GB2033882A - Production of ammonia - Google Patents

Abstract

Hydrogen for ammonia production is recovered (e.g. by pressure swing absorption) in high purity from a methanol synthesis plant purge gas stream. High purity nitrogen is produced by nitrogen enrichment of air and combined with the high purity hydrogen for passage to an ammonia synthesis reactor. Schemes are described for effecting ammonia synthesis at or above the pressure at which the hydrogen is produced, with ammonia recovery by condensation against cooling water, by refrigeration or by absorption in a liquid absorbent, such as water or dilute aqueous ammonia solution.

Description

SPECIFICATION Process This invention relates to a process for the co-production of methanol and ammonia.

In the conventional methods of manufacturing methanol a hydrocarbon feedstock is converted to a synthesis gas by steam reforming or by partial oxidation using pure oxygen followed by appropriate purification. The resulting approximately 2:1 hydrogen: carbon monoxide mixture is contacted under suitable temperature and presure conditions with a methanol synthesis catalyst. Suitable catalysts include mixed oxides of copper, zinc, chromium, manganese or aluminium, for example, copper oxide-promoted zinc oxide (containing, e.g., greater than 20% by weight of copper as metal atoms).

Typical methanol synthesis conditions include temperatures of from 230"C to about 2700C and pressures of from about 50 to about 110 kg/cm2 absolute. Upon exiting the methanol synthesis reactor the product methanol is separated by condensation whilst unreacted synthesis gas is recirculated to the inlet side of the reactor. A purge stream is taken from this gas recycle loop and is conventionally used as a fuel.

Ammonia is conventionally synthesised on a commercial scale from a 3:1 hydrogen:nitrogen mixture.

Such a mixture can be produced, for example, by steam reforming with air. The resulting mixture which consists predominantly of hydrogen, carbon monoxide, nitrogen and carbon monoxide to carbon dioxide and to generate further hydrogen, following which carbon dioxide is removed. One method of carbon dioxide removal comprises subjecting the gas stream to a washing step with a liquid amine absorbent, such as monoethanolamine, followed by methanation to convert residual carbon oxides to methane. The quantity of air used in the secondary reformer is adjusted so that the final gas mixture is an approximately 3:1 hydrogen:nitrogen mixture. This mixture is then compressed to the operating pressure of the ammonia synthesis loop.Besides hydrogen and nitrogen the synthesis gas contains significant quantities of inert gases such as argon and of methane produced as a result of the final methanation step used to remove carbon oxides. After passing through the ammonia synthesis reactor, the product ammonia is condensed from the gas stream by external cooling with cooling water or by refrigeration or can be washed out of the gas stream with, for example, water. Unreacted gas is, if necessary, dried and recompressed before being recycled to the ammonia synthesis reactor. Typically, the conversion to ammonia per pass through the reactor is about 15% to about 20%.Since the degree of conversion depends, inter alia, on the partial pressures of ammonia, of hydrogen and of nitrogen in the reactant gas mixture, it is usual to reduce the ammonia concentration in the recycle stream as far as possible and in as economical a fashion as possible. A purge stream is normally taken from the recycle gas stream in order to prevent build-up in the system of inert materials such as argon and methane. These inert materials exert significant partial pressures in the ammonia synthesis loop and a correspondingly higher overall pressure must be used than if these inert materials were absent in order to attain the desired conversion to ammonia per pass through the reactor.

This means that additional energy must be expended to compress the make-up synthesis gas and recycle gas. Typically, the energy requirement of such an ammonia synthesis production process is in the region of 8 x 1 06k cal per tonne of ammonia. (The term "k cal" whenever used throughout this specification refers to net or lower heat value).

It is an object of the present invention to provide a process whereby ammonia can be efficiently synthesised and whereby the energy requirement ascribable to ammonia production is significantly reduced compared with the conventional process.

According to the present invention, we provide a process for the co-production of ammonia and methanol which comprises: (a) providing a synthesis gas containing carbon oxides and hydrogen in a proportion and of a purity suitable for methanol synthesis; (b) contacting resulting synthesis gas in a methanol synthesis zone with a methanol synthesis catalyst maintained under temperature and pressure conditions conducive to synthesis of methanol; (c) recovering from the methanol synthesis zone a methanol-containing product stream; (d) separating from resulting methanol-containing product stream methanol and a methanol-depleted stream; (e) separating essentially pure hydrogen from at least a part of the methanol-depleted stream; (f) subjecting air to a nitrogen enrichment step so as to produce an essentially pure nitrogen stream;; (g) mixing essentially pure hydrogen from step (e) with essentially pure nitrogen from step (f) in a proportion suitable for ammonia synthesis; (i) recovering a gaseous ammonia-containing product stream from the ammonia synthesis zone; and (j) separating ammonia from resulting ammonia-containing product stream.

The invention thus contemplates, in one form, utilising the purge gas stream from a methanol synthesis loop as a source from which a high purity hydrogen stream can be produced for ammonia production, utilising a high purity nitrogen stream as the other reactant.

Synthesis gas for use in the process of the invention can be produced, for example, by conventional steam reforming of a hydrocarbon. The hydrocarbon may be methane, natural gas, naphtha or any other hydrocarbon feedstock capable of being subjected to steam reforming. Unless the feedstock is already sulphur4ree, it will be necessary to desuiphurise the hydrocarbon feedstock prior to steam reforming.

Alternatively, the hydrocarbon feedstock may be subjected to partial oxidation using pure oxygen.

The synthesis gas used for methanol production in the process of the invention preferably comprises a stoichiometric or near stoichiometric mixture, i.e. a substantially 2:1 hydrogen:carbon monoxide mixture or a similar mixture with the hydrogen in excess, e.g. an approximately 2.25:1 H2:CO mixture. The synthesis gas may further comprise a minor amount of carbon dioxide. In the methanol synthesis zone, the following reactions take place:2H2+ CO

CH3OH 3H2+ CO2

CH3OH + H2O In the methanol synthesis zone the temperature and pressure conditions are conducive to synthesis of methanol. Typically the temperature ranges from about 200 C to about 300"C and the pressure ranges from about 50kg/cm2 absolute up to about 110kg/cm2 absolute.The temperature and pressure conditions in the methanol synthesis zone should be so selected in relation to the selected catalyst as to promote efficient formation of methanol from the synthesis gas.

In accordance with conventional practice the methanol synthesis may be conducted as a "once through" operation but it will usually be preferred to recycle unreacted gas to the methanol synthesis zone. In the former case all of the unreacted gas is passed on for use in the hydrogen purification step (step (e)); in the latter case the usual purge stream is taken for this purpose.

Any suitable method can be used to separate high purity hydrogen from such unreacted gases from the methanol synthesis zone. Conveniently a pressure swing adsorption system can be used. Such systems are commercially available and are capable of yielding from a methanol plant purge stream a hydrogen stream of 99.999% purity or better.

Any suitable method of producing high purity nitrogen from air may be used. Forms of plant are now commercially available for the production of at least 99.999% pure nitrogen.

Since both the hydrogen stream and the nitrogen streams are extremely pure, ammonia synthesis can be effected substantially in the absence of inert materials such as argon and methane. This means that the convertion per pass at a given total pressure is increased. Hence the operating pressure of the ammonia synthesis section can be reduced significantly compared with conventional plants without reducing efficiency.

The essentially pure hydrogen from step (e) will usually be produced at a superatmospheric pressure, for example 34kg/cm2 absolute or greater. Usually the essentially pure nitrogen from step (f) will be at a lower pressure than that of the hydrogen from step (e) and will require pre-compression at least to this value prior to mixing. Depending in large part upon the method chosen for ammonia recovery the pressure of the ammonia synthesis zone may be substantially the same as that of the hydrogen from step (e) or it may be greater. In this latter case further compression of the mixed gases from step (g) will be required.

Ammonia recovery downstream from the ammonia synthesis zone may be by condensation against cooling water, by refrigeration (e.g. to a temperature in the range of from about -20"C) or by absorption in a liquid absorbent such as water or a dilute aqueous ammonia solution.

In order that the invention may be clearly understood and readily carried into effect some preferred processes in accordance with the invention, together with plants suitable for carrying out such processes, will now be described, by way of example only, with reference to the accompanying drawings, wherein: Figure lisa diagrammatic flow sheet of a plant for the co-production of ammonia and methanol operating according to the present invention; Figures 2 and 3 are diagrammatic flow sheets of the respective ammonia synthesis sections of two modified forms of plant for the co-production of ammonia and methanol.

Referring to Figure 1 of the drawings, a plant for the co-production of ammonia and methanol is supplied at super-atmospheric pressure with natural gas via line 1.

After passage through desulphurisation unit 2, the desulphurised gas passes on via line 3 to a steam reformer 4. Steam is supplied via line 5 in the appropriate proportion. After passing through heat exchanger 6 located within the furnace of steam reformer 4, the natural gas/steam mixture passes on via line 7 through reformertubes 8 (only three of which are indicated in Figure 1 for the sake of clarity).

Natural gas is supplied to the burners of the furnace of steam reformer 4 via line 9 and the combustion products thereof pass to the flue stack via line 10. In accordance with conventional practice water can be passed through heat exchanger 11' located within the body of the furnace of steam reformer 4 in order to raise steam.

The gas mixture exiting the reformer tubes 8 passes on via line 11 to cooling stage 12 in which it is cooled by a coolant supplied via line 13. The partially cooled gases pass on via line 14to a second cooling stage 15 which is likewise supplied with coolant, via line 16. Although each of cooling stages 12 and 15 is shown in the drawing as a single stage, each of these stages may in practice comprise several heat recovery stages. Thus heat recovery from the gas mixture maybe effected in cooling stages 12 and 15, as appropriate by one or more of the following methods, inter alia: raising steam at elevated pressure; preheating boiler feed water; utilizing the sensible heat of the gas mixture to provide heat for distillation of product methanol or for other process uses; and air cooling. As is known in the art air cooling can be used as the final stage of cooling stage 15.The cooled gases from cooling stage 15 are then compressed by means of first stage compressor 17 to an intermediate pressure and pass on via line 18 to second stage compressor 19 which serves to bring the gas up to methanol synthesis pressure.

The stoichiometric ratio of hydrogen;carbon monoxide for the synthesis of methanol is 2:1, but in practice the ratio is usually increased to about 2.25:1.

If it is desired to produce a greater amount of ammonia than would be given by the normal H2:CO ration leaving the reformer tubes 8, then the reformed gas mixture may, if desired, be subjected to a shift reaction in shift converter 20. In this case, valve 21 in line 14 is closed and valves 22 and 23 are opened so that the gas passes from cooling stage 12 to second cooling stage 15 via line 24, shift converter 20 and line 25.

The compressed gases pass via line 26 to methanol converter 27 which contains a suitable methanol synthesis catalyst, for example, the methanol synthesis catalyst sold by Imperial Chemical Industries Limited under designation 51/2. According to the manufacturer's published literature this is a copper-based catalyst which contains A1203 and ZnO as promoters. As an example of a suitable form of methanol synthesis reactor there can be mentioned the form of reactor illustrated in Figure 5 on page 377 of volume 13 of "Kirk-Othmer's Encyclopaedia of Chemical Technology", Second Edition, published by John Wiley & Sons, Inc.The pressure at the inlet to the methanol synthesis reactor 27 lies, for example, in the range of from about 50 kg/cm2 absolute to about 110 kg/cm2 absolute whilst the temperature ranges from about 230"C to about 270"C. The temperature and pressure conditions in the methanol synthesis zone are not critical factors of the invention, so long as they are conducive to efficient formation of methanol. The optimum temperature and pressure conditions depend on the nature of the chosen catalyst.

The gaseous reaction mixture passes on via line 28 through a cooling stage (not shown) to knock-out pot 29. Unreacted gases are recycled via line 30 to line 26. Methanol product is removed via line 31 and can be further purified, for example by distillation in conventional manner.

A purge stream of unreacted gases consisting mainly of hydrogen, together with unreacted methane, minor amounts of carbon monoxide, carbon dioxide and nitrogen and traces of water, methanol and dimethyl ether is taken via line 32 and passed to a pressure swing adsorption unit. In Figure 1 two pressure swing adsorber vessels are shown; in practice four or more pressure swing adsorber vessels may be requires. Suitable pressure swing adsorption plants are sold, for example, by Union Carbide Corporation.

The valves and other items of ancillary equipment required for operation of the pressure swing adsorption unit are not shown in Figure 1 for the sake of clarity. These will be provided in accordance with conventional practice.

If desired, the gas in line 32 can be subjected to an optional CO2 removal step in absorber 35 by closing valve 36 and opening valves 37 and 38. In this case, gas passes along lines 32 and 39 to absorber 35 and thence to pressure swing adsorber vessels 33 and 34.

Another optional step in the methanol synthesis section of the plant is the removal of carbon dioxide before the methanol synthesis reactor. This can be effected, for example, in unit 40. In this case, valve 41 is closed and valves 42 and 43 are opened so that gas from first stage compressor 17 passes through unit 40 and then via lines 44 and 18 to second stage compressor 19 and line 26 to methanol synthesis reactor 27.

Absorber 35 and/or unit 40 may, for example, comprise a washing stage e.g. a plant utilising a physical absorption process to remove CO2 from the gas and requiring little or no heat energy for the regeneration of the circulating liquid.

Air is supplied via line 45 to a high purity nitrogen plant 46. A suitable form of high purity nitrogen plant is that supplied by Petrocarbon Developments Limited of Petrocarbon House, Manchester M22 4TB. A stream of high purity nitrogen consisting, for example, of 99.9998% nitrogen is supplied from plant 46 line 47 to a primary compressor 48. Air which is enriched to some extent with oxygen, is exhausted to atmosphere via line 49 from plant 46.

Primary compressor 48 raises the pressure of the nitrogen to the exit pressure of the gas from pressure swing adsorber vessels 33 and 34. The compressed nitrogen passes from primary compressor 48 via line 50.

High purity hydrogen (preferably having a purity of at least 99.999%) exits pressure swing adsorber vessels 33 and 34 via line 51. The flows of high purity hydrogen and nitrogen in lines 50 and 51 are adjusted so as to produce a substantially 3:1 hydrogen nitrogen mixture. This gas mixture then passes to first stage ammonia compressor 52 and thence via line 53 to secondary ammonia compressor 54. The compressed mixture passes on via line 55 to cooling stage 56 which is supplied with cooling water via line 57.The cooled gases pass on via line 58 to ammonia converter 59 which contains a suitable ammonia synthesis catalyst, for example, l.C.l.Ltd's 35/4 catalyst and which is maintained at a temperature of, for example, from about 250"C to about 480"C and at a pressure of, for example, from about 100kg/cm2 absolute to about 380kg/cm2 absolute. The design of a suitable ammonia synthesis converter is described, for example, in an article by J.

B. Allen (Chemical & Process Engineering Sept. 1965, pages 473-483), as well as in the article on "Ammonia" in Ullman's Encyklopaedie der technischen Chemie, 4th Edition (1973) Vol.7, pages 445 to 513, published by Verlag Chemie GmbH. Suitable "cold shot" ammonia converters are illustrated, for example, in United States Patent Specification No.3254967 and in United Kingdom Specification No.1213045 as well as in Figure 24 on page 287 of Vol.2 of Kirk-Othmer's "Encyclopaedia of Chemical Technology", 2nd Edition, published by John Wiley & Sons, Inc.

From ammonia converter 59 the reaction mixture passes via line 60 to an air cooler 61 and thence via line 62 to ammonia separator 63. Unreacted gases are recycled via line 64, whilst ammonia product is removed via line 65.

If necessary, a purge stream can be taken via line 66. However, since the nitrogen and hydrogen introduced via lines 59 and 51 respectively are of extremely high purity (preferably at least 99.999% purity), it will normally be possible to dispense with any purge stream.

Line 67 is used to remove from pressure swing adsorber vessels 33 and 34 a largely hydrogen-free gas stream. This consists mainly of methane, together with small quantities of hydrogen, carbon monoxide, carbon dioxide and nitrogen and traces of water, methanol and dimethyl ether. This gas stream can be used as a fuel.

The plant of Figure 2 has a methanol synthesis section and a pressure swing adsorption unit that are identical to those of Figure 1. Its ammonia synthesis section is operated at a somewhat lower pressure than that of the plan of Figure 1. The high purity hydrogen from the pressure swing adsorption unit passes to the ammonia synthesis section along line 51.

As in the plant of Figure 1, air is supplied via line 45 to a high purity nitrogen plant 46. Air, somewhat enriched in oxygen is vented to atmosphere via line 49 whilst high purity nitrogen passes on via line 47 to primary compresser 48. The high purity nitrogen stream in line 50 combines in a ratio of substantially 1:3 with the high purity hydrogen stream in line 51 and with dried recycled gas in line 71 to pass on via line 72 to recirculator 73. From the recircu lator 73 the gas mixture passes via line 74 to ammonia converter 75. The gas exiting ammonia concerter 75 passes via line 76 through a cooling stage (not shown) to washing column 77 which contains a packing 78 of any suitable design.In washing column 78 the gas passes in counter current to a dilute aqueous ammonia solution which trickles down through packing 78 which is designed to promote intimate gas-liquid contact. For example, packing 78 can be of the bubble cap type.

The now essentially ammonia-free gas exits washing column 77 via line 79 and passes to drying stage 80 in which it is dried, for example, by contact with a suitable molecular sieve, before passing on via line 71 for recycle to recirculator 73.

From washing column 77 a concentrated aqueous ammonia solution is withdrawn via line 81; this passes to spray nozzles 82 mounted near the top of a stripping column 83 which is provided with a similar packing 84to packing 78. Column 83 is heated by means of heating coil 85 which is supplied with steam by means of line 86. Gaseous ammonia escapes from stripping column 83 via line 87 to condenser 88 which is supplied with cooling water via line 89. Liquid ammonia passes on via line 90 to catch pot 91. A portion of the liquid ammonia is removed as produce via line 93 whilst the remainder is recycled to stripping column 83 via line 94. Line 95 is a vent line.

Weter or dilute ammonia exits stripping column 83 at the bottom via line 96 and passes via cooling stage 97, which is supplied with cooling water via line 98 to spray nozzles 99 at the top of washing column 77; If necessary a purge stream can be taken from the gas recycle loop via line 100.

The plant of Figure 3 has a methanol synthesis section that is identical to that of Figure 1. As in the plant of Figure 1, the purge gas is passed through pressure swing adsorber vessels (not shown) and the resulting essentially pure hydrogen (preferably at least 99.999% pure) passes on via line 51.

As in the plants of Figures 1 and 2, air is supplied via line 45 to a high purity nitrogen plant 46. High purity nitrogen (preferably at least 99.999% pure) passes on via line 47 to primary compressor 48. Air, somewhat enriched in oxygen, is vented to atmosphere via line 49.

From primary compressor 48 the high purity nitrogen, now at essentially the same pressure as the high purity hydrogen in line 51, is admixed with the hydrogen in line 51 in a substantially 1:3 ratio and passes on to primary ammonia compression stage 52 and thence via line 53 to secondary ammonia compression stage 54. The 3:1 hydrogen:nitrogen gas mixture, now compressed to ammonia synthesis pressure passes on via line 55, is cooled in cooler 56 and is then passed to ammonia converter 59 via line 58. Line 57 indicates a cooling water supply line.

From ammonia synthesis converter 59 the reacted gas mixture passes on via line 60 to refrigeration zone 101 which is cooled by means of a refrigerant circulating through line 102. In refrigeration zone 101 the temperature is typically reduced to a value in the range of from about - 1 0"C to about -20"C. The refrigerated gas/liquid mixture passes on via line 103 to product separator 104. Liquid ammonia product is removed via line 105 whilst unreacted gas is recirculated via line 106.

The invention is further illustrated in the following Example.

Example - Natural gas, having the following compositions: Component Volume% CO2 0.1 CH4 92.9 N2 2.4 C2H5 3.4 C3H8 0.8 n-C4H10 0.2 C5H12 0.2 100.0% is supplied to the plant of Figure 1 via line 1 at a pressure of 40kg/cm2 absolute. Steam is supplied via line 5 at a steam :carbon ratio of 3 moles of steam: atom of carbon. The exit temperature from the reformer tubes 8 is 880"C.

399.738 rm3 per hour of make-up synthesis gas are supplied via line 18 to the methanol synthesis loop. (1 rm3 means 1 m3 dry gas measured at 20"C and at 1.03kg/cm2 absolute.) The inlet pressure to methanol synthesis reactor 28 is 81.57kg/cm2 absolute. 106.124 tonnes/hr of methanol (i.e., 2,547 tonnes/day) are removed via line 132.

The composition of the make-up gas is: Component Volume % CO 13.9 CO2 8.1 H2 72.0 CH4 5.4 N2 0.6 H2O Trace 100.0% The purge gas in line 32 has the following composition: Component Volume% CO 2.3 CO2 2.7 H2 76.2 CH4 16.5 N2 2.0 H20 Trace CH3OH 0.3 CH30CH3 Trace 100% The pressure swing adsorber unit has a hydrogen recovery efficiency of approximately 65-76%. The rate of supply of nitrogen via line 47 is selected so as to produce, upon admixture with the high purity nitrogen in line 51, an approximately 3:1 hydrogen:nitrogen mixture. The rate of production of ammonia is 32.13 tonnes/hour (i.e., 771.3tonnes/day).

Utilising the same rate of supply of natural gas via line 1, it is a simple matter to adjust the proportion of purge gas removed via line 32 compared to the volume of gas recycled via line 30 and thereby to vary the relative proportions of ammonia and methanol that are synthesised. For example, instead of producing 2,547 tonnes/day of methanol and 771.3 tonnes/day of ammonia, it is possible to design the plant of Figure 1 such that it is equally possible to operate it so as to produce 2,199 tones/day of methanol and 1,000 tonnes/day of ammonia or so as to produce 597 tonnes/day of methanol and 2,233 tonnes/day of ammonia.

Hence the invention permits the plant operator to vary output of ammonia and of methanol to meet market demands.

By utilising the process of the present invention for the production of 2,547 tonnes/day of methanol, it is possible to produce 771.3 tonnes/day of ammonia at an energy investment typically in the range of approximatey 6.3 x 106 to 6.7 x 106k cal/tonne. This energy may be supplied in the form of natural gas or other fuel to a gas turbine, or to a boiler for steam generation, or for generation of electricity, so as to provide the power required for the various gas compressors used in the process and also to replace the heat value of the hydrogen in the gas in line 32 which would, in a conventional methanol plant, be used typically as fuel for the reformer 4 and not as a potential source of hydrogen for ammonia manufacture.

By comparison, in a conventional ammonia synthesis plant, in which the synthesis gas is produced by steam reforming of a hydrocarbon followed by secondary reforming with air injection, followed by conventional shift reaction, removal of carbon dioxide and methanation, the heat investment amounts to approximately 8x1 O6kcals/tonne ammonia. If the pressure swing adsorption unit of Figure 1 were supplied directly with an H2/CO2 mixture produced by shift reaction of the output stream from the steam reformer, with no methanol production, the heat investment would be approximately7.83x106kcal/tonne ammonia.

Since energy savings of 0.1x106kcals/tonne ammonia are considered significant in the design of ammonia plant (see for example, the paper "Energy Reduction on a Modern Ammonia Plant" by Frank C. Brown, "ampo 78" paper 5), it will be appreciated by those skilled in the art that the present invention represents a significant advance in the art.

Claims (14)

1. A process for the co-production of ammonia and methanol which comprises: (a) providing a synthesis gas containing carbon oxides and hydrogen in a proportion and of a purity suitable for methanol synthesis; (b) contacting resulting synthesis gas in a methanol synthesis zone with a methanol synthesis catalyst maintained under temperature and pressure conditions conducive to synthesis of methanol; (c) recovering from the methanol synthesis zone a methanol-containing product stream; (d) separating from resulting methanol-containing product stream methanol and a methanol-depleted stream; (c) separating essentially pure hydrogen from at least a part of the methanol-depleted stream; (f) subjecting air to a nitrogen enrichment step so as to produce an essentially pure nitrogen stream;; (g) mixing essentially pure hydrogen from step (e) with essentially pure nitrogen from step (f) in a proportion suitable for ammonia synthesis; (h) contacting resulting mixed gases in an ammonia synthesis zone with an ammonia synthesis catalyst maintained under temperature and pressure conditions conducive to synthesis of ammonia; (i) recovering a gaseous ammonia-containing product stream from the ammonia synthesis zone; and (j) separating ammonia from resulting ammonia-containing product stream.

2. A process according to claim 1, in which in step (e) essentially pure hydrogen is separated from at least a part of the methanol-depleted stream by means of pressure swing absorption.

3. A process according to claim 1 or claim 2, in which in step (e) essentially the whole of the methanol-depleted stream is subjected to the hydrogen separation procedure.

4. A process according to claim 1 or claim 2, in which in step (e) only a part of the methanol-depleted stream is subjected to the hydrogen separation procedure and the remainder of the methanol-depleted stream is recycled to the methanol synthesis zone.

5. A process according to any one of claims 1 to 4, in which the essentially pure hydrogen is at least 99.999% pure.

6. A process according to any one of claims 1 to 5, in which the essentially pure hydrogen from step (e) is produced at a pressure of 35 kg/cm2 absolute or greater.

7. A process according to any one of claims 1 to 6, in which the essentially pure nitrogen from step (f) is at a lower pressure than that of the essentially pure hydrogen from step (e) and is pre-compressed to a pressure at least as high as that of the essentially pure hydrogen prior to admixture therewith.

8. A process according to any one of claims 1 to 7, in which in step (h) the pressure maintained in the ammonia synthesis zone corresponds substantially to the pressure at which the essentially pure hydrogen from step (e) is produced.

9. A process according to any one of claims 1 to 8, in which in step (j) ammonia recovery is effected by condensation against cooling water.

10. A process according to any one of claims 1 to 8, in which in step (j) ammonia recovery is effected by refrigeration.

11. A process according to any one of claims 1 to 8, in which in step (j) ammonia recovery is effected by absorption in a liquid absorbent.

12. A process according to claim 11, in which the liquid absorbent is water or a dilute aqueous ammonia solution.

13. A process for the co-production of ammonia and methanol conducted substantially as herein described and exemplified.

14. A process for the co-production of ammonia and methanol conducted substantially as herein described with particular reference to any one of Figures 1 to 3 of the drawings.