Coke deposited within the gas passages of a dual tower type, fluidized bed apparatus for thermally cracking hydrocarbon oils is removed by combustion through contact with a stream of an oxygen-containing, high temperature combustion gas without a need to disjoint the apparatus. The apparatus is comprised of heating and cracking towers each adapted for containing a mass of fluidized solid particles continuously recirculating between the two towers. In one embodiment, all of the solid particles are discharged from the apparatus and a stream of the oxygen-containing combustion gas, produced in a combustion furnace connected to the heating tower, is allowed to pass through the gas passages.

BACKGROUND OF THE INVENTION 
This invention relates to a method of removing coke deposited within gas 
passages of an apparatus for the thermal cracking of hydrocarbons. 
For thermally cracking hydrocarbon oils such as crude oils, reduced crude 
oils and residual oils, it is known to use a dual tower type apparatus 
composed of heating and cracking towers each containing a fluidized bed of 
solid particles continuously recirculating between the two towers. In such 
an apparatus, the feedstock is fed to the cracking tower where it is 
subjected to thermal cracking conditions by contact with the heated, 
fluidized soild particles. The cracked product is withdrawn overhead from 
the cracking tower for recovery while the solid particles are introduced 
into the heating tower, where they are heat-regenerated by contact with a 
combustion gas introduced into the heating tower from a combustion furnace 
provided adjacent thereto. The thus heat-regenerated solid particles are 
then recycled to the cracking tower. 
When the apparatus is operated for a certain period of time, for example 
for five weeks, there arises a need to conduct a decoking operation since 
a large amount of coke is deposited and accumulated within the gas 
passages downstream of the cracking tower, such as pipes, cyclone, 
quenching device, etc. In such a case, it has been the general practice to 
disjoint the apparatus for the removal of the coke deposited on the inside 
surfaces of the pipes, cyclone and other parts, the separated parts being 
assembled after being cleaned of coke. The decoking of the thus separated 
parts are generally effected physically using suitable cleaning devices 
and, thus, is very troublesome. Moreover, since the apparatus is generally 
high and heavy, the disjointing and assembling procedures themselves are 
also time-consuming. Therefore, the conventional decoking method is 
disadvantageous from an economic point of view. 
There is known a so-called steam-air decoking method in which a hot 
combustion gas and steam are alternately allowed to flow through the gas 
passages of the apparatus so that the coke deposited within the gas 
passages is subjected to alternate heating and cooling, whereby the coke 
is spalled. The spalled coke pieces are carried with the stream of the 
high speed steam. This method does not require disjointing work. However, 
this method is applicable only to apparatuses of a small diameter cracking 
tube formed of a material of a large thermal expansion coefficient, such 
as a metal, since such an apparatus alone enables easy exfoliation of the 
coke from the surfaced of the tube and high speed flow of steam. The 
steam-air decoking method cannot be applied to the above-mentioned dual 
tower type cracking apparatus which is generally made of an inorganic 
refractory material and which is large in pipe diameter. 
BRIEF SUMMARY OF THE INVENTION 
It is, therefore, an object of the present invention to provide an 
economical method capable of removing coke deposited on the inside wall of 
a dual tower-type apparatus for cracking hydrocarbons efficiently without 
a need to disjoint the apparatus.

DETAILED DESCRIPTION OF THE INVENTION 
Referring to FIGURE, the reference numerals 2 and 3 denote a heating tower 
and a cracking tower, respectively, each of which has generally a tubular 
shape. The heating tower 2 and the cracking tower 3 are each adapted for 
enclosing a mass of solid particles 35 acting as a heat transfer medium. 
The heating tower 2 is provided with an opening 29 at its lower position 
and an opening 18 at a position above the opening 29. The cracking tower 3 
is also provided with an opening 19 at its lower position and an upper 
opening 20 at a position above the opening 19. A first transport leg or 
pipe 14 is connected at one end to the upper opening 18 of the heating 
tower 2 and at its other end to the lower opening 19 of the cracking tower 
3 so that the solid particles may descend through the transport leg 14 by 
gravity. Similarly, a second transport leg or pipe 14' extends between the 
opening 20 of the cracking tower 3 and the lower opening 29 of the heating 
tower 2 so that the solid particles may flow downward through the leg 14' 
by gravity. 
First and second supply means are provided in the heating and cracking 
towers 2 and 3, respectively, for supplying a fluidizing gas to respective 
towers therethrough. Thus, a first fluidizing gas is fed from a line 8 and 
introduced into the heating tower 2 through a line 21 branched from the 
line 8 so that the solid particles in the heating tower 2 may be 
maintained in a fluidized state. A second fluidizing gas which may be the 
same as or different from the first fluidizing gas is fed from a line 8' 
and introduced into the cracking tower 3 via line 22 branched from the 
line 8' so that the solid particles contained in the cracking tower may be 
maintained in a fluidized state. Lines 23 and 24, branched from the lines 
8 and 8', respectively, open into the heating and cracking towers 2 and 3, 
respectively, for the supply of the fluidizing gases therethrough to 
maintain the respective fluidized beds in a suitable fluidizing condition. 
Provided adjacent to the heating tower 2 is a combustion means 1 including 
a burner 1a, a combustion furnace 1b and a combustion gas discharge port 
1c connected to the heating tower 2, so that the combustion gas produced 
by the combustion means 1 is fed to the heating tower 2 for heating the 
solid particles contained in the heating tower 2. The combustion gas and 
the first fluidizing gas are discharged from the heating tower 2 through a 
discharge conduit means which includes a discharge pipe 25 and a valve 15 
connected to the top of the heating tower 2. 
Connected to the middle portion of the cracking tower 3 is a feed means 9 
through which a hydrocarbon feedstock is streamed into the cracking tower 
3 for cracking treatment therein. A discharge line 10 is connected to the 
top of the cracking tower 3 through which a gas containing the cracked 
gaseous product and the second fluidizing gas is discharged from the 
cracking tower 3. Indicated as 4 and 5 are respectively a gas-solid 
separator such as a cyclone, and a cooling means such as a quencher 
connected with each other by a pipe 11. Indicated as 12 is a solids return 
line through which the solids separated in the cyclone 4 are recycled to 
the cracking tower 3. The quencher 5 has a cooled gas discharge line 38 
which is divided into a line 27 connected to a fractionating tower 6 and a 
line 28 connected to a knockout drum 7. Selective introduction means such 
as valves 16 and 17 are provided between the quencher 5, and the 
fractionating tower 6 and the knockout drum 7 for selectively introducing 
the cooled gas from the quencher 5 into either the fractionating tower 6 
or the knockout drum 7. Designated as 37 is a gas discharge line connected 
to the knockout drum 7 through which the gas supplied into the drum 7 is 
discharged after the removal of solids components entrained therein. 
A conduit 30 and a conduit 31 is located adjacent to the lower portion of 
the cracking tower 3 and the heating tower 2, respectively, for the 
removal of the solid particles from each tower, and a line 31 is provided 
through which solid particles are introduced into the heating tower. Feed 
lines 32 and 33 are provided to introduce gas into the upper portion of 
the cracking tower 3, for purposes described below. 
The thus constructed cracking apparatus is operated as follows. In start 
up, the lines 30 and 30' and the valve 16 are closed. A suitable amount of 
solid particles such as sand, coke, alumina or any other conventionally 
employed heat transfer medium is introduced through the line 31 into the 
heating and cracking towers 2 and 3. First and second fluidizing gases 
such as steam are fed to the lines 8 and 8', respectively. As a 
consequence, there is formed in each of the heating and cracking towers 2 
and 3 a bed of fluidized solid particles which continuously recirculate 
between the heating and the cracking towers 2 and 3 through the first and 
second transport legs 14 and 14'. 
Meanwhile, a fuel such as a fuel oil or a fuel gas is combusted in the 
combustion means 1 and the resulting high temperature combustion gas is 
fed to the heating tower 2 to heat the solid particles in the heating 
tower 2. The combustion gas after being contacted with the solid particles 
is discharged together with the first fluidizing gas through the valve 15 
and the line 25. 
A portion of the solid particles in the cracking tower 3 is continuously 
streamed downward through the second transport leg 14' by gravity and 
introduced into the lower portion of the heating tower 2, where the solid 
particles are heated by contact with the combustion gas supplied from the 
combustion means 1. A portion of the thus heat-regenerated solid particles 
is continously flown downward through the first transport leg 14 and is 
introduced into the cracking tower 3 for the utilization of the heat 
thereof for effecting the thermal cracking of the feedstock. 
When the fluidized bed in the cracking tower 3 is heated to a temperature 
sufficient enough to effect the cracking operation, the valve 16 is opened 
and the valve 17 is closed. The hydrocarbon feedstock is then continuously 
fed through the feeding means 9 to the cracking tower 3 where it is 
subjected to thermal cracking conditions by contact with the solid 
particles which have been heated in the heating tower 2. 
The gaseous cracking product is withdrawn from the cracking tower 3 
together with the second fluidizing gas and is introduced into the 
gas-solid separator 4. The solids separated in the separator 4 are 
returned to the cracking tower 3 through the return line 12 while the gas 
is fed to the quencher 5. The cooled gas from the quencher 5 is introduced 
into the fractionating tower 6 through the line 27, thereby to obtain 
desired fractions. The knockout drum 7 connected to the line 28 branched 
from the line 38 serves to remove the solid particles entrained in the 
cooled gas from the quencher 5. During the start up operation, the cooled 
gas from the quencher 5 is introduced into the knockout drum rather than 
the fractionating tower 6. When the thermal cracking process reaches a 
steady state, the valve 17 is closed and the valve 16 is opened for 
introducing the cooled gas into the fractionating tower 6. 
When the operation is continued for a long period of time, coke produced in 
the cracking step in the cracking tower 3 deposits on the inside wall of 
the gas passages such as the line 10, gas-solid separator 4, line 11, and 
quencher 5 so that it becomes impossible to continue the cracking 
operation in a stable manner. The accumulation of the coke may be detected 
by a differential indicator 26 provided to measure the difference in 
pressure between, for example, the cracking tower 3 and the quencher 5. 
Thus, when the differential indicator raises an alarm, the cracking 
operation is stopped to conduct decoking operation. 
In one method according to the present invention, decoking is conducted as 
follows: 
(a) The feed of the hydrocarbon feedstock is stopped. 
(b) The valves 15 and 16 are closed and the valve 17 is opened. 
(c) Substantially all of the solid particles in the heating tower 2, 
cracking tower 3 and first and second transport legs 14 and 14' are 
discharged from the apparatus through the lines 30 and 30'. 
(d) The combustion means 1 is operated to produce a high temperature, 
oxygen-containing combustion gas. The combustion gas generally has a 
temperature of 700.degree.-2000.degree. C., preferably 
800.degree.-1500.degree. C. and an oxygen content of 0.1-15 vol %, 
preferably 1-10 vol %. The oxygen-containing combustion gas may be 
produced by combustion of a fuel, such as a fuel oil or any other suitable 
fuel, with an excess air ratio. 
As a result of the above operation, the high temperature oxygen-containing 
combustion gas in the combustion means 1 is allowed to pass through the 
heating tower 2, cracking tower 3, line 10, gas-solid separator 4, line 11 
and quencher 5 so that the coke deposited within the gas passages is 
decomposed by combustion. The decoking operation is generally continued 
until the concentration of carbon dioxide in the gas discharged through 
the line 37 decreases to less than about 0.1 vol. %. 
In an alternative embodiment of the present invention, the decoking is 
performed as follows: 
(a) The feed of the hydrocarbon feedstock is stopped. 
(b) The valve 16 is closed and the valve 17 is opened. 
(c) The combustion means 1 is operated to produce high temperature 
combustion gas for heating the solid particles in the heating tower 2 
through direct contact therewith. 
(d) The first and second fluidizing gases, which may be the same as or 
different from with each other, are fed to the lines 8 and 8', 
respectively, to maintain the solid particles in each tower in a fluidized 
state and in continuous recirculation between the heating and cracking 
towers 2 and 3 through the transport legs 14 and 14', whereby the 
temperature of the solid particles in the cracking tower 3 is maintained 
generally in the range of 600.degree.-800.degree. C. 
(e) An oxygen-containing gas such as air is fed to the cracking tower 3. 
The content of the oxygen in the oxygen-containing gas is generally such 
that the oxygen concentration in the tower 3 is maintained in the range of 
0.1-15 vol. %. The oxygen-containing gas may be supplied through the line 
22, 24 or 9. It is possible to provide, as shown in the drawing, one or 
more gas feed lines 32 and 33 for the introduction of the 
oxygen-containing gas therethrough into the upper space of the cracking 
tower 3. When the apparatus is in the thermal cracking operation, the gas 
feed lines 32 and 33 may be closed or supplied with steam. 
In the above-mentioned alternative embodiment, if the solid particles are 
formed of a combustible material such as coke, then it is necessary to 
minimize the contact between the solid particles and oxygen, since 
otherwise the oxygen is consumed by reaction with the solid particles and 
the decoking cannot be achieved effectively. In such a case, therefore, 
the combustion in the combustion means 1 is conducted so that the 
resulting combustion gas is substantially free of oxygen. Further, the 
oxygen-containing gas should be fed to the cracking tower 3 at a position 
over the top surface of the bed of the fluidized solid particles. To 
achieve this purpose, it is preferred that the level of the fluidized bed 
in the cracking tower 3 be maintained as low as possible, i. e. adjacent 
to the opening 20. The oxygen-containing gas is supplied from the line 32 
and/or 33. The lowering of the height of the fluidized bed in the cracking 
tower 3 can be done by increasing the pressure in the cracking tower 3 by 
controlling the degree of opening of the valves 15 and/or 17. For the 
purpose of minimizing the discharge of fine particulate of the solid 
particles from the cracking tower 3, it is preferable to decrease the 
velocity of the second fluidizing gas supplied to the cracking tower 3. 
The velocity may be decreased to any extent so far as the recirculation of 
the solid particles between the heating and cracking towers 2 and 3 is 
maintained. 
According to the method of the present invention, the decoking can be 
effected without disjointing the cracking apparatus and without using any 
particular equipment. Thus, as soon as the decoking operation is 
terminated, it is possible to resume the cracking operation. 
The following examples will further illustrate the present invention. 
EXAMPLE 1 
A heavy hydrocarbon oil was thermally cracked with the use of the apparatus 
shown in the accompanying drawing. Coke particles having diameters ranging 
from 0.2 to 2.0 mm were used as a heat transfer medium. The apparatus was 
operated under the following conditions: 
Combustion means 1: The combustion gas had a temperature of about 
2000.degree. C. at the furnace outlet and contained substantially no 
oxygen. 
Heating tower 2: The coke particles have a temperature of about 800.degree. 
C. 
Cracking tower 3: The temperature of coke particles was 750.degree. C. 
The feedstock oil was fed at a rate of 5000 Kg/H. The cracking operation 
had been continued for about 1000 hours when the differential indicator 26 
showed the need to perform decoking. Thus, the feed of the feedstock (line 
9) was stopped. The entire amount of the coke particles was discharged 
from the apparatus. The valves 15 and 16 were closed and the valve 17 was 
opened. The combustion means 1 was then operated under the following 
conditions: 
Fuel: A fuel gas was fed at a rate of 170 Nm.sup.3 /H for combustion. 
Combustion air: Supplied at a rate of 2500 Nm.sup.3 /H. 
Steam: Supplied at a rate of 2300 Nm.sup.3 /H. 
The combustion gas had a temperature of 810.degree. C. at the outlet of the 
combustion furnace and an oxygen content of about 2.5 vol %. About 12 
hours after the initiation of the decoking operation, the temperature at 
the top of the cracking tower was found to be stabilized at 700.degree. C. 
The decoking operation had been further continued for about 72 hours when 
the concentration of carbon dioxide in the gas discharged from the 
knockout drum 7 was reduced to about 0.1 vol %. Then the feed of the fuel 
to the burner was stopped. The temperature of the cracking tower 3 was 
lowered to room temperature after about 12 hours from the stop of the feed 
of the fuel. The inspection of the inside wall surfaces of the apparatus 
revealed that the decoking was satisfactorily accomplished. 
EXAMPLE 2 
The thermal cracking operation in Example 1 was repeated in the same manner 
as described therein. After about 1000 hour cracking process, coke was 
found to accumulate within the gas passages of the apparatus in a 
significant amount. Thus, the feed of the feedstock (line 9) was stopped, 
and the valve 17 was opened and the valve 16 was closed. The decoking was 
conducted under the following conditions: 
Combustion means 1: A fuel gas was fed to the burner and combusted at a 
rate of 190 Nm.sup.3 /H with combustion air of 1900 Nm.sup.3 /H. 
Heating tower 2: The temperature of the coke particles was 780.degree. C. 
Cracking tower 3: Steam was fed in an amount of 1500 Kg/H through the line 
8'. Air was fed at a rate of 800 Nm.sup.3 /H from the line 32 and 200 
Nm.sup.3 /H from the line 33. 
In the initial stage of the decoking operation, fine coke particles in the 
cracking tower 3 was entrained in the gas withdrawn from the tower 3 and 
burnt in the upper portion of the tower 3. Therefore, the oxygen 
concentration in the gas flowing through the line 37 was 3%. After about 
12 hours from the commencement of the decoking operation, the oxygen 
concentration was increased to about 5%. The decoking had been continued 
for about 72 hours when the oxygen concentration and the carbon dioxide 
concentration at the line 37 were found to be about 6% and about 0.1%, 
respectively, indicating the completion of the decomposition of the coke 
accumulated within the gas passages of the apparatus. The inspection 
within the apparatus after the decoking operation revealed that the 
decoking was ended with satisfactory results. 
For the purpose of comparison, decoking was carried out manually after 
disjointing the apparatus. The disjointing and the assembling works 
required a crane and other devices. A total of 11 days were required for 
completing the decoking work with about 8 workers per day in average. 
The invention may be embodied in other specific forms without departing 
from the spirit or essential characteristics thereof. The present 
embodiments are therefore to be considered in all respects as illustrative 
and not restrictive, the scope of the invention being indicated by the 
appended claims rather than by the foregoing description, and all the 
changes which come within the meaning and range of equivalency of the 
claims are therefore intended to be embranced therein.