GB2231040A - Using residual gases in an ammonia synthesis - Google Patents

Description

1 - PROCESS FOR MATERIAL UTILIZATION OF RESIDUAL GASES FROM A RARE GAS

UNIT The invention relates to a process for the material utilization of gases which are produced at low pressures in a rare gas unit downstream of an ammonia plant and mainly comprise such components as hydrogen, nitrogen and methane as well as a small quantity of argon The process can be applied in any ammonia plants in which the recycle flash gas is treated in a downstream hydrogen recovery unit and the residual gas fraction is fed to a rare gas unit.

A considerable-portion of the ammonia produced at present is manufactured on the basis of a synthesis gas which is obtained by the steam reforming or partial oxidation of hydrocarbons Dominant are ammonia plants based on natural gas, which represent the development state of the 60 S and 70 s Thus, the following description refers to the plants on natural gas basis, but it can 2 - also be applied to ammonia plants on the basis of other hydrocarbons to the same extent.

The production of ammonia according to the steam reforming process is usually 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 contained in the natural gas into hydrogen, carbon monoxide and carbon dioxide by addition of steam in the primary reformer 4 Further conversion of methane into hydrogen, carbon monoxide and carbon dioxide by addition of process air in the secondary reformer 5 Conversion of the carbon monoxide into carbon dioxide in a normally two-stage carbon monoxide conversion unit 6 Removal of the formed carbon dioxide with chemical and/or physical washing solutions in special absorption towers 7 Conversion of the remaining carbon monoxide and carbon dioxide contained in the gas into methane in the methanation section 8 Compression of the synthesis gas to the required synthesis pressure 9 Partial conversion of the hydrogen and nitrogen 3 - contained in the synthesis recycle gas into ammonia Separation of the ammonia product from the synthesis recycle gas 11 Separation of the synthesis recycle gas for removing 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 The generation of the synthesis gas is commonly carried out at a pressure of about 2 5 to 5 M Pa It is known that the feed natural gas often has considerably higher pressures at the battery limit and hence is expanded to the pressure in the synthesis gas generation section As the feed natural gas mostly contains a small quantity of sulfur which acts as a catalyst poison in the subsequent process stages, it is necessary to remove the sulfur in a desulfurization unit In case of a small sulfur content, e g in the range up to 100 mg/Nm, it is usual to use zinc oxide for that purpose, which binds hydrogen sulfide by forming zinc sulfide However, since not the entire sulfur exists as hydrogen sulfide, but is also partially combined as organic sulfur, e g.

in the form of mercaptane, it must first be transformed into hydrogen sulfide by conversion with hydrogen The hydrogen thereby required is usually provided by recycling 4 - a partial stream of the synthesis gas, which contains, as is known, about 75 vol% hydrogen, from an intermediate stage of the synthesis gas compressor, normally at pressures from 5 to 10 M Pa As this gas is returned to the synthesis gas generation section, it must be expanded to the pressure of that section The expansion is carried out without utilization of the existing pressure difference as energy The conversion of the hydrocarbons contained in the natural gas down to a residual methane content of 10 to 12 vol% takes place after mixing with excess process steam in a fuel heated tubular furnace, the so-called primary reformer, at a temperature of about 8000 C In the following secondary reformer, the conversion of the not yet converted methane down to a residual content of about 0 5 vol% is performed at about 1,0000 C by using process air, with which the nitrogen component required for the ammonia synthesis is introduced In the succeeding, normally two-stage carbon monoxide conversion step, the carbon monoxide contained in the gas is converted by means of steam at 2000 C to 4500 C into hydrogen and carbon dioxide, down to a residual content of 0 2 to 0 5 vol% The removal of the carbon dioxide is conducted by washing with chemically and/or physically functioning washing solutions in special towers down to a residual content of about 0 1 vol% Since the remaining carbon monoxide and carbon dioxide contained in the gas act as a poison to the catalyst for the ammonia synthesis, it is necessary to remove them to a minimum level, normally less than 3 mg/Nm In most cases, the removal is effected by conversion of the carbon oxides into methane by means of hydrogen in the methanation section at temperatures of about 300 WC to 350 'C After this process stage, the synthesis gas appears in the final composition for the ammonia synthesis, i e it contains hydrogen and nitrogen in the ratio of 3 to 1 and a small quantity of methane, commonly 0 5 to 1 5 vol%, which is derived, as already mentioned, from the residual methane after the primary reforming and/or secondary reforming, the residual carbon monoxide after the carbon monoxide conversion, and the residual carbon dioxide after the carbon dioxide removal, as well as rare gases, mainly argon, in the range of from 0 3 vol% to 0 7 vol%, which are introduced primarily by the process air fed to the secondary reforming and partially by the feed natural gas Methane and rare gases are enriched in the synthesis circulation because they act as inert gases in the ammonia forming reaction.

Therefore, it is necessary to remove the same quantity of inert gases which reaches the synthesis circulation continuously This removal is carried out, in accordance with the existing technical and technological 6 - conditions, e g in the case of ammonia separation from the synthesis recycle gas by 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 sysnthesis 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 The recycle flash gas branched is generally refrigerated first to minimize the content of ammonia In the past, it was usual that this gas was used only as energy as a fuel gas in the primary reform- ing In the meantime, however, most of ammonia plants have been equipped at least with downstream units for the recovery of the hydrogen contained The principle of the majority of these plants is that the recycle flash gas to be processed is cooled down to a temperature, at which methane, rare gases and nitrogen condense but hydrogen still remains gaseous, after removal of the still contained residual ammonia by, for example, refrigeration, separation of the condensed ammonia and/or water-washing with subsequent gas drying, by means of an external chilling source by the use of a separate refrigeration circulation system or from the own chilling 7 - source by utilizing Joul-Thomsom Effect (e g DD 47,120).

Both cases yield a hydrogen fraction which comprises mainly hydrogen and also a small quantity of nitrogen as well as methane and rare gases in the range of a few percent, and a residual gas fraction which comprises methane, rare gases and nitrogen as well as a small quantity of hydrogen which has been dissolved in the initial liquid mixture of methane, rare gases and nitrogen Further, it is possible to conduct the separation of hydrogen by means of separation membranes (e.g DE-OS 2,910,742) By this means, a hydrogen-rich gas with a similar composition to that of the low temperature separation processes is obtained However, the loss of hydrogen is more than in the above-mentioned processes, i e a larger quantity of the residual gas fraction with a hydrogen content of not more than 40 vol% is produced The pressure in the hydrogen recovery unit is generally selected in such a manner that the hydrogen fraction obtained can be fed to the suction side of the synthesis gas compressor The residual gas produced in the above processes is often used only as energy as a fuel gas for, e g the primary reforming in the ammonia plant However, since the rare gas content of the gas generally lies between 15 and 25 vol%, it is possible to obtain argon and occasionally other gases contained therein in a highly purified form by subjecting 8 - the gas to further low temperature separation, while other gas components are also produced as relatively pure Lractions The low temeprature separation is generally conducted by utilizing a separate refrigeration circulation.

Besides the liquid argon product and other rare gases (e g krypton and xenon), such rare gas plants generate a hydrogen/nitrogen fraction containing to 80 vol% of hydrogen and a small portion of argon and methane, a methane fraction with a methane content of more than 95 vol% and a nitrogen fraction of various purities, but generally of at least 95 vol% nitrogen.

The utilization of these gas streams is very limited due to their relatively low pressures These gas streams are generally produced at a pressure of from 0 4 M Pa to 1.0 M Pa Thus, their use in ammonia plants, for example, is only possible after additional compression The methane separated is usually utilized as a fuel gas in ammonia plants For this purpose, compression to the fuel gas pressure is eventually required The nitrogen produced is fed to a low pressure nitrogen supply network or returned to the ammonia plant entirely or partially together with the hydrogen/nitrogen fraction The connection of these return gases to the ammonia production process is dependent on the pressures of the return gases and the natural gas fed to the ammonia plant In case of 9 - a positive pressure difference between the return gases and the natural gas, the connection can be made on the suction side of the natural gas compressor In the absence of a positive pressure difference, the pressure must be increased by additional compression at least to the suction pressure of the natural gas compressor.

The purpose of the invention is to utilize materially residual gases, which may comprise such components as hydrogen, nitrogen, methane and argon and are produced at low pressures in a rare gas unit processing recycle flash gas, without additional compression cost in the synthesis gas generation section of an ammonia plant.

Fig 1 shows a complex comprising an ammonia plant, a hydrogen recovery unit and-a rare gas unit, in which:

1 Natural gas compressor 2 Desulfurization section 3 Primary reformer 4 Secondary reformer Conversion section - 6 Carbon dioxide removal system 7 Methanation section 8 Synthesis make-up gas compression 9 Synthesis recycle gas compression 10 Ammonia reactor 11 Separator 12 Hydrogen recovery unit 13 Rare gas unit 14 Jet compressor (ejector) 20 to 43 Lines It is the task of the invention to provide a process according to which the material utilization of the residual gases of a rare gas unit for treating the recycle flash gas of an ammonia plant is made possible without additional compression cost.

According to the invention, this task is achieved in such a manner that when a portion of the recycle flash gas produced in the ammonia plant is returned to the synthesis gas generation section after additional refrigeration and separation of the condensed ammonia product or removal of ammonia through a water scrubbing unit, this recycle flash gas at a high pressure than that in the reforming stage, preferably between 4 0 and 40 M Pa, 11 - more preferably between 20 and 30 M Pa, is used as a driving gas for a jet compressor (ejector) by the utilization of the pressure difference to the synthesis gas generation section, where the reforming stage is preferably operated at a pressure between 2 5 and 4 5 M Pa.

By this jet compressor, the gas streams produced in the rare gas unit, which may comprise such components as hydrogen, nitrogen, methane and argon and preferably at a pressure between 0 2 and 1 3 M Pa, more preferably between 0 4 and 1 0 M Pa, are compressed to such a pressure that allows connection to the synthesis gas generation section in the ammonia plant.

The components such as hydrogen and nitrogen contained in the recycle flash gas and in the gases from the rare gas unit are used as a feedstock for the ammonia synthesis without any consumption of raw material and energy for the compression of the required natural gas and process air as well as for the cracking of methane.

Contrary to the returned rare gases-which are fed without change together with the synthesis make-up gas to the synthesis circulation again, the methane acting as an inert gas in the synthesis circulation no more reaches the synthesis circulation together with the synthesis make-up gas, because it is converted into hydrogen and carbon monoxide by the steam reforming 12 - reaction in the primary reforming and/or secondary reforming unit.

According to the invention, any gas may also be used as the driving gas for the jet compressor, so far as it is fed to the synthesis gas generation section with sufficient pressure and in sufficient quantity (e g.

synthesis gas from an intermediate stage of the synthesis gas compressor used for the conversion of organic sulfur, or natural gas) as well as the recycle flash gas mentioned above For typical example, instead of the hydrogen containing gas such as the above-mentioned recycle flash gas, as a driving gas for the ejector, hydrocarbon containing gas such as feed gas for the reformer unit preferably at a pressure between 5 and 10 M Pa may be utilized in the present invention.

Example 1:

The invention will be explained in detail by reference to an example Fig 1 shows a complex consisting of an ammonia plant, a hydrogen recovery unit 12 and a rare gas unit 13 About 38,000 Nm /h of natural gas is fed as a raw material for ammonia production to the ammonia plant through line 20 and compressed to about 4 M Pa by a natural gas compressor 1 The gas is fed to a desulfurization unit 2 via line 21 and, after about 6,850 Nm 3/h of a gas consisting of recycle flash gas and 13 - the gas from the rare gas unit 13 has been added to this gas through line 43, a two-stage refining is carried out there, down to a residual sulfur content of less than 3 0.5 mg/Nm The resulting gas is mixed with about 100 t/h of process steam through line 23 and fed to a primary reformer 3 through line 22 Here catalytic conversion of methane takes place down to a methane content of about 11 5 vol% Further conversion of methane down to a residual methane content of about 0 3 vol% is carried out in a secondary reformer 4 after feeding the gas via line 24 About 55,00 Nm 3/h of process air required for this purpose is fed through line 25.

The conversion of the formed carbon monoxide into hydrogen and carbon dioxide down to a carbon monoxide content of about 0 3 vol% is carried out in a two-stage conversion unit 5 to which the raw-synthesis gas has been conveyed through line 26 The carbon dioxide rich gas is sent to a carbon dioxide removal system 6 through line 27 By means of an activated hot potassium based solution, carbon dioxide is removed from the raw synthesis gas down to a residual content of about 0 1 vol% before the gas is fed to a methanation section 7 through line 28, where the remaining carbon monoxide and carbon dioxide are converted to methane with hydrogen.

After mixing about 3,900 Nm 3/h of a hydrogen fraction containing about 90 vol% hydrogen, fed through 14 - line 38 from the hydrogen recovery unit 12, and about 600 Nm 3/h of a hydrogen/nitrogen fraction containing about 50 vol% hydrogen, sent through line 39 from the rare gas unit 13, the synthesis gas is passed to a synthesis gas compression unit 8 through line 29 and compressed to about 30 M Pa The mixing with the synthesis recycle gas is carried out through lines 30 and 31.

The feeding to an ammonia reactor 10 is conducted through line 32 Catalytic conversion of hydrogen and nitrogen into ammonia takes place in the ammonia reactor 10.

After cooling the synthesis recycle gas and condensing liquid ammonia, the synthesis recycle gas is sent through line 33 to a separator 11, where about 1,520 t/d of liquid ammonia is separated Before returning the synthesis recycle gas to a synthesis recycle gas compression section 9 via line 34, about 6,000 Nm 3/h are drawn out to the hydrogen recovery unit 12 via line 35, and further 6,000 Nm 3/h to a jet compressor 14 through line 36 In the hydrogen recovery unit 12, the low temperature sepa- ration of the recycle flash gas is carried out The hydrogen fraction is returned to the ammonia plant via line 38 About 2,000 Nm 3/h of the remaining residual gas fraction is fed to the rare gas unit 13 through line 37.

Further low temperature separation is carried out in this unit, so that the argon product (about 18 t/d) is taken out via line 42 About 600 Nm 3/h of the generated - hydrogen/nitrogen fraction is compressed and returned to the ammonia plant through line 39 The nitrogen fraction ( 3 t/d) is fed to respective consumers through line 40.

A gas mixture ( 850 Nm 3/h) with a pressure of about 1 0 M Pa, which consists of the methane fraction and the generated residual gases and has a composition of approx 77 vol% methane, approx 9 vol% hydrogen, approx 9 vol% nitrogen and approx 5 vol% argon, is fed via line 41 to the jet compressor 14 driven by the recycle flash gas This gas is returned together with the recycle flash gas at a pressure of about 4 4 M Pa to the delivery side of the natural gas compressor 1 through line 43.

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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 feedstock containing hydrocarbons, the synthesis gas containing inert gases, especially rare gases and methane, at the inlet to the synthesis circulation, the ammonia product being separated by partial condensation from the synthesis recycle gas, the recycle flash gas being further processed in a downstream hydrogen recovery unit and/or rare gas unit, and one or several gas streams of higher pressure than that in the reforming stage are required to be fed as feedstocks to this reforming stage, a process for the material utilization of residual gases from the rare gas unit in the ammonia plant characterized in that the residual gas from the rare gas unit, comprising components as hydrogen, nitrogen, argon and methane, with a pressure below the pressure of the reforming stage is fed to an ejector and compressed there to the pressure in the reforming stage by using one or several gas streams with a higher pressure than that in the reforming stage as a driving medium and the resulting mixture is fed to the synthesis gas generation section before the reforming stage.

2 The process according to Claim 1 wherein a hydrogen-containing gas is used as the driving medium, 17 - which is returned from the plant sections downstream of the synthesis gas generation, especially from the synthesis gas compression and the synthesis circulation, to the synthesis gas generation before the desulfurization stage.

3 The process according to Claim 1 wherein a hydrocarbon-containing gas is used as the driving medium, which is fed upstream of the desulfurization stage as a raw material for the synthesis gas generation.

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

Published 1990 aw The Patent Office, State House, 60 '71 High Holborn London WC 1 R 4 TP Further copies maybe obtainedfrom The Patent Office.

Sales Bri:nch, St Mary Cray, Orpington, Kent BB 5 3 BD Printed by Multiplex techniques ltd St Mary Cray Kent Con 1/87