FR2653779A1 - Process for decoking a plant for steam-cracking hydrocarbons and corresponding plant - Google Patents

Abstract

Process for decoking a plant for steam-cracking hydrocarbons, comprising pipes (12) of small diameter joined via a manifold (16) to a pipe (18) of larger diameter, consisting in decoking the small pipes (12) in turn by injecting steam at their upstream ends and in decoking the pipe (18) of large diameter by means of erosive solid particles injected into the manifold (16).

Description

 The invention relates to a method of decoking an installation for steam cracking of hydrocarbons, as well as the corresponding installation, allowing the execution of this process.

 We know that usually it is necessary to stop, at more or less regular intervals, the operation of an installation for steam cracking of hydrocarbons, to decoker it, that is to say to remove the layer of coke deposited on the internal walls of the installation which are in contact with the hydrocarbon charge during the cracking reaction. These interruptions in the operation of the installation reduce the overall efficiency of the latter.

 To avoid this drawback, it has been proposed to decoke the installation during its normal operation, by injecting into the hydrocarbon passage tubes, a small quantity of erosive solid particles of small particle size, which make it possible to eliminate by erosion the coke layer which forms on the internal walls of the tubes, as well as inside an indirect quenching exchanger for gaseous effluents, to which the tubes are connected at the outlet of the cracking furnace.

 This technique is preferably applicable to single pass ovens, in which the hydrocarbon charge circulation tubes are straight and do not have a bend. It is in fact understood that the erosion of the elbows by the solid particles conveyed by the charge of hydrocarbons circulating at high speed, risks being significant and causing more or less rapid deterioration or destruction of these elbows.

 The subject of the invention is a method for decoking an installation for steam cracking of hydrocarbons, during the normal operation of this installation, when the installation comprises a steam cracking oven with several passes, and in particular with two passes, in which . tubes of small diameter are joined by a collector to a tube of larger diameter, the outlet of which is connected to an indirect quench exchanger situated outside the furnace.

 The invention also relates to a steam cracking installation of this type, specially designed to allow the execution of this process.

 The invention therefore proposes, firstly, a method of decoking an installation for steam cracking of hydrocarbons, this installation comprising a steam cracking oven in several passes of a load of hydrocarbons and steam, a first at least pass consisting of at least one series of small diameter tubes joined by a collector to a larger diameter tube constituting the last pass, and an indirect quenching exchanger for gaseous effluents mounted between the outlet of the larger diameter and means for direct quenching of the effluent, characterized in that it. consists of decoking the small diameter tubes in turn by injecting water vapor at their upstream ends, and decoking the large diameter tube and the indirect quench exchanger by injecting solids. means less than approximately 300 microns, in the aforementioned collector, the injection of water vapor taking place first in at least one tube of small diameter, then in at least another, and so on, the steam flow water injected into a tube being determined to prevent the passage of the hydrocarbon charge in this tube, decoking being carried out during normal operation of the installation.

 The decoking of the furnace tubes is therefore carried out in two different ways, depending on which one. these are small diameter tubes or larger diameter tubes, the decoking of small diameter tubes being carried out by chemical reaction of the coke and the water vapor injected, that of the larger diameter tube being produced by erosive solid particles, injected into the collector which connects the small diameter tubes to this larger diameter tube. The solid particles passing through this tube also decoke the indirect quench exchanger mounted at the outlet of this tube, and can then be separated from the stream of gaseous effluents, for example in a cyclone.

 The hydrocarbon and water vapor feed to the steam cracking installation normally passes through the small diameter tubes which are not subjected to steam decoking, then through the large diameter tube for final cracking. The water vapor leaving the tube or tubes of small diameter subjected to decoking, mixes in the collector with the load of hydrocarbons which gains the tube of large diameter. This additional water vapor has a favorable influence on the final cracking of the charge in the large diameter tube.

 This mixed decoking according to the invention is particularly advantageous because it. allows continuous operation of the installation, without risk of erosion of the elbows in the first passes of the installation, which are decoked by water vapor.

 According to another characteristic of the invention, the decoking with steam of the tubes of small diameter and that of the tube of large diameter by the erosive particles, can take place simultaneously, or not.

 To improve the decoking of the quench exchanger, an additional quantity of erosive particles can be injected at the downstream end of the large diameter tube, outside the furnace, immediately upstream of the inlet of the quench exchanger. indirect.

 According to a variant of the invention, this method also consists in carrying out a partial decoking of each tube of small diameter by injection into this tube of a small amount of erosive solid particles.

 This partial decoking can be carried out during normal operation of the installation or during decoking with steam.

 Finally, the invention also provides for injecting, at the inlet of the indirect quenching exchanger, a gas colder than the gaseous effluents, in order to carry out a limited pre-quenching of these effluents, less than approximately 1500C.

The gas used can be ethane, cracked and cooled or recycled pyrolysis gasoline, in particular hydro-treated, for example C5 fractions or
C6 after benzene extraction.

 The invention also provides a steam cracking installation of the aforementioned type, allowing the execution of the process which has just been described, characterized in that it comprises water vapor injection conduits connected to the upstream ends of the tubes of small diameter and comprising members such as valves for opening and closing these conduits, means for controlling these members making it possible to decoker the small diameter tubes in turn by injecting water vapor into these tubes, and means for injecting erosive solid particles, having an average diameter of less than approximately 300 microns, into the collector connecting the small-diameter tubes to the large-diameter tube.

This installation can also include:
- Means for injecting an additional flow of erosive solid particles at the downstream end of the large diameter tube;
- Means for injecting a low flow rate of erosive solid particles at the upstream ends of the small diameter tube;
means for injecting a gas colder than the gaseous effluents at the inlet of the indirect quenching exchanger, in order to carry out a limited pre-quenching, less than approximately 1500C, of the gaseous effluents.

 The invention will be better understood and other details, characteristics and advantages thereof will appear more clearly on reading the description which follows, given by way of example with reference to the appended drawing, which is a schematic view of part of a steam cracking installation sel.on the invention.

 In this drawing, the reference 10 designates a steam cracking oven, which comprises a series of straight tubes 12 of small diameter, fed at their upstream ends by a collector 14 located outside the oven and joined at their downstream ends by a collector 16, possibly insulated, located inside the furnace 10. The col.reader 16 feeds a tube 18 of larger diameter, straight, the outlet end of which is connected, outside the furnace, to a heat exchanger 20 forming means for indirect quenching of the gaseous steam cracking effluents. The outlet of the exchanger 20 is connected to means 22 for direct quenching of the gaseous effluents.

 In this installation, the steam cracking charge, consisting of a mixture of hydrocarbons and water vapor, is brought to the collector 14, circulates in the small tubes 12, then circulates in the opposite direction in the tube 18 of larger diameter, leaves the oven to pass through the indirect quenching exchanger 20 and reaches the means 22 for direct quenching.

 The cracking of the hydrocarbons in the small tubes 12, the collector 16 and the large tube 18 results in the deposition of coke on the walls of these tubes and of this collector. The coke is also deposited, in a significant way, on the walls of the exchanger 20, which are in contact with the gaseous steam cracking effluents.

 The invention allows the installation to be decoked, without interrupting its normal operation and without risk at the level of the connection between the small tubes 12 and the large tube 18.

 For this, the installation comprises conduits 24 for injecting water vapor (or water vapor-hydrogen mixture) into the small tubes 12 at the upstream ends thereof, outside the oven 10 .

Each conduit 24 comprises a valve or other similar means 26 for opening and closing, and is connected to a means 28 for supplying steam (or steam-hydrogen mixture). The valves 26 of the various conduits 24 are connected to a means 30 for sequential opening and closing control, so that only one valve 26 or a very small number of valves 26 can be opened at a time, the others valves 26 being closed. The flow rate of water vapor (or of water vapor-hydrogen mixture) injected into a small tube 12 is adjusted so that it opposes the entry of the steam cracking charge into this tube. The invention also provides means for injecting erosive solid particles at the upstream end of the large tube 18, preferably at the upstream ends of the reader collar 16 supplying this large tube.

These means (for example a suspension of solid particles in a stream of water vapor) are shown diagrammatically in the drawing and are designated by the reference 32.

 The erosive solid particles have an average diameter or particle size of less than approximately 300 microns, preferably between 4 and 85 microns approximately. The flow rate of particles injected is between 0.001 and 6% by weight of the steam cracking charge (hydrocarbons plus water vapor) and preferably between 0.01 and 3% by weight of this charge. This small quantity of solid particles conveyed by the steam cracking charge then constitutes an erosive gas which, by flow in the tube 18 and the exchanger 20, eliminates the deposited coke or at least limits the thickness of the deposited coke layer. on the internal walls of the tube 18 of the exchanger 20.

 One can also provide, as shown schematically in the drawing, in its right part, means 34 for injecting a very small amount of these erosive solid particles into the tubes 12 of small diameter, at their upstream ends. Another substantially equivalent possibility consists in injecting the particles into the inlet col.lector 14 or upstream of this collector. In this case, it is possible first of all to partially decoke the tubes 12 by means of the erosive solid particles, and to terminate their decoking by injecting steam.

 It is advantageous to provide means 36 for injecting additional erosive solid particles into the stream of gaseous effluents leaving the furnace 10, immediately at the inlet of the indirect quench exchanger 20 to improve the decoking of the latter.

 The invention also provides for injecting at this level, that is to say immediately at the inlet of the exchanger 20, a gas 38 colder than the gaseous steam cracking effluents, so as to carry out a pre-quenching of these effluents, limited to approximately 1500C, and for example between 50 and 1300C.

 This pre-quenching gas can be cooled cracked ethane, or possibly recycled pyrolysis gasoline, preferably hydro-treated, for example C5 or C6 fractions having a low octane number after extraction of the benzene.

 The injection of a pre-quenching gas, immediately at the outlet of the tube 18, upstream of the indirect quenching exchanger 20, is advantageous in a hydrocarbon steam cracking installation, whatever the type of this installation or the type of decoking process used. Indeed, the pre-quenching makes it possible to avoid or limit a post-cracking of the effl.uents at the outlet of the oven 10; moreover, when the injected gas comprises fractions of pre-cracked ethane or of C4 or C5 or C6 cuts as described above, there is a slight additional cracking. of these compounds in the inlet of exchanger 20, which leads to an additional production of sought-after olefins.

This steam cracking installation is used as follows:
The steam cracking charge (hydrocarbons plus water vapor) is brought by the collector 14, circulates at high speed inside the furnace 10, in the tubes 12 of small diameter, the collector 16 and the tube 18 of large diameter, leaves the oven 10, undergoes a pre-quenching limited to 38 (less than approximately 1500C, and preferably between 50 and 1300C), then an indirect quenching in the exchanger 20 and a final quenching in the means 22.

 When it is desired to decoke the installation, the circuit 30 controls the opening of one or more valves 26 and water vapor is injected into the tube or tubes 12 via the conduit or conduits. correspondents. By a gas to water reaction, the coke deposited in the small tubes 12 is removed. The water vapor leaving these small tubes 12 at their downstream ends mixes in the collector 16 with the steam cracking charge which circulates in the other tubes 12 (those which do not undergo steam decoking). After a determined interval of time, the valves 26 which were open are closed, and one or some other valves 26 are opened, in order to decoke other tubes 12.

Simultaneously or not, erosive sol.ide particles can be injected into the collector 16 at 32 and at the inlet of the exchanger 20, at 36.

 It is also possible, simultaneously, to inject a small amount of erosive solid particles into the tubes 12 of small diameter, at their upstream end, by means 34.

 The solid particles conveyed by the gas stream erode the coke layer deposited on the internal walls of the tubes and of the exchanger 20 and carry out decoking. A cyclone, interposed for example between the exchanger 20 and the direct quenching means 22, makes it possible to separate the erosive solid particles from the stream of gaseous effluents.

 As the quantity of particles circulating in the tubes 12 is relatively small, or is zero, there is no risk of erosion at the outlet of these tubes in the collector 16. The greater quantity of particles fed in 32 , which circulates in the tube 18, does not lead to erosion problems because the tube 18 does not include bends. The installation is therefore very reliable.

 The collector 16 may be straight or bent and the point of injection of the particles may be located at the inlet of the collector or at other points, for example at its outlet, at the connection with the large diameter tube.

 He. will advantageously be reinforced in thickness, thermally insulated and jacketed or made up of an alloy which is not very cathalic for coking, for example an alloy containing from 2 to 15%, in particular from 3 to 10% of aluminum.

Claims (11)

 1. Method for decoking an installation for steam cracking of hydrocarbons, this installation comprising a steam cracking oven (10) in several passes of a charge of hydrocarbons and water vapor, at least one pass consisting of at least one series of tubes (12) of small diameter joined by a collar-reader (16) to a tube (18) of larger diameter constituting the last pass, and an exchanger (20) for indirect quenching of gaseous effluents, mounted between the outlet of the tube (18) of large diameter and means (22) for direct quenching of the effluents, characterized in that it. consists in decoking the small diameter tubes in turn by injecting water vapor at their upstream end, and decoking the large diameter tube (18) and the exchanger (20) for indirect quenching by injecting soil particles. erosive ides, having an average particle size of less than approximately 300 microns, in the aforementioned collector (16), the injection of water vapor taking place first in at least one tube (12) of small diameter then in at least one other tube (12) of small diameter and so on, the flow of water vapor injected into a tube (12) being determined to prevent the passage of the hydrocarbon charge in this tube, decoking being carried out during normal operation. of the installation.  2. Method according to claim 1, characterized in that the decoking with steam of the tubes (12) of small diameter and cel.ui of the tube (18) of large diameter by the erosive particles, take place either simultaneously, either non-simultaneously.  3. Method according to claim 1 or 2, characterized in that it consists in adding hydrogen to the water vapor injected into the tubes (12) of small diameter.  4. Method according to one of the preceding claims, characterized in that it consists in injecting an additional quantity of erosive particul.es downstream of the tube (18) of large diameter, at the inlet of the exchanger (20) indirect quenching, to improve the decoking of the latter.  5. Method according to one of the preceding claims, characterized in that it consists in carrying out a partial decoking of the tubes (12) of small diameter by injection into these tubes or upstream of a small quantity of erosive solid particles.  6. Method according to one of the preceding claims, characterized in that it consists in injecting at the inlet of the exchanger (20) indirect quenching, a gas colder than the gaseous effl.uents to achieve a limited pre-soaking of these effluents, less than approximately 1500C.  7. Method according to claim 6, characterized in that the aforementioned gas is cooled cracked ethane or recycled pyrolysis gasoline, in particular hydro-treated, for example.e C5 fractions or C6 after benzene extraction.  8. Installation for steam cracking of hydrocarbons, comprising a steam cracking oven (10) in several passes of a charge of hydrocarbons and steam, at least one pass consisting of at least one series of tubes ( 12) of small diameter, joined by a collector (16) to a tube (18) of larger diameter constituting the last pass, and an exchanger (20) for indirect quenching of the gaseous effluents, mounted between the outlet of the tube (-18 ) large diameter and means (22) for direct quenching of the effluents, characterized in that it comprises conduits (24) for injecting water vapor connected to the upstream ends of the tubes (12) of small diameter and comprising members (26) such as valves for opening and closing these conduits, means (30) for controlling these members, making it possible to decoker in turn the tubes (12) of small diameter by injection of steam d in these tubes, and means (32) for injecting erosive solids, ayan t an average particle size of less than approximately 300 microns, in the collector (16) connecting the small diameter tubes to the large diameter tubes.  9. Installation according to claim 8, characterized in that it comprises means (36) for injecting an additional flow of solid particles at the downstream end of the tube (18) of large diameter.  10. Installation according to claim 8 or 9, characterized in that it comprises means (34) for injecting a low flow rate of particul.es solids into the upstream ends of the tubes (12) of small diameter, or in upstream of these ends.  11. Installation according to one of claims 8 to 10, characterized in that it comprises means (38) for injecting a gas colder than the gaseous effluents at the outlet of the tube (18) of large diameter, to carry out a limited pre-quenching, less than approximately 1500C, of the effluents.