EP0001946A1 - Reactive gases mixing apparatus for oxygen-reforming reactors - Google Patents

Abstract

An apparatus intended for mixing very rapidly reactive gases entering in a reforming oxygen reactor, in which a highly exothermic reaction takes place between an oxidizing gas having a high oxygen content, and a process gas containing hydrocarbons and eventually water-vapor, hydrogen and carbon oxides. This apparatus is mainly composed, on the one hand, of a refractory casing (4), into which the process gas is injected tangentially so as to impart the gas a helicoid motion, and on the other hand, of a plurality of parallel channels (7, 10), into which the oxidizing gas is injected at one end to go out at the other end through an orifice of which at least one of the sizes is smaller than 20 mm. The apparatus, according to the present invention, is particularly suitable for reforming oxygen reactors, for producing synthesis gas, operating in more severe conditions than usual.

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 internally with refractory, packed with a catalyst bed. 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 parameters of the process, 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 to the reactor, said device mixing the gases at too slow a speed relative to the speed of action in the gas phase.

• (a) Dann la préparation du gaz de synthèse d'ammoniac par réformage primaire à la vapeur suivi d'un réfor- .mage secondaire à l'air, la température d'entrée des gan reactionnels est suffisamment élevée pour que la réaction commence instantanément; neanmoins, la pression partielle de l'oxygène dans le mélange desdits gas 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, airsi que le revêtement réfractaire dans la zone de mélange, sont conçus pour résister a des températures sensiblement nlus élevées que celle du lit cataly- ticue.
• (b) Dans la précaration des gaz de synthèse par réformage direct à l'oxygène des hydrocarbures, on limite en général la température de préchauffe des deun gaz réactionnels à un niveau inférieur à 450 °C environ, 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 réactions 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 ire supérieures à 500 °C, et des proportions de vapeur plu faibles.

In the oxygen reforming reactors present widely 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 the ammonia synthesis gas by primary steam reforming followed by secondary reforming in air, the inlet temperature of the reaction gans 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 coming from the primary reforming does not contain any 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 for mixing the reaction gases, except that the upper layer of catalyst, airsi that the refractory lining in the mixing zone, are designed to withstand substantially higher temperatures than that of the catalytic bed.
• (b) In the preparation of synthesis gases by direct oxygen reforming of hydrocarbons, the preheating temperature of the reaction gases is generally limited to a level below approximately 450 ° C., and a large amount is also used. amount of water vapor in process gas, and sometimes even in oxygen. Under these conditions, the reactions hardly start in the gas phase, before coming into 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 had to start appreciably in the gas phase, by the play of the higher preheating temperatures, that is to say higher than 500 ° C., and higher steam proportions. weak.

In the process derives from the patent application French 77-08459, the operating conditions for secondary oxygen reforming are precisely more severe than those currently practiced, essentially because the partial pressure of oxygen in the reaction mixture is much higher, i.e. 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 can contain hydrocarbons heavier than methane, while having temperatures of preheating comparable or superior to those practiced for the secondary reactor in the gas industry; ammonia synthesis. Thus, for the conditions of the aforementioned patent application, it is necessary to use a new design of gas mixing apparatus.

Similarly, if one wishes to carry out a direct reforming of a hydrocarbon raw material with oxygen, 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, a new design of mixing apparatus would be required for the reasons explained above.

The object of the present invention is precisely to respond to this need, by designing a gas mixing apparatus allowing a quasi-homogeneous mixture to be obtained very quickly before the oxygen has time to react. significantly.

In the field of synthesis gas production by partial oxidation of hydrocarbons, the reaction of oxygen on the raw material is carried out without. using catalyst, and consequently, the formation of a small amount of carbon black particles 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 soue is major is to protect the mixing device from excessive claleur of the reactor, by an internal circulation of water for example.

Dan the ox domain; Partial relationship mentioned above, alusiears mixing devices have been designed, as described in US Patents 2,582,938 - 2,772,149 - 2,521, 117 - 2,838,105. and Great Britain patents 26, Co- 780,120 and 832,385. In these mixing devices, oxygen is generally injected through a single channel, which must have a sufficient section 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 surface for the diaphragm, which makes it expensive and impractical.

In the case of the Great Britain patent 726,206 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, 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.

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 black formation must be completely eliminated.

The basic 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 impress 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 large flow rate of oxidizing gas used, this implies that it is injected into the process gas via a multitude of parallel channels, each terminating 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 oxygen in the process gas, the latter is driven by a violent helical 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 reactor _'éformage to oxygen, and the need to place it appa mixer near the catalyst, this one and that one 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 wrapped in its preferred form: the metal and cylindrical wall 1 extends downwards through a frustoconical section 3 for the connection with the reforming reactor carrying the catalyst, and upwards through a spherical bottom 2 through which the pipe 5 passes, from which the oxidizing gas arrives. The process gas inlet pipes have not been shown in FIG. 1 to facilitate reading; nevertheless their axes are at level n-A ', and they are arranged so as to obtain a tangential flow of 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 here a cone shape, but which may have other fe mes such as: torus, sphere or saucer. The central distributor 6 supplies all the parallel pipes qi> 1 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, ie 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 represented in FIG. 1, to facilitate reading, but FIG. 2 represents a section along AA-A 'with all the channels represented; 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 high, we are ob 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 crescent-shaped, double-walled 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 together the two walls of the crescent, which gives it the same shape as the channel. himself. Thus, each film of oxidizing gas, when 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 by 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 'u high speed of reaction 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 (12)

1. An apparatus intended for very rapid mixing of an oxidizing gas, containing a high content of free oxygen, and a process gas containing hydrocarbons, and optionally water vapor, hydrogen and carbon oxides, and essentially composed: a) a refractory envelope into which the process gas is tangentially injected, in order to impart a helical movement to it inside said envelope, b) a distributor supplying the oxidizing gas in a multitude of parallel channels, all housed inside said envelope, while the oxidizing gas exits at the other end of each channel through an orifice, one of which at the less of the dimensions is less than 20 mm. 2. A device according to claim 1, in which the oxidizing gas outlet orifice has the form of a slot. 3. An apparatus according to claim 2 wherein the slot has a width less than 8 mm. 4. An apparatus according to claim ₂ wherein the slot has a width less than 3 mm. 5. An apparatus according to any one of claims 2 to 4, wherein each channel consists of a tube having a diameter less than 80 mm, and has, at the end where the oxidizing gas comes out, a double fan wall whose end is provided with a slot. 6. An apparatus according to any one of claims 2 to 4, wherein, at least for part of the channels, the section of each channel in its main part is crescent-shaped, oriented in the same direction as that of movement helical of the process gas, and the end of said channels, at the point where the oxidizing gas exits, is provided with a slot having the same crescent shape. 7. An apparatus according to claim 1 wherein the oxidizing gas outlet orifice has a circular or elliptical shape. 8. An apparatus according to claim 7 in the. which the diameter or minor axis is less than 8 mm. 9. An apparatus according to claim 1 in which the oxidizing gas outlet orifice is in the form of a cross or a star, in which the width of the slot forming any one of the branches is less than 20 mm. 10. An apparatus according to claim 9 wherein the width of the slot forming any one of the branches is less than 8 mm. 11. An apparatus according to claim 9 in which the width of the slot forming any one of the branches is less than 3 mm. 12. An apparatus according to any one of claims 1 to 11, wherein certain channels have a helical shape oriented in the same direction as that of the helical movement of the process gas.

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