A nitric acid plant

Abstract

A plant for the synthesis of nitric acid from a feed gas comprising oxygen and ammonia, comprising an ammonia oxidation reactor and a steam network, wherein said reactor includes at least one internal heat exchanger connected to the steam network and arranged to produce steam with heat of the ammonia combustion reaction, wherein the steam network includes a steam drum separated from the reactor, the steam drum being connected to said heat exchanger via a feed line to feed water to the heat exchanger and a steam line to collect steam formed in the heat exchanger, wherein the steam network includes an electrical heater mounted internally in the steam drum and arranged to heat the feed water sent to the reactor.

Description

[0001] A nitric acid plant

[0002] DESCRIPTION

[0003] Field of application

[0004] The invention is in the field of industrial production of nitric acid from catalytic combustion of ammonia.

[0005] Prior art

[0006] The production of nitric acid starting from catalytic combustion of ammonia (Ostwald process) includes the following main steps. The catalytic oxidation of ammonia is performed under pressure in a suitable ammonia oxidation reactor (“ammonia burner”), obtaining a gas (“nitrous gas”) containing nitrogen oxides NOx (NO and NO2) and nitrous oxide (N2O) at a high temperature. The oxidation of ammonia occurs over a suitable catalyst, usually a catalyst gauze, in the presence of oxygen, normally ambient air fed by an air compressor.

[0007] The hot nitrous gas is cooled through a set of heat exchangers (“cooling train”) including a cooler-condenser; from the cooler-condenser, a stream of cooled nitrous gas and a stream of acid condensate are sent to an absorber, where the nitrogen oxides are absorbed in water. The absorber, usually an absorption tower, produces a nitric acid solution and a tail gas withdrawn from top of the absorber. The tail gas is usually treated for abatement of NOx and N2O before being discharged. Being under pressure, the tail gas may also be expanded to recover energy.

[0008] Various implementations of the above process exist and are described e.g. in the Ullmann’s Encyclopaedia, 2012 Edition, “Nitric Acid”. Background art of processes and plants for the preparation of nitric acid can be found in GB 2041900, US 2024 / 246819 and US 2014 / 377157.

[0009] In the ammonia oxidation reactor, the considerable heat released by the exothermic conversion is generally removed by an internal heat exchanger connected to a steam network (also termed steam system). The heat of reaction can be used to produce steam at a considerable pressure, which is an attractive way of recovering energy within the process.

[0010] More recently, the production of ammonia from renewable energy sources has emerged to reduce emissions and reduce the carbon intensity of the entire process. Conventionally, the sustainability of the process is identified by a colour. A coal-based process is named “brown”; a natural gas-based process is named “grey”; a process featuring carbon dioxide capture is named “blue” and a process where ammonia is produced from biofuels or renewable electricity is named “green”. In recent years, the interest towards green ammonia has increased dramatically. In green ammonia production processes, steam is not a product and therefore any needs of steam of other tied-in processes, such as nitric acid production processes, cannot be fulfilled by said ammonia processes.

[0011] Under certain circumstances, it may be desired to provide a heat input to the nitric acid process. This is typically the case of start-up when the catalyst contained in the ammonia oxidation reactor and the catalyst used for tail gas treatment must be heated up to the minimum operational temperature. Equally, the whole process must be heated up to a suitable temperature to avoid nitric acid condensation on the cold wall of the equipment.

[0012] One way to heat the process before and during startup phase, is to supply heat to the steam network and provide heat to the process through the heat exchangers of the steam network. These heat exchangers can be located for example in the same vessel of the ammonia oxidation reactor. The required heat can be supplied for example with a steam flow generated outside the nitric acid process and injected in the existing steam network, preferably in the existing steam drum. Once the steam drum is heated and pressurized through the external steam input, the heat is propagated to the rest of the process that is flushed with air.

[0013] Typically, a nitric acid production plant requires at least one planned shutdown per year. During transitional operating phases, such as startup, shutdowns or operating load lower than 50-70% with respect to nominal load, steam needs to be produced with auxiliary alternative methods. Steam boilers fed with natural gas are well known and reliable, but are source of CO2 emissions. Steam boilers fed by renewable hydrogen are not fully deployed. Auxiliary steam boilers fed with other renewable sources, like biomass, need biomass inventories and are neither feasible or suitable for quick startups. Electric steam boilers may be used for their reliability, wide range of duty and fast response, but they require a dedicated steam drum and a control system. Said drum and control system would have a significant impact on plant costs, and are not needed for most of the operating time.

[0014] A still unsolved challenge of prior art is thus to allow steam production during startups of plants for nitric acid production in green fertilizer complexes, without adding new equipment that would result in a significant increase of investment costs.

[0015] Summary of the invention

[0016] The invention addresses the above-mentioned problems in connection with a lack of hot steam for heating the ammonia oxidation reactor when required (e.g. during start-up) and / or for control of the oxidation reactor under load variations.

[0017] The above problems are solved with a plant for the production of nitric acid according to the claims. Other aspects of the invention relate to a method for controlling a system for producing a nitrous gas in a nitric acid plant, and to a method of revamping an existing nitric acid plant, according to the claims.

[0018] The plant of the invention comprises an ammonia oxidation reactor where nitrous gas is produced by catalytic oxidation of ammonia, a nitrous gas cooling section where the hot nitrous gas is cooled before being fed to an absorption step, and a steam network.

[0019] In the nitrous gas cooling section, heat is removed from the hot nitrous gas and may be transferred to other process streams (e.g. for heating absorption tail gas) or to water / steam. Said nitrous gas cooling section includes at least one heat exchanger within which at least part of the heat produced by the ammonia oxidation reaction is used to produce a steam flow which is sent to the steam network.

[0020] The steam produced in the steam network may be used internally in the process or exported. Said steam network includes a steam drum separated from the ammonia oxidation reactor. Said steam drum is connected to said at least one heat exchanger of the cooling section via a feed line to feed water and via a steam line to collect steam. The steam network further includes an electrical heater mounted internally in the steam drum and arranged to heat the feed water sent to the at least one heat exchanger. Accordingly, said heat exchanger may remove heat from the hot nitrous gas in a first mode of operation, or may provide a heat input to the process in a second mode of operation, when fed with water previously heated by the electrical heater. Said second mode of operation may be selected when an additional heat input is required by the process, for example during transients.

[0021] In a highly preferred embodiment, said at least one heat exchanger is housed internally in the ammonia oxidation reactor. In this embodiment, the electrical heater can be used to heat the boiling feed water which is sent to the internal heat exchanger of the reactor. Accordingly, heat generated in the electrical heater is transferred to the ammonia oxidation reactor and compensates for a reduced or no availability of hot steam from the process.

[0022] Said electrical heater is strategically positioned inside the pressure vessel of the steam drum connected to the reactor. Said electrical heater can be fixed or removable according to embodiments of the invention.

[0023] The electrical heater provides an additional means for controlling the reactive system (ammonia oxidation reactor) during transients or when operation at reduced load is required. Said additional electric heater may also produce steam to feed a steam turbine and / or other external users during transients or during operation at reduced load.

[0024] The location of the electrical heater inside the steam drum has important advantages compared to an in-line electrical heater. First, it does not require a separate vessel for the electrical heater. Second, the in-line electrical heater introduces a significant pressure drop which is avoided in the present invention. If the circulation from the steam drum to the reactor is a forced circulation via a pump, this means that less power is absorbed by the pump.

[0025] Description of the invention

[0026] The invention concerns a plant for the oxidation of ammonia and production of nitric acid. The plant comprises an ammonia oxidation reactor wherein ammonia is oxidised and a nitrous gas is produced, a nitrous gas cooling section and a steam network.

[0027] Said nitrous gas cooling section provides cooling to the nitrous gas and at least part of the heat removed from the nitrous gas is used to produce steam. Accordingly, said nitrous gas cooling section includes at least one heat exchanger connected to the steam network and arranged to produce steam which is then sent to the steam network. Preferably, at least one heat exchanger is provided internally in the ammonia oxidation reactor to produce steam with heat released by the oxidation reaction.

[0028] Said steam network includes a steam drum separated from the reactor, said steam drum being connected to said heat exchanger via a water feed line arranged to feed water from the steam drum to said heat exchanger, and a steam collection line adapted to collect steam formed in the heat exchanger. According to the invention, an electrical heater is mounted internally in the steam drum and is arranged to heat the feed water sent to the reactor.

[0029] In a preferred embodiment, said electrical heater includes a bundle of electrically heated tubes inside the pressure vessel of the steam drum.

[0030] The electrical heater may have a modular structure, for example being formed by a plurality of electrically heated elements and may be integral with the steam drum or removable from said steam drum.

[0031] In an interesting embodiment of the invention, the steam network includes a steam superheater that superheats the steam produced within the nitrous gas cooling section. The steam network preferably includes also an additional electric steam superheater placed downstream the steam drum and upstream the steam turbine.

[0032] The plant includes at least one line configured to feed a stream of gas containing oxygen, such as air, to a suitable position of the nitric acid plant, preferably to the ammonia oxidation reactor, to provide the oxygen required for the oxidation of ammonia in the ammonia oxidation reactor (ammonia burner) and possibly a further amount of oxygen for the oxidation of nitrous gas in the cooling section. Said oxidation of nitrous gas in the cooling section converts NO into NO2, which is preferred in the absorption stage. Oxygen for oxidation of nitrous gas may be introduced together with oxygen for ammonia oxidation, or separately. In some embodiments, a stream containing oxygen (such as air) for the oxidation of nitrous gas is introduced downstream of the ammonia oxidation reactor.

[0033] In a preferred embodiment, the nitrous gas produced in the ammonia oxidation reactor is cooled and at least in part condensed through a set of coolers and condensers before being introduced in an absorber. Within the absorber, a nitric acid solution is separated from a tail gas, said tail gas is then at least partially recycled to ammonia oxidation burner and / or used as nitrogen source for the production of the ammonia feed of the process. For example, ammonia is synthesized from an ammonia make-up gas wherein said tail gas of the absorber provides some or all of the nitrogen of said ammonia make-up gas. The ammonia make-up gas is a mixture of nitrogen and hydrogen. In a highly preferred embodiment, some or all of the hydrogen in the ammonia make-up gas is produced from electrolysis of water, preferably powered with renewable energy (so called “green” hydrogen). The invention is applicable to new nitric acid plants and revamping of existing plants. A revamping according to the invention includes the provision of the above-described electrical heater in the steam drum of the steam network. A revamping according to the invention may also be performed to allow smooth operation under variable loads, particularly when the plant is required to run at low load such as less than 50% of nominal capacity, for example 20% or less.

[0034] Another aspect of the invention is a method for controlling a reactive system for the oxidation of ammonia for the production of nitric acid, wherein the system is controlled selectively according to a first mode wherein heat is removed from the ammonia oxidation reactor or according to a second mode wherein heat is supplied to said reactor. In the first mode, heat is removed predominantly by means of evaporation of the feed water sent to the reactor. In the second mode, heat is transferred to the reactor by means of the feed water heated by said electrical heater in the steam drum.

[0035] Normally, the oxidation of ammonia releases heat (exothermic reaction) and, accordingly, the system operates in the first mode. The second mode can be selected during transients wherein the reactor requires a heat input. For example, the second mode is selected during start-up or shutdown of the plant. The second mode may also be selected during a load variation of the plant. In an embodiment, said load variation includes operating the plant at a partial load not greater than 70% of the nominal load of the system, preferably not greater than 60%, more preferably not greater than 50%.

[0036] In a highly preferred embodiment, the production of at least part of the ammonia depends on a renewable energy. Example of renewable sources include solar, wind, biomass, hydroelectric, and other sources that are naturally replenished. In such embodiments, periods of reduced availability of the renewable source (e.g. day / night for solar energy) decreases the production of ammonia that can be sent to the reactive system. An intermittent input of renewable energy may also be dictated by variations of the price of energy. Said second mode of operation can be selected during said periods of temporary lack or reduced availability of said renewable energy and consequently to maintain a proper temperature in the reactor.

[0037] When the nitric acid plant is not in operation, said electrical heater can still be used to produce steam for export to one or more external users. Said external users may include equipment of: a plant connected to the nitric acid plant (e.g. for production of ammonium nitrates); an ammonia synthesis plant; a hydrogen generation unit. Said external users may include preferably one or more of the following: a heater of an ammonium nitrates solution; an ammonium nitrates solidification operation; a chilling unit used in an ammonia synthesis process; a reboiler for ammonia distillation in ammonia synthesis; a high-temperature electrolysis unit for generation of hydrogen from water.

[0038] In an embodiment, heat produced by the oxidation of ammonia is used to produce superheated steam; said superheated steam is saturated in a steam saturator vessel; heat removed from the superheated steam during the saturation process is used for the solidification of ammonium nitrates. Optionally, the saturation process in the saturator vessel includes generation of saturated steam using an internal electrical heater installed in said steam saturator vessel. This electrical heater may be used to generate saturated steam in a condition when the required amount of heat for sustaining the solidification process is not fully available.

[0039] Description of the figures

[0040] Figs. 1 and 2 are simplified diagrams of an ammonia oxidation system according to embodiments of the invention. The figures show the following items:

[0041] 1 ammonia burner

[0042] 2 internal heat exchanger (evaporator) of the burner 1

[0043] 2a steam superheater

[0044] 2b heat exchangers

[0045] 3 steam drum 4 electrical heater

[0046] 5 additional electrical heater

[0047] 6 feed gas of the ammonia burner 1 , containing ammonia and air

[0048] 7 product effluent of the ammonia burner 1 containing NOx

[0049] 8 feed water to the heat exchanger 2

[0050] 9 saturated steam withdrawn from the steam drum 3

[0051] 10 superheated steam

[0052] 11 ammonia oxidation reactor

[0053] 12 steam line

[0054] Fig. 1 shows a reactive system of a nitric acid plant wherein the ammonia burner 1 processes a feed gas 6 containing ammonia and air to produce an NOx stream that is cooled in at least one heat exchanger 2, resulting in a cooled gas 7 containing NOx. The burner 1 is part of an ammonia oxidation reactor 11 .

[0055] The feed gas 6 is a mixture of air and ammonia obtained in a suitable section of the nitric acid plant. In a preferred embodiment, the ammonia contained in the feed gas 6 is produced partially or entirely from renewable energy.

[0056] The heat exchanger 2 inside the reactor 11 removes some of the heat of reaction to produce steam that is collected in a steam drum 3. An electrical heater 4 is housed in the pressure vessel of the steam drum 3. Said electrical heater 4 for example is a bundle of electrically heated tubes. Said heater 4 can raise the temperature of water sent and withdrawn from the heat exchanger 2 via lines 8 and 12, to provide a temporary heat input to the reactor 11 when needed. A saturated steam 9 may be removed from the steam drum 3 to feed an additional electrical heater 5 for producing superheated steam 10 that may be fed to a steam turbine (not shown).

[0057] In the embodiment of Fig. 2, the NOx stream produced in the burner 1 is cooled in a steam superheater 2a that produces superheated steam 10 by heating a stream of saturated steam 9. Further heat exchanger(s) 2b remove some heat of reaction to heat a boiler feed water 8 that is introduced in a steam drum 3, via line 12. The electrical heater 4 is housed in the pressure vessel of the steam drum 3, said heater 4 being for example a bundle of electrically heated tubes. The stream 10 may be heated with an additional electric heater 5 provided with electrical energy placed downstream the steam drum 3 and upstream a steam turbine.

[0058] The electrical heaters 4, 5 are preferably operated during start-up, shutdown or during a load variation of the system. A method for controlling a reactive system comprising the reactor 11 in a nitric acid plant is as follows. The system is operated selectively according to a first mode wherein heat is removed from the ammonia oxidation reactor 11 by means of evaporation of the feed water 8 sent to the reactor 11 , or according to a second mode wherein heat is supplied to the ammonia oxidation reactor 11 by means of the feed water 8 heated by said electrical heater 4 in the steam drum 3. This second mode, wherein the electrical heater 4 provides a heat input to the reactor 11 , may be selected during startup, shutdown or during transients of the plant.

Claims

CLAIMS1 ) A plant for the production of nitric acid, comprising a reactor wherein ammonia is oxidized obtaining a nitrous gas; a nitrous gas cooling section and a steam network, wherein said nitrous gas cooling section comprises at least one heat exchanger within which at least part of the heat produced by the ammonia oxidation reaction is used to produce steam sent to said steam network, said steam network including a steam drum (3) separated from the reactor, the steam drum being connected to said at least one heat exchanger of the cooling section via a feed line (8) to feed water and via a steam line (12) to collect steam, wherein the steam network further includes an electrical heater (4) mounted internally in the steam drum (3) and arranged to heat the feed water sent to the at least one heat exchanger.2) A nitric acid plant according to claim 1 , wherein said electrical heater (4) includes electrically heated tubes arranged inside the pressure vessel of the steam drum (3).3) A nitric acid plant according to claim 1 or 2, the electrical heater (4) being integral with the steam drum (3) or removable from said steam drum.4) A nitric acid plant according to any of the previous claims wherein the at least one heat exchanger (2) arranged to produce steam with the heat of the ammonia oxidation reaction is housed in the same vessel of the reactor.5) A nitric acid plant according to any of the previous claims wherein the steam network includes a steam super heater (2a) that provides superheating to the steam produced in the nitrous gas cooling section.6) A nitric acid plant according to any of the previous claims, wherein the steam network includes an additional electric steam super heater (5) that is provided with electrical energy and that is placed downstream the steam drum (3) andupstream a steam turbine.7) A nitric acid plant according to any of the previous claims including a line arranged to feed a stream of gas containing oxygen, wherein said stream is fed to a suitable position of the nitric acid plant, preferably to the reactor, to provide oxygen required for oxidation of ammonia and / or for further nitrous gas oxidation.8) A nitric acid plant according to any of the previous claims wherein nitrous gas produced in the reactor is cooled and at least in part condensed through a set of coolers and condensers before being introduced in an absorber wherein a nitric acid solution is separated from a tail gas, said tail gas being at least partially recycled to the reactor and / or used as nitrogen source for production of an ammonia make-up gas.9) A method for controlling a system for the production of nitrous gas by oxidation of ammonia in a nitric acid plant, wherein: the system comprises a reactor wherein ammonia is oxidized obtaining a nitrous gas, a nitrous gas cooling section comprising at least one heat exchanger (2) within which at least part of the heat produced by the ammonia oxidation reaction is used to produce steam in a steam network of the nitric acid plant, wherein said exchanger (2) is housed internally in the vessel of the ammonia oxidation reactor; said steam network including a steam drum (3) separated from the reactor, said steam drum being connected to said at least one heat exchanger of the cooling section via a feed line (8) to feed water and via a steam line (12) to collect steam, wherein the steam network further includes an electrical heater (4) mounted internally in the steam drum (3) and arranged to heat the feed water sent to the at least one heat exchanger, wherein the method includes: controlling the system selectively according to a first mode wherein heat isremoved from the ammonia oxidation reactor by means of evaporation of the feed water sent to the reactor, or according to a second mode wherein heat is supplied to the ammonia oxidation reactor by means of the feed water heated by said electrical heater (4) in the steam drum (3).10) A method according to claim 9 wherein the second mode is selected during start-up or shutdown of the nitric acid plant.11 ) A method according to claim 9 or 10 wherein the second mode is selected during a load variation of the nitric acid plant.12) A method according to claim 11 wherein said load variation includes operating the nitric acid plant at a partial load not greater than 70% of the nominal load of the nitric acid plant, preferably not greater than 60%, more preferably not greater than 50%.13) A method according to claim 11 or 12 wherein the production of at least part of the ammonia fed to the reactor depends on a renewable energy and said second mode is selected during a temporary lack or reduced availability of said renewable energy.14) A method according to any of claims 11 to 13 wherein heat produced by the oxidation of ammonia is used to produce superheated steam; said superheated steam is saturated in a steam saturator vessel; and resulting saturated steam is used for the solidification of ammonium nitrates.15) A method according to claim 14 wherein the saturation process in the saturator vessel includes generation of saturated steam using an internal electrical heater installed in said steam saturator vessel.16) A method of revamping an existing nitric acid plant comprising a reactor wherein ammonia is oxidized and a nitrous gas cooling section, wherein said cooling section includes at least one heat exchanger (2) arranged to produce steam with heat of the ammonia oxidation reaction in a steam network, said at least one heat exchanger (2) being housed in the same vessel of thereactor, wherein the steam network further includes a steam drum (3) separated from the reactor, said steam drum (3) being connected to said heat exchanger via a feed line to feed water to the heat exchanger and a steam line to collect steam formed in the heat exchanger, the method including: modifying said steam drum (3) by mounting an electrical heater (4) inside the steam drum, said electrical heater being arranged to heat the water feed directed to said internal heat exchanger of the reactor.

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