FR2645874A1 - Process for decoking a hydrocarbon vapour cracking installation - Google Patents

Abstract

A process for decoking a hydrocarbon vapour cracking installation, which consists in injecting, into the hydrocarbon batch to be treated, solid particles consisting of a core of a material of low fragility, coated with a film (skin) of coke. The mixture of hydrocarbons and solid particles circulates in the tubes 12 of the vapour cracking furnace 10 and in the indirect quenching boiler 18, there produces decoking by erosion of the layer of coke formed on the internal walls of these parts of the installation, the particles being separated from the gaseous effluents from the vapour cracking in a cyclone 10 and being recycled by reinjection, for example at the inlet of the furnace 10 and at the inlet of the indirect quenching boiler 18. The invention permits decoking of a hydrocarbon vapour cracking installation during the normal operation of the installation.

Description

DECOKAGE PROCESS OF A VAPOCRACKING SYSTEM
HYDROCARBONS
The invention relates to a method of decoking an installation for steam cracking of hydrocarbons.

 It is already known, in order to eliminate the coke which is deposited on the internal walls of an installation for steam cracking of hydrocarbons, in particular on the walls of the tubes of the steam cracking oven through which the charge of hydrocarbons passes, to use a oxidizing chemical decoking process by an air-steam mixture, or by superheated steam with optionally the addition of hydrogen. However, this requires stopping the normal operation of the steam cracking installation.

 It has also been proposed to carry out this decoking by an erosion of the coke layer deposited by means of particles of solid material conveyed by a gaseous current circulating at high speed in the tubes of the steam cracking furnace. In general, the normal operation of the installation is stopped and the steam cracking furnace is isolated from the other parts of the installation located downstream, to be connected to the atmosphere.

 It has also been proposed to carry out continuous decoking of the installation, during its normal operation, by means of sand particles injected into the hydrocarbon charge to be treated. It is planned to trap the sand particles downstream of the steam cracking furnace, during the direct quenching of the steam cracking effluents by injection of heavy oil. However, it is then almost impossible to separate the sand particles from the quenching oil, which makes it impossible to reuse the quenching oil afterwards, even for burning it. In addition, the furnace tubes and the elbows connecting the tubes are subjected to intense erosion, which quickly leads to their destruction. This process has therefore never been used in practice.

 The invention aims to provide a simple, effective and inexpensive solution to these problems.

 It relates to a process for decoking an installation for steam cracking of hydrocarbons, which can be implemented without the normal operation of the installation being stopped, and without this resulting in a risk of deterioration of the components of the installation.

 To this end, it proposes a process for decoking an installation for steam cracking of hydrocarbons which comprises a steam cracking tube furnace and means for indirect quenching of steam cracking effluents, this process consisting in removing the coke deposited on the walls. internal of the installation, in particular of the furnace tubes and the indirect quenching means, by erosion by means of solid particles conveyed by a gaseous current at high speed, characterized in that said solid particles comprise a core of not very fragile material, coated with a thin layer or film of coke and have on average a size less than 250 μm.

 These solid particles make it possible to almost completely erode and eliminate the layer of coke deposited on the internal wall of the installation, without risking damaging the metal walls covered by the layer of coke. Indeed, when these solid particles strike a metallic paroli covered with a layer of coke, they can at most fracture this layer of coke and remove small pieces of it, but they are incapable of damaging the metal wall located under the coke layer. The coke film is fragile with respect to the metal of the tubes; in the event of a non-elastic impact on the metal, it fractures, which would not be the case if sand particles were used, for example, which would attack the metal.

 Consequently, it is possible to carry out continuous or substantially continuous decoking of the installation, by injecting these solid particles into the charge of hydrocarbons to be treated. The decoking can therefore be carried out without stopping the normal operation of the steam cracking installation, which results in significant gains.

 According to another characteristic of the invention, the method also consists in separating these solid particles from the gas stream which carries them by means of a cyclone, then mixing these solid particles with a hydrocarbon liquid devoid of heavy aromatic pyrolysis compounds, re-compress the particle-liquid mixture and reinject it into the installation, at least at one point in the steam cracking oven.

 The coke film which in fact covers the core of the solid particles prevents the particles from sticking and sintering together. It also allows an excellent flow of the particles, which facilitates their recovery by separation of the gas stream in a cyclone.

 The coke film also facilitates their wetting with a hydrocarbon liquid, which makes it easy to mix them with this liquid to recompress them by means of a pump and then reinject them into the steam cracking installation.

 According to another characteristic of the invention, the coke film formed on the core of the solid particles has a thickness of between 0.08 and 10 microns approximately. preferably between 0.15 and 5 microns.

 The size of the solid particles is on average less than about 250 microns. in particular less than 150 microns, and preferably between 4 and 45 microns approximately. In the latter case, their presence does not substantially modify the flow properties of the gas stream which carries them, but gives this gaseous stream a capacity for slight and regular erosion of the coke layer deposited on the internal walls of the installation, which allows this layer of coke to be eliminated as it is formed. By adjusting the quantity of solid particles transported by the gas stream, it is possible either to leave a very thin layer of coke on the internal walls of the installation, to protect them from any risk of erosion, or to completely rid them of the layer of coke deposited.

In addition, the tubes of steam cracking ovens are generally made of a metal alloy
Fe-Ni-Cr, and a hard layer of metallic oxide (essentially chromium oxide) is formed on their internal surface which will not be, or very little, attacked by these solid particles and which will therefore permanently protect the metal tubes.

 The invention also provides for adding make-up particles to the recycled solid particles, to compensate for the low loss of particles in the cyclone, these make-up particles may advantageously contain a coke catalyst, such as iron or nickel.

 It is therefore possible to use as make-up particles metal particles not coated with coke, or even particles, not regenerated, of a catalyst used in fluid catalytic cracking processes.

 As a variant, these make-up particles may also include a surface layer ~ of coke.

 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 schematically represents a part of an installation for steam cracking of hydrocarbons according to the invention.

 This installation essentially comprises an oven 10 for steam cracking a hydrocarbon charge, comprising tubes 12 through which the hydrocarbons pass, which are for example connected at their ends to manifolds 14 for supplying hydrocarbons and 16 for discharging effluents. steam cracking. The outlet manifold 16 is connected to an exchanger or an indirect quenching boiler 18, the outlet of which is connected, by one or more cyclones 20, to means 22 for distillation and direct quenching by injection of heavy oil.

 During the normal operation of this installation, the charge of vaporized hydrocarbons is brought to the supply manifold 14 and circulates at high speed in the tubes 12 where it undergoes steam cracking. The steam cracking effluents are transmitted by the outlet manifold 16 to the exchanger 18 where they undergo indirect quenching and are cooled to an intermediate temperature, less than approximately 600 ° C., but sufficient to avoid any condensate or liquid. The steam cracking effluents then reach the means of distillation and indirect quenching.

 The steam cracking of the hydrocarbons in the furnace 10 and their passage through the exchanger 18 results in a deposit of coke on the walls with which they are in contact.

 In the prior art, it is necessary to periodically stop the installation in order to carry out decoking, in general chemically by means of steam, the periods of continuous operation of the installation not being able to exceed a few weeks in the best case.

 The invention provides for continuous or discontinuous decoking of the installation, without stopping its normal operation, by means of solid particles which are injected into the charge of vaporized hydrocarbon brought into the tubes 12 via the manifold 14. These solid particles circulate with the hydrocarbon charge in the tubes 12, then in the indirect quench boiler 18, and are finally separated from the gaseous steam cracking effluents in the cyclone (s) 20.

The effluents leave the cyclone (s) 20 to gain the means of distillation and direct quenching, while the solid particles leaving the bottom of the cyclone (s) 20 are mixed in a tank 24 with a liquid, preferably a substantially free hydrocarbon liquid. of heavy aromatic pyrolysis compounds, to form a suspension of solid-liquid particles, the pressure level of which is raised by pumping, so that this suspension can be reinjected into the steam cracking installation, for example at the inlet of the supply manifold 14 and at the inlet of the quench boiler 18, as shown diagrammatically in the drawing.

 An addition of solid particles, intended to compensate for the losses of solid particles in cyclone 20, is carried out at 26.

 The solid particles which escape from the cyclone 20 with the gaseous steam cracking effluents are in very small quantity (the separation efficiency of the cyclone can be greater than 99%) and are trapped by the quench oil of the gaseous effluents.

 The nature of the solid particles and their size, as well as their quantity in relation to the flow of gas circulating in the installation, are determined so as to produce an erosion of the coke layer deposited in particular on the internal walls of the tubes 12 and of the indirect quenching boiler 18, without risking damaging these walls themselves.

 For this, the invention provides that the solid particles comprise a core of slightly brittle material, which is coated with a thin layer or film of coke. The overall size of these particles is, on average, less than 250 microns, more precisely less than 150 microns and is preferably between 4 and 45 microns approximately. The coke film formed on the core of the particles has a thickness of between about 0.08 and 10 microns, preferably between about 0.15 and 5 microns. These dimensions depend on the mass of the nucleus of the metallic particles, and therefore on the nature of this nucleus. In a particular embodiment of the invention, the core of the solid particles used is metallic and has an average size of between 4 and 30 microns, preferably between 4 and 15 microns approximately. The particle flow rates are very low, for example between 0.05 and 3 * by weight of the gas flow rate, and preferably between 0.1 and 1% by weight.

 The coke film which coats the nucleus of the particles will generally erode or fragment in the event of a violent impact on metal walls. The risk of erosion of the metal of these walls is therefore reduced, all the more so since this metal is generally protected by a very hard oxide layer (chromium oxide, silicon oxide, aluminum oxide).

 The make-up particles which are added at 26 optionally contain a coke catalyst, such as iron or nickel. Metallic cores which are not initially coated with coke can therefore be used as make-up particles. When these particles circulate in the steam cracking installation and are recycled there, they gradually cover themselves with coke.

 These make-up particles may also be particles of catalysts, which have been used in processes for fluid catalytic cracking of hydrocarbons, and which have not been regenerated.

 It is of course also possible to use make-up particles which are already provided with a surface layer of coke.

 The make-up particles can be stored in the form of a solid-liquid mixture, the liquid possibly being water, or a hydrocarbon liquid devoid of heavy aromatic pyrolysis compounds, for example a fraction of the hydrocarbon charge to treat. The addition of these particles to the stream of recycled particles therefore poses no problems, nor does their addition to the hydrocarbon charge. In addition, for the storage of these particles in the form of a solid-liquid suspension, the presence of the coke film makes it possible to benefit from the experience acquired on coal-water or coal-fuel oil mixtures, for the choice of additives for stabilizing the suspension (the surface condition of the particles being of the same nature). The erosion of the suspension feed pump is also reduced, thanks to the coke film.

The invention therefore confers decisive advantages to techniques of this type.

Claims (9)

 1) Method for decoking an installation for steam cracking of hydrocarbons, which comprises a steam cracking tube furnace (12) and means (18) for indirect quenching of steam cracking effluents, the process consisting in removing the coke deposited on the internal walls of the installation, in particular tubes (12) and means (18) for indirect quenching, by erosion by means of solid particles conveyed by a gas stream at high speed, characterized in that said solid particles comprise a core of slightly brittle material coated with a thin layer or film of coke and have on average a dimension of less than 250 μm.  2) A method according to claim l, characterized in that said solid particles are injected into the installation during normal operation thereof and are conveyed by the hydrocarbon feed to be treated, the method also consisting in separating these solid particles gas stream by means of a cyclone (20), to mix them with a liquid, for example hydrocarbon free of heavy aromatic pyrolysis compounds, to recompress the mixture of solid-liquid particles, and to reinject it into the installation, at least at a point in the steam cracking oven.  3) Method according to claim 1 or 2, characterized in that the coke film formed on the core of the particles has a thickness between about 0.08 and 10 microns, in particular between about 0.15 and 5 microns.  4) Method according to one of the preceding claims, characterized in that the solid particles have a diameter between about 4 and 45 microns.  5) Method according to one of the preceding claims, characterized in that the core of said particles is metallic and has a size between 4 and 15 microns approximately.  6) Method according to one of the preceding claims, characterized in that it consists in adding make-up particles to the recycled solid particles, these make-up particles possibly containing a coke catalyst, such as iron or nickel.  7) Method according to claim 6, characterized in that the make-up particles comprise a metal core not coated with coke.  8) Process according to claim 6, characterized in that the make-up particles are particles of catalysts used in fluid catalytic cracking processes, and not regenerated.  9) Method according to claim 6 or 8, characterized in that the make-up particles comprise a surface layer of coke.