Note: Descriptions are shown in the official language in which they were submitted.
121028Z
The present invention relates to apparatus :for thermo-cracking
a hydrocarbon starting material to alkenes, the apparatus comprising a cracking
furnace with externally heated reactor tubes (coils) and a shell and tube
heat exchanger connected to the cracking furnace in order to quench the
reactor effluen~ ("quench", cooler, "transfer line" heat exchanger, TLX) where-
in steam is generated on the shell side.
Such installations (plants), which are generally used in the
preparation of alkenes like ethene and propene from starting materials which
may vary from natural gas to naphthas and gas oil, are described in Kirk-Othmer,
Encyclopedia of Chemical Technology, third edition, vol. 9 (1980) pages 400-
408, in particular pages 403-408.
In the course of time a number of general conditions have been
found for the cracking furnaces of those installations which should be met
regardless of the hydrocarbon starting material, and even control programs
controlled by a "computer" have been designed which, as to the power balance,
guarantee an optimum operation of the cracking furnaces so that they can be
operative for some months at a time.
The reactor effluent of the cracking furnaces is quenched in a
shell and tube heat exchanger from 750-900C to 350-560C (Kirk-Othmer l.c.,
page 407, table 5) to prevent further reactions from taking place under
adiabatic conditions after the effluent has left the cracking furnace, since
such reactions would affect adversely the yield of alkenes. Simultaneously
steam with a pressure of 105-125 bara (bar absolute) :i.s generated.
Ilowever, when quenching the reactor effluent, the inside surfaces
of the heat exchanger tubes are fouled, said fouling leading to a decrease
in heat transfer while also the sensible heat of the reactor effluent is
increasingly used less for the generation of the high pressure steam. The
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12~0282
effluent coming from the shell and tube heat exchanger thus has an ever increas-
ing temperature.
Up to now it has been assumed that this phenomenon cannot be
prevented. Generally the phenomenon was ascribed to condensation of heavy
hydrocarbon components from the effluent of the cracking furnace onto the
colder heat exchanger surfaces followed by continuing dehydrogenation reactions
in the condensate at the temperature prevailing on the wall of the heat
exchanger tubes (vide Lohr. B ~ H Dittmann, OGJ, 1978, May 15).
According to Dutch patent application 70 07556 in a different
quenching system, wherein a cracker gas mixture is quenched by introducing said
gas mixture via an inlet into a quench liquid which is present in a quenching
barrel, the problem of fouling and even clogging of the inlet pipe by deposi-
tion of tar and carbonaceous materials on the inside of the inlet pipe is
prevented, by insulating the inlet pipe on the outer side so that the tempera-
ture of the inside of the inlet pipe remains relatively high and condensation
of tar and carbonaceous materials appears less easy on the inside. The in-
sulating layer has a thickness of some centimeters.
This solution is not possible if a shell and tube heat exchanger
is used, as insulation of the tubes of the shell and tube heat exchanger
nullifies ~overrides) the entire cooling of the reactor effluent.
It has now been found that the fouling on the inside of the heat
exchanger can be decreased and/or inhibited, so that the shell and tube heat
exchanger can be in operation for much longer times, if the internal surfaces
of the tubes of the heat exchanger are coated with an inert layer, impermeable
to the materials from the reactor effluent which are responsible for the foul-
ing, said layer masking the alloy of which the heat exchanger tubes consist.
Thus, according to the present invention, there is provided
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a shell and tube heat exchanger adapted for use in an apparatus for cracking
a hydrocarbon starting material to alkenes, wherein tubes of the heat ex-
changer are formed of alloy and internal surfaces of the heat exchanger tubes
are coated with an inert layer, said layer being impermeable to materials in
the reactor effluent from a cracking furnace used for the preparation of
alkenes, said inert layer effectively masking said alloy of the heat exchanger
tubes from said materials in the reactor effluent.
Such a layer should preferably have such a thickness, that it is
impermeable to the reactor effluent, but on the other hand it should not be
so thick that it impedes the heat transfer.
The minimum thickness should preferably be 0.5 ~m. Preferably
it has a thickness of not more than 20 ~m, for, with greater thicknesses,
the effect, the temperature drop on the layer, should be too big.
In another aspect, the invention provides apparatus for thermo-
cracking a hydrocarbon starting material to alkenes, comprising a cracking
furnace with externally heated reactor tubes or coils and a shell and tube heat
exchanger in fluid communication with said cracking furnace and usable for
quenching reactor effluent from the cracking furnace, said shell and tube heat
exchanger including a shell side and one or more tubes formed of alloy, steam
being generatable on the shell side of the heat exchanger, wherein internal
surfaces of tubes of the heat exchanger are coated with an inert layer im-
permeable to the materials in the reactor effluent which are responsible for
the fouling, said inert layer effectively masking said alloy of the heat
exchanger tubes from said materials in the reactor effluent.
The invention also provides a process for the treatment of a shell
and tube heat exchanger, for use in apparatus for cracking a hydrocarbon
starting material to alkenes and intended for quenching effluent coming
~2~1L(328;2
from a cracking reactor of such apparatus, wherein internal surfaces of heat
exchanger tubes of the exchanger are sprayed with a mixture of an oily fraction,
obtained when quenching effluent from a cracking reactor for the preparation
of alkenes, and an initiator forming free radicals, draining off the excess
of the mixture from the heat exchanger tubes and heating the tubes at a tempera-
ture at which the mixture is cured.
According to a preferred embodiment of the invention the inert
layer substantially comprises graphite, and/or metal and/or metal oxides, metal
salts and/or silicates.
A particularly suitable process which can be used to obtain such
a layer uses a viscous mixture of a powdered graphite, metals, metal oxides,
metal salts (particle size generally <5 ~m) with a silicone based resin in an
aromatic solvent. Said mixture is applied with current spraying methods and is
thermoset. Thermosetting takes suitably place at temperatures between 275C
and 375C for 1 1/2 - 5 h. Said thermosetting ~curing) is necessary to vapor-
ize the solvent, and to have reticulation take place in the resin component,
and optionally to have the resin component decomposed, while silicon remains
enclosed in the layer. The result is that a quasi-continuous layer is formed,
with a small specific area. Such a layer is highly wear-resistant and
resistant to high temperatures.
The impermeability of the layer can be increased by repeating the
process several times. Beside graphite especially metals from Group III or IV
of the periodic table and their oxides are suitable, e.g. aluminium, titanium,
zirconium. Also silicates and aluminates can be used.
Graphite and aluminium particularly appear to provide satisfactory
results (decrease and/or inhibition of the fouling phenomena), while both are
cheap and can be easily applied.
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Other processes which can be used to apply a metal layer are
known techniques like vaporization under vacuum, (vacuum coating or vacuum
metalizing) J forming a deposit of metal by decomposition of a vaporous metal
compound (gas plating).
According to a second embodiment of the invention the impermeable
layer on the internal surface of the heat exchanger tubes consists of an inert
polymeric layer.
Preferably this is a polymeric layer, formed by applying a mix-
ture of the oily fraction, which is recovered when quenching the effluent
from the cracking reactor (ethylene quench oil), and an initiator forming
free radicals, in particular a peroxide, such as benzoyl peroxide, cumene
hydroperoxide, on the internal surface of the tubes, draining the excess and
thermosetting the remaining mixture.
Such a layer has a structure which highly resembles the fouling
layer which normally appears, and it is stable at the temperatures prevailing
in the heat exchanger, so that it does not influence the phenomena which
appear in the heat exchanger. On this layer, once formed, only a small foul-
ing layer appears.
In the process of the present invention preferably a peroxide is
used as catalyst, in particular benzoyl peroxide, as peroxides in the polymeri-
sation of alkenes and alkene mixtures are effective catalysts.
The amount of catalyst may vary within wide ranges but preferably
a mixture is used which comprises 0.5 - 3% of catalyst, such a mixture quickly
providing a good polymer layer.
The effect which is obtained with the apparatus according to the
invention is elucidated in the following examples, with reference to the
accompanying drawing which graphically illustrates the variation in quenched
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effluent -temperature with time for two diferent groups of heat exchanger tubes.
Example I
In a known installation for the preparation of ethene, with a
capacity of 40000 tons/year ethene, gas oil was cracked. The effluent of thè
cracking furnace had the following composition.
~LZ~02~3%
Effluent composition (weight %)
2 0.49
CH4 8.21
C0 0.02
_________________________________
8.72
C2H2 0.16
C2H4 21.22
C2H6 2.87
__________________________________
24. 25
methyl-acetylene 0.16
prepadiene 0.12
C3H6 13.61
_3_8_______________ ________ _____
14.30
C4H6 4.86
C4H8 6.57
_4_10______________ ________ ____
11.48
C5 until 6.21
benzene 2.82
C6 (NA) 2.05
toluene 2.22
C7 (NA) 0.67
o-xylene 0.26
m-xylene 0.43
p-xylene 0.20
ethyl benzene0.32
styrene 0.40
C8 (NA) 0.15
Cg until 2.68
C10 and higher11.83
_________________________________
41.25
100.0
NA = non - aromatic
~2~ 21~;~
Said effluent, which had a temperature of 800-850C and a pressure
of 1.6 bara was quenched in two newly cleaned shell and tube heat exchangers
~TLX) connected in parallel, while on the shell side of the heat exchangers
steam with a pressure of llO bara was generated.
One TLX (A) had heat exchanger tubes made from a nickel-chromium-
alloy which is usual for these type of tubes.
The other TLX (B) had heat exchanger tubes from the same nickel-
chromium-alloy, the internal surface of which was coated with a 5 ~m thick
aluminium based layer applied in 3 steps in accordance with the present in-
vention.
The temperature of the quenched effluent coming from the TLX (A)
in the beginning of ~he test was 420C and the temperature of the quenched
effluent coming from TLX (B) was 450C.
The variation in the temperature of the effluents coming from
both TLX against the time duration of the test is elucidated in the figure.
Curve A shows the result for TLX (A) .
Curve B shows the result for TLX (B).
One sees that with TLX (A) (Curve A) the temperature of the
effluent coming from the TLX, increased to 500C in about 5 days and during
the rest of the test the temperature gradually further increased, until after
26 days the maximum allowed temperature of 560C was obtained.
The fouling rate in TLX (B) (Curve B) was substantially constant
and the extrapolated attainable hours of service is 60 days instead of 26 as
for TLX (A).
Thus it follows that with the second TLX the heat transfer during
the whole test was better than with the first TLX.
Both TLX's were thrown out of operation and were inspected.
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TLX (A) appeared to comprise a thick fouling layer.
In TLX ~B) only a slight fouling was present.
Coating the internal surface of the heat exchanger tubes of TLX (B~was carried out by spraying a mixture of 12% by weight of aluminium powder
with a particle size of < 2 ~m, 48% by weight of a silicone resin comprising
methyl groups and phenyl groups, and 40% by weight of toluene into the tubes,
draining the excess and heating the remaining layer for 2 hours at 300C, thus
vaporizing the toluene and reticulating the resin~ repeating this processing
once, and finally repeating the treatment once with a mixture of 10% by weight
of aluminium powder with a particle size of < 2 ~m, 40% by weight of the same
silicone resin and 50% by weight of toluene.
Example II
The test of example I was repeated, while a TLX (C) was used, the
heat exchanger tubes of which were coated on the inside with a 5 ~m thick layer
based on graphite, which was applied as follows:
A mixture of 24% by weight of graphite having a particle size of
< 1 ~m, 36% by weight of the same silicone resin as was used when forming the
coating according to example I and 40% by weight of toluene was introduced
into the tubes, the excess was drained off and the remaining layer was heated
for 2 hours at 300C at which temperature the toluene was vaporized and the
resin was subjected to reticulation. This processing was repeated once more
and finally the processing was repeated Wit}l a mixture of 20% by weight of
graphite having a particle size < 1 ~m, 30% by weight of the same silicone
resin and 50% by weight of toluene.
The variation in the temperature of the effluent coming from said
TLX (C) against the time corresponded to the variation in the temperature of
the effluent coming from TLX (B) (example I) against time.
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At the end of the test the heat exchanger tubes were inspected;
only a slight fouling was observed.
Example III
A similar test was carried out as in example I wherein a TLX (D)
~as used having heat exchanger tubes the internal surfaces of which were coated
with a polymeric layer, the latter having been formed by mixing the oily
fraction obtained when quenching the effluent from the cracking reactor (ethy-
lene quench oil) with 1.5% of benzoyl peroxide. The mixture was then introduced
into the heat exchanger tubes, the excess drained off and the tubes externally
heated at 400C.
The variation in the temperature of the effluen~ coming from TLX
~D) also corresponded to curve B of the figure. At the end of the test the
heat exchanger tubes of TLX (D) were inspected. On the polymeric layer a slight
fouling had been deposited.
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