molecules, they can however further react viacondensation to form a chain of its benzene ring structure. These condensation products leave the gasphase and settle on the inner walls of the radiant coils as a layer of hard coke that is very difficult toremove. Various mechanism of coke formation will be discussed later in detail.

3 REACTOR PARAMETERS

The pyrolysis coils essentially behaves as a plug flow reactor that receives heat from its surrounding, i.e.,the radiant heat of the firebox. Much of the early development of furnace technology found inspirationsin the works of Schutt (1959). In industrial pyrolysis of ethane, it was found that the methane yieldincreases and the ethylene yield decreases with the approach to the equilibrium of ethane

dehydrogenation reaction. The equilibrium approach can be defined as:

( )( )( )

2 2 4

2 6

H C H

p C H

y yP

K y= (13)

where P = hydrocarbon partial pressure, PaKp = equilibrium constant for the ethane dehydrogenation reaction at the coil outlettemperature, Pa

y = mole fraction of constituent in the hydrocarbon mixture of the coil effluent

Based on Schutts data and assumption that every decomposing mole of ethane not converted to ethylenewill yield 2 moles of other pyrolysis products, the molal yield of ethylene per converted mole of ethanecan be represented by the following formula:

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installed vertically to reduce the investment cost. Large wall burners were replaced with smaller but more

efficient floor burners (Bowen, 1992).

Yield in Weight Percent

Residence Time 0.10 s 0.20 s 0.50 s

Methane 15.48 15.78 16.16Ethylene 34.16 32.16 29.37

Propylene 17.02 17.35 17.78Butadiene 5.20 5.10 5.00Benzene 5.89 6.00 5.75Toluene 2.59 2.65 2.52

Fuel Oil 3.12 3.35 3.61

Table 1: Effect of Residence Time in Cracking of Typical Light Naphtha (Kolmetz, et al, 2002)

4-pass /

W-coil

0.4 s

Two-pass /

U-coil

(0.2 0.25 s)

Single-pass

coil

(0.08 - 0.12 s)

Hybrid coil

0.2 0.4 s

Figure 1: Types of Coils in Industrial Pyrolysis

3.2 Reaction Temperature

Pyrolysis is a highly endothermic reaction. For a given tube dimension and operating pressure, increase in

reaction temperature drives the reaction to the right to produce smaller hydrocarbon molecules. Over the

last few decades, the operating temperature of pyrolysis furnace has steadily been spread from the region

of 750 850oC to close to 900

oC.

The downside of higher operating temperature is more rapid coking rate and carburization, which

shortens the tube-life. Hence, engineers have continuously worked to developed technology to suppresscoke formation, better metallurgy to sustain the elevated temperatures, as well as, techniques to reduce

carburization. As the initiation step of coking and carburization is the same, i.e., formation of catalytic

coke, recent developments have been collaborative works of both fields of interest.

3.3 Reaction Pressure

Pyrolysis is a gas-phase reaction, which produces more moles of gas molecules for its reaction to the

right. Hence, low operating pressure is favored. Modern furnaces operate under low pressure of 175-240

kPa. Although innovative coil design has reduced coil pressure and contributed better yields, the major

credit goes to improvement in the compressor technology. The coil outlet pressure is indirectly controlled

by the suction pressure of a process gas compressor located downstream.

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The compression is necessary to achieve the high pressures (~3.5 MPa) for separation of hydrogen and

methane. To achieve lower coil outlet pressure, the required compressor horsepower becomes very large.

This becomes an optimization problem between better yield, processing capacity, capital investment and

energy cost. Fortunately, advancement in turbo machinery designs has provided the economic of scale to

construct high horsepower compressor and allowing the furnaces to operate at low pressures.

3.4 Inert

Dilution steam is an inert that premixed with hydrocarbon feed before feeding to the pyrolysis coils. The

early attempts of industrial pyrolysis were carried out without any dilution steam. It was found that tube

carburization rate becomes very rapid. Dilution steam was added then forth to reduce coking and

carburization. The second function of dilution steam is to lower the hydrocarbon partial pressures. This is

to minimize undesirable secondary reactions and higher ethylene yield as shown by Schutt.

In industrial pyrolysis, the mass ratio of steam to hydrocarbon feed is a controlled parameter in furnace

operations. This ratio varies from 0.3 for ethane feed to 0.6 for gas oil cracking. The general rule of

thumb is that less dilution steam is required for smaller hydrocarbon molecules. In true operations, higher

ratio may be employed if the hydrocarbon feed is less than rated so as the bulk residence time remains

low as desired.

26.0

27.0

28.0

29.0

30.0

31.0

32.0

33.0

34.0

810 830 850 870 890

Coil Outlet Temperature (oC)

EthyleneYield(Wt%)

200 kPa

220 kPa

240 kPa

26.0

27.0

28.0

29.0

30.0

31.0

32.0

33.0

34.0

35.0

810 830 850 870 890

Coil Outlet Temperature (oC)

EthyleneYield(Wt%)

S/H 0.6

S/H 0.5

S/H 0.4

Figure 2: Effect of coil out temperature, pressure and steam-to-hydrocarbon ratio to ethylene yield in

naphtha cracking

4 MATHEMATICAL MODEL4

The steam cracking process can be mathematically described using the following fundamental balanceequations.

Material balance for component -j:

( ),j j

j

i

dw Mr S i j

dz G= (17)

where wj = mass fraction of component j, kg/kg

z = length along coil, m

Mj = molecular weight of component j, kg/kmol

G = mass flux, kg/m2.s

ri = reaction rate, kmol/m3

.sS = stoichiometric constant of component jin reaction I

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Energy balance:

( )4

Tubeside :p i i o e

i

dTG C r H U T T

dz D= + (18)

( ) ( ) ( ) ( )4 4 4 41 2Fireboxside : o e w e g e g eU T T C T T C T T U T T = + + (19)

wherepC

= process gas specific heat, kcal/kg.K

T = process gas bulk temperature, K

Hi = heat of reaction for reaction i, kcal/kmol

D = inside tube diameter, m

Uo = overall inside heat transfer coefficient, kcal/s.m2.K

comprising - tube wall thermal conductivity

- coke resistance`

- fouling coefficient

- process gas heat transfer` coefficient

- tube inside radiation

` Te = outside tube wall temperature, K

Tw = refractory wall temperature, K

Tg = flue gas temperature, KU = flue gas convective heat transfer coefficient, kcal /s.m

2.K

C1 = constants containing emissivity factors, kcal /s.m2.K

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C2 = factors and Stefan-Boltzmann constant

Mechanical energy balance:2 2

2

2dP G d fG

dz dz gDg

= (20)

where P = pressure, kgf /m2

= process gas density, kg/m3

G = dimensional constant, kg.m /kgf.s2f = friction factor

In order to solve these equations, an initial guess of product spectrum, temperature and pressure profile

needs to be made. This initial guess, especially the product spectrum can be made based on past data or

using simple empirical relationships. Then, using iterations and convergence criteria, the equations can be

solved rigorously using high-speed computers.

5 COKE FORMATION

Coke formation is a severe problem in industrial pyrolysis. A progressive fouling of the internal walls of

pyrolysis coil and on surface of quench exchanger takes place during the running time, due to presence ofunsaturated species and depending on the operating conditions. Four main consequences of coking

process can be singled out in cracking furnaces:

The external tube skin temperature continuously rises and can reach its maximum allowable value.

This fact can limit the on-stream time of the unit.

The pressure drop increases with the running time and can influence the process selectivity.

The furnace thermal efficiency is progressively reduced.

The reaction volume progressively declines.

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5.1 Classification of Coke

Although there are differing schools of thought on coke formation mechanisms, it is generally accepted

that coke is formed through two separate reaction pathways, i.e. catalytic step and pyrolytic step. The two

types of coke are formed during different age of the coil run and hence, exhibit different characteristic.

Catalytic coke is formed by the dehydrogenation of the hydrocarbon with catalysts of metal components

on the surface of the reactor tube. Tiny particles of iron and nickel leave the tube surface as they absorbcarbon (from CH- radicals) on the cooler side of the particle and deposit it on the hotter side. The

deposition is in the form of hollow tubules, which grow in length. This form of carbon has a lot of open

space between tubules and is a good insulator. These cokes are very hard and not easy to remove by

decoking. Catalytic coking is coupled with carburization because the carbon deposit displaces the metal

particles and dissolves into the metal solution under high temperature.

(a) Filamentous coke (b) Coil-type coke

Figure 3: Catalytic Coke (SK Corporation, 2003)

Pyrolytic coke is soft and can be divided into two, gaseous and condensation coke. The gaseous coke isformed by the dehydrogenation of light olefinic hydrocarbon like acetylene and the condensation coke is

formed by condensation /polymerization /dehydrogenation of heavy aromatic compounds. The decoking

of pyrolytic coke can be easier than that of catalytic coke. Pyrolytic coke has various types and it is very

difficult to define the shape. It is classified into globular, black mirror, fluffy and amorphous type

according to the morphology and classified into gaseous and condensation coke according to the nature of

the hydrocarbon precursor.

Figure 4: Pyrolytic Coke (SK Corporation, 2003)

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5.2 Coke Formation Mechanism

The primary reactions involved in the formation of coke on the metal surface of radiant tubes and quench

section are as follows:

4 3* *CH CH H+ (21)

CH 3 2* *CH H+* (22)

2* 2 2CH CO 2HH O ++ (23)

22 2CO + H O CO H+ (24)

C2 2

*2H CH C+ (25)

2CO + CO CO C+ (26)

22 2

1

2

O H+ H O (27)

2* *H HS + HS

H

(28)

Fresh from a decoke, elements at the internal tube and outlet piping surfaces are in a highly oxidized

state, namely Fe2O, MnO, Cr2O, NiO, etc. The oxides act as catalytic sites for dehydrogenation. There are

also considerable absorbed O2 and H2O absorbed on these surfaces. Experimental data showed that

equations (22) and (26) are kinetic rate limited while the remaining reactions are at or very near equilibria

at the metal surface.

As the run progresses with time, the O2and H2O partial pressure at the metal surfaces decrease. The rate

of reduction of the highly oxidized elements at the surface decreases and the coke already formed

provides a diffusion barrier, which inhibits the transport of H2O to the surface. As a result, the right side

of reaction (23) is less favored because of lower H2O partial pressure. CO production and C2H2decomposition decreases consequently. The ratio of CO to CO2 increases per equation (24) and this

favors the carbon reversion reaction.

Ultimately, the H2O partial pressure at the metal surface is dominated by the rate of diffusion of H 2O

through the coke layer. If the operating conditions are held constant, the coking rate will decrease with

time.

5.3 Decoking Methods

Whenever a pyrolysis run is terminated, the furnace needs to be decoked to remove the coke layer on the

tube wall and quench exchanger surface. In modern industrial pyrolysis, decoking is carried out by

feeding a mixture of air and steam into the pyrolysis coils and fire the furnace to a temperature higher

than the normal cracking temperature (~ 880-900oC). The primary reaction is the water gas shift reaction,

similar to those in coal gasification.

H C 2 2OO + C + (29)

In some prior decoking process, hydrogen is admixed with steam before feeding to the coils. An earlier

thought was that addition of hydrogen would promote methanation and drives reaction (29) to the

extreme right. However, in practice this helps very little to hasten the gasification reaction. This method

is no longer in use.

2 4 2CO + 3H CH + H O (30)

On the contrary, at the quench exchanger surface, high H2O partial pressure will actually inhibit thegasification of coke. Hence, an air/steam mixture is unable to effectively decoke the quench exchangers.

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Typically, towards the end of decoking the pyrolysis coils, the air/steam ratio is increased to allow more

air into the furnace. This helps to remove the coke on the quench exchanger surface more effectively.

Some modern furnaces exhibit an injection nozzle where air is fed directly to the quench exchangers for

air-only decoke.

6 RECENT DEVELOPMENTS

Modern pyrolysis furnaces have evolved a great way compared to the early furnace designs. Most

industrial furnaces built in the last decade can achieve thermal efficiency up to 98%. High olefins yield

has been achieved with novel coil designs, which offer very short residence time. Efficient burners have

also been designed to operate at low excess air to save fuel but still meet the NO xemission limits. The

greatest challenge for engineers today is to improve the on-stream factor by reducing the coke formation

and to extend furnace life between tube replacements. The following are examples of technologies

developed in recent years.

6.1 Chemical Treatment

Sulfur-based compounds e.g. mercaptan, dimethyl sulfide and dimethyl disulfide, have been traditionally

dosed into the pyrolysis coils after a fresh decoke cycle. These sulfur compounds converts the metaloxide sites on the tube wall surfaces into metal sulfides. Although the primary aim is to reduce

carburization rate, it also reduces catalytic coking.

Sulfur treatment has limitations as the metal sulfides layer tends to be destroyed by flaking or even

liquefied in the case of nickel sulfide. Other chemical additives for the same objective are aqueous salt of

IA and IIA metals, as well as proprietary silicon and phosphorus-based compounds. Each works on the

same principle of forming a layer of diffusion barrier. By forming this barrier, catalytic coke is reduced.

These techniques are not widely used, as they are relatively expensive.

6.2 Tube Coating

Tube coatings practice the same principle of diffusion barrier but the pyrolysis coils are pre-coated duringmanufacture instead of online chemical dosing. These are typically glass ceramic coatings onto the tube

walls.

A variant of this technology had a very successful campaign in a Canadian ethylene plant. The on-stream

time was improved from an average 33 days to over 500 days before decoking was required. This

technology has yet to be commercialized.

6.3 Metallurgy

Interesting development in the field of novel tube design has been achieved in recent years. Two fine

examples are the development of cast-finned tube (Tillack et al, 1998) and tube with mixing element

welded on the internal walls (Kubota Corporation, 2003). Both designs aimed to improve the heat andmass transfer by novel geometry of the tube internals. In the case of cast finned tubes, very short

residence time can be achieved and hence, improving the selectivity for ethylene production. With

regards to tubes with mixing element, due to the superior heat transfer, the tube metal temperature is

lower for a desired gas temperature. This saves fuel, promoting a longer run length and increasing the

tube-life as carburization is reduced.

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Figure 5: Cast finned tube (Tillack, 1998) Figure 6: Tube with mixing element (Kubota, 2003)

7 CONCLUSIONS

Pyrolysis furnace is an active research area for improved yields, increased capacity and fuel reduction.

Pyrolysis is a series of free radicals reactions utilized in olefins production. High olefin yields are favored

by short residence time, high temperature and low hydrocarbon pressure. Steam is an inert added to

reduce carburization and the hydrocarbon partial pressure. The reaction product spectrum can be

estimated using empirical methods, rigorous solutions or a combination of both. Coke formation is a

challenge to engineers trying to improve the furnace on-stream factor. Several new technologies havebeen developed to mitigate the coke formation challenge.

8 REFERENCES

1. Rice, F.O. and Herzfeld, K.F.,J. Am. Chem. Soc., 56, 284 (1934).

2. Jacob, S.M., Gross, B., Voltz, S.E., and Weekman, V.W.,A.I.Ch.E. J., 22, 701 (1976).

3. Kivlen, J.A., US Patent 4,904,604 (1990).

4. Dente, M., Ranzi, E. and Goossens, A.G., Computer & Chemical Engineering, 3, 61 (1979).

5. van Goethem, M.W.M., Kleinendorst, F.I., van Leeuwen, C. and van Velzen, N., Computer &

Chemical Engineering, 25, 905 (2001).

6. Shutt, H.C., Chem. Eng. Prog., 55(1), 68 (1959).

7. Zdonik, S.B., Hallee, L.P. and Green, E.J.,Manufacturing Ethylene, reprinted from TheOil & Gas

Journal, Petroleum Publishing Co., Tulsa, Oklahoma (1970).

8. Bowen, C.P. and Brewer, J.R., US Patent 5,151,158, assigned to Stone & Webster Engineering

9. Corporation (1992).

10. Kolmetz, K, et al.,Refining Technology Conference, Dubai, UAE (2002).

11. Bozzano, G., Dente, M, Faravelli, T. and Ranzi, E.,Applied Thermal Engineering, 22, 919 (2002).

12. SK Corporation, URL: http://eng.skcorp.com/product/pycoat/technology/ethylene_plant.html, viewed

on 13 February 2003.

13. Kivlen, J.A., Struth, B.W. and Weiss, C.P., US Patent 3,641,190, assigned to Esso Research and

Engineering Company (1972).

14. Kivlen, J.A. and Koszman, I., US Patent 3,557,241, assigned to Esso Research and Engineering

Company (1971).15. Sliwka, A, US Patent 4,420,343, assigned to BASF Aktiengesellschaft (1983).

16. Gandman, Z, US Patent 6,228,253 B1 (2001).

17. Grossman, D.G., US Patent 6,358,618 B1, assigned to Corning Incorporated (2002).

18. Benum, L., Spring 2002 AIChE National Meeting, New Orleans, LA, USA (2002).

19. Tillack, D.J. and Guthrie, J.E.,Nickel Development Institute Technical Series,# 10,071 (1998).

20. Kubota Corporation, URL: http://www.kubota.co.jp/infra/sc-j/mert/mert-e.html, viewed on 26

February 2003.

21. Froment, G.F. and Bischofff, K.B., Chemical Reactor Analysis and Design, 2nd

Ed., John Wiley &

Sons, Inc (1990).

22. Kivlen, J.A., US Patent 3,579,601, assigned to Esso Research and Engineering Company (1971).

23. Kniel, L., Winter, O. and Stork, K., Ethylene Keystone to the Petrochemical Industry, Marcel

Dekker, Inc (1980).

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http://eng.skcorp.com/product/pycoat/technology/ethylene_plant.htmlhttp://www.kubota.co.jp/infra/sc-j/mert/mert-e.htmlhttp://www.kubota.co.jp/infra/sc-j/mert/mert-e.htmlhttp://eng.skcorp.com/product/pycoat/technology/ethylene_plant.html

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Pyrolysis-Furnace-Rev-1.pdf