Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
36,~
PROCESS OF THERMALLY CRACKING
HEAVY PETROLEUM OIL
Technical Field:
This invention relates to a process of continuously
thermally cracking a heavy petroleum oil.
Prior Art:
Various methods are known for thermally cracking a heavy
petroleum oil and for producing a liquid pitch and a cracked
light oil. For example, United States patent No. 4,477,344
proposes a method of thermally cracking a heavy hydrocarbon oil
with the use of one cracking furnace and two or more perfect
mixing type reactors so as to obtain a pitch suitable as a fuel
and a cracked light oil. This method is advantageous because
the thermal cracking can be conducted in a continuous manner
while effectively preventing the occurrence of coking troubles.
However, this method is not fully satisfactory from the
standpoint of economy and apparatus efficiency because the
method requires the use of two or more perfect mixing type
reactors operated at temperatures which are gradually increased
in the later stages.
United States patent No. 4,581,124 discloses a method in
which a heavy feed stock is thermally cracked with the use of a
combination of one cracking furnace and one tank reactor. In
this method, since the conversion rate in the cracking furnace
is suppressed to a low degree and the thermal cracking is mainly
effected in the tank reactor, coking troubles are apt to occur
in the tank reactor. To prevent the occurrence of coking
troubles in the tank reactor, a mixture of mesophase pitch and
matrix pitch (isotropic pitch) is continuously withdrawn from
the tank reactor and is introduced into a separator for the
sepration of the mixture into the mesophase pitch and the matrix
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pitch. The mesophase pitch thus separated is recovered
while the matrix pitch thus separated is recycled to the
tank reactor to keep the ratio of the matrix pitch to
the mesophase pitch within the rank reactor high. Thus,
the prior art method has a drawback because the
operation in the separating step becomes complicated due
to the necessity for keeping the ratio of the matrix
pitch to the mesophase pitch high.
Obiect:
It is an object of an aspect of the present
invention to provide a process of thermally cracking a
heavy petroleum oil which is devoid of the drawbacks of
the conventional technique and which is free of coking
troubles.
An object of an aspect of the present invention is
to provide a process of thermally cracking a heavy
petroleum oil with the utilization of a combination of
one thermal cracking furnace and one tank reactor while
preventing the occurrence of coking troubles in the tank
reactor.
Constitution:
An aspect of this invention is as follows:
A process of thermally cracking a heavy petroleum
oil characterized in that the heavy petroleum oil is
treated with the use of a combination of one cracking
furnace and one perfect mixing type tank reactor, that
the thermal cracking in the cracking furnace is
performed at a temperature of 450-520C and a pressure
of from ambient pressure to 20 kg/cm2 with a conversion
of 50-80% of the total conversion rate and that the
thermal cracking in the tank reactor is performed at a
temperature of 400-450C and a pressure of 100 mmHg to 5
kg/cm2 for 10-120 minutes with a conversion of not
higher than 50% of the total conversion rate while
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feeding steam in such an amount as to maintain the
partial pressure of hydrocarbons in the range of 100-600
mmHg to produce a liquid pitch having a volatile matter
content of 30-45 wt % and useful as a fuel along with a
gaseous product including cracked light oil.
The present invention will be described in detail
below with reference to the accompanying drawing, in
which the sole FIGURE illustrates a flow diagram for
carrying out the process according to the present
invention.
Examples of the heavy petroleum oils used in the present
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invention include atmospheric and vacuum residues of petroleum
crude oils, various cracking residues, asphalt products from
solvent deasphalting and native natural asphalt.
When such a heavy hydrocarbon oil is subjected to a thermal
cracking treatment in a known manner with the use of a
combination of one cracking furnace and one tank reactor,
mesophase is formed in a large amount. The mesophase tends to
coalesce with each other to grow to large particles. Namely,
carbon is apt to form so that coking troubles are liable to
occur. Upon study by the present inventors, it has been found
that such coking troubles in the tank reactor can be effectively
prevented from occurring by increasing the conversion in the
cracking furnace. When the thermal cracking product obtained in
the cracking furnace with a high conversion rate is treated in
the succeeding tank reactor, the mesophase produced in the tank
reactor is small in size and excellent in dispersibility.
Therefore, precipitation of carbon (coking) hardly occurs and,
hence, coking troubles in the tank reactor are prevented.
The process of the present invention is conducted with the
use of a combination of one cracking furnace and one perfect
mixing type tank reactor. In this process, the feed stock is
thermally cracked in the cracking furnace until at least 50 % of
the total thermal cracking conversion is reachedO
The term "total thermal cracking conversion" used in the
present specification is intended to mean the total conversion
accomplished in the cracking furnace and the tank reactor and is
defined by the following equation:
R = [(A - C)/A]x100 (I)
= ~(A - B)/A + (B - C)/A] x 100 (II)
30 wherein
R: Total thermal cracking conversion (~)
A: The weight of the components in the feed
stock which have a boiling point of at least
538 C
C: The weight of the components contained
in the thermal cracking product which are
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obtained in the tank reactor and which ha~e a
boiling point of at least 538 C
B: The weight of the components contained
in the thermal cracking product which are
obtained in the cracking furnace and which
have a boiling point of at least 538 C
[~A - s)/~]x100: Conversion (%) in the cracking furnace
~(B - C~/A]x100: Conversion (%~ in the tank reactor
The total thermal cracking conversion is selected according
to the kind of the feed stock and the like. Generaly speaking,
the total conversion required for obtaining pitch having
volatile matter content of 30-45 wt % is 65-75 %.
The reaction conditions adopted in the cracking furnace
include a temperature of 450-520 C and a pressure of from
ambient pressure to 20 kg/cm2~ In the cracking furnace, the
thermal cracking is conducted so that the conversion in this
step is at least 50 - 80 %, preferably 60-75 % of the total
thermal cracking conversion. The conversion in the cracking
furnace may be controlled by the control of the reaction
temperature, reaction pressure and residence time.
The reaction conditions in the perfect mixing type tank
reactor involve a reaction temperature of 400-450 C, a reaction
pressure of 100 mmHg to 5 Kg/cm2 and a reaction time of 10-120
min, preferably 20-60 min. The perfect mixing type tank reactor
is operated under a reduced pressure or under a partial pressure
of hydrocarbons of 100-600 mmHg by feeding an inert gas such as
steam. The thermal cracking in the perfect mixing type tank
reactor is conducted so that the conversion does not exceed 50
%, preferably falls within the range of 25-40 % of the total
thermal cracking conversion. The conversion can be controlled
by controlling the reaction temperature, partial pressure of
hydrocarbon and reaction time. When the reaction temperature
and pressure are kept constant, the conversion can be controlled
by control of the reaction time. In the perfect mixing type
tank reactor, liquid pitch and gaseous product including cracked
light oil are produced. The liquid pitch thus produced contain
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mesophase. The amount of the mesophase is relatively small, i.e.
generally 30 vol % or less, especially 15-25 vol ~. The
mesophase has a particle size of 20-50~m and is excellent in
dispersibility in the pitch. ~he mesophase is hardly carbonized
(formation of coke).
The cracking furnace may be, for example, an external
heating type tubular reactor while the tank reactor may be, for
example, of a type equipped with an agitating apparatus within
the reactor. If desired, the tank reactor can be further
provided with a wet wall system or a scraper to keep the inside
wall of the reactor clean. Any known cracking furnace and the
tank reactor may be suitably used for the purpose of the present
inventionO
In the process according to the present invention, a
cracked heavy oil produced in the process may be recycled to the
cracking furnace to increase the overall yield of cracked light
oil. The cracked heavy hydrocarbon oil suitably used for this
purpose has generally a boling point of 340 C or more. The
amount of the cracked oil recycled is 0.1-0.3 part by weight per
one part by weight of the feed stock. Too large an amount of
the recycled heavy cracked oil causes the increase of the cracked
gas and pitch and reduction of total liquid yield.
Referring now to the FIGURE, the feed stock is fed to a
cracking furnace 1 through a line 5. In this case, before being
introduced into the cracking furnace 1, the feed stock may be
mixed with a cracked heavy oil recycled through a line 17 from
the bottom of a ~ractionating tower 4, if desired. The feed
stock which is mixed with the cracked heavy oil is subjected to
a thermal cracking treatment in the cracking furnace 1 and the
resulting cracked product is fed through a line 7 to a perfect
mixing type tank reactor 2 where it is subjected to a further
thermal cracking treatment. To the bottom of the tank reactor 2
is supplied through a line 8 high temperature steam ~with a
temperature of about 400-700 C) which has been fed through a
line 6 and heated in the cracking furnace 1 and, if necessary,
by means of a steam super heater 12. The steam serves to heat a
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liquid pitch contained in the reactor 2 for the further thermal
cracking thereof, to accelerate the stripping of volatile
components from the liquid pitch and to decrease the partial
pressure of hydrocarbons in the space within the reactor 2. The
steam supplied to the reactor 2 is not necessarily high
temperature steam heated by means of a steam super heater 12.
When steam which is not heated by means of the super heater is
supplied to the reactor 2, the steam serves to strip volatile
components from the liquid pitch and to decrease the partial
pressure of hydrocarbons in the space within the reactor 2.
The gas components including the cracked oil produced in
the reactor 2 are introduced into a fractionating tower 4
through a line 10, whereas the liquid pitch obtained in the
reactor 2 is discharged threfrom through a line 9 and introduced
into a pitch cooling drum 3 where the liquid pitch is cooled for
the termination of the reaction. The liquid pitch in the
cooling drum 3 is discharged therefrom through a line 11 and
recovered as a pitch product. The pitch product has a volatile
matter content of 30-45 wt % and a softening point of 160-220 C
and is suitable for use as a fuel pitch.
The gas components introduced into the fractionating tower
4 through the line 10 are fractionated into a cracked gas
discharged through a line 14, a cracked light oil (boling point
of C5-370 C) discharged through a line 15, a cracked heavy oil
25 (boling point of 370-538 C) discharged through a line 16 and a
recycling cracked heavy oil (boiling point of 535 ~C or more)
discharged through a line 17. The cracked heavy oil discharged
through the line 17 is recycled for mixing with the feed stock
to be fed to the cracking furnace 1.
The process shown in the FIGURE can be varied and modified
in various manners. For example, the fractionating tower 4 may
be composed of a combination of two or more fractionating
towers. Further, in stead of directly feeding the feed stock to
the cracking furnace, the feed stock can be previously
introduced into the fractionating tower 4, introducing the
mixture of the feed stock and the cracked heavy oil obtained in
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the bottom of the fractionating tower 4 into the cracking
furnace 1. Furthermore, the cracked heavy oil discharged
through the line 16 may be added to the cracked heavy oil
discharged from the bottom of the tower 4 for mixing with the
feed stock. The recycling of the cracked heavy oil for mixing
with the feed stock is not essential but can be omitted.
The above-described process according to the present
invention uses a combination of one cracking furnace and one
perfect mixing type tank reactor for the thermal cracking
treatment of petroleum heavy hydrocarbon oil and permits one to
continuously obtain a sufficiently lightened, cracked oil and a
pitch which is suitable as a fuel and which contains a volatile
matter in the amount of 30-45 wt ~, without encountering coking
troubles in the tank reactor. In the conventional thermal
cracking treatment of heavy petroleum hydrocarbon oils using a
combination of one cracking furnace and one perfect mixing type
tank reactor, the mesophase produced in the tank reactor tends
to coalesce to form precipitates of carbon, namely tends to
encounter coking troubles. In contrast, the mesophase in the
pitch produced according to the process of the present
invention, in which the thermal cracking conversion rate in the
cracking furnace is made high, has a small particle size, hardly
coalesces, and is excellent in dispersibility in the pitch, so
that precipitation of carbon scarecely occurs.
In addition to the above-described merits, the present
invention has the following advantages:
(1) Since majority of the reaction hea~ may be supplied
from the cracking furnace, the amount of the heating gas medium
can be reduced so that the operation cost can be considerably
reduced.
(2) Since the feed to be subjected to the thermal cracking
treatment in the tank reactor is a product obtained by thermal
cracking of a feed stock at a high conversion rate in a cracking
furnace in which the reaction time is not distributed, it is
possible to reduce the reaction load in the tank reactor.
Therefore, the tank reactor can be constructed into a small
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sized reactor. Moreover, the pitch obtained in the tank reactor
has uniform properties and is low in content of highly
polycondensed components such as mesophase and quinoline
insolubles. Such a pitch when used as binder for the
preparation of metallugical coke exibits excellent properties as
a binder and when used as a fuel is excellent in perfect
combustibility.
The present invention will described in more detail by way
of examples.
Example 1
Using the apparatus as shown in the FIGURE, a heavy
petroleum oil having the properties shown in Table 1 was treated
for thermal cracking. Thus, the feed stock was fed to a
cracking furnace at a feed rate of 300kg/hr where it was
thermally cracked and the resulting-thermally cracked product
was introduced into a perfect mixing type reactor ~stirring tank
reactor with an inside diameter of 500 mm and a height of 3000
mm) to which steam was supplied from the bottom for further
thermal cracking. The conversion rates in the cracking furnace
and the tank reactor were varied by varying the reaction time,
with the total conversion being kept constant (about 67 %). The
pitch products obtained were examined for their coking -tendency
in the perfect mixing type reactor, the results of which are
summarized in Table 2.
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Table 1
Properties of Feed Stock
Specific gravity (15/4 C) 1.021
Molecular weight 934
Heptane insolubles (wt%) 8.93
Conradson carbon residue (wt%) 20.8
Elementary analysis (wt %)
Carbon 84.56
Hydrogen 10.37
Sulfur 3.95
Nitrogen 0.51
Table 2
Properties of Pitch
_ _ _
Experiment Conversion (%) ¦ Properties of Pitch
~ _
No. Crac]cing Tank ¦ Particle size Coking
Furnace Reactor I of Mesophase ~ m) Tendency
1 ___ _ 70 r 100< _ great
2 45 55 50-100 fair
3 60 40 30-50 none
4 _ 75 _5_ _ 20 30 _ _ none
The coking tendency was determined as follows:
Mesophase in the pitch was observed by polarizing microscope to
evaluate the coking tendency by the particle size of the
mesophase unit and the degree of coalescence. Evaluation was
made according to the following ratings.
None: No coking
Fair: Coking occurred slightly
Great: Coking occurred considerably
