FR2645875A1 - Process and device for trapping and recycling solid particles in a hydrocarbon vapour cracking installation - Google Patents

Abstract

A device for recycling solid particles used for the decoking of a hydrocarbon vapour cracking installation, this device comprising a cyclone 30 for separating the solid particles and the gaseous effluents from the vapour cracking and a flask 36 for trapping solid particles by means of a liquid 38 devoid of heavy (low volatility) hydrocarbons from pyrolysis, the mixture of liquid 38 and solid particles being recompressed by a pump 40 and reinjected into the vapour cracking installation. The invention is generally concerned with the decoking of hydrocarbon vapour cracking installations.

Description

METHOD AND DEVICE FOR CAPTURING AND RECYCLING PAR
SOLID TUBES IN A STEAM PLANT
HYDROCARBONS
The invention relates to a method and a device for capturing and recycling solid particles in an installation for steam cracking of hydrocarbons.

 It has already been proposed to decoke the internal walls of a hydrocarbon steam cracking installation, by means of erosive solid particles which are mixed with the charge of vaporized hydrocarbons and which circulate with this charge in the installation, in particular in the tubes of the steam cracking oven and in the exchanger carrying out an indirect quenching of the gaseous steam cracking effluents leaving the oven.

 These solid particles carry out an erosion and / or fracturing, more or less violent depending on their size, of the coke layer deposited on these internal walls and must then be separated from the gaseous steam cracking effluents. For this, it has been proposed to trap these solid particles with heavy oil used for the direct quenching of gaseous effluents. However, it then proved impossible to separate the solid particles from the heavy oil, which prohibits recovering them for recycling, and also prohibits the use of the heavy oil containing these particles, even as fuel.

 This technique could therefore not be implemented in practice.

 The subject of the invention is a method and a device for capturing and recycling solid particles in a hydrocarbon steam cracking installation, which make it possible to avoid the abovementioned drawbacks.

 It also relates to a process and a device of this type, which make it possible to decoke the steam cracking installation during its normal operation, to capture the erosive solid particles used for decoking, and then to recycle them in the installation.

 To this end, it proposes a process for capturing and recycling solid particles in a hydrocarbon steam cracking installation, in which solid particles are transported by the charge of hydrocarbons to be treated and circulate in the installation to carry out its decoking, characterized in that it consists in cooling the gaseous effluents, at the outlet of the steam cracking furnace, to an intermediate temperature where the cracking reactions are very reduced, and for which there is no condensation in the liquid phase, then passing the gaseous effluents through at least one cyclone to separate most of the solid particles therefrom, bringing the separated particles into contact with a liquid containing substantially no heavy pyrolysis hydrocarbons, and recompressing the particle-liquid mixture and injecting it into the steam cracking installation, to recycle the solid particles.

This results in a number of advantages
- due to the limited cooling of the gaseous effluents at the outlet of the steam cracking furnace, there is no over-cracking reaction in the separation cyclone,
- any pollution of the quenching oil or of the sections of the installation located downstream is avoided by solid particles,
- unlike the known methods of washing a gas for recovering particles, any contact between the gas and the liquid is avoided, which prevents significant capture of heavy aromatics, which cannot be separated from the particles subsequently,
- It is possible to decoke the steam cracking installation during its normal operation, resulting in significant productivity gains.

 According to another characteristic of the invention, the abovementioned liquid is a small fraction of the hydrocarbon charge, for example less than 5% by weight thereof, or else recycled pyrolysis gasoline, or else hydrocarbons having 5 to 23 carbon atoms, or even water.

 This liquid can be added without disadvantage to the load of hydrocarbons to be treated.

 According to yet another characteristic of the invention, the flow rate and the temperature of the particle-liquid mixture are determined in order to obtain an almost instantaneous vaporization of the liquid upon injection of this mixture into the steam cracking installation.

 This avoids solid deposits on the walls of the installation downstream of the injection points.

 This mixture is preferably atomized to promote its vaporization, and very fine atomization or spraying can be obtained with steam or by adding to the mixture, before its injection, a cut of light hydrocarbons, comprising for example from 2 to 4 carbon atoms, so that the mixture is at a temperature higher than its bubble temperature corresponding to the pressure of the injection point (s) in the installation.

 This avoids having to heat the particle-liquid mixture before recycling.

 Advantageously, the cut of light hydrocarbons comes from a C4 pyrolysis cut, recycled, preferably after extraction of the butadiene.

 The residual C4 cut, after extraction of the butadiene, is an acceptable cracking charge, readily available on site.

 According to yet another characteristic of the invention, in order to bring the solid particles leaving the cyclone into contact with the aforementioned liquid, a continuous flow of liquid is carried out, with possibly a vortex movement, on a wall situated around and below the zone arrival of particles.

 This prevents particles from accumulating on the aforementioned wall, and also prevents the liquid from forming droplets which would be liable to obstruct the supply pipe for solid particles, by bonding of particles on a wet wall. .

 This last drawback can also be avoided if a very small part of the gaseous steam cracking effluent flow is taken from the cyclone with the solid particles to avoid any possible rise of droplets in the particle passage duct. It is then possible to carry out a washing of the effluent-gas-solid particles mixture leaving the cyclone, using the abovementioned liquid. The heavy hydrocarbons contained in the withdrawn fraction of the gaseous effluents are trapped by the aforementioned liquid and finally recycled in the installation.

However, their quantity is so small that their effect can be considered negligible.

 The invention also provides a device for capturing and recycling solid particles in an installation for steam cracking of hydrocarbons, by carrying out the aforementioned process, this device being characterized in that it comprises at least one cyclone for separating pavticles. solids and gases, supplied with a stream of gaseous steam cracking effluents from a steam cracking furnace and an indirect quenching exchanger, means for bringing the separated solid particles into contact with a liquid not substantially containing heavy pyrolysis hydrocarbons, means, such as a pump, for recompressing the solid-liquid particle mixture, and means for injecting this mixture at at least one point of the steam cracking installation.

 In a first embodiment of the invention, the abovementioned contacting means comprise a balloon, supplied with solid particles by the cyclone and comprising a wall surrounding the arrival of these solid particles, means for producing a continuous flow, for example with a rotating component, liquid on this wall, and means for recycling the liquid between the outlet and the inlet of the balloon.

 In another embodiment of the invention, these contacting means comprise a downward conduit connected to the outlet of the cyclone and through which the solid particles pass and a very small fraction of the flow of gaseous effluents produced by the installation , points of injection and spraying of the aforementioned liquid into this conduit, and a reservoir to which the conduit terminates. A pump makes it possible, on the one hand, to supply the injection and spraying points of the liquid in the abovementioned descending conduit and, on the other hand, to recycle in the installation the liquid mixture of solid particles contained in the tank.

The invention will be better understood and other characteristics, details and advantages thereof will appear more clearly on reading the description which follows, given with reference to the appended drawings, in which:
FIG. 1 schematically represents a ligation of hydrocarbon steam cracking comprising a device for capitation and recycling of solid particles according to the invention
Figure 2 schematically shows an embodiment of this device
FIG. 3 schematically represents an alternative embodiment of this device.

 The hydrocarbon steam cracking installation shown diagrammatically in FIG. 1 comprises a steam cracking furnace 10, means 12 for indirect quenching of the steam cracking ef fluents leaving the furnace 10, and means, not shown, for distillation and direct quenching ( by injection of heavy oil) steam cracking effluents.

 The furnace 10 comprises, for example, single-pass tubes 14 supplied with hydrocarbons by an intake manifold 16 and connected, at their opposite end, to individual indirect quench exchangers, forming the aforementioned means 12 and themselves connected to a manifold of exit 18.

 The load of hydrocarbons to be vaporized is brought to the liquid state by a line 20 in a zone 22 of the furnace 10 used to heat and vaporize the hydrocarbons. A pipe 24 for supplying water vapor joins the pipe 20 in this zone of the furnace 10 and a preheating pipe 26 makes it possible to bring the vaporized hydrocarbon-water vapor mixture to the manifold 16 for supplying the tubes 14.

 To carry out the decoking of the installation, and in particular that of the tubes 14 of the furnace 10 and of the indirect quenching exchangers 12, solid particles are added to the charge of vaporized hydrocarbons, for example as indicated diagrammatically at 28 at the inlet. of the preheating duct 26, and erode the coke layer formed on the internal walls of the tubes 14 of the furnace and of the indirect quench exchangers 12.

 These solid particles, the nature and particle size of which are determined as a function of the results sought, must be separated from these gaseous effluents by steam cracking before distillation and direct quenching of the latter.

 For this, the invention provides at least one cyclone 30 for gas-solid separation, to which the outlet manifold 18 is connected, and which comprises an upper pipe 32 for the outlet of the gases and a lower pipe 34 for the outlet of the solid particles. This outlet conduit 34 opens into a balloon 36 containing a substantially de-liquid 38 provided with heavy pyrolysis hydrocarbons. In this balloon 36, the solid particles are captured by the liquid 38 and the solid particle-liquid mixture is taken from the bottom of the balloon 36 and recompressed by a pump 40 to be reinjected into the steam cracking installation, in particular at the inlet 28 of the preheating duct 26 and possibly at the inputs of the exchangers 12.

 Part of the recompressed mixture supplied by the pump 40 is returned to the upper part of the balloon 36, to form a continuous film of liquid flowing over the internal wall of this balloon. This avoids agglomerations of solid particles on the inner wall of the balloon.

It also avoids the formation of drops of liquid which could be fixed at the outlet of the conduit 34 and would risk blocking it.

 The conditions for injecting the solid-liquid particle mixture into the installation are adjusted so that there is an almost instantaneous vaporization of the liquid, in order to avoid a deposit of solid material on the internal walls of the installation. . This vaporization is favored by a very fine atomization or spraying of the mixture of solid particles and liquid. For this, it is possible to add, as schematically indicated in 41, light hydrocarbons to the mixture, before its injection into the installation. By this means, the temperature of the mixture can be higher than its bubble temperature for the pressure at the injection point in the installation. This avoids preheating the solid particles-liquid mixture before injecting it into the installation. The liquid can also be atomized with steam.

 The hydrocarbons used for this can be a C4 pyrolysis cut, recycled, after extraction of the butadiene.

 Cyclone 30 has a very high efficiency of separation of solid particles and gas, this efficiency being able to be greater than 99%. Most of the solid particles will therefore be separated from the gaseous steam cracking effluents in the cyclone and can be recycled. To compensate for the very small quantity of solid particles which leave the cyclone 30 with the gaseous effluents via the pipe 32, an addition of solid particles is provided in the tank 36, for example as indicated diagrammatically at 42 on a supply pipe 44 of the balloon 36 in liquid. In the example shown in FIG. 1, this pipe 44 comes from the supply of liquid hydrocarbons to the installation, the liquid in the tank 36 being constituted by a small fraction, less than 5% by weight, of the hydrocarbon charge. to crack.

 To this fraction of the charge feeding the flask 36, it is possible to add pyrolysis gasoline, as indicated diagrammatically in 46. It is also possible to use only pyrolysis gasoline, or virgin diesel fuel (not cracked).

 The solid particles, to facilitate their capture and recycling, are preferably wettable by the liquid 38. If necessary, additives can be added to this liquid improving the wetting of the solid particles and the stability of the suspension, and preventing foaming of the liquid. The amounts of solid particles will preferably be low, relative to the gaseous charge, for example between 0.05 and 3% by weight and preferably between 0.1 and 1% by weight.

 Generally, the solid particles used have a relatively small particle size, just sufficient to be able to be separated from the gaseous steam cracking effluents in cyclone 30 with a predetermined efficiency (for example 99% or more). These solid particles may have dimensions of less than about 150 microns, preferably -85 microns, depending on their nature and their density. It is possible in particular to use solid particles formed of a metal core covered with a thin film of coke, or alternatively particles of fluid catalytic cracking catalysts, preferably not regenerated, or fines of cement, etc.

 The very small quantity of solid particles which leaves cyclone 30 with the gaseous steam cracking effluents 32, is trapped by the oil used for the direct quenching of these effluents, and therefore has no effect on the parts of the installation. located downstream of the cyclone.

 FIG. 2 represents, in more detail, a device for capturing solid particles according to the invention.

 This device consists of the balloon 36, the upper part of which is traversed by the outlet pipe 34 of the cyclone 30. The lower part of the balloon 36, for example of conical shape, is connected to the pump 40.

The upper part of the balloon 36 comprises, around the lower part of the duct 34 a chamber 48 for continuous flow of liquid, delimited by an external wall 50 and an internal wall 52, which are of revolution and which provide a free space around the duct 34. From the chamber 48, which is supplied with liquid by the pump 40, the liquid flows in continuous movement, advantageously swirling, from a "source line" on the wall 50, without making droplets, and captures the solid particles leaving the duct 34. The non-wetted parts of the device can be kept at a temperature sufficient to avoid any condensation. To further reduce the risks of obstruction of the lower end of this duct, it is possible, by a duct 54, to sampling of a low gas flow rate inside the balloon 36 around the pipe 34. A very small fraction of the flow of gaseous vapor effluents is therefore passed through the pipe 34 cracking circulating in the cyclone 30, to prevent any entry of droplets in the conduit 34 and to facilitate the descent of the solid particles.

 The flow of gaseous effluents taken at 54. can be between 1 per 100 and 1 per 1000, for example the flow of gaseous effluents in cyclone 30.

 It is also possible to supply the chamber 48 with liquid comprising no particles, by means of a pump 55 sucking the liquid slightly below its free surface in a decanted zone of the balloon 36.

 It may also be advantageous to provide a loopback circuit on the pump 40, as shown in dotted lines, in order to increase the flow rate of liquid flowing through this pump.

 In order to avoid or reduce the deposition of particles in this pump, it is also possible to inject additives for stabilizing the particle-liquid suspension, for example medium hydrocarbons (such as diesel) to increase the viscosity of the liquid.

 In the variant embodiment shown in FIG. 3, the outlet pipe 34 for the particles of the cyclone 30 is connected, by an elbow pipe 56, to a downcomer 58 into which a certain quantity of capture liquid is sprayed, at several points. 60.62.64 respectively. The conduit 58 terminates in a reservoir 66 for storing the solid-liquid particle mixture. A pump 40 makes it possible, as in the previous embodiments, to withdraw this mixture from the reservoir, to recompress it, and to reinfect it in the installation. steam cracking, while supplying the injection points 60, 62 and 64 in the downcomer 58. The top-up 44 of capture liquid takes place in a part 68 of the tank, which is separated by a partition 70, provided of a communication orifice in its lower part, of the other part of the tank into which the conduit 58 opens. A conduit 72 makes it possible to take a gas flow rate in the upper part of the reservoir 66.

 In this variant of ~ embodiment of the device, a very small fraction of the flow of gaseous effluents circulating in the cyclone 30, passes with the solid particles in the conduit 34, the conduit 56, the descent conduit 58 and ends at the reservoir 66 , to come out through the sampling conduit 72.

 This variant embodiment of the capture device makes it possible to treat a larger quantity of solid particles than the device in FIG. 2. It also makes it possible to treat particles that are less easily wettable by the capture liquid.

Claims (13)

 1) Method for capturing and recycling solid particles in a hydrocarbon steam cracking installation, in which the solid particles are transported by the hydrocarbon charge to be treated and circulate in the installation to carry out its decoking, characterized in that '' it consists in cooling the gaseous effluents leaving the steam cracking furnace (10) to an intermediate temperature where the cracking reactions are very reduced and for which there is no condensation in the liquid phase, then passing the gaseous effluents in at least one cyclone (30) to separate most of the solid particles therefrom, to bring the separated particles into contact with a liquid (38) containing substantially no heavy pyrolysis hydrocarbons, to recompress the particle-liquid mixture and inject it into the steam cracking installation, to recycle the solid particles.  2) Method according to claim 1, characterized in that the aforementioned liquid (38) is a small fraction of the hydrocarbon charge, for example less than 5% by weight thereof, or recycled pyrolysis gasoline , or also hydrocarbons having 5 to 23 carbon atoms, or even water.  3) Method according to claim 1 or 2, characterized in that the aforementioned solid particles are wettable by the liquid (38 !.  4) Method according to the urn of claims 1 to 3, characterized in that the flow rate and the temperature of the particle-liquid mixture are determined to obtain an almost instantaneous vaporization of the liquid upon injection of the mixture into the installation of steam cracking.  5) Method according to one of the preceding claims, characterized in that it consists in adding to the particle-liquid mixture, before its injection into the installation, a cut of light hydrocarbons so that the mixture is at a temperature higher than its bubble temperature corresponding to the pressure of the injection point in the installation.  6) Method according to claim 5, characterized in that the cut of light hydrocarbons comes from a C4 pyrolysis cut, recycled, in particular after extraction of butadiene.  7) Method according to one of the preceding claims, characterized in that, in order to bring the solid particles leaving the cyclone (30) and the liquid (38) into contact, a continuous flow of liquid is carried out, possibly with a vortex movement, from a source line on a wall (50) situated around and below the zone (34) of arrival of the particles.  8) Method according to one of claims 1 to 7, characterized in that it also consists in taking in the cyclone (30), with the solid particles, a very small fraction of the flow of gaseous steam cracking effluents.  9) Method according to claim 8, characterized in that it then consists in washing the gaseous effluent-solid particles mixture leaving the cyclone, by means of the aforementioned liquid.  10) Device for capturing and recycling solid particles in a hydrocarbon steam cracking installation, by carrying out the method according to one of the preceding claims, characterized in that it comprises at least one cyclone (30) for separating particles solids and gases, fed by a stream of gaseous steam cracking effluents from a steam cracking furnace (10) and means (12) for indirect quenching, means (36,58,66) for contacting the particles solids and a liquid (38) containing substantially no heavy pyrolysis hydrocarbons, means, such as a pump (40), for recompressing the solid-liquid particle mixture, and means for injecting this mixing at at least one point in the steam cracking installation.  11) Device according to claim 10, characterized in that it comprises means for adding light hydrocarbons to the mixture of solid particles and liquid, before its injection into the installation.  12) Device according to claim 10 or 11, characterized in that the contacting means comprise a balloon (36), supplied with solid particles by the aforementioned cyclone (30) and comprising a wall (50) surrounding the arrival of solid particles, means for carrying out a continuous flow, for example with a vortex component of rotation, of the liquid on this wall, and means for recycling the liquid between the outlet and the inlet of this balloon.  13) Device according to claim 10 or 11, characterized in that the contacting means comprise a downward conduit (58) connected to the outlet (34) of the cyclone and through which passes the solid particles and a very small fraction of the flow gaseous effluents produced by the steam cracking installation, points (60, 62, 64) for injecting and spraying the abovementioned liquid into the conduit (58), and a reservoir (66) to which this conduit terminates.