Process for producing low-carbon ammonia from natural gas

Abstract

The invention relates to technologies for producing low-carbon ammonia by converting natural gas by primary and secondary reforming into a synthesis gas containing hydrogen and nitrogen. The obtained synthesis gas is purified of carbon dioxide, and the purified synthesis gas is methanated, followed by the high-pressure catalytic synthesis of ammonia. The technical result is that the entire volume of the synthesis gas from the process line is used for the synthesis of ammonia and the process produces sufficient cold to support the ammonia recovery stage.

Description

[0001] METHOD FOR PRODUCING LOW-CARBON AMMONIA FROM NATURAL GAS

[0002] Field of technology

[0003] The invention relates to technologies for producing low-carbon ammonia by converting natural gas into synthesis gas containing hydrogen and nitrogen, followed by catalytic synthesis of ammonia at high pressure.

[0004] State of the art

[0005] A method for producing ammonia from conventional natural gas includes a stage of converting natural gas into synthesis gas containing hydrogen and nitrogen, and a stage of synthesizing ammonia, in which said synthesis gas reacts at high pressure to form ammonia.

[0006] At the conversion stage, natural gas is sent to preparation, where gas is obtained that is purified from undesirable impurities. Next, the prepared natural gas is sent to the reforming stage, where converted gas is obtained, mainly containing nitrogen, hydrogen, carbon monoxide CO and carbon dioxide CO2. Then, the synthesis gas is sent to the CO conversion stage, where carbon monoxide is oxidized to carbon dioxide CO2. Carbon dioxide CO2 is removed from the resulting gas mixture and the synthesis gas is sent to the final purification stage - methanation.

[0007] At the ammonia synthesis stage, the synthesis gas is compressed and sent to the synthesis stage, where converted synthesis gas is obtained, mainly containing ammonia, nitrogen and hydrogen. The resulting mixture is sent to the ammonia extraction stage.

[0008] When using the method of producing ammonia from natural gas of the conventional type, the most common method is reforming, which takes place in two stages: preliminary steam reforming (primary reforming) and secondary reforming of the resulting gas with the addition of a steam-oxygen or steam-air-oxygen mixture (secondary reforming). In this method, the heat required for the primary reforming reaction is obtained by burning part of the natural gas in a reforming furnace. In this case, it is envisaged to use a fire heater to heat the process media and / or generate high-pressure steam for the needs of the ammonia production plant.

[0009] The above-described furnaces and fired heaters in conventional natural gas ammonia production plants are a source of carbon dioxide (CO2) emissions into the atmosphere due to the use of natural gas as fuel.

[0010] A method is known for producing ammonia from natural gas, including compression, heating and purification of natural gas from sulfur compounds, two-stage catalytic conversion of methane under pressure, including steam conversion in the first stage and steam-air conversion 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 conversion in the first stage of the conversion process, catalytic conversion of carbon monoxide contained in the converted 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 obtained ammonia with its subsequent delivery to the consumer, as well as the utilization of the heat of flue gases and their release into the environment (RU 2445262 C 1 , published 20.03.2012).The disadvantage of this method is the use of heat from natural gas burned in a burner, which causes the emission of carbon dioxide into the atmosphere.

[0011] The closest to the proposed method is the production of ammonia from natural gas, characterized by a low level of CO2 emissions into the atmosphere, which consists in the fact that natural gas and steam are heated in a furnace (VSP) and in preparatory (primary) reforming (PDR) and autothermal (secondary) reforming (ATR) units they are converted into synthesis gas containing H2, CO and CO2; ATR operates using a flow of air enriched with oxygen or oxygen; the synthesis gas is converted in the conversion reaction section (RCS) and then CO2 is removed in the decarbonization section (DCS) to obtain lean synthesis gas; part of the lean synthesis gas is used as fuel in the VSP furnace; the remaining part of the synthesis gas depleted in carbon dioxide is processed in a methanation unit (MET); the nitrogen leaving the air separation section (ASS) is added to the synthesis gas fraction leaving the MET and sent to the ammonia synthesis section (ASS), where ammonia and purge gas are obtained;hydrogen is extracted from the purge gas (H2O) and added to the nitrogen and synthesis gas stream before being fed to the SIN; the off-gas obtained in the H2O is added to a portion of the carbon dioxide-depleted synthesis gas stream sent as fuel to the VSP; in the RVR section, a stream of oxygen-enriched air or oxygen is also obtained, which is used in the ATR (RU 2759379 C2, published 12.11.2021).;

[0012] In this method, natural gas is not used as a fuel, so the method is characterized by a low level of carbon dioxide emissions into the atmosphere. The known method has the following disadvantages. Part of the synthesis gas, which can be sent to the ammonia synthesis stage in order to increase the yield of the marketable product, is sent for combustion as a fuel. In addition, the potential energy of the pressure of the purge gas flow, also sent for combustion as a fuel, is not used to produce cold, electricity. In addition, production is provided with water necessary for the process only from external sources.

[0013] The essence of the invention

[0014] The technical problem solved by the proposed method is to increase the overall efficiency of the method for producing low-carbon ammonia.

[0015] The technical result achieved by using the proposed method consists in ensuring the possibility of using the entire volume of synthesis gas on the process line for the synthesis of ammonia and self-sufficiency of the stage of ammonia isolation by cold.

[0016] The technical result is achieved by a method for producing ammonia, which consists in the fact that natural gas and water vapor 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 for carbon monoxide conversion, the obtained synthesis gas is purified from carbon dioxide, then the purified synthesis gas is methanated, after which ammonia is synthesized and isolated, which is removed as a finished product, and the purge gas after the isolation of ammonia is used as fuel for heating natural gas and water vapor, wherein according to the invention, after methanation the synthesis gas is compressed and sent for ammonia synthesis, after the isolation of ammonia the pressure of the purge gas is reduced by expanding it, the isolation of ammonia is carried out using a heat carrier,cooled as a result of the expansion of the purge gas, and the ammonia condensed from the purge gas during its expansion is removed as a finished product.

[0017] In addition, it is advisable to recycle part of the purge gas for compression and subsequent feeding to ammonia synthesis together with synthesis gas.

[0018] It is also advisable to carry out primary reforming using the heat of the converted gas after secondary reforming.

[0019] In addition, condensate is separated from the flue gases obtained during combustion of fuel by heating natural gas and water vapor, which is sent to generate water vapor, which is carried out using the heat of the converted gas after secondary reforming, and the resulting water vapor is used in primary reforming and to obtain a steam-air-oxygen mixture for secondary reforming.

[0020] In addition, it is advisable to use the purge gas after the release of ammonia and the reduction of pressure as fuel for heating the steam-air-oxygen mixture.

[0021] The technical result is achieved by the fact that the entire volume of the produced synthesis gas of the process line is sent to the synthesis stage, and only the purge gas is used as a low-carbon fuel, ensuring a reduction in CO2 emissions. Since a large flow of purge gas is sent for combustion, after the release of ammonia, it is necessary to reduce its pressure by expanding it, and the cold obtained during expansion is used in the release of ammonia.

[0022] List of drawings

[0023] Fig. 1 shows a diagram of the implementation of the proposed method for producing low-carbon ammonia.

[0024] Examples of the invention

[0025] The method for producing low-carbon ammonia from natural gas, which has received the conventional name “Decarbonized Ammonia-2500” (“AmDek-2500”), is carried out as follows.

[0026] Raw natural gas 101 with a pressure of about 5.0-7.5 MPa enters the sulfur compound removal line 1, where gas purified from undesirable impurities is obtained. Water vapor 103 is added to purified natural gas 102 and sent to the fire heater 2. Water vapor 103 is a mixture of high-pressure steam 104 coming from outside the process unit and high-pressure steam 114 from the process steam generation unit 6. Steam-gas mixture 107 heated to the temperature of the start of the steam reforming reaction enters the reaction tubes of the primary reforming reactor 3, where the process occurs due to the heat of the converted gas 110 leaving the secondary reforming stage 4. Then the converted gas 108 enters the secondary reforming reactor 4.

[0027] Atmospheric air 106 enters the process air compression line 5, where it is pumped to a pressure of about 5.0-7.0 MPa. Water vapor 104 and oxygen 105 are added to the compressed air and sent to the fire heater 2. High-pressure steam 114 from the process steam generation unit 6 (not shown in the diagram) can also be added to the water vapor 104. The heated steam-air-oxygen mixture 109 enters the secondary reforming reactor 4, where converted gas is obtained, containing mainly nitrogen, hydrogen, water vapor, carbon monoxide and carbon dioxide.

[0028] The heat of the outgoing converted gas PO is used to carry out the primary reforming reaction 3. The converted gas 111 from the tubular reactor enters the process steam generation unit 6, where the heat of the flow is used, among other things, to generate process steam 114 from the flue gas condensate 113.

[0029] The converted gas stream 112 enters the carbon monoxide conversion line 7, where in the presence of water vapor, catalytic conversion of carbon monoxide into carbon dioxide occurs with the formation of hydrogen. The synthesis gas 115 obtained as a result of the conversion, containing carbon dioxide, is sent to the stage of purification from carbon dioxide 8 by the amine purification method. Carbon dioxide 116 separated from the synthesis gas is sent for disposal.

[0030] Carbon dioxide-depleted synthesis gas 117 is sent to methanation reactor 9, where catalytic conversion of traces of carbon dioxide remaining after the carbon dioxide removal stage occurs. Next, synthesis gas flow 118 enters synthesis gas compression unit 10, where the synthesis gas pressure is increased to 20-22 MPa. Compressed synthesis gas 119 is sent to ammonia synthesis reactor unit 11.

[0031] Converted gas 120 from ammonia synthesis reactor unit 11, containing mainly nitrogen, hydrogen and ammonia, is sent to ammonia recovery unit 12. In ammonia recovery unit 12, converted gas 120 is cooled to a temperature below zero degrees Celsius, wherein ammonia contained in converted gas 120 partially condenses and is removed as a product - stream 121. To cool converted gas 120 in ammonia recovery unit 12, coolant stream 124 is used, which transfers cold from pressure reduction unit 13 to ammonia recovery unit 12. Coolant stream 125, heated in ammonia recovery unit 12, is returned to the pressure reduction unit for cooling.

[0032] The ammonia-depleted converted gas from the ammonia separation unit 12 is divided into two streams of the same composition: recycle gas 122 is returned to the compression unit 10 for mixing with fresh synthesis gas and subsequent use as a feedstock for ammonia synthesis, and purge gas 123 is fed to the pressure reduction unit 13.

[0033] The pressure of the purge gas 123 in the pressure reduction unit 13 is reduced by means of a Joule-Thompson valve or an expander to a level of about 0.2 MPa. As a result of the gas expansion, cooling to a temperature below minus fifty degrees Celsius occurs, wherein the ammonia contained in the purge gas 123 condenses and is removed as a product - stream 126. The cold obtained in the pressure reduction unit is removed by the heat carrier 125.

[0034] The resulting low-pressure purge gas 127 is intended for use as fuel in the fire heater 2. Part of the purge gas - stream 128, if necessary, is directed outside the installation for use as fuel in auxiliary production.

[0035] Flue gases 129 from the fire heater 2 with a temperature of about 100-120 degrees Celsius enter the flue gas water condensate extraction line 14. After the water condensate is extracted, flue gases 130, which consist of nitrogen, residual water and air impurities, are directed into the atmosphere.

[0036] Extraction of carbon dioxide 116 for subsequent utilization, together with the absence of carbon dioxide in flue gases, ensures a low level of CO2 emissions, amounting to less than 0.2 t / t (the number of tons of CO2 calculated per 1 ton of ammonia produced), which corresponds to a CO2 capture level of more than 90%.

Claims

Invention formula 1. A method for producing ammonia from natural gas, which consists in heating natural gas and steam and subjecting it to primary reforming, subjecting the resulting converted gas to secondary reforming using a steam-air-oxygen mixture, after which the resulting converted gas is sent for carbon monoxide conversion, the resulting synthesis gas is purified to remove carbon dioxide, then the purified synthesis gas is methanated, after which ammonia is synthesized and isolated, which is removed as a finished product, and the purge gas after the isolation of ammonia is used as fuel for heating the natural gas and steam, characterized in that after methanation the synthesis gas is compressed and sent for ammonia synthesis, after the isolation of ammonia the pressure of the purge gas is reduced by expanding it, the isolation of ammonia is carried out using a heat carrier cooled as a result of the expansion of the purge gas,and the ammonia condensed from the purge gas during its expansion is removed as a finished product.

2. The method according to paragraph 1, characterized in that part of the purge gas is sent as recycle for compression and subsequent feeding to ammonia synthesis together with synthesis gas.

3. The method according to paragraph 1, characterized in that the primary reforming is carried out using the heat of the converted gas after secondary reforming.

4. The method according to paragraph 1, characterized in that condensate is separated from the flue gases obtained during combustion of fuel by heating natural gas and water vapor, which is sent to generate water vapor, which is carried out using heat. converted gas after secondary reforming, and the resulting water vapor is used in primary reforming and to obtain a steam-air-oxygen mixture for secondary reforming.

5. The method according to paragraph 1, characterized in that the purge gas after the release of ammonia and the reduction of pressure is also used as fuel for heating the steam-air-oxygen mixture.

Images