EP0001946B1 - Reactive gases mixing apparatus for oxygen-reforming reactors - Google Patents

Description

The present invention relates to an apparatus having for object to very quickly mix the reaction gases entering an oxygen reforming reactor intended for the production of synthesis gas.

In the oxygen reforming technique, a process gas is subjected to partial oxidation by reaction with an oxidizing gas, in a reactor coated with an internal refractory, provided with a bed of catalyst based on nickel, and operating under practically adiabatic conditions. Said process gas is composed of one or more hydrocarbons, and optionally water vapor, hydrogen, oxides of carbon and nitrogen. Said oxidizing gas contains a high content of free oxygen, and optionally nitrogen, water vapor and carbon dioxide. In industrial practice, it is advantageous to preheat the reaction gases as much as possible before injecting them into the reactor, in order to save oxygen consumption and thus improve the general yield of the reaction.

It is then found that beyond a certain preheating temperature, which varies according to the process parameters, the reaction begins instantly as soon as the two reaction gases are brought into contact, well before coming into contact with the catalyst. However, the reaction which takes place in the gas phase can lead to undesirable effects, such as excessive overheating of the corresponding zone of the reactor, or parasitic side reactions leading to the formation of carbon black particles, which are then deposited on the catalyst and reduce performance. In a large number of cases, these undesirable effects are attributable to the device for mixing the reaction gases at the inlet of the reactor, said device mixing the gases at a rate too slow relative to the rate of reaction in the gas phase.

In fact, when the reaction begins instantaneously in contact with the two reaction gases, while the mixture is still far from being homogeneous, the strongly exothermic reaction which takes place in the oxygen-rich fractions causes the temperature of the said gases to rise to a very high level. fractions, and the heat thus released is transmitted by radiation to the other fractions which are poor in oxygen and rich in hydrocarbons, thus causing a thermal cracking of these and, consequently, the formation of carbon particles in the reaction mixture.

• (a) Dans la préparation du gaz de synthèse d'ammoniac par réformage primaire à la vapeur suivi d'un réformage secondaire à l'air, telle que décrite dans les brevets USA 2 829 113,' 3 278 452, 3 264 066 et 3 388 074 par exemple, la température d'entrée des gaz réactionnels au réacteur de réformage secondaire est suffisamment élevée pour que la réaction commence instantanément; néanmoins, la pression partielle de l'oxygène dans le mélange desdits gaz réactionnels est très faible, de l'ordre de 1,0 à 1,5 bar seulement, et le gaz provenant du réformage primaire ne contient aucun autre hydrocarbure que le méthane, dont la molécule est beaucoup plus stable thermiquement que celles des autres hydrocarbures; de plus, la pression partielle du méthane dans le mélange des gaz réactionnels est très faible, de l'ordre de 1,3 à 1,8 bar; dans ces conditions, aucune précaution particulière n'est nécessaire pour procéder au mélange des gaz réactionnels, sauf que la couche supérieure de catalyseur, ainsi que le revêtement réfractaire dans la zone de mélange, sont conçus pour résister à des températures sensiblement plus élevées que celle du lit catalytique.
• (b) Dans la préparation des gaz de synthèse par réformage direct à l'oxygène des hydrocarbures, telle que décrite dans le brevet français 1 158 617 par exemple, on limite en général la température de préchauffe des deux gaz réactionnels à un niveau inférieur à 450°C environ, comme indiqué à la page 2 ligne 4 dudit brevet, et l'on utilise aussi une importante quantité de vapeur d'eau dans le gaz de procédé, et parfois même dans l'oxygène. Dans ces conditions, la réaction commence à peine dans la phase gazeuse, avant d'arriver au contact du catalyseur, et les dispositifs de mélange présentement utilisés permettent d'obtenir un mélange quasi-homogène sur le catalyseur; néanmoins, ces dispositifs ne seraient plus satisfaisants si la réaction devait commencer d'une manière appréciable dans la phase gazeuse, par le jeu des températures de préchauffe plus élevées, c'est à dire supérieures à 500°C, et des proportions de vapeur plus faibles.

In oxygen reforming reactors currently used in industry, which can be classified into two categories, undesirable effects are avoided for reasons which are specific to each category:

• (a) In the preparation of ammonia synthesis gas by primary steam reforming followed by secondary air reforming, as described in US Patents 2,829,113, 3,278,452, 3,264,066 and 3,388,074 for example, the inlet temperature of the reaction gases to the secondary reforming reactor is high enough for the reaction to start instantly; nevertheless, the partial pressure of oxygen in the mixture of said reaction gases is very low, of the order of only 1.0 to 1.5 bar, and the gas originating from the primary reforming contains no other hydrocarbon than methane, whose molecule is much more thermally stable than those of other hydrocarbons; in addition, the partial pressure of methane in the mixture of reaction gases is very low, of the order of 1.3 to 1.8 bar; under these conditions, no particular precaution is necessary to proceed with the mixing of the reaction gases, except that the upper layer of catalyst, as well as the refractory lining in the mixing zone, are designed to withstand temperatures substantially higher than that of the catalytic bed.
• (b) In the preparation of synthesis gases by direct oxygen reforming of hydrocarbons, as described in French patent 1,158,617 for example, the preheating temperature of the two reaction gases is generally limited to a level below 450 ° C approximately, as indicated on page 2 line 4 of said patent, and a large amount of water vapor is also used in the process gas, and sometimes even in oxygen. Under these conditions, the reaction hardly begins in the gas phase, before arriving in contact with the catalyst, and the mixing devices presently used make it possible to obtain a quasi-homogeneous mixture on the catalyst; nevertheless, these devices would no longer be satisfactory if the reaction were to start appreciably in the gas phase, by the play of the higher preheating temperatures, that is to say higher than 500 ° C., and more vapor proportions. weak.

In the process described in French patent application FR-A-2,372,116, the operating conditions for secondary oxygen reforming are precisely more severe than those practiced at present, essentially because the partial pressure of the oxygen in the reaction mixture is much higher, that is to say of the order of 6 to 8 bars, that the partial pressure of methane in said mixture is of the order of 10 to 15 bars, and that the process gas may contain heavier hydrocarbons than methane, while having comparable preheating temperatures or higher than those used for the secondary reactor in the ammonia synthesis gas industry. Thus, in said method, the reaction begins instantaneously on contact with the two reaction gases, and there is a need to use a new design of gas mixing apparatus to avoid the risks of formation of carbon particles.

Likewise, if it is desired to carry out a direct reforming with oxygen of a hydrocarbon raw material, with a view to producing a synthesis gas having a low molar ratio H2 / CO, implying the use of high preheating temperatures and low vapor proportions, there would also be a need for a new design of mixing apparatus for the reasons explained above.

The object of the present invention is precisely to respond to this need, by designing a gas mixing device making it possible to very quickly obtain a quasi-homogeneous mixture before the oxygen has time to react in a manner significant.

In the field of the production of synthesis gases by partial oxidation of hydrocarbons, the reaction of oxygen on the raw material is carried out without the aid of a catalyst, and consequently, the formation of a small amount of black particles. carbon is not a problem for satisfactory operation of the process. In addition, the partial oxidation reaction in these processes takes place at a much higher temperature (1300 to 1500 ° C) than that of oxygen reforming (900 to 1100 ° C), and therefore the major concern is to protect the mixing device from excessive reactor heat, for example by internal circulation of water.

In the aforementioned partial oxidation field, several mixing devices have been designed, as described in the US patents 2,582,938 - 2,772,149 - 2,621,117 - 2,838,105 and the Great Britain patents 726,206 - 780 120 and 832 385. In these mixing devices, oxygen is generally injected through a single channel, which must have a cross section sufficient to admit the entire flow; therefore, even if oxygen is injected at high speed through said section, the speed of dispersion of the oxygen molecules in the reaction mixture is slow compared to that of the reaction. In addition, the oxygen jet, at the point where it leaves its orifice, is generally surrounded by the gas at low speed of the reactor, which is not favorable to a rapid dispersion of the oxygen molecules.

In the particular case of US Pat. No. 2,772,149 the mixture of reaction gases takes place on the surface of a porous diaphragm; this has the advantage of rapidly mixing the reaction gases. However, because of the slow speed of the gas passing through the pores of the diaphragm, the reaction takes place essentially at the outlet surface of said diaphragm, which must therefore be designed to withstand high temperatures. In large capacity units, this device would require a large area for the diphragm, which makes it expensive and impractical.

In the case of Great Britain patent 726206, the mixing device is designed essentially for a liquid raw material, which is injected into the reactor in the form of a thin film receiving on both sides at high speed jets of oxygen at a certain angle. . This device is particularly suitable for dispersing the liquid into fine droplets; however, this does not allow a homogeneous gas mixture to be obtained quickly, since the oxygen jets are not surrounded by gas at high speed. If a gaseous raw material is used in this device, and especially if it contains a high hydrogen content as in French patent application FR - A - 2 372 116, the corresponding volumetric flow rate would be much higher than that of oxygen or equivalent liquid; as the oxygen jet only comes into contact with the process gas jet on one side, the speed of dispersion of the oxygen molecules would be too slow compared to that of the reaction which starts instantly on contact with the two gases.

For all the reasons developed above, the mixing devices designed for the partial oxidation processes cannot be suitable for the reforming with oxygen of a gaseous hydrocarbon raw material, under severe conditions as described above, since the risk of carbon particle formation must be completely eliminated.

One could consider using a reaction gas mixing device such as that described in US Pat. No. 3,998,934 for producing carbon black by oxygen reforming hydrocarbons, for example by injecting oxygen into the central pipes. 13-14-15 of said apparatus, and the process gas through the tangential side pipes 16. When the reaction begins immediately in contact with the two reaction gases, as a result of high preheating temperatures, the process gas molecules which come into contact with those of oxygen furthest from the axis react first to produce a mixture at high temperature, as a result of the exothermicity of the reaction; then, the oxygen molecules a little closer to the axis react with said mixture to produce a gaseous mixture at an even higher temperature, and so on to the axis of the apparatus where the temperature will be maximum. A central zone is thus formed having a temperature very much higher than that of the process gas on the periphery, which does not react at the start With oxygen, but receives by radiation the heat from said central zone and then undergoes thermal cracking leading to training of carbon particles. Thus, whatever the degree of turbulence, a large part of the oxygen molecules come into contact with gas which has already reacted and which is therefore brought to a high temperature, which creates an excessive temperature in the central zone and causes the formation of carbon particles which deposit on the catalyst and make the process unusable. The apparatus of US Pat. No. 3,998,934 cannot therefore be suitable either for mixing the two reaction gases entering an oxygen reforming reactor where the reaction begins instantly on contact with the two gases.

Typically, in processes for producing carbon black, such as that described in U.S. Patent 3,998,934, heat is transmitted by radiation from a hot gas flowing around the periphery of the cylindrical apparatus to a dispersed liquid hydrocarbon. very finely in droplets by the effect of turbulence along the axis of the device; there is therefore no need to mix the fluids to cause thermal cracking of the charge which will produce carbon black. However, in the oxygen reforming processes, it is precisely sought to do the opposite, because it is imperative to avoid the formation of carbon particles, which requires very quickly to obtain a homogeneous mixture of the reaction gases before arrive in contact with the catalyst, and even before the oxygen has time to react significantly in the gas phase in the case of processes using severe conditions, such as that described in French patent application 77-08459, In processes in which the reaction begins instantly in the gas phase as soon as the two reaction gases are brought into contact. Thus, for these reasons, it is not possible to use in said processes a "mixing apparatus of reaction gases similar to those used in the processes for producing carbon black by thermal decomposition of hydrocarbons.

The fundamental idea in the design of the apparatus which is the subject of the present invention is that the section for the passage of the oxidizing gas, at the precise place where it begins to come into contact with the process gas, must imprint on the flow of oxygen a form of fine mesh or thin layer, so that the oxygen molecules have very little way to go before being dispersed in the other gas. Given the high flow rate of oxidizing gas used, this implies that it is injected into the process gas through a multitude of parallel channels, each ending in an outlet orifice, at least one of the dimensions is very small. This outlet orifice may have the form of a slit, continuous or discontinuous, the width of which is less than 20 mm, and preferably less than 8 mm; said orifice can also have a circular or elliptical shape, the smallest of the diameters of which must be less than 20 mm, and preferably less than 8 mm. In addition, in order to increase the speed of dispersion of the oxygen in the process gas, the latter is driven by a violent heficoidal movement around said channels, this movement being preferably obtained by a tangential injection of this gas onto the interior walls of the appliance.

Given the high temperatures and pressures in the oxygen reforming reactor, and the need to place the mixing apparatus close to the catalyst, this and that are both placed in the same metal container, coated internally with a layer of bricks or refractory cement. In addition, the housing of the catalyst requires a large diameter, while the mixing apparatus requires a fairly small space, the volume of which is on the order of 3 to 9 percent of that of the catalyst.

The apparatus which is the subject of the present invention thus consists of two parts: on the one hand an envelope coated internally with refractory, and inside which the process gas is injected, and on the other hand a distributor and a multitude parallel channels in which the oxidizing gas circulates, and which are housed inside said envelope. The shape of the envelope can be either cylindrical, with a circular or elliptical section, or frustoconical. The connection with the catalyst bed is generally made by a frustoconical section.

Fig. 1 shows the envelope in its preferred form: the metallic and cylindrical wall 1 is extended downwards by a frustoconical section 3 for the connection with the reforming reactor carrying the catalyst, and upwards by a spherical bottom 2 through which pass it. pipe 5 from which the oxidizing gas arrives. The process gas inlet pipes have not been shown in FIG. 1 for easier reading; nevertheless their axes are at level A-A ', and they are arranged so as to obtain a tangential flow of the gas along the internal walls of the refractory, as indicated in FIGS. 2 and Fig. 3 which represent sections according to A-A '. As the gas then flows downward, it follows a helical movement within the envelope, around and between the multiple parallel channels of the oxidizing gas.

The oxidizing gas enters through the pipe 5, which leads to a distributor 6 having a cone-shaped ice, but which may have other forms of revolution around the axis of the envelope such as: torus, sphere or saucer. The central distributor 6 supplies all the pipes parallel to the axis of the envelope and which are welded, and arranged regularly over the entire section; the diameter of the distributor 6 is substantially less than the internal diameter of the refractory lining 4, in order to leave space for good circulation of the process gas. Of course, the distributor and channel assembly is made of a stainless and refractory alloy, and is held in position by all appropriate mechanical means.

According to a first representation of the present invention, indicated in FIG. 1, each channel is composed of a pipe 7 of small diameter, that is less than 80 mm, the end of which ends in a fan 8 with double wall: the oxidizing gas exits through the continuous or discontinuous slot formed at the edge of the 'fan by the two walls, and whose width is less than 20 mm, and preferably less than 8 mm. The plane of each fan is oriented parallel to the wall closest to the envelope, or makes a slight angle with it, for example from 10 to 30 °, in the direction facilitating the flow of the process gas. Only three channels have been shown in FIG. 1, to facilitate reading, but FIG. 2 shows a section on A-A 'with all of the channels shown; in this last figure are shown the inlet pipes 9 for the process gas, tangentially to the walls.

According to a variant of this first representation, the end of the channels or tubes is in the form of a star or cross with double walls, which is obtained by crushing on four or more sides the walls of the tube with pliers. Two or more crossed slits are thus obtained, the width of each being less than 20 mm, and preferably less than 8 mm.

The representation described above is particularly suitable for small or medium capacities, for which a limited number of parallel tubes can be used, and each having a fairly small diameter. When the flow of oxidizing gas becomes very large, it is necessary either to increase the number of tubes, or to increase their diameter, or both simultaneously; all these measures have the effect of counteracting the helical movement of the process gas and of converting it very quickly into a movement parallel to the tubes. This drawback can be overcome, at least partially, by giving certain tubes, especially those located on the periphery, a helical shape oriented in the same direction as the process gas.

Fig. 3 shows, in section along A-A 'of FIG. 1, a second representation of the present invention, designed to avoid the drawback mentioned above, and therefore perfectly suited to very large capacities. Each oxidant gas channel 10 has a double-walled crescent-shaped section, oriented so as to facilitate the circulation of process gas between the channels. In addition, the slot through which the oxidizing gas exits has a width of less than 20 mm, and preferably less than 8 mm, and is obtained by bringing the two walls of the crescent closer together, which gives it the same shape as the channel. himself. Thus, each film of oxidizing gas, as it leaves the slot, is sandwiched between two layers of process gas, which disperses it very quickly therein.

As in the previous representation, it is also possible in this case to give, at least to certain channels, a helical shape oriented in the same direction as that of the process gas, in order to facilitate the dispersion of the oxygen therein, while reducing the impact of too hot gas on the reactor catalyst.

From the description developed above, it can be seen that the smallest dimension of the outlet opening of the oxygen channels, or the width of the slot therein, will essentially depend on the degree of severity of the oxygen reforming reaction, i.e. the rate of reaction of oxygen with the process gas in the gas phase, and the risk of carbon black formation or excessive temperatures, which would result from high reaction speed in a very heterogeneous gas mixture. Consequently, the width of said slit, or the smallest dimension of said orifice, will be all the more reduced the greater the reaction speed, or the greater the risk of black formation. In the most severe cases, this slot width or this smallest dimension must be less than 3 mm.

While the particular representations of the present invention have been described above, it is understood that the present invention is not confined thereto, and, therefore, the object of the following claims is to cover the whole scope, and translate the whole spirit of the present invention.

Claims (7)

1. An apparatus for mixing very rapidly, without formation of carbon particules, the reacting gases entering oxygen reforming reactors containing a catalyst and where a highly exothermic reaction occurs instantaneously, before reaching said catalyst, between a free oxygen rich gas and a process gas containing hydrocarbons, and possibly steam, hydrogen and carbon oxides, which comprises essentially a metallic shell (1) containing a distributor (6) and a multitude of parallel channels (7), and which is characterized by

- said metallic shell (1) having a cylindrical or truncated conical shape, being intemally lined with refractory (4), and being provided with one or several conduits (9) through which said process gas is injected tangentially into said shell, thereby subjecting it to a helical flow therein,

- said distributor (6) having a shape of revolution around the axis of said metallic shell (1), and receiving said free oxygen rich gas through a conduit (5) crossing said metallic shell and its refractory lining (4),

- said channels (7) being parallel to the axis of said metallic shell (1), and arranged so as to allow said process gas to flow around and between said channels before coming into contact with said free oxygen rich gas,

- each one of said parallel channels (7) being welded at one end to distributor (6) and open at the other end, so that free oxygen rich gas is injected in each channel at one and issues at the other end, where it comes into contact with said process gas,

- the orifice through which said free oxygen rich gas issues from each parallel channel (7) having at least one very reduced dimension, thus allowing to inject each jet of said free oxygen rich gas into said process gas in the form of a thin film or a thin thread.

2. An apparatus according to claim 1, wherein the outlet orifice of said free oxygen rich gas has the shape of a slot.

3. An apparatus according to claim 1 or 2, wherein each channel (7) is made of a circular tube having, at the end where said oxygen rich gas issues, a double walled fish tail (8) ending by a slot.

4. An apparatus according to claim 1 or 2, wherein, at least for some of the channels, each channel has the shape of a crescent (10), oriented in the direction which facilitates the helical flow of said process gas, and has, at the end where said oxygen rich gas issues, a slot having the same crescent shape (10).

5. An apparatus according to claim 1 wherein the outlet orifice of the oxygen rich gas has the form of a cross or a star, of which each branch is a slot.

6. An apparatus according to claim 1 wherein the outlet orifice of the oxygen rich gas has a circular or elliptical shape.

7. An apparatus according to any one of claims 1 to 6, wherein some channels (7) have a helical form oriented in the same direction as that of the helical flow of the process gas.

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