Process for producing ammonia from natural gas

Abstract

A process for producing ammonia from natural gas and air consists in purifying natural gas of sulphurous impurities by catalytic hydrotreatment and the chemical adsorption of hydrogen sulphide, heating the purified natural gas, subjecting same to primary and secondary reforming and subsequently converting carbon monoxide into carbon dioxide. The obtained synthesis gas is purified of carbon dioxide then subjected to methanation, adsorption drying and low-temperature rectification, followed by the synthesis of liquefied ammonia. Process media are heated by differential cooling of the flue gases from a primary reforming furnace, and high-pressure steam is generated by heat from secondary reforming. A spent synthesis gas line between the methanation and ammonia synthesis stages is supplemented with adsorption drying and low-temperature rectification. The flue gases from the primary reforming furnace are selectively purified of nitrogen oxides using catalysts based on oxides of vanadium, titanium, tungsten or aluminium. The process line is duplicated. The technical result is that of reducing the use of fuel-grade natural gas, reducing the environmental impact on the surroundings and reducing the risk of the emergency shutdown of ammonia production.

Description

[0001] METHOD FOR PRODUCING AMMONIA FROM NATURAL GAS

[0002] AREA OF TECHNOLOGY

[0003] The invention relates to the field of ammonia production from natural gas and air and can be used in gas chemical industry enterprises.

[0004] Ammonia is the most important source of raw material for the production of nitrogen fertilizers, mainly urea and ammonium nitrate.

[0005] The raw materials for ammonia production are nitrogen and hydrogen in a 1:3 ratio. Nitrogen is primarily obtained through energy-intensive low-temperature air distillation, while hydrogen is produced by steam reforming of natural gas (66%), gasification of solid fuels, electrolysis, or thermal decomposition of water.

[0006] PRIOR ART

[0007] Given the current global economic and technological conditions, the priority is large-scale production of blue ammonia (approximately 4,000-4,500 tons / day) and urea (approximately 6,000-7,000 tons / day) on a single process line with optimal (low) natural gas consumption and electricity supply to "cover" urea consumption by 2030. In Russia, ammonia production and its main processing stages are export-oriented, which, given sanctions restrictions on natural gas exports, should offset Gazprom's losses in the European gas market. Given domestic demand and exports, the ammonia industry in Russia is rapidly developing, and to date, at least nine new major projects with urea as the end product have been announced. Their total capacity will be 6 million tons of ammonia and 10 million tons of urea per year (Analytical report Module Energy Plus, [Electronic resource] URL: http: / / solidgas.ru>fag / l_spr_ammiak.pdf). In this regard, the development of new technical solutions aimed at improving the technology and equipment for the ammonia production process is becoming increasingly important.

[0008] A method is known for producing ammonia from natural gas, which includes compressing, heating and purifying natural gas from sulfur compounds, two-stage catalytic conversion of methane under pressure, including steam reforming in the first stage and steam-air reforming in the second stage, using the heat of the gas converted in the second stage, as well as a portion of the natural gas, purge and tank gases additionally burned on a burner to carry out the reforming in the first stage of the reforming process, catalytic conversion of carbon monoxide contained in the reformed gas to obtain a nitrogen-hydrogen mixture, its purification from carbon dioxide, purification from oxygen-containing compounds by methanation, compression of the purified nitrogen-hydrogen mixture, synthesis of ammonia in a closed cycle and separation of the ammonia obtained with its subsequent delivery to the consumer, as well as the utilization of the heat of flue gases and their release into the environment,wherein the two-stage conversion of natural gas is carried out in a radial-spiral reactor divided into two sections with the steam conversion process carried out in the first section at a temperature of 800-1000°C and steam-air conversion in the second section at a temperature of 900-1400°C, the catalytic conversion of carbon monoxide is carried out at a temperature of 200-220°C in one stage in a radial-spiral reactor, and the required process temperature is maintained by water evaporative cooling with the delivery of commercial saturated water vapor to the consumer, the purification of natural gas from sulfur compounds, the purification of a nitrogen-hydrogen mixture from oxygen-containing compounds and the synthesis of ammonia are also carried out in radial-spiral reactors, and the temperature of the flue gases after the burner before the first-stage natural gas conversion reactor is maintained within the range of 900-1100°C by recirculating a portion of the cooled flue gases with their mixing with air, supplied to the burner,wherein the heat of the converted gas is used to preheat the initial natural gas before desulfurization and to generate steam, which is then sent to carry out steam and steam-air reforming of natural gas, and the heat of the flue gases after the first stage natural gas reforming reactor is used to preheat the mixture of air with flue gases, purge and tank gases supplied to the burner (patent RU 2445262 C1, IPC C01C 1 / 04, filed 11.01.2011, published 20.03.2012). The disadvantages of the invention are:

[0009] - lack of information on the method used to purify natural gas from sulfur compounds and the ways of utilizing the extracted sulfur;

[0010] - catalysts for numerous catalytic processes are not indicated, since different catalysts require different operating temperature ranges to implement the same process;

[0011] - the complexity of regulating the process to ensure an optimal ratio of nitrogen and hydrogen in the ammonia synthesis reactor by changing the flow rates simultaneously in several previous reactors;

[0012] - a complex system for regulating the temperature of flue gases after the burner before the first stage natural gas conversion reactor by recirculating part of the cooled flue gases with their mixing with air;

[0013] - the use of multiple types of fuel media on a single burner device, the composition and quantities of which may vary depending on the operating mode of the installation;

[0014] The flow chart shown in the patent does not depict the carbon dioxide and expansion gas flows from the carbon dioxide purification unit 21. There is no understanding of their intended uses. The hydrogen source for the desulfurization unit 4 is not shown.

[0015] A method for producing ammonia is known, which contains the following stages:

[0016] - distribute the supplied gas containing nitrogen and hydrogen into an ammonia converter, wherein the converter contains:

[0017] - a first shell having two or more separate layers of catalyst placed therein;

[0018] - a second shell located around the first shell;

[0019] - a first heat exchanger located outside the first shell and in fluid communication with it;

[0020] - a second heat exchanger located outside the second shell and in fluid communication with it; and

[0021] - a flow channel located within the first shell, wherein two or more separate layers of catalyst within the first shell are arranged around the flow channel, wherein a first portion of the feed gas is introduced into the first shell and a second portion of the feed gas is introduced into the second shell;

[0022] - provide reaction of the first part of the supplied gas in the presence of a catalyst to obtain an ammonia output stream;

[0023] - providing for the transfer of at least a portion of the heat of reaction from the ammonia outlet stream to produce steam within the first heat exchanger;

[0024] - provide transfer of at least a portion of the heat of reaction from the ammonia outlet stream of the second portion of the supplied gas within the second heat exchanger;

[0025] - introduce the heated second part of the supplied gas into the first shell;

[0026] - provide a reaction of the heated second part of the supplied gas in the presence of a catalyst (patent RU 2469953 C2, IPC C01C 1 / 04, B01J 8 / 04, filed on 09 / 08 / 2008, published on 12 / 20 / 2012). The disadvantages of the invention are: - excessive complication of the design of the ammonia converter used, making it impossible to replace the catalyst in the converter without dismantling at least part of the converter;

[0027] - the presence in the ammonia converter of a large gap between the two shells - the first, providing the catalytic process of ammonia synthesis, and the second (the converter body itself), providing heat transfer through the wall of the first shell from the reaction mixture in the first shell to the flow of the second part of the supplied gas passing through the gap - increases the metal consumption of the body with the diameter of the first shell Di and the size of the gap Z by at least (1 + Z / Di) times, in addition, with a low heat transfer coefficient in the gas-gas system, the relatively small surface of the first shell does not allow for sufficient heat removal of the heat of the ammonia synthesis reaction released inside the shell (92 kJ / mol) for the entire reaction mixture by only the second part of the feedstock flow;

[0028] - the temperature control system in the ammonia converter becomes more complex, as it must be maintained while balancing the heat supply and heat removal systems on two feed streams of the feed gas in four heat exchangers and a furnace (Fig. 2 of patent RU 2469953 C2).

[0029] A method for producing ammonia and ammonia derivatives from natural gas feedstock is known, which includes:

[0030] - conversion of natural gas into synthesis gas at the inlet;

[0031] - synthesis of ammonia from synthesis gas in the synthesis circuit;

[0032] - using at least part of the ammonia to obtain an ammonia derivative, wherein the method is carried out with energy consumers (2) requiring mechanical energy for operation, and heat consumers (3) requiring heat supply for operation, wherein

[0033] - part (15) of the natural gas raw material is used to supply fuel to a piston gas engine (6); - the energy (7) generated by the gas engine (6) is used to meet, at least partially, the energy needs of energy consumers (2);

[0034] - heat is recovered from the exhaust gas of the gas engine and at least part of the heat is recovered to provide it to at least one of the said heat consumers (3), wherein the heat recovered from the exhaust gas of the gas engine is at least part of the low-temperature heat (10) transferred to at least one of the heat consumers by means of a heat transfer medium that is heated by indirect heat exchange with the exhaust gas to a temperature of no higher than 200°C (patent RU 2682584 C2, IPC C01C 1 / 04, C07C 273 / 00, B01J 19 / 00, declared on 05.08.2015, published on 19.03.2019). The disadvantages of the claimed invention are:

[0035] - the efficiency of the internal combustion engine in terms of fuel consumption - natural gas - is low (efficiency at the level of 50%) and is significantly lower than the efficiency of fire heaters (efficiency at the level of 80-85%);

[0036] - the use of heat recovered from the exhaust gas of one gas engine cannot significantly improve the heat balance of the ammonia production process as a whole, and the installation of a large number of gas engines will complicate equipment maintenance.

[0037] Also known is the method for producing low-carbon ammonia from natural gas, "Decarbonized Ammonia-3000", which is closest in a set of technological procedures to the claimed invention, consisting in the fact that on the technological line natural gas and steam are heated and subjected to primary reforming, the obtained converted gas is subjected to secondary reforming using a steam-air-oxygen mixture, after which the obtained converted gas is sent to the conversion of carbon dioxide, the obtained synthesis gas is subjected to purification from carbon dioxide, then methanation of the purified synthesis gas is carried out, after which ammonia is synthesized, and the purge gas from the ammonia synthesis process is used as fuel for heating natural gas and steam, while on the fuel line natural gas and steam are heated and subjected to primary reforming, the obtained converted gas is subjected to secondary reforming using a steam-air-oxygen mixture,after which the resulting converted gas is sent to carbon dioxide conversion, the resulting synthesis gas is purified to remove carbon dioxide, then the pressure of the purified synthesis gas is reduced and part of it is used as fuel for heating natural gas and steam in the fuel line and the other part of it together with the purge gas as fuel for heating natural gas and steam in the process line (patent RU 2808874 C1, IPC C01C 1 / 04, SOSh 3 / 02, C01B 3 / 36, filed on 23.08.2023, published on 05.12.2023). The disadvantages of the claimed invention are:

[0038] - a sharp increase in the costs of ammonia production associated with the creation of an additional fuel line for syngas production, duplicating in terms of equipment almost entirely the process line with the exception of the methanation and ammonia production stages;

[0039] - the additional fuel line processes natural gas in quantities that are several times lower than the capacity of the process line, which requires the use of different capacity devices for the same purpose on each of the lines;

[0040] - the almost double number of units operated on two lines sharply increases the risk of emergency situations leading to the shutdown of the entire ammonia production;

[0041] - it is necessary to additionally include a high-pressure oxygen production line in the ammonia production scheme to create the required flow of steam / air-oxygen mixture; a source of hydrogen for the needs of purifying raw natural gas from sulfur compounds is not provided;

[0042] - there is no provision for the extraction of ammonia from the purge gas, which leads to losses of the target product and to the risk of the formation of nitrogen oxides in the combustion heater during gas combustion;

[0043] - there is no provision for cleaning flue gases from nitrogen oxides, which will lead to emissions of toxic substances into the atmosphere;

[0044] - no information is provided on the process condensate formed both during the methanation of synthesis gas and during the cooling and separation of synthesis gas before purification from carbon dioxide.

[0045] DISCLOSURE OF THE INVENTION

[0046] The objective of the claimed invention is to reduce the consumption of natural fuel gas by redistributing internal energy-saturated flows in the ammonia production system, reducing the environmental load on the external environment and reducing the risks of emergency shutdown of ammonia production.

[0047] The stated problem is solved due to the fact that in the method for producing ammonia from natural gas, which consists in the fact that the initial natural gas is successively purified from sulfur-containing impurities by catalytic hydrotreating, heated and subjected to primary reforming in stages of the process line, the obtained converted gas is subjected to secondary reforming followed by the conversion of carbon monoxide into carbon dioxide, after which the obtained synthesis gas is purified from carbon dioxide, then methanation of the purified synthesis gas is carried out, after which ammonia is synthesized, while catalytic hydrotreating is supplemented by chemical adsorption of the formed hydrogen sulfide, heating of the process media - a mixture of the initial natural gas with hydrogen, a mixture of natural gas and water vapor, process air, superheating of high-pressure steam - is carried out due to differential cooling of the flue gases of the primary reforming furnace;High pressure steam is generated using the heat from the secondary reforming synthesis gas; the process line for the produced synthesis gas between the synthesis gas methanation stage and the ammonia synthesis and condensation stage is supplemented by stages of adsorption drying and low-temperature rectification of the synthesis gas, while the ammonia synthesis and condensation stage ensures the production of liquefied ammonia; the flue gases from the primary reforming furnace are selectively purified to remove nitrogen oxides using catalysts based on vanadium, titanium, tungsten or aluminum oxides; the process line is duplicated by a similar second process line, and both process lines are connected by direct and reverse flows to off-site facilities.

[0048] Sulfur is a catalytic poison for the catalysts used in the ammonia production process, so sulfur is removed from the feed gas through hydrotreating and chemical adsorption processes. Purge gas obtained during the ammonia synthesis stage is used as a hydrogen source for the hydrotreating process.

[0049] Purification of the original natural gas from sulfur-containing impurities by catalytic hydrotreating allows the conversion of impurities such as carbon disulfide CS2, carbonyl sulfide COS, mercaptans, sulfides and disulfides into hydrogen sulfide according to reactions (1)-(5):

[0050] CS2+ 4H22H2S + CH4, (1) and

[0051] COS + H2-► H2S + CO, (2) and

[0052] RSH + H2-► H2S + RH, (3) and

[0053] Ri SR2+ 2Н2-> H2S + RiH + R2H, (4) and where R, Ri and R2 are hydrocarbon radicals.

[0054] In the second stage of natural gas desulfurization, hydrogen sulfide is chemisorbed on a catalyst containing metal oxide MeO, forming metal sulfide MeS according to reaction (6):

[0055] Natural gas, purified from sulfur compounds, is mixed with water vapor, heated by recovering heat from flue gases, and then undergoes a process of steam (primary) reforming.

[0056] Primary reforming is carried out by breaking the H-C bonds of methane and O-H bonds of water at high temperature and proceeds according to the following reactions (7) and (8), limited by equilibrium:

[0057] CH + CO + 3H2 (heat of reaction - 206.4 kJ / mol) (7) and

[0058] CO + H2O -> CO2 + H2 (heat of reaction - 41.2 kJ / mol) (8).

[0059] The combination of primary reforming reactions is generally endothermic.

[0060] The reformed gas from the reformer and heated process air enter the secondary reformer, the upper section of which is a combustion chamber, and the lower section of which contains a catalyst bed. Secondary reforming involves a sequential combination of combustion reactions:

[0061] CH4 + 3 / 2O2-► CO + 2H2O (9),

[0062] 2H2+ O2— ► 2H2O (10) and the steam reforming reactions described above.

[0063] The heat of combustion of hydrogen and methane provides both heating of the reaction mixture and the subsequent endothermic steam reforming reaction. Excess heat from secondary reforming is used to generate high-pressure steam, which is superheated by the heat of the flue gas in the convection chamber of the steam reforming furnace.

[0064] Steam methane reforming occurs both during primary reforming in a steam reforming furnace and during secondary reforming in a secondary reforming reactor. Controlling the methane conversion rate during the primary and secondary reforming stages allows the production of reformed gas from the steam reforming furnace and secondary reforming reactor at the required temperatures, maximizing the efficient use of both the heat from the generated syngas and the heat from the steam reforming furnace flue gases. Increasing the methane conversion rate in the secondary reforming reactor is possible by introducing excess air into the system compared to that required for ammonia synthesis (typically, process air is supplied in the amount necessary to achieve a hydrogen-to-nitrogen molar ratio in the syngas of three to one). Controlling the required hydrogen-to-nitrogen ratio and removing excess nitrogen is possible during the syngas preparation stage through cryogenic purification.

[0065] Since the heating of all process media - the mixture of the original natural gas with hydrogen entering the sulfur removal process, the mixture of natural gas and water vapor entering the primary reforming, the process air entering the secondary reforming, and the superheating of high-pressure steam - is carried out by differential cooling of the flue gases of the primary reforming furnace, this allows for the most complete recovery of heat from the flue gases by forming an optimal sequence of recuperative heat exchangers, taking into account the temperature differences of the heated flows and the differential sections of the flue gas flow.

[0066] Efficient redistribution of internal energy-rich flows in the ammonia production system reduces fuel consumption. Since ammonia synthesis requires a high-purity mixture of nitrogen and hydrogen, the purified syngas process line between the syngas methanation stage and the ammonia synthesis and condensation stage is supplemented with adsorption drying and low-temperature syngas distillation stages. Low-temperature distillation produces syngas with the nitrogen-to-hydrogen ratio required for ammonia synthesis, while adsorption drying of the syngas is necessary for the low-temperature distillation process to prevent water vapor from condensing from the syngas and freezing in the equipment at low temperatures.

[0067] The stage of ammonia synthesis and condensation ensures the production of high-purity liquefied ammonia as a commercial product.

[0068] The flue gases from the primary reforming furnace are selectively purified to remove nitrogen oxides, which reduces the emission of toxic substances into the atmosphere and lowers the environmental burden.

[0069] Duplicating an ammonia production line with a similar second production line, with both production lines connected by direct and reverse flows to off-site facilities, allows for doubling ammonia production within the region and reducing the risk of an emergency shutdown of ammonia production, ensuring the production of marketable products in the event of the failure of one or more units, or their scheduled repair, or modernization on one of the production lines.

[0070] High efficiency of the numerous catalytic stages of the ammonia production process is ensured by the selection of active catalysts that enable the corresponding reactions to occur at high rates, which reduces catalyst loading in the reactors and indirectly reduces ammonia production amortization costs. For natural gas hydrotreating, it is advisable to use a reactor with a fixed-bed catalyst based on cobalt and molybdenum oxides, and for chemical adsorption of hydrogen sulfide formed during hydrotreating, one or more units with a fixed-bed chemisorbent based on zinc oxide, installed in series and / or in parallel, should be used. The use of hydrogen sulfide chemical adsorption reactors installed in series allows for the discharge of spent catalyst without interrupting the process.

[0071] It is recommended to use a steam reforming furnace with a tube bundle filled with a nickel-based catalyst for primary reforming, and to use a reactor with a fixed bed of catalyst based on Cr, and / or Mn, and / or Fe, and / or Co, and / or Ni, and / or Cu, and / or Zn, and / or Ga, and / or In, and / or Ag, and / or Ti, and / or Y, and / or Zr, and / or La, and / or Ca, and / or Sr, and / or Ba, or mixtures of two or three oxides of this set, for secondary reforming.

[0072] In order to produce an additional amount of hydrogen, as well as to purify the converted gas from the main volume of carbon monoxide, it is advisable to subject the synthesis gas from secondary reforming, cooled by the generation of high-pressure steam, to catalytic conversion of carbon monoxide into carbon dioxide in two sequentially installed reactors for high-temperature and low-temperature conversion of carbon monoxide with intermediate heat removal, while for high-temperature conversion of carbon monoxide into carbon dioxide, use fixed catalyst beds based on Fe / Cr / Cu, and for low-temperature conversion of carbon monoxide into carbon dioxide, use fixed catalyst beds based on Cu / Al / Zn.

[0073] In the final stages of converting gas purification, it is useful to use a reactor with fixed-bed nickel-based catalyst for syngas methanation, and a reactor with fixed-bed iron-based catalyst for ammonia synthesis. Adsorption drying of the syngas is advisable, allowing for the reduction of residual moisture concentration in the gas stream to a few ppm, using KA-type zeolites as adsorbents, which possess unique selectivity and high moisture capacity. The load on syngas dehydrators can be reduced by pre-condensation and subsequent separation of water formed during methanation, as well as by sequential gas cooling with heat recovery to heat the methanation reactor feedstock at the initial stage and using ammonia as a refrigerant in the final cooling stage.

[0074] When cleaning flue gases from nitrogen oxides, it is recommended to use vanadium, titanium, tungsten or aluminum oxides.

[0075] It is beneficial to maintain the low-temperature synthesis gas distillation stage so that the synthesis gas enters the distillation stage with a nitrogen:hydrogen molar ratio of 1:2, and enters the ammonia synthesis and condensation stage with a nitrogen:hydrogen molar ratio of 1:3. This allows for the recovery of excess nitrogen and optimal feedstock for ammonia synthesis during the distillation process. The excess nitrogen recovered from the dried synthesis gas during the low-temperature synthesis gas distillation stage is sent as regeneration gas to the adsorption drying stage of the synthesis gas. The waste regeneration gas from the adsorption drying stage of the synthesis gas is recommended to be sent as fuel to the natural gas reforming stage.

[0076] Hydrogen-containing blow-offs generated at the stage of ammonia synthesis and condensation form streams of high- and low-pressure hydrogen-containing blow-offs, with the high-pressure hydrogen-containing blow-offs being divided into three parts, the first of which is sent to the stage of natural gas desulphurization, the second is fed for recycling to the stage of synthesis gas methylation, and the third is fed for mixing with low-pressure hydrogen-containing blow-offs.

[0077] It is useful to use low-pressure hydrogen-containing blowdowns as a carbon-free fuel at the stage of natural gas reforming,

[0078] 5 thereby sharply reducing the environmental burden on the environment.

[0079] LIST OF DRAWINGS

[0080] The figure shows a schematic diagram of one of the possible options for implementing the method for producing ammonia from natural gas along the first process line using the following

[0081] 10 designations:

[0082] 101 - process air compression section;

[0083] 102 - desulfurization department;

[0084] 103 - primary and secondary reforming department;

[0085] 104 - carbon monoxide conversion section;

[0086] 105 - amyl gas purification department;

[0087] 106 - methanation department;

[0088] 107 - gas drying section;

[0089] 108 - cryogenic gas purification department;

[0090] 109 - gas compression section;

[0091] 20 110 - ammonia synthesis department;

[0092] 111 - refrigeration cycle compartment;

[0093] 112 - exhaust gas washing section;

[0094] 113 - separation of process condensate stripping;

[0095] 114 - separation of the steam and boiler water system;

[0096] 1-33 - pipelines.

[0097] The second process line is identical in structure to the first process line and is not shown in the figure.

[0098] BRIEF DESCRIPTION OF DRAWINGS

[0099] The feedstock natural gas stream from the plant boundaries is sent via pipeline 1 to desulfurization section 102. Since sulfur is a catalytic poison for the catalysts used in the ammonia production process chain, the feedstock natural gas stream is purified from sulfur by means of hydrotreating and chemical adsorption processes in desulfurization section 102. The feedstock natural gas stream is also sent via pipeline 2 to primary and secondary reforming section 103 for fuel purposes. For the hydrotreating process, a stream of purified purge gas from purge gas washing section 112 is sent to desulfurization section 102 via pipeline 25 as a hydrogen source. The hydrotreating process in desulfurization section 102 is carried out in one reactor. The process of chemical binding of sulfur from hydrogen sulfide formed during hydrotreating is carried out in two reactors installed in series.The reactor piping allows for the discharge of spent catalyst without stopping the process.

[0100] The natural gas stream, purified from sulfur compounds, supplied through pipeline 3, is mixed with water vapor supplied through pipeline 10 from the process condensate stripping section 113, heated and then subjected to the primary reforming process in the steam reforming furnace in the primary and secondary reforming section 103. Synthesis gas is produced in furnace tubes filled with a nickel-based catalyst.

[0101] The sources of heat required for the steam reforming process are the flows of purified blowdown gas, exhaust regeneration gas, expansion gas, and also raw natural gas supplied to the primary and secondary reforming section 103 to the furnace burners via pipelines 24, 17, 12, and 2, respectively.

[0102] Provision is made for the recovery of heat from flue gases to heat the flows of a mixture of raw natural gas purified from sulfur compounds and water vapor supplied through pipelines 3 and 10, process air supplied through pipeline 5, high-pressure water vapor (not shown in the figure), a mixture of raw natural gas and purified blowdown gas supplied through pipelines 1 and 25 for purification from sulfur compounds in the desulfurization section 102.

[0103] The converted gas from the steam reforming furnace and the heated process air enter the secondary reforming reactor, where they undergo secondary reforming.

[0104] The source of nitrogen for ammonia synthesis is a stream of atmospheric air supplied through pipeline 4. After filtration and compression in the compressor in process air compression section 101, it is supplied through pipeline 5 to the primary and secondary reforming section 103 as process air. Interstage cooling of the compressed process air stream and separation of the resulting condensate are provided.

[0105] To generate additional hydrogen and to purify the converted gas from the bulk of carbon monoxide after separation of primary and secondary reforming 103, the converted gas stream is fed through pipeline 6 to the carbon monoxide conversion section 104. Conversion is carried out in two reactors installed in series with intermediate heat removal for heating the boiler water. The first reactor uses an iron-based catalyst, the second - a copper-based catalyst. The use of two reactors is due to the reduced load on the copper catalyst, which is more expensive, but which provides a deeper conversion of CO to CO2.

[0106] The converted gas stream from the carbon monoxide conversion section 104 is fed via pipeline 7 to the amine gas scrubbing section 105, which removes carbon dioxide from the converted gas stream through absorption with an aqueous amine solution. Carbon dioxide absorption is carried out at elevated pressure and reduced temperature. The saturated amine solution is regenerated at reduced pressure and elevated temperature and then reused for absorption (not shown in the figure). Intermediate removal of light gases dissolved in the amine is provided, forming the expansion gas stream, which is sent via pipeline 12 to the primary and secondary reforming section 103 and used as fuel in the steam reforming furnace. The recovered carbon dioxide stream is discharged from the unit via pipeline 8.

[0107] Oxygen-containing components are contaminants for the ammonia synthesis catalyst, so the final step in syngas purification is methanation—a process in which residual carbon monoxide is converted to methane, which is an inert gas during ammonia synthesis. The converted gas stream obtained in amyl gas purification section 105 is sent via pipeline 13 to methanation section 106.

[0108] The process condensate stream generated during the methanation of synthesis gas in methanation section 106 is sent via pipeline 14 to amine gas purification section 105 for mixing with the condensate stream generated during the cooling and separation of the converted gas. The total process condensate stream is sent via pipeline 9 to process condensate stripping section 113, where the condensate is purified by stripping with a medium-pressure steam stream supplied via pipeline 29 to the bottom of the stripping column as a stripping agent.

[0109] The stripping gas stream is fed through pipeline 10 to the primary and secondary reforming section 103 for mixing with the stream of raw natural gas purified from sulfur compounds, fed through pipeline 3. The process condensate stream, purified from impurities of ammonia, methanol and carbon dioxide in the process condensate stripping section 113, is discharged through pipeline 11 to the boundaries of the plant.

[0110] The converted gas stream from methanation section 106 enters gas drying section 107 via pipeline 15, where the synthesis gas is purified from water by moisture adsorption on zeolites. The dried gas descends through drying adsorbers and is then filtered to remove entrained zeolite dust. Desiccant regeneration is accomplished at reduced pressure by an ascending flow of regeneration gas supplied via pipeline 18. The heat required for desiccant regeneration is supplied by medium-pressure steam in a regeneration gas heater (not shown).

[0111] The flow of dried converted gas is fed through pipeline 16 to cryogenic gas purification section 108 to remove excess nitrogen, the main amount of methane and part of the argon from the synthesis gas, which are then used as a flow of regeneration gas sent through pipeline 18 to gas drying section 107. In cryogenic gas purification section 108, a flow of purified converted gas is produced, fed through pipeline 19 to gas compression section 109, with the nitrogen:hydrogen ratio required for ammonia synthesis, equal to 1:3.

[0112] The flows of purified converted gas and recycled gas are fed through pipelines 19 and 22, respectively, to gas compression section 109, where the flows are compressed to the pressure required for ammonia synthesis, after which the flow of compressed converted gas is sent through pipeline 20 to the ammonia synthesis section PO, where it is heated and sent to a reactor in which ammonia is formed from hydrogen and nitrogen (an exothermic reaction limited by chemical equilibrium).

[0113] The flow of gas product mixture entering the refrigeration cycle section 111 via pipeline 21 from the ammonia synthesis reactor of the ammonia synthesis section PO is cooled in sequentially installed heat exchangers and then sent to a separator in which the liquid phase, consisting of condensed ammonia, is separated from the gas phase, consisting primarily of unreacted nitrogen and hydrogen, returned in the form of a flow of recirculated gas via pipeline 22 after cold recovery to the gas compression section 109.

[0114] Refrigeration cycle section 111 consists of an ammonia cooling system, which is an open refrigeration cycle consisting of a refrigerant compressor, a refrigerant condenser, a refrigerant buffer tank, and one or more refrigerant evaporators (not shown in the figure). Ammonia is condensed from the gas product mixture stream supplied through pipeline 21 from the ammonia synthesis section PO in one or more refrigerant evaporators in refrigeration cycle section 111. Refrigerant vapor from the evaporators is directed to the corresponding stages of the refrigerant compressor, which ensures its circulation. The refrigerant from the compressor enters the condenser and then, in the liquid phase, enters the refrigerant collector (not shown in the figure). The liquefied ammonia stream is withdrawn from the plant as a marketable product through pipeline 27 from refrigeration cycle section 111.

[0115] In order to prevent the accumulation of inert gases in the ammonia synthesis circuit, provision is made for blowing off a portion of the recirculated synthesis gas flow, which is sent as a purge gas, which is a high-pressure hydrogen-containing purge, through pipeline 23 to the purge gas washing section 112. In order to capture ammonia, which is part of the purge gas transported through pipeline 23, and uncondensed gases, which are low-pressure hydrogen-containing purges, of the refrigeration cycle section 111 transported through pipeline 33, provision is made for water washing of high- and low-pressure gases in the purge gas washing section 112.The purified purge gas stream obtained in the purge gas scrubbing section 112 and representing a high-pressure hydrogen-containing purge is partially directed via pipeline 25 to the desulfurization section 102 and is used as a source of hydrogen for the hydrotreating of the natural gas feedstock, partially fed via pipeline 32 to the methanation section for recycle, and partially directed for mixing with low-pressure hydrogen-containing purges. The purified purge gas stream obtained in the purge gas scrubbing section 112 and representing a low-pressure hydrogen-containing purge is fed via pipeline 24 to the primary and secondary reforming section 103 and is used as fuel. The ammonia stream recovered from the purge gases is returned via pipeline 26 to the refrigeration cycle section 111.

[0116] The steam and boiler water system section 114 provides for the deaeration of the demineralized water flow coming from the boundaries of the plant through pipeline 28. The steam and boiler water system section 114 also supplies treated boiler water to the process sections for heat recovery and generation of high-pressure (in the primary and secondary reforming section 103) and low-pressure (in the amine gas cleaning section 105) steam, and distributes steam between the high-, medium-, and low-pressure headers, as well as supplies steam from the headers to consumers. The flow of excess medium-pressure steam through pipeline 31 and the flow of turbine condensate through pipeline 30 are discharged from the steam and boiler water system section 114 from the plant.

[0117] The second technological line operates similarly to the first.

[0118] Using the technology discussed above, a calculation was performed for one ammonia production line according to the claimed invention. The capacity of the two production lines for incoming natural gas is 300 tons / hour. One production line receives 150 tons / hour of natural gas. The calculation results for determining the compositions of the main streams and their flow rates are presented in Table 1.

[0119] In addition to hydrogen sulfide, the original natural gas contains mercaptans (the concentration of mercaptan sulfur is 0.0063 g / m3 3), therefore, as a result of hydrotreating, about 1.4 kg / h of mercaptans are converted into hydrogen sulfide, which is then chemisorbed by zinc oxide. The highly purified natural gas stream, containing 96.6% methane and about 2.7% Cr-C6 hydrocarbons, is almost completely converted during primary and secondary reforming into synthesis gas, in which the residual methane concentration is 1.32%. The resulting synthesis gas with a carbon monoxide concentration of 8.1% during carbon monoxide conversion ensures the production of additional hydrogen, and the resulting synthesis gas contains about 42% hydrogen, 21% nitrogen, and 36% ballast carbon dioxide and water vapor. Next, in the amine purification section, water vapor is removed from the synthesis gas stream by condensation with the utilization of condensation heat in recuperative heat exchangers, and carbon dioxide is extracted from the synthesis gas stream cooled from 231°C to 45°C with an aqueous solution of diethanolamine, which is released as a by-product of the process during the regeneration of the absorbent.Carbon dioxide can then be used at gas or oil production facilities to maintain reservoir pressure, at gas chemical plants to produce urea, methanol and other products, at food and other industrial plants.

[0120] Following methanation, the syngas, with a hydrogen:nitrogen ratio of 2:1 (64.6% hydrogen and 32.3% nitrogen), undergoes deep drying and low-temperature distillation, which removes excess nitrogen from the syngas and brings the hydrogen:nitrogen ratio to 3:1. This enabled the production of 276 t / h of commercial liquid ammonia with a purity of 99.975% on a single process line during ammonia synthesis, as well as 8 t / h of bleeds via pipelines 24 and 25, consisting largely of hydrogen. This ensures the hydrotreating of the feedstock and produces carbon-free fuel for the production process.

[0121] In total, from 300 t / h of hydrocarbon feedstock supplied to the ammonia plant, 552 t / h of high-quality liquid ammonia can be produced, which meets the requirements of the Interstate Standard for exported ammonia (according to GOST 6221-90, the purity of liquid ammonia is at least 99.9%), as well as 16 t / h of hydrogen-containing gas.

[0122] Thus, the technical result of the claimed method for producing ammonia from natural gas is a reduction in the consumption of natural gas fuel due to the redistribution of internal energy-saturated flows in the ammonia production system, the production of hydrogen-containing gas for the hydrotreating of the feedstock, an extension of the service life of catalysts deactivated by sulfur, due to the complete extraction of sulfur-containing impurities from natural gas by two-stage sulfur removal using catalytic hydrotreating and chemisorption methods, a reduction in the environmental load of the external environment due to the use of carbon-free fuel and the removal of nitrogen oxides from flue gases and a reduction in the risks of an emergency shutdown of ammonia production as a whole.

Claims

CLAUSES OF THE INVENTION 1. A method for producing ammonia from natural gas, which consists in the fact that the initial natural gas is successively purified in stages of a process line to remove sulfur-containing impurities by catalytic hydrotreating, heated and subjected to primary reforming, the resulting converted gas is subjected to secondary reforming followed by the conversion of carbon monoxide into carbon dioxide, after which the resulting synthesis gas is purified to remove carbon dioxide, then the purified synthesis gas is methanated, after which ammonia is synthesized, characterized in that the catalytic hydrotreating is supplemented by chemical adsorption of the formed hydrogen sulfide, the heating of the process media - a mixture of the initial natural gas with hydrogen, a mixture of natural gas and water vapor, process air, superheating of high-pressure steam - is carried out due to differential cooling of the flue gases of the primary reforming furnace;High pressure steam is generated using the heat from the secondary reforming synthesis gas; the process line for the produced synthesis gas between the synthesis gas methanation stage and the ammonia synthesis and condensation stage is supplemented by stages of adsorption drying and low-temperature rectification of the synthesis gas, while the ammonia synthesis and condensation stage ensures the production of liquefied ammonia; the flue gases from the primary reforming furnace are selectively purified to remove nitrogen oxides using catalysts based on vanadium, titanium, tungsten or aluminum oxides; the process line is duplicated by a similar second process line, and both process lines are connected by direct and reverse flows to off-site facilities.

2. The method according to paragraph 1, characterized in that a reactor with a fixed catalyst bed based on cobalt and molybdenum oxides is used to carry out hydrotreating of natural gas.

3. The method according to paragraph 2, characterized in that one or more sequentially and / or parallel-installed apparatuses with a fixed layer of chemisorbent based on zinc oxide are used for chemical adsorption of hydrogen sulfide formed during hydrotreating.

4. The method according to paragraph 1, characterized in that a steam reforming furnace with a tube bundle of nickel-based catalyst is used to carry out primary reforming.

5. The method according to claim 1, characterized in that for carrying out secondary reforming, a reactor with a fixed bed of catalyst based on Cr, and / or Mn, and / or Fe, and / or Co, and / or Ni, and / or Cu, and / or Zn, and / or Ga, and / or In, and / or Ag, and / or Ti, and / or Y, and / or Zr, and / or La, and / or Ca, and / or Sr, and / or Ba, or mixtures of two or three oxides of this set is used.

6. The method according to paragraph 1, characterized in that the conversion of carbon monoxide into carbon dioxide is carried out in two sequentially installed reactors for high-temperature and low-temperature conversion of carbon monoxide with intermediate heat removal.

7. The method according to claim 6, characterized in that fixed catalyst layers based on Fe / Cr / Cu are used for high-temperature conversion of carbon monoxide into carbon dioxide.

8. The method according to claim 6, characterized in that fixed catalyst layers based on Cu / Al / Zn are used for low-temperature conversion of carbon monoxide into carbon dioxide.

9. The method according to paragraph 1, characterized in that a reactor with fixed beds of nickel-based catalyst is used for methanation of synthesis gas.

10. The method according to paragraph 1, characterized in that a reactor with fixed layers of iron-based catalyst is used for the synthesis of ammonia. I. The method according to paragraph 1, characterized in that zeolites of the KA type are used for drying the synthesis gas.

12. The method according to paragraph 1, characterized in that the synthesis gas is sent to the stage of low-temperature rectification of the synthesis gas with a nitrogen:hydrogen molar ratio of 1:2, and the synthesis gas is sent to the stage of synthesis and condensation of ammonia with a nitrogen:hydrogen molar ratio of 1:

3.

13. The method according to paragraphs 1 and 12, characterized in that the excess nitrogen extracted from the dried synthesis gas at the stage of low-temperature rectification of the synthesis gas is sent as regeneration gas to the stage of adsorption drying of the synthesis gas.

14. The method according to paragraphs 1 and 13, characterized in that the waste gas from the regeneration stage of the adsorption drying of the synthesis gas is sent as fuel to the stage of reforming natural gas.

15. The method according to claim 1, characterized in that the hydrogen-containing purges formed at the stage of synthesis and condensation of ammonia form high- and low-pressure hydrogen-containing purge streams.

16. The method according to paragraph 15, characterized in that the high-pressure hydrogen-containing bleeds are divided into three parts, the first of which is sent to the natural gas desulfurization stage, the second is fed for recycling to the synthesis gas methanation stage, and the third is fed for mixing with low-pressure hydrogen-containing bleeds.

17. The method according to claim 15, characterized in that the low-pressure hydrogen-containing blowdowns are used as a carbon-free fuel at the stage of reforming natural gas.

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