Method and device for preparing methanol from natural gas

Abstract

The invention belongs to the technical field of natural gas reforming, and particularly relates to a method and a device for preparing methanol from natural gas. The invention provides a method and a device for preparing methanol from natural gas, balance can be maintained only by lower external heat or electric energy input through waste heat recovery and control of the molar ratio of O2 to CH4 in feed gas, methanol can be prepared from natural gas without heat input under extreme conditions, meanwhile, the energy efficiency of the whole process is 63%, and the cost is low. The energy efficiency is high; and 0.23 ton of carbon dioxide is discharged when one ton of methanol is produced, so that the carbon emission is low.

Description

Technical Field

[0001] The present invention belongs to the technical field of natural gas reforming, and particularly relates to a method and an apparatus for producing methanol from natural gas. Background Art

[0002] Methanol is a raw material for a variety of chemical products. With the shortage of global petroleum resources, the technology of producing methanol from natural gas reforming has also attracted much attention. The conventional process for producing methanol from natural gas generally converts natural gas into syngas through methane steam reforming technology, and the syngas is further compressed to synthesize methanol. The existing technology consumes a large amount of external heat and electricity in the raw material gas preheating, methane (CH4) reforming, and syngas compression sections, resulting in energy waste and low energy efficiency. At the same time, the carbon dioxide emissions per ton of methanol produced are 1.5 - 1.8 tons, which has a negative impact on the environment.

[0003] Therefore, there is an urgent need for a method for producing methanol from natural gas with high energy efficiency and low carbon emissions. Summary of the Invention

[0004] The purpose of the present invention is to provide a method and an apparatus for producing methanol from natural gas. The method provided by the present invention only needs a relatively low external heat or electric energy input to maintain balance. In extreme cases, methanol can be produced from natural gas without heat input. At the same time, the energy efficiency of this process is 63%, with high energy efficiency; the carbon dioxide emissions per ton of methanol produced are 0.23 tons, with low carbon emissions.

[0005] The present invention provides a method for producing methanol from natural gas, comprising the following steps:

[0006] Mix CH4, CO2, and O2 to obtain a mixed gas, and sequentially perform first preheating and reforming reaction on the mixed gas to obtain reformed gas. Then, sequentially perform first cooling, first water removal, and compression on the reformed gas to obtain syngas; the molar ratio of O2 to CH4 in the mixed gas is 1 - 2; the reforming reaction releases reaction heat;

[0007] Second preheat the syngas and then perform methanol synthesis reaction. Sequentially perform second cooling and gas-liquid separation on the obtained product to obtain a gas-phase stream and a liquid-phase stream; part of the gas-phase stream is returned to the methanol synthesis reaction step, and part is used as purge gas; the methanol synthesis reaction releases reaction heat;

[0008] Perform distillation on the liquid-phase stream to obtain methanol and dimethyl ether;

[0009] Burn the purge gas and dimethyl ether to obtain flue gas, and sequentially perform third cooling and fourth cooling on the flue gas;

[0010] Part of the reaction heat released by the reforming reaction serves as the heat source for the first preheating, and part is converted into electric energy through the Rankine cycle for the compression process; the sensible heat generated by the first cooling and the third cooling serves as the heat source for the first preheating;

[0011] The sensible heat generated by the second cooling and the fourth cooling serves as the heat source for the second preheating;

[0012] The reaction heat released by the methanol synthesis reaction serves as the heat source for rectification.

[0013] Preferably, the molar ratio of CH4 to CO2 in the mixed gas is 0.3 - 0.4.

[0014] Preferably, the temperature of the first preheating is 800 - 1000 °C; the temperature of the reforming reaction is 800 - 1000 °C, and the pressure is 0.1 - 0.6 MPa; the reforming reaction is carried out under the catalysis of a catalyst.

[0015] Preferably, the temperature of the first cooling is 10 - 30 °C; the pressure of the compression is 6 - 8 MPa.

[0016] Preferably, the temperature of the second preheating is 230 - 260 °C; the temperature of the methanol synthesis reaction is 230 - 260 °C, and the pressure is 6 - 8 MPa; the methanol synthesis reaction is carried out under the catalysis of a catalyst.

[0017] Preferably, the temperature of the second cooling is 10 - 30 °C.

[0018] Preferably, the rectification includes the first rectification and the second rectification; the bottom temperature of the first rectification is 160 - 180 °C, the temperature of the top condenser is 0 - 30 °C, the reflux ratio is 0 - 1, and the pressure is 1 - 3 MPa; crude methanol is obtained at the bottom of the first rectification tower, and dimethyl ether is obtained at the top; the bottom temperature of the second rectification is 100 - 120 °C, the temperature of the top condenser is 60 - 70 °C, and the reflux ratio is 0 - 2; methanol is obtained at the top of the second rectification tower, and water is obtained at the bottom.

[0019] Preferably, the temperature of the third cooling is 400 - 1000 °C;

[0020] The temperature of the fourth cooling is 200 - 400 °C.

[0021] The present invention also provides a device for the method of producing methanol from natural gas according to the above technical solution, including:

[0022] A natural gas reforming unit, a methanol synthesis unit, a methanol refining unit, and a waste heat recovery unit;

[0023] The natural gas reforming unit includes a first preheater, a natural gas reforming reactor, a first cooler, a first flash tank and a compressor connected in sequence;

[0024] The methanol synthesis unit includes a second preheater, a methanol synthesis reactor, a second cooler and a second flash tank connected in sequence; the inlet of the second preheater is connected to the outlet of the compressor;

[0025] The methanol refining unit includes a pre-separation column, a methanol separation column, a boiler, a third cooler and a fourth cooler connected in sequence; the inlet of the pre-separation column is connected to the liquid-phase stream outlet of the second flash tank; the inlet of the methanol separation column is connected to the bottom outlet of the pre-separation column; the boiler is connected to the top outlet of the pre-separation column;

[0026] The waste heat recovery unit is used to recover the sensible heat generated by the first cooler, the second cooler, the third cooler and the fourth cooler, as well as the reaction heat released by the natural gas reforming reactor and the methanol synthesis reactor;

[0027] Part of the reaction heat released by the natural gas reforming reactor is used as the heat source of the first preheater, and part is converted into electric energy through the Rankine cycle for the compression process of the compressor; the sensible heat generated by the first cooler and the third cooler is used as the heat source of the first preheater; the sensible heat generated by the second cooler and the fourth cooler is used as the heat source of the second preheater; the reaction heat released by the methanol synthesis reactor is used as the heat source for rectification.

[0028] Preferably, the number of trays of the pre-separation column is 10 to 20; the number of trays of the methanol separation column is 20 to 40.

[0029] Beneficial effects:

[0030] The present invention provides a method for producing methanol from natural gas. By controlling the molar ratio of oxygen to methane in the feed gas and simultaneously recovering the available waste heat and partial reaction heat generated in other sections of the natural gas to methanol system, the heat is provided together for preheating the feed gas, thereby achieving low external heat supply in the feed gas preheating section. At the same time, a Rankine cycle is introduced to convert the unused reaction heat into electrical energy to provide power for the compression section, achieving low power supply in the compression section. Without considering heat transfer losses, the present invention continuously feeds CH4, CO2, and O2, and uses the reaction heat generated during the reaction process or the sensible heat generated during the cooling process of the high-temperature stream to supply energy to the energy-consuming sections, thereby achieving zero input of external heat or electrical energy. During the process of producing methanol from natural gas, part of the chemical energy of the feed gas is converted into heat energy to supply energy to the energy-consuming sections, part is converted into the chemical energy of methanol along with the formation of methanol, and the energy efficiency is 63%. The rest is lost in other forms. Therefore, the present invention needs to continuously feed natural gas as a raw material instead of a one-time input. In summary, the present invention achieves the energy balance of the three major energy-consuming units of feed gas preheating, feed gas reforming, and syngas compression through low energy input. In extreme cases, it can achieve zero heat and zero power consumption in the process of producing methanol from natural gas, greatly improving the energy utilization efficiency of the process of producing methanol from natural gas and reducing the carbon dioxide emissions at the same time. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] In order to more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the drawings required to be used in the embodiments.

[0032] Figure 1 It is a schematic diagram of the process for producing methanol from natural gas in the embodiment; where: H represents heat recovery; S represents material flow; E represents electrical energy transportation. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0033] The present invention provides a method for producing methanol from natural gas, comprising the following steps:

[0034] Mix CH4, CO2, and O2 to obtain a mixed gas, and sequentially perform first preheating and reforming reaction on the mixed gas to obtain a reformed gas. Then, sequentially perform first cooling, first water removal, and compression on the reformed gas to obtain syngas; the molar ratio of O2 to CH4 in the mixed gas is 1-2; the reforming reaction releases reaction heat;

[0035] Second preheat the syngas and then perform methanol synthesis reaction. Then, sequentially perform second cooling and gas-liquid separation on the obtained product to obtain a gas-phase stream and a liquid-phase stream; part of the gas-phase stream is returned to the methanol synthesis reaction step, and part is used as purge gas; the methanol synthesis reaction releases reaction heat;

[0036] Perform distillation on the liquid-phase stream to obtain methanol and dimethyl ether;

[0037] The off-gas and dimethyl ether are combusted to obtain flue gas, and the flue gas is sequentially subjected to third cooling and fourth cooling;

[0038] Part of the reaction heat released by the reforming reaction is used as the heat source for the first preheating, and part is converted into electric energy through the Rankine cycle for the compression process; the sensible heat generated by the first cooling and the third cooling is used as the heat source for the first preheating;

[0039] The sensible heat generated by the second cooling and the fourth cooling is used as the heat source for the second preheating;

[0040] The reaction heat released by the methanol synthesis reaction is used as the heat source for rectification.

[0041] In the present invention, CH4, CO2 and O2 are mixed to obtain a mixed gas, and the mixed gas is sequentially subjected to first preheating and reforming reaction to obtain reformed gas, and the reformed gas is sequentially subjected to first cooling, first water removal and compression to obtain syngas. In the present invention, the molar ratio of O2 to CH4 in the mixed gas is 1 to 2, specifically 1.2, 1.5 or 1.8; the molar ratio of CH4 to CO2 in the mixed gas is preferably 0.3 to 0.4, specifically 0.35; the temperature of the first preheating is preferably 800 to 1000 °C, specifically 850 °C, 900 °C or 950 °C.

[0042] In the present invention, the reaction formulas of the reforming reaction are shown in Formula 1 and Formula 2:

[0043] CH4 + CO2 → 2CO + 2H2 Formula 1;

[0044] CH4 + O2 → CO2 + 2H2 Formula 2.

[0045] Formula 1 is the CH4-CO2 dry reforming reaction, and the reaction of Formula 1 absorbs heat;

[0046] Formula 2 is the CH4-O2 partial oxidation reforming reaction, and the reaction of Formula 2 releases reaction heat. In the present invention, the reforming reaction as a whole releases reaction heat; part of the reaction heat is used as the heat source for the first preheating for the heating process, and part of the reaction heat is converted into electric energy through the Rankine cycle for the subsequent compression process.

[0047] In the present invention, the temperature of the reforming reaction is preferably 800 to 1000 °C, specifically 850 °C, 900 °C or 950 °C; the pressure of the reforming reaction is preferably 0.1 to 0.6 MPa, specifically 0.3 MPa or 0.5 MPa; the reforming reaction is preferably carried out under the catalysis of a catalyst; the catalyst is preferably a Ni / Al2O3 catalyst. In the present invention, a high-temperature gas-phase stream is obtained after the reforming reaction; the high-temperature gas-phase stream includes CO2, CO, H2, CH4 and H2O.

[0048] In the present invention, the temperature of the first cooling is preferably 10 to 30 °C, specifically it can be 20 °C or 25 °C; the first cooling is to cool the high-temperature gas-phase stream obtained after the reforming reaction, and the sensible heat released by the first cooling is used as the heat source for the first preheating.

[0049] In the present invention, the first water removal preferably removes the liquid-phase water by flash evaporation; the stream after the first water removal is compressed, and the pressure of the compression is preferably 6 to 8 MPa.

[0050] In the present invention, the compressed syngas is second preheated and then undergoes a methanol synthesis reaction, and the obtained product is successively subjected to second cooling and gas-liquid separation to obtain a gas-phase stream and a liquid-phase stream. In the present invention, the temperature of the second preheating is preferably 230 to 260 °C, specifically it can be 240 °C or 250 °C.

[0051] In the present invention, the reaction formulas of the methanol synthesis reaction are shown in Formulas 3 and 4:

[0052] CO2 + 3H2 → CH3OH + H2O Formula 3;

[0053] CO + 2H2 → CH3OH Formula 4.

[0054] The syngas is converted into methanol through the CO2 hydrogenation and CO hydrogenation reactions, and at the same time, a side reaction of methanol dehydration to dimethyl ether shown in Formula 5 will also occur:

[0055] 2CH3OH → CH3OCH3 + H2O Formula 5.

[0056] In the present invention, the methanol synthesis reaction releases reaction heat, which is used as the heat source for subsequent rectification.

[0057] In the present invention, the temperature of the methanol synthesis reaction is preferably 230 to 260 °C, specifically it can be 240 °C or 250 °C; the pressure of the methanol synthesis reaction is preferably 6 to 8 MPa, specifically it can be 7 MPa; the methanol synthesis reaction is preferably carried out under the catalysis of a catalyst; the catalyst is preferably a Cu / ZnO / Al2O3 catalyst. In the present invention, a high-temperature stream is obtained after the methanol synthesis reaction; the high-temperature stream includes unreacted CO, CO2 and H2, the inert component CH4, the main product methanol, the by-product dimethyl ether and water.

[0058] In the present invention, the temperature of the second cooling is preferably 10 to 30 °C, specifically it can be 20 °C or 25 °C; the second cooling is to cool the high-temperature stream obtained after the methanol synthesis reaction, and the sensible heat released by the second cooling is used as the heat source for the second preheating. The gas-liquid separation is preferably carried out by flash evaporation.

[0059] In the present invention, after the high-temperature stream undergoes secondary cooling and gas-liquid separation, a gas-phase stream and a liquid-phase stream are obtained; a part of the gas-phase stream is recycled to the methanol synthesis reaction, and a part is used as purge gas; the liquid-phase stream participates in the subsequent rectification process. In the present invention, the components of the gas-phase stream include CO, CO2, H2, and CH4; 95% of the gas-phase stream is recycled to the methanol synthesis reaction at the front end, 5% is purged as purge gas, and is then combusted with the dimethyl ether obtained by subsequent rectification; the liquid-phase stream includes methanol, H2O, and dimethyl ether.

[0060] In the present invention, the liquid-phase stream is rectified to obtain methanol and dimethyl ether; in the present invention, the rectification preferably includes first rectification and second rectification; the bottom temperature of the first rectification is preferably 160-180°C, the temperature of the top condenser is preferably 0-30°C, the reflux ratio is preferably 0-1, and the pressure is preferably 1-3 MPa; the first rectification obtains crude methanol at the bottom of the column and dimethyl ether at the top of the column. In the present invention, the dimethyl ether obtained by the first rectification is subsequently combusted; the heat source required for the first rectification is preferably continuously introduced into the bottom of the column in the form of steam. In a specific embodiment of the present invention, the first rectification is preferably carried out in a pre-separation column; the number of trays of the pre-separation column is preferably 10-20, specifically 15; the top of the pre-separation column obtains gaseous dimethyl ether and a small amount of H2, CO, and CO2 dissolved in dimethyl ether, and the bottom of the column obtains crude methanol.

[0061] In the present invention, the bottom temperature of the second rectification is preferably 100-120°C, the temperature of the top condenser is preferably 60-70°C, the reflux ratio is preferably 0-2, and the pressure is preferably 0.1-0.2 MPa; the second rectification obtains methanol at the top of the column and water at the bottom of the column. In the present invention, the heat source required for the second rectification is preferably continuously introduced into the bottom of the column in the form of steam. In a specific embodiment of the present invention, the second rectification is preferably carried out in a methanol separation column; the number of trays of the methanol separation column is preferably 20-40, specifically 30; the purity of the methanol obtained at the top of the second rectification is 99.9%.

[0062] In the present invention, the purge gas and dimethyl ether are combusted to obtain flue gas, and the flue gas is sequentially subjected to third cooling and fourth cooling; the temperature of the third cooling is preferably 400-1000°C; the temperature of the fourth cooling is preferably 200-400°C. In the present invention, the purge gas and dimethyl ether are preferably combusted in a boiler, and the sensible heat generated during the combustion process is released during the third cooling and fourth cooling processes. In the present invention, the sensible heat generated by the third cooling is used as the heat source for the first preheating; the sensible heat generated by the fourth cooling is used as the heat source for the second preheating.

[0063] The present invention also provides an apparatus for the method of producing methanol from natural gas according to the above technical solution, including:

[0064] A natural gas reforming unit, a methanol synthesis unit, a methanol refining unit, and a waste heat recovery unit;

[0065] The natural gas reforming unit includes a first preheater, a natural gas reforming reactor, a first cooler, a first flash tank, and a compressor that are connected in sequence;

[0066] The methanol synthesis unit includes a second preheater, a methanol synthesis reactor, a second cooler, and a second flash tank that are connected in sequence; the inlet of the second preheater is connected to the outlet of the compressor;

[0067] The methanol refining unit includes a pre-separation tower, a methanol separation tower, a boiler, a third cooler, and a fourth cooler that are connected in sequence; the inlet of the pre-separation tower is connected to the liquid-phase stream outlet of the second flash tank; the inlet of the methanol separation tower is connected to the bottom outlet of the pre-separation tower; the boiler is connected to the top outlet of the pre-separation tower;

[0068] The waste heat recovery unit is used to recover the sensible heat generated by the first cooler, the second cooler, the third cooler, and the fourth cooler, as well as the reaction heat released by the natural gas reforming reactor and the methanol synthesis reactor;

[0069] Part of the reaction heat released by the natural gas reforming reactor is used as the heat source of the first preheater, and part is converted into electric energy through the Rankine cycle for the compression process of the compressor; the sensible heat generated by the first cooler and the third cooler is used as the heat source of the first preheater; the sensible heat generated by the second cooler and the fourth cooler is used as the heat source of the second preheater; the reaction heat released by the methanol synthesis reactor is used as the heat source for rectification.

[0070] In the present invention, the number of trays of the pre-separation tower is preferably 10 to 20, and specifically can be 15; the number of trays of the methanol separation tower is preferably 20 to 40, and specifically can be 30.

[0071] In the present invention, unless otherwise specified, all raw material components are commercially available products well-known to those skilled in the art.

[0072] To further illustrate the present invention, the method for producing methanol from natural gas provided by the present invention will be described in detail below with reference to the accompanying drawings and embodiments, but they should not be construed as limiting the protection scope of the present invention.

[0073] The calculation formulas for energy efficiency and carbon emissions in the examples and comparative examples of the present invention are as follows:

[0074]

[0075] Example 1

[0076] After mixing CH4, CO2 and O2, the mixed gas (the molar ratio of CH4 to O2 is 1, and the molar ratio of CH4 to CO2 is 0.33) is first preheated to 850 °C, and then reforming reaction is carried out in the natural gas reforming unit. The operating temperature of the reforming reactor is 850 °C, the operating pressure is 0.6 MPa, and the supported catalyst is a commercial Ni / Al2O3 catalyst. The high-temperature gas stream at the outlet of the reforming reactor mainly includes CO2, CO, H2, CH4 and H2O. After cooling to 10 °C and flashing to remove water, it is compressed to 7.5 MPa by a compressor and sent to the methanol synthesis section.

[0077] The syngas is preheated to 250 °C, and then methanol synthesis reaction is carried out. The operating temperature of the methanol synthesis reactor is 250 °C, the operating pressure is 7.5 MPa, and the supported catalyst is a commercial Cu / ZnO / Al2O3 catalyst. The high-temperature stream at the outlet of the methanol reactor is cooled to 10 °C and then undergoes gas-liquid separation. 95% of the gas stream is recycled to the front-end methanol synthesis reactor, 5% is used as purge gas and sent to the subsequent combustion step, and the liquid stream is sent to the rectification stage.

[0078] The methanol rectification stage includes pre-separation and methanol separation processes to obtain by-products dimethyl ether and methanol. The by-product dimethyl ether and purge gas are burned in a boiler to obtain high-temperature flue gas, and sensible heat is released during the third cooling and fourth cooling processes.

[0079] 1400 GJ / h of reaction heat is released in the natural gas reforming unit, 60 GJ / h is used for preheating the mixed gas, and 1340 GJ / h is converted into electric energy work through the Rankine cycle. The sensible heat released by cooling the high-temperature flue gas obtained after natural gas reforming is 52 GJ / h, 378 GJ / h of reaction heat is released by the methanol synthesis reactor, the sensible heat released by cooling the high-temperature flue gas obtained by the methanol synthesis reactor is 8 GJ / h, and the sensible heat released by the third cooling and fourth cooling of the high-temperature flue gas obtained after burning dimethyl ether and purge gas is 10 GJ / h and 8 GJ / h respectively.

[0080] 122 GJ / h of heat source is required for preheating the raw material gas, 16 GJ / h of heat source is required for preheating the syngas, 123 GJ / h of heat source is required for pre-separation in the rectification stage, 255 GJ / h of heat source is required for methanol separation, and finally heat source balance is achieved.

[0081] If heat loss during heat transfer is not considered, the entire process flow of Example 1 does not require external heat or electric energy input, the energy efficiency is 63%, and the carbon emission is 0.23 tons of CO2 / ton of methanol.

[0082] Example 2

[0083] After mixing CH4, CO2 and O2, the mixed gas (the molar ratio of CH4 to O2 is 1.2, and the molar ratio of CH4 to CO2 is 0.33) is first preheated to 850 °C, and then reforming reaction is carried out in the natural gas reforming unit. The operating temperature of the reforming reactor is 850 °C, the operating pressure is 0.6 MPa, and the supported catalyst is a commercial Ni / Al2O3 catalyst. The high-temperature gas stream at the outlet of the reforming reactor mainly includes CO2, CO, H2, CH4 and H2O. After cooling to 10 °C and flashing to remove water, it is compressed to 7.5 MPa by a compressor and sent to the methanol synthesis section.

[0084] The syngas is preheated to 250 °C, and then methanol synthesis reaction is carried out. The operating temperature of the methanol synthesis reactor is 250 °C, the operating pressure is 7.5 MPa, and the supported catalyst is a commercial Cu / ZnO / Al2O3 catalyst. The high-temperature stream at the outlet of the methanol reactor is cooled to 10 °C and then undergoes gas-liquid separation. 95% of the gas stream is recycled to the front-end methanol synthesis reactor, 5% is used as purge gas and sent to the subsequent combustion step, and the liquid stream is sent to the distillation stage.

[0085] The methanol distillation stage includes pre-separation and methanol separation processes to obtain by-products dimethyl ether and methanol. The by-product dimethyl ether and purge gas are burned in a boiler to obtain high-temperature flue gas, and sensible heat is released during the third cooling and fourth cooling processes.

[0086] In the natural gas reforming unit, the reaction heat released is 1860 GJ / h, 90 GJ / h is used for preheating the mixed gas, and 1770 GJ / h is converted into electric energy work through the Rankine cycle. The sensible heat released by cooling the high-temperature flue gas after natural gas reforming is 33 GJ / h, the reaction heat released by the methanol synthesis reactor is 350 GJ / h, the sensible heat released by cooling the high-temperature flue gas obtained by the methanol synthesis reactor is 11 GJ / h, and the sensible heat released by the third cooling and fourth cooling of the high-temperature flue gas obtained after burning dimethyl ether and purge gas is 8 GJ / h and 6 GJ / h respectively.

[0087] The raw material gas preheating requires a heat source of 131 GJ / h, the syngas preheating requires a heat source of 17 GJ / h, the pre-separation of the distillation stage requires a heat source of 180 GJ / h, and the methanol separation requires a heat source of 170 GJ / h, finally achieving heat source balance.

[0088] If the losses during the heat transfer process are not considered, the entire process flow of Example 2 does not require external heat or electric energy input, the energy efficiency is 59%, and the carbon emission is 0.21 tons of CO2 / ton of methanol.

[0089] Example 3

[0090] After mixing CH4, CO2 and O2, the mixed gas (the molar ratio of CH4 to O2 is 1.3, and the molar ratio of CH4 to CO2 is 0.33) is first preheated to 850 °C, and then reforming reaction is carried out in the natural gas reforming unit. The operating temperature of the reforming reactor is 850 °C, the operating pressure is 0.6 MPa, and the supported catalyst is a commercial Ni / Al2O3 catalyst. The high-temperature gas stream at the outlet of the reforming reactor mainly includes CO2, CO, H2, CH4 and H2O. After cooling to 10 °C and flashing to remove water, it is compressed to 7.5 MPa by a compressor and sent to the methanol synthesis section.

[0091] The syngas is preheated to 250 °C, and then methanol synthesis reaction is carried out. The operating temperature of the methanol synthesis reactor is 250 °C, the operating pressure is 7.5 MPa, and the supported catalyst is a commercial Cu / ZnO / Al2O3 catalyst. The high-temperature stream at the outlet of the methanol reactor is cooled to 10 °C and then undergoes gas-liquid separation. 95% of the gas stream is recycled to the front-end methanol synthesis reactor, 5% is used as purge gas and sent to the subsequent combustion step, and the liquid stream is sent to the distillation stage.

[0092] The methanol distillation stage includes pre-separation and methanol separation processes to obtain by-products dimethyl ether and methanol. The by-product dimethyl ether and purge gas are burned in a boiler to obtain high-temperature flue gas, and sensible heat is released during the third cooling and fourth cooling processes.

[0093] In the natural gas reforming unit, the reaction heat released is 2300 GJ / h, 122 GJ / h is used for preheating the mixed gas, and 2178 GJ / h is converted into electric energy work through the Rankine cycle. The sensible heat released by cooling the high-temperature flue gas obtained after natural gas reforming is 10 GJ / h, the reaction heat released by the methanol synthesis reactor is 320 GJ / h, the sensible heat released by cooling the high-temperature flue gas obtained by the methanol synthesis reactor is 14 GJ / h, and the sensible heat released by the third cooling and fourth cooling of the high-temperature flue gas obtained after burning dimethyl ether and purge gas is 6 GJ / h and 4 GJ / h respectively.

[0094] The raw material gas preheating requires a heat source of 138 GJ / h, the syngas preheating requires a heat source of 18 GJ / h, the pre-separation of the distillation stage requires a heat source of 112 GJ / h, and the methanol separation requires a heat source of 208 GJ / h, finally achieving heat source balance.

[0095] If heat loss during heat transfer is not considered, the entire process flow of Example 3 does not require external heat or electric energy input, the energy efficiency is 56%, and the carbon emission is 0.19 tons of CO2 per ton of methanol.

[0096] Figure 1 It is a schematic diagram of the natural gas to methanol process for the example; where: H represents heat recovery; S represents material flow; E represents electric energy transportation.

[0097] Comparative Example 1

[0098] The difference from Example 1 is only that: the reaction heat released by the reforming reactor is not recovered, that is, it no longer supplies heat to the first preheater, nor is it converted into electric energy through the Rankine cycle to supply energy to the compressor. The energy required for both devices comes from external supply. Under the above process configuration and heat exchange scheme, the heat still required to be input inside the system is 0.3 GJ / ton of methanol, the electric energy is 1.2 GJ / ton of methanol, the energy efficiency is 60%, and the carbon emission is 0.5 tons of CO2 / ton of methanol.

[0099] Although the above embodiments have described the present invention in detail, they are only some embodiments of the present invention, not all embodiments. People can also obtain other embodiments based on these embodiments without creative efforts, and these embodiments all fall within the protection scope of the present invention.

Claims

1. A method for producing methanol from natural gas, characterized in that, It includes the following steps: Mix CH4, CO2 and O2 to obtain a mixed gas, sequentially perform first preheating and reforming reaction on the mixed gas to obtain reformed gas, and sequentially perform first cooling, first water removal and compression on the reformed gas to obtain syngas; the molar ratio of O2 to CH4 in the mixed gas is 1-2; the reforming reaction releases reaction heat; Second preheat the syngas and then perform methanol synthesis reaction, and sequentially perform second cooling and gas-liquid separation on the obtained product to obtain a gas-phase stream and a liquid-phase stream; part of the gas-phase stream is returned to the methanol synthesis reaction step, and part is used as purge gas; the methanol synthesis reaction releases reaction heat; Rectify the liquid-phase stream to obtain methanol and dimethyl ether; Burn the purge gas and dimethyl ether to obtain flue gas, and sequentially perform third cooling and fourth cooling on the flue gas; Part of the reaction heat released by the reforming reaction is used as the heat source for the first preheating, and part is converted into electric energy through the Rankine cycle for the compression process; The sensible heat generated by the first cooling and the third cooling is used as the heat source for the first preheating; The sensible heat generated by the second cooling and the fourth cooling is used as the heat source for the second preheating; The reaction heat released by the methanol synthesis reaction is used as the heat source for rectification.

2. The method according to claim 1, wherein The molar ratio of CH4 to CO2 in the mixed gas is 0.3-0.

4.

3. The method according to claim 1 or 2, characterized in that, The temperature of the first preheating is 800-1000 °C; the temperature of the reforming reaction is 800-1000 °C, and the pressure is 0.1-0.6 MPa; the reforming reaction is carried out under the catalysis of a catalyst.

4. The method according to claim 3, wherein The temperature of the first cooling is 10-30 °C; the pressure of the compression is 6-8 MPa.

5. The method according to claim 1 or 2, characterized in that, The temperature of the second preheating is 230-260 °C; the temperature of the methanol synthesis reaction is 230-260 °C, and the pressure is 6-8 MPa; the methanol synthesis reaction is carried out under the catalysis of a catalyst.

6. The method according to claim 5, characterized in that, The temperature of the second cooling is 10-30 °C.

7. The method according to claim 1, wherein The rectification includes first rectification and second rectification; the bottom temperature of the first rectification is 160-180 °C, the temperature of the top condenser is 0-30 °C, the reflux ratio is 0-1, and the pressure is 1-3 MPa; crude methanol is obtained at the bottom of the first rectification tower, and dimethyl ether is obtained at the top; the bottom temperature of the second rectification is 100-120 °C, the temperature of the top condenser is 60-70 °C, the reflux ratio is 0-2, and the pressure is 0.1-0.2 MPa; methanol is obtained at the top of the second rectification tower, and water is obtained at the bottom.

8. The method according to claim 1, characterized in that, The temperature of the third cooling is 400-1000 °C; The temperature of the fourth cooling is 200-400 °C.

9. The device used in the method for producing methanol from natural gas according to any one of claims 1 to 8, characterized in that, It includes: A natural gas reforming unit, a methanol synthesis unit, a methanol refining unit and a waste heat recovery unit; The natural gas reforming unit includes a first preheater, a natural gas reforming reactor, a first cooler, a first flash tank and a compressor connected in sequence; The methanol synthesis unit includes a second preheater, a methanol synthesis reactor, a second cooler and a second flash tank connected in sequence; the inlet of the second preheater is connected to the outlet of the compressor; The methanol refining unit includes a pre-separation tower, a methanol separation tower, a boiler, a third cooler, and a fourth cooler that are connected in sequence; the inlet of the pre-separation tower is connected to the liquid-phase stream outlet of the second flash tank; the inlet of the methanol separation tower is connected to the bottom outlet of the pre-separation tower; the boiler is connected to the top outlet of the pre-separation tower; The waste heat recovery unit is used to recover the sensible heat generated by the first cooler, the second cooler, the third cooler, and the fourth cooler, as well as the reaction heat released by the natural gas reforming reactor and the methanol synthesis reactor; Part of the reaction heat released by the natural gas reforming reactor is used as the heat source of the first preheater, and part is converted into electric energy through the Rankine cycle for the compression process of the compressor; The sensible heat generated by the first cooler and the third cooler is used as the heat source of the first preheater; the sensible heat generated by the second cooler and the fourth cooler is used as the heat source of the second preheater; the reaction heat released by the methanol synthesis reactor is used as the heat source for rectification.

10. The device according to claim 9, characterized in that, The number of trays in the pre-separation tower is 10 to 20; the number of trays in the methanol separation tower is 20 to 40.

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