Million-ton-scale methanol synthesis technology

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This post was last edited by Qingfeng Liushui on 2011-1-22 at 12:43. 0 Introduction: The scale effect in methanol production is quite significant; as a result, the scale of production facilities continues to increase, moving toward larger and super-large sizes. In 2004, when Shandong Hualu Hengsheng Chemical Co., Ltd. put into operation the first large-scale domestic nitrogen fertilizer plant with a capacity of “300 kt/a of synthetic ammonia + 200 kt/a of methanol”, the 200 kt/a methanol production capacity represented the highest such capacity for a single methanol production unit in China at that time; prior to that, most domestic methanol production units had capacities of only a few tens of thousands to hundreds of thousands of tons per year. In December 2005, China’s first 400 kt/a methanol plant was put into operation at Sichuan Lutianhua Co., Ltd. In September 2006, China’s first 600 kt/a methanol plant began trial production at CNOOC Jiantao Chemical Co., Ltd. To make effective use of coal resources and enhance economies of scale, the National Development and Reform Commission issued a regulation in July 2006 stipulating that the production capacity of newly built methanol plants from coal must be at least 1 million tonnes per year. The Datang Dolun Coal Chemical Project currently under construction, as well as the methanol plant of Baotou Shenhua Coal Chemical Co., Ltd., have an annual production capacity of over 1 Mt per unit. In 1997, Lurgi was the first company to introduce the concept of MegaMethanol on a million-ton scale. Since then, methanol production technologies with a capacity of over 1 Mt/a per plant have become the focus of research in this industry. Currently, there are more than a dozen methanol plants around the world with an annual production capacity of over 1 Mt, three of which have a capacity of over 1.7 Mt/a. Below, several million-ton-scale methanol synthesis technologies with proven application records are introduced. 1 Lurgi two-stage isothermal methanol synthesis process 1.1 Introduction to structure and process flow: Considering the high heat generation during the methanol synthesis reaction and the poor heat resistance of existing copper-based catalysts, Lurgi has adopted a tubular reactor. For large-scale methanol production of over 1 million tons, a two-stage isothermal methanol synthesis approach using a \"gas-cooled reactor + water-cooled reactor\" is employed; there is no requirement to install a protection bed, and the process flow is shown in Figures 1 and 2. In water-cooled reactors, the reaction heat is removed by generating secondary medium-pressure steam, while in air-cooled reactors, the reaction heat is removed through counterflow heat exchange with fresh syngas, thereby achieving heat coupling. In the air-cooled reactor, the fresh gas flowing inside the tubes (at around 125°C) exchanges heat in counterflow with the catalyst outside the tubes, being heated to 250°C before proceeding to the water-cooled reactor for the methanol synthesis reaction, which takes place at a temperature of 265°C; thereafter, it returns to react outside the tubes of the air-cooled reactor. In Lurgi’s two-stage isothermal methanol synthesis process, most of the catalyst is installed in a gas-cooled reactor. http://pub2.hi2000.com/upload1/0810151553414999.jpg Figure 1: Lurgi methanol synthesis process
http://pub2.hi2000.com/upload1/0810151554067800.jpg Figure 2: Schematic diagram of the reactor
1.2 Main features
(1) Water-cooled reactor. The reaction is easily and accurately controlled, operates under near-isothermal conditions, features a high methanol yield (the catalyst’s yield of methanol is 1.2 kg/L), high energy efficiency, and high reliability; the heat generated by the reaction produces high-pressure steam. (2) Air-cooled reactor. Optimized reaction pathway (high equilibrium driving force, high conversion rate); no risk of catalyst poisoning; no need for a reactor feed preheater. (3) High single-series production capacity. (4) Low investment cost. (5) The recycle ratio is 1.5. The methanol content at the reactor outlet can reach 17% (mole fraction). 1.3 Performance: Lurgi introduced the concept of megaton-scale methanol in 1997, and in the same year it signed a contract with TITAN in Trinidad for a methanol plant with a capacity of 2500 t/d. In 2000, a 5,000 t/d methanol plant was contracted with ATLAS in Trinidad, and it came online in June 2004. In 2000 and 2004 respectively, methanol plants with a capacity of 5,000 t/d each were contracted with Iran’s National Petroleum Company (NPC). In 2004, a contract was signed with QAFAC of Qatar for a 6,750 t/d methanol plant. In 2005, contracts were signed with PETRONAS in Malaysia and China’s Datang International for a 5,000 t/d methanol plant each. In 2006, contracts were signed with Shenhua Ningxia Coal Industry for two 5,000 t/d methanol plants respectively. 1.4 Main shortcomings: high cooling water consumption and low amount of by-product steam ； The equipment is large, and its transportation is restricted ； The equipment resistance is too high ； Large temperature difference between water cooling and air cooling ； Special materials such as dual-phase steel are required. 2 Davy (ICI) radial-flow steam-driven methanol synthesis technology 2.1 Davy’s early large-scale methanol production technologies When it comes to Davy’s methanol synthesis technology, people usually think of its multi-stage cold-quench methanol synthesis towers. These towers utilize fixed-bed reactors and are of the 4-stage cold-quench adiabatic axial-flow type; a diamond-shaped distribution system is used between the stages to inject cold gas in order to reduce the temperature and thus control it within the synthesis tower. Due to the simple structure of this type of tower, it is also suitable for large-scale applications; among the world’s early large-scale methanol plants, those that used multi-stage quench-type methanol synthesis towers were the most common. However, it also has the following obvious problems: (1) It is unable to recover the high-grade heat energy generated during methanol synthesis, and the volume of gas circulating in the synthesis loop is large ； (2) Backmixing occurs between catalyst sections, resulting in a low methanol content at the outlet of the synthesis reactor ； (3) The space-time yield of the catalyst is not high, and a large amount of gas is used. Therefore, large-scale devices are rarely used these days. 2.2 Davy Radial Flow Steam Rising Methanol Synthesis Technology 2.2.1 Overview of Structure and Process In recent years, DAVY Company has developed radial flow steam rising methanol synthesizers specifically for methanol production plants with capacities of millions of tons. The main feature of these reactors is that the catalyst is located on the shell side, while the feed gas enters through the central tube and flows radially from the center outward. This process requires the installation of a guard bed before the reactor to further purify the syngas. Figure 3 shows a schematic diagram of the process currently designed by Davy for large-scale methanol plants, while Figure 4 depicts a schematic diagram of a radial-flow steam-rise type large-scale methanol synthesizer. http://pub2.hi2000.com/upload1/0810151554305183.jpg Figure 3: Schematic diagram of the process flow in Davy’s large-scale methanol plant
http://pub2.hi2000.com/upload1/0810151554531148.jpg Figure 4: Davy’s large-scale methanol synthesis reactor
2.2.2 Main features
(1) It utilizes a structure where gas flows radially, steam rises upward, and the synthesizers are connected in series/parallel. (2) The gas passes through the catalyst bed radially from inside to outside, with a small pressure drop. (3) The boiler feedwater enters from the bottom of the reactor and flows upward through a vertically arranged series of tubes; this process generates medium-pressure steam that carries away the heat generated by the reaction. By controlling the steam pressure, the temperature of the catalyst bed can be regulated, resulting in a temperature distribution that is nearly isothermal. (4) Convert CO + H2 into methanol as efficiently as possible, with a conversion rate of up to 98.5%. (5) There is no need to use precious metals; the requirements for materials are relatively low, and the cost of the synthesizer is low. 2.2.3 Performance: The methanol plant built by DAVY Company in Trinidad, consisting of two steam-driven upward-flow reactors connected in series/parallel to produce 5,400 tons of methanol per day, came online in October 2005. In December 2006, Baotou Shenhua Coal Chemical Co., Ltd. signed a technology introduction contract for 5,500 t/d methanol production with DAVY Company, utilizing two synthesis towers connected in a series-parallel configuration. The 3,600 t/d methanol plant at EMethanex in Egypt, as well as the 4,430 t/d methanol plant at Kharg Petrochemicals in Iran, also utilize Davy technology. 2.2.4 Main shortcomings: The concentration of exported methanol needs to be further increased to reduce the circulation volume and lower the energy consumption associated with circulation compression ； The gas flows outward in a diverging manner, with the flow speeds varying by several times, making it difficult to ensure a uniform distribution ； The by-product steam has a pressure of 2.2 MPa and a temperature of about 220°C; its usability is poor. 3 Topsøe’s adiabatic shell-and-tube methanol synthesis technology
3.1 Brief introduction to the structure and process
The Danish company Topsøe has designed a single-train, million-tonne-scale methanol production process in which several adiabatic shell-and-tube synthesis reactors are connected in series and parallel. The catalyst is placed in the tube side, while the shell side contains boiling water. This process requires a protection bed to be installed before the reactor in order to further purify the syngas, reducing the total sulfur content in it to less than 5×10‑9. 3.2 Main features: (1) The company’s proprietary RM101 copper-based catalyst is required; this type of catalyst boasts advantages such as high activity, high strength, small particle size, high selectivity, and low usage amount. (2) The recycle ratio is 1.5–2.5, with a high one-pass conversion rate (usually 50%–60%), and the methanol content at the reactor outlet can reach 13.86% (mole fraction). (3) High carbon conversion rate. Methanol plants that use coal as a raw material can typically convert 95% to 96% of the carbon in the feed gas into methanol. (4) The pressure drop across the synthesis reactor is usually less than 0.2 MPa; the net energy consumption per ton of methanol is approximately 22.15 GJ (taking into account the feed gas, turbine steam, and steam output). (5) Good heat recovery. 3.3 Performance: The 2,500 t/d methanol plant in Norway uses a three-series TopsΦe process. Large-scale methanol projects that adopted the TopsΦe technology during 2006–2007 included: a methanol plant in Saudi Arabia with a production capacity of 5,000 metric tons per day using gas as feedstock (expected to come online in 2009), and a methanol plant in Nigeria with a production capacity of 10,000 metric tons per day using gas as feedstock (expected to come online in 2010). 3.4 The shortcomings are largely similar to those of Lurgi’s older designs; there is no significant innovation in terms of structure ； Special materials need to be used ； Large number of devices ； The investment is substantial. 4 Casale’s IMC plate-type heat exchange methanol synthesis technology 4.1 Introduction to structure and process Casale developed an IMC plate-type reactor in which the heat exchange plates are arranged radially and in a concentric fan shape; these plates, which serve as cooling elements, are embedded within the catalyst bed. Boiler feed water flows through these plates to generate medium-pressure saturated steam, thereby removing the heat generated by the reaction from the catalyst bed. The heat exchange plate is supported at the bottom of the bed, while the catalyst is supported by the bed of inert material at the bottom. The central tube serves as a passage in the lower part of the synthesis tower, allowing the catalyst to be removed through the discharge port at the bottom. Figures 5 and 6 are schematic diagrams of Casale’s plate-type heat-exchange methanol synthesis tower. http://pub2.hi2000.com/upload1/0810151555281202.jpg Figure 5: Schematic diagram of the Casale methanol synthesis tower. http://pub2.hi2000.com/upload1/0810151555557209.jpg Figure 6: Schematic diagram of the structure of the Casale plate-type heat-exchange methanol synthesis tower. 4.2 Main features: (1) Large heat-exchange area for the internal components. (2) The alcohol purity is high, reaching 10%. (3) The gas can flow co-currently or counter-currently, axially or radially, with low pressure drop. (4) High catalyst loading factor, high utilization of high-pressure space, and high production intensity. (5) Domestic catalysts are used, but they must be approved by Casale testing and come with performance guarantees; this is the biggest difference compared to the technologies of foreign companies such as Lurgi, Davy, and TopsΦe. 4.3 Performance In November 2002, after Casale carried out an “on-site” retrofit on a 320 MTD methanol plant belonging to Nevinnomyssk Azot in Russia, utilizing the IMC plate heat exchanger design, the plant’s production increased by 33%. China’s Shanghai Coking Co., Ltd.’s 450 kt/a methanol plant, Xinneng Energy Co., Ltd.’s 600 kt/a methanol plant, and Shandong Jiutai Energy (Inner Mongolia) Co., Ltd.’s 1 Mt/a methanol plant have also adopted Casale’s IMC plate-type heat-exchange methanol synthesis technology. Using IMC plate-type isothermal methanol synthesis towers, the largest methanol project undertaken by Casale is the 7,000 t/d natural gas-to-methanol project in Iran, which is currently in the design phase. 4.4 Shortcomings: Difficult catalyst loading ； Forced circulation is used, which places high demands on the forced circulation pump ； Methanol synthesizers have a complex structure, high requirements for materials, are difficult to manufacture, and involve significant investment ； When the methanol synthesis pressure is relatively high (e.g., greater than 8 MPa), the structural reliability and process feasibility require verification through practice. 5 MHI/MGC shell-coil cold-tube composite SPC methanol synthesis technology 5.1 Structure and brief introduction This technology is a SPC (Superconverter) methanol synthesizer jointly developed by Japan’s Mitsubishi Heavy Industries (MHI) and Mitsubishi Gas (MGC); it functions as a vertical, simple double-tube heat exchanger. The catalyst is placed between the inner tube and the outer tube, while boiling water circulates between the tubes. The feed gas enters the inner tube from below; the heated gas then passes through the catalyst bed. The reaction gas is cooled by both the boiling water on the outside and the gas inside, thereby bringing the operating temperature closer to the optimal level. The air currents flowing along the inner and outer tubes are in opposite directions; the inlet temperature of the syngas to the catalyst layer is the highest, and it gradually decreases as it moves toward the outlet. Such a temperature distribution ensures an optimal reaction rate, allowing for high conversion rates while reducing the amount of catalyst used. The SPC methanol synthesizer can also be used for methanol production on a million-ton scale, and its structure is shown in Figure 7. http://pub2.hi2000.com/upload1/0810151556324232.jpg Figure 7 Schematic diagram of the SPC methanol synthesizer 5.2 Key features (1) Compared with Lurgi tubular reactors, the SPC methanol reactor has a lower circulation ratio and a higher space-time yield; at a space velocity of 5000 h⁻¹ and a pressure of 8.0 MPa, an outlet methanol concentration of 14% can be achieved. (2) The synthesizer acts as a preheater to preheat the inlet gas; the fresh syngas is preheated within the reactor, thereby eliminating the need for a heat exchanger. (3) Good energy recovery: 1 t of methanol can produce 1 t of steam at 4.0 MPa as a by-product. It is claimed that the energy consumption per ton of methanol produced using this reactor can be reduced to 29 kJ/t. (4) Good process stability. 5.3 The pressure drop due to deficiencies is relatively large ； The equipment structure is relatively complex; each inner tube must be connected to the gas collection pipe using flexible tubes in order to eliminate thermal stress ； The catalyst is installed between the casings, which causes inconvenience in the loading and unloading of the catalyst as well as in the installation and maintenance of the equipment ； The cold wall effect of catalysts cannot be ignored, requiring the catalyst to have good low-temperature selectivity and activity. Four 800 kt/a methanol plants using SPC technology have been put into operation in Saudi Arabia. 6 MRF Multi-stage Radial Flow Methanol Synthesis Technology 6.1 Introduction to Structure and Features The Japanese company TEC and Mitsui Toyo Chemical have jointly developed a new type of energy-saving and cost-reducing multi-stage indirect-cooling type radial flow (abbreviated as MRF) methanol synthesis reactor. It is said that this system can be easily scaled up from the current capacity of 750–850 kt/a to a capacity of 1.5 Mt/a. The MRF methanol synthesis tower is a vertical cylindrical pressure vessel, consisting of a pressurized shell, a catalyst basket with a central tube, and vertical boiler tubes (cooling tubes) connected to the boiler feedwater vaporization header and the steam collection header. The tubes are arranged in several layers of concentric circles and mounted vertically above the catalyst bed. Syngas enters through the central tube and then flows radially through the catalyst layer to undergo reaction; the gas after the reaction collects in the annular space between the catalyst basket and the shell of the synthesis tower, from where it exits at the top. The boiler feed water enters the cooling tubes from the bottom of the furnace; the steam generated collects in the steam collection main, from where it is then discharged from the top of the tower. The structure of the MRF multi-stage radial flow methanol synthesis tower is similar to that of the Davy radial flow steam rising methanol synthesis tower. 6.2 Main features (1) Radial gas flow, short flow channels, low space velocity, and low pressure drop. (2) The syngas passes vertically through the surface of the boiler tubes; even at low gas flow rates, the heat transfer efficiency between the bed and the cold tubes remains high. (3) High one-way conversion rate and low circulation gas volume. (4) The arrangement pattern of the boiler tubes ensures that the reaction temperature closely follows the ideal temperature distribution curve; the reaction proceeds along the optimal temperature profile, resulting in a high methanol yield. The concentration of crude methanol at the outlet of the synthesis tower exceeds 8.5%. (5) The reaction heat is removed promptly and effectively, the bed temperature remains stable, a small amount of catalyst is required, operation takes place under mild conditions, and the catalyst has a long service life. 6.3 Drawbacks: Complex structure and high manufacturing difficulty ； The pressure of the by-produced steam is lower than that in shell-and-tube towers, making it difficult to utilize the steam ； The boiler water is forced to circulate within double tubes; the circulation pump requires significant power, which results in substantial maintenance workload for the pump. Technical requirements and development trends of 7-million-ton methanol synthesis reactors (1): Adoption of low-pressure synthesis technology. (2) Recover reaction heat at a higher potential energy, generating by-product steam. (3) The temperature of the catalytic bed is easy to control; temperature regulation is convenient, rapid, and effective, allowing for flexible adjustment. (4) Isothermality is the direction of development; the bed temperature should be as uniform as possible, and the reaction temperature should remain within the optimal range in order to extend the catalyst’s service life. (5) High space-time yield, high catalyst production intensity, low recycle ratio, and high single-pass conversion rate. (6) Good selectivity, high quality of crude methanol, and few by-product reactions and impurities. (7) It has strong operational adaptability and can cope with changes in various operating conditions. (8) It has a high volume factor; for a reactor of a given volume, more catalyst can be loaded. (9) The catalyst is easy to load, unload, and reduce. (10) A guard bed is installed before the reactor to further reduce the total sulfur content in the syngas, thereby protecting the methanol catalyst. (11) The use of radial or axial-radial reactors ensures uniform gas distribution within the bed and low resistance. (12) Try to avoid using special and expensive materials to reduce manufacturing and assembly difficulties as well as manufacturing costs.