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Steam Cracking of Hydrocarbons
The most important olefins and diolefins used to manufacture petrochemicals are ethylene,
propylene, butylenes, and butadiene. They are mostly produced by steam cracking .
The feedstocks for steam cracking units range from light paraffinic hydrocarbon gases to
various petroleum fractions and residues.
The cracking reactions are principally bond breaking, and a substantial amount of energy is
needed to drive the reaction toward olefin production.
The simplest paraffin (alkane) and the most widely used feedstock for producing ethylene is
ethane. Cracking ethane can be visualized as a free radical dehydrogenation reaction,
where hydrogen is a coproduct:
The reaction is highly endothermic, so it is favored at higher temperatures and lower
pressures. Superheated steam is used to reduce the partial pressure of the reacting
hydrocarbons . Superheated steam also reduces carbon deposits that are formed by the
pyrolysis of hydrocarbons at high temperatures
For example, Pyrolysis of ethane produces carbon and hydrogen:
Ethylene can also pyrolyse in the same way. Additionally, the presence of steam as a diluent
reduces the hydrocarbons’ chances of being in contact with the reactor tube-wall. Deposits
reduce heat transfer through the reactor tubes, but steam reduces this effect by reacting
with the carbon deposits (steam reforming reaction).
Many side reactions occur when ethane is cracked. A probable sequence of reactions
between ethylene and a formed methyl or an ethyl free radical could be represented:
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Propene and l-butene, respectively, are produced in this free radical reaction. Higher
hydrocarbons found in steam cracking products are probably formed through similar
reactions. When liquid hydrocarbons such as a naphtha fraction or a gas oil are used to
produce olefins, many other reactions occur. The main reaction, the cracking reaction,
occurs by a free radical and beta scission of the C-C bonds. This could be represented as:
The newly formed free radical may terminate by abstraction of a hydrogen atom, or it may
continue cracking to give ethylene and a free radical. Aromatic compounds with side
chains are usually dealkylated. The produced free radicals further crack to yield more
In the furnace and in the transfer line exchanger, coking is a significant problem. Catalytic
coking occurs on clean metal surfaces when nickel and other transition metals used in
radiant tube alloys catalyze dehydrogenation and formation of coke. Coke formation
reduces product yields, increases energy consumption, and shortens coil service life.
Coking is related to feedstock, temperature, and steam dilution. The radiant tubes
gradually become coated with an internal layer of coke, thus raising the tube metal
temperature and increasing pressure drop through the radiant coils. When coke reaches an
allowable limit as indicated by a high pressure drop, it should be removed.
Steam Cracking Process
A typical ethane cracker has several identical pyrolysis furnaces in which fresh ethane feed
and recycled ethane are cracked with steam as a diluent. Figure 1 is a block diagram for
ethylene from ethane. The outlet temperature is usually in the 800°C range. The furnace
effluent is quenched in a heat exchanger and further cooled by direct contact in a water
quench tower where steam is condensed and recycled to the Pyrolysis furnace. After the
cracked gas is treated to remove acid gases, hydrogen and methane are separated from the
pyrolysis products in the demethanizer. The effluent is then treated to remove acetylene,
and ethylene is separated from ethane and heavier in the ethylene fractionator. The
bottom fraction is separated in the deethanizer into ethane and C
fraction. Ethane is then recycled to the pyrolysis furnace.
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Figure 1, Block diagram for producing ethylene from ethane
The important process variables are reactor temperature, residence time, and
steam/hydrocarbon ratio. Feed characteristics are also considered, since they influence the
Steam cracking reactions are highly endothermic. Increasing temperature favours the
formation of olefins, high molecular weight olefins, and aromatics. Optimum temperatures
are usually selected to maximize olefin production and minimize formation of carbon
Reactor temperature is also a function of the feedstock used. Higher molecular weight
hydrocarbons generally crack at lower temperatures than lower molecular weight
compounds. For example, a typical furnace outlet temperature for cracking ethane is
approximately 800°C, while the temperature for cracking naphtha or gas oil is about 675–
In steam cracking processes, olefins are formed as primary products. Aromatics and higher
hydrocarbon compounds result from secondary reactions of the formed olefins.
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Accordingly, short residence times are required for high olefin yield. When ethane and light
hydrocarbon gases are used as feeds, shorter residence times are used to maximize olefin
production and minimize BTX and liquid yields; residence times of 0.5–1.2 sec are typical.
Cracking liquid feedstocks for the dual purpose of producing olefins plus BTX aromatics
requires relatively longer residence times than for ethane. However, residence time is a
compromise between the reaction temperature and other variables.
A higher steam/hydrocarbon ratio favours olefin formation. Steam reduces the partial
pressure of the hydrocarbon mixture and increases the yield of olefins. Heavier
hydrocarbon feeds require more steam than gaseous feeds to additionally reduce coke
deposition in the furnace tubes.
Liquid feeds such as gas oils and petroleum residues have complex polynuclear aromatic
compounds, which are coke precursors. Steam to hydrocarbon weight ratios range between
0.2–1 for ethane and approximately 1–1.2 for liquid feeds.
Feeds to steam cracking units vary appreciably, from light hydrocarbon gases to petroleum
residues. Due to the difference in the cracking rates of the various hydrocarbons, the
reactor temperature and residence time vary. As mentioned before, long chain
hydrocarbons crack more easily than shorter chain compounds and require lower cracking
For example, it was found that the temperature and residence time that gave 60%
conversion for ethane yielded 90% conversion for propane.
Feedstock composition also determines operation parameters. The rates of cracking
hydrocarbons differ according to structure. Paraffinic hydrocarbons are easier to crack than
cycloparaffins, and aromatics tend to pass through unaffected. Isoparaffins such as
isobutane and isopentane give high yields of propylene. This is expected, because cracking
at a tertiary carbon is easier:
As feedstocks progress from ethane to heavier fractions ,the yield of ethylene decreases,
and the feed per pound ethylene product ratio increases markedly. Table 1 shows yields
from steam cracking of different feedstocks, and how the liquid by-products and BTX
aromatics increase dramatically with heavier feeds.
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Cracking Gas Feeds
The main gas feedstock for ethylene production is ethane. Propane and butane or their
mixture, LPG, are also used, but to a lesser extent. They are specially used when coproduct
propylene, butadiene, and the butenes are needed. The advantage of using ethane as a feed
to cracking units is
a high ethylene yield with minimal coproducts. For example, at 60% per pass conversion
level, the ultimate yield of ethylene is 80% based on ethane being recycled to extinction.
Ultimate yields from steam cracking various feedstocks
The following are typical operating conditions for an ethane cracking unit and the products
Propane cracking is similar to ethane except for the furnace temperature, which is relatively
lower. However, more by-products are formed than with ethane, and the separation section
is more complex. Propane gives lower ethylene yield, higher propylene and butadiene
yields, and significantly more aromatic pyrolysis gasoline. Residual gas (mainly H
methane) is higher that produced from ethane. Increasing the severity of a propane cracking
unit increases ethylene and residual gas yields and decreases propylene yield. Figure 2
shows the influence of conversion severity on the theoretical product yield for cracking
Cracking n-butane is also similar to ethane and propane, but the yield of ethylene is even
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Figure 2, The influence of conversion severity on the theoretical product yield for the
cracking of propane. Acetylene, methyl acetylene, and propadiene are hydrogenated and
both ethane and propane are recycled to extinction (wt%)
Cracking Liquid Feeds
Liquid feedstocks for olefin production are light naphtha, full range naphtha, reformer
raffinate, atmospheric gas oil, vacuum gas oil, residues, and crude oils. The ratio of olefins
produced from steam cracking of these feeds depends mainly on the feed type and, to a
lesser extent, on the operation variables. For example, steam cracking light naphtha
produces about twice the amount of ethylene obtained from steam cracking vacuum gas oil
under nearly similar conditions. Liquid feeds are usually cracked with lower residence times
and higher steam dilution ratios than those used for gas feedstocks. The reaction section of
the plant is essentially the same as with gas feeds, but the design of the convection and the
quenching sections are different. This is necessitated by the greater variety and quantity of
coproducts. An additional pyrolysis furnace for cracking coproduct ethane and propane and
an effluent quench exchanger are required for liquid feeds. Also, a propylene separation
tower and a methyl acetylene removal unit are incorporated in the process. Figure 3 is a
flow diagram for cracking naphtha or gas oil for ethylene production.
As with gas feeds, maximum olefin yields are obtained at lower hydrocarbon partial
pressures, pressure drops, and residence times.
One advantage of using liquid feeds over gas feedstocks for olefin production is the wider
spectrum of coproducts. For example, steam cracking naphtha produces, in addition to
olefins and diolefins, Pyrolysis gasoline rich in BTX.
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Table 2 shows products from steam cracking naphtha at low and at high severities. It should
be noted that operation at a higher severity increased ethylene product and by-product
methane and decreased propylene and butenes. The following conditions are typical for
Steam cracking raffinate from aromatic extraction units is similar to naphtha cracking.
However, because raffinates have more isoparaffins, relatively less ethylene and more
propylene is produced.
Cracking gas oils are not as desirable feeds for olefin production as naphtha because they
have higher sulfur and aromatic contents. The presence of a high aromatic content in the
feed affects the running time of the system and the olefin yield; gas oils generally produce
less ethylene and more heavy fuel oil.
Products from steam cracking naphtha at high severities
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Figure 3, Flow diagram of an ethylene plant using liquid feeds.
Processes used to crack gas oils are similar to those for naphtha.
However, gas oil throughput is about 20–25% higher than that for naphtha. The ethylene
cracking capacity for AGO is about 15% lower than for naphtha. Table 3 shows the product
composition from cracking AGO and VGO at low and high severities.
Production of Diolefins
It is known that diolefins are more stable than monoolefins and have different reactivities.
The most important industrial diolefinic hydrocarbons are butadiene and isoprene.
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Product composition from cracking atmospheric gas oil and vacuum gas oil
= CH-CH = CH
Butadiene is the raw material for the most widely used synthetic rubber, a copolymer of
butadiene and styrene (SBR). In addition to its utility in the synthetic rubber and plastic
industries, many chemicals could also be synthesized from butadiene.
Butadiene is obtained as a by-product from ethylene production. It is then separated from
fraction by extractive distillation using furfural. Butadiene could also be produced by
the catalytic dehydrogenation of butanes or a butane/butene mixture.
The first step involves dehydrogenation of the butanes to a mixture of butenes which are
then separated, recycled, and converted to butadiene.
Figure 4 is the Lummus fixed-bed dehydrogenation of C
mixture to butadiene. In the
process, the hot reactor effluent is quenched, compressed, and cooled. The product mixture
is extracted: unreacted butanes are separated and recycled, and butadiene is recovered.
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Figure 4, Flow diagram of the Lummus process for producing butadiene.
(1) reactor, (2) quenching, (3) compressor, (4) cryogenic recovery,
(5) stabilizer, (6) extraction.
The Phillips process uses an oxidative-dehydrogenation catalyst in the presence of air and
steam. The C
mixture is passed over the catalyst bed at 900 to 1100°C. Hydrogen released
from dehydrogenation reacts with oxygen, thus removing it from the equilibrium mixture
and shifting the
reaction toward the formation of more butadiene.
Isoprene (2-methyl 1,3-butadiene) is the second most important conjugated diolefin after
butadiene. Most isoprene production is used for the manufacture of cis-polyisoprene,
which has a similar structure to natural rubber. It is also used as a copolymer in butyl
There are several different routes for producing isoprene. The choice of one process over
the other depends on the availability of the raw materials and the economics of the selected
process, among them are: