FR2649761A1 - Method and device for distributing a gaseous flow charged with solid particles - Google Patents

Abstract

Method and device for distributing a flow of gas charged with solid particles into a series of straight tubes 10 supplied by a manifold 12, comprising means producing a turbulent flow of the gas/particles flow in the manifold 12; supply connectors 20 provided at the ends of the tubes 10 in the manifold 12 and comprising an inlet orifice directed towards upstream of the manifold, and means 28, 30 for sampling a significant fraction of the gas/particles flow of the downstream end 26 of the manifold 12 and for recycling this sampled fraction by injection at the upstream end of the manifold. The invention applies particularly to hydrocarbons steam-cracking plant.

Description

METHOD AND DEVICE FOR DISTRIBUTING A GASEOUS FLOW
LOAD OF SOLID PARTICLES
The invention relates to a method and a device for distributing a gas flow loaded with solid particles in a series of parallel tubes, fed by a collector.

 The invention is applicable, in particular, but not exclusively to single-pass steam cracking furnaces for hydrocarbons.

 It is known that hydrocarbon steam cracking ovens comprise tubes for circulation of the gaseous charge of hydrocarbons and water vapor which, outside the furnace, are connected to heat exchangers carrying out an indirect quenching of the effluents gaseous resulting from the steam cracking of hydrscarbons. he internal walls of the furnace tubes and heat changers quickly cover themselves with a layer of coke, the spreader of which grows more or less quickly depending on the nature of the charge treated, so that the heating must be stopped regularly. operation of the steam cracking installation to chemically or hydraulically decoke the walls of the furnace tubes and quench exchangers.

 It has recently been proposed to carry out this decoking by means of solid particles of small particle size, which are mixed with the gaseous charge by injection upstream of the furnace or the interior thereof, and which therefore circulate with the gas charge in the furnace and heat exchanger tubes. These solid particles can then be separated from the gaseous effluents in a cyclone provided between the direct quenching exchangers and the direct quenching means by injection of quenching oil, so that the decoking of the tubes by erosion by means of these solid particles can be produced during the normal operation of the steam cracking installation. The quantity of solid particles injected into the gaseous charge is relatively small, whether decoking takes place permanently or discontinuously, at regular intervals.

 When the steam cracking furnaces are of the multi-pass type, the tubes have a relatively large diameter and the distribution of the gaseous charge and of the solid particles in the different tubes poses hardly any problem. One can for example make separate injections of particles at the entry of the coils which are limited in number: the coils however have the drawback of risks of erosion in the elbows. On the other hand, when it is a single-pass type oven, the very numerous tubes have a much smaller diameter and are supplied with hydrocarbons by an inlet manifold. I1 then poses a problem of distribution of the gas flow and solid particles in these tubes, which the invention aims in particular to solve.

 The invention particularly relates to a method and a device for distributing a gas flow charged with solid particles in a series of parallel tubes supplied by a collector.

 The invention also relates to a method and a device of this type, allowing a uniform distribution of the gas flow rate and of the solid particles in the various tubes supplied by the manifold.

 The invention also relates to a process and a device of this type, making it possible to improve the decoking, permanent or discontinuous, of a single-pass steam cracking furnace for hydrocarbons, and the associated steam cracking process.

 It proposes a method for distributing a gas flow loaded with solid particles in a series of parallel tubes supplied by a manifold, in particular in a steam cracking furnace for hydrocarbons.

characterized in that it consists in injecting into the gas flow, upstream of the collector or at the inlet of celul-cl, a small quantity of solid particles of small particle size making it possible to clean by erosion, in particular to decoker, the walls internal tubes, to ensure. in the manifold a flow with high turbulence of the gas flow at an average speed notably lower than the speed of circulation in the tubes, but sufficient to avoid substantially deposits of solid particles in the manifold, to be provided at each end of the through tube in the collector a supply nozzle whose inlet section has a component in a plane perpendicular to the mean direction of flow in the collector and is oriented upstream relative to this direction, and to connect the part terminal of the downstream end of the collector to a volume making it possible to capture an excess of particles.

 Thanks to the turbulence of the flow in the collector, a homogeneity of the gas / particle mixture is obtained throughout the collector. The end fittings which are provided at the ends of the tubes in the collector make it possible to supply these tubes in a regular and constant manner with particles, whatever the place of the tubes in the collector. The inlet section in the end pieces which has a front component facing the flow, makes it possible to avoid too pronounced changes of direction at the entry into the small tubes, which would be the cause of gas-particle separation phenomena and would lead to an irregular distribution of the particles; moreover, the tips constitute very effective turbulence generators in the collector.Finally, the volume for the capture of an excess of particles (and in particular coarser particles which can circulate, in the semi-entrained state, along of the lower generator of the collector), makes it possible to avoid overfeeding of the last collector tube, or clogging of the latter by an excess of particles.

 According to another characteristic of the invention, a fraction of the gas-particle flow rate is taken at the downstream end of the manifold; this then behaves like an infinite collector, not comprising a last "tube fed by the residual fraction of the gas-particle mixture.

 According to another characteristic of the invention, the method also consists in recycling at the upstream end of the collector the fraction taken from the gas / particle flow, after having recompressed it, for example by ejecto-compression.

 Advantageously, provision is also made for a section restriction, such as a neck or preferably a venturi at the inlet of each tube downstream of the aforementioned nozzle.

 This section restriction makes it possible to regularize and standardize the flow rates of gases circulating in the various tubes.

 It therefore has an advantageous effect on the cleaning of the internal walls of these tubes, by erosion by the solid particles conveyed by the gas, the speed of which is substantially uniform from one tube to another.

 The invention also provides a device for distributing a gaseous-charged flow of solid particles in a series of parallel tubes fed by a manifold, in particular in a hydrocarbon steam cracking furnace, characterized in that it comprises means of injecting solid particles into the gas flow upstream or at the inlet of the manifold, means producing a turbulent flow of the gas / particle flow in the manifold, at a speed sufficient to substantially avoid any deposit of solid particles in the manifold , supply end pieces mounted at the ends of the tubes and extending inside the manifold, each end piece comprising an inlet section which is oriented towards the upstream end of the manifold and has a component in a plane perpendicular to the mean direction of flow in the collector, and a volume connected to the downstream end of the collector.

 The device also comprises means for sampling at the downstream end of the manifold a fraction of the gas-particle flow flowing in the manifold.

 The aforementioned volume can be a static volume allowing a retention of excess particles, or a conduit, allowing a dynamic capture of the excess particles, entrained by the sampling of the gas-particle mixture, or a combination of these two embodiments. the device also comprises means for recompression and recycling, upstream of the collector, of the fraction taken from the gas / particle flow rate.

 The ends of the tubes include section restrictions, such as for example ventu rls, located for example immediately downstream of the supply ends.

 The solid particles enabling the collector to be fed are preferably stored in a tank, in the form of a suspension in a liquid such as water or light or medium hydrocarbons, the tank being connected to pumping and spray injection into a passage for the aforementioned gas flow.

The invention will be better understood and other characteristics, details and advantages thereof will appear more clearly on reading the description which follows, made with reference to the accompanying drawings and in which
- Figure 1 is a partial tick diagram view of a steam cracking installation comprising distribution means according to the invention;
- Figures 2 and 3 show variants of the distribution means according to the invention;
- Figures 4, 5, and 6 schematically show alternative embodiments of the tube ends
- Figure 7 schematically shows a steam cracking installation according to the invention.

 FIG. 1 shows schematically a part of a hydrocarbon steam cracking furnace, which comprises a series of parallel rectilinear tubes 10 of small diameter, the ends of which are connected respectively to a supply manifold 12 -and to a outlet collector (not shown).

 The collector 12 is supplied, at one of its ends; by a charge 14 of vaporized hydrocarbons and water vapor which is for example at a temperature of the order of 55 ° C. and into which a small quantity of solid particles of small particle size is injected which are stored in the form of a suspension in a liquid such as water or light or medium hydrocarbons, in a tank 16. A pump 18 makes it possible to take the liquid / solid particles mixture in the tank 16 to inject it upstream of the collector 12, in a conduit in which the charge of vaporized hydrocarbons and water vapor circulates.

 As shown schematically in Figure 1, the tubes 10 form one or more parallel rows and open at regular intervals into the manifold 12, the latter having a section which decreases progressively from its upstream end to its downstream end, relative in the direction of flow of the charge, which makes it possible to maintain a minimum speed of the mixture in the collector, and to avoid deposits of particles.

 The end of each tube 10 opening into the manifold 12 comprises a feed nozzle 20 extending inside the manifold and comprising an inlet section or an orifice 22 oriented towards the upstream end of the manifold 12 and having a significant component in a plane perpendicular to the mean direction of flow of the charge in the collector 12.

In addition, each tube 10 comprises, immediately downstream of the supply nozzle 20, a section restriction 24 such as a neck or a venturi, for example, making it possible to standardize and make the gas flow rate substantially constant. in each tube 10. Advantageously, a sonic venturi will be used.

 The downstream end 26 of the manifold 12 is connected by a conduit 28 of suitable size, to an electric compressor 30 comprising an axial conduit 32 for supplying a flow 34 of engine gas, such as steam. A valve 36 makes it possible to adjust the flow rate 34 of engine gas.

 The outlet of the ejector-compressor 30 is connected by a conduit 38 to the upstream end of the manifold 12 or to the conduit 40 bringing the hydrocarbon charge to the manifold 12.

 Advantageously, the valve 36 for adjusting the flow rate 34 of engine gas can be controlled by a system 42 comprising means for detecting the skin temperature of the first and last tubes 10 of the furnace, to control the flow rate 34 unlike these temperatures, as will be explained in more detail below.

 The device of Figure 1 operates as follows.

 The flow of vaporized hydrocarbons and water vapor, charged with solid particles of small particle size, is brought by the pipe 40 to the upstream end of the manifold 12 and flows with high turbulence in this manifold.

 The size of the solid particles is between 5 and 150 microns approximately (preferably between 5 and 60 microns), their quantity is between 0.01 and 3% by weight of the flow rate of hydrocarbons and water vapor. The gas / solid particles mixture circulating in the collector 12 behaves substantially like a homogeneous gas endowed with erosive properties, because of the high turbulence of the flow in the collector 12, turbulence which is further reinforced by the presence of the end caps. supply 20. The average flow speed in the collector is between 20 and 120 meters per second, for example between 30 and 80 meters per second and is notably lower than the circulation speed in the tubes 10, which is between 130 and approximately 300 meters per second, in particular between 160 and 270 meters per second. This flow speed in the collector 12 is however sufficient to avoid any gas / solid segregation in the collector and therefore, any deposit of solids in this collector , which promotes a uniform distribution of the gas / solids flow in the tubes 10.

 The sampling of a significant fraction of the gas / solid particle flow rate at the downstream end 26 of the manifold, in a way transforms this manifold into a manifold of infinite length, from which it follows that the downstream end of the manifold has no more significant influence on the distribution of the gas / solid particle flow rate in the various tubes 10, whether they are close to or distant from the downstream end of the manifold.

 The supply of a flow rate 34 of engine gas into the ejector 30 allows, on the one hand, to take the desired fraction from the gas / solid flow rate in the collector, and on the other hand, to recompress this fraction for recycling, by injection into the conduit 40 or at the upstream end of the manifold. The system 42 makes it possible to adjust the flow rate of engine gas 34 by action on the valve 36, and therefore to increase or decrease the fraction withdrawn from the flow rate flowing in the manifold 12, which makes it possible to influence the supply of particles. solids of the first tubes compared to the last tubes and therefore to correct an irregularity of distribution, detected by differences between the skin temperatures of these tubes.

In fact, the solid particles which circulate at high speed in the tubes 10 have an erosive action on the coke layer which forms on the internal wall of the tubes. The variations in the skin temperatures of the tubes make it possible to evaluate the degree of fouling of the tubes, and therefore the efficiency of the erosion of the coke layer by the solid particles. The increase in the sampling rate will lead to a greater increase in the average speed in the collector at the end of the collector (of small section) than at the start of the collector.

This non-homogeneous aerodynamic modification has an influence on the particle supply of the first tubes compared to the last; this influence must be appreciated experimentally, for a given type of particles and an internal geometry of the collector. The sampling flow at the end of the collector can therefore be modulated, as a function of information on the relative fouling of the various tubes. More simply, we can set it to an adequate value.

The section restrictions 24 formed at the upstream end of the tubes 10 have the effect of standardizing and making the gas flows circulating in these tubes substantially constant. This results in a possibility of automatic regulation of the cleaning of these tubes by solid particles, for the following reason
If, for some reason, a tube 10 receives less erosive solid particles than the other tubes, it will clog up faster and will have a smaller internal gas passage cross-section than that of the other tubes, due to the 'faster increase in the thickness of the coke layer which forms on the inner wall of the tube. This reduction in section will result in an increase in the speed of circulation of the gas, the flow rate being maintained at a constant value by the venturi 24. As the erosion produced by the solid particles increases like the cube of the speed, any increase in velocity in tube 10 will result in a much greater increase in erosion, which will reduce the thickness of the coke layer formed on the inner wall of the tube. Thus, in this tube, any faster tendency to fouling due to a smaller quantity of particles, will be compensated for and canceled out by a tendency to increase the speed of circulation in this tube, and this thanks to the presence of venturis.

 Another means for correctly regulating and distributing the gas / particle flow rate in the various tubes consists in providing in the manifold 12, upstream of the first tubes 10, a dummy supply end 44, identical to the supply ends 20 and which will create a turbulence substantially identical to that of the tips 20. The first tubes 10 will therefore be, from this point of view, in the same situation as the following tubes.

 In the alternative embodiment shown in FIG. 2, a volume 50 of static particle retention is connected to a lower orifice 51 of the downstream end of the manifold 12, slightly upstream of the last tube 10. The heavy particles entrained on the lower part of the collector fall into the volume 50 through the orifice 51.

 This volume of capture of excess heavy particles makes it possible to avoid overfeeding of the last tube with solid particles, which is very important in order to avoid erosion of this tube.

 At 52, provision is made for a small sweep flow rate of fine particles into the volume 51.

 In the variant of FIG. 3, the two solutions of FIGS. 1 and 2 are combined = the downstream end of the manifold 12 is connected to an ejector-compressor 30 (allowing dynamic capture of the particles), associated with a volume 50 of static capture of the heaviest solid particles.

 There are shown in Figures 4, 5 and 6 alternative embodiments of the ends of the tubes 10 and their supply ends.

 In the embodiment of Figure 4, the tip 20 is identical to those shown in Figure 1, but the section restriction 24 is formed by a venturi preferably sonic neck. This venturi or any other section restriction is formed from a particularly hard material, to resist erosion, for example boron carbide or silicon carbide.

 In the embodiment of FIG. 5, each tube 10 ends in an end 46 cut in a bevel which forms the orifice for the entry of the gas / solid particle flow rate in the tube 10.

 In the embodiment of FIG. 6, each supply nozzle is constituted by an elbow 48 to 90 ′, fixed on the internal wall of the manifold 12, and into which the end of the corresponding tube 10 opens, comprising the restriction of section 24.

 The solid particles used can be metallic particles, alumina, silica, etc.

 It is also advantageous to use petroleum coke with a very fine particle size (or ground), of approximately 10 to 80 m. This coke can be removed by combustion from all parts of the installation where the particles can settle. Its use also avoids pollution of the quenching oil from steam cracking effluents. Indeed, the particles not recovered at the outlet of the installation will be captured in the pyrolysis oil. If these particles are combustible, which is the case with coke, and very fine, this will not pose any problems for the use of pyrolysis oil as fuel.

 According to another characteristic variant, it is possible to inject erosive solid particles, having the dual property of having a low hardness at steam cracking temperatures, and of having a chemically active surface as a coke catalyst. These particles will be ineffective, relatively from the point of view of the erosive effect; however, upon impact with the hard surface of the steam cracking tubes, where they will be exposed (without a protective preformed coke layer), they will leave traces of coke catalyst elements which will help to form a coke layer protective minimum.

 Non-stainless steel powders may be suitable, or better, powders of iron, nickel or a mixture of these two components (for example iron powders or ex-carbonyl nickel of average particle size between 5 and 20 microns.

 One can for example use such powders with a percentage between 1 and 20% by weight, the balance being petroleum coke.

 FIG. 7 schematically represents an improved steam cracking installation, which is protected against coke deposits, and this particularly for tubes of very small hydraulic diameter. This steam cracking installation uses tubes of very small hydraulic diameter, for example between 10 and 16 mm approximately, the length of which is approximately 4 to 6 m. The residence times are then in the range of about 20 to 40 milliseconds and the ethylene yields are increased by several percentage points compared to the best current installations.

 In a particularly advantageous manner, an oven cell 62 of about 10 m in height is kept, the inlet manifold 12 being placed in the middle part of the oven, and supplying two layers of tubes with opposite circulation, of length about 5 m; the tubes of the same ply initially have, at room temperature, a weak curvature, identical for all these tubes, so that, after temperature rise, the tubes have a curvature little different from their initial curvature, depending on their temperature clean. This avoids that a sheet of tubes is divided into two groups of tubes having opposite curvatures. Each sheet of tubes is connected to an outlet manifold 61 supplied with a pre-tempering gas introduced at the upstream end, at 60.

 This arrangement makes it possible to carry out a steam cracking with an ultra-short residence time, in an existing oven cell, or similar to existing oven cells: moreover, the inlet manifold 12 is common to the two layers of tubes, which is economic.

 According to a characteristic arrangement of the invention, the pressure downstream of at least one venturi 24 is measured, the pressure in the outlet manifold, and the difference between these two pressures is regulated (pressure drop of the tube outside the venturi ), at a value significantly higher than that of the pressure drop of the clean tube, for the same charge flow (this pressure drop can be calculated from the measurement of the charge flow, for example by measuring the manifold pressure 12). Thus, excessive coking of the tubes 10 is avoided, but without completely decoking these tubes, therefore leaving a layer of minimal protective coke.

 The erosion can be regulated by adjusting the flow of particles injected by the pump 18.

Claims (17)

 1. A method of distributing a gas flow loaded with solid particles in a series of parallel tubes (10) supplied by a manifold (12), in particular in a steam cracking furnace for hydrocarbons, characterized in that it consists in inject into the gas flow, upstream of the manifold, a small quantity of solid particles of small particle size making it possible to clean by erosion, in particular decoker, the internal walls of the tubes 110). to ensure in the manifold (12) a high turbulence flow of the gas flow, at an average speed significantly lower than the speed of circulation in the tubes (10), but sufficient to avoid the deposition of solid particles in the manifold, to be provided at each end of the tube (10) opening into the manifold a supply nozzle (20, 46, 48) whose inlet section (22) has a component in a plane perpendicular; re to the mean direction of flow in the collector and is oriented upstream relative to this direction and to connect the downstream end part of the collector (12) to a volume (50) for capturing an excess of solid particles.  2. Method according to claim 1, characterized in that it consists in taking at the downstream end (26) a fraction of the gas flow / solid particles circulating in the collector.  3. Method according to claim 2, characterized in that it also consists in recycling at the upstream end of the collector, or in the vicinity thereof, the fraction taken from the gas / solid particle flow rate.  4. Method according to claim 3, characterized in that it consists in recompressing, for example by ejecto-compression, said fraction - taken from the gas / particle flow rate, before the insect at the upstream end of the collector or in the vicinity of it.  5. Method according to one of the preceding claims, characterized in that it also consists in forming a section restriction (24), such as a neck or a venturi, at the inlet of each tube (10), downstream of the aforementioned nozzle (20).  6. Method according to one of the preceding claims, characterized in that it consists in having, upstream of the end piece of the first tube (10) in the manifold (12), an obstacle formed by a dummy end piece (44) , substantially identical to the aforementioned end caps (20).  7. Method according to one of the preceding claims, characterized in that the solid particles have a particle size between 5 and 150 microns approximately, and the amount of solid particles injected is between 0.01 and 3% by weight of the flow gaseous.  8. Method according to one of claims 1 to 7, characterized in that the particles injected are essentially fine particles of petroleum coke.  9. Method according to one of claims 1 to 8, characterized in that particles are injected whose surface is chemically active as a coke catalyst, having a low hardness at steam cracking temperatures, for example particles of iron powder or nickel or a mixture of these two components.  10. Device for distributing a gas flow loaded with solid particles in a series of parallel tubes (10) supplied by a manifold (12), in particular in a steam cracking furnace for hydrocarbons, characterized in that it comprises means for injecting solid particles into the gas flow upstream or at the inlet of the collector (12), means producing a turbulent flow of the gas flow in the collector (12), at a speed sufficient to avoid any deposition of particles solids in the manifold, supply ends (20) mounted at the end of the tubes (10) and extending inside the manifold (12), each end piece comprising an inlet section (22) which is oriented towards the upstream end of the collector and has a component in a plane perpendicular to the mean direction of flow in the collector, and means (28, 30; 50) for collecting solid particles at the downstream end of the collector .  11. Device according to claim 10, characterized in that it comprises means (28, 30) for sampling at the downstream end of the collector, of a fraction of the gas / solid particle flow rate flowing in the collector, and means recycling, upstream or at the inlet of the collector, of the fraction taken from the gas / solid particle flow.  12. Device according to claim 11, characterized in that the recycling means comprise an ejector-compressor (30), supplied by an engine flow (34) of auxiliary gas such as steam.  13. Device according to one of claims 10 to 12, characterized in that section restrictions (24) such as, for example, venturis, are provided at the inlet of the tubes (10) downstream of the supply ends ( 20).  14. Device according to one of claims 10 to 13, characterized in that the collector (12) comprises, upstream of the outlet of the first tube (s) (10), an obstacle formed by a dummy nozzle (44) substantially identical to the power tips (20).  15. Device according to one of claims 10 to 14, characterized in that it comprises a reservoir (16) for storing solid particles in the form of a suspension in a liquid such as water or light or medium hydrocarbons , the reservoir (16) being connected to a pump (18) and to injection means by spraying into the conduit (40) for passage of the abovementioned gas flow.  16. Device according to one of claims 10 to 15, characterized in that the inlet manifold (12) is connected to two layers of tubes of opposite circulation, and is located in the middle part of the radiation area of the furnace , the two layers of tubes extending respectively from one side and the other from the manifold (12).  17. Device according to one of claims 10 to 16, characterized in that it comprises means for measuring the pressure drop of at least one tube (10), excluding venturi (24), means for regulating this pressure drop to a value significantly greater than the pressure drop of the clean tube, thanks to means making it possible to modify the quantity of erosive particles injected.