Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
~2~9943
'rhis inve~tion relates to a process for the thermal
cracking of hydrocarbon oils, in which process the hydrocarbons
are heated to reaction temperature and conducted into a reaction
zone, where the ~low is l~pward from below.
In ~he the~ïrlal cracking of hydrocarbon oils, heavy oil
frlcti~ns are cracked to lighter fractions to increase the yield
of the latter. In the cracking process, the Eeed oil is heated
in the heating tubes of the cracking furnace to cracking tempera-
ture. As a rule, two alternative methods are available. In one
of them, cracking takes place in the heating tubes of the crackinq
furnace and partly in the pipelines which lead to the process
steps following the cracking. In this cracking process the delay
times are not exactly known, but they are relatively short, in
the order of one minute. The pressure varies greatly, going down
from the furnace entrance to the furnace exit. In the other
cracking process, the hydrocarbon feed is first heated in the
cracking furnace to a suitable reaction temperature, and the
actual cracking reaction takes place in a separate reaction zone,
where the delay time is considerably lon~er than in the preceding
process, that is, in the order of 10 to 30 minutes. No heat is
introduced to the reaction zone.
In the last-mentioned process, the reaction zone as a
rule consists of an upright, cylindrical pressure vessel, at one
end of which the oil feed heated in the cracking furnace is in-
troduced and at the other end of which extracted a mixture of
liquid and gas to go to further refining steps extracted, for in-
stance for distillation. The flow direction in the reaction One
has been either downward from above or upward from helow.
In the thermal cracking of hydrocarbon oils, reactions
of substantiall~ two kinds take place. One of them is the crack-
ing reaction proper, the long-chain molecules being split into
smaller molecules, causing reduction of viscosity. The other
.,
,j'~l" '~
-- 1
~Z~9g~3
- reactlon type is called polycondensati~n, whereby the molecules
combine and prod~ce pitch and coke as hydrogen is set free. The
last-mentioned reaction is an undesired reaction because it re-
sults in greater quantities of asphaltelles. Since the condensing
reac~ions grow to be signiEicant al hi~ er telllL~(-rat~lres, cn~ea-
vours ace made to use lower reactio~ cm~ rt~ Lcs ~nd c~rrc~ )olld-
ingly lon~er delay times.
The delay time is very important for thermal cracking.
The cracking has not time to ta~e place if the delay time is too
lG short. In a case where the delay time is too long, the cracking
products begin to react and to form undesired reaction products.
As a result, an unstable product is formed which causes difficul-
ties in the further use of fuel. The aim is therefore to achieve
a cracking as uniform as possible. If the flows in the pressure
vessel serving as reaction zone are non-uniform, the result will
be varying delay times.
In the cracking reaction, light components are formed
which evaporate at the temperature and pressure in the reaction
zone. Therefore, the density of the liquid/gas mixture decreases
as the mixture flows upward in the pressure vessel. Owing to the
~ hydrostatic pressure differential in the pressure vessel, the
density of the gas part also decreases as the mixture flows up-
ward. The liquid fractions formed in the cracking reactor have
a lower density than the feed, which also lowers the density of
the liquid/gas mixture. Therefore, the flow velocity is not con-
stant in the usually employed cylindrical reactor with uniform
thickness, but accelerates as the mixture flows upward.
The thermal cracking procedure disclosed in U~S. Patent
No. 4,2~7,387 has a cylindrical vertical pressure vessel serving
as reaction zone. With a view to preventing refluxes within the
reactor, perforated intermediate plates are provided at the bottom
of the reaction zone to form a plurality of mixing sites in the
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reactor. The object is to achievc a delay time as uniform as
possible for the fraction fed into the zone. The use of inter-
mediate plates has its drawbacks. ~aulty operation of the reac-
tor may ca~lse the who]e reactor to be coked to occlusion. The
intermeclilte l~lates rnake the co~e removal and reactor cleaning
inconvc~ ier~t ~nd e~pensive.
~ n object of the invention is to achieve an in~E~rov~mellt
in the processes known in the art. A more ~etailed object of the
invention is to provide a process in which a uniform delay time
can be attained without the need for intermediate plates that
hamper the cleaning process.
According to the present invention there is provided a
process for the thermal crac~ing of hydrocarbons, wherein the
hydrocarbons are heated to reaction temperature and conducted into
a reaction zone as a fluid/gas mixture, where the flow is upwards
from below, and wherein the fluid/gas mixture is set in tangential
rotation in a pressure vessel defining the reaction zone.
A tangentially rotating, but vertically uniformly up-
wards progressing fluid/gas flow with no return flows causing
non-uniform delay times is generated in the reaction zone.
The tangentially rotating flow of the fluid/gas mixture
can be obtained in a number of ways. In a preferred embodiment,
the rotary motion is produced by means of helical members that
form a helically ascending corridor in the pressure vessel serv-
ing as reactor. In this passage, the f~ow is always in the up-
wards direction and no downflows occur. The helix system may
extend over the entire length of the reaction zone, or only over
part of it. In some instances, it may suffice to restrict the
helix system to the entrance section of the reaction zone.
It is also possible to provide in the reactor two or
more helix-like members, which reverse the direction of rotation
of the fluid/gas mixture. In this manner, one or several mixing
12~9943
steps for the fluid/gas mixture flowin~ in the reaction zonc are
produced.
Another embodiment serving to set the fluid~gas mixture
in tan~ntial rotary motion is that in which tangentially mounted
nozzles ar~ uscd. Irhrouc~h the nozzl~s part of the feed or 2nother
fluid, e.g. s-t~am, may be introduced to set the feed proper in
rotary rno~ion. ~rhe number of the nozzles is selected according
to the need, for instance from2 to 20 nozzles. It is also pos-
sible to dispose the feed pipes for the hydrocarbon being cracked
entering the reaction zone tangentially in the entrance section
of the zone.
According to still another advantageous embodiment, the
reaction 20ne has the shape of an outwardly expanding cone over
its entire length or in part, for instance only on the part of
the supply section. Such a conical shape has the effect of making
the distribution of delay time uniform.
It has been established that the appropriate temperature
for the cracking reaction is between 410 to 470 degrees and
the pressure between 2 and 20 bar. The ratio of the average
diameter and the length of the reaction zone is preferably in
the range from 1:1 to 1:20.
The invention will now be described in more detail, by
way of example only, with reference to the accompanying drawings,
in which:-
Fig. 1 shows an advantageous embodiment of a methodaccording to the invention as a schematlc process diagram;
Fig. 2 shows an advantageous embodiment of the reactor
used in the invention in schematic elevational view;
Fig. 3A shows another advantageous embodiment of the
3~ reactor employed in the process of the invention viewed from above;
Fig. 3B shows the reactor of Fig. 3A in elevational view;
Fig. 4A shows a further advantageous embodiment of the
~99~3
- reactor used in the invention view~d from above; and
Fig. 4B shows the reactor of Fig. 4A in elevational view.
In Fig. l, the feed oil is conducted through -the pipe
11 into the furnace 12, wh~re its t~mperature is raised to betw~en
~lO ~r~ 70 d~(3r~es. Irom ~he ~]rnace J2, the oil is conducted
t~llOU(j]-l the p;pe 13 into the reactor 14, ~here it flows upward
and leaves at the top of the reactor through the pipe 15 to a
separate unit (not shown), wherein for insiance gas, petrol,
light and heavy fuel oil may be separated from each other. The
average delay tiïne in the reaction zone is between 5 and lOO
minutes.
In the embodiment of Fig. 2, a helical member 16 is
provided within the reactor 14. The hydrocarbons to be cracked
are conducted into the reactor 14 upwards from below, whereby
they enter a helical corridor formed by the spiral member 16, and
where the actual cracking takes place.
In the embodiment of Fig. 2, the reaction zone 18 may
also have two helical members 16 and 17, with opposite directions
of the helix. Hereby, the fluid/gas mixture flowing in the reac-
tion zone 18 will reverse its rotation.
Figs. 3A and 3B show the lower part of the pressurevessel 14 acting as reaction zone 18, and in which the hydro-
carbon flow to be cracked with the lower part of the pressure
vessel 14 is introduced upward from below. The nozzles l9 com-
municate tangentially through which either part of the feed or
another fluid, such as steam for instance, may be introduced in
order to set in rotation the hydrocarbon flow that is being
cracked.
In the embodiment of Figs. 4A and 4B, on the end of
the feed pipe 20 for the hydrocarbons to be cracked, are formed
nozzles 21 which force the feed into rotary motion.
Example I:
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On a pilot plant scale, thermal cracking of crude oil
was carried out, using a reactor as in Fig. 1, and a similar
reactor having no helix system. In other respects, the conditions
were equal. The feed oil was the vacuum distilling base of
Soviet (ru(le oil. The res~lts are shown in the tab]e attached:-
Char~ic~-erislic Feed L'~ er~ics of the base product
(Distillltion fr;~ction 180 C+)
Without With
helix system helix system
_____________________________________ ____________________________
Density (g/cm 20 C) 1.0011 1.001 1.002
Asphaltene content
t% by weight) 6.28 10.70 11.10
Sulphur content
(% by weight) 3.65 3.38 3.54
Viscosity cSt (50 C) 43000 4200 3300
Stability 1) - 2.0 2.1
____________________~_____________________________________________
1~ The concept of stability is described in more detail in: van
Kerkvoort, ~.J., Nieuwstad) A.J.J. IV: E Congres Intern. du
Chauffage Industriel, paper number 220, Paris, 1952.
2~
