CH622964A5 - Process for starting or maintaining a chemical reaction which takes place on heating and reactor for its use - Google Patents

Abstract

A transparent coolant fluid circulates in an annular channel bounded by the concentric walls (14, 15) of the tube (11). The interior forms a reaction chamber (17). The tube is made, for example, of glass or quartz. An inert fluid, also transparent, enters the chamber (17) through two diffusers (21, 22) with a laminar flow, and protects the inner wall of the tube (11). …<??>The reactants follow an axial path (25) confined by the inert fluid and out of contact with the tube (11). An external source, for example an arc or filament, produces intense electromagnetic radiations which are reflected by a reflector onto a section of the trajectory (25) to cause the reaction. …<??>The reactor is employed especially for the dissociation or combustion of organic and inorganic compounds. …<IMAGE>…

Description

The present invention relates to a process for starting or maintaining a chemical reaction taking place while hot. The invention also relates to a reactor with a wall protected by a fluid for the implementation of this process.

High temperature reactors are currently used for pyrolysis, thermolysis, dissociation, decomposition and combustion reactions of organic and inorganic compounds. Practically all reactors of this type transfer heat to the reactants by convection and / or conduction, but this characteristic poses two problems which limit the nature and the scope of the reactions implemented. These problems are due to the fact that, in a conventional reactor which transfers heat to the reactants by convection, the highest temperature in the system necessarily prevails at the interface of the internal wall of the reactor and the current of the reactants.

The first problem is posed by the limitation of the reaction temperatures available imposed by the resistance of the known materials of the walls of the reactors at high temperature. The decreasing ability of these materials to maintain their integrity when the temperature increases is obviously well known. However, since these materials must be heated, so that thermal energy can be transferred to the reactant stream, the available reaction temperatures are limited by the temperature of safely heating the reactor wall. This is particularly important when the expected reaction is to take place at high pressure or create high pressures.

The second problem is due to the fact that the wall of a conventional reactor is at the highest temperature in the system and to the fact that the transfer of heat by convection and conduction requires contact between the wall and the flow of reactants. As it is at elevated temperature, the reactor wall is an ideal, if not the most desirable, reaction site, and in many cases the products of the reaction accumulate on that wall. Consequently, the device can no longer transfer the heat to the reactants and this increasing thermal impedance requires the progressive increase in the temperature of the source, so that the initial transfer of heat to the current of the reactants is maintained. Obviously, as the accumulation of materials progresses, the temperature required for the source exceeds that which the material of the wall can withstand. In addition, as an additional amount of energy is required to maintain the reaction, the process becomes less efficient from a technical and economic point of view. Thus, when the planned reaction can no longer be maintained for reasons of heat transfer, mechanical resistance of the materials or profitability, the installation must be stopped and cleaned.

Usually, cleaning is carried out mechanically by scraping the wall or chemically by burning deposits. In some continuous processes, attempts have been made to scrape the wall while the reaction is in progress. However, the scraper tool itself necessarily heats up, becomes a reaction site and must therefore be cleaned afterwards. In all cases, this downtime represents a significant economic loss. In many cases, a second reactor is mounted so that the time lost for production is minimal. However, this additional apparatus generally represents a significant investment. Such high temperature chemical reactors include a tube brought to a temperature at which the internal walls emit enough radiation for the reaction to be started and maintained. However, as in the case of conduction and convection reactors, in reactions forming solid products, the product often accumulates undesirably on the walls of the tube so that the heat transfer is reduced and the tube can even butcher.

The reactor described in US Patent No. 2,962,073 is intended for the manufacture of carbon black and hydrogen by pyrolysis of natural gas. The process is given as continuous, but in practice, convective heat transfer, used during reactor operation, poses significant problems for maintenance and control of the reaction. Since the heated reactor tubes are ideal sites for the reaction, carbon invariably accumulates and eventually clogs the reactor. However, the problem of thermal runaway is much more important because it can cause explosions. Under these conditions, it is determined that, during the pyrolysis of natural gas, the thermal conductivity of the gas phase suddenly increases by a factor of between 5 and 30, depending on the composition of the gas. As the temperature in a conventional convection reactor cannot be regulated with sufficient precision and speed to compensate for the phenomenon, the reactor becomes unstable in some cases and can even explode. These conditions arise in the case of conventional reactors and a solution to this problem has not yet been found.

U.S. Patent No. 3,565,766 describes a recent attempt to increase the quality of coal by pyrolysis. The apparatus described comprises a series of hollow steel reactors forming multi-stage fluidized beds at gradually increasing temperatures up to approximately 870 ° C. Fluidization at low temperatures is ensured by an inert gas which can itself transmit heat, although external heating is provided. At high temperatures, the fluidization is carried out by the head gas of the final stage and, in this last stage, the temperature is maintained by internal combustion of the carbonized materials in air or oxygen. As this device is essentially based on convective heat transfer, it has many of the defects and drawbacks described above.

The carbon black manufacturing apparatus described in U.S. Patent No. 2,062,358 includes a porous tube placed in a heating chamber. A hot gas is directed from an oven remote to the chamber, then expelled through the wall of the porous tube so that it mixes with the reagents. Thus, only the transfer of heat by convection of a fluid to the reactants is used. Given the absence of a cavity forming a black body, this arrangement requires the circulation of a large volume of fluid in the heating chamber to compensate for heat losses.

U.S. Patent No. 2,769,772 describes a heat treatment reactor for fluids such as hydrocarbons, comprising two concentric tubes placed in an oven heated by a flame. The reagents circulate axially in the permeable concentric internal tube. A gas constituting a gaseous vehicle flows in the annular chamber between the concentric tubes and it is heated by contact with the external wall. Fluids in the inner tube are heated by convection when the gaseous vehicle passes through the permeable wall and mixes with them. Radiative transfer is expressly avoided. In reality, the inner tube cannot be heated without simultaneously heating the outer tube to at least as high a temperature.

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The surface combustion cracking oven described in United States patent No. 2436282 implements the principle of a gaseous convective heat transfer vehicle similar to that of the aforementioned patent No. 2769772. The oven comprises a porous refractory tube surrounded by a double envelope. A combustible fluid from an annular chamber is expelled through the porous wall towards the interior of the tube and is ignited. It is however obvious that the combustible fluid of the annular chamber explodes unless it is expelled through the porous wall at a speed greater than the speed of propagation of the flame through the wall. Likewise, the temperature in the annular chamber must be kept below the ignition temperature of the gas-air mixture. The products of combustion of the surface flame mix with the reactants in the oven with dilution and possibly reaction with them. Heat is applied to the reactants by convection mixing of the combustion products and reactants.

U.S. Patents Nos. 2670272, 2670275, 2750260, 2915367, 2957753 and 3499730 describe combustion chambers for the preparation of titanium dioxide for pigments by combustion of titanium tetrachloride in oxygen. In patent No. 2670275 which is representative of this group, titanium tetrachloride is burnt in a porous refractory tube. An inert gas constantly diffuses through the porous tube and enters the combustion chamber where it forms a protective envelope for the internal face of the tube. This porous shell significantly reduces the tendency of titanium dioxide particles to adhere to the walls of the reactor. As the combustion of titanium tetrachloride is exothermic, no precaution is taken for the transfer of heat to the reaction mixture as it passes through the tube. In reality, this patent indicates that it is advantageous to remove heat from the chamber either by exposure of the assembly comprising the porous tube to the atmosphere, or by circulation of a cooling fluid in a coil placed in the vicinity of the porous tube.

The invention relates to the process which is the subject of claim 1, and to the reactor which is the subject of claim 10, to special embodiments which are the subject of other claims.

The invention allows an essential breakthrough in the technical field considered. As it makes available for the first time a source of thermal energy, which has never been mastered in this way, its possible applications are many and varied. In addition, the resolution of the problem posed by the resistance of materials, a very important problem for many years, makes possible the practical use of many useful chemical reactions which have been known for a long time but which cannot be carried out being given the temperature limits imposed on reactors operating by convection and / or conduction transfer.

The invention will be better understood using the description of some embodiments given by way of example and with reference to the drawings in which:

- fig. 1 is an elevation in partial section of an embodiment of the reactor according to the invention;

- fig. 2A is a section through the inlet end of a second embodiment of the reactor according to the invention;

- fig. 2B is a section through the outlet ends of the second embodiment of the reactor according to the invention, FIGS. 2A and 2B represent the halves of the same structure, separated along the line A-A so that the drawing can clearly represent certain details;

- fig. 2C is a perspective in partial section of the second embodiment of the reactor of the invention from which certain elements have been removed or shown diagrammatically so that the operation appears clearly;

- fig. 3 is a section along line 3-3 of FIG. 2A;

- fig. 4 is a section along line 4-4 of FIG. 2B;

- fig. 5 is a section along line 5-5 of FIG. 2A; and

- fig. 6 is a perspective view of part of the device for heating the reactor tube of the second embodiment of the invention.

Fig. 1 shows a first embodiment of a high temperature chemical reactor 10 according to the invention which comprises a tube 11 having an inlet end 12 and an outlet end 14. The tube 11 has an internal wall 15 and an external wall 16 delimiting an annular channel therebetween, and the interior of the tube 11 forms a chamber 17 of the reactor. The tube 11 is made of material that is practically transparent to radiation. Suitable materials having a very low absorption coefficient a are glass, quartz, hot sintered alumina, hot sintered yttrium oxide, Pyrex (borosilicate glass), Vycor (silicate glass ) and sapphire, organic polymers such as Plexiglas (acrylic), Lucite (acrylic), polyethylene, polyprolylene and polystyrene, and mineral salts such as sodium, potassium, cesium, lithium halides or lead.

In this specification, the term radiation and the term radiant energy refer to all types of radiation including high energy nuclear particles. However, as they cannot be used in practice at present, only electromagnetic radiation from the black body or the like is considered, in particular at wavelengths between approximately 100 and 0.01 (i are the primary source of energy which 'we basically use.

During the operation of the reactor 10, a fluid which is practically transparent to radiation penetrates through the inlet 18, circulates in the annular channel and cools the tube 11, then exits through the outlet. fluids with low absorption coefficients a are liquid or gaseous water, heavy water, nitrogen, oxygen and air.

A device for introducing an inert fluid into the chamber 17 through an inlet 20 comprises two laminar diffusers 21 and 22 placed near the inlet end 12. The diffusers 21, 22 can have a honeycomb or other configuration such that the fluid sent under pressure flows in a practically laminar manner. The inert fluid thus penetrates practically axially into the chamber 17 and forms a protection of the internal face of the tube 11, and it is collected so that it circulates again after the outlet 23. The inert fluid is practically transparent to radiation because it has a low coefficient a. Fluids suitable for this purpose are simple gases such as helium, neon, argon, krypton and xenon, complex gases, which do not decompose to form a solid product, such as hydrogen , nitrogen, oxygen and ammonia, and liquid or carbonated water. The inert term used in this specification applies to two properties, the ability of the fluid to react chemically with the material of the tube 11 and the ability of the fluid to react chemically with the materials being treated. Thus, the selection of an inert fluid depends in all cases on the particular atmosphere. Unless otherwise indicated, it is advantageous for the fluid to be inert with respect to the tube and it is usually desirable for it to be inert in the case of the reaction carried out. However, in certain cases, the inert protective fluid also takes part in the reaction, for example, when the iron or carbon particles react in the presence of a water vapor protection with formation of iron oxide and hydrogen or carbon monoxide and hydrogen respectively.

The reagents enter the chamber 17 through the inlet 24 at the end 12 of the tube 11. They follow a predetermined path 25 axially to the tube 11 and are confined by the inert protective fluid practically in the center of the chamber 17 out of contact of tube 11.

A source of very intense radiation (not shown) is disposed in a polished reflector 31 mounted on a frame 32 outside the tube 11. The source of radiation can be a

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plasma, a heated filament, a seeded film, a pulse-operated flashlight or other suitable device; a laser can also constitute the source but, currently, the technology is not sufficiently developed so that its use is profitable. The radiation created by the source is collected by the reflector 31 and transmitted through the tube 11 in the chamber 17 so that it coincides with at least part of the path 25. A sufficient quantity of radiation is thus absorbed so that the temperature of the reagents reaches the value necessary to initiate and maintain the desired chemical reaction. As indicated above, the tube 11, the cooling fluid and the inert protective fluid are practically transparent to radiation. Thus, they do not significantly interfere with the transmission of energy to the reactants and remain practically cold. Thus, the tube 11 does not undergo appreciable thermal stresses and does not receive precipitates or other deposits which could accumulate normally.

The foregoing description assumes that the reagents themselves have a relatively high radiation absorption coefficient a. However, in the opposite case, a radiation absorbing material, called auxiliary material, can be introduced into the chamber 17 so that it coincides at least at one point with the path 25. In the embodiment of FIG. 1, the auxiliary absorption material is a finely divided solid such as carbon powder or another suitable material entering the chamber 17 with the reagents through the inlet 24 and absorbing enough radiation for the temperature of the reactants to reach the desired value.

Alternatively, the auxiliary material may be a liquid, for example a pitch, asphalt, linseed oil or diesel fuel, and it may contain solutions, dispersions, gels and suspensions of various materials which can be easily chosen from the available materials according to particular criteria. The auxiliary material can be a gas advantageously having an absorption in the electromagnetic spectrum between approximately 100 and 0.01 µm; such gases are especially ethylene, propylene, nitrogen oxides, bromine, chlorine, iodine and ethyl bromide. The auxiliary material can also be solid, for example carbon, placed in the chamber 17 along at least part of the path 25 of the reactants.

Another device raising the reaction temperature to the desired value may comprise an electrically heated element, an electric arc or a flame, placed in the chamber 17 and coinciding with at least part of the path 25. In this case, the source of heat of release is autonomous and is not formed by the device creating the radiations. Such a device is particularly useful when the reagents themselves are transparent to radiation, but when at least one of the products of the reaction absorbs. Thus, when the intended reaction is started, the device for raising the temperature can be stopped because the products of the reaction absorb enough radiation for the reaction to be maintained. Similarly,

when using an auxiliary material, this may no longer be introduced or may be withdrawn when the reaction has started, for example using a control device. An example of a reaction, in which an auxiliary material or a triggering device is required only at the start, is the pyrolysis of methane in the form of carbon and hydrogen.

As indicated above, certain reactions are partially or totally reversed when the reaction products are not immediately and quickly cooled. To this end, a device 40 for cooling the reaction products is placed in the chamber 17 near the outlet end 14 of the tube 11. An embodiment of the device 40 is placed practically in the center in the chamber 17 and comprises a tubular member 41 having an internal channel 42 in which a cooling fluid such as water circulates. The internal surface of the tube 42 is intended to absorb radiation. When the reaction products, the remaining reactants and any auxiliary materials pass through the tube 41, the heat is quickly transferred by radiative coupling and the material is quickly cooled so that undesirable chemical reactions are avoided.

We now consider figs. 2A to 6 and in particular FIGS. 2A to 2C which represent a second embodiment of reactor 60 which comprises a tube 61 having an inlet end 62 and an outlet end 63, the interior of the tube 61 forming a chamber 65 of the reactor. The tube 61 is made of porous material which can emit radiation, the diameter of the pores being advantageously between approximately 0.025 and 0.5 mm,

so that an inert fluid can flow uniformly in sufficient quantity through the wall of the tube and form a suitable protection. Other walls, for example in the form of fabrics, screens or various types of perforations, can be used for the same result. The tube 61 can be made of a material such as graphite, carbon, sintered stainless steel, sintered tungsten or sintered molybdenum, or a mineral material such as oxides of thorium, magnesium, zinc, aluminum or zirconium in particular. Tungsten, nickel and molybdenum can also be used in the form of a canvas or a screen.

A sealed tubular container 70 under pressure, advantageously made of stainless steel, surrounds the tube 61. The integrity of the container 70 is ensured by a series of sealing flanges 71, 72, 73, 74, 75 and 76, which connect the various parts of the reactor 60. The flanges 72, 73 and 76 have grooves housing O-rings 77, 78 and 79 made of stainless steel respectively, forming pressure-resistant seals. The tube 61 of the reactor can slide at one end in a sleeve 81 of graphite which allows the elongation of the tube 61 which can take place during operation at high temperature.

The container 70 further comprises an inlet 83 for admission of an inert fluid which is practically transparent to radiation. The inert fluid is first directed under pressure into a chamber 85 formed between the tube 61 and the wall of the container 70. Then, the fluid passes through the wall of the tube 61 in the chamber 65 and forms a protection of the internal face. of tube 61.

A container cooling device 70 comprises coils 87 placed around the external surface of the container 70. The coils 87 are preferably covered with a coating of flame sprayed aluminum which improves the thermal contact between the container 70 and the coils 87 and increases the cooling efficiency. The coils 87 are also placed around an observation channel 88 formed in the wall of the container.

As shown clearly in figs. 2A and 3, the reagents enter the chamber 65 through the inlet end 62 of the tube 61. A device for introducing the reagents comprises an inlet portion 90, mounted in leaktight manner on the flanges 71, 72, near from the inlet end 62 of the tube 61. The reactants are entrained in a gas stream through the inlet 91, in front of a tangential partition 92 and in a chamber 93 under pressure delimited between an outer wall 94 and a diffuser 95. A suitable material for 95 diffusers, which minimizes current turbulence, is porous carbon, steel wool and a wire mesh. As in the case of fig. 1, the reagents follow a predetermined axial path relative to the tube 61 and are delimited by the gaseous protective envelope practically in the center of the chamber 65 out of contact with the internal wall of the tube.

In the second embodiment, the tube 61 itself creates the very intense radiation directed towards the center, practically coinciding with at least part of the path of the reagents. The heating is provided by several lOOa-lOOf carbon electrodes placed radially outside the tube 61 and around it, at regular intervals; the heat from the electrodes 100 is transmitted to the tube 61 by radiation. The electrodes 100a and 100b are buried at

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one end in a curved element 101a of carbon, the electrodes 100c and 100d on the one hand and 100e and lOOf on the other hand being themselves buried in an element 101b and 101c respectively of carbon. Tubular spacers 102a-102c of alumina have the dual function of centering the tube 61 and dividing the three circuits. As shown more precisely in FIGS. 2B and 4, each electrode 100a-100f is mounted at the other end in a copper electrode 104 forming a bus bar. Although the apparatus comprises six electrodes 104, only one has been shown in FIG. 4 for convenience. Each electrode 104 comprises a flange 105 made of phenolic resin and a ceramic insulator 106. The electrode 104 is cooled by water which circulates in an internal channel 107, by an inlet 108 and an outlet 109. An electrical connection 110 transmits a high intensity. A seal 111 made of polytetrafluoroethylene facilitates the elimination of leaks from the container 70. The electrical system shown is particularly suitable for a three-phase supply. However, other facilities may be used depending on the circumstances. The porous tube 61 can itself be heated directly by electrical resistance, the electrodes 100 then being eliminated.

The thermal efficiency of the tube heater is further improved by a thermal shield 120 of molybdenum which forms the surface which delimits the black body reflecting the electromagnetic radiation from the electrodes 100 to the tube 61. Thus, the shield 120 reflects heat rather than 'it does not transmit it, and constitutes an insulator so that it can be of any material having this characteristic and capable of withstanding the temperatures created by the electrodes 100. The shield 120 is placed in the container 70 radially outside by relative to the electrodes 100 and it preferably comprises a flat strip of rectangular section wound in the form of a series of helical turns. This construction allows the inert protective gas to penetrate through the inlet 83 and its free circulation in the chamber 85.

As in the embodiment of FIG. 1, an auxiliary absorption material or another triggering device can be used if necessary. Auxiliary absorption materials enter the chamber 65 through an inlet 121. In addition, a device 125 for cooling the reaction products of the type already described or different can be used and prevents any undesirable chemical reaction which may take place when the reaction products are not cooled immediately after their formation.

The essential advantage of the second embodiment compared to the first is that, in the second embodiment, the inert fluid is introduced into the chamber 65 in the radial direction inwards while, in the first embodiment, it is introduced axially in chamber 17. It should be noted that a laminar current can be maintained over a relatively short distance only before the turbulence causes mixing and destroys the integrity of the gas protection. Since the radial introduction of the gas does not require a laminar flow of the fluid, it is possible to use much longer axial lengths of the reaction chamber. It suffices that, in the second embodiment, the absolute pressure of the inert fluid is greater than that of the current of the reactants so that the reactants and / or the products of the reaction do not strike the tube 61. This characteristic facilitates the production of such a device on a large scale industrially.

Another difference between the two embodiments is that the tube 11 of FIG. 1 is cooled while the tube 61 of FIG. 2 must be heated and can operate at temperatures exceeding 2980 ° C, for example when porous thorium oxide is the material. Although the cold wall is more able to withstand pressure because it does not undergo thermal stress, the hot wall 61 does not undergo a pressure gradient, except possibly the very small difference in pressures prevailing between the inert fluid and the current of reactive. The pressure is supported by the wall of the container 70 made of stainless steel which is obviously cooled by the coils 87 and is not subjected to thermal stress. Thus, a refractory material such as carbon or thorium oxide, capable of withstanding temperatures much higher than those which can be tolerated by the conventional materials of the walls of the reactors and which is not suitable in conventional convection reactors, can be used to the first time in a device usable in practice and operating at extremely high temperature.

A third embodiment combines characteristics of the first two. Thus, the reactor tube can be made of porous material which is practically transparent to radiation. Such materials are porous quartz, porous sintered glass and porous sapphire. An inert fluid which is practically transparent to radiation can also be introduced into the chamber in the radial direction inwards through the porous wall and not axially in laminar form as described in the first embodiment. Radiation is created, collected and directed into the reaction chamber as described for the first embodiment.

The third embodiment gives the high power density of the first embodiment and the circulation of fluid injected radially from the second embodiment. However, currently, the second embodiment is better suited to large-scale industrial applications because the radiation is obtained by ordinary electric heating in a resistor. The second embodiment can therefore be easily produced and maintained. In addition, this second embodiment can implement all the methods and all the reactions considered by simple adjustment of the residence time of the reactants in the chamber, so that the reduction in power density is compensated.

The high temperature chemical reactions carried out according to the invention require the use of an annular sheath or protection of inert fluid substantially transparent to radiation. The sheath or envelope has a significant axial length. This annular sheath can be created substantially parallel to the axis, or substantially perpendicular to its axis, towards the inside of the external circumferential surface.

In the first case, as described in the first embodiment of the reactor, the sheath fluid must be maintained in the form of a laminar stream preventing mixing with the reactant stream. This criterion imposes certain limits on the axial length of the sheath given the laminar flow, and the integrity of the protection cannot be maintained over indefinite lengths downstream, in particular when a particularly violent reaction is expected. Thus, such a sheath creation process is especially suitable for laboratory and small-scale applications.

In the last case described above, concerning the second, third and fourth embodiment, the integrity of the sheath or fluid protection is independent of flow considerations and can be maintained over an axial distance much greater than that one obtains in the case of a laminar protection injected parallel to the axis. The essential criterion is the maintenance of an inert fluid stream at a pressure higher than that of the reagent stream so that the reagents cannot pass through the fluid protection or no longer be confined by the sheath.

When the sheath or protection has been created, at least one reagent passes through the center "along a predetermined path which practically coincides with the axis of the sheath. This confines the reagents inside and out of contact with the surfaces of the reactor chamber.

Finally, very intense radiation is transmitted to the center of the sheath and coincides with at least part of the predetermined path of the reagents. These radiations can be directed at at least one point on the path of the reagents as in the first or third embodiment, or over a finite length of this.

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path as in the second and fourth embodiment. In all cases, a sufficient quantity of radiation is absorbed in the central part so that the temperature of the reactants reaches the value necessary to initiate the desired chemical reaction.

When the reagents do not absorb the radiation themselves, an absorbent auxiliary material can be introduced along the path of the reagents, advantageously before direction of the radiation in the central part. The auxiliary material then absorbs enough radiation for the temperature in the central part to reach the value necessary to initiate the desired chemical reaction. As previously stated,

when the intended reaction is such that the transparent reagents create at least one radiation absorbing product, the auxiliary material can be deactivated after initiating the reaction.

The process under consideration may further comprise cooling the reaction products and the reagents and / or auxiliary materials remaining just after the end of the desired reaction. The purpose of this is to stop the desired reaction and suppress any unwanted reactions. The remaining products, auxiliaries and reagents can be advantageously and efficiently cooled by radiative transfer to a cold surface that absorbs radiation.

We now consider the implementation of reactors with a wall protected by a fluid. The reactors according to the invention can be used for practically all chemical reactions at high temperature, which often have been considered impossible in practice or only theoretically possible. The most important criterion for the use of such reactors in a particular chemical reaction at high temperature is that the reaction must be thermodynamically possible under the conditions considered. When using these reactors, the chemical reactions considered can be carried out at temperatures up to around 3315 ° C., by (1) creation inside the porous tube of the reactor of a tubular sheath made of practically transparent inert fluid to radiation and forming a protection of the internal surface of the reactor tube, the annular sheath having a significant axial length and its internal part delimiting a reaction chamber, (2) circulation of at least one reagent (in the solid, liquid state or gaseous) in the reaction chamber along a predetermined path substantially coinciding with the longitudinal axis of the sheath, the reagents being confined in the reaction chamber, (3) direction of very intense radiation in the reaction chamber so that they coincide with at least part of the predetermined path of the reactants, a sufficient quantity of radiation being absorbed in the chamber for the temperature of the reactants to reach l has the value necessary to initiate and maintain the desired chemical reaction.

Among the reactions which can be carried out in the reactors described, mention may be made of the dissociation of hydrocarbons and hydrocarbon materials such as coal and various petroleum fractions, into hydrogen and carbon black, steam reforming of coal, petroleum fractions, oil shale, asphalt sands, lignite and other carbon or hydrocarbon feedstocks in the form of syngas mixtures, the processes may also include the possible use of one or more mineral carbonates (such as that limestone or dolomite) or mineral oxides which react chemically with sulfur-containing impurities which can thus be removed from mixtures formed of synthesis gas, the partial dissociation of hydrocarbon materials and hydrocarbons into compounds with lower molecular weight, partial pyrolysis of saturated hydrocarbons to unsaturated hydrocarbons such as ethylene, propylene and acetylene, the conversion of organic waste such as sludge from waste water or by-products containing lignin into combustible gases, the total or partial desulfurization of hydrocarbon charges containing sulfur, the reduction of ores or organic compounds to

a lower valence using hydrogen, carbon, synthesis gas or another reducing agent, and the total or partial reaction of an inorganic element or compound with a carbonaceous material so that the corresponding inorganic carbide be trained.

If necessary, one or more catalysts can be added to these chemical reactions at elevated temperature so that they accelerate the reaction or change the course thereof to a desired sequence. When the process uses carbon or hydrocarbon reactants, the addition of a suitable catalyst to the system can promote the formation of free radicals, carbonium ions or carbanions influencing the course of the reaction.

Obviously, a single set of operating conditions is not optimal or suitable for all the reactions which can be carried out in the reactor. Operating conditions such as temperatures, pressures, charge rates, residence time in the reactor tube and cooling rates may vary depending on the particular reaction carried out. For example, among the parameters which have an influence on the products of the pyrolysis of a hydrocarbon, mention may be made of the temperature of heating of the hydrocarbon and the time of maintenance at this temperature. We know, for example, that methane must be heated to around 1232 ° C so that it forms acetylene. The formation of ethylene from ethane begins at a relatively low temperature, of the order of 830 ° C. In an example of pyrolysis of hydrocarbons, acetylene, ethylene, hydrogen, carbon black and hydrocarbon oils are formed. Reaction times of the order of a millisecond generally make the yield of acetylene maximum, since reactions of duration greater than 1 ms generally favor the production of ethylene and other products to the detriment of acetylene , while reaction times of less than one millisecond generally reduce the yield of ethylene and acetylene at the same time. Very high temperatures exceeding, for example, 1650 ° C. generally favor the production of carbon black and hydrogen to the detriment of acetylene and ethylene. The reaction times in the reactors of the invention can be reduced by shortening the reactor tube and by increasing the flow rate of the reactants introduced into the tube. For very short reaction times, the mixing of an auxiliary radiation absorbing material such as carbon black with the reagents may be advantageous so that the coupling between the stream of reagents and the radiation coming from the wall of the tube is favored. and thus facilitates rapid heating of the reagents.

For carrying out various chemical reactions at high temperature, the high temperature reactor with a wall protected by a fluid, shown in FIGS. 2A to 6, is used for the implementation of the particular reaction. The tube 61 of the reactor is a porous graphite tube 91.4 cm long, having an internal diameter of 7.6 cm and an external diameter of 10.2 cm, the mean radius of the pores being 20 µm. porous tube is housed in a pressure vessel 70 in steel 25.4 cm in diameter. The tube 61 is heated by electrodes 100a at lOOf of carbon placed in the chamber 85. The thermal shielding 120, also placed in the chamber 85, is made of molybdenum. A collar 125 cooled by water is placed near the outlet end of the tube 61 and cools the reaction products formed by radiative coupling. After continuous processing for a variable time, the tube 61 is inspected to determine the presence of an accumulation of carbon black or the like. We never find out.

Unlike conventional convection reactors, the process described uses radiative coupling for the transfer of heat to the current of the reactants. The amount of heat transferred is independent of the physical contact between the reactor wall and the flow of reactants and the turbulent mixture in the flow. The essential consideration for heat transfer in the apparatus of the invention is the coefficient of absorption of radiation a

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by reagents. The inert fluid for protecting the wall of the reactor is advantageously practically transparent to radiation, so that it has a very low coefficient. In this way, the radiation can be transferred through the inert fluid to the current of the reactants with little or no energy loss. In an ideal case, the reagents themselves or an associated absorbent material have a high coefficient a and therefore absorb large amounts of energy, or the reagents can be finely divided (for example in a fog) so that the radiation is absorbed by trapping between the particles. As the materials which absorb radiation well are generally good emitters of radiation, when the reagents or the associated absorbent materials are brought to a sufficiently high temperature, they form secondary radiators which re-emit energy throughout the reaction volume and increase therefore the heat transfer characteristics of the installation. This phenomenon occurs almost instantly and can be quickly and precisely resolved. In addition, the re-emission phenomenon ensuring rapid and uniform heating of the reactants is completely independent of the turbulent mixture in the current of the reactants.

The apparatus and the chemical process at high temperature allow the resolution of extremely troublesome problems and thus the implementation of reactions which until now were impossible or possible only in theory. As the heat is transmitted by radiative coupling and not by convection and / or conduction, the temperature of the current of the reactants can be independent both of the temperature of the wall and of the condition of the current of the reactants, and the important problem posed by the mechanical strength of the materials is resolved.

The use of an inert protective fluid made possible essentially by radiative coupling isolates the wall of the reactor from the flow of reagents and makes impossible under normal operating conditions the accumulation of precipitates or deposits and the plugging of the device.

The use of radiative coupling also allows precise and almost instantaneous adjustment of the heat transfer rate, this adjustment being practically impossible in the case of conventional convection reactors. In addition, the reactor of the invention can give a power density, at the reaction site, exceeding 10,000 W / cm2. This figure represents a great improvement compared to the value of 2 to 3 W / cm2 usually obtained with conventional reactors.

The reactions which can be carried out according to the process of the invention, in the reactor of the invention, are numerous and varied. For example, organic compounds such as hydrocarbons in particular, can be pyrolyzed in the form of carbon and hydrogen without the problems of accumulation and thermal runaway posed in a conventional manner. Saturated hydrocarbons can be partially pyrolyzed as unsaturated hydrocarbons; thus, propane and ethane can be dehydrogenated to propylene and ethylene respectively. Unsaturated hydrocarbons can be partially pyrolyzed in the absence of hydrogen and form saturated hydrocarbons and more specifically, petroleum products can be thermally cracked. Diesel can therefore be easily transformed into diesel fuel, kerosene, petrol and even methane. Halogenated intermediates can be added to partially pyrolyzed hydrocarbons so that the compounds formed have an increased molecular weight. Hydrocarbons can be partially or totally pyrolyzed in the presence of water vapor and form carbon monoxide and hydrogen; additional hydrogen can be added and the reaction can be carried out to form alkanes which are high calorific fuel gases.

Mineral compounds can be pyrolyzed analogously. For example, salts or oxides of iron, mercury, silver, tungsten and tantalum in particular can be dissociated and give pure metals. Oxides of iron, nickel, cobalt, copper and silver, to name a few, can be directly reduced in the presence of hydrogen with the same result. This list is by no means exhaustive.

It is also possible to prepare new composite products by implementing the process of the invention. For example, carbon or talc particles coated with silicon carbide can be produced. This product is an excellent abrasive because, during use, it constantly breaks and forms new sharp surfaces. The particles of certain elements such as U235 can also be coated in a sealed envelope in another material such as carbon; this particular product is useful as a fuel element in a nuclear reactor.

It is also possible to carry out the final stage of conventional aerobic incineration of garbage, in particular household garbage and waste water. The relatively low temperatures used in common incineration processes allow the formation of organic peroxides and nitrogen oxides which contribute greatly to photochemical fog and other forms of air pollution. As these compounds are not stable at the high treatment temperatures permitted according to the invention, the incineration effluent obtained can be very little polluted.

In addition, anaerobic destructive distillation can also be carried out at high temperature and / or the disassociation of waste with the formation of useful products such as carbon black, activated carbon, hydrogen and cullet, among others. The addition of water vapor to this waste causes the creation of hydrogen and carbon monoxide which can be treated in conventional form and give combustible gases.

Finally, the addition of hydrogen to this waste allows the preparation of heavy oils equivalent to those of petroleum and other petroleum products. Thus, significant reductions in air pollution and significant economic gains can be made by implementing the invention.

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Claims (28)

622964 2 1. A process for starting or for maintaining a chemical reaction taking place hot, characterized in that it comprises the following steps: 1) maintain an annular sheath of inert fluid substantially transparent to radiation, the sheath having an axial length greater than its transverse dimension, the inside of the sheath defining a reaction chamber, 2) passing at least one reagent into the reaction chamber along a path substantially coinciding with the longitudinal axis of the sheath, the reagents being confined in the reaction chamber, 3) produce electromagnetic radiation at a location outside the reaction chamber, and 4) collect and focus the radiation by directing it into the reaction chamber so that it coincides with at least part of the determined path of the reactants, a sufficient quantity of radiation being absorbed in the reaction chamber for the temperature of the reactants to reach the value required to initiate and maintain the desired chemical reaction. 2. Method according to claim 1, characterized in that, to maintain the annular sheath of inert fluid, the fluid is sent through a diffuser and it is passed axially in substantially laminar flow inside a practically transparent reaction tube having an internal wall delimiting the reaction chamber, in order to pass at least one reagent into the reaction chamber, the reagent is introduced through an inlet end of the tube and is directed into this tube along an axial path relative to the tube, in that the radiation is produced by means of a radiant energy source, in that the radiation is collected and focused by directing it into the reaction chamber by a reflection, and in that a practically transparent coolant is passed to the radiation in an annular passage formed between the internal wall and a concentric external wall belonging to the reaction tube. 3. Method according to claim 1, characterized in that, to maintain the annular sheath of inert fluid, the fluid is continuously directed from the outside to the internal surface of the sheath perpendicular to the axis of the sheath. 4. Method according to claim 1 or 2, characterized in that an auxiliary material absorbing the radiations is introduced along the path of the reagents, a sufficient quantity of radiations being absorbed by the auxiliary material so that the temperature at the center reaches the value necessary to initiate the desired chemical reaction. 5. Method according to claim 4, characterized in that the auxiliary radiation absorption material is deactivated after triggering of the desired reaction. 6. Method according to claim 1 or 2, characterized in that the radiation is directed over a determined length of the path of the reagents. 7. Method according to claim 1 or 2, characterized in that it comprises the cooling of the reaction products and of the remaining reactants just after the exit of the reactants from the reactor tube, so that the desired chemical reaction is completed and that an undesirable chemical reaction is not possible. 8. Method according to claim 7, characterized in that the reaction products and the remaining reagents are cooled by radiative transfer to a cold surface absorbing radiation. 9. The method of claim 7, characterized in that it further comprises the introduction of an auxiliary material absorbing radiation along the path of the reagents before introduction of these into the reactor tube, the auxiliary absorbent material being cooled right after. 10. Reactor for implementing the method according to claim 1, in which practically all the heat is transmitted by radiative coupling, characterized in that it comprises: a) a reaction tube having an inlet end and an outlet end and the interior of which delimits a reaction chamber, the tube being of a material substantially transparent to radiation, b) a device for introducing at least one reagent into the reaction chamber through the inlet end of the tube, the reagent being directed along a path determined axially relative to the reaction chamber and being delimited by the sheath protective of inert fluid practically in the center of the reaction chamber and out of contact with the internal wall of the tube, c) a source of radiation placed in a reflector placed outside the reaction tube, the radiation being collected and focused by the reflector then directed into the reaction chamber, so that they coincide with at least part of the determined path of the reagent, and d) means for maintaining the annular sheath of inert fluid transparent to radiation, said means consisting either of a diffuser placed near the inlet end of the tube and intended to admit the inert fluid under pressure, so that the fluid then has a substantially laminar flow along the axis of the reaction tube, the latter being double-walled, delimiting an annular channel for the passage of a cooling fluid substantially transparent to radiation outside the reaction chamber , or in a device for bringing the inert fluid under pressure around the reaction tube so that it crosses the wall of the tube, the latter being porous. 11. Reactor according to claim 10, characterized in that it further comprises a device intended to introduce an auxiliary material for absorbing radiation into the reactor chamber, coincident with at least one point in the path of the reactants. 12. Reactor according to claim 11, characterized in that the auxiliary material is a liquid. 13. Reactor according to claim 11, characterized in that the auxiliary material is a gas which absorbs radiation of the electromagnetic spectrum between 100 n and 0.01 | i. 14. Reactor according to claim 11, characterized in that the auxiliary material is finely divided carbon powder introduced by the inlet end of the reactor tube along a path which coincides with the path of the reactants. 15. Reactor according to claim 11, characterized in that the auxiliary material is a solid element, preferably carbon, placed in the reactor chamber along at least part of the path of the reactants. 16. Reactor according to claim 10, characterized in that it further comprises a device placed in the reaction chamber in coincidence with at least part of the path of the reactants and intended to raise the temperature of the latter to a necessary value upon initiation of the desired chemical reaction. 17. Reactor according to claim 16, characterized in that the device intended to raise the temperature of the reactants comprises an electrically heated element. 18. Reactor according to claim 16, characterized in that the device intended to raise the temperature of the reactants comprises an electric arc. 19. Reactor according to claim 16, characterized in that the device intended to raise the temperature of the reactants comprises a flame. 20. Reactor according to claim 10, characterized in that it further comprises a device for cooling the reaction products, placed near the outlet end of the reactor tube. 21. Reactor according to claim 10, characterized in that the reactor tube is made of glass, quartz, hot sintered alumina or hot sintered yttrium oxide. 22. Reactor according to claim 10, characterized in that the reactor tube is made of organic polymer. 23. Reactor according to claim 10, characterized in that the reactor tube is made of a mineral salt. s 10 15 20 25 30 35 40 45 50 55 60 65 3 622,964 24. Reactor according to claim 10, characterized in that the fluid is a gas. 25. Reactor according to claim 10, characterized in that the fluid is liquid or gaseous water, heavy water, nitrogen, air or oxygen. 26. Reactor according to claim 10, characterized in that the source of the radiation is a plasma arc, a heated filament or a seeded flame. 27. Application of the method according to claim 1 to the dissociation of hydrocarbon materials into hydrogen and carbon black. 28. Application according to claim 27, characterized in that the hydrocarbon materials are hydrocarbons.