FR2811976A1 - PROCESS AND DEVICE FOR PRODUCING A GASEOUS MIXTURE CONTAINING HYDROGEN AND CO BY STAGE OXIDATION OF A HYDROCARBON - Google Patents

Abstract

Process and installation for producing a gaseous mixture containing hydrogen and CO by oxidation of a gaseous hydrocarbon with an oxygen medium in e, according to which said gaseous hydrocarbon and said oxygen medium in e are introduced into a reactor containing a catalytic bed. Said hydrocarbon is introduced partly upstream of said catalytic bed and, for the remainder, within said catalytic bed at at least one level situated inside said catalytic bed.

Description

The invention relates to the field of the production of reducing atmospheres

  containing hydrogen and CO such as H2 / CO or H2 / CO / N2

  as obtained by oxidation of a gaseous hydrocarbon.

  Protective atmospheres consisting of a reducing mixture H2 / CO or H2 / CO / N2 are commonly used in the metallurgical industry for various metal processing operations such as: annealing, tempering, heating before quenching, decarburizing annealing , soldering or sintering. These atmospheres must be strictly controlled at the level of their water and CO2 contents in order to protect the parts against oxidation. Examples of applications are given in the patent

EP-A-482,992.

  The H2 / CO or H2 / CO / N2 mixture is advantageously obtained by partial oxidation of methane (or another gaseous hydrocarbon) by an oxygenated medium on a catalytic bed contained in a reactor, and in the stoichiometric ratio of 1 mole methane per 1/2 mole of oxygen depending on the overall reaction (when the oxygenated medium is a mixture of oxygen and nitrogen such as air): CH4 + 1/202 + xN2 -> CO + 2H2 + xN2 If the atmosphere obtained is too rich in CO and H2 for the use to be made of it, dilution by adding additional nitrogen may be practiced. This type of reactor can also be used for the production of hydrogen, after separation with CO. It is also possible to add a gas reactor to the outlet water to convert carbon monoxide to

CO2 and H2.

  The generally accepted reaction mechanism can be schematized in two global reactions taking place essentially in two successive zones of the reactor: - an exothermic zone where the exothermic reaction takes place:

CH4 + 202 -> C02 + 2H20

  - an endothermic zone where endothermic reforming reactions take place:

CH4 + H20 - CO + 3H2

CH4 + CO2 -> 2CO + 2H2

  It is also known to preheat the oxygenated medium to a temperature of the order of 500 C before entering the reactor, by heat exchange with the gases leaving the reactor before they are sent to their place of use. But this preheating alone is insufficient, and must be supplemented by an intake of calories, carried out inside the reactor. The catalyst generally used in this reactor is based on Ni, or else Pt, Rh, Pd or another noble metal, deposited on a porous silica support

or alumina.

  Since the reactors have an endothermic zone, it is necessary to equip them with an additional heating device by burners or electrical resistances and / or to carry out a strong preheating of at least the oxygenated medium. Thus it is possible to reach a temperature level such that the endothermic reforming reactions take place at a speed sufficient for the outlet temperature of the H2 / CO mixture to be high enough to obtain a small amount. residual CO2 and H20. On the other hand, the radial temperature inside the catalytic bed of the reactor must be as homogeneous as possible in order to avoid the presence of cooler zones than desired, which would inevitably lead to an increase in the

  H20 and CO2 content in the gases produced.

  Usually, the injection of the hydrocarbon into the reactor is carried out at a single point thereof, upstream of the catalytic bed, in a space which constitutes a chamber where the gaseous species mix. It may be advantageous to inject the hydrocarbon and the oxygenated medium concentrically to form a burner, provided that the upper part of the catalytic bed is protected by a refractory insulating medium.

which stops the burner flame.

  One drawback of this type of reactor is the risk that the combustion of the hydrocarbon will be incomplete and lead to the formation of soot. However, this formation of soot is very damaging for the performance of the catalytic bed, the pores of which are progressively blocked. And, in general, the formation of soot causes progressive fouling of the installation pipes, which must therefore be stopped.

and clean periodically.

  The object of the invention is to propose a process for operating a reactor for forming a CO / H2 or CO / H2 / N2 atmosphere by oxidation of a hydrocarbon on a catalytic bed in which the

  soot formation, as well as a reactor for the implementation of this process.

  To this end, the subject of the invention is a process for producing a gaseous mixture containing hydrogen and CO by oxidation of a gaseous hydrocarbon by an oxygenated medium, according to which said gaseous hydrocarbon and said oxygenated medium are introduced. in a reactor containing a catalytic bed, and characterized in that the combustion of the mixture is carried out

  hydrocarbon / oxygen in a stepped fashion inside the reactor.

  According to one of the embodiments of the invention, the combustion of the hydrocarbon / oxygen mixture is carried out in a staggered manner by the fact that said hydrocarbon is introduced partly upstream of said catalytic bed and, for the remainder, within said bed at least one level catalytic

  I5 inside said catalytic bed.

  Advantageously the stoichiometric ratio 02 / hydrocarbon

  implemented on each floor is greater than 0.5.

  The subject of the invention is also an installation for producing a gaseous mixture containing hydrogen and CO by oxidation of a gaseous hydrocarbon by an oxygenated medium, introduced into a reactor comprising a catalytic bed, characterized in that it includes means

  to inject said hydrocarbon in a stepped manner inside the reactor.

  According to one of the embodiments of the invention, said means for injecting said hydrocarbon allow its injection in part at the inlet of said reactor upstream of the catalytic bed and, for the remainder, at least one

  level located inside said catalytic bed.

  The means for injecting said hydrocarbon at the inlet of said reactor can be arranged concentrically with the means for injecting

  said oxygenated medium in said reactor.

  The installation then preferably comprises a layer of

  inert material disposed on the upper surface of the catalytic bed.

  The installation may include means for preheating said oxygenated medium before it enters the reactor, at least some of said means operating thermally independently from the remainder of

installation.

  It can also include means for preheating

  the hydrocarbon before entering the reactor.

  As will be understood, according to the invention the combustion of the hydrocarbon / oxygen mixture is carried out in a staggered manner by the fact that one introduces

  the hydrocarbon on different levels (depths) inside the reactor.

  Advantageously, only the fraction of the hydrocarbon is injected into the upper part of the reactor, upstream of the catalytic bed, the remainder being injected into the catalytic bed itself, at one or more levels (depths). inside of it. In this way, o we make sure that all of the hydrocarbon injected at a given level will be burned completely, without any significant reactions occurring

parasites that form soot.

  The invention will be better understood on reading the description which

  follows, given with reference to the appended figures: - Figure 1 which shows schematically seen in section a first example of installation for producing an H2 / CO or H2 / CO / N2 mixture according to the invention; - Figure 2 which similarly shows a second example of a

  such installation according to the invention.

  The installation shown in FIG. 1 comprises, in a known manner, a pipe I for transporting an oxygenated gaseous medium, for example an oxygen / nitrogen mixture such as compressed air and freed of its impurities (CO2, water vapor , sulfur compounds in particular). This mixture 02 / N2 is preheated inside a heat exchanger 2, then is introduced into a reactor 3 comprising a catalytic bed 4. This is usually formed by coated silica or alumina beads externally by nickel or by a precious metal such as platinum, rhodium, palladium, etc. The catalytic bed 4 is enclosed between an upper layer 5 and a lower layer 6 composed of an inert permeable material, such as silica or alumina beads. The reactor 3 comprises at its periphery heating means such as electrical resistances 7 which heat the catalytic bed 4 and the gases which pass through it so that the endothermic reforming reactions mentioned above are sufficiently advanced therein. In the upper part of the reactor 3, upstream of the upper layer 3s 5 of inert material on the gas path, there is an empty space 8 forming a mixing chamber for the different gases introduced into

reactor 3.

  The installation also includes a pipe 9 for supplying a gaseous hydrocarbon CxHy such as methane, or a mixture of such hydrocarbons. According to the invention, this pipe 9 is split into several sub-pipes 10, 11, 12, 13 in which the admission of the hydrocarbon is controlled and regulated by a set of valves not shown. The first sub-pipe 10 introduces a given proportion of the hydrocarbon

CxHy in the mixing chamber 8.

  As is known, the first sub-pipe 10 can be concentric with the pipe 1 for introducing the O 2 / N 2 mixture so as to form a burner whose flame is stopped by the upper layer 5 of inert material. This layer 5 protects the catalytic bed 4 from degradation which would be caused by direct contact with the flame or with its radiation. The other sub-conduits 11, 12, 13 each introduce a fraction of the remaining hydrocarbon, at a given level of the catalytic bed 4. The injection of the hydrocarbon into the mixing chamber 8 is therefore no longer carried out either. (o, as we said, there would be a risk of incomplete combustion of the hydrocarbon), but a "staged" injection of this

  hydrocarbon distributed over the height of the reactor 3.

  For the sake of clarity, FIG. 1 shows pipes, 11, 12, 13 offset radially, but it would be preferable for all the gaseous injections into the reactor 3 to be carried out in its vertical axis, in order to obtain a temperature also radially homogeneous as possible at a

given level of reactor 3.

  This staged injection of the hydrocarbon makes it possible to limit the risk of incomplete combustion, therefore the formation of soot. It is thus possible, for example, to carry out the injection of 10% of the total quantity of hydrocarbon at the inlet of the generator 3 by the first sub-pipe 9, and O of 30% of this quantity to each of the three other injection levels, located respectively 10, 20 and 30 cm from the upper surface of the catalytic bed 4 when the latter has a total height of 80 cm. This injection mode also has the advantage of prolonging the zone of the catalytic bed 4 where heat is produced, which is favorable to the regular establishment of endothermic reforming reactions over at least most of the height of the catalytic bed 4. It is thus possible to reduce the CO 2 and water vapor contents of the gaseous mixture produced. In total, it is possible to obtain a gas outlet temperature higher by a few tens of degrees than in the case where there is a single point of injection of the hydrocarbon at the inlet of generator 3. On the other hand, avoids the strong overheating of the generator 3, located in the vicinity of the single injection point of the hydrocarbon, which can rapidly degrade the catalytic bed 4 and the walls of the

generator 3.

  As is known, the H2 / CO / N2 mixture produced by the reactor o 3 leaves therefrom via a pipe 14 which ensures its transport to its place of use. In the example shown, this pipe 14 passes through the heat exchanger 2 so that the H2 / CO / N2 mixture contributes to the preheating of the

  mixture 02 / N2 flowing in line 1.

  The staged injection of the hydrocarbon, by distributing the exothermic reaction over a large part of the height of the catalytic bed 4, instead of concentrating it in its upper part, can even contribute to making the heating installation 7 useless if, moreover, the O 2 / N 2 mixture is preheated to a sufficiently high value of the order of 550 C or more. This preheating can be carried out using the exchanger 2 only, or also using an additional device inserted in the pipe 1 between

exchanger 2 and reactor 3.

  This additional device for heating the oxygenated medium can be constituted, as shown in FIG. 2, by a chamber 15 arranged on the pipe 1 between the exchanger 2 and the reactor 3, inside which an electrical resistance 16 is placed. This communicates to the oxygenated medium the few tens or a few hundred degrees which are necessary to dispense with a heating device integrated into the reactor 3, or at least to limit its power and / or the extent of its zone d 'action. Such an additional device has the advantage of being thermally independent of the other members of the installation, and can be adjusted flexibly without

  disrupt the operation of the rest of the installation.

  For the same purpose, it is also possible to preheat the hydrocarbon before it enters the reactor 3, provided that this preheating does not lead to a premature cracking of the hydrocarbon. This preheating can advantageously be carried out by attaching a portion of the pipe 9 to the exchanger 2 and / or to the independent preheating device 15. If a heating device integrated into the reactor 3 is not used or if no uses a very localized or not very powerful heating device, thus avoiding the establishment of strong radial thermal gradients in the catalytic bed 4. They could lead either to a rapid degradation of the catalyst at the periphery of the bed 4, or to a insufficient temperature in the central region of said bed 4. In all cases, the smoothing of the radial thermal gradients facilitates optimal exploitation of the catalytic bed 4 and allows o to obtain lower contents of C02, H2, O and unburned hydrocarbon in the gas mixture produced by the installation. In addition, the internal carcass of reactor 3 undergoes less thermal stress than when

  7 powerful heater should be used.

  It goes without saying that the invention is applicable to the use of any oxygenated medium and of any hydrocarbon capable of leading to the formation of the desired atmosphere. The oxygenated medium can be not only air, but any O 2 / N 2 mixture (for example a gaseous mixture containing 35 to 40% of oxygen and 60 to 65% of nitrogen taken initially in the liquid state at the bottom of the gas). 'a device for producing cryogenic nitrogen), or oxygen / argon, or even pure oxygen. Hydrocarbon can be not only natural gas, but

  for example propane, butane, LPG.

Claims (9)

  1. Method for producing a gaseous mixture containing hydrogen and CO by oxidation of a gaseous hydrocarbon by an oxygenated medium, according to which said gaseous hydrocarbon and said oxygenated medium are introduced into a reactor containing a catalytic bed, characterized in this the combustion of the hydrocarbon / oxygen mixture is carried out in a stepped manner at inside the reactor.   2. Method according to claim 1, characterized in that the combustion of the hydrocarbon / oxygen mixture is carried out in a staggered manner by the fact that said hydrocarbon is introduced into the reactor partly upstream of said catalytic bed and, for the remainder, within said catalytic bed in at   minus a level located inside said catalytic bed.   3. Method according to claim 1 or 2, characterized in that the stoichiometric oxygen / hydrocarbon ratio used at each stage is greater than 0.5.   4. Installation for producing a gaseous mixture containing hydrogen and CO by oxidation of a gaseous hydrocarbon by an oxygenated medium, introduced into a reactor (3) comprising a catalytic bed (4), characterized in that it includes means (10, 11, 12, 13) for injecting   said hydrocarbon staged inside the reactor.   5. Installation according to claim 4, characterized in that said means for injecting said hydrocarbon allow its injection partly at the inlet of said reactor (3) upstream of the catalytic bed (4) and, for the   remaining, at least at a level located inside said catalytic bed (4).   6. Installation according to claim 5, characterized in that the means (10) for injecting said hydrocarbon at the inlet of said reactor (3) are arranged concentric with the means for injecting said oxygenated medium into said reactor (3).   7. Installation according to one of claims 4 to 6, characterized   in that it comprises a layer (5) of inert material disposed on the   upper surface of the catalytic bed (4).   8. Installation according to one of claims 4 to 7, characterized   in that it comprises means (2, 15, 16) for preheating said oxygenated medium before it enters the reactor (3), at least some (15, 16) of said means operating in a thermally independent manner remaining from the installation.   9. Installation according to one of claims 4 to 8, characterized   in that it includes means for preheating the hydrocarbon before its entry into the reactor (3).