FR2605305A1 - PROCESS FOR PRODUCING AMMONIA - Google Patents

Abstract

THE INVENTION RELATES TO A PROCESS FOR PRODUCING AMMONIA FROM AN AZOTHYDRIC MIXTURE. THE PURPOSE OF THE INVENTION IS TO INCREASE THE RATE OF CONVERSION OF THE INITIAL AZOTHYDRIC MIXTURE INTO AMMONIA. THIS PURPOSE IS ACHIEVED BY USING A PROCESS FOR THE PRODUCTION OF AMMONIA FROM AN AZOTHYDRIC MIXTURE, CHARACTERIZED IN THAT THIS PROCESS IS CARRIED OUT IN A CATALYST LAYER, SEPARATED IN AT LEAST TWO PARTS FOLLOWING A PERPENDICULAR SECTION WITHIN THE MEANING OF ADMISSION OF THE INITIAL AZOTHYDRIC MIXTURE, AS THE MIXTURE OF THE AZOTHYDRIC MIXTURE BETWEEN THESE PARTS IS PERFORMED UNTIL A TEMPERATURE GRADIENT AT THE MOST EQUAL TO 20 C BETWEEN THE FIRST PART AND THE FOLLOWING PART OF THE CATALYST, AND IN THAT THE PROCESS IS CARRIED OUT AT A PRESSURE OF 10 TO 100 MP AND AT A TEMPERATURE OF 20 TO 250 C. THIS PROCESS APPLIES MORE PARTICULARLY TO A MANUFACTURE FROM AN AZOTHYDRIC MIXTURE OBTAINED DURING THE TREATMENT OF GASES NATURAL OR ASSOCIATED IN THE NITROGEN INDUSTRY.

Description

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PROCESS FOR PRODUCING AMMONIA

  The claimed invention relates to a process for the production of ammonia from an azotic D5 mixture which is obtained during the treatment of gases

  natural or associated in the nitrogen industry.

  Ammonia finds application in the production of nitric acid, nitrogenous salts, urea, sodium carbonate, hydrocyanic acid, fertilizers, and diazotypic layer-pulling materials. The best ammonia production processes are carried out at a high pressure of 10 to 35 MP and at a temperature of 600 ° C from nitrogen and from D5 hydrogen by the following reaction:

N2 + 3 H2 _ _2 NH3 + Q

  Since the synthesis reaction of 2C ammonia is exothermic and reversible, the maximum ammonia yield is achieved in a theoretically optimal regime characterized by a constant decrease in temperature and an increase in the concentration of ammonia (see " Spravochnik azochika ", edited by E.Ya.Melnikov, 1986, p510, LD Kuznetsov et al.," Sintez ammiaka ", 1982," Khimia "(Moscow), p.296). As a result, modern ammonia synthesis technology is geared towards creating temperature regimes approaching the theoretically optimal 3-0 regime. In addition, during the production of ammonia with iron catalysts, the temperature must not exceed 550 ° C., since above this temperature the catalyst rapidly loses its activity. To increase the conversion rate of the initial azothydric mixture, into ammonia, the process is carried out, by carrying out a repeated recycling of the mixture

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  Hydrogen chloride which is in the course of reaction by reintroducing it several times to the inlet of the synthesis reactor, separating the ammonia formed from the mixture and directing a part of the recirculable mixture to the blowing. The recycling rate and the pressure at which the process is carried out are then determined by

  according to the economic indices sought.

  In the industry the application of a process for the production of ammonia has been found (see LD Kuznetsov 1 et al., "Sintez ammiaka", 1982, Khimia (Moscow), p.296) in which the temperature and the composition of the reaction gas does not change substantially over time. This process comprises preheating the initial azotic mixture before feeding it into the catalyst layer, then an intermediate cooling of the reaction gas between the catalyst layers, or a cooling directly in the catalyst layer through the use of tubes. heat dissipation. In the case of the intermediate cooling of the arothydric mixture, the catalyst is placed in the form of 3 to 4 layers, between which cooling is carried out on behalf of stirring of the azotic mixture with the initial gas, or using the heat released for the heating. water or to obtain steam. However, this process is characterized by the fact that the temperature regime obtained is much lower than the theoretically optimal regime, which results in conversion rates of the initial azotic mixture to the desired product,

  insufficiently high (about 16% by volume).

  In addition, in order to carry out the indicated process, it is necessary to use bulky reactors,

complicated construction.

  A method for producing ammonia produced in a catalyst layer is known. The catalyst is heated beforehand to a temperature corresponding to the reaction temperature,

  a temperature ranging from 120 to 590 C.

  Then, the initial hydrazide mixture is heated to a temperature of 20 to 150 ° C. on the catalyst. After 5 to 30 minutes, the direction of admission of the azotic mixture into the catalyst layer (US, A, 865796) is reversed. By periodically reversing the direction of admission of the azotic mixture into the catalyst layer, respecting equal passage times for each direction, that is to say by alternately switching the inlet and the outlet of the azotic mixture, the process Ammonia synthesis is carried out continuously, in a non-stationary regime, with a variable temperature profile on the layer. This temperature profile is characterized by three zones: the zone of heating of the azotic mixture, the zone of high temperatures in the center of the catalyst layer and the zone of fall of the temperature being located near the place where the mixture 2 Cazothydric comes out of the diaper. The temperature drop zone is formed by the thermal inertia of the catalyst, as a result of the heat exchange with the cold gas mixture. This temperature profile in principle makes it possible to obtain quite high concentrations

  ammonia at the outlet of the catalyst layer.

  However the indicated process of production of ammonia

  in practice encounters certain difficulties.

  During the implementation of this process, only a low ammonia concentration equal to 15.8% by volume of the azohydric mixture at the outlet of the catalyst layer is obtained, due to an unequal density of the catalyst arranged in bulk. The heterogeneous structure of the bulk catalyst layer promotes the formation, in accordance with the layer section, perpendicular to the direction of movement of the reaction mixture, both of overheating zones and of cooled zones, which leads to overheating of the catalyst. and the loss of his activity. In other words, the ammonia concentration of the azotic mixture at the outlet of the catalyst layer is decreased. It was therefore proposed to develop a production process for ammonia ensuring the obtaining of high concentration amoniac, by maintaining level

  constant temperature regime of the process.

  The solution consists in carrying out the process for producing ammonia from the azotic mixture, by introducing said mixture into the catalyst layer having a temperature of 420 to 590 C, periodically reversing the direction of admission of said mixture, and evacuating subsequently the catalyst layer, ammonia formed and unreacted azothydric mixture. According to the claimed invention, the production process is carried out in a catalyst layer, separated into at least two parts in a section perpendicular to the direction of admission of the initial azothydric mixture, the stirring of the azotic mixture is carried out between these parts. until a temperature gradient of this mixture at most equal to 20 C is obtained between the first part and the next part of catalyst, the process is then carried out at a pressure of 10 to 100 MP, and at a temperature of inlet temperature of the

  azotic mixture from 20 to 250 C.

  With this process, it is possible by using an initial hydropathic mixture comprising: about 22% of nitrogen, about 66% of hydrogen, inert 3-0 gases such as for example argon, methane (about%) and ammonia (about 2%), to obtain the desired product at a concentration of 20% by volume, while reducing the energy consumption for carrying out the ammonia synthesis and simplifying general technology compared to

  known methods of producing ammonia.

  According to the claimed invention and in order to increase the yield of ammonia, as well as to have the possibility of stirring the azotic mixture in the high temperature zone, during the passage of the gas mixture through the catalyst layer, it is rational to use the separated catalyst in at least two parts in a section perpendicular to the direction of admission of the initial mixture, these two parts having a ratio of volumes of 0.5

at 1: 1.

  According to the claimed invention, it is advantageous to use the catalyst in the form of grains

with a dimension of 15 to 50 mm.

  Other purposes and advantages of the claimed invention will be better understood by reading the

  detailed description of the ammonia production process

  and examples of implementation of this method.

  the ammonia production process according to the invention is carried out using as starting material a gaseous mixture having the following composition: nitrogen (approximately 22%), hydrogen (approximately 66%), inert gases

  (argon, methane) (about 10%), ammonia (about 2%).

  The synthesis of the desired product is carried out by the following reaction:

N2 + 3H2 2 NH3 + Q

  which takes place in the catalyst layer. Many elements of the Periodic Table

  elements have a catalytic activity vis-à-

vis-à-vis the synthesis of ammonia.

  However, according to the invention, use will be made of iron-promoting catalysts which have a high catalytic activity. Preferably, the iron promoter catalyst contains 90 to 93% magnetite with an excess of FeO over stoichiometric proportions and 7 to 10% of additives.

  promoters (A1203, CaO, K20, SiO2 and other oxides).

  The catalysts used have the appearance of a granulated or milled material. The size of the fractions is 1 to 3 mm, 3 to 5 mm, 5 to 7 mm, 7 to 10 mm, 10 to 15 mm. The claimed process was carried out using a catalyst which could have both the above-mentioned dimensions of the fractions, and the grain size of 15 to 50 mm. By using the catalyst according to the fractions indicated, the intensity of the heat exchange between the initial filterable azo-hydride mixture and the catalyst layer decreases, which leads to the increase in the duration of the contact required for heating the azotic mixture. cold to high temperatures. During this time, some of the starting reagents are able to react in order to synthesize the ammonia, the partially reacted azothydric mixture arrives in the high temperature zone and the reaction proceeds with a lower heat release which results in the decrease of the maximum temperature. To prevent the diffusion inhibition of the ammonia formation reaction it is possible to execute holes opening into the interior

large grains of catalyst.

  By using a coarse-grained catalyst, the maximum temperature is lowered and the temperature profiles are reduced, as a result the temperature of the process approaches the optimum speed and the hydraulic resistance of the equipment used decreases appreciably. The use of catalyst grains larger than 50 mm lowers the intensity of the heat and mass exchange between the filterable azo-hydride mixture and the catalysts so much as to maintain a high ammonia concentration of the azotic mixture. at the outlet of the catalyst layer, it is necessary to substantially increase the amount of catalyst, that is to say to lower the flow rate

initial gas.

  According to the invention, the ammonia synthesis process is carried out in the separated catalyst layer in at least two parts, so that the ratio of the volumes of the two parts is from 0.5 to 1: 1. synthesizes in the catalyst layer

  previously heated to a temperature of 420 to 590 C.

  After having reached the indicated temperature, the initial reaction gas of the composition mentioned above is brought into the catalyst layer at a pressure of 10 to 10 MPa and at a temperature of 20 to 250 C. The direction of admission of the gas The reaction is perpendicular to the section where the catalyst layer is separated. In accordance with the claimed process, the direction of admission of the azotic mixture into the catalyst layer is periodically reversed. Periodicity

  such a change is 1 to 30 minutes.

  In the gap between the portions of the catalyst layer formed during the separation, the initial azotic mixture is stirred until a temperature gradient of at most

   C at the entry into the next part of catalyst.

  It is rational to stir the azotic mixture in the high temperature zone, this zone occupying about 1/3 of the total volume 3V0 of the catalyst layer and lying in its central part. If the azotic stirring is carried out outside the high temperature zone, its effectiveness is reduced. As it has been proved, it is necessary to achieve efficient stirring of the azotic gas that the ratio of the volumes of the parts of the

catalyst are 0.5 to 1: 1.

  The equalization of the temperature differences of the azotic mixture by stirring is related to the pressure loss D5. During stirring of the azotic mixture until a temperature gradient of not more than 20C is reached at the inlet of the catalyst layer portion following the first part, the pressure loss of the gas and the expenditure for brewing have increased significantly. When the temperature gradient is greater than 20 ° C, there are zones of overheating and cooled sections in the catalyst layer, which reduces by more than 10% the ammonia concentration of the azotic mixture outgoing

of the catalyst layer.

  The azohydric mixture is brought to a temperature of 20 to 250 C at a pressure of 10 to 100 MP in the first part of the catalyst layer depending on the movement of the gas. If the 2C azothydriqum mixture is heated to a temperature below 20 ° C., the increase in the concentration of ammonia at the exit of the layer is not achieved, and if it is heated to a temperature above 250 ° C. This results in a decrease in the ammonia concentration of the azotic mixture at the outlet of the catalyst layer as a result of the

  formation in the catalyst layer of a non-conventional

  optimal o the temperature decreases and also a significant decrease in the duration between the changes of direction of admission of the initial azotic mixture. If the synthesis of ammonia is carried out at pressures below 10 MP or above 100 MP, its efficiency is considerably reduced, since the ammonia concentration of the azotic mixture at the outlet of the catalyst layer is substantially reduced at low pressure, and conversely the use of high pressures requires the use of high-priced equipment. The azotic mixture from the first catalyst layer portion at an elevated temperature differs in the section perpendicular to the initial gas flow of 150-200 ° C. To equalize the temperature of the azotic mixture according to the indicated section, it is stirred until a temperature gradient of at most 20 C at the entrance to the part is reached.

  following the first part of the catalyst layer.

  Since the characteristic time of establishing the temperature imbalance of the azotic mixture is longer than the time between the changes in direction of admission of the initial azotic mixture due to the structural heterogeneity of the catalyst layer, the stirring of the azotic mixture leads to the equalization of the temperature in any section of the catalyst layer perpendicular to the initial gas flow and then makes it possible to obtain an optimal regime of the ammonia production process

generally.

  The change of the direction of admission of the azotic mixture in the catalyst layer is carried out

  with a change period equal to 1 to 30 minutes.

  A decrease in the maximum allowable period of the change in direction of admission of the initial gas results in a decrease in the ammonia concentration of the azotic mixture at the outlet of the catalyst layer. If this period increases, the heat is removed by blowing the catalyst layer with the admitted starting mixture and the forming reaction.

ammonia ceases.

  As indicated above in detail, filtration of the initial azotic mixture through the catalyst layer results in the formation of ammonia at a concentration of about 20% by volume in the azotic mixture. The ammonia formed and the unreacted mixture are removed from the

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  catalyst and directed to the separation achievable by known methods and to purification. In order to better understand this invention, the following will be

  examples of its concrete execution.

Example 1

  A nitrogen-containing nitrogen mixture is introduced at a rate of 22% by volume, hydrogen at a rate of 66% by volume, inert gases at a rate of 10% by volume and ammonia at a rate of 2%. in volume, at the temperature of 20 C, at a pressure of 32 MP, at a flow rate of the mixture of 12000 h -1 in a catalyst layer whose grains are of a size of 3 to mm, this layer being separated into two parts and heated to 500 C, the ratio of the volumes of each part of the bed being 1: 1. The azohydric mixture is stirred between the catalyst layer portions until a temperature gradient of said 20 C mixture is obtained at the inlet of the catalyst layer portion following the first portion. The inversion of the G inlet direction of the initial mixture is carried out every minute. The average yield of ammonia is 16.1%,

  the increase in ammonia was 14.1%.

Example 2

  The synthesis of ammonia is carried out in; conditions similar to those indicated in Example 1, however the temperature of the azotic

  the inlet of the catalyst layer is 100 C.

  The inversion of the direction of admission of the initial mixture is carried out every 4 minutes. The average yield of ammonia is 19.7%, the increase in ammonia

being 17.7%.

Example 3

  The synthesis is carried out under the conditions analogous to those indicated in Example 1, except that analogous to those indicated in Example 1, except that the temperature of the azohydric mixture at the entry into the catalyst layer is 250 ° C. The inversion of the direction of admission of the starting mixture is carried out every minute. The average yield of ammonia is 19%, the increase in ammonia is 17%.

Example 4

  The ammonia synthesis is carried out under the conditions analogous to those indicated in Example 1, except that the pressure of the initial azothydric mixture is 10 MP, and that the temperature of the azotic

  the entry into the catalyst layer is 100 ° C.

  The inversion of the direction of admission of the initial mixture is carried out every 5 minutes. The average yield of ammonia is 10%, the increase in ammonia is 8%.

Example 5

  The procedure is as in Example 1, except that the

  2C pressure of the initial azotic mixture is 100 MP.

  The average yield of ammonia is 32%,

  the increase in ammonia is 30%.

Example 6

  The procedure is as in Example 2, however the size of the catalyst grains is 15 mm. The average yield of ammonia is 20.2%,

  the increase in ammonia is 18.2%.

Example 7

  3-0 is carried out as in Example 2, except that the grain size of the catalyst is 50 mm. The average ammonia yield is 16%, the increase

in ammonia is 14%.

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Example 8.

  The procedure is as in Example 1, except that the catalyst layer is separated into two parts and that the ratio between the volumes of these parts is 0.5: 1. The average yield of ammonia is 15.8 %

  the increase in ammonia is 13.8%.

Example 9.

  The procedure is as in Example 1, except that the catalyst layer is separated into two parts and the ratio between the volumes of these parts is 0.6: 1. The average yield of ammonia is 15.9% and

  the increase in ammonia is 13.9%.

Example 10

  The procedure is as in Example 1 except that the catalyst layer is separated into two parts and that

  the ratio of the volume of these parts is 0.7: 1.

Example 11

  The procedure is as in Example 1, except that the catalyst layer is separated into two parts and the ratio between the volumes of these parts is 0.8: 1. The average yield of ammonia is 16%, and

  the increase in ammonia is 14%.

Z5

Example 12.

  The procedure is as in Example 1, except that the catalyst layer is separated into two parts and that the ratio between the volumes of these parts is 0.9: 1. The average yield of ammonia is 16.1%,

  the increase in ammonia is 14.1%.

Example 13

  The procedure is as in Example 1. However, the azotic mixture is stirred between the parts of the

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  catalyst until a temperature gradient of 10 C is obtained at the inlet to the part of the catalyst layer which follows the first part. The average yield of ammonia is 16.4%,

  the increase in ammonia is 14.4%.

Example 14.

  The procedure is as in Example 1. However, the initial azotic mixture contains 20% by volume C of nitrogen, 60% by volume of hydrogen, 15% by volume of inert gas, 5% by volume of ammonia. The inversion of the direction of admission of the initial mixture is carried out every 2 minutes, the average yield with ammonia is

  16.6%, the increase in ammonia is 11.6%.

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Claims (2)

  1) A process for producing ammonia from an azotic mixture by introducing said mixture into the catalyst layer having a temperature of 420 to 590 C, periodically reversing the direction of admission of said mixture, and subsequently evacuating the catalyst layer, the amoniac formed and the unreacted mixture, characterized in that the process of obtaining ammonia in a catalyst layer, separated into at least two parts in a section, is carried out. perpendicular to the direction of admission of the initial azothydric mixture, the stirring of the azothydric mixture is carried out between these parts until a temperature gradient of this mixture at most equal to 20 C is obtained between the first part and the part catalyst, the process is then carried out at a pressure of 100 MP and at an inlet temperature of the mixture azothydric acid from 20 to 250 C.   2C 2D) Method according to claim 1, characterized in that the catalyst is used separated in at least two parts in a section perpendicular to the direction of admission of the initial mixture, these two parts   having a volume ratio of 0.5 to 1: 1.   3) Method according to one of claims 1 to   2, characterized in that the catalyst under   grain shape with a size of 15 to 50 millimeters. 3-