Naphtha Cracking

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Naphtha Cracking
Thermal cracking is well known and widely accepted technology for olefin
production. This technology is also called steam cracking, since steam is added to
hydrocacbon before cracking to reduce the parial pressure of hydrocacbon and to
produce a better yield performance. In the petrochemical industry, steam cracking
is a core technology for producing olefins, although there are alternative routes
from off-gas of fluid catalvtic cracking units in oil refineries or by
dehydrogenation or prepane or outanes.
Thermal cracking reactions are bisically uses to break the C – C bonds of
hydrocacbons non-catalytically at high temperature of around 800 – 900
0
C and at a
low pressure of 0.16 – 0.2Mpa in the coils located in the radiant sections of
furnaces. They finally produce lower molecular weight olefins. In including radical
reactions, whereas gas oil cracking is more complex with more than 3000
reactions.
In industrial practie, non-reaced ethane or propane is separated in the cold
separation sestion and recycled back to cracking furnaces where ethane and
propane are cracked again. Therefore, the ultimate yields of olefins are much
higher than those of once-through yields.
I.1.1. Petrochemical complex in Japan.
Naphtha is used asa base feedstock for petrochemical complexes in Japan. LPG
is also used as an alternative feedstock, but accounts for about 60% of such
feedstock and the rest of the feedstock are LPG and gas oil. However, in the
United States, ethane and LPG separated from natural gas are major feedstock,
which are for 70 – 80% of total demand, while the rest are naphtha and gas oil.
Based on naphtha cracking, the product pattern consists of ethylene at 28%,
propylene at 17%, butene and butadiene products at 11%, off-gas and pyrolysis
heavy oil at 24% and pyrolysis gasoline at 20%.

A typical petrochemical complex consists of an ethylene plants such as
polyolefin and aromatic plants using olefin products. The configuration of the
complex depends on the final product types and the available feedstock. A
complex based on naphtha-cracking ethylene plants is more elaborate than a
complex based on cracking gases such as ethane or propane.
Some 60 – 70% of pyrolsis gasoline fractions consist of aromatic such as
benzene, toluene and xylenes called a BTX fraction. These are recovered by
extractive separations at 12 – 14% based on the feedstock rate. However, in the
recent product pattern, the BTX extraction rate tends to decrease due to advances
in cracking technology and a parafinic and lighter naphtha used as the feedstock. In
the 1970s, about 50% of BTX demand was supplied from ethylene plants;
however, in the 1990s, this has decreased to 40%. The rest of BTX demand is
supplied from catalytic naphtha reforming plants.
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I.1.2Cracking furnace for naphtha.
Feed naphtha is preheated at the converstion section of a cracking furnace and
introduced to the radiation coils, togerther with dilution steam when the naphtha is
to thermally cracked into olefin fractions.
I.1.2.1. Type of Cracking furnace.
As shown in Figure .. for a typical configuration of the cracking furnace, it is
generally designed as a fire-box type which can be grouped into several sub-types
with regard to radiant coils and burner types. In particular, with increasing capacity
of the cracking furnace and requirement of high severty operation, the accurate
control of heat flux in the radiant coils is important. This is attained by using both
floor burners and either radiant wall or stage burners. Fuel gas is mainly used for
firing, but fuel oil is also used in some cases. Cracking reactions ocurs in the
radiant coils, and the convection section is used for heat recovery by feed
preheating, steam superheating and boiler feed-water preheating. To avoid
overcracking of reacted gas, tranfers line exchanges for rapid cooling are installed

just at the exit of the radiant coil.
The radiant coils are generally located in a single row. The burners are placed
on the floor and side wall, or only on the floor. A long-flame type is used for the
floor burners, and the flame pattern is formed upward in parallel with the radiant
coils. The side wall burners are generally the radiant wall type and are located at
several stages every 1–2 metres. Several sets of side wall burners with long and
oval flame may also be installed at the terrace of the radiant wall. The use of side
wall burners makes it possible to achieve an accurate control of the heat flux over
the radiant coils.
I.1.2.2. Radiant Tube and Coil.
In the early days, horizontal straight radiant coils were used, connected with
bends at their ends. A long with the temperature rose higher. It caused the
defiection of coils so that it was difficult to support the coils horizontally. The coil
was therefore set vertically and suspended from the ceiling.
Obtaining better olefin yields requires a short residence time, so coil diameter
has been reduced and coil length shortened. The inner diameter of the coil has been
reduced to 40 – 50 mm and the length is now about 10 – 20 metres. Typical coil
arrangements are shown in Figure..
With respect to coil materials, HK- 40 or Incoloy 800 was previously used.
With the increase in the severity of operating conditions, the coil skin temperature
has risen higher to about 1100
0
C. Keeping coil life longer reqiures carburization-
resistance material, and high chomium and high nickel materials with the addition
of tungsten, molybdenum or nicbium have been developed ( refer to Table..). The
radiant coils are mainly manufactured by a centrifugal casting divots. This cause
the acceleration of carburization. So machining on the inner surface requires
removal of casting divots, particularly on the surface of the exit part operated at the
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highest temperature. Wrought coils have been developed to overome this casting
defeet.
I.1.3. TREATMENT OF A CRACKED GAS.
An ethylene process consists of a hot section, meluding cracking furnace, a heat
recovery of cracked gas, and a cold section to separate into ethylene, propyiene and
other olefin products.
The hot section generally consists of the units shown in Figure… Cracked gas is
quenched by a series of transfer line exchangers to recover heat and to terminate
cracking reactions. The exchangers generate high-presure steam ( about 10Mpa
and 500
0
C). Cracked gas is further cooled down in the oil quench tower and the
water quench tower, where several levels of heat are recovered. The gas from the
top of the water quench tower enters the four to six stage cracking gas compressor
to pressurize from 40 – 50kPa to 3 – 3.5 Mpa. In the compression stages H
2
S and
CO
2
are removed from cracked gas by treating caustic soda. The gas is then dried
and sent to the cold section, which can be divided into two configurations: the
front-end demethanizing system and the front-end depropanzing system. These
systems are shown in Figures……
Acetylene as the by-product is a catalyst poison for downstream polyethylene
productions, so it needs to be removed either by hydrogenation or absorption.
Acetylene concentration is up to about 1.5% by volume in an ethylene and ethane
mixture in the front-end demethanizing system. Accordingly, controlling the
temperature on hydrogenation is relatively difficult. However, the recent advances
on the catalyst improve the reaction performance such as the selectivity of
acetylene hydrogenation. This hydrogenation system is called back-end

hydrogenation because the reactor is sited after the dementhannizer. To moderate
the reaction and increase selectivity, carbon monxide may be added the moderator.
However, if high-purity ethylene is required, carbon monoxide is sometimes not
used.
On the other hand, in the front-end demethanizer system, the hydrogenation
reactor is sited before the demethanizer, and this system is called front-end
hydregenation. In this case, the acetylene content is relatively low and the
hydrogenation reaction is milder than that of the back-end hydrogenation.
Hydrogen is contained in the cracked gas, so an additional supply of hydrogen is
usually not necessary. Carbon monoxide is also contained in the gas and may
improve reaction selectivity. However, the reaction conditions are subject to
change due to the furnace operations and hence the cracked gas compositions.
I.1.4. QUENCH AND HEAT RECOVERY.
The purposes of the quencher are to terminate the cracking reactions and to
prevent the formation of the heavy materals by polymerization, and to recover
energy at a high temperature. Generally, cracked gas is quenched by the transfer
line exchangers, which are directly connected to the radiant coil exit and generate
high pressure steam.
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In th case of naphtha or gas oil feedstock, cracked gas may be condensed in the
exchanger if the quench temperature is too low. The condensed liquid will
accelerate the fouling rate. Therefore, the quench temperature is kept higher and
the heat recovery is limited.
In the case of ethane or propan cracking, as less heavier materials are formed by
cracking reactions, it is possible to lower the condensing temperature. As the
fouling rate may be suppressed, heat is recovered effectively in the transfer line
exchangers. Therefore, an oil quench tower is usually not required.
Cracked gas from the transfer line exchangers then enters the oil and water
quench towers, and further heat is recovered. At the oil quench tower, cracked gas

is cooled down from about 350
0
C to 100
0
C by direct contact with quench oil.
Quench oil is supplied at about 100
0
C, and the quench oil temperature is controlled
at the bottom of the tower to below about 190
0
C to avoid polymerization.
However, for heat recovery, a high temperature is more efficient. If the
temperature is too high, it tends to accelerate polymerization. Therefore, an
optimal temperature should be selected such as 190
0
C. Heat is recoverd by
generating the dilution steam required for cracking. The purpose of dilution steam
is to reduce the partial pressure of hydrocacbons and to facilitate cracking
reactions. The heat is also used as reboiler heat in downstream separators. At the
water quench tower, heat is recovered at the relatively low temperature of 80 –
90
0
C and is used mainly as reboiler heats for propylene and propane and other
similar separations.
In order to get better olefin yields, the pressure of the cracking reactions is kept
as low as possible. On the other hand, the lower suction pressure of the cracked gas
compressor requires more power. Therefore, the coil outlet pressure can be
lowered only by reducing the pressure drop between the coil outlet and the suction
of the cracked gas compressor. It is important to minimize pressure drops in the
transfer line exchangers, the oil quench tower and the water quench tower, for high

olefin yields. The coil outlet pressure is mostly designed at 0.16 – 0.2 Mpa.
Figure… shows typical types of tranfers line exchangers, which can be divided
into a double-tube exchanger type and a shell-and-tube exchanger type. Generally,
the double-tube exchangers are followed by a shell-and-tube exchanger. During
operation, fouling of the transfer line exchanger tubes will gradually increase and
energy efficiency will drop accordingly. Therefore, easy cleaning of exchanger
tubes is needed. The double-tube type has the advantage of easy cleaning, but the
shell-and-tube type can also be mdified for easier cleaning. Other design criteria
are intended to release heat stress, prevent erosion or maintain a low pressure drop.
I.1.5. THERMODYNAMICS OF THERMAL CRACKING REACTIONS.
Cracking reactions break the C – C bond of hydrocacbon at a high temperature
(800 ÷ 900
0
C) and low pressure (0.16 – 0.2Mpa) non-catalytically.
A large positive value of the Gibbs free energy means that a system is not stable
thermodynamically. According to the standard Gibbs energy of formation per
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carbon atom for various materials as shown in Figure…, each hydrocacbon tends
to be cracked down into hydrogen and carbon atoms.
Therefore, cracking reactions should be terminated by rapid quench control, and
residence time kept short to avoid overcracking. This gives higher yields can be
predicted quantitatively by a simulator based on the dynamic reaction model for
many elementary reactions. But, the following can be explained qualititaitvely by
reforring to Figure….
As the cracking temperature increases:
(1) pyrolysis of paraffins proceeds.
(2) More ethylene than propylene is produced.
(3) More acetylene is produced.
(4) More hydrogen and carbon deposition are produced,and

(5) Bezene and naphthalene production tends to increase gradually.
I.1.6. MECHANISM OF THERMAL CRACKING.
Components meluded in cracking reactions are so many and reaction paths are
also so compheated that the… theoretically yet. However, Rice and Herzfield first
imtrojuced a free radical chain reaction machanism. The mechanism has been
modified several times by many researchers and is widely accepted as the basis for
explaining the mechanism of the cracking reactions.
I.1.6.1. Free radical chain reaction Mechanism.
Free radical chain reactions can be divided into three reaction steps, namely
initiation, propagation and termination.
(1) Inititation.
The reaction is initiated thermally, and pyrolysis initiation of a higher paraflin is
caused by homolytic cleavage of a carbon – carbon bond and a carbon – hydrogen
bond. It forms free radicals into the reaction system.
(a) Cleavage of carbon – carbon bond
C
n+m
H
2m+2n+2
C
n
H
0
2n+1
+ C
m
H
0
2m+1
(b) Hydrogen transfer between paraffins and olefins

C
n
H
2n+2
+ C
m
H
2m
C
n
H
0
2n+1
+
(c) Cleavage of C – H bonds.
C
n
H
2n+2
C
n
H
0
2n+1
+ H
0
Generally, larger moleculer weight compounds have less activation energy
per carbon bonds and can be reacted more easily. C – C bond energy per
molecule is about 300kj/mol, which is less than that of C – H bonds of
420kj/mol. Therefore, C – C bonds are less stable than C – H bonds. The chain

reaction is mainly initiated with the cleavage of the C – C bond that releases
two sets of radicals.
(2) Propagation of free radical chain reactions
(a) Abstraction of hydrogen
C
n
H
0
2n+1
+ C
m
H
2m+2
C
n
H
2n+2
+ C
m
H
0
2m+1
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