GB2536996A

GB2536996A - Process

Info

Publication number: GB2536996A
Application number: GB1600454.1A
Authority: GB
Inventors: David Pach John; Sheldon Daniel
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2015-02-05
Filing date: 2016-01-11
Publication date: 2016-10-05
Also published as: GB201600454D0; GB201501953D0

Abstract

The production of ammonium nitrate (NH4NO3) from ammonia and nitric acid (HNO3) comprising the steps of: a) providing a reaction gas mixture comprising hydrogen 20 and nitrogen 18; b) passing the reaction gas mixture over a catalyst to form ammonia 34; c) catalytically oxidising a first portion of the ammonia to form a gas mixture comprising nitrogen, oxygen and nitric oxide (NO); and d) converting the NO into HNO3 40, thereby forming a vent gas mixture 14 comprising nitrogen, oxygen and nitrogen oxides (NOx), wherein the process comprises treating the vent gas mixture to produce a nitrogen gas stream and using at least a portion of the nitrogen gas stream 18 (e.g. having less than 10 ppm oxygen) in the reaction gas mixture, and reacting a second portion of the ammonia recovered 44 in step (b) with a portion of the nitric acid recovered in step (d) to form NH4NO3 46. The hydrogen may be generated by steam reforming of hydrocarbons 30 to produce syngas, followed by water-gas shift (WGS) stage, carbon dioxide removal and methanation stage. The vent gas treatment of NOx may comprise reducing it with ammonia from the ammonia production unit 22 to form nitrogen.

Description

Process The present invention relates to a process for the production of ammonia and nitric acid. More particularly, it relates to an integrated process for the production of ammonia and nitric acid optionally with the production of ammonium nitrate. In particular, the present invention relates to a process in which air used for the production of nitric acid is treated to provide nitrogen for the production of ammonia.

Conventionally, ammonia is produced by the Haber-Bosch process in which hydrogen and nitrogen are reacted at high pressure. The overall stoichiometry for this reaction is: 3 H2 + N2.= 2 NH3 Typically the hydrogen is obtained by steam reforming hydrocarbon feedstocks, such as natural gas, in a process known as primary reforming to produce a stream comprising un-reacted hydrocarbon, hydrogen, carbon dioxide and carbon monoxide. Nitrogen may be provided from a number of sources but often is provided by secondary reforming the product of primary reforming with air to produce a raw synthesis gas. A catalytic water-gas shift conversion is then used to convert at least some of the carbon monoxide to carbon dioxide and form additional hydrogen. The carbon dioxide can then be removed. The remaining stream is often subjected to catalytic methanation to convert residual amounts of carbon monoxide to methane. The stream from the methanator, which will primarily consist of hydrogen and nitrogen, with trace amounts of methane, is then compressed and passed to the ammonia reactor in which the hydrogen is reacted with the nitrogen to form ammonia.

Nitric acid is typically produced by the catalytic oxidation of ammonia with excess air to generate nitric oxide, followed by the non-catalytic oxidation of the nitric oxide to nitrogen dioxide, which then is absorbed into water to form nitric acid. The overall stoichiometry for this reaction is: NH3+ 2 02 -> HNO3 + H2O Air is generally used as the source of oxygen although the oxygen content may be enriched or additional oxygen may have to be supplied prior to, or during, the reaction.

A portion of the ammonia may be reacted with a portion of the nitric acid to form ammonium nitrate. Ammonium nitrate is conventionally made by reacting ammonia with nitric acid in accordance with the following reaction: NH3 + HNO3 NH4NO3 Whilst ammonia is used in the production of nitric acid, they are produced separately and rely on separate feeds of air to supply the nitrogen and oxygen required as starting materials to the ammonia and nitric acid reactions respectively.

We have realised that one air feed may be used to provide the nitrogen for the ammonia production reaction and the oxygen for the nitric acid reaction.

Thus, according to the present invention there is provided a process for the production of ammonia and nitric acid, said process comprising the steps of: (a) providing a reaction gas mixture comprising hydrogen and nitrogen, (b) passing the reaction gas mixture over an ammonia synthesis catalyst in an ammonia converter to form ammonia and recovering the ammonia, (c) oxidising a first portion of the ammonia with air over an ammonia oxidation catalyst in an ammonia oxidation reactor to form a gas mixture comprising nitrogen, oxygen and nitric oxide, 15 and (d) converting the nitric oxide into nitric acid in a nitric acid production unit and recovering the nitric acid, thereby forming a vent gas mixture comprising nitrogen, oxygen and one or more nitrogen oxides, wherein the process comprises treating the vent gas mixture in a vent gas treatment unit to produce a nitrogen gas stream and using at least a portion of the nitrogen gas stream in the reaction gas mixture, and reacting a second portion of the ammonia recovered in step (b) with a portion of the nitric acid recovered in step (d) to form ammonium nitrate, and recovering the ammonium nitrate.

As the present invention utilises one air feed for both the production of the nitrogen for the ammonia reaction and the production of the oxygen for the nitric acid reaction, substantial benefits in the reduction of capital and operating costs are achieved. For example, one benefit of the present process is the ability to operate without an air-fed secondary reformer and without an air separation unit (ASU) which would be required to provide oxygen and nitrogen streams.

The reaction gas mixture in the present invention comprises a hydrogen stream. The hydrogen stream may be provided by a hydrogen production unit comprising a syngas generation stage, a water-gas shift stage, a carbon dioxide removal stage and an optional methanation stage.

The syngas generation stage produces a gas mixture comprising hydrogen, steam and carbon oxides, the water-gas shift stage converts carbon monoxide to carbon dioxide and increases the hydrogen content of the syngas. The carbon dioxide removal stage provides a hydrogen-rich stream and the optional methanation stage removes residual carbon monoxide and carbon dioxide to generate a hydrogen stream suitable for ammonia synthesis.

The syngas generation stage may be based on steam reforming of a hydrocarbon such as natural gas, naphtha or a refinery off-gas; or by the gasification of a carbonaceous feedstock, such as coal or biomass. Preferably the syngas generation stage comprises steam reforming a hydrocarbon. This may be achieved by primary reforming a hydrocarbon with steam in externally-heated catalyst-filled tubes in a fired-or gas-heated steam reformer and, where the methane content of the primary reformed gas is high, secondary reforming the primary-reformed gas mixture in a secondary reformer, by subjecting it to partial combustion with an oxygen-containing gas and then passing the partially combusted gas mixture through a bed of steam reforming catalyst. The oxygen-containing gas may be air, oxygen or oxygen-enriched air. The primary reforming catalyst typically comprises nickel at levels in the range 5-30% wt, supported on shaped refractory oxides, such as alpha alumina or magnesium-or calcium aluminates. If desired, catalysts with different nickel contents may be used in different parts of the tubes, for example catalysts with nickel contents in the range 5-15% wt or 30-85% wt may be used advantageously at inlet or exit portions of the tubes. Alternatively, structured catalysts, wherein a nickel or precious metal catalyst is provided as a coated layer on a formed metal or ceramic structure may be used, or the catalysts may be provided in a plurality of containers disposed within the tubes. Steam reforming reactions take place in the tubes over the steam reforming catalyst at temperatures above 350°C and typically the process fluid exiting the tubes is at a temperature in the range 650-950°C. The heat exchange medium flowing around the outside of the tubes may have a temperature in the range 900-1300°C. The pressure may be in the range 10-80 bar abs. In a secondary reformer, the primary-reformed gas is partially combusted in a burner apparatus mounted usually near the top of the reformer. The partially combusted reformed gas is then passed adiabatically through a bed of a steam reforming catalyst disposed below the burner apparatus, to bring the gas composition towards equilibrium. Heat for the endothermic steam reforming reaction is supplied by the hot, partially combusted reformed gas. As the partially combusted reformed gas contacts the steam reforming catalyst it is cooled by the endothermic steam reforming reaction to temperatures in the range 900-1100°C. The bed of steam reforming catalyst in the secondary reformer typically comprises nickel at levels in the range 5-30% wt, supported on shaped refractory oxides, but layered beds may be used wherein the uppermost catalyst layer comprises a precious metal, such as platinum or rhodium, on a zirconia support. Such steam reforming apparatus and catalysts are commercially available.

Alternatively, the steam reforming maybe achieved by passing a mixture of the hydrocarbon and steam through an adiabatic pre-reformer containing a bed of steam reforming catalyst and then passing the pre-reformed gas mixture to an autothermal reformer which operates in the same way as the secondary reformer to produce a gas stream containing hydrogen, carbon oxides and steam. In adiabatic pre-reforming, a mixture of hydrocarbon and steam, typically at a steam to carbon ratio in the range 1-4, is passed at an inlet temperature in the range 300- 620°C to a fixed bed of pelleted nickel-containing pre-reforming catalyst. Such catalysts typically comprise 40% wt nickel (expressed as NiO) and may be prepared by co-precipitation of a nickel-containing material with alumina and promoter compounds such as silica and magnesia. Again, the pressure may be in the range 10-80 bar abs.

Gasification of carbonaceous feedstock to produce a syngas may be achieved using known fixed bed, fluidised-bed or entrained-flow gasifiers at temperatures in the range 900-1700°C and pressures up to 90 bar abs. The crude synthesis gas streams require additional treatments known in the art to remove unwanted sulphur and other contaminants.

In a preferred process, the syngas generation stage comprises primary reforming a hydrocarbon, particularly natural gas, in a fired steam reformer to produce a gas stream comprising hydrogen, carbon monoxide, carbon dioxide and steam, and optionally a secondary reforming stage in which the primary reformed gas is further reformed in a secondary reformer using oxygen.

The gas stream comprising hydrogen, carbon monoxide, carbon dioxide and steam is preferably subjected to one or more catalytic water-gas shift stages by passing the gas mixture at elevated temperature and pressure over a water-gas shift catalyst. Any suitable catalytic shift conversion reactor and catalyst may be used. If insufficient steam is present, steam may be added to the gas stream before it is subjected to the water-gas shift conversion. The reaction may be depicted as follows; H2O + CO # H2 + CO2 The reaction may be carried out in one or more stages. The, or each, stage may be the same or different and may be selected from a high temperature shift process, a low temperature shift process, a medium temperature shift process and an isothermal shift process.

High temperature shift catalysts may be promoted iron catalysts such as chromia-or alumina-promoted magnetite catalysts. Other high temperature shift catalysts may be used, for example iron/copper/zinc oxide/alumina catalysts, manganese/zinc oxide catalysts or zinc oxide/alumina catalysts. Medium, low temperature and isothermal shift catalysts typically comprise copper, and useful catalysts may comprise varying amounts of copper, zinc oxide and alumina. Alternatively, where sulphur compounds are present in the gas mixture, such as synthesis gas streams obtained by gasification, so-called sour shift catalysts, such as those comprising sulphides of molybdenum and cobalt, are preferred. Such water-gas shift apparatus and catalysts are commercially available.

For high temperature shift catalysts, the temperature in the shift converter may be in the range 300-360°C, for medium temperature shift catalysts the temperature may be in the range 190300°C and for low-temperature shift catalysts the temperature may be 185-270°C. For sour shift catalysts the temperature may be in the range 200-370°C. The flow-rate of synthesis gas containing steam may be such that the gas hourly space velocity (GHSV) through the bed of water-gas shift catalyst in the reactor may be 6000 hour'. The pressure may be in the range 10-80 bar abs.

In a preferred embodiment, the water-gas shift stage comprises a high temperature shift stage or a medium temperature shift stage or an isothermal shift stage with or without a low temperature shift stage.

The shifted gas mixture may be subjected to a carbon dioxide removal stage. Any suitable carbon dioxide removal unit may be used. Carbon dioxide removal units may function by reactive absorption, such as those known as aMDEATm or BenfieldTM units that are based on using regenerable amine or potassium carbonate washes, or by physical absorption, based on using methanol, glycol or another liquid at low temperature, such as RectisolTM, SelexolTM units. Carbon dioxide removal may also be performed by pressure-swing adsorption (PSA) using suitable solid adsorbent materials. PSA may be preferred where the shifted gas mixture contains methane. The carbon dioxide removal unit may also function to simultaneously remove residual steam, primarily by condensation due to the low operating temperatures. Such carbon dioxide removal apparatus and solvents are commercially available. Some or all of the carbon dioxide formed in the shifted gas mixture may be removed to produce a gas stream comprising mainly hydrogen and nitrogen with low levels of carbon monoxide. The carbon dioxide removed by the carbon dioxide removal unit may be sold, or it may be captured and stored using known CCS techniques or it may be used in enhanced oil recovery processes or, less desirably, emitted as effluent from the process.

In a methanation stage, residual carbon monoxide and carbon dioxide in the hydrogen gas stream may be converted to methane over a methanation catalyst in a methanator. Any suitable arrangement for the methanator may be used. Thus the methanator may be operated adiabatically or isothermally. One or more methanators may be used. A nickel-based methanation catalyst may be used. For example, in a single methanation stage the gas from the carbon dioxide removal stage may be fed at an inlet temperature in the range 200-400°C to a fixed bed of pelleted nickel-containing methanation catalyst. Such catalysts are typically pelleted compositions, comprising 20-40% wt nickel. Such methanation apparatus and catalysts are commercially available. The pressure for methanation may be in the range 10-80 bar abs.

The hydrogen gas stream recovered from the hydrogen production unit is combined with a nitrogen stream recovered from the vent gas treatment unit, compressed to the ammonia synthesis pressure and passed to an ammonia production unit. If air or oxygen-enriched air is used in the syngas generation step then some nitrogen may already be present in the hydrogen gas stream, but preferably the hydrogen stream is free of nitrogen so that essentially all of the nitrogen in the reaction gas mixture is provided by the vent gas mixture.

The ammonia production unit comprises an ammonia converter containing an ammonia synthesis catalyst. The nitrogen and hydrogen react together over the catalyst to form the ammonia product. Ammonia synthesis catalysts are typically iron based but other ammonia synthesis catalysts may be used. The reactor may operate adiabatically or may be operated isothermally. The catalyst beds may be axial and/or radial flow and one or more beds may be provided within a single converter vessel. The conversion over the catalyst is generally incomplete and so the synthesis gas is typically passed to a loop containing a partially reacted gas mixture recovered from the ammonia converter and the resulting mixture is fed to the catalyst. The synthesis gas mixture fed to the loop may have a hydrogen to nitrogen ratio of 2.2-3.2. In the ammonia production unit, the hydrogen/nitrogen mixture may be passed over the ammonia synthesis catalyst at high pressure, e.g. in the range 80-350 bar abs, preferably 150-350 bar abs for large-scale plants, and a temperature in the range 300-540°C, preferably 350-520°C.

A purge gas stream containing methane and hydrogen may also be taken from the loop and fed to the reaction gas generation step or used as a fuel. In a preferred embodiment, methane and/or hydrogen recovered from the purge gas may be used to remove oxygen from the vent gas mixture according to methods described below.

Where the catalyst to be used in the ammonia production unit is poisoned by oxygen or its compounds, the oxygen content of the nitrogen stream produced by the vent gas treatment unit will generally be 10 ppm or less and may be less than about 5 ppm, or less than about 1 ppm.

A nitric acid production unit is fed with a first portion of the ammonia from the ammonia production unit. The nitric acid production unit generally comprises an ammonia oxidation vessel and an absorption unit. The ammonia oxidation vessel comprises an ammonia oxidation catalyst over which a mixture of the first portion of ammonia and air at an ammonia concentration typically of 8-12% vol, is reacted at 700-1000°C and a pressure in the range 1-50 bar abs to produce a gas mixture comprising nitrogen, oxygen and nitrogen oxides including nitric oxide (NO). For safety reasons, the process is operated with an excess of air to avoid ammonia slip. The ammonia oxidation catalyst may comprise one or more platinum-based ammonia oxidation gauze catalysts. Additionally or alternatively a particulate cobalt-based ammonia oxidation catalyst may be used. Such catalysts may advantageously produce lower levels of nitrous oxide (N20), which is a potent greenhouse gas.

The nitric oxide produced catalytically in the ammonia oxidation vessel reacts downstream of the catalyst with oxygen present in the product gas mixture and/or with air, oxygen or oxygen-enriched air added to the product gas downstream of the burner in a bleacher unit to produce nitrogen dioxide (NO2). The product gas mixture comprising nitrogen dioxide is passed to an absorption unit where it is contacted with water in which it dissolves to form nitric acid (HNO3). The nitric acid may be recovered from the absorber using known methods. Such nitric acid production apparatus and catalysts are commercially available.

A vent gas mixture may be recovered from the absorption unit of the nitric acid production unit. The vent gas mixture comprises nitrogen, oxygen and one or more nitrogen oxides. The vent gas is passed to a vent gas treatment unit where oxygen and nitrogen oxides are removed to provide a nitrogen stream suitable for the ammonia synthesis stage. The residual nitrogen oxides present in the vent gas mixture may comprise NO, NO2 and N20. Preferably, the nitrogen oxides present in the vent gas are also removed in the vent gas treatment unit. NO and NO2 are desirably removed to avoid the possibility of forming ammonium nitrate or ammonium nitrite in the ammonia synthesis loop.

Any suitable vent gas treatment unit may be used. Suitable units include those utilising catalytic and/or combustion processes. In combustion processes, the vent gas and a fuel gas such as hydrogen and/or a hydrocarbon may be passed to a vent gas treatment unit comprising a combustion reactor where the hydrogen and/or a hydrocarbon is combusted catalytically or non-catalytically with oxygen in the vent gas mixture to form a combusted gas mixture. The nitrogen oxides may be reduced in the same unit over a suitable reduction catalyst. Hence, the vent gas may be treated in a catalytic combustion reactor containing a non-selective reduction catalyst to promote the reduction of nitrogen oxides, typically in the form of pellets or a shaped honeycomb structure. The reduction catalyst may comprise a precious metal selected from platinum, palladium and rhodium; or comprise one or more of vanadium pentoxide, iron oxide and titanium. The vent gas and a fuel gas are fed to the reactor above the ignition temperature of the fuel, in the case of hydrogen typically 150-200°C; higher with other fuels up to about 450°C or higher. The exothermic combustion reactions can cause operating temperatures in excess of 800°C. The maximum temperature can be managed by using a plurality of catalytic beds, each with a dedicated fuel supply and/or inter-stage cooling. The fuel is typically supplied in excess as the reaction with oxygen is preferential to the NOx reduction reactions. Methane and/or hydrogen are particularly suitable fuels for the combustion and reduction reactions. In a preferred embodiment, the hydrogen and/or methane are recovered from the ammonia production unit, e.g. from a purge gas.

Where methane is used as the fuel, the following reactions may occur with the oxygen-containing impurities: CH4+ 02 CO2+ 2 H2O CH4+ 4 NO -> CO2+ 2 H20 + 2 N2 CH4+ 2 NO2-) CO2+ 2 H20 + N2 Other reactions may also take place.

Where hydrogen is used as the fuel, the following reactions may occur with the oxygen-containing impurities: 2 H2 + 02 -^ 2 H20 H2 + NO2 -> NO + 2 H20 2 H2 + 2NO N2 + 2 H2O H2 + N20 N2 + H2O Such apparatus and catalysts are commercially available.

Alternatively or in addition the nitrogen stream may be produced by passing the vent gas or the combusted gas mixture and a reductant such as ammonia to a vent gas treatment unit comprising a reduction reactor containing a selective reduction catalyst and selectively reducing the nitrogen oxides to nitrogen over the reduction catalyst. Hence, the vent gas may be treated in a selective catalytic reduction reactor containing selective reduction catalyst typically in the form of shaped honeycombs or catalyst-coated flat plates comprising base metal oxides, preferably titanium or vanadium oxides. Platinum and palladium selective reduction catalysts may also be used. The catalyst may also be a zeolite. The reductant may be added to the gas steam and passed to the catalyst in the range 210-410°C. An excess of reductant may be used to maximise conversion. Ammonia is a particularly suitable reductant, which may conveniently be recovered from the ammonia production unit.

Where ammonia is used as the reductant, the following reactions may occur with the nitrogen oxides: 4 NH3 + 6 NO -> 5 N2 + 6 H20 8 NH3 + 6 NO2-, 7 N2 + 12 H2O Any suitable catalysts and reaction conditions may be used. However, in the present invention the production of ammonia as a side reaction or the presence of ammonia in the treated vent gas is not deleterious as it may be passed to the ammonia production unit.

The stream from the vent treatment unit may be supplied directly to the ammonia production unit. However, in some circumstances there may be advantages in supplying it to a water gas shift converter and/or a carbon dioxide removal unit within the synthesis gas production unit, such than any carbon monoxide or carbon dioxide present can be removed. Water may also be present in the stream from the vent gas treatment unit. Where present, any water may be recovered in chilling or compression stages before feeding the nitrogen gas stream to the ammonia production unit.

The vent gas stream may contain nitrogen in excess of that required for the ammonia production step. Excess nitrogen recovered from the vent gas treatment unit that is not required for the ammonia production, is suitably pure for use in a nitrogen production unit. Therefore excess nitrogen-containing gas from the vent gas treatment may be sent to a nitrogen production unit.

The ammonia produced in the ammonia production unit and the nitric acid produced in the nitric acid production unit may then be passed to an ammonium nitrate production unit where they may be reacted to produce ammonium nitrate. Any suitable process for producing ammonium nitrate may be used.

The present invention will now be described by way of example with reference to the accompanying drawing in which Figure 1 is a schematic representation of a process according to one aspect of the present invention, in which the ammonia produced in the ammonia production unit is used in the nitric acid production unit and the ammonia and nitric acid are utilised in the production of ammonium nitrate.

It will be understood by those skilled in the art that the drawings are diagrammatic and that further items of equipment such as reflux drums, pumps, vacuum pumps, temperature sensors, pressure sensors, pressure relief valves, control valves, flow controllers, level controllers, holding tanks, storage tanks, and the like may be required in a commercial plant. The provision of such ancillary items of equipment forms no part of the present invention and is in accordance with conventional chemical engineering practice.

In Figure 1, air is fed in line 10 to a nitric acid production unit 12. A nitrogen-rich vent gas mixture stream is removed in line 14 and passed to the vent gas treatment unit 16 where oxygen and nitrogen oxides are removed. A purified nitrogen stream in line 18 is combined with a hydrogen stream 20 and the mixture passed to an ammonia production unit 22. The vent gas treatment unit 16 comprises a combustion reactor and/or an oxidation reactor, wherein the oxygen and nitrogen oxides in the vent gas mixture 14 are combusted and/or oxidised catalytically with a fuel gas comprising hydrogen and/or methane. Alternatively, the vent gas treatment unit 16 comprises a reduction reactor containing a reduction catalyst, wherein the oxygen and nitrogen oxides in the vent gas mixture 14 are reacted with ammonia over the reduction catalyst. The vent gas treatment unit 16 provides a purified nitrogen gas stream comprising 10 ppm or less of oxygen. Excess nitrogen-containing stream is removed from the treatment unit 16 in line 24.

Hydrogen stream 20 is produced by passing natural gas in line 26 and steam in line 28 into a hydrogen production unit 30 comprising a primary steam reformer and optionally an oxygen-fed secondary reformer, where they are reacted in a steam reforming reaction to produce a syngas comprising hydrogen and carbon oxides. The reformed gas mixture is subjected to one or more water-gas shift stages to convert the carbon monoxide to carbon dioxide and increase the hydrogen content of the syngas. Following carbon dioxide absorption from the syngas, a methanation stage may be included to reduce the carbon monoxide levels of the hydrogen stream. The by-product carbon dioxide is recovered from the hydrogen production unit 30 in line 32. The nitrogen gas stream recovered in line 18 is supplied to the ammonia production unit 22 with the hydrogen gas stream in line 20 obtained from the hydrogen production unit 30. Here they are reacted in the ammonia production unit 22 under suitable conditions to produce ammonia.

Ammonia is recovered from the ammonia production unit 22 via line 34. Hydrogen and/or methane recovered from a purge gas 36 taken from the ammonia production unit 22 may be returned to the hydrogen production unit 30 or may be fed to the vent gas treatment unit 16 as a fuel. A first portion of the recovered ammonia 34 is fed via line 38 to the nitric acid production unit 12 where it is reacted with air 10 to form nitric oxide. Reactions take place in the nitric acid production unit 12 under suitable conditions to produce nitric acid from the nitric oxide. The produced nitric acid is recovered via 40 and supplied to an ammonium nitrate production unit 42 where it is reacted with a second portion of the recovered ammonia 34, fed to the ammonium nitrate production unit 42 by line 44. In the ammonium nitrate production unit 42, the ammonia 44 and the nitric acid 40 are reacted under suitable conditions to form ammonium nitrate which is removed in line 46.

The present invention will now be described with reference to the following examples.

Example 1

In a process depicted in Figure 1, 3955 mtpd air is fed to the nitric acid production unit 12.

About 3035 mtpd of a nitrogen-rich vent gas stream is removed therefrom and passed to the vent gas treatment unit 16. After treatment, 380 mtpd of a purified nitrogen-rich stream is removed from the vent gas treatment unit 16 combined with a hydrogen stream and passed to the ammonia production unit 22. 80 mtpd hydrogen is recovered from the hydrogen production unit 30 and passed to the ammonia production unit 22 where it is reacted with the nitrogen-rich stream from line 18 to form 450 mtpd ammonia. Of this, 225 mtpd is passed to the nitric acid production unit 12 and 225 mtpd is passed to the ammonium nitrate production unit 42. In the ammonium nitrate production unit 42, the 225 mtpd ammonia is reacted with 780 mtpd nitric acid recovered from the nitric acid production unit 12 to produce 1000 mtpd of ammonium nitrate.

Claims

Claims 1. A process for the production of ammonia and nitric acid, comprising the steps of: (a) providing a reaction gas mixture comprising hydrogen and nitrogen, (b) passing the reaction gas mixture over an ammonia synthesis catalyst in an ammonia converter to form ammonia and recovering the ammonia, (c) oxidising a first portion of the ammonia with air over an ammonia oxidation catalyst in an ammonia oxidation reactor to form a gas mixture comprising nitrogen, oxygen and nitric oxide, and (d) converting the nitric oxide into nitric acid in a nitric acid production unit and recovering the nitric acid, thereby forming a vent gas mixture comprising nitrogen, oxygen and one or more nitrogen oxides, wherein the process comprises treating the vent gas mixture in a vent gas treatment unit to produce a nitrogen gas stream and using at least a portion of the nitrogen gas stream in the reaction gas mixture, and reacting a second portion of the ammonia recovered in step (b) with a portion of the nitric acid recovered in step (d) to form ammonium nitrate, and recovering the ammonium nitrate.
2. A process according to claim 1 wherein the hydrogen in the reaction gas mixture is provided by a hydrogen production unit comprising a syngas generation stage, a water-gas shift stage, a carbon dioxide removal stage and optionally a methanation stage.
3. A process according to claim 2 wherein the syngas generation stage is based on steam reforming of a hydrocarbon such as natural gas, naphtha or a refinery off-gas; or by the gasification of a carbonaceous feedstock, such as coal or biomass.
4. A process according to claim 2 or claim 3 wherein the syngas generation stage comprises steam reforming a hydrocarbon by primary reforming in a fired or gas-heated steam reformer.
A process according to any one of claims 1 to 4 wherein essentially all of the nitrogen in the reaction gas is provided by the vent gas mixture.
6. A process according to any one of claims 1 to 5 wherein the vent gas treatment unit comprises a combustion reactor containing a non-selective reduction catalyst wherein the oxygen is combusted and the nitrogen oxides reduced with a fuel over the catalyst to form a combusted gas mixture.
A process according to claim 6 wherein the fuel is a gas stream comprising hydrogen and/or methane, preferably hydrogen and/or methane recovered from the ammonia production unit.
8. A process according to any one of claims 1 to 5 wherein the vent gas treatment unit comprises a reduction reactor containing a selective reduction catalyst, wherein the nitrogen oxides are reacted with a reductant over the reduction catalyst.
9. A process according to claim 8 wherein the reductant is ammonia, preferably ammonia recovered from the ammonia production unit.
10. A process according to any one of claims 1 to 9 wherein the nitrogen gas stream used to form at least part of the reaction gas mixture comprises 10 ppm or less of oxygen.
11. A process according to any one of claims 1 to 10 wherein excess nitrogen recovered from the vent gas treatment unit is sent to a nitrogen production unit.