Comparison of dimethyl ether production process technologies

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Comparison of Dimethyl Ether Production Process Technologies Xu Meitong, Wang Lihua (Shandong Yankuang Lunan Fertilizer Factory, Tengzhou 277527) 【Abstract】 This paper introduces the production methods of dimethyl ether (DME) as well as the progress in related technologies, and analyzes and compares the characteristics of these processes. The production technology of dimethyl ether using methanol as a raw material is relatively mature; however, this process involves a long sequence of steps, requires significant investment in equipment, results in high product costs, and is highly susceptible to fluctuations in the methanol market. The production cost of dimethyl ether via the syngas method is low, and it can replace LPG and diesel as a new type of fuel, which is of great significance for the optimized use of energy in our country. 【Author’s institution】: Yankuang Lunan Fertilizer Factory; Yankuang Lunan Fertilizer Factory, Tengzhou 277527; Tengzhou 277527 【Keywords】: Dimethyl ether; production technology; new type of fuel 【Journal】: Fuel & Chemical Engineering, Issue 03, 2008 【Classification number】: TQ223.24 Dimethyl ether (CH3OCH3) is a colorless gas or compressed liquid with a slight ether-like odor; it is non-toxic and non-corrosive, and does not form peroxides upon prolonged exposure to air. It can be used as a propellant for aerosols, a refrigerant, and a foaming agent. High-concentration dimethyl ether can be used as a **agent. Dimethyl ether can also replace LPG and diesel as a new type of fuel; when mixed with methanol in certain proportions, it can serve as a substitute for city gas and liquefied gas. The large-scale production of dimethyl ether began in Europe in 1966. China started producing dimethyl ether relatively late; large-scale production only began in 1995. The main domestic manufacturers and their production capacities are shown in Table 1. In terms of aerosol products, China’s current annual production is around 350 million bottles. If half of these use dimethyl ether as a propellant, and assuming each bottle weighs 100 grams, then about 17,500 tons of dimethyl ether are needed per year. It is estimated that by 2010, around 30,000 tons of dimethyl ether will be required for aerosol products. Experts predict that China’s diesel consumption will reach 150 million tons in 2010. If the price of dimethyl ether drops to the level of diesel, and assuming it replaces 10% of diesel, then 15 million tons of fuel-grade dimethyl ether would be required. China’s current annual consumption of liquefied natural gas is between 35 and 40 million tons; if part of this is replaced by dimethyl ether, about 25 million tons would be required each year. Therefore, dimethyl ether holds great potential as a fuel for domestic use. Table 1: Major producers and production capacities of dimethyl ether in China
Producers | Production capacity (10,000 t/a) | Production method
Shandong Jiutai Chemical Co., Ltd. | 3.50 | Liquid-phase dehydration
Jiangsu Kunshan Chemical Raw Materials Factory | 0.10 | Liquid-phase dehydration
Guangdong Zhongshan Fine Chemicals Company | 0.50 | Gas-phase dehydration
Zhejiang Yiwu Guangming Chemical Company | 0.25 | Gas-phase dehydration
Chengdu Huayang Weiyuan Natural Gas Chemical Plant | 0.20 | Gas-phase dehydration
Hubei Tianli Industrial Company | 0.15 | One-step gas-phase process
Guangdong Jiangmen Nitrogen Fertilizer Factory | 0.25 | Gas-phase dehydration
Shaanxi New Type Fuel and Appliance Co., Ltd. | 0.50 | One-step gas-phase process
Zhejiang Zhuji Xinya Chemical Company | 0.10 | Gas-phase dehydration
Ningxia Yinchuan China National Coal Energy Group Corporation | 83.00 | Gas-phase dehydration
Anhui Mengcheng County Fertilizer Factory | 0.30 | Gas-phase dehydration
Luzhou Natural Gas Chemical Company | 100.00 | Gas-phase dehydration

1. Production methods and technological advancements of dimethyl ether
Dimethyl ether was initially produced by distilling by-products from high-pressure methanol production. With advances in methanol synthesis technology, industrial production methods for producing dimethyl ether from methanol dehydration and syngas synthesis have developed rapidly. 1.1 Liquid-phase dehydration of methanol: The traditional method for producing dimethyl ether involves using methanol as a raw material; under the catalysis of concentrated sulfuric acid, methyl hydrogen sulfate is formed, which then reacts with methanol to produce dimethyl ether. This reaction is characterized by a low reaction temperature (130–160°C), selectivity and conversion rates both exceeding 90%, the possibility of batch or continuous production, low investment requirements, and simple operation. Due to the strong carbonization effect of concentrated sulfuric acid on methanol, the catalyst has a short service life. Moreover, the removal reaction generates large amounts of residual acid and wastewater, causing severe environmental pollution ； The intermediate methyl hydrogen sulfate is highly toxic and harmful to human health. The production scale of traditional crafts is relatively small. The Shanghai Research Institute of Petrochemical Technology effectively suppressed the carbonization of organic substances by integrating the reaction and separation processes in a conventional methanol liquid-phase dehydration unit. At the same time, sulfuric acid is not lost and remains contained within the reactor for long-term use (with a first-use period of over 6 years), **reducing production costs and minimizing pollution. 1.2 Gas-phase methanol dehydration method In 1965, the American company Mobil was the first to report a method for producing dimethyl ether using the gas-phase methanol dehydration process. The basic principle involves passing methanol vapor through a solid acidic catalyst (alumina or crystalline aluminum silicate) in a fixed-bed catalytic reactor, where a heterogeneous reaction takes place to produce dimethyl ether from methanol. The mixture resulting from this dehydration process is then separated and purified to obtain dimethyl ether of fuel-grade or aerosol-grade quality. Domestic patent reports describe the production of dimethyl ether gas through dehydration at lower temperatures (100–125°C), normal pressure (0–0.05 MPa), and with the use of new catalysts; this approach effectively solves technical issues such as esterification dehydration, catalyst regeneration, and the synchronization of the reaction process. The Shanghai Research Institute of Petrochemicals also achieved success by using the D-4 alumina catalyst it developed, constructing an industrial plant for the gas-phase catalytic dehydration of methanol to dimethyl ether with a capacity of 2,000 tons per year, which was successfully put into operation in 1995. The methanol conversion rate of this device is ≥60%, the dimethyl ether selectivity is ≥99%, the catalyst has a service life of over 6 months, and the product can achieve a high purity level suitable for use in aerosol formulations. Table 2 lists the progress in the synthesis of dimethyl ether using the gas-phase methanol dehydration method at home and abroad. Table 2 Comparison of processes for synthesizing dimethyl ether using the gas-phase methanol dehydration method at home and abroad. Company Name, Catalyst Model, Reaction Temperature, °C, Reaction Pressure, Conversion Rate, %, Selectivity, %. American Mobil Company: HZSM-5 (alumina silicate), 200, atmospheric pressure, 80%, 98%. Japanese Mitsubishi Company: γ-Al2O3, –, 74.2%, 99%. Southwest Research Institute: ZSM-5, 200, 75–85%, 98%. Shanghai Wujing Chemical Plant: Silicate powder crystals, 130–200, atmospheric pressure, 85%, 100%. Shanghai Petrochemical Research Institute: D-4 alumina catalyst, –, –, –, 1.3%. Synthesis of dimethyl ether in one step from syngas. Extensive research has been conducted both domestically and internationally on the kinetics of methanol synthesis and methanol dehydration reactions. In recent years, the Dalian Institute of Chemical Physics, Chinese Academy of Sciences, has developed high-performance bifunctional catalysts for the one-step synthesis of dimethyl ether from syngas, and on this basis has created a new process for the one-step synthesis of dimethyl ether from syngas using a fixed-bed reactor. This process uses a fixed-bed reactor, with a H2/CO ratio of 1–2 for the syngas feedstock, a CO/CO2 ratio of 15–25, an operating pressure of 2.5–4.0 MPa, and a reaction temperature of 230–300°C. The space velocity of the syngas feedstock is 700–1500 h-1, and the catalyst used is a self-made metal zeolite catalyst. In addition, research has also been conducted on the technologies of producing syngas through catalytic oxidation of methanated air with partial oxidation, as well as on the production of dimethyl ether from nitrogen-containing syngas. The goal is to produce cheap syngas using inexpensive oxygen sources, thereby reducing the production costs of dimethyl ether. The one-step synthesis of dimethyl ether, particularly the slurry-bed one-step synthesis of dimethyl ether, is a newly developed technology. Compared with traditional methanol dehydration processes, this process features a simpler design, facilitates the removal of reaction heat, and enables constant-temperature operation. It allows the reaction process to be coupled with the heat transfer process, thereby achieving the optimal reaction temperature and preventing catalyst deactivation. It can directly utilize coal-based syngas with a high CO content, and it allows for the online loading and unloading of catalysts; it features high one-pass conversion rates for CO and high dimethyl ether yields, thereby giving dimethyl ether a cost advantage. Currently, this technology is in the pilot and pilot-scale testing stages. 1.4 Direct synthesis via CO2 hydrogenation In recent years, research on the hydrogenation of CO2 to produce oxygen-containing compounds has received increasing attention; the effective utilization of CO2 can reduce environmental pollution caused by industrial emissions. Kansai Electric Power Company and Mitsubishi Heavy Industries have designed a process in which CO2 is converted into methanol, which is then dehydrated to produce dimethyl ether, within a fixed-bed reactor. The reaction conditions are a temperature of 250–300°C and a pressure of 4–10 MPa; a patented dual-function catalyst is used in this process. In pilot tests, the conversion rate of CO2 was 90%, while the selectivity for dimethyl ether was 45%. Research on the synthesis of dimethyl ether from CO2 and hydrogen is also being conducted in China. Using the Cu-ZnO-ZrO2/ZrO2/HZSM-5 bifunctional catalyst developed by the Lanzhou Institute of Chemical Physics, Chinese Academy of Sciences, and a fixed-bed tubular reactor, a CO2 conversion rate of 34.5% was achieved at 240°C and under H2/CO=2.8 conditions, with a dimethyl ether selectivity of 60.7%. Fang Dingye and others from East China University of Science and Technology used a composite catalyst composed of a domestically produced C302 copper-based catalyst and a CM-3-1 modified molecular sieve to investigate the reaction performance of CO2 hydrogenation to dimethyl ether in a stirred-tank reactor under feed gas conditions of 260°C, 5.0 MPa, 1000 h-1, and H2/CO = 4. The CO2 conversion rate was 65%, and the dimethyl ether selectivity was 60%. 2 Comparison of dimethyl ether production processes: The liquid-phase sulfuric acid catalysis method using methanol and the gas-phase method for dimethyl ether production are both well-established technologies, with industrial plants operating using each of these methods. The methanol dehydration method uses high-purity methanol as raw material; it produces few by-products during the dehydration reaction, and the purity of dimethyl ether reaches 99%. It is suitable for aerosol products with high quality requirements, and can also be used as a propellant for refrigerants or medical aerosols. This process is relatively mature; new facilities can be built using existing enterprises, or separate plants can be constructed for production. However, this method involves processes such as methanol synthesis, methanol distillation, methanol dehydration, and dimethyl ether distillation; the process is lengthy, which results in high equipment investment and high product costs. It is also highly susceptible to fluctuations in the methanol market. The syngas-based process for producing dimethyl ether is a technical approach suitable for China’s national conditions. Especially in a slurry bed, the reaction temperature is evenly distributed, thermal balance is easier to control, the operation is simple and stable, and the production cost is low. The syngas used in the syngas method can be produced from coal, heavy oil, slag oil gasification, and natural gas conversion; the raw materials are inexpensive and readily available, which enables this process to be used in fertilizer plants and methanol plants. These plants can adapt their methanol facilities to produce dimethyl ether on a commercial scale. The key issue facing the one-step production of dimethyl ether at present is the need for efficient and cost-effective coal-to-gas processes and equipment ； Reactors capable of meeting the needs of large-scale dimethyl ether production are required ； Solve the issue of utilizing CO2 in the production of dimethyl ether from coal as a raw material ； Development and production of related catalysts ； Advanced and cost-effective dimethyl ether separation and purification technology. 3 Conclusion: The scale of plants for producing dimethyl ether using the methanol method is generally in the thousand-ton range, making it difficult to meet the demands of large-scale production. Additionally, due to the influence of methanol prices, this method is not suitable for producing fuel-grade dimethyl ether. However, the dimethyl ether produced by this method has advantages such as high purity, and is widely used in fine chemical industries such as propellants for aerosols, refrigerants, and foaming agents, offering good economic benefits. Dimethyl ether produced in a one-step process from syngas can be manufactured on a large scale; individual production facilities typically have a capacity of over 10,000 tons. The dimethyl ether produced can be used as a fuel for domestic use and as fuel for diesel engines. This process is more rational than the methanol dehydration process, and it is a key focus of research and development both domestically and internationally. China is rich in coal and natural gas, but relatively short of oil; using dimethyl ether as an energy carrier to partially replace oil is of great significance for the optimal utilization of energy and the achievement of sustainable development in the country. Last edited by ryn on 2009-3-13 15:03]