GB2231039A - Recycling flash gas in ammonia synthesis - Google Patents

Description

:2:23 1 O B PROCESS FOR MATERIAL USE OF PRODUCT FLASH GAS The invention

relates to a process for the material utilization of the product flash gas in an ammonia plant The process can be applied to all ammonia plants, wherein the synthesis hydrogen is produced by the steam reforming process from a hydrocarbon-rich feedstock, certain quantities of the synthesis recycle gas being dissolved in the liquid ammonia, and product flash gases are produced at low pressures.

The major part of the ammonia produced at present is formed on the basis of natural gas Here, plants representing the development state of the sixties and seventies prevail.

The ammonia production of this generation is commonly divided into the following process stages.

1 Compression of feed natural gas 2 Desulfurization of the natural gas 3 Conversion of the major portion of hydrocarbons in the natural gas to hydrogen, carbon monoxide and carbon dioxide by injection of steam in the primary reformer 4 Further conversion of methane by addition of process air to hydrogen, carbon monoxide and carbon dioxide in the secondary reforming unit - Conversion of carbon monoxide to carbon dioxide in a two-stage carbon monoxide conversion unit 6 Removal of the formed carbon dioxide by chemical and/or physical scrubbing solutions in special absorption towers 7 Conversion of the remaining carbon monoxide and carbon dioxide in the gas to methane in the methanation unit 8 Compression of the fresh synthesis gas to the required synthesis pressure 9 Partial conversion of hydrogen and nitrogen in the synthesis gas to ammonia in synthesis loop Separation of the ammonia product from the synthesis recycle gas 11 Separation of the synthesis recycle gas to remove inert methane and rare gases 12 Recovery of hydrogen and rare gases from the synthesis recycle gas separated 13 Recompression of the synthesis recycle gas 3 - Generation of the synthesis gas is commonly carried out at a pressure of approx 2 5 M Pa to 5 M Pa.

Depending on the pressure of used feedstock, e g.

natural gas, it is therefore necessary to reduce it at the battery limit in case of a higher pressure or to increase it to the desired pressure for the gas generation, e.g with a centrifugal compressor in case of a lower pressure As the feed natural gas mostly contains small quantities of sulfur, which acts as a catalyst poison for the downstream process steps, it is necessary to remove this sulfur in a desulfurization unit For low sulfur contents, e g within the range of up to 100 mg/Nm 3, it is common to use zinc oxide for that purpose, which binds hydrogen sulfide by forming zinc sulfide As not all the sulfur is present as hydrogen sulfide, however, but is partly also fixed as organic sulfur, e g in the form of mercaptanes, it must first be converted to hydrogen sulfide with hydrogen The hydrogen required for this purpose is usually supplied by recycling from an inter- mediate stage of the synthesis gas compressor a partial stream of the synthesis gas, which is known to contain approx 75 vol% hydrogen, normally at a pressure of 5 M Pa to 10 M Pa As this gas is recycled to the synthesis gas generation section, it must be reduced to the pressure of that section This reduction is carried out without utilization of the existing pressure difference as energy.

4 - Depending on the sulfur content of the natural gas, the required hydrogen content is 5 to 10 vol%, i e the natural gas and the hydrogen-rich gas are mixed in a ratio of 14: 1 to 6 5: 1 Actual conversion of the organic sulfur to hydrogen sulfide takes place at temperatures between 3500 C to 4000 C in the presence of a cobalt/molybdenum or nickel/molybdenum catalyst After passing through the following reactor filled with zinc oxide, the total sulfur content of the gas is reduced to 3 usually below 0 5 mg/Nm Conversions of the hydrocarbons in the gas down to a methane content of 10 to 12 vol% takes place after injection of excess process steam in a fuel- fired tubular furnace, the so-called primary reformer, at temperatures of approx 800 'C In the subsequent, so- called secondary reformer, conversion of the not yet converted hydrocarbons down to a residual methane content of approx 0 5 vol% takes place at approx 1,000 C by injection of process air, with which the nitrogen component required for the ammonia synthesis is introduced.

In the succeeding, commonly two-stage carbon monoxide conversion unit, the carbon monoxide in the process gas is converted to hydrogen and carbon dioxide at temperatures of 2000 to 450 'C with steam down to a residual content of 0 2 to 0 5 vol%.

Removal of the carbon dioxide is carried out by scrubbing with chemically and/or physically functioning - scrubbing solutions in special towers down to a residual content of approx 0 1 vol% As the residual quantities of carbon monoxide and carbon dioxide in the process gas are poisons for the catalyst in ammonia synthesis, it is necessary to remove them down to a minimum content, 3 usually below 10 mg/Nm This removal is effected in most cases by conversion of these carbon oxides with hydrogen to methane in the methanation unit at temperatures of about 3000 to 3500 C.

After this process stage, the synthesis gas is available in its final form for the ammonia synthesis, i.e it contains hydrogen and nitrogen at a ratio of 3: 1, small quantities, usually 0 5 to 1 5 vol% methane, which is resulted, as already described, from the residual methane from the primary reforming and/or secondary reforming, the residual carbon monoxide from the carbon monoxide conversion and the residual carbon dioxide from the carbon dioxide removal, as well as rare gases, predominantly argon in the range of 0 3 to 0 7 vol%, which are introduced mainly by the process air supplied to the secondary reforming unit and partly also by the feed natural gas.

Methane and rare gases are enriched in the synthesis loop, as they are inert components with regard to the ammonia synthesis It is therefore essential to continuously remove from the loop the same quantities 6 - of inert gases, which reaches the synthesis loop This is carried out, in accordance with the prevailing technical- technological conditions, e g in the case of ammonia separation from the synthesis recycle gas through partial condensation, by the dissolution in the ammonia product on one hand, whereby up to 50 % of the inert gases can be removed depending on the synthesis pressure and the tolerated inert gas concentration in the synthesis recycle gas, and by branching a portion of the synthesis recycle gas, the so-called recycle flash gas, on the other hand, in which case the recycle flash gas volume may generally be up to 5 % of the synthesis gas supplied to the synthesis loop.

In the case of separation of the ammonia product from the synthesis recycle gas by partial condensation, the liquid ammonia is then usually reduced in pressure in stages from the synthesis pressure to the pressure of downstream consumers or down to nearly atmospheric pressure The major part of the dissolved synthesis recycle gas is thereby released again The volume of this gas mixture known as product flash gas is between 3 3 Nm /t ammonia and 70 Nm /t ammonia, depending on the pressure in the synthesis loop and the final pressure of the flashed ammonia product The components are always hydrogen, nitrogen, ammonia, methane and rare gases, mainly argon It is known (e g US-PS 3,705,009, US-PS 7 - 3,432,265, US-PS 3,598,527, DE-OS 2,129,307, DE-OS 1,767,214) that this product flash gas can be used as energy for heating purposes.

It is also known (DD-PS 235,385) that the hydrogen, nitrogen and ammonia in the product flash gas can be utilized materially by recycling to the gas generation section Simutlaneous return of the rare gases, primarily argon, increases the efficiency and output of the downstream unit for rare gas recovery, because the rare gas losses accompanied by the ammonia product are prevented In the case of stepwise flashing of the liquid ammonia, there are different joining possibilities of the product flash gases into the gas generation section depending on the selected flashing pressures.

In the above-mentioned patent, the ammonia is flashed in a flash drum to a pressure of approx 4 M Pa.

After refrigeration the evolved product flash gas is returned to the natural gas-process steam mixture up- stream of the primary reformer The remaining liquid ammonia product is further depressurized to 2 2 M Pa in another flash drum The product flash gas generated there is fed to the suction of the natural gas compressor after simple refining (refrigeration).

The disadvantage of this process is the additional compression energy required by the natural 8 - gas compressor to pressurize the product flash gases formed at low pressures and returned to the suction of the natural gas compressor.

It is the task of the invention to utilize product flash gas comprising hydrogen, nitrogen, methane and rare gases, without additional compression requirements, in the synthesis gas generation section of an ammonia plant.

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Fig 1 illustrates the flow diagram of the ammonia wherein:

Natural gas compression section Desulfurization section Primary reforming section Secondary reforming section Conversion section Carbon dioxide removal system Methanation section Fresh synthesis gas compression section Synthesis recycle gas compression section Ammonia reactor Separator Hydrogen recovery unit Rare gas unit 9 - 14 Ejector Flash drum 16 Refrigeration separator 17 Flash drum 18 Refrigeration separator 52 Lines The basic task of the invention is to recycle product flash gases with pressures below the pressure, preferably between 2 5 and 4 5 M Pa, of the reforming stage to the synthesis gas generation section of the ammonia plant, without requiring additional compression energy.

The task is solved by the invention in such a way that a portion of the withdrawn recycle flash gas of the ammonia plant at high pressure, preferably between 5 and 30 M Pa, more preferably between 10 30 M Pa, and the product flash gas yielded at a pressure, preferably between 1 0 and 2 5 M Pa, below that of the discharge pressure of the natural gas compressor are combined, after refrigeration and separation of the condensed ammonia product, in an injection compressor (ejector) by balancing the internal energy of both gas streams, and then the recycle-product flash gas stream is sent back to the process gas stream upstream of the desulfurization section at a pressure - exceeding the discharge pressure of the natural gas compressor.

The components hydrogen and nitrogen in the recycle flash gas and the product flash gas are available as feedstock for the ammonia synthesis without energy and additional raw material for the compression of the necessary natural gas and process air as well as for the cracking (reforming) of methane The returned ammonia portion is cracked in the primary reforming and/or secondary reforming section into the basic components hydrogen and nitrogen, which are also thus available as feedstock for the ammonia synthesis Contrary to the rare gases returned with the recycle flash gas, which are again fed unchanged to the synthesis loop with the fresh synthesis gas, the methane acting as inert gas in the synthesis loop is not returned to the synthesis loop together with the fresh synthesis gas, because it is converted to hydrogen and carbon monoxide by the steam reforming reaction in the primary and/or secondary 2-0 reforming section.

By applying the invented process, the inert gas proportion in the recycle synthesis gas is clearly changed in favor of rare gases, due to the fact that only the rare gases returned with the recycle flash gas, but not the methane contained therein, are fed to the synthe- sis loop, which leads to an increase and improvement of 11 - rare gas recovery in the downstream rare gas unit.

As the recycle flash gas returned to the gas generation section according to the invention consists primarily of hydrogen, commonly between 55 and 65 vol%, the possibility exists to use this gas as a hydrogen supplier for the conversion of organically fixed sulfur to hydrogen sulfide, when it is returned upstream of the desulfurization unit Depending on the sulfur content of the natural gas, it is thus possible to completely or partly shut down the synthesis gas recycling from an intermediate pressure stage of the synthesis gas compressor.

In the present invention, instead of a hydrogen containing gas such as the above-mentioned recycle flash gas, a hydrocarbon containing gas to be the feedstock for the reformer unit preferably at a pressure between 5 and 10 M Pa may be used as a driver gas in the ejector.

Example:

The invention will be explained in detail on the basis of an application example Figure 1 shows the flow diagram of an ammonia plant.

The feedback natural gas, approx 38,000 Nm 3/h, is fed to the natural gas compression section 1 via line and compressed there to approx 4 M Pa The gas then reaches, via line 21 at which 6,000 Nm 3/h recycle flash gas and 200 Nm /h product flash gas supplied through 12 - line 52 are added, the desulfurization section 2, where the sulfur is removed in two stages down to a residual 3 content of less than 0 5 mg/Nm Before entering the primary reforming unit 3, the natural gas in line 22 is mixed with approx 100 t/h process steam, fed through line 23, and a product flash gas fed from a flash drum via line 45, and heated to 5000 C in the convection zone of the primary reforming section Catalytic crakcing of methane takes place down to a residual content of approx 11 5 vol% The raw synthesis gas is heated there to a temperature of approx 800 'C Further conversion of the methane to a residual content of approx 0 3 vol% is carried out in the secondary reforming unit 4, to which the raw synthesis gas is fed via line 24 The heat thereby required is supplied by burning hydrogen 3 with oxygen introduced with approx 55,000 Nm /h of process air necessary for the nitrogen supply via line 25.

Conversion of the carbon monoxide formed in the primary and secondary reforming units down to a residual carbon monoxide content of approx 0 3 vol% takes place under the formation of hydrogen and carbon dioxide in a two- stage conversion section 5, to which the raw synthesis gas is fed via line 26 The carbon dioxide-rich raw synthesis gas is fed to the carbon dioxide removal system 6 via line 27 Here, the carbon dioxide is washed out with an activated hot potassium based solution 13 - down to a residual content of approx 0 1 vol% The removal of the residual carbon monoxide and carbon dioxide in the raw synthesis gas down to a residual content of less than 10 ppm is accomplished by conversion with hydrogen to methane in the methanation section 7, to which the raw synthesis gas is fed through line 28.

After addition of approx 3,900 Nm 3/h of hydrogen fraction, supplied through line 38 from the outlet of the hydrogen recovery unit 12, and approx 350 Nm 3/h of hydrogen- nitrogen fraction, supplied through line 39 from the outlet of the rare gas unit 13, the fresh synthesis gas via line 29 is fed to the synthesis gas compression unit 8, where it is compressed to approx 30 M Pa Mixing with the synthesis recycle gas is effected via lines 30 and 31.

Then the mixed synthesis recycle gas is led to the ammonia reactor 10 through line 32 Here ammonia is formed by the catalytic conversion of hydrogen and nitrogen.

After cooling the synthesis recycle gas and condensing the liquid ammonia, the liquid gas mixture is fed through line 33 to a separator 11 where the liquid ammonia is separated Before the synthesis recycle gas is returned to the synthesis recycle gas compression section 9 through line 34, approx 6,000 Nm 3/h of a recycle flash gas is drawn off via lien 35 to the hydrogen recovery unit 12, followed by the withdrawal of approx 6,000 Nm 3/h of another recycle flash gas via line 36 to the inlet of an 14 ejector 14 In the hydrogen recovery unit 12, the recycle flash gas is separated by low-temperature separation into the above-described hydrogen fraction, which is mixed with the fresh synthesis gas via line 38, and approx 2,000 Nm 3/h of a residual gas fraction, which is supplied to the rare gas unit 13 via line 37 In the rare gas unit 13, low-temperature separation is carried out yielding the above-described hydrogen/nitrogen fraction, which is joined via line 39 to the fresh synthesis gas, a nitrogen fraction, which is fed via line to the respective consumers, a methane fraction, which is added via line 41 to the fuel gas of the ammonia plant, and an argon fraction, which is supplied via line 42 to the users The argon production of 18 t/d, which is the case when the invention is not applied, may be increased to 19 2 t/d by the application of the invention The liquid ammonia from the separator 11 is supplied via line 43 to a flash drum 15 and depressurized there to approx 4 M Pa The product flash gas containing ammonia is fed to a refrigeration separator 16 and cooled down to -20 'C The condensed ammonia is separated and enters via line 46 another flash drum 17, as does the liquid ammonia from the flash drum 15 via line 47 Approx.

1,500 Nm 3/h of the treated product flash gas is sent via line 45 to the primary reforming section 3 and is joined there to the natural gas-process steam mixture The - liquid ammonia product fed to the flash drum 17 is depressurized there to approx 2 2 M Pa The released product flash gas is fed via line 48 to a downstream refrigeration separator 18 The ammonia condensed there is led via line 49 to an ammonia discharge line 50 The ammonia, product in an amount of 1,520 t/d is taken out of the system through line 50 The remaining product flash gas is sucked in via line 51 by the recycle flash gas flowing through the ejector 14, pressurized to a pressure of 4 M Pa or higher, and then supplied via line 52 to the compresed natural gas stream upstream of the desulfuriza- tion section 2.

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Claims (4)

CLAIMS:

1 In an ammonia plant where the synthesis hydrogen is produced by the steam reforming process from a hydrogen- rich feedstock, the synthesis gas at the inlet to the synthesis loop containing inert gases, particularly methane and rare gases, the ammonia product being separated from the synthesis recycle gas by partial condensation, and dissolving hydrogen, nitrogen, rare gases and methane, and the ammonia product is flashed in stages or in one step to a pressure below that in the reforming section of the ammonia plant, thereby partially releasing the dissolved gases as product flash gas and needing one or more gas streams thereof with a higher pressure than in the reforming stage to be fed as feed- stock to this reforming stage, a process for the material use of the product flash gas in the ammonia plant characterized in that the product flash gases at a pres- sure below that of the reforming stage are fed to an ejector, compressed there to the pressure of the reforming stage by using as a driving medium one or several gas streams with a higher pressure than required in the reforming stage and then returned to the synthesis gas generation section upstream of the reforming stage.

2 The process according to Claim 1 wherein the driving medium for the ejector is a hydrogen-rich gas 17 - with a pressure of particularly from 5 M Pa to 30 M Pa, which is recycled from units downstream of the synthesis gas generation section, particularly from the synthesis gas compression section and the synthesis loop, to the synthesis gas generation section upstream of the desulfu- rization section.

3 The process according to Claim 1 wherein the driving medium for the ejector is a hydrocarbon-rich gas with a pressure of particularly from 5 M Pa to 10 M Pa, which is fed as feedstock for the synthesis gas generation section upstream of the desulfurization section.

4 A process according to claim 1 carried out substantially as hereinbefore described with reference to the accompanying drawing.

P.xbllshed 1990 a, The Patent Office, State House, 66/71 High Holborn, London W Cl B 4 TP Frther copies maybe obtained from The Patent Office.

Sales Branch, St Mary Cray, Orpington, Kent BR 5 3RD Printed by Multiplex techniques ltd, St Mary Cray, Kent, Con 1/87