Low-cost and water-saving system for preparing methanol from natural gas

Abstract

A low-cost and water-saving system for preparing methanol from natural gas. The system comprises: a natural gas combined conversion system, a methanol synthesis rectification system and a process condensate steam stripping system. Compared with traditional combined conversion under the same product scale, the same water-carbon ratio and the same operation conditions, the stoichiometric ratio (molar ratio) ((H2-CO2) / (CO+CO2)) required by methanol synthesis of fresh gas can be flexibly adjusted in the present invention by arranging pre-conversion and a large steam reforming bypass or a small steam reforming bypass, hydrogen does not need to be additionally supplemented, that is, a hydrogen recovery unit does not need to be arranged, and the thermal load of a steam reforming furnace can be effectively reduced by arranging the bypass, thereby reducing the size of the steam reforming furnace.

Description

A low-cost, water-saving natural gas-to-methanol system Technical Field

[0001] The present invention belongs to the field of chemical equipment, and in particular relates to a low-cost, water-saving natural gas methanol production system. Background Art

[0002] Natural gas is an important chemical raw material and environmentally friendly energy source. It serves as a crucial feedstock in the production of various chemical products. Natural gas resources are particularly abundant in countries and regions such as Africa, the Middle East, Russia, Southeast Asia, and North America. Natural gas production of downstream chemical products is widely adopted internationally due to its low investment, simple process, minimal environmental pollution, and easy operation.

[0003] Currently, the world's mainstream patent holders for producing methanol using natural gas as raw material include Lurgi, Topsoe, Casali, David, etc. Each company has its own unique advantages in natural gas conversion and methanol synthesis, and each patent holder masters one or more natural gas conversion technologies, such as Casali's Process and plant for methanol production, PCT / EP2022 / 051764.

[0004] At present, the process routes for producing methanol from natural gas mainly include the one-stage conversion process, namely the traditional natural gas steam catalytic conversion process; the autothermal conversion process, and the combined conversion process, namely the one-stage + two-stage combined conversion process.

[0005] One of the conversion processes has the following characteristics: 1) The conversion system pressure is low, generally less than 2.5 MPaG, resulting in high power consumption of the downstream methanol synthesis gas compressor; 2) The temperature of the conversion gas at the outlet of the converter is relatively high, generally between 800 and 900°C, and the wall temperature of the converter tube is relatively high, resulting in stringent requirements on materials; 3) The H2 content in the conversion gas is high, the hydrogen-carbon ratio in the synthesis gas is relatively high, and the residual methane content is high, which will increase the investment in synthesis system equipment; 4) The process has a high unit natural gas consumption and is generally suitable for a single set of methanol units below 2500 MTPD.

[0006] Characteristics of the autothermal reforming process: The partial oxidation reaction is exothermic, and the heat released can meet the heat requirements of the steam reforming reaction, eliminating the need for external heating, which can reduce operating costs and fuel consumption. Compared to steam catalytic reforming, where the energy required is provided externally, this process can save on furnace tubes and reduce equipment investment costs. This process offers the advantages of low investment, simplicity, and operational flexibility, but it does require the addition of an air separation unit and a high oxygen demand. Furthermore, the synthesis gas produced by the autothermal reforming process typically has an insufficient hydrogen-to-carbon ratio for methanol production, requiring a hydrogen recovery unit, which also increases investment.

[0007] Features of the combined conversion process: The hydrogen-carbon ratio in the conversion gas can directly meet the needs of methanol synthesis, eliminating the problem of excess hydrogen, significantly reducing the consumption of raw natural gas, and also eliminating the need for a hydrogen recovery device; the system pressure of the combined conversion can be higher than that of the single-stage conversion process, and the outlet methane content is low, which can reduce the power consumption of the synthesis gas compressor and the circulating gas compressor; at the same time, due to the reduction of the single-stage conversion load, the water-carbon ratio of the single-stage conversion reaction can be reduced, and the outlet temperature of the single-stage furnace in the combined conversion process is lower than that of the single-stage conversion process, reducing the requirements for the reformer furnace tube material.

[0008] Purpose of the Invention

[0009] The purpose of the present invention is to improve the comprehensive utilization efficiency of energy, reduce equipment investment and operating costs, and provide a low-cost, water-saving natural gas methanol production system.

[0010] In order to achieve the above objectives, the present application adopts the following technical solutions: a low-cost, water-saving natural gas to methanol system, comprising: a natural gas combined conversion system, a methanol synthesis distillation system and a process condensate stripping system;

[0011] The combined natural gas reforming system includes a natural gas compressor subsystem, a natural gas hydrodesulfurization subsystem, a pre-reformer, a steam reformer, an autothermal reformer, a reformed gas waste heat recovery and cooling subsystem, and a bypass regulation subsystem. The natural gas is hydrodesulfurized after being pressurized, and the desulfurized natural gas undergoes pre-reformation, steam reforming, and autothermal reforming to produce synthesis gas CO2, CO, and H2;

[0012] The bypass regulating subsystem includes a bypass pipeline and a regulating subsystem. The bypass pipeline is used to combine a portion of the pre-converted reformed gas with the other portion of the reformed gas through steam reforming as bypass gas and then feed the resultant gas into an autothermal reformer for a pure oxygen catalytic reforming reaction, or to combine a portion of the desulfurized natural gas with the other portion of the reformed gas through pre-converting and steam reforming and then feed the resultant gas into an autothermal reformer for a pure oxygen catalytic reforming reaction. The regulating subsystem is used to regulate the stoichiometric ratio (H2-CO2) / (CO+CO2) of the synthesis gas entering the methanol synthesis distillation system for synthesizing methanol by controlling the proportion of the bypass gas to maintain it between 2.0 and 2.1.

[0013] The methanol synthesis distillation system is used to synthesize synthesis gas into methanol and perform refining and purification;

[0014] The process condensate stripping system is used to recover and cool the process condensate separated from the reformed gas process waste heat in the natural gas combined reforming system and the wastewater in the methanol synthesis distillation system through stripping purification, and the by-product process steam is fed into the natural gas combined reforming system. A portion of the stripping wastewater is sent to the medium-pressure waste boiler of the reforming waste heat recovery subsystem, and the by-product medium-pressure steam is fed into the natural gas combined reforming system. The remaining portion recovers waste heat and is cooled before being sent to an external system. The oxygen required by the natural gas combined reforming system is provided by an external air separation unit.

[0015] Furthermore, the ratio of the bypass gas is adjusted between 0% and 50%.

[0016] Furthermore, process steam is added to the desulfurized natural gas so that the water-carbon molar ratio entering the pre-reformer is 1-2, and the natural gas is preheated to 400-500°C before being fed into the pre-reformer.

[0017] Furthermore, process steam is added to the reformed gas entering the steam reformer from the pre-reforming device so that the water-to-carbon molar ratio entering the steam reformer is 1.5 to 2.5.

[0018] Furthermore, oxygen is added to the autothermal reformer through the reforming burner, and the reformer outlet temperature is controlled to be less than 1050° C., and the outlet methane dry basis content is less than 2% mol.

[0019] Furthermore, after the natural gas is hydrodesulfurized by the natural gas hydrodesulfurization subsystem, the sulfur content in the natural gas is reduced to below 0.1 ppmv.

[0020] Furthermore, the methanol synthesis distillation system includes a synthesis gas compression system, a methanol synthesis system and a methanol distillation system.

[0021] Furthermore, both the methanol synthesis system and the methanol distillation system are provided with a washing tower, which recovers methanol in the tail gas from the pre-tower top in the system, and the methanol recovery rate in the methanol distillation system is greater than 99.5%.

[0022] Furthermore, the residual amount of methanol in the methanol distillation wastewater in the methanol distillation system is controlled to be less than 200 ppmw.

[0023] Furthermore, the purge gas, flash gas and non-condensable gas generated by the methanol synthesis system and the methanol distillation system are fed into the steam reformer for use as fuel gas.

[0024] Compared with the traditional combined reforming (steam reforming (SMR) + ATR reforming) under the same product scale, the same water-to-carbon ratio and operating conditions, the present invention has the following advantages and beneficial effects:

[0025] (1) By setting up a pre-reforming + steam reforming (SMR) large bypass or a steam reforming (SMR) small bypass, the stoichiometric ratio ((H2-CO2) / (CO+CO2)) required for methanol synthesis fresh gas can be flexibly adjusted without the need for additional hydrogen, that is, there is no need to set up a hydrogen recovery unit.

[0026] (2) By setting up a bypass, the heat load of the steam reformer can be effectively reduced, thereby reducing the size of the steam reformer. At the same time, because the steam reforming load with low thermal efficiency is reduced, the thermal efficiency of the entire device is improved.

[0027] (3) By setting up a bypass, the amount of steam entering the autothermal reformer can be reduced, and the amount of by-product high-pressure steam can be reduced, thereby reducing the load required for steam superheating, and further reducing the size of the steam reformer superheating coil and the size of the high-pressure steam piping system.

[0028] (4) By setting up a bypass, hydrogen recovery can be eliminated and the size of the steam conversion equipment can be reduced, thereby saving investment in equipment, instruments, and interlocking control systems.

[0029] (5) Through the above-mentioned large and small bypasses, this technology can flexibly respond to fluctuations in the upstream natural gas composition, always ensuring the stability of the hydrogen-to-carbon ratio required for demethanol synthesis of fresh gas, and making the downstream methanol synthesis unit operation stable.

[0030] (6) This technology can reduce the need for additional steam from the pipeline for pre-conversion and steam reforming (SMR) by 1) producing steam from process condensate stripping and feeding the steam produced from the process condensate medium-pressure waste boiler into the reforming device; and 2) reducing the steam reforming (SMR) load and the steam feed demand for the steam reforming (SMR) through the above-mentioned large and small bypass settings, thereby achieving the purpose of water saving.

[0031] Figures in the specification

[0032] FIG1 is a schematic structural diagram of a natural gas to methanol system according to Example 1.

[0033] FIG2 is a schematic structural diagram of a natural gas to methanol system according to Example 2. DETAILED DESCRIPTION

[0034] In order to enable those skilled in the art to better understand the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the drawings in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments in the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of this application.

[0035] It should be noted that the terms "including" and "having" and any variations thereof in the specification and claims of this application and the above-mentioned drawings are intended to cover non-exclusive inclusions. For example, a process, method, system, product or apparatus that includes a series of steps or units is not necessarily limited to those steps or units clearly listed, but may include other steps or units that are not clearly listed or are inherent to these processes, methods, products or apparatuses.

[0036] The process system involved in the present invention includes a natural gas combined reforming (CR) system, a methanol synthesis system, a methanol distillation system, a process condensate stripping system, etc.

[0037] The combined reforming system of the present invention includes a natural gas compressor, a hydrogenation reactor, a desulfurization tank, a fine desulfurization tank (optional), a pre-reformation reactor, a steam reformer (square box furnace), an autothermal reformer, a waste heat recovery system, etc.

[0038] The methanol synthesis system of the present invention includes a synthesis gas compressor, a methanol synthesis reactor, a synthesis gas-gas heat exchanger, a water cooler, a methanol separator, a methanol flash tank and other equipment.

[0039] The methanol distillation system of the present invention includes a pre-distillation tower, a pressure distillation tower, an atmospheric distillation tower, a recovery tower (optional), a washing tower (optional) and other equipment.

[0040] The process condensate stripping system of the present invention includes a stripping tower, a tower bottom heat exchanger, a desalted water preheater, a water cooler, an unqualified condensate storage tank and other equipment.

[0041] In this system, natural gas undergoes hydrodesulfurization to reduce the sulfur content in the natural gas to below 0.1ppmv, meeting the sulfur content requirements of downstream catalysts.

[0042] After desulfurization, the raw gas can be divided into two settings:

[0043] Option 1) Small Bypass Option: By adjusting the steam-to-feed gas ratio entering the pre-reformer, the water-to-carbon ratio of the feed gas is controlled to approximately 1-2. The feed gas is then preheated to 400-500°C before being fed to the pre-reformer, where higher carbon hydrocarbons in the feed gas are converted to methane. A portion of the reformed gas from the pre-reformer is heated by the steam reformer's convection coils and fed directly to the steam reformer for the steam reforming reaction. Another portion bypasses the steam reformer (SMR), is heated by the steam reformer's convection coils, and then mixed with the reformed gas from the steam reformer and fed to the autothermal reformer (ATR). Natural gas reacts in the steam reformer and autothermal reformer to produce the CO and H2 required for downstream processes.

[0044] Option 2) Large Bypass Option (optional): A portion of the feed gas can be preheated to 400-500°C before being sent to the pre-reformer, where the higher carbon hydrocarbons in the feed gas are converted to methane. The other portion is preheated through the pre-reformer + steam reforming (SMR) bypass coil and then mixed with the steam reforming (SMR) outlet gas before entering the autothermal reformer (ATR).

[0045] Part of the reformed gas after entering / exiting the pre-reforming reactor is mixed with the reformed gas at the outlet of the steam reformer (SMR) through a bypass and then enters the autothermal reformer. The proportion of the reformed gas passing through the bypass is 0-50%. By controlling the opening of the bypass, the stoichiometric ratio (molar ratio) ((H2-CO2) / (CO+CO2)) of the fresh gas entering the methanol synthesis can be effectively adjusted to be maintained between 2.0 and 2.1, meeting the requirements of the downstream methanol synthesis for the hydrogen-to-carbon ratio of the fresh gas, and no additional hydrogen needs to be added, that is, no additional hydrogen recovery unit needs to be set up.

[0046] In addition, on the basis of adjusting the bypass opening, the stoichiometric ratio (molar ratio) ((H2-CO2) / (CO+CO2)) can also be fine-tuned through various measures such as auxiliary adjustment of the conversion water-carbon ratio (including pre-conversion, steam conversion (SMR)), oxygen addition amount (ATR outlet temperature), etc., to ensure that the requirements of the downstream synthesis for the fresh gas hydrogen-carbon ratio are met.

[0047] By adjusting the ratio of steam to raw gas entering the steam reformer, the steam reformer inlet water-to-carbon ratio is controlled to be approximately between 1.5 and 2.5, and the steam reformer outlet temperature is controlled to be approximately 700-800°C.

[0048] Oxygen is fed into the autothermal reforming reactor through a reforming burner, and the reforming furnace outlet temperature is controlled to be less than 1050° C., and the outlet methane content is controlled to be less than 2% mol (dry basis).

[0049] The reformed gas at the outlet of the autothermal reformer (ATR) is sent to the methanol synthesis section as synthetic fresh gas after a series of waste heat recovery measures to recover waste heat and cool and separate the liquid.

[0050] The process condensate of the converted gas after the waste heat recovery process is sent to the process condensate stripping section.

[0051] The converted fresh synthetic gas is directly sent to the methanol synthesis process to produce crude methanol products.

[0052] The crude methanol product produced by methanol synthesis is then fed into the methanol distillation system to produce refined methanol products.

[0053] By setting up an additional washing tower to recover methanol in the tail gas from the top of the pre-tower, the methanol recovery rate in the methanol distillation section is greater than 99.5%.

[0054] The residual methanol content in the methanol distillation wastewater is controlled to less than 200ppmw and sent to the process condensate stripping section.

[0055] Saturated steam is produced as a by-product at the top of the stripping tower and serves as a supplementary source of steam for the conversion section.

[0056] After the waste liquid at the bottom of the stripping tower is heat-exchanged with the condensate from the feed process, part of it is used as water to make up the conversion medium-pressure waste boiler and to feed the by-product medium-pressure steam into the conversion section. The remaining part recovers the waste heat and is sent to the condensate for refining treatment after cooling.

[0057] Example 1

[0058] Referring to FIG1 , the natural gas raw material from outside the boundary enters (1) the inlet buffer tank of the natural gas compressor system, and after gas-liquid phase separation, it is sent to the natural gas compressor for compression. The compressed natural gas is sent to the raw gas gas-gas heat exchanger in the (4) steam reformer (SMR) convection section and (6) reformed gas waste heat recovery and cooling system for heating. The temperature after heating is about 350℃~380℃. The steam reformer fuel gas is used according to the actual situation of the factory using the downstream (8) methanol synthesis system purge gas / flash gas and (9) methanol distillation system non-condensable gas. The insufficient part is supplemented by natural gas. The preheated natural gas enters the cobalt-molybdenum hydrogenation reactor of (2) natural gas hydrodesulfurization system to convert the organic sulfur in the natural gas raw material into inorganic sulfur, mainly H2S. The hydrogenated natural gas is sent to the (refined) desulfurization tank for desulfurization, and the sulfur content is reduced to below 0.1ppmv. Two desulfurization tanks are set up, which can be operated in parallel or in series, so that when the desulfurizer in one desulfurization tank is replaced, the other desulfurization tank can still remain online. After desulfurization, the natural gas is mixed with process steam to adjust the water-carbon ratio of the natural gas entering the pre-conversion to 1-2. After being heated by the convection section coil of the (4) steam reformer (SMR), it is sent to the (3) pre-conversion for pre-conversion reaction to convert the high-carbon hydrocarbons in the raw gas into low-carbon hydrocarbons. A certain amount of steam is mixed with a portion of the reformed gas after the pre-conversion to adjust the water-carbon ratio of the natural gas entering the SMR to 1.5-2.5. Then, it is heated to 500-600°C by the convection section coil of the (4) steam reformer (SMR) and sent to the catalyst furnace tube of the radiation section of the (4) steam reformer (SMR) for steam reforming reaction; the other portion enters the convection section coil of the (4) steam reformer (SMR) through the steam reformer (SMR) bypass and is heated to 600-700°C. Then, it is combined with the reformed gas at the outlet of the (4) steam reformer (SMR) and enters the (5) autothermal reformer (ATR) system for reforming reaction. The flow rate through the steam reformer (SMR) bypass is 0-50%. The oxygen required for the reaction of the (5) autothermal reformer (ATR) system comes from the (11) air separation system. The reformed gas from the (5) autothermal reformer (ATR) system enters the (6) reformed gas waste heat recovery and cooling separation system to recover the process waste heat and complete the process gas cooling and purification. The purified synthesis gas enters the (7) synthesis gas compression system for compression to 7.0-9.0 MPaG and is then sent to the (8) methanol synthesis system. The crude methanol produced by the synthesis is sent to the (9) methanol distillation system. After four-tower distillation, the qualified refined methanol product is sent to the factory storage tank for storage or sold outside. The purge gas / flash gas / non-condensable gas produced by the (8) methanol synthesis system and the (9) methanol distillation system is sent to the (4) steam reformer (SMR) for use as fuel gas.The process condensate and wastewater generated by the (6) conversion gas waste heat recovery and cooling system and the (9) methanol distillation system are sent to the (10) process condensate stripping system for stripping. The steam from the top of the stripping tower is sent back to the conversion system. The wastewater at the bottom of the stripping tower is partially sent to the (6) conversion gas waste heat recovery and cooling system. The by-product process steam from the medium-pressure waste boiler is replenished into the conversion system. The remaining wastewater is sent to the factory's supporting desalted water refining system after waste heat recovery and cooling.

[0059] Example 2

[0060] Referring to FIG2, the main difference from Example 1 is that: a portion of the natural gas after desulfurization is added with process steam to adjust the water-carbon ratio entering the pre-conversion to 1-2, and then it is heated by the convection section coil of the steam reformer (4) and sent to the pre-conversion (3) for pre-conversion reaction to convert the high-carbon hydrocarbons in the raw gas into low-carbon hydrocarbons. A certain amount of steam is added to the reformed gas after the pre-conversion to adjust the water-carbon ratio entering the SMR to 1.5-2.5, and then it is heated by the convection section coil of the steam reformer (4) and sent to the pre-conversion (3) for pre-conversion reaction to convert the high-carbon hydrocarbons in the raw gas into low-carbon hydrocarbons. The tube is heated to 500-600°C and sent to the catalyst furnace tube of the radiation section of the (4) steam reformer (SMR) for steam reforming reaction; the other part enters the convection section coil of the (4) steam reformer (SMR) through the large bypass of the pre-reformer + steam reformer (SMR) and is heated to 600-700°C, and then is combined with the reformed gas at the outlet of the (4) steam reformer (SMR) and enters the (5) autothermal reformer (ATR) system for reforming reaction. The flow rate through the pre-reformer + steam reformer (SMR) bypass is 0-50%.

[0061] The above is a further detailed description of the present invention and should not be considered as a limitation on the specific implementation of the present invention. For those skilled in the art, simple deductions or substitutions that do not depart from the concept of the present invention are within the scope of protection of the present invention.

Claims

1. A low-cost, water-saving natural gas to methanol system, characterized in that include: Combined natural gas conversion system, methanol synthesis distillation system and process condensate stripping system; The combined natural gas reforming system includes a natural gas compressor subsystem, a natural gas hydrodesulfurization subsystem, a pre-converter, a steam reformer, an autothermal reformer, a reformed gas waste heat recovery and cooling subsystem, and a bypass regulation subsystem. The natural gas is hydrodesulfurized after being pressurized, and the desulfurized natural gas is pre-converted, steam reformed, and autothermal reformed to produce synthesis gas CO2, CO, and H2; The bypass regulating subsystem includes a bypass pipeline and a regulating subsystem. The bypass pipeline is used to combine part of the pre-converted conversion gas with other parts of the conversion gas through steam conversion as bypass gas and then send them into the autothermal reformer for pure oxygen catalytic conversion reaction, or to combine part of the desulfurized natural gas as bypass gas with other parts of the conversion gas through pre-conversion and steam conversion and then enter the autothermal reformer for pure oxygen catalytic conversion reaction; the regulating subsystem is used to adjust the stoichiometric ratio (H2-CO2) / (CO+CO2) of the synthesis gas entering the methanol synthesis distillation system for synthesizing methanol to be maintained between 2.0 and 2.1 by controlling the proportion of the bypass gas; The methanol synthesis distillation system is used to synthesize synthesis gas into methanol and perform refining and purification; The process condensate stripping system is used to recover and cool the process condensate separated from the process waste heat of the conversion gas in the natural gas combined conversion system and the waste water in the methanol synthesis distillation system through stripping purification, and the by-product process steam is fed into the natural gas combined conversion system. A part of the stripping waste water is sent to the medium-pressure waste boiler of the conversion waste heat recovery subsystem, and the by-product medium-pressure steam is fed into the natural gas combined conversion system, and the rest of the waste water is recovered and cooled and then sent to the external system; the oxygen required by the natural gas combined conversion system is provided by an external air separation unit; 2. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: The proportion of the bypass gas is adjusted between 0 and 50%.

3. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: The desulfurized natural gas is mixed with process steam so that the water-carbon molar ratio entering the pre-converter is 1-2, and is preheated to 400-500°C before being sent to the pre-converter.

4. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: Process steam is added to the reformed gas entering the steam reformer from the pre-reformer so that the water-to-carbon molar ratio entering the steam reformer is 1.5 to 2.

5.

5. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: The self-heating reformer is supplemented with oxygen through the reforming burner, and the outlet temperature of the reformer is controlled to be less than 1050° C., and the dry basis content of methane at the outlet is less than 2% mol.

6. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: After the natural gas is hydrodesulfurized by the natural gas hydrodesulfurization subsystem, the sulfur content in the natural gas is reduced to below 0.1 ppmv.

7. The low-cost, water-saving natural gas to methanol system according to claim 1, characterized in that: The methanol synthesis and distillation system comprises a synthesis gas compression system, a methanol synthesis system and a methanol distillation system.

8. The low-cost, water-saving natural gas to methanol system according to claim 7, characterized in that: The methanol synthesis system and the methanol distillation system are both provided with a washing tower, and the washing tower recovers methanol in the tail gas at the top of the pre-tower in the system. The methanol recovery rate in the methanol distillation system is greater than 99.5%.

9. The low-cost, water-saving natural gas to methanol system according to claim 7, characterized in that: The residual methanol content in the methanol distillation wastewater in the methanol distillation system is controlled to be less than 200 ppmw.

10. The low-cost, water-saving natural gas to methanol system according to claim 7, characterized in that: The purge gas, flash gas and non-condensable gas generated by the methanol synthesis system and the methanol distillation system are sent to the steam reformer for use as fuel gas.

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