Introduction to methanol production methods

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1. Briefly describe the industrial methods for methanol production. 　There are various methods for producing methanol; the early methods that involved the dry distillation of wood or lignin have been phased out in industry today. Methanol can also be produced by the chloromethane hydrolysis method, but this approach is expensive, so it has not been used in industry. The partial oxidation of methane can be used to produce methanol. This method for manufacturing methanol features a simple process flow and reduced construction costs; however, this oxidation process is difficult to control, as it often results in the formation of carbon oxides and water due to excessive oxidation. This causes significant losses to both raw materials and products. Therefore, the method of producing methanol through partial oxidation of methane has not yet been industrialized. But it has the aforementioned advantages, and research in this area abroad has never ceased. It should be a methanol production method with great industrial prospects. 　Currently, the pressurized catalytic hydrogenation of carbon monoxide and carbon dioxide is almost universally used in industry to produce methanol. The typical process includes steps such as raw gas production, raw gas purification, methanol synthesis, and crude methanol distillation. 　Natural gas, naphtha, heavy oil, coal and its processed products (coke, coke oven gas), acetylene off-gases, etc., can all be used as raw materials for producing methanol synthesis gas. The steam reforming of natural gas and naphtha requires the use of complex and expensive reformers. The converter is equipped with a radiant chamber and a convection chamber, where the conversion of hydrocarbon vapors takes place at high temperatures in the presence of a catalyst. Partial oxidation of heavy oil must be carried out in a high-temperature gasifier. When solid fuel is used as the raw material, water gas can be produced by batch gasification or continuous gasification. The intermittent gasification method uses air and steam as gasifying agents, carrying out the blowing and gas production stages separately ； Continuous gasification uses oxygen and steam as gasifying agents, with the process proceeding continuously. 　Various catalysts used in methanol production, such as natural gas and naphtha steam reforming catalysts as well as methanol synthesis catalysts, are prone to deactivation due to sulfide poisoning, and it is necessary to remove sulfides completely. Gas desulfurization methods can be divided into two categories: dry desulfurization and wet desulfurization. Dry desulfurization equipment is simple, but due to the slow reaction rate, it is relatively large in size. Wet desulfurization can be divided into three categories: physical absorption, chemical absorption, and direct oxidation. 　The synthesis of methanol takes place at high temperature and pressure in the presence of a catalyst, and it is a typical gas-solid catalytic reaction process. With the continuous advancement of methanol synthesis catalyst technology, the overall trend is currently shifting from high pressure to low and medium pressure. 　Crude methanol contains water, higher alcohols, ethers, and other impurities, and requires purification. The refining process includes distillation and chemical treatment. Chemical treatment mainly uses alkalis to destroy impurities that are difficult to separate during distillation and to adjust the pH. Distillation is primarily used to remove volatile components such as dimethyl ether, as well as less volatile components like ethanol, higher alcohols, and water. 　The overall process for methanol production is long and complex, and it can take on various forms depending on the different raw materials and purification methods used. 　2. Briefly describe the three methods—high-pressure, medium-pressure, and low-pressure methods—and their differences. 　The high-pressure process generally refers to the synthesis of methanol using a zinc-chromium catalyst at high temperatures and pressures of 300–400°C and 30 MPa. Since the first successful synthesis of methanol using this method in 1923, for almost 50 years, this approach has been used worldwide for methanol production, with only some differences in design; for example, the methods for heat transfer within the methanol synthesis reactor fall into two main categories: the cold-tube continuous heat exchange type and the cold-jump multi-stage heat exchange type ； The flow pattern of the reaction gas can be axial and radial, or a mixed mode that includes both ； There are processes with by-product steam and those without by-product steam, etc. In recent years, our country has developed a technology for synthesizing methanol on copper-based catalysts at a pressure of 25–27 MPa; the methanol content in the outlet gas is around 4%, with a reaction temperature of 230–290°C. 　The ICl low-pressure methanol process is a methanol production method developed successfully by the British company ICl in 1966. This broke the monopoly of the high-pressure method for methanol synthesis, marking a significant advancement in methanol production technology; it uses a copper-based catalyst of type 51-1 and operates at a synthesis pressure of 5 MPa. The reactor used in the ICl process is of the hot-wall multi-stage quench type, with a simple structure; at the top of each catalyst layer, there is a diamond-shaped quench gas distributor that ensures even distribution of the quench gas into the catalyst layer, thereby regulating the temperature inside the reactor. Other types of low-pressure synthesis towers include the tube bundle type by-product steam synthesis tower developed by Lurgi in West Germany, and the three-phase methanol synthesis system from the Electric Power Research Institute in the United States. In the 1970s, the Sichuan Vinylon Plant under China’s Light Industry Ministry introduced from the French company Speichim a low-pressure methanol production plant capable of producing 300 tons per day, using acetylene off-gases as raw material (based on British ICI patented technology). In the 1980s, the Second Fertilizer Plant of Qilu Petrochemical Company introduced a low-pressure methanol synthesis unit from the German company Lurge. 　The medium-pressure method was further developed on the basis of research on the low-pressure method. Due to the low operating pressure in the low-pressure method, the equipment required is quite large in size, which hinders the scale-up of methanol production. Therefore, a medium-pressure methanol synthesis method with a pressure of around 10 MPa was developed. It can more effectively reduce plant construction costs and methanol production costs. For example, ICI has successfully developed the 51-2 type copper-based catalyst, whose chemical composition and activity are similar to those of the low-pressure synthesis catalyst 51-1; the only difference lies in the crystal structure of the catalyst, and its production cost is higher than that of the 51-1 type. Since this catalyst can maintain a long service life even at higher pressures, ICI was able to increase the synthesis pressure from the original 5 MPa to 10 MPa. The synthesis reactor used is of the four-stage quench type, just like in the low-pressure process, and its process flow and equipment are similar to those of the low-pressure method. 3. Briefly describe the production method of methanol from natural gas. 　Natural gas is the main raw material for producing methanol. The main component of natural gas is methane, with small amounts of other alkanes, olefins, and nitrogen. Methods for producing methanol feed gas from natural gas include steam reforming, catalytic partial oxidation, and non-catalytic partial oxidation, among which steam reforming is the most widely used. It is carried out in a tubular furnace at atmospheric pressure or under pressure. Since the reaction is endothermic, external heat supply is required to maintain the desired conversion temperature; this is generally achieved by burning some fuel gas between the tubes, while the steam used for conversion is produced directly on the unit using the heat from the flue gases and the converted gases. 　In the syngas produced by the natural gas steam reforming process, there is an excess of hydrogen while the amounts of carbon monoxide and carbon dioxide are insufficient. An industrial solution to this problem is to use a steam reforming process with the addition of carbon dioxide in order to achieve an appropriate ratio; carbon dioxide can be supplied externally or recovered from the flue gases of the reformer. Another method is the two-stage conversion process using natural gas as the feedstock; in the first stage of conversion, steam reforming of natural gas takes place, with only about 1/4 of the methane undergoing reaction ； In the second stage, partial oxidation of natural gas takes place; not only is the composition of the syngas obtained optimal, but the increased reaction temperature to over 800°C also reduces the amount of residual methane, thereby increasing the effective gas components for methanol synthesis. 　Before entering the steam reformer, natural gas must be purified to remove harmful impurities, with the sulfur content in the purified gas required to be less than 0.1 mL/m3. The transformed gas is compressed and sent to the synthesis section to produce methanol. 　4. Briefly describe the production methods for methanol from coal and coke. 　Coal and coke are the main solid fuels used to produce the raw gas for methanol production. The process route for producing methanol from coal and coke includes fuel gasification, gas desulfurization, shift reaction, decarburization, as well as methanol synthesis and purification. 　The thermal processing of coal and coke using steam and oxygen (or air, oxygen-enriched air) is known as solid fuel gasification. The combustible gas produced as a result of this process is commonly referred to as gas, and it serves as the initial raw material for manufacturing methanol. The main equipment used for gasification is a gas generator. Depending on the way in which the coal moves within the furnace, gasification methods can be divided into fixed-bed (moving-bed) gasification, fluidized-bed gasification, and pneumatic-bed gasification. For the production of methanol in China using coal and coke, gasification generally employs the fixed-bed batch gasification method, with UCJ gasifiers being used for this purpose. Abroad, for coal gasification, the industrially established gasification furnaces include the Koppers-Totzek, Lurgi, and Winkler types. The types of second and third-generation gasifiers mainly include those from Texaco and Shell-Koppers, among others. 　The hydrogen-to-carbon ratio in the crude feed gas produced from coal and coke is too low; therefore, a shift process is required after gas desulfurization. The excess carbon monoxide is converted into hydrogen and carbon dioxide, and the excess carbon dioxide is removed through a decarburization process. 　Methanol is produced by compressing the feed gas, carrying out methanol synthesis, and refining it through distillation. 　5. Briefly describe the production method of oil-based methanol. 　There are mainly two types of oils used industrially to produce methanol: one is naphtha, and the other is heavy oil. 　The fraction obtained from crude oil distillation at temperatures below 220°C is called light oil, also known as naphtha. Methods for producing syngas from naphtha include pressurized steam reforming, catalytic partial oxidation, pressurized non-catalytic partial oxidation, and batch catalytic reforming. The main method currently used to produce methanol feed gas from naphtha is pressurized steam reforming. The pressurized steam conversion of naphtha must be carried out in a complexly structured converter. The converter is equipped with a radiant chamber and a convective chamber, where the conversion of hydrocarbon vapors takes place at high temperatures in the presence of a catalyst. After steam reforming, the composition of naphtha is exactly suitable for methanol synthesis. There is no need to add carbon dioxide before or after conversion or to employ a two-stage conversion process, nor is it necessary to adjust its composition through transformation or decarburization. 　Heavy oil is a product of the petroleum refining process; depending on the refining method, it can be classified into atmospheric heavy oil, vacuum heavy oil, cracking heavy oil, and their mixtures. There are two methods for producing methanol feed gas from heavy oil as raw material: partial oxidation and high-temperature cracking. The pyrolysis method requires cracking heavy oil in a regenerative furnace at high temperatures of over 1400°C. Although oxygen is not necessary, the equipment is complex, operation is cumbersome, and a large amount of carbon black is produced. 　Partial oxidation of heavy oil refers to a combustion reaction between heavy hydrocarbons and oxygen; this reaction releases heat, causing some of the hydrocarbons to undergo thermal cracking. The products of this cracking then undergo further oxidation and reforming reactions, ultimately yielding syngas that consists mainly of H2 and CO, along with small amounts of CO2 and CH4, for use in methanol synthesis. The syngas produced by the partial oxidation of heavy oil contains excessive amounts of carbon monoxide and carbon dioxide due to the high hydrocarbon ratio in the feedstock heavy oil; therefore, part of this syngas must be converted by reacting carbon monoxide with steam to produce hydrogen and carbon dioxide, after which the carbon dioxide is removed to achieve the composition required for methanol synthesis. 　The synthesized crude methanol needs to be refined to remove impurities and water, thereby obtaining pure methanol. 　6. Briefly describe the production method of diols. 　Methanol production in combination with ammonia synthesis, abbreviated as combined methanol production, is a purification process for syngas; it is a new process developed to replace the copper-ammonia solution used in many ammonia production facilities in China for removing trace amounts of carbon oxides. 　The process conditions for producing polyol involve adding a methanol synthesis unit between the outlet of the fifth stage of the compressor and the inlet of the copper washing process. This unit includes equipment such as a methanol synthesis tower, a circulator, water coolers, separators, and storage tanks for crude methanol. In this process, the gas coming from the outlet of the fifth stage of the compressor first enters the methanol synthesis tower, where most of the carbon monoxide and carbon dioxide that would otherwise need to be removed in the copper washing process react with hydrogen to produce methanol. As a result, the carbon monoxide content in the gas that proceeds to the copper washing process is significantly reduced, thereby lowering the workload on the copper washing process ； At the same time, the carbon monoxide requirements for the shift process can be relaxed to a certain extent, which reduces the steam consumption in the shift process; moreover, the carbon monoxide delivered by the first few cylinders of the compressor becomes useful gas, thereby lowering the electrical power consumption of the compressor. 　The energy consumption is reduced significantly after co-producing methanol; this allows for a savings of 50 kW·h of electricity per ton of ammonia, as well as 0.4 tons of steam, resulting in an overall energy saving of 2 million kJ. The polyol production process must pay attention to processes such as precise desulfurization and distillation of the feed gas, in order to ensure the service life of the methanol catalyst and the quality of the methanol product.