DK156638B - PROCEDURE FOR THE PREPARATION OF AN AMMONIA SYNTHESIC GAS - Google Patents

Description

DK 156638 B

The present invention relates to a process for producing an ammonia synthesis gas containing hydrogen and nitrogen, containing hydrocarbon-air reaction in an amount sufficient to substantially reduce the amount of hydrocarbons and to provide a stoichiometric excess nitrogen in addition to what is required for ammonia synthesis, treatment of the obtained raw gas stream to remove carbon oxides, drying of the raw gas stream and cooling of the gas stream so that residual hydrocarbons and nitrogen are condensed therefrom to produce synthesis gas stream for ammonia production.

Commercial production of hydrogen is frequently carried out by a series of successive process steps, which generally comprise: (i) the preparation of a gas containing carbon oxides and hydrogen as major constituents of reaction 20 of the hydrocarbon starting material. (ii) oxidation of the carbon monoxide with steam to form carbon dioxide and hydrogen (so-called "shift con version"), (iii) removal of the carbon dioxide, leaving a substantially pure stream of hydrogen, (iv) a final purification to suitably remove any remaining impurities.

Two main variants of this sequence of machining steps currently in use are: 35 2

DK 156638 B

A. Catalytic Reform with Water Vapor

This process is currently limited due to the availability of suitable catalysts for use in natural gas, naphtha and similar light starting materials. The catalysts are sensitive to sulfur and, as a result, the hydrocarbon is thoroughly freed of sulfur prior to contact with the catalyst. The desulfurized starting material is mixed with 2 to 4 moles of steam / carbon atom 10 and then passed over the catalyst as it leaves at high temperature in the form of a mixture mainly containing hydrogen, carbon oxides, residue and unreacted steam. The heat needed to raise the temperature of the reaction components to the initial temperature and to provide the endothermic reaction heat is supplied by enclosing the catalyst in tubes which are heated externally in an oven.

Alternatively, the steam vapor reforming process may be carried out fully or partially autothermally by the addition of air and / or oxygen to allow combustion within the catalytic reactor. Particularly in the production of ammonia-synthesis gas from natural gas, the indirect heat supply to the reaction components of the primary 25 ... converter is supplemented by internal combustion of air in the secondary converter, which ia. supplies the nitrogen needed for ammonia synthesis. In another steam reforming process, all the high temperature heating required is provided by autothermal combustion of oxygen or oxygen of oxygen-enriched air in the catalyst zone, and no indirectly heated converter is present at all.

The gas from the reforming process is subjected to so-called "shift conversion" of CO, removal of CO2 and final purification, such as so-called methanation, depending on the requirements for individual applications.

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3 B. Partial Oxidation

The partial oxidation reactions are based on the combustion of the added hydrocarbons with a limited supply of oxygen or air. Examples include some processes, such as the so-called Texaco and Shell processes, which are capable of receiving all types of hydrocarbons ranging from natural gas to coal, and others such as the so-called Koppers-Totzek and Lurgi processes that are specific to coal.

Since no catalyst is used in these processes, the sulfur content of the added hydrocarbons is not critical.

The gases produced from the partial oxidation processes contain hydrogen, carbon oxides, residual methane and water vapor in various proportions, the sulfur compound, mainly hydrogen sulfide, to the extent that the sulfur is present in the starting material. , as well as other impurities in trace amounts. The gases produced, especially in the case of Lurgi and other processes in which the coal is kept in the gasification apparatus at relatively low temperatures, may contain significant amounts of organic matter of greater molecular weight such as benzene and tar components.

The desirability of releasing the resulting gas stream for 30 impurities in trace amounts in connection with the difficulty of working with a so-called "low" carbon monoxide conversion catalyst (about 200-250 ° C) with sulfur-laden gases has frequently led to the choice of a sink. with nitrogen for the final purification of the gas after the "shiff" treatment and the removal of carbon dioxide and of hydrogen sulfide corresponding to the use of 4

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of the so-called methanation by reforestation with steam.

It will be understood that by using the partial oxidation processes described above and by the autothermal steam reforming processes, the use of air as the internal oxidant will be limited by the extent to which the nitrogen thus produced is acceptable in the gas stream produced. .

Thus, in the usual natural gas-based ammonia process, the amount of air that can be supplied to the secondary converter is limited to the nitrogen supply needed in the ammonia synthesis stage. Also, in the processes involving partial oxidation and autothermal reforming, it is usually necessary to rely on at least partial supply of oxidizing agent in the form of substantially pure oxygen, except when the process is used solely for the preparation of a low-quality fuel gas. The need for a supply of substantially pure oxygen means that a system for fractionating air must be provided. The resulting additional capital and operating costs result in such processes appearing less attractive as high hydrogen content gas generators, except when the hydrocarbon starting material is very cheap or when full flexibility is desired with respect to the source of the starting material used.

30 An exception to this limitation is found in the so-called Braun "Purifier" process for the manufacture of ammonia disclosed in OSA Patent No. 3,442,613.

In the process described, a synthesis gas stream is obtained by primary reforming of methane and other hydrocarbons with water vapor followed by a secondary reforming in which air is present in a sufficiently large volume.

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amount to provide a stoichiometric excess of nitrogen of 2 mole% to 150 mole% based on the synthesis gas needs. The excess nitrogen is condensed in one step after the reforming unit.

5

The present invention aims to provide a process for producing a gas stream suitable for the ammonia synthesis.

The present invention provides a method of the kind described in the preamble of claim 1, and this method is peculiar to the characterizing part of claim 1.

The invention is based on the fact that mixtures of hydrogen and nitrogen are readily separated due to the great difference in their properties, and that the nitrogen content of such mixtures can be controlled with great accuracy. The simplest method of separating the 20 gases is cryogenic treatment, although other methods of separation are based on the difference in the molecular size of the gases, e.g. differential adsorption or diffusion methods can also be used. The invention allows any gas containing predominantly hydrogen and nitrogen to be used, and the source of the gas may therefore be derived from partial oxidation of oil, coal or gas in air.

In a preferred embodiment of the invention, the desired amount of nitrogen is separated out in a cryogenic separator. The separator used can be used so-called Joule Thomson cooling and regenerating heat exchange, expansion devices for low temperature work, supplemental cooling or any combination thereof. Suitable cryogenic separators are well known and known in the art.

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6 commercially available. The secreted hydrogen containing the desired amount of nitrogen usually leaves the cryogenic separator at a slightly lower pressure than the feed pressure and it is injected into an ammonia synthesis system.

The nitrogen stream usually leaves the cryogenic separator at a somewhat lower pressure than its supply pressure, but it can still deliver useful energy as it is heated and passed through an expansion turbine.

The process of the present invention has a number of advantages as compared to that disclosed in United States Patent No. 3,442,613. The synthesis gas used in the invention may be produced by partial oxidation e.g. combustion of many different hydrocarbons including coal, which is less complex than the preparation by catalytic reforming with water vapor, and the sulfur content of the hydrocarbon raw material is not critical since no catalyst is used. The combustion can be carried out in a single step, avoiding the use of primary and secondary reform units. While the amount of air used in the combustion stage leads to nitrogen in an amount of 25 at least 200 mole% in excess of the synthesis gas requirements, this large amount of excess nitrogen can be used to produce useful power after the separation step, since it is at a higher pressure than the prior art, thereby contributing to the economy of the overall System.

The invention will now be described with reference to the accompanying drawings, in which: FIG. 1 shows a flow chart of a method according to the present invention / 7

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FIG. 2 is a flow chart of an alternative method in accordance with the present invention; FIG. 3 shows a diagram of a high temperature open circuit gas turbine suitable for use in the process.

10 Referring to FIG. 1, natural gas, oil or coal, or a combination thereof is oxidized with air or with oxygen-enriched air, which is generally preheated and pressurized. The gas stream thus formed contains hydrogen, nitrogen, carbon oxides, methane and hydrogen sulfide if sulfur is present, the nitrogen being present in excess of at least 200 mole percent of the required-for-ammonia synthesis. late. The partial oxidation process is carried out at a pressure up to 150 bar, generally 15 to 150 bar, preferably 30 to 100 bar, and at a temperature of 300 to 2000 ° C, generally up to 1000 ° C. The oxidation can be carried out at atmospheric pressure, in which case the gas stream can be pressurized at a later stage in the treatment process.

25

The gas thus produced is passed over a so-called "shift" catalyst, e.g. iron oxide or cobalt molybdate, generally at a temperature in the range of 200 to 500 ° C to convert the carbon monoxide present to carbon dioxide and hydrogen. The gas stream is then treated to remove carbon dioxide and hydrogen sulfide which are present as impurities. There are many types of methods for such gas removal including a hot potassium carbonate scrubber system, ie. 70 to 110 ° C 35 hot, as well as the so-called "Rectisol" process. The sulfur content of the gas may have been removed by any of 8

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preferably prior steps. Carbon oxides which may have remained can be removed by methanation, generally at 250 to 450 ° C. The gas thus produced consists of a mixture of hydrogen and nitrogen with methane, inert gases such as argon and water vapor as the main pollutants. This gas is then initially dried by cooling and then by contact with a desiccant, e.g. a molecular sieve adsorbent (which would also remove any remaining traces of carbon dioxide). The dried gas is then fed to a cryogenic nitrogen / hydrogen separator, e.g. one that uses Joule Thomson cooling and regenerating heat exchange. The gas is brought into contact with heat exchanger elements which cool the gas to approx. In the cryogenic capacitor, the nitrogen content of the hydrogen is reduced to the level required for the ammonia synthesis gas, which is typically 25% N 2 for the ammonia synthesis. The cryogenic condensation of nitrogen will lead to a partial reduction in the content of methane and argon in the feed gas, removing the 20 impurities found in the waste stream of nitrogen. The hydrogen nitrogen stream leaving the condenser at a pressure slightly less than the gas supply pressure is injected into the ammonia synthesis system.

In the embodiment depicted in FIG. 2, the nitrogen capacitor includes a liquid nitrogen wash to remove residual carbon monoxide down to an acceptable level in ammonia synthesis. This agent allows the release of 30 methane emissions and allows for the convenient use of a higher level of CO from the so-called "shift" conversion, leading to a final synthesis gas which is largely is free of CH4 as well as inert gases.

The pure nitrogen needed for washing can be conveniently recovered from that contained in the cryogenic separator.

densified nitrogen and thus there would be no 9

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dependence on an external source of liquid nitrogen, as is the case with the classic nitrogen washing plants. It is also possible that the cryogenic condensation of nitrogen is applied to the methanation process, as shown in FIG. First

In all embodiments, it is advantageous for the waste stream of nitrogen to be discharged from the cryogenic condenser at more or less ambient temperature and at an elevated pressure of up to 50 bar, generally 5 to 10 bar, since it can be heated thereon. to a suitable supply temperature in a high temperature turbine and therein is expanded to about atmospheric pressure, thereby forming a useful proportion of the force necessary to compress the gas stream, e.g. to compress the process air used in the partial oxidation or in the reforming steps-using water vapor.

The waste stream of nitrogen can be heated to the inlet temperature of the turbine 20, e.g. 500 to 2000 ° C, generally 500 to 1000 ° C, by indirect heat exchange and / or by direct combustion of such combustible contents, e.g. traces of methane, hydrogen, carbon monoxide with supplemental air, and with additional fuel, if desired, 25 in the turbine flow diagram.

In the expansion turbine arrangement shown in FIG. 3, constitutes the expansion gas turbine for the hot gas turbine element in an open-circuit gas turbine. The nitro gene is mixed with supplementary fuel and fed to the combustion chamber of the gas turbine as fuel. At the same time, the process air required for the partial oxidation or the reforming treatments is extracted from the outlet of the gas turbine compression section. By this means, a large equilibrium is obtained between the mass balance in the compressor part and in the expansion part in the gas turbine, and

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10, effective means of compression and expansion are provided using already developed systems for industrial equipment.

Alternatively, nitrogen wastewater can be expanded at a low temperature, e.g. ambient temperature, to produce power, and it can be used to cool desired parts in the ammonia synthesis plant.

10 If it is possible to develop the excess stream of carbon dioxide from the plant to remove the acidic gas directly to the atmosphere in impure form, then it is convenient to use the pressurized waste stream of nitrogen from the cryogenic separator to strip a 15 a substantial part of the carbon dioxide from the wash solution to carry the combined streams of nitrogen and carbon dioxide, which are still at high pressure, for heating and subsequent expansion during work.

20

In summary, the above-described System for Air Oxidation, Nitrogen Condensation, Nitrogen Expansion provides the following advantages over the prior art including oxidation of the starting materials: 1. Exemption from air fractionation plants, oxygen compressors, pipelines, etc., 2. In many cases, a reduction in the total energy consumption of the compressors of the plant is obtained; 3. a significant reduction in the total energy demand of the plant can be obtained, taking into account the potential high effect of the gas expansion for the waste stream containing nitrogen and the associated conventional heat recovery.

The pressures quoted in this connection are relative pressures.

Claims (8)

A process for freeing a synthesis gas stream 5 for producing ammonia comprising reacting hydrocarbons with air in an amount sufficient to substantially reduce the amount of hydrocarbons and to provide a stoichiometric excess of nitrogen in addition to it. required for ammonia synthesis, treatment of the thus obtained raw gas stream for removal of carbon oxides, drying of the natural gas stream and cooling of the gas stream so that residual hydrocarbons and nitrogen are condensed therefrom to obtain the synthesis gas stream to the ammonia stream to the ammonia stream between the hydrocarbons and air, partial oxidation of a substance selected from a group consisting of oil, coal, natural gas or any combination thereof, in the presence of air at a pressure of 15-150 bar and at a temperature of 300- 2000 ° C 20 to form a hydrogen stream containing hydrogen and nitrogen with a large metric excess of nitrogen on at least 200 mole% based on the amount needed for the synthesis of ammonia, the dried raw gas stream being introduced into a cryogenic separator at a pressure of 15-1000 bar, dividing it by 25 to form two stream leaving the cryogenic separator, namely, (1) the synthesis gas stream having a pressure of 15-100 bar, and (2) a nitrogen rich waste stream having a pressure of 5-50 bar, which nitrogen rich stream is heated to a temperature of 500-2000 ° C and expanded in a turbine to formation of force. Process according to Claim 1, characterized in that the raw gas stream resulting from the partial oxidation is fed over a so-called "shiff" catalyst and reacted with water vapor at elevated temperature to convert the carbon monoxide contained in the raw gas. - DK 156638 B stream, for carbon dioxide and hydrogen. Process according to claim 2, characterized in that the gas stream after passage through the "shift" catalyst is subjected to hot potassium carbonate scrubbing at a temperature of 70-110 ° C to remove the gas's acid content before the separation step. Process according to claim 3, characterized in that, after scrubbing with hot potassium carbonate, the raw gas stream is subjected to washing with liquid nitrogen to remove carbon monoxide in the raw gas stream before the separation step. Process according to claim 4, characterized in that the separation step is carried out in a cryogenic separator and that the liquid nitrogen in the washing step is obtained from the nitrogen condensed in the cryogenic separator. 20 Process according to Claim 3, characterized in that, after scrubbing with hot potassium carbonate, the raw gas stream is subjected to methanation to remove any carbon oxides before the separation step. 25 Process according to claim 1, characterized in that the nitrogen-rich stream is mixed with supplementary fuel and introduced as fuel into the combustion chamber in a gas turbine. 30 Method according to claim 7, characterized in that the partial oxidation process air demand is extracted from the output stream of the gas turbine compressor. 5 10 15 20 25 30 35