GB2573885A

GB2573885A - Process

Info

Publication number: GB2573885A
Application number: GB1905369.3A
Authority: GB
Inventors: John Cousins Matthew; Gray David; David Pach John
Original assignee: Johnson Matthey PLC
Current assignee: Johnson Matthey PLC
Priority date: 2018-05-14
Filing date: 2019-04-16
Publication date: 2019-11-20
Also published as: GB201807759D0; WO2019220074A1; GB201905369D0

Abstract

A method for revamping an ammonia production facility said ammonia production facility having a front end for producing a synthesis gas comprising nitrogen and hydrogen from a hydrocarbon feedstock, comprising a fired primary steam reformer 16, an air-fed secondary reformer, a water gas shift unit 26, and a carbon dioxide removal unit connected in series, said method comprising the steps of: installing a gas-heated reformer 54 having tube and shell sides in parallel to the fired steam reformer: connecting the tube side of the gas-heated reformer to an inlet of the secondary reformer 20; connecting an outlet of the secondary reformer to the shell side of the gas-heated reformer: installing an ASU 76 with an oxygen outlet 80 and a nitrogen outlet 72: disconnecting the air feed to the secondary reformer: and connecting the oxygen stream outlet of the ASU to the secondary reformer and the nitrogen outlet of the ASU to the front end downstream of one or more of the water gas shift unit, the carbon dioxide removal unit 30 and the methanation unit 36, and upstream of an ammonia synthesis unit.

Description

The present invention relates to a method for revamping an ammonia plant.

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:

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 unreacted hydrocarbon, hydrogen, carbon dioxide and carbon monoxide. Nitrogen is often provided by secondary reforming the product of primary reforming with air to produce a raw synthesis gas. Catalytic water-gas shift conversion is then used, typically in two stages including a high-temperature shift stage over a bed of an iron-containing catalyst, to convert at least some of the carbon monoxide to carbon dioxide and form additional hydrogen. The carbon dioxide then be removed, for example by absorption. The remaining stream is often subjected to catalytic methanation to convert residual amounts of carbon monoxide and carbon dioxide 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.

WO2015067436 discloses a procedure for revamping a front-end of an ammonia plant for producing ammonia make-up synthesis gas, wherein the front-end includes a primary reformer, a secondary reformer, a shift conversion section, a CO2 removal section and optionally a methanation section; in which a shell-and-tube gas-heated reformer is installed after the secondary reformer, a portion of the available feedstock is reformed in the tubes of the gasheated reformer, and heat is provided to the shell side of the gas-heated reformer from at least a portion of product gas leaving the secondary reformer, possibly mixed with product gas leaving the tubes of said gas-heated reformer. The secondary reformer is fed with air or oxygen-enriched air to provide the nitrogen in the synthesis gas. This has the disadvantage that the shift conversion and CO2 removal sections have to be very large to accommodate the additional flow from the enlarged reforming section.

We have realised that an alternative method may be employed that overcomes the problems of this method.

Accordingly the invention provides a method is for revamping an ammonia production facility said ammonia production facility having a front end for producing a synthesis gas comprising nitrogen and hydrogen from a hydrocarbon feedstock, said front end comprising a fired primary steam reformer, an air-fed secondary reformer, a water gas shift unit, a carbon dioxide removal unit and optionally a methanation unit connected in series, said method comprising the steps of: (i) installing a gas-heated reformer having a tube side and a shell side in parallel to the fired steam reformer; (ii) connecting the tube side of the gas-heated reformer to an inlet of the secondary reformer; (iii) connecting an outlet of the secondary reformer to the shell side of the gas-heated reformer; (iv) installing an air separation unit with an oxygen stream outlet and a nitrogen stream outlet; (v) disconnecting the air feed to the secondary reformer; and (vi) connecting the oxygen stream outlet of the air separation unit to the secondary reformer and the nitrogen stream outlet of the air separation unit to the front end downstream of one or more of the water gas shift unit, the carbon dioxide removal unit and the methanation unit, and upstream of an ammonia synthesis unit.

The invention further provides a process for producing a synthesis gas comprising hydrogen and nitrogen in the front end of an ammonia production facility comprising the steps of: (i) passing a first portion of a mixture of hydrocarbon feedstock and steam through externallyheated catalyst filled tubes in a fired steam reformer to form a first crude synthesis gas; (ii) passing a second portion of the mixture of gaseous hydrocarbon feedstock and steam through externally-heated catalyst filled tubes in a gas-heated reformer having a shell-side and a tube side to form a second crude synthesis gas; (iii) passing the first and second crude synthesis gases to a secondary reformer and secondary reforming the gases to form a hot secondary reformed gas stream; (iv) passing the hot secondary reformed gas stream to the shell-side of the gas heated reformer to form a cooled secondary reformed gas; (v) subjecting the cooled secondary reformed gas to the water-gas shift reaction in a water-gas shift unit to form a shifted gas stream; (vi) removing carbon dioxide from the shifted gas stream in a carbon dioxide removal unit to form a hydrogen gas stream; and (vii) optionally subjecting residual carbon oxides in the hydrogen gas stream to methanation in a methanation unit, wherein the secondary reforming is performed using an oxygen gas stream recovered from an air separation unit, and a nitrogen gas stream from the air separation unit is fed to the front end of the ammonia production facility downstream of one or more of the water-gas shift unit, the carbon dioxide removal unit and the methanation unit, and upstream of an ammonia synthesis unit.

The invention further comprises an ammonia production process including the synthesis gas production process.

The term “revamping” in the present application means a method of adapting an existing ammonia production facility and process to increase the ammonia production or process efficiency.

The method and process are distinguished from that in WO2015067436 by the installation of the gas heated reformer in parallel to the primary reformer, such that the reformed gases from both the primary reformer and gas-heated reformer are fed to the secondary reformer, and by the use of an oxygen stream, rather than air or enriched air in the secondary reformer. The process of the present invention also produces a synthesis gas having a lower methane content, which is advantageous.

The hydrocarbon feedstock may be natural gas, naphtha or a refinery off-gas. Natural gas is preferred. The hydrocarbon feedstock is desirably passed through a purification unit upstream of the steam reformers for the removal of contaminants that reduce the effectiveness of the steam reforming catalysts. Preferably the hydrocarbon feedstock is desulphurised. Methods and adsorbents for the desulphurisation of hydrocarbons are known.

The hydrocarbon feedstock is combined with steam. The steam may be added directly to the hydrocarbon feedstock by injection or by passing the hydrocarbon through a saturator. The steam to carbon ratio, i.e. the molar ratio of steam to hydrocarbon carbon in the hydrocarbon feedstock may be in the range 2-5. The steam to carbon ratio in the feed to the primary reformer is preferably in the range 2.5-3.5. The steam to carbon ratio in the feed to the gas heated reformer may be the same or different to that fed to the primary reformer and may advantageously be lower, preferably in the range 2-3.5.

The fired steam reformer may be any fired steam reformer such as a top-fired or side-fired steam reformer. A feed gas comprising hydrocarbon feedstock and steam, and optionally carbon dioxide, is passed through externally heater, catalyst-filled tubes to reform the hydrocarbon into a first crude synthesis gas containing hydrogen, carbon monoxide, carbon dioxide and steam. In a fired steam reformer, the catalyst-filled tubes are externally heated by combusting a fuel gas with air. The heating of the tubes is primarily by radiation. A primary reforming catalyst typically comprises nickel at levels in the range 1-30% wt, supported on shaped refractory oxides, such as alpha alumina or magnesium- or calcium aluminates. Alternatively, structured catalysts, wherein a nickel or precious metal catalyst is provided as a coated surface 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 steam reformer is fired, i.e. heated by the combustion of a gaseous fuel mixture flowing around the outside of the tubes. The hot combustion gases may have a temperature in the range 9001300°C. The pressure may be in the range 10-80 bar abs. The fired steam reformer is a primary reformer in that it provides a first reforming step. The revamp method does not change the primary reformer itself but may adjust the amount of hydrocarbon and steam mixture fed to it.

A gas-heated reformer is installed into the existing front end as part of the revamp method. The gas heated reformer may be a heat exchange reformer having a tube side through which the reactants flow, and a shell side through which a heating gas is passed to heat the tubes. The gas-heated reformer therefore may have a tube side inlet for the hydrocarbon and steam mixture, a tube side outlet for discharging the reformed gas mixture, a shell-side inlet for receiving the hot secondary reformed gas and a shell side outlet for discharging the cooled secondary reformed gas. The tube side may be separated from the shell side, for example by tube sheets or otherwise, to create a heat exchange zone in which the tubes are heated. The gas-heated reformer contains a plurality of externally heated catalyst-filled tubes. The tubes are heated by a hot gas passing around the tubes and therefore the heating is primarily by convection. Nevertheless, the catalysts and operating conditions may be similar to those in a fired steam reformer. The second crude synthesis gas formed in the tubes of the gas heated reformer also comprises hydrogen, carbon monoxide, carbon dioxide and steam. The compositions of the first and second crude synthesis gases may be the same or different. Because the gas heated reformer is upstream of the secondary reformer, the tubes do not discharge reformed gas into the heat exchange zone.

The installation of the gas-heated reformer increases the capacity of the process to make additional synthesis gas and consequently more ammonia.

The secondary reformer is revamped by connecting it to a feed line from the gas-heated reformer, by disconnecting the air feed and connecting an oxygen gas stream line from an installed air separation unit. In the secondary reformer, the primary-reformed gas mixture and the reformed gas mixture from the gas-heated reformer are partially combusted with the oxygen gas stream in a burner apparatus mounted usually near the top of the reformer. Steam may be added to the oxygen gas stream. The partially combusted reformed gas is then passed adiabatically through a bed of a steam reforming catalyst disposed within the secondary reformer 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 9001100°C. The bed of steam reforming catalyst in the secondary reformer typically comprises nickel at levels in the range 1-30% 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.

Whereas the first and second crude synthesis gas streams may be provided separately to the secondary reformer, it may be desirable to combine them and connect the secondary reformer to a mixed reformed gas feed comprising the primary reformed gas and the reformed gas from the gas-heated reformer.

The hot secondary reformed gas is passed to the gas-heated reformer as a heat exchange medium to heat the catalyst filled tubes and thereby form the cooled secondary reformed gas. The cooled secondary reformed gas is further processed in the front end to form the synthesis gas for the ammonia synthesis.

The air separation unit installed in the revamp method may be any air separation apparatus that produces high purity nitrogen and oxygen gas streams. The oxygen gas stream preferably has a purity of >90% vol O2, more preferably >92% vol O2, with a nitrogen content, for example of <1% vol. The purity of the nitrogen stream is preferably >90% vol, ISfimore preferably >95% vol N2, more preferably > 98% vol N2. The oxygen gas stream outlet of the air separation unit is connected by a line or conduit to the secondary reformer. The nitrogen stream outlet from the air separation unit is connected to the front end downstream of one or more of the water gas shift unit, the carbon dioxide removal unit and an optional methanation unit and upstream of an ammonia synthesis unit. In the present invention, the nitrogen is not passed through the water gas shift unit. Where the nitrogen stream is divided and fed downstream of each of the water-gas shift unit and the carbon dioxide removal unit or methanation unit, preferably >51% vol, more preferably >75% vol, most preferably > 90% vol, of the nitrogen required for ammonia synthesis is fed downstream of the carbon dioxide removal unit or methanation unit. Preferably all of the nitrogen stream is fed downstream of the carbon dioxide removal unit or methanation unit, if present, as this reduces the impact of the additional synthesis gas on the existing equipment.

The existing secondary reformer may not have the ideal dimensions for the increased volume of the reformed gases or a burner suitable for using the oxygen gas stream. Therefore, the revamp method may include replacing the burner in the secondary reformer with a burner adapted for use with an oxygen gas stream, or replacing the secondary reformer entirely with a secondary or autothermal reformer having different dimensions and containing a different burner adapted for use with an oxygen gas stream.

A secondary reformer contains two reaction zones. The first zone is defined by the region between the burner and the inlet to the catalyst bed. The burner in a secondary reformer is often located at the tip of a neck region, so that it discharges into a volume in the shape of a frustum cone, with a vertical axis and the larger diameter at its base. In this zone, the process gas and process oxidant mix together and the oxygen is consumed. The second reaction zone is defined by the bed of secondary reforming catalyst. This zone is typically cylindrical in shape, with the cylinder axis arranged vertically. In this second zone, residual hydrocarbons present in the process gas are reformed with steam to generate synthesis gas.

An objective in the design of a secondary reformer is to ensure uniform conditions, both spatially and temporally, of the temperature and composition of the process gas stream leaving a first reaction zone (characterized by homogenous, gas phase combustion reactions) and entering a second reaction zone (characterized by heterogenous, catalyst- supported reforming reactions). Non-uniform conditions can manifest themselves as “hot spots”, which can lead to catalyst damage and/or loss of catalyst activity. On the other hand, uniform conditions at the inlet to the catalyst bed can provide an optimum performance of the catalyst bed, for example as measured by the approach to equilibrium at the exit from the catalyst bed.

In order to obtain a uniform gas mixture at the inlet to the secondary reformer catalyst, it is necessary to intimately mix the process gas from the primary reformer and gas-heated reformer with the process oxidant. In a conventional secondary reformer, the process oxidant is air, and the mass flowrates of process gas and process oxidant are typically the same order of magnitude. It is then often convenient to introduce the process air into the process gas using a burner gun that features a multiplicity of separate nozzles. In this way, the overall task of dispersing the process airstream into the process gas stream is performed at the many separate burner nozzles. The operating temperature in a secondary reformer is high enough for auto-ignition to occur where process gas and process air mix, so flames are formed at each burner nozzle.

In principle, increasing the number of burner nozzles will help facilitate dispersion but, in practice if the nozzles are too close together, then adjacent flames may coalesce, which adversely affects mixing. For a particular application, there is thus an optimum number of burner nozzles. Various arrangements of burner nozzles may be used, for example, in a “shower-head” arrangement in which they may be arranged in a cluster, and a ring burner arrangement, where the nozzles are mounted on one or more concentric circular rings.

In contrast, oxygen-based secondary reformers, usually referred to as autothermal reformers desirably have a different basic design. The requirement remains to intimately mix the process oxidant with the process gas, but in this case, the mass flowrate of the oxidant is typically much less than that of the process gas. Dispersing a relatively smaller flowrate of oxidant into a relatively larger flowrate of process gas requires the oxidant to be accelerated to higher velocity, which in turn requires increased burner pressure drop as compared with air-based operation. Therefore, rather than employing many small diameter burner orifices, with attendant difficulties in fabrication, a preferred approach is to employ a single burner nozzle. In this case, the nozzle is mounted in a cylindrical neck region of the autothermal reformer, above the conical expansion referred to above. The dimensions of the burner and reformer neck are selected to stabilise the flame on the burner and to enhance mixing between the streams of process oxidant and process gas.

The ammonia production facility front end further comprises a water gas shift unit. The cooled secondary reformed gas stream, which typically comprises hydrogen, carbon monoxide, carbon dioxide and steam, and potentially also some residual methane, is subjected, optionally after temperature adjustment, 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. If insufficient steam is present, steam may be added to the gas stream before it is subjected to the water-gas shift conversion. The water-gas shift reaction may be depicted as follows;

H₂O + CO H2 + CO₂

The reaction may be carried out in one or more stages. The stages may include a step of high temperature shift, performed in a high temperature shift reactor containing a high temperature shift catalyst or a step of medium temperature shift, performed in a medium temperature shift reactor containing a bed of medium temperature shift catalyst. For high temperature shift catalysts, the inlet temperature to the shift reactor is preferably in the range 280-500°C more preferably 300-450°C and most preferably 310-380°C so that the performance of the catalyst over an extended period is maximised. The shift process is preferably operated adiabatically without cooling of the catalyst bed, although if desired some cooling may be applied for example by passing cooling water under pressure through tubes disposed within the catalyst bed. The exit temperature from the shift reactor is preferably < 600°C, more preferably < 550°C to maximise the life and performance of the catalyst. Optionally, the shifted gas stream from the high temperature shift reactor may be cooled and subjected to one or more further stages of water-gas shift selected from medium temperature shift and/or low temperature shift. Medium temperature shift and low temperature shift catalysts are typically copper-zinc oxidealumina compositions. For medium temperature shift catalysts, the inlet temperature may be in the range 190-300°C and for low-temperature shift catalysts the inlet temperature may be 185270°C. The flow-rate of synthesis gas containing steam may be such that the gas hourly space velocity (GHSV) through the one or more beds of water-gas shift catalyst in the reactors is >6000 hour¹. The shift stage may be operated at a pressure in the range 1-100 bar abs, more preferably 15-50 bar abs.

If desired the revamp method may further comprise replacing an iron-containing high temperature water gas shift catalyst with a low-steam water-gas shift catalyst. This allows the steam-to-carbon ratio through the front end adjusted from the existing steam-to-carbon ratio to a lower, steam-to-carbon ratio. The adjustment may be made by reducing the steam flowrate to the process before the steam reformers. The existing steam to carbon ratio may be >1.5 but more typically is >2, e.g. 2.5 to 3.5. By adjusting the steam-to-carbon ratio, the steam to dry gas ratio at the inlet to the water-gas shift reactor may usefully be reduced to <0.45:1, preferably <0.42:1. In addition, the pressure drop through the front end may be reduced by reducing the steam ratio. This enables the hydrocarbon feedstock feed rate to the reformers to be increased. Increasing the hydrocarbon feedstock feed rate increases the front-end pressure drop back towards the original value and increases the hydrogen and so ammonia production.

The low steam water gas shift catalyst may be an enhanced iron-containing high temperature shift catalyst, an iron-free high-temperature shift catalyst or a medium-temperature shift catalyst.

The enhanced iron-containing water gas shift catalyst has properties that enable it to be operated at lower steam to carbon ratios than the replaced catalyst. Thus, the enhanced ironcontaining high temperature shift catalyst may have a higher geometric surface area (GSA) and/or may contain one or more promoters.

The higher GSA is suitably provide in a cylindrical pellet having a length C and diameter D, wherein the surface of the cylindrical pellet has two or more flutes running along its length, said cylinder having domed ends of lengths A and B such that (A+B+C)/D is in the range 0.25 to 1.25, and (A+B)/C is in the range 0.03 to 0.3. Preferably (A+B+C)/D is in the range 0.50 to 1.00, more preferably 0.55 to 0.70, most preferably 0.55 to 0.66. Preferably (A+B)/C is in the range 0.05 to 0.25, more preferably 0.1 to 0.25. The pellet may have 3 to 12, preferably 3 to 7, more preferably 3 to 5 semi-circular, elliptical, or U shaped flutes, evenly spaced about the circumference of the pellet and running axially along its length. The flutes may have a width d” in the range of 0.1 D to 0.4D, preferably 0.1 D to 0.25D when 5 flutes are present, preferably 0.2-0.30D when 4 flutes are present and preferably 0.25-0.4D when 3 flutes are present, and desirably the total flute width is < 35% of the circumference of the pellet. A preferred shape is a cylindrical pellet with a length 4 to 5mm, diameter 8 to 9mm and having three four or especially five evenly-spaced peripheral flutes running axially along the length of the pellet.

Preferably the enhanced iron-containing high temperature shift catalyst comprises acicular iron oxide particles. Such catalysts compositions containing iron and chromium oxides are described in US5656566. Alternatively, it may be desirable to at least partially replace the chromium oxide in the iron-based catalyst with alumina or another stabilising oxide. Zinc oxide and copper may desirably also be present. Such catalysts are described for example in EP2237882.

Alternatively, the low steam high temperature shift catalyst may be an iron-free water gas shift catalyst. Suitable catalysts of this type include those comprising a zinc-aluminate spinel. Thus, the low steam water gas shift catalyst may comprise a mixture of zinc alumina spinel and zinc oxide in combination with an alkali metal selected from the group consisting of Na, K, Rb, Cs and mixtures thereof. Such catalysts are described for example in EP2141118 and EP2300359. Alternatively, the low steam high temperature shift catalyst may comprise a metal-doped zinc oxide/alumina composition. For example, a suitable catalyst containing oxides of zinc and aluminium together with one or more promoters selected from Na, K, Rb, Cs, Cu, Ti, Zr, rare earth elements and mixtures thereof. Such catalysts are described for example in EP2924002.

Alternatively, a medium temperature shift catalyst as described above may be installed in an adiabatic or isothermal water gas shift reactor.

The shifted gas may be cooled to below the dew point to condense steam and form a dry shifted gas, which may be separated from condensate using conventional gas-liquid separation apparatus.

The shifted gas stream is 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 aMDEA^RTM or Benfield^R™ 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 Rectisol^R™, Selexol^R™ units. Carbon dioxide removal may also be performed by pressure-swing adsorption (PSA) using suitable solid adsorbent materials. 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 may be already present in the existing front end.

If desired, the effluent from the carbon dioxide removal unit may be subjected to a methanation stage in a methanation unit. In a methanation stage, residual carbon oxides (i.e. carbon monoxide and carbon dioxide) in the hydrogen gas stream are 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. Methanation generates steam which is desirably removed from the methanated gas by cooling it to below the dew point and removing the condensate using a conventional gasliquid separator. Further drying of the methanated gas stream may also be carried out, for example using a zeolite. Such methanation apparatus and catalysts are commercially available. The pressure for methanation may be in the range 10-80 bar abs.

If nitrogen from the air separation unit is not fed to the shifted gas stream, the effluent from the carbon dioxide removal unit is a hydrogen gas stream containing small amounts of methane, carbon monoxide and carbon dioxide. The purity of such a hydrogen stream is preferably > 95% vol H₂.

Where a portion of the nitrogen stream from the air separation unit is added downstream of the water gas shift unit, i.e. upstream of the carbon dioxide removal unit, the effluent from the carbon dioxide removal unit will comprise both hydrogen and nitrogen. As set out above it is preferable that not all of the nitrogen is added to the shifted gas stream, in which case nitrogen from the air separation unit may need to be added to the hydrogen- and nitrogen-containing gas downstream of the carbon dioxide removal unit, in order to arrive at the desired 3:1 H2:N2 stoichiometry for ammonia synthesis

The resulting hydrogen and nitrogen containing synthesis gas (syngas), may be compressed in a first and one or more further compression stages to the ammonia synthesis pressure and passed to an ammonia synthesis unit in the back end of the ammonia production facility

An ammonia synthesis unit typically comprises an ammonia converter containing an ammonia synthesis catalyst. The nitrogen and hydrogen in the syngas 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 synthesis 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 150350 bar abs for large-scale plants, and a temperature in the range 300-540°C, preferably 350520°C. The ammonia is typically recovered from the loop by refrigeration.

The revamp method may further include the installation of one or more additional ammonia converters, ammonia loops, compressors and refrigeration systems to handle the increased amount of synthesis gas from the front end.

The ammonia produced in the ammonia production unit may be sold and/or a portion converted into nitric acid, ammonium nitrate or urea. Any suitable process for producing nitric acid, ammonium nitrate or urea may be used.

The present invention will now be described by reference to the accompanying drawings in which;

Figure 1 depicts a conventional ammonia production facility front end not according to the present invention, and

Figure 2 depicts a revamped front end according to one embodiment of the present invention.

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

In Figure 1, a desulphurised natural gas feedstock 10 is mixed with steam fed via line 12 to form a feed gas 14. The feed gas 14 is passed to a fired steam reformer 16 in which it is reacted in steam reforming catalyst-filled tubes heated by the combustion of a fuel gas to form a primary reformed gas mixture comprising hydrogen, carbon dioxide, carbon monoxide, steam and methane. The primary reformed gas mixture is fed from the primary reformer 16 via line 18 to a secondary reformer 20 where it is partially combusted with air fed via line 22 and the partially combusted gases, passed adiabatically through a fixed bed of steam reforming catalyst to form a secondary reformed gas mixture containing hydrogen, nitrogen, carbon dioxide carbon monoxide and steam. The secondary reformed gas mixture is cooled and fed from the secondary reformer 20 via line 24 to a water-gas shift unit 26 where it is subjected to a stage of high temperature shift in a high temperature shift vessel containing a bed of an ironcontaining high temperature shift catalyst followed by low temperature shift on a low temperature shift vessel containing a bed of copper-containing low temperature shift catalyst, to form a shifted gas stream. The shifted gas stream is cooled and optionally condensate removed and fed from the water gas shift unit 26 via line 28 to a carbon dioxide removal unit 30, where it is subjected to a chemical or physical absorption process using an absorbent to remove carbon dioxide and form a carbon dioxide-depleted synthesis gas consisting essentially of hydrogen and nitrogen with small levels of methane and carbon oxides. Carbon dioxide is recovered from the carbon dioxide removal unit 30 via line 32. The carbon dioxide-depleted synthesis gas is heated and fed from the carbon dioxide removal unit 30 via line 34 to a methanation unit 36 comprising a methanator containing a methanation catalyst in which residual carbon oxides are reacted with hydrogen to form a methanated synthesis gas. The methanated synthesis gas is cooled to below the dew point to condense steam. The resulting condensate is recovered from the methanation unit 36 via line 38. An ammonia synthesis gas consisting essentially of hydrogen and nitrogen is recovered from the methanation unit 36 via line 40.

In Figure 2, the conventional process of Figure 1 has been revamped by the addition of a gas heated reformer in parallel to the fired steam reformer, by the addition of an air separation unit, by replacement of the air feed line with an oxygen gas stream from the air separation unit, and by use of the secondary reformed gas as the heat exchange medium in the gas heated reformer. Nitrogen from the air separation unit is fed to the effluent from the methanation unit to form the synthesis gas.

Accordingly, a desulphurised natural gas feedstock 10 is mixed with steam fed via line 12 to form a feed gas 14. The feed gas 14 divided into a first portion 50 and a second portion 52. The first portion of the feed gas is passed via line 50 to the fired steam reformer 16 in which it is reacted in steam reforming catalyst-filled tubes heated by the combustion of a fuel gas to form a first crude synthesis gas comprising hydrogen, carbon dioxide, carbon monoxide, steam and methane. The second portion of the feed gas is fed via line 52 to a gas heated reformer 54 having a tube side and a shell side, where it is reacted on the tube side in steam reforming catalyst-filled tubes 56 heated by a hot secondary reformed gas fed via line 58 to the shell side, to form a second crude synthesis gas comprising hydrogen, carbon dioxide, carbon monoxide, steam and methane. The first crude synthesis gas is recovered from the fired steam reformer 16 via line 60 and combined with the second crude synthesis gas recovered via line 62 from the gas heated reformer, and the combined crude synthesis gas fed via line 64 to the secondary reformer 20, where it is partially combusted with a steam-containing oxygen gas stream fed via line 66 and the partially combusted gases, passed adiabatically through a fixed bed of steam reforming catalyst to form a secondary reformed gas mixture containing hydrogen, carbon dioxide carbon monoxide and steam. Because an oxygen gas stream is used in the secondary reformer, nitrogen is essentially absent from the secondary reformed gas. The hot secondary reformed gas mixture is fed via line 58 to the shell side of the gas heated reformer 54 where it is cooled. The cooled secondary reformed gas is fed from the gas heated reformer via line 68 to the water-gas shift unit 26 where it is subjected to a stage of high temperature shift in a high temperature shift vessel containing a bed of an iron-based high temperature shift catalyst followed by low temperature shift on a low temperature shift vessel containing a bed of copper-containing low temperature shift catalyst, to form a shifted gas stream. The shifted gas stream is cooled and optionally condensate removed and fed from the water gas shift unit 26 via line 28 to the carbon dioxide removal unit 30, where it is subjected to a chemical or physical absorption process using an absorbent to remove carbon dioxide and form a carbon dioxide-depleted synthesis gas consisting essentially of hydrogen with small levels of methane and carbon oxides. Carbon dioxide is recovered from the carbon dioxide removal unit 30 via line 32. The carbon dioxide-depleted synthesis gas is heated and fed from the carbon dioxide removal unit 30 via line 34 to a methanation unit 36 comprising a methanator containing a methanation catalyst in which residual carbon oxides are reacted with hydrogen to form a methanated synthesis gas. The methanated synthesis gas is cooled to below the dew point to condense steam. The resulting condensate is recovered from the methanation unit 36 via line 38. A hydrogen gas stream consisting essentially of hydrogen is recovered from the methanation unit 36 via line 70 and combined with a nitrogen gas stream provided by line 72 to form an ammonia synthesis gas 74 consisting essentially of hydrogen and nitrogen.

An air separation unit 76 is provided with an air feed 78 and provides an oxygen gas stream 80 and the nitrogen gas stream 72. The oxygen gas stream 80 is mixed with steam provided by line 82 to form the steam-containing oxygen gas stream 66 used in the secondary reformer 20.

Optionally, the secondary reformer 20 may be revamped by replacing the existing burner with a different burner more suited to operation with an oxygen gas stream.

Optionally, the water gas shift unit may be revamped by replacement of the iron-based water gas shift catalyst with an enhanced iron-based water gas shift catalyst, an iron-free water gas shift catalyst or a medium temperature shift catalyst. If desired the low-temperature shift stage may be removed.

The invention will now be described by reference to the following example.

Example 1

An ammonia process as depicted in Figure 1 in was revamped in accordance with the process depicted in Figure 2 by steps comprising:

(i) installation of a gas heated reformer having a tube side and a shell side in parallel to the existing primary steam reformer, (ii) connecting the outlet of the gas heated reformer to the effluent of the primary reformer to combine the reformed gases from the primary reformer with those of the gas heated reformer, (iii) installing an air separation unit to supply an oxygen gas stream and a nitrogen gas stream, (iv) disconnecting the air feed from the existing secondary reformer and adapting the secondary reformer to use the oxygen gas stream, (v) connecting the outlet from the secondary reformer to the shell side of the gas heated reformer so that the secondary reformed gas acts as the heat exchange medium in the gas-heated reformer, (vi) connecting the outlet from the shell side of the gas heated reformer to the downstream process comprising the water-gas shift unit, the carbon dioxide removal unit and the methanation unit, and (i) connecting the nitrogen gas stream from the air separation unit to the effluent from the methanation unit.

The hydrocarbon feedstock feed rate was then increased to correspond to an uprate in production of ammonia from 1200 tonnes/day to 1600 tonnes/day. The following tables set out the compositions and flowrates of the streams as depicted in Figure 2. The air separation unit was fed with 3356 kmol/h (96378 kg/h) air at 25°C, 1 bara.

Stream No.	10	12	14	28	32	34	38	50	52
Composition (mol%)
CO	0.00	0.00	0.00	0.24	0.00	0.43	0.00	0.00	0.00
CO2	0.00	0.00	0.00	16.09	36.08	0.03	0.00	0.00	0.00
Hydrogen	2.00	0.00	0.50	53.48	0.00	96.47	0.00	0.50	0.50
Methane	91.00	0.00	22.75	0.39	0.00	0.69	0.00	22.75	22.75
Nitrogen	3.00	0.00	0.75	0.50	0.00	0.91	0.00	0.75	0.75
H2O	0.00	100.0	75.00	29.30	63.92	1.47	100.0	75.00	75.00
Ethane	3.00	0.00	0.75	0.00	0.00	0.00	0.00	0.75	0.75
Propane	1.00	0.00	0.25	0.00	0.00	0.00	0.00	0.25	0.25
Oxygen	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Argon	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Molar Flow (kmol/h)	2006	6018	8024	12000	5348	6652	127	6018	2006
Mass Flow (kg/h)	33747	108415	142161	164506	146487	18019	2285	106621	35540
Temp. (°C)	380.0	350.0	351.6	230.0	102.0	75.0	10.0	351.6	351.6
Pressure (bara)	34.0	35.0	34.0	26.5	1.5	27.0	25.5	34.0	34.0

Stream No.	60	62	64	66	58	68	70	72	74
Composition (mol%)
CO	3.04	6.27	3.63	0.00	10.42	10.42	0.00	0.00	0.00
CO2	5.94	5.85	5.52	0.00	5.92	5.92	0.00	0.00	0.00
Hydrogen	32.27	41.65	32.41	0.00	43.30	43.30	97.83	0.00	73.77
Methane	11.53	6.82	9.59	0.00	0.39	0.39	1.19	0.00	0.90
Nitrogen	0.62	0.57	0.57	0.05	0.50	0.50	0.94	100	25.3
H2O	46.61	38.85	42.06	7.69	39.48	39.48	0.05	0.00	0.04
Ethane	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Propane	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Oxygen	0.00	0.00	6.23	92.27	0.00	0.00	0.00	0.00	0.00
Argon	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00	0.00
Molar Flow (kmol/h)	7335	2648	10706	723	11999	11999	6464	2108	8572
Mass Flow (kg/h)	106622	35541	164506	22344	164506	164506	15733	59045	74778
Temp. (°C)	730.0	810.0	722.1	248.6	969.9	725.5	10.0	10.0	9.0
Pressure (bara)	30.0	30.0	30.0	33.0	29.0	28.5	25.5	30.0	25.5

The process offers an improved method for uprating production from the ammonia process by increasing the volume of synthesis gas containing <1% vol methane.

Claims

1. A method for revamping an ammonia production facility said ammonia production facility having a front end for producing a synthesis gas comprising nitrogen and hydrogen from a hydrocarbon feedstock, said front end comprising a fired primary steam reformer, an air-fed secondary reformer, a water gas shift unit, a carbon dioxide removal unit and optionally a methanation unit connected in series, said method comprising the steps of: (i) installing a gas-heated reformer having a tube side and a shell side in parallel to the fired steam reformer; (ii) connecting the tube side of the gas-heated reformer to an inlet of the secondary reformer; (iii) connecting an outlet of the secondary reformer to the shell side of the gas-heated reformer; (iv) installing an air separation unit with an oxygen stream outlet and a nitrogen stream outlet; (v) disconnecting the air feed to the secondary reformer; and (vi) connecting the oxygen stream outlet of the air separation unit to the secondary reformer and the nitrogen stream outlet of the air separation unit to the front end downstream of one or more of the water gas shift unit, the carbon dioxide removal unit and the methanation unit and upstream of an ammonia synthesis unit.

2. A method according to claim 1 wherein the gas heated reformer is a heat exchange reformer having a tube side through which the reactants flow, and a shell side through which a heating gas is passed, said tube side having a tube side inlet for a hydrocarbon feedstock and steam mixture, and a tube side outlet for discharging a reformed gas mixture, and said shell side having a shell-side inlet for receiving a hot secondary reformed gas and a shell side outlet for discharging a cooled secondary reformed gas.

3. A method according to claim 1 or claim 2 wherein the secondary reformer is connected to a mixed reformed gas feed line comprising a primary reformed gas line from the fired steam reformer and a reformed gas line from the gas-heated reformer.

4. A method according to any one of claims 1 to 3 wherein the burner in the secondary reformer is replaced with a burner adapted for use with an oxygen gas stream.

5. A method according to any one of claims 1 to 3 wherein the secondary reformer is replaced entirely with a secondary or autothermal reformer having different dimensions and containing a different burner adapted for use with an oxygen gas stream.

6. A method according to any one of claims 1 to 5 further comprising replacing an iron-containing high temperature water gas shift catalyst in the water gas shift unit with a low-steam water-gas shift catalyst.

7. A method according to claim 6 wherein the low-steam water gas shift catalyst is an enhanced iron-containing high temperature shift catalyst, an iron-free high-temperature catalyst or a medium-temperature shift catalyst.

8. A method according to any one of claims 1 to 7 further comprising a methanation unit connected to the carbon dioxide removal unit to receive a carbon-dioxide depleted gas wherein the nitrogen stream outlet of the air separation unit is connected to the front end downstream of the methanation unit and upstream of an ammonia synthesis unit.

9. A process for producing a synthesis gas comprising hydrogen and nitrogen in the front end of an ammonia production facility, comprising the steps of: (i) passing a first portion of a mixture of gaseous hydrocarbon feedstock and steam through externally-heated catalyst filled tubes in a fired steam reformer to form a first crude synthesis gas; (ii) passing a second portion of the mixture of gaseous hydrocarbon feedstock and steam through externally-heated catalyst filled tubes in a gas-heated reformer having a shell-side and a tube side to form a second crude synthesis gas; (iii) passing the first and second crude synthesis gases to a secondary reformer and secondary reforming the combined gases to form a hot secondary reformed gas stream; (iv) passing the hot secondary reformed gas stream to the shell-side of the gas heated reformer to form a cooled secondary reformed gas; (v) subjecting the cooled secondary reformed gas to the water-gas shift reaction in a water-gas shift unit to form a shifted gas stream; (vi) removing carbon dioxide from the shifted gas stream in a carbon dioxide removal unit to form a hydrogen gas stream; and (vii) optionally subjecting residual carbon oxides in the hydrogen gas stream to methanation in a methanation unit, wherein the secondary reforming is performed using an oxygen gas stream recovered from an air separation unit, and a nitrogen gas stream from the air separation unit is fed to the front end of the ammonia production facility downstream of one or more of the water gas shift unit, the carbon dioxide removal unit and the methanation unit and upstream of an ammonia synthesis unit.

10. A process for producing ammonia including the synthesis gas production process of claim 9.