FR2675499A1 - Process and device for the steam cracking of a hydrocarbon charge comprising a gas generator and a heat exchanger/reactor - Google Patents

Abstract

The invention relates to a process and to a device for steam cracking a hydrocarbon charge. The heating stage of the charge is carried out by passing, at an appropriate flow rate, an amount of air and an amount of fuel into at least one gas generator (1), known as "Jet Engine", which delivers heating gases at a suitable pressure and a suitable temperature. These hot gases are conveyed into the shell (14) of a heat exchanger/reactor (11) which operates under pressure so as to crack the hydrocarbons which move in reaction tubes (12) of the exchanger/reactor. The hot heating gases are then conveyed into a second heat exchanger (18) and then to a power recovery turbine (22). The steam cracking effluent is subjected to a quench (27) and recovered.

Description

The invention relates to a method and a device for carrying out endothermic chemical reactions by passing the charge through a heat exchanger under pressure and at high temperature.

The invention applies to the implementation of a non-catalytic reaction such as the pyrolysis of gases containing methane or the steam cracking of a hydrocarbon feed such as naphtha or ethane, leading to the production of ethylene .

The reactions involved in these applications involve quantities of heat which must be supplied for a very short time and therefore it is advantageous to be able to have a reactor capable of delivering a high thermal flux, according to a square type temperature profile, or rectangular.

Heating at very high temperature (for example 100,000 and above) of reaction systems such as steam cracking of hydrocarbons involves, according to the prior art, burners supplied with fuel in an oven of the radiative type.

The tubes in which the reactants flow, are subjected to a heat exchange of mainly radiative type, the contribution of convection heating being minimal.

The radiative heat exchange implies that there is no screen between the heat source and the tubes to be heated. Under these conditions, the number of tubes inside a multi-wall burner chamber can only be limited. In addition, the pressure drop and residence time constraints mean that the length of the individual tubes cannot be adjusted at will in order to compensate for this limited number of tubes. This generally results in a compromise in the heat transfer system between a large burner chamber and a rather small total surface area for heat exchange. These are in fact the radiative heat transfer mechanism and exposure to a very high temperature. (close to that of the flame) which are the compensating factors on which it is possible to intervene to reach the required level of heat exchange.

On an industrial scale, construction constraints as well as economic constraints require the parallel installation of a good number of identical ovens to reach the maximum heating capacity. For example, it may be necessary to use up to thirteen identical pyrolysis furnaces in a unit in the case of steam pyrolysis of naphtha to produce 400,000 tonnes of ethylene per year.

One drawback of the technology described above concerns the size requirement of the installations, hence the need for a large quantity of structural steel and a large floor area.

Another drawback of this type of technology is linked to exposure to a very high temperature affecting the resistance to stress of the materials. Indeed, a tube subjected to very high temperature tends to expand both axially and transversely. This transverse expansion can be exalted by the pressure prevailing inside the tubes all the more that the pressure outside the tubes is substantially close to atmospheric pressure. It follows that the thickness of the tubes must be increased, to prevent the risk of ruptures, which ultimately costs more.

Another drawback linked to exposure to a very high temperature concerns an increase in coking inside the reaction tubes, which requires more frequent shutdowns of the installations.

The invention overcomes these drawbacks and makes it possible to obtain excellent results, in particular in terms of energy balance, but also in terms of yield.

More specifically, the invention relates to a process for steam cracking from a gaseous mixture of a hydrocarbon feed containing from 2 to 20 carbon atoms and water vapor, comprising a heating phase in a heating zone under appropriate conditions, which delivers a hot ethylene-rich steam cracking effluent and a rapid cooling phase of said effluent in a cooling zone under appropriate conditions and said cooled steam cracking effluent is recovered, characterized in that: - said mixture is preferably introduced beforehand, in an adequate ratio
hydrocarbons on water vapor at one end of a reactor-heat exchanger
under tube and shell pressure, and the mixture is circulated within a plurality
preferably substantially mutually parallel tubes contained in said reactor
exchanger, - the gas mixture is heated in the reactor-exchanger of
heat according to the following stages: a) an amount of air and a quantity of air are passed under appropriate flow conditions
of fuel in at least one gas generator, adapted to deliver heating gases
at adequate pressure and temperature; b) said gases are sent to an inlet of the shell of the heat exchanger-reactor to one
ends, preferably on the introduction side of the mixture, and said gases are circulated
in the shell of the heat exchanger-reactor so as to exchange heat
indirectly with the gas mixture; c) said heating gases are evacuated through an outlet from the calender at the other end, and
collects a hot ethylene-rich steam cracking effluent that is sent
immediately in the cooling zone.

The gas generator delivers pressurized gases which comprise from 10 to 20% of oxygen by volume and combustion products resulting from the combustion of the fuel, by the volume of air introduced and then compressed. This mixture will therefore be designated by the term heating gas. Generally the pressure of these gases is from 4 to 20 bar (1 bar = 105 Pa) and their temperature from 600 to 1400 "C.

According to a characteristic of the process, the temperature thus reached and more precisely the highest temperature range, can be obtained, at least in part, by the combustion of an appropriate quantity of fuel, thanks to at least part of the combustion gases. which contain the required amount of oxygen from the gas generator, in an afterburner downstream of the gas generator. Advantageously, this quantity of oxygen can reach 15 to 19% by volume at the outlet of the gas generator. The temperature of this gas is generally higher than the initiation temperature of the combustion of the injected fuel.

Excellent operating characteristics of the heat exchanger-reactor were obtained when the pressure of the gases ensuring the heating of the reaction tubes was 6 to 10 bar and their temperature from 1000 to 1200 "C.

According to another characteristic of the process, the heating gases are generally introduced into the calender at a mass flow rate at least equal to twice that of the gas mixture introduced into the tubes, for example from 2 to 10 times, and preferably from 5 at 8 times, so that the temperature drop of the gases at the outlet of the grille is at most equal to 1500C and preferably between 50 and 100 C.

According to another characteristic of the process, the residence time of the gaseous mixture (hydrocarbons and water vapor) in the reactor-exchanger is generally from 80 to 400 ms and advantageously from 100 to 300 ms.

According to another characteristic of the process, all of the gases are usually evacuated, that is to say the combustion gases at the outlet of the gas generator, and those resulting from the afterburning which circulated through the calender to heat the reaction tubes, at a temperature generally between 580 "C and 12500C, preferably between 900 and 10500C and under a pressure of 3 to 19 bar, preferably 5 to 7 bar and these gases can be sent in a second heat exchanger gas-gas heat, in which the mixture of hydrocarbons and steam is preheated, preferably against the flow direction of these gases at a temperature generally 50 to 100 "C below the initial cracking temperature of the load, for example between 500 and 650 "C.

According to another particularly advantageous characteristic of the process, the gases having passed through the second heat exchanger can be recovered, at a temperature of 550 to 950 "C and under a pressure of 3 to 19 bar, preferably at 700-950" C under 4 to 10 bar to send them to at least one power recovery turbine capable of supplying utilities for the operation of the steam cracking unit (load compression for example).

The invention also relates to the device for carrying out an endothermic reaction such as a steam cracking reaction, comprising a heating reactor and a member adapted to produce a quench and connected to said heating reactor, characterized in that the heating reactor comprises in combination: a) a gas generator (1) comprising a first air inlet (3) and a second inlet
(5) a fuel, adapted to deliver by an outlet of the heating gases to a
temperature and pressure determined; b) an elongated pressure heat exchanger reactor (11), preferably
vertical, with tubes (12) and grille (14) having at one of its ends an inlet
of said gas connected to the outlet of the gas generator and connected to the calender (14) of the
reactor-exchanger in which said gases circulate, and supply means (13)
in a suitable gas mixture comprising at least one hydrocarbon connected to a
plurality of reaction tubes (12), said tubes preferably being substantially
parallel to each other and preferably substantially parallel to the longitudinal axis of the
reactor-exchanger, said shell comprising means for turbulence of the gases
circulating, allowing indirect heating of said tubes, said reactor-exchanger
comprising at the other end, an outlet (17) of said gases having circulated through the calender
and means for discharging (24, 26) a hot gaseous effluent connected to said tubes
reaction vessels, said reactor-exchanger being connected to said quench member (27),
an inlet is connected to the means for discharging (26) the hot gaseous effluent and one of which
outlet is connected to means (28) for collecting cooled effluent.

The gas generator generally comprises an axial compressor (2) comprising a transmission shaft (7) connected to the first air inlet (3), a chamber (4) for combustion of fuel by compressed air connected to said compressor and at the second fuel inlet (5) and a power recovery turbine (6) connected to the compressor shaft and adapted to discharge compressed air and combustion gases under pressure into the exchanger reactor, said turbine comprising further said gas outlet.

The gas generator may be the equivalent of a jet engine of a so-called "Jet Engine", the noise of which is reduced by appropriate arrangements.

According to a preferred configuration, the reactor-heat exchanger is vertical and it is suitable for co-current circulation of the gas mixture in the reaction tubes and of the heating gases in the shell, from one end to the other and from preferably from the bottom up.

To increase the temperature of the heating gases at the inlet of the reactor-exchanger, it may be useful to interpose a post-combustion chamber at the outlet of the gases from the power recovery turbine, generally integrated with the gas generator. .

Thanks to the introduction of gases at an appropriate mass flow rate, it is possible to ensure a temperature difference between the temperature of the heating gases and that of the gaseous effluent in the tubes, at least equal to 50 "C and preferably between 100 and 150 "C.

The process implemented by the above device makes it possible to carry out the steam cracking of the hydrocarbon molecules at very high temperature, maintained substantially constant all along the reaction tubes. Furthermore, the residence time of the product in the exchanger reactor is very short. The reaction tubes are generally of small diameter, for example 10 to 40 mm. The difference in pressure of the heating gases between the outside of the tubes and the inside is such that it prevents, at least in part, the radial expansion of the tubes, so that the risks of rupture of the walls of the tubes are minimized. It follows that the steel thickness of these walls can be reduced. Certainly, the axial expansion remains, but a thermal expansion joint at the outlet end of the tubes can generally absorb variations in elongation of these latter.

In addition, the quench is immediate at the exit of the heating zone. There is in fact not the equivalent of connecting the tubes of the different chambers as in the units of the prior art. According to the invention, the volume of piping required for transfer to the quench exchanger is minimal and the temperature profile of the reaction is substantially more of the square type than it is according to the prior art.

Finally, the mechanical energy recovered from the power recovery turbines is substantially the same as that available at the output of the gas generator. It can be judiciously used in order to make the process very economical.

The invention will be better understood from the attached figure illustrating an embodiment applicable to the production of ethylene and propylene by naphtha steam pyrolysis combining the recovery of mechanical energy by a power recovery turbine. .

A gas generator 1, which may be a jet engine, comprises a compressor 2 for axial air supplied by a line 3 which compresses it to around 10 to 20 bar. The air temperature increases from around 300 to 4500C by compression. The compressed air passes through a combustion chamber 4 which is an integral part of the jet engine and which is supplied with fuel by a line 5. This fuel can be from methane to fuel oil. Exothermic combustion is initiated and takes place at constant pressure to increase the air temperature to around 1000 to 1200 "C. The air and very hot combustion products are then expanded to a high pressure power recovery turbine which drives the air compressor 2 through the drive shaft 7.

The air and the products of combustion leave the gas generator 1 at a pressure of approximately 5 to 8 bar and at a temperature of approximately 700 to 800 "C and are sent by a line 8 to the outlet of the turbine 6 , to a post-combustion chamber 9 into which fuel is introduced via a line 5a The temperature of the combustion fumes and of the air (heating gas) which may still contain from 15 to 19% by volume of oxygen can reach about 1100 to 1400 "C. These gases are used as a heat source in a reactor-heat exchanger below and their temperature can be controlled by suitable means controlling a fuel supply valve connected to the post-combustion.

They are sent by a line 10 to the lower end of a reactor-heat exchanger 1 1 which is vertical and elongated. This tube-and-shell type reactor-exchanger comprises an enclosure 50 coated with refractory material containing a shell 14 made of normal steel without alloy, for example, adapted to withstand a pressure of approximately 20 bar. This grille includes internals 51 adapted to achieve a gas baffle circulation and promoting turbulence and the mixing of these gases, for example internals comprising alternately a central solid disc and a pierced disc (disc and donut; GA Skrotzki,
Power, June 1954).

The reactor-exchanger also contains a plurality of reaction tubes 12, about a thousand 2.5 cm in diameter, made of Incoloy type steel, with high nickel content, substantially parallel to each other and substantially parallel to the axis of the reactor. heat exchanger maintained by substantially transverse walls, one 40 being floating and the other 41 being welded to the enclosure 50. These tubes are adapted to receive, thanks to a parallel supply, a mixture preheated to 580-650 "C at 1.5 to 3 bar, water vapor and hydrocarbons, a naphtha cut for example, by a line 13 opening at the lower end of the reactor 11, so that the gaseous hydrocarbon mixture circulates from below above in the reactor-exchanger under conditions such that its residence time is limited to approximately 100 to 300 ms.

The tubes 12 are heated essentially by convection substantially at the temperature of the heating gases which circulate in the calender 14 connected to the supply line 10 by means, for example, of a nozzle not shown in the figure at the base of the grille.

Thus, the heating gases circulate in the same direction of flow as that of the gaseous mixture in the tubes, which promotes a greater supply of heat at the very start of the reaction.

The reactor-exchanger comprises at one of its ends, on the discharge side of the gaseous effluent, a cooling chamber 29, substantially cylindrical, impervious to the heating gases and to the gaseous mixture constituting the charge. It is delimited, on the end side of the reactor-exchanger, by the floating circular wall 40 which supports the reaction tubes passing through this chamber over part of their length and on the shell side by a circular and floating wall 42 traversed by the reaction tubes. The chamber is connected to means 5c for introducing inert gas such as CO2, N2, water vapor or their mixture which preferably enters it tangentially. The inert gas can leave it either tangentially by appropriate means or by at least one tube 15 pierced with at least one orifice preferably situated at its end on the inlet side of the heating gases. This tube preferably parallel to the axis of the reactor is suitable for transferring the inert gas, advantageously the carbon dioxide recovered downstream from the device, from the cooling chamber in the calender thanks to an orifice drilled in the wall 42 which communicates with said tube. The chamber 29 is suitable for cooling the walls of the chamber, which reduces the variations in volume of the chamber due to the phenomenon of thermal expansion. According to this particularly advantageous embodiment, it allows a pre-tempering of the effluent from 5 to 50 "C for example, in the reaction tubes cooled by inert gas, over a length representing from 1/50 to 1/10 of their length and advantageously from 1/25 to 1/15 of their length.

Finally, the possibility of carrying out a very high rapid heat transfer contributes to inducing a substantially square temperature profile throughout the reaction, which promotes a better yield of ethylene.

The heating gases are collected and evacuated at the top of the reactor-heat exchanger at approximately 900-1050 "C and at approximately 5 to 7 bar by a line 17 and directed to the upper end of a conventional gas heat exchanger 18. This gas is suitable for preheating counter-current by convection in suitable tubes the gaseous reaction mixture of hydrocarbons and steam at 500-650 "C which is introduced through a supply line 19 at the bottom of these tubes. The mixture, once preheated, is sent to the lower end of the reactor-heat exchanger by line 13.

The heating gases circulating in the shell of this preheater exchanger 18 leave the latter at its lower end at a temperature of 700-950 "C and approximately 4 to 6 bar. They have, under these conditions, as much potential mechanical energy they would have had in an equivalent gas turbine configuration, that is to say at the outlet 8 of the power recovery turbine 6. The heating gases are therefore directed by a discharge line 20 to a guard container 21 (called KO Drum) then are expanded in a power recovery turbine 22 to extract the mechanical energy which will be used for example to compress the load. The expanded gases, substantially at atmospheric pressure and at a temperature of 400-5500C are brought by a line 23 towards the entry of a heat recovery chamber 30 allowing to carry out by convection a first preheating of the hydrocarbon charge and of the water vapor around 350 to 450 "C.

The gaseous effluent at the outlet of the reaction tubes 12 of the reactor-exchanger 11 is collected at a temperature of 800 to 900 "C by a suitable internal collector 24 and provided with an expansion joint. A line 26 for transfer of very short length is connected to the upper end of the reactor-exchanger and leads the effluent from the collector to a vertical member 27, of the conventional type, adapted to achieve rapid cooling (quench) by circulation against the current of water demineralized and which generates water vapor at very high pressure, of the order of 100 bar.

The cooled effluent is then collected at the lower end of the quench member 27 by a line 28 which leads it to downstream separation units, not shown in the figure.

The reactor-exchanger finally comprises means for controlling its operation. They include an automatic controller 31 connected by a transmission line 32 to a temperature probe 33 preferably arranged on the evacuation line of the steam cracking effluent 26. The controller controls a valve 35 by a transmission line 34 partial opening of the gaseous fuel in the gas generator.

The following example illustrates, without limitation, the process and the device according to the invention and in particular the quantity of energy involved. For a production of 400,000 tonnes / year of ethylene from steam pyrolysis d naphtha water, the following operating conditions are adopted:
Water to naphtha ratio 0.5
Quantity injected with naphtha 135 T / h
Mechanical energy required downstream 60 MW
Excess energy 60 MW
Gas generator (GE LM 5000) 4 X 30 MW
Air quantity 4 x 450 T / h
Quantity of heating gas produced 4 x 460 T / h at 1 1500C and 7 bar
Preheating temperature of
load 620 "C at 2.5 bar
Reactor-heat exchanger:
tube and grille type
Number 4
Number of tubes 1200
Outside diameter 20 mm
Length 12m
Heat transfer area 1150 m2 per reactor-exchanger
To supply all the reaction heat to the charge and to the steam (687 GJ / hour) by the sensible heat of the hot gas without combustion in situ in the reactor-exchanger, the weight flow rate of hot gas is 1,840 T / h, so as not to exceed 1000C in loss of temperature of the hot gas, which will result in a final recovery of power in excess of 60 MW.

The gases leave the reactor-exchanger at approximately 950 "C at 6 bar and pass through an exchanger where they are cooled to approximately 890" C providing 117 GJ / h of preheating heat to the load. These cooled gases, at a pressure of 5 bar, are expanded in four power recovery turbines delivering 30 MW each for utilities, i.e. 419 GJ / h. The exhaust gases, at around 5000C, from these turbines are then sent to a common heat recovery chamber where 670 GJ / h of heat can be recovered for a first preheating of the load and the production of steam while the gases cooled at the end to 2000C are discharged through a chimney into the atmosphere. The heat balance shows a thermal efficiency of 82% calculated as follows:
Amount of energy consumed by combustion GJ / h
Gas generator - primary combustion 1,336
- post combustion 967
TOTAL 2,303
Amount of heat absorbed
Reaction 687 Preheating 117. Recovery 670
SUBTOTAL 1,474
Amount of mechanical power recovered
209 60 MW 209
Amount of excess electrical power
209 60 MW 209
SUB TOTAL 418
1,892
TOTAL1,892

Claims (16)

 immediately in the cooling zone.  collects a hot ethylene-rich steam cracking effluent that is sent  heat indirectly with the gas mixture; c) said heating gases are evacuated through an outlet from the calender at the other end, and  said gases in the shell of the reactor-heat exchanger so as to exchange  one of the ends, preferably on the introduction side of the mixture, and circulate  heating to a suitable pressure and temperature; b) said gases are sent to an inlet of the shell of the reactor-heat exchanger at  of fuel in at least one gas generator, adapted to deliver gases from  heat according to the following stages: a) an amount of air and a quantity of air are passed under appropriate flow conditions  the heating phase of the gas mixture is carried out in the reactor-exchanger of  exchanger,  mixture inside a plurality of tubes, contained in said reactor  pressurized tube and shell heat exchanger, and circulate the  adequate hydrocarbons on water vapor at one end of a reactor  - Said mixture is preferably introduced beforehand, in a ratio  and recovering said cooled steam cracking effluent, characterized in that:  rapid said effluent in a cooling zone under appropriate conditions  hot ethylene-rich steam cracking effluent and a cooling phase  heating in a heating zone under appropriate conditions, which delivers a  containing from 2 to 20 carbon atoms and water vapor, comprising a phase of  CLAIMS 1- Steam cracking process from a gas mixture of a hydrocarbon feed 2 - Process according to claim 1 characterized in that the pressure of said gases  heating is from 4 to 20 bar and their temperature from 600 to 1400 "C. 3 - Process according to claims 1 and 2, wherein the temperature of 600 to 1400 "C is  at least in part obtained by burning an appropriate amount of fuel  thanks to at least a portion of said gases containing oxygen from the  gas generator, in a post-combustion zone downstream of the latter. 4 - Method according to one of claims 1 to 3 wherein the pressure of said gases is 6  at 10 bar and their temperature from 1000 to 12000C. 5 - Method according to one of claims 1 to 4 wherein the residence time of the mixture  in the reactor-exchanger is 80 to 400 ms and advantageously 100 to 300 ms. 6 - Method according to one of claims 1 to 5 wherein said gases are removed which have  circulated through the grille at a temperature of 580 "C at 12500C and under pressure  from 3 to 19 bar and they are sent to a second heat exchanger in which  preheats the mixture of hydrocarbons and steam, preferably against the current of the  direction of flow of said gases, at a temperature of 50 to 100 "C lower than the  initial cracking temperature of the mixture. 7 - Method according to one of claims 1 to 6 wherein recovering the gases having  passed through the second heat exchanger, at a temperature of 550 "C to 950" C and under  a pressure of 3 to 19 bar, preferably at 700-950 "C at 4 to 10 bar and we send them  in at least one power recovery turbine. 8 - Method according to one of claims 1 to 7 wherein the heating gases are  introduced into the grille at a mass flow rate at least twice that of the  gas mixture introduced into the tubes and preferably at a mass flow rate of 5 to 8  times that of the gas mixture. 9 - Device for carrying out an endothermic reaction such as a reaction of  steam cracking, comprising a heating reactor and a member adapted to produce a  quenching and connected to said heating reactor characterized in that the reactor  heating comprises in combination: 0 a gas generator (1) comprising a first air inlet (3) and a second inlet  (5) a fuel suitable for delivering, via an outlet, heating gases to a  temperature and pressure determined; b) an elongated pressure heat exchanger reactor (11), preferably  vertical, with tubes (12) and grille (14) having at one of its ends an inlet  of said gas connected to the outlet of the gas generator and connected to the calender (14) of the  reactor-exchanger in which said gases circulate, and supply means (13)  in a suitable gas mixture comprising at least one hydrocarbon connected to a  a plurality of reaction tubes (12), said calender comprising means for  turbulence of the gases flowing therein, allowing indirect heating of said tubes, said  reactor-exchanger comprising at the other end, an outlet (17) of said gases having  circulated through the radiator grille and means (26) for evacuating a hot gaseous effluent  connected to said reaction tubes (14), said reactor-exchanger being connected  to said quench member (27), one input of which is connected to the discharge means (26) of  hot gaseous effluent and one outlet of which is connected to collection means (28)  cooled effluent. 10 - Device according to claim 9 wherein the gas generator generally comprises  an axial compressor (2) comprising a transmission shaft (7) connected to the first  air inlet (3), a chamber (4) for combustion of fuel by compressed air  connected to said compressor and to the second fuel inlet (5) and a turbine  (6) power recovery connected to the compressor shaft and adapted to  discharge compressed air and combustion gases (heating gas) under pressure into the  exchanger reactor, said turbine further comprising said gas outlet. 11 - Device according to one of claims 9 to 10 wherein a chamber (9) to post  combustion comprising fuel supply means is interposed between  said gas recovery turbine outlet (6) and said gas inlet  gas in the reactor-exchanger (11). 12 - Device according to one of claims 9 to 1 1 characterized in that it comprises a  second gas-gas heat exchanger (18) downstream of the heating gas outlet of the  heat exchanger reactor, suitable for preheating, preferably against the current,  by indirect exchange with said gases, the feed gas mixture before entering  in the pressurized heat exchanger-reactor. 13 - Device according to one of claims 9 to 12, characterized in that it comprises at  least one power recovery turbine (22) connected to the exhaust gas outlet  heating of the second heat exchanger. 14 - Device according to one of claims 9 to 13 wherein the exchanger reactor  has at one of its ends on the discharge side of the effluent a  cooling (29) of the hot gaseous effluent, impermeable to said heating gases and  said gaseous effluent, delimited on the end side by a floating circular wall on a  part of their length which supports the reaction tubes (7) passing through said chamber  and by a floating circular wall, grille side, crossed by said tubes, said  chamber being connected to means for introducing inert gas (5c) and to at least  an inert gas outlet, said chamber being adapted to perform cooling  said walls of the chamber and to make a pre-tempering of the effluent in the tubes  reaction over a length representing from 1/50 to 1/10 of their length and  advantageously from 1/25 to 1/15 of their length. 15 - Device according to claim 14 wherein said inert gas outlet comprises at  at least one inert gas evacuation tube in the shell, said tube being connected to a  end to said chamber by at least one orifice drilled in the wall 42 and comprising  at least one orifice preferably at the other end, suitable for introducing the inert gas  in the grille. 16 - Device according to one of claims 9 to 15 characterized in that it comprises  control and servo means comprising an automatic controller (31) connected  a temperature probe (33) arranged on the effluent discharge means (26)  and adapted to control at least one fuel flow valve (35) supplying the  gas generator.