EP0110486B1

EP0110486B1 - Installation (plant) for thermo-cracking a hydrocarbon starting material to alkene, shell and tube heat exchanger for use in such an installation and process for manufacturing shell and tube heat exchanger

Info

Publication number: EP0110486B1
Application number: EP19830201725
Authority: EP
Inventors: Mario Prof. Dente; Eliseo Prof. Ranzi; Simon Barendregt
Original assignee: Pyrotec NV
Current assignee: Pyrotec NV
Priority date: 1982-12-07
Filing date: 1983-12-06
Publication date: 1986-12-10
Also published as: EP0110486A1; JPH0519080B2; CA1210282A; JPS59157494A; NL8204731A; DE3368282D1

Description

The invention relates to a plant for thermo- cracking a hydrocarbon starting material to alkenes, 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 effluent ("quench", cooler, "transfer line" heat exchanger, TLX) wherein 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 were found for the cracking furnaces of those installations, which should be met regardless the hydrocarbon starting material, and even control programs controlled by a "computer" were designed, which as to the power balance guarantee an optimum operation of the cracking furnces and by which is reached, that the cracker furnaces can be operative for some months together.
The reactor effluent of the cracking furnaces is quenched in the shell and tube heat exchanger from 750―900°C to 350―560°C (Kirk-Othmer I.c., page 407, table 5) to prevent that after leaving the cracker furnace, in said effluent still reactions take -place under adiabatic conditions, which would affect adversely the yield of alkenes, and simultaneously steam with a pressure of 105-125 bara (bar absolute) is generated.
However 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 even less used for the generation of the high pressure steam. The effluent coming from the shell and tube heat exchanger has an ever increasing temperature.
Up to now it was assumed that this phenomenon cannot be prevented. Generally the phenomenon was ascribed to condensation of heavy hydrocarbon components from the effluent from 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 quench liquid which is present in a quenching barrel, the problem of fouling and even clogging of the inlet pipe by deposition 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 temperature of the inside of the inlet pipe remains relatively high and condensation of tar and carbonaceous material appears less easy on the inside. The insulating 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 shell and tube heat exchanger nullifies (overrides) the entire cooling of the reactor effluent.
It was now 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 a much longer time, if the internal surface of the tubes of the heat exchanger are coated with a layer which is insensitive for and impermeable to the materials in the reactor effluent which are responsible for the fouling, said layer having been formed from a mixture of particles of graphite, or aluminium, in a silicone resin, or if the internal surfaces of the tubes of the heat exchanger are coated with a layer which is insensitive for and impermeable to the materials in the reactor effluent which are responsible for the fouling, said layer being a polymer layer, formed by applying a mixture of the oily fraction which is recovered when quenching the effluent from the cracking reactor (alkylene quench oil) and a free radical forming initiator onto the inner surface of the tubes, e.g. by spraying, followed by thermosetting said layer.
From GB-A-831 912 it is known per se to provide the heat-transfer surfaces and/or tubes of condensers and heat exchangers with a protective coating which comprises a silicone, However, the apparatus concerned are condensers such as are used in power plants where corrosion problems arise on the exposed metal of the tube walls due to matter suspended in the cooling water, especially particles of carbon, which cause local decomposition of the metal and the protective coating which is applied according to this document, is applied on the water side of the condenser or heat exchanger.
From BE-A-348 252 it is known per se to prevent the deposition of "suie" i.e. carbon black from flue gases formed in the combustion of carbonaceous materials, onto heat exchanger surfaces, by applying a graphite layer on said exchanger surfaces from an aqueous suspension consisting of graphite powder, water glass and water. The composition of flue gases which comprise large amounts of oxidation products like carbon oxides (CO and C0₂), and further nitrogen and oxygen besides carbon black and tarry components, however, is completely different from the effluent from a cracking reactor wherein no combustion phenomena take place and neither carbon dioxide, oxygen nor nitrogen are present. A graphite powder, water glass and water moreover is insufficiently impervious to be suitable in the apparatus (quench coolers) according to the invention.
From EP-A-22349 it is known per se to form a protective metal oxide film on surfaces of structures made from metals or alloys, for use at high temperatures. The metal oxide film is said to prevent various problems, e.g. coke formation or carburisation, corrosion, catalytic reactions in chemical reactors, which may arise, dependent on the conditions under which the structures are used.
None of these prior art references mention the problem of fouling of the inside surfaces of the heat exchanger tubes of quench coolers used in installations for thermo-cracking a hydrocarbon starting material to alkenes and that this specific problem can be solved in the above-mentioned ways, cannot be derived from those publications.
The coating layer according to the invention should 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. This means that the thickness at least must be .0.5 um. Preferably the thickness is not more than 20 um, for with greater thickness, the effect, the temperature drop on the layer, should be too big.
For applying a coating layer formed from a mixture of particles of graphite, or aluminium in a silicon resin, one prepares a viscous mixture of these particulate fillers (particle size generally <5 pm) with a silicone based resin in an aromatic solvent. Said mixture is applied with current spraying methods and is thermoset. Thermosetting takes place suitably at temperatures between 275°C and 375°C for 1 1/ 2―5h. Said thermosetting (suring) is necessary to vaporize the solvent, and to have cross-linking 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. Layers comprising graphite and a silicone resin give a good result and further are cheap.
Particularly coating layers comprising aluminium and a silicone resin appear to give an optimum decrease and/or inhibition of the fouling phenomena and are preferred.
Other processes which can be used to apply a metal layer are current techniques like vaporization under vacuum, (vacuum coating or vacuum metalizing), forming a deposit of metal by decomposition of a vaporous metal compound (gas plating).
Coating layers formed by applying a mixture of alkylene (ethylene) quench oil and a free radicals forming initiator have a structure which highly resembles the fouling layer which normally appears, and which 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 fouling layer appears.
For forming this layer a mixture of alkylene quench oil (ethylene quench oil) and a free radicals forming initiator is applied on the internal surfaces of the tubes, followed by draining the excess and thermosetting the remaining mixture.
The free radicals forming initiator preferably is a peroxide, like benzoyl peroxide, cumene hydroperoxide, which gives optimum results in thermosetting the mixture.
The invention also relates to a shell and tube heat exchanger to be used in an apparatus for cracking a hydrocarbon starting material to alkenes, wherein the internal surfaces of the heat exchanger tubes are coated with a layer which is insensitive for and impermeable to the reactor effluent of a cracking furnace for the preparation of alkenes, said layer meeting the above-mentioned conditions.
The invention also relates to a process for manufacturing a shell and tube heat exchanger to . be used in an installation for cracking a hydrocarbon starting material to alkenes and resistant to quenching of the effluent from the cracking reactor of such an installation wherein the internal surfaces of the heat exchanger tubes are sprayed with a mixture of an oil fraction obtained when quenching the effluent from a cracking reactor for the preparation of alkenes and of an initiator forming free radicals, draining off the excess of the mixture from the heat exchanger tubes and heating the tubes at a temperature at which the mixture is cured.
In said process preferably a peroxide is used as catalyst, in particular benzoyl peroxide, as peroxides in the polymerisation 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, as with such a mixture quickly a good polymer layer can be obtained.
The effect which is obtained with the installation according to the invention is elucidated in the following examples.

Example I

In a current installation for the preparation of ethene, with a capacity of 40000 tons/year ethene, gas oil was cracked. The effluent of the cracking -furnace had the following composition.
Said effluent, which had a temperature of 800―850°C and a pressure of 1.6 bara was quenched in two just cleaned shell and tube heat exchangers (TLX) connected in parallel, while on the shell side of the heat exchangers steam with a pressure of 110 bara was generated.
One TLX (A) had heat-exchanger tubes made from a nickel-chromium-alloy which is usual for this 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 µm thick aluminium based layer applied in 3 steps.
The temperature of the quenched effluent coming from the TLX (A) in the beginning of the test was 420°C and the temperature of the quenched effluent coming from TLX (B) was 450°C.
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 500°C 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 560°C was obtained.
The fouling rate in TLX (B) (Curve B) was substantially constant and the extrapolated attainable hours of service will be 60 days instead of 26 like for TLX (A).
Herefrom 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.
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 pm, 48% by weight of a silicone resin comprising methyl groups and phenyl groups, and 40% by weight of toluene into the tube, draining the excess and heating the remaining layer for 2 hours at 300°C, 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 ^pm, 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 ₁₁m thick layer based on graphite, which was applied as follows:
A mixture of 24% by weight of graphite having a particle size of <1 pm, 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 300°C at which temperature the toluene was vaporized and the resin was subjected to reticulation. This processing was repeated once and finally the processing was repeated once with a mixture of 20% by weight of graphite having a particle size <1 pm, 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 the time.
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 now a TLX (D) was used having heat exchanger tubes the internal surface of which was coated with a polymeric layer, formed by mixing the oily fraction obtained when quenching the effluent from the cracking reactor (ethylene quench oil) with 1.5% of benzoyl peroxide and introducing said mixture into the heat exchanger tubes, draining off the excess and externally heating the tubes at 400°C.
The variation in the temperature of the effluent coming from TLX (D) is 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 polymer layer a slight fouling had been deposited.

Claims

1. Installation (plant) for thermocracking a hydrocarbon starting material to alkenes, comprising a cracking furnace with externally heated reactor tubes (coils) and a shell and tube heat exchanger ("quench" cooler, "transfer line" heat exchanger, TLX) to be used for quenching the reactor effluent and connected to the cracking furnace, wherein on the shell side steam is generated, characterized in that, the internal surfaces of the tubes of the heat exchangers are coated with a layer which is insensitive for and impermeable to the materials in the reactor effluent which are responsible for the fouling, said layer having been formed from a mixture of particles of graphite, or aluminium in a silicone resin.

2. Installation (plant) for thermocracking a hydrocarbon starting material to alkenes, comprising a cracking furnace with externally heated reactor tubes (coils) and a shell and tube heat exchanger "quench" cooler, "transfer line" heat exchanger, TLX) to be used for quenching the reactor effluent and connected to the cracking furnace, wherein on the shell side steam is generated, characterized in that, the internal surfaces of the tubes of the heat exchanger are coated with a layer which is insensitive for and impermeable to the materials in the reactor effluent which are responsible for the fouling, said layer being a polymer layer, formed by applying a mixture of the oily fraction which is recovered when quenching the effluent from the cracking reactor (alkylene quench oil) and a free radical forming initiator onto the inner surface of the tubes, e.g. by spraying, followed by thermosetting said layer.

3. Installation according to claim 1, characterized in that the coating layer has been formed from a mixture of aluminium particles and a silicone resin.

4. Installation according to claim 2, characterized in that the polymeric layer has been .formed by applying a mixture of the oily fraction which is recovered when quenching the effluent from the cracking reactor, and a peroxide as initiator onto the surface followed by thermosetting.

5. Shell and tube heat exchanger to be used in an installation for cracking a hydrocarbon starting material to alkenes, characterized in that the internal surfaces of the heat exchanger tubes are coated with a layer which is insensitive for and impermeable to the reactor effluent of a cracking furnace for the preparation of alkenes, said layer having been formed from a mixture of particles of graphite, or aluminium in a silicone resin.

6. Shell and tube heat exchanger to be used in an installation for cracking a hydrocarbon starting material to alkenes, characterized in that the internal surfaces of the heat exchanger tubes are coated with a layer which is insensitive for and impermeable to the reactor effluent of cracking furnace for the preparation of alkenes, said layer being a polymeric layer, formed by spraying a mixture of the oily fraction which is recovered when quenching the effluent from the cracking reactor (alkylene quench oil) and of an initiator forming free radicals, onto the internal surface of the lines followed by thermosetting.

7. Shell and tube heat exchanger according to claim 5, characterized in that the coating layer has been formed from a mixture of aluminium particles and a silicone resin.

8. Shell and tube heat-exchanger according to claim 6, characterized in that the polymeric layer has been formed, by applying a mixture of the oily fraction which is recovered when. quenching the effluent from the cracking reactor and a peroxide as initiator onto the surface followed by thermosetting.

9. Process for the manufacture of a shell and tube heat-exchanger, to be used in an installation for cracking a hydrocarbon starting material to alkenes and intended for quenching the effluent coming from the cracking reactor of such an installation, characterized in that the internal surfaces of the heat exchanger tubes are sprayed with a mixture of an oily fraction obtained when quenching the effluent from a cracking reactor for the preparation of alkenes and of an initiator forming free radicals, draining off the excess of the mixture from the heat exchanger tubes and heating the tubes at a temperature at which the mixture is cured.

10. Process according to claim 9, characterized in that as initiator a peroxide is used.

11. Process according to claim 9, characterized in that benzoyl peroxide is used as initiator.

12. Process according to claim 9-11, characterized in that a mixture is used which comprises 1-5% initiator.