EP0157691B1 - Device to pulverize a liquid into a gaseous flux comprising several aligned venturis, and application of this device - Google Patents

Description

The invention relates to a device for spraying a liquid into a gas stream, said device comprising several venturis arranged in series, in which the gas stream flows with sonic flow in line with the necks of the venturi and with supersonic flow in part of the divergent following each pass.

There are known, in the art, devices for spraying liquid with a nozzle, of the venturi type, that is to say comprising a converging part, a neck and a diverging part - in which the gaseous spraying fluid flows at speed. sonic at the neck, and at supersonic speed downstream of the latter, in the divergent, with a sudden passage at a subsonic speed a little further downstream, with the production of a shock wave. The liquid to be sprayed is generally injected at the level of the neck of the venturi and the drops which result from a first dispersion, under the effect of the speed gradient of the gas flow at the level of the neck, are fractionated further downstream, under the effect of the shock wave in multiple droplets.

Various types of liquid fuel burners or charge injectors in chemical reactors using spraying systems of this type have been proposed, but the nozzles thus produced do not always allow a sufficient degree of fineness of the droplets and of dispersion of these to obtain an excellent reaction yield, in particular when the fuel or the charge is a heavy petroleum derivative with high viscosity or a suspension of solid particles in a liquid.

On the other hand, the structures proposed are generally complex, which complicates their assembly and disassembly, in particular on the burners for maintenance.

The invention aims to remedy these drawbacks by proposing a spraying device which, by its simplicity of structure and operation, is well suited to the injection of various fuels and which, by its relatively large orifices compared to the known devices of this type limits clogging by solid or pasty particles and is suitable for maintenance without disassembly. This invention further aims to provide a spraying device usable for all applications in which a liquid charge must be finely atomized and therefore vaporized on contact with a hot fluidized granular mass, such as for example the application for injection into catalytic cracking or hydrocracking reactors.

To this end, the subject of the invention is a device for spraying a liquid into a gas flow, characterized in that it comprises at least two venturis arranged coaxially in series, in which said gas flow flows, and, at the neck of the venturi arranged the most upright, at least one means for injecting said liquid into said gas stream, the dimensions of the venturis being such that, in each venturi, the speed of the gas stream is sonic at the neck and supersonic in downstream of this, on a fraction of the divergent which follows this neck and that, if we designate by Pi _n the fluid generating pressure at the neck of the venturi of rank n and by S _n the section of said neck, we has the relation Pi _n S _n = Pi _{n + l} S _{n + l} .

The spraying device according to the invention makes it possible to obtain two-phase mixtures (liquid particles-gas) with high degree of fineness, particularly suitable for the combustion of heavy derivatives in burners, since the combustion time of the fuel drops is in first approximation proportional to the square of their diameter.

This type of device is also particularly suitable for injecting heavy petroleum charges into catalytic conversion reactors, such as cracking or hydrocracking reactors, since the quality of the atomization of these charges allows almost instantaneous vaporization of the hydrocarbons in the reaction zone and therefore a considerable improvement in the quality of the heat transfers when brought into contact with the catalyst grains. This instantaneous vaporization also makes it possible to limit the coking of the cracking catalyst to the strict minimum and to preserve the active sites of the latter, hence better activity and selectivity.

Due to the fineness of the liquid particles produced, the spraying device is also suitable for injecting liquid products into any other reactor in which a charge such as a heavy charge must be atomized as finely as possible with an auxiliary fluid. .

It is also possible to combine this sprayer with a “mechanical” sprayer (that is to say operating only using the fuel pressure) and which would be located upstream, the assembly forming a spraying system. says "mechanical assistance".

These applications of the spraying device according to the invention constitute other objects of the present invention.

In order to stabilize the gas flow between two successive venturis of the device, a cylindrical part or neck will be interposed between the divergent of each venturi and the convergent of the following venturi.

At the end of the diverging part located most downstream of the spraying device, a diffuser may be provided in order to split the gas flow into several jets, or a profiled body with variable radius of curvature, intended to spread the final jet obtained according to a given angle, the profile being defined so as to avoid detachment of the jet.

According to a preferred characteristic of the invention, the angle of the convergent of each venturi will be between 20 and 45 with, preferably, a rounded connection profile with the neck, and the angle of each divergent will be at most 14 °. The divergent can also have a curvilinear profile instead of frustoconical. The cylindrical neck will have a variable length depending on the application, but at least 3 times the diameter. The Applicant has indeed established that, for these values, it is still possible to obtain a speed close to the sonic speed in the first neck for low pressures of gaseous fluid, less than 0.5 bar relative (or 1.5 absolute bar if the ambient pressure is 1 bar). Indeed, a small angle of the divergent less than or equal to 14 ° - allows a recompression of the flow with the minimum pressure drop in the divergent.

The injection of the liquid at the neck of the first venturi can be carried out in any manner known in the art and various embodiments of injectors will be described below.

• la figure 1 est une représentation schématique du corps du pulvérisateur;
• les figures 2a à 2e représentent différentes formes d'injecteurs au droit du col du premier venturi;
• les figures 3a et 3b représentent respectivement l'orifice de sortie à corps profilé et un diffuseur placé à la sortie du dernier divergent du pulvérisateur;
• la figure 4 est une représentation graphique de l'évolution de la vitesse de l'écoulement dans les venturis successifs;
• la figure 5 est une représentation graphique du diamètre moyen de Rosin-Rammler en fonction du taux de fluide pour un pulvérisateur selon l'invention [courbe (C)] et pour un pulvérisateur d'un type connu désigné sous l'appellation commerciale «Foyer-Turbine type MV» [courbe (C')]; et
• la figure 6 est une représentation graphique du diamètre moyen de Rosin-Rammler en fonction du taux de fluide de pulvérisation pour des dispositifs possédant de un à quatre venturis successifs.

Various forms of implementation of the invention will now be described with reference to the accompanying drawings, in which:

• Figure 1 is a schematic representation of the body of the sprayer;
• Figures 2a to 2e show different forms of injectors to the right of the neck of the first venturi;
• FIGS. 3a and 3b respectively represent the outlet orifice with a profiled body and a diffuser placed at the outlet of the last diverging point of the sprayer;
• Figure 4 is a graphical representation of the evolution of the speed of flow in successive venturis;
• FIG. 5 is a graphical representation of the mean Rosin-Rammler diameter as a function of the fluid level for a sprayer according to the invention [curve (C)] and for a sprayer of a known type designated under the trade name “Foyer - MV type turbine ”[curve (C ')]; and
• FIG. 6 is a graphic representation of the mean Rosin-Rammler diameter as a function of the rate of spraying fluid for devices having from one to four successive venturis.

The body of the spraying device according to the invention consists, as shown in Figure 1, of a series of venturis, whose convergers are referenced respectively 10, 11, 12, 13, whose necks are designated by 1, 15, 16 , 17, and whose divergences are designated by 3, 4, 5, 6. The liquid is injected at the neck 1 of the first venturi; the output of the two-phase gas (or spray fluid) and liquid mixture takes place at the end of the last venturi. The angle of the convergent of each venturi is, in this case, about 27 ° and the angle of each diverging is of the order of 14 ° (total angle at the top). The neck of each venturi is cylindrical and of adequate length. At the outlet of each divergence is arranged a cylindrical body 7, 8, 9 respectively, which aims to stabilize the flow before approaching the next convergent.

The diameter of the neck of each of the venturis is defined in such a way that, for a first given supply of the spraying fluid, the flow has a sonic speed at the level of each of the necks (FIG. 4, respectively in A ₁ , A ₂ and A ₃ and A ₄ ) and a supersonic speed in each divergent (respectively from A ₁ to B ₁ , A ₂ to 8 ₂ and A ₃ to B ₃ ), the recompression being established through a straight shock wave (respectively in B _i , B ₂ and 8 ₃ ) which promotes the spraying of the liquid.

To this end, bearing a generating pressure Pi _i given to the first neck of section S of the first venturi, the section S of the neck of the second venturi is defined as a geometric parameter satisfying the relation Pi ₁ S ₁ = Pi ₂ S ₂ where Pi ₂ is the generating pressure at the neck of the second venturi, the fluid flow velocities at the necks amounting to Mach 1 under the usual operating conditions. Likewise, the relation is extended to a venturi of order n, namely Pi ₁ S ₁ = Pi _n S _n where Pi _n and S _n are the fluid-generating pressure and the section of the neck of said venturi of order n respectively, in the case of an adiabatic flow, that is to say without heat exchange by the walls of the sprayer, which is practically the case with commercial fuel sprayers. The generating pressures at the necks decreasing from upstream to downstream, this requires that the neck sections are increasing from upstream to downstream.

The injection of liquid and spraying fluid can advantageously be carried out in a manner known per se using a premix upstream of the device according to the invention. The liquid injection can also be carried out (Figures 2a to 2e) in line with the neck of the first venturi. An injector giving good results can consist of a tube 21 (FIG. 2a) coaxial with the first venturi, which delivers in the same direction as the flow of the spraying fluid (nitrogen in the case of the tests of the Applicant, generally of steam in industry). The outlet diameter of this liquid injector can be variable (from 1 to 4 mm in the case of tests carried out by the Applicant), which makes it possible to inject a given quantity of liquid at different pressures depending on the diameter of the injector.

The annular flow of the spray fluid, around the injector in line with the neck, is sonic. Variants of this type of injector (which also give good results) consist in extending the injector with a central body 22 downstream in the divergent. The liquid is injected through orifices in the form of holes 24 (FIG. 2b) or slots 23 (FIG. 2c) which flow transversely to the flow (sonic at the neck) of the spraying fluid.

Another solution consists in injecting the liquid by means of orifices 25 (or slits) perpendicular to the flow (FIGS. 2d and 2e), these orifices being located on the outer ring at the neck 1 of the first diverging part.

Depending on the use that is made of this injector (burner, atomization in the atmosphere, injection into a chemical reactor, etc.), it may be useful to distribute the two-phase mixture in different ways, at a certain angle.

Thus, the outlet orifice of the injector can be constituted by the end of the divergent (FIG. 1), the angle of the two-phase jet being substantially equal to the angle of the last divergent. If a more open jet angle is desired, the end 26 of the diverging portion (FIG. 3a) can be profiled so that the two-phase fluid adheres to the wall over a certain length. The angle of the jet a 'is then more open than the angle a of the divergent.

If we want to rigorously control the dispersion of the mixture in space, we can split the main flow (Figure 3b) into several jets (6 to 8 generally), through the orifices 27 of a diffuser 20 arranged at the end of the downstream divergent spray device, the passage section S _{n + 1} of all of said orifices being greater than the section of the neck of the last venturi n and corresponding to the expression Pi ₁ · Si = Pi _n · Sn = Pi _{n + 1} · S _{n + 1} · These orifices 27 are oriented at an angle a ", each orifice consisting of a single hole or of a profiled shape (convergent, neck, divergent).

Finally, in the case where the length of the spraying device should be minimum, the conical divergences can be replaced by diverging ones with shorter evolving profile.

FIG. 5 represents, by way of comparison, for this sprayer [curve (C)] and for a sprayer of known type, designated by the trade name "Turbine hearths type MV" [curve (C ')] the average diameter of Rosin-Rammler as a function of the spraying nitrogen rate, in the case of tests carried out on a spraying bench, with oil of viscosity 15 cSt and nitrogen as atomizing fluid, under operating conditions analogues.

In practice, water vapor is used as the spraying fluid and, below 5% of vapor, the drops are quite large. On the other hand, as soon as the consumption of atomizing fluid increases, the sprayer with successive sonic necks according to the invention gives drops that are much finer than the sprayers of the prior art (twice as fine, at 50% vapor) .

FIG. 6 finally graphically shows the improvements in the quality of the atomization due to the increase in the number of successive venturis.

The comparison of curve D (only one venturi), with curves E (two successive venturis), F (three successive venturis) and G (four successive venturis) shows that, with steam injections which are generally of the order from 5 to 30%, the quality of the atomization increases with the number of successive venturis.

Claims (11)

1. A device for atomising a liquid into a gaseous flux, characterised in that it comprises at least two venturis disposed coaxially in series, in which said gaseous flux flows, and in the neck (1) of the venturi disposed furthest upstream at least one means (21) for injecting said liquid into said gaseous flux, the dimensions of the venturis being such that in each venturi the velocity of the gaseous flux is sonic at the level of the neck and supersonic downstream thereof, over a fraction of the divergent which follows this neck, and if Pin designates the generating pressure of fluid at the neck of the venturi of row n and S_n designates the section of said neck, the equation Pin Sn ₌ Pin+i S_n+i results.

2. A device for atomising a liquid into a gaseous flux according to Claim 1, characterised in that the divergent (3, 4, 5) of each venturi and the convergent (11, 12, 13) of the following venturi are connected by a cylindrical member (7, 8, 9) intended to stabilise the flow of the gaseous flux between the venturis.

3. A device for atomising a liquid into a gaseous flux according to either of Claims 1 and 2, characterised in that with a view to dividing the gaseous flux into a plurality of jets it comprises, at the outlet of the final divergent, a diffuser (20) having a plurality of cylindrical or profiled orifices (27) whose axes diverge so as to form between them an angle (a"), the passage cross-section of all the orifices being equal to or greater than the cross-section of the neck of the final venturi.

4. An atomising device according to either of Claims 1 and 2, characterised in that the outlet orifice comprises a profiled body (26) whose radius of curvature is variable and which is intended to spread the final jet obtained in accordance with a given angle (a'), so as to avoid breakdown of the jet.

5. An atomising device according to any of the preceding claims, characterised in that the angle of the convergent of each venturi is between 20 an 45° and, moreover, the angle of each divergent is equal to 14°.

6. An atomising device according to any of the preceding claims, characterised in that the injection means (21) is extended downstream, in the divergent, by a central body (22).

7. An atomising device according to Claim 6, characterised in that central body (22) comprises an orifice (28) provided for discharging the liquid to be atomised parallel to and in the direction of the gaseous flow.

8. An atomising device according to Claim 6, characterised in thatthe central body (22) comprises orifices in the form of slots (23) or cylindrical holes (24) provided for discharging the liquid to be atomised transversely to the gaseous flow.

9. An atomising device according to any of the preceding claims, characterised in that the injection of the liquid to be atomised is effected perpendicularly to the gaseous flow at the level of the neck (1) through orifices (25) in the form of holes or slots provided in the neck itself.

10. Application of the atomising device according to any of the Claims 1 to 9, for supplying a burner with fuel in liquid form or in the form of a suspension of a solid in a liquid.

11. Application of the device according to any of the Claims 1 to 9, for supplying a chemical reactor, in particular a catalytic cracking or hydrocracking reactor.

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