Methanol synthesis tower

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Understanding of the material selection for methanol synthesis towers 1. Requirements of the medium on reaction tube materials: Since the fluid flowing through the tubes in a typical shell-and-tube methanol synthesis tower is syngas, syngas can react with nickel (Ni) present in the reaction tubes under certain conditions to form carbonyl compounds, which leads to catalyst deactivation. Therefore, when selecting materials for equipment design, reaction tubes should be made of duplex stainless steels with low nickel content, such as S31803 and S31500 specified in the ASME SA-789M standard. The corresponding domestic duplex steel grades are 00Cr18Ni5Mo3N and 00Cr18Ni5Mo3Si2; these were also used in methanol synthesis towers with capacities of 50,000 tons, 80,000 tons, and 100,000 tons in the past. In recent years, due to the widespread construction of methanol production facilities, imported tube materials are more often chosen, although their lead times are relatively long. Domestically produced pipes are also quite common. When designing a 250,000-ton synthetic tower for a project at our company, we selected S31803 pipes with a diameter of 44 mm, a wall thickness of 2 mm, and a length of 7 meters, based on the synthetic process conditions specified by East China University of Science and Technology. 2. Selection of shell material: For the selection of shell materials, in methanol synthesis towers with a capacity of 100,000 tons or less, 13MnNiMoNbR is generally used as the material for the shell tubes and reinforcement sections. In large-scale methanol production projects with capacities of 200,000 tons, 250,000 tons, and 300,000 tons, 20MnMoNi55 is typically used as the shell material. This is due to the differences in their physical properties, specifically in terms of linear expansion coefficients; the linear expansion coefficient of 20MnMoNi55 is closer to that of duplex steel (see the table below for comparison). Therefore, in the design of larger methanol synthesis towers, 20MnMoNi55 should be given priority as the shell material, and it is thus used as the material for both the reinforcement sections and the shell tubes in most synthesis tower designs, in order to reduce the axial thermal stresses resulting from temperature differences between the tubes and the shell. Table: Comparison of thermal expansion coefficients α
Temperature (°C): 100, 200, 300
Thermal expansion coefficient α (10-6 mm/°C):
S31803: 13.00, 13.50, 14.00
13MnNiMoNiR: 11.53, 12.25, 12.90
20MnMoNi55: 12.7, 13.20, 13.60

III. Design of the shell-side structure
Under operating conditions, an exothermic reaction takes place inside the tubes, while the shell side consists of a boiler water circulation system that removes the heat generated by this reaction. The faster the circulation of the boiler water, the more promptly the reaction heat can be removed, which is beneficial for controlling the synthesis temperature. Additionally, a smaller temperature gradient exists along the axial direction of the tubes, resulting in a smaller temperature difference between the tubes and the shell. This in turn reduces the axial stress on these components, making the equipment safer and more reliable. To ensure the circulation efficiency of the shell-side boiler water, certain requirements must be met regarding the number of internal support plates on the shell side, as well as the water flow area and the number and distribution of the water flow holes. In the design of a 250,000-ton synthetic tower in China, we arranged more than 4,300 reaction tubes based on the process conditions, as well as over 500 water passage holes with a diameter of Φ44. The total water passage area accounted for approximately 6%. When arranging these water passage holes, it was necessary to ensure a reasonable distribution along the perimeter and in six radial directions, keeping them as close as possible to the axis of the equipment; this prevented the formation of dead zones, allowing the heat generated in the reaction tubes located at the center of the equipment to be dissipated quickly, which helped to achieve a more uniform temperature across the same cross-section. Furthermore, since the inner support plates on the shell side provide support for the reaction tubes, an excessive number of such plates increases the circulation resistance of the boiler water on the shell side, which hinders its circulation. However, the number of support plates is not determined arbitrarily; it is determined based on calculation results, more precisely, by the average wall temperature in both the tube side and the shell side. Among the numerous synthesis towers in use domestically, most have three sections; however, there are also some that have four or even five sections. This not only increases the resistance to the water circulation in the boiler but also raises the rigidity required to support the reaction tubes. Therefore, during the design of synthesis towers, it is necessary to determine an appropriate number of support plates, as well as the number and placement of water flow holes, based on the average wall temperature of the tube and shell sides. Requirements for boiler water quality. IV. The impact of the catalyst loading height in the insulation layer on the equipment. Since the production capacity of a synthesis tower is calculated based on the catalyst loading amount at a certain synthesis pressure, in shell-and-tube type insulation-temperature-equalizing synthesis towers, a certain height of catalyst is generally loaded above the tube sheet of the tower. After the reduction reaction occurs during the initial startup, the catalyst sinks, but it still remains at a certain height above the tube sheet, where the synthesis reaction continues to take place. The heat generated by this reaction is carried away by the gas entering the tower at a lower temperature. If the insulation layer is too thick, more heat will be released during the reaction, and the gas entering the tower will not be able to remove enough of this heat, resulting in excessively high temperatures in the insulated section above the tube sheet. Additionally, the temperature range suitable for duplex steel is limited, so this can lead to \"overheating\" of the weld joints between the tubes and the tube sheet, causing leaks and ultimately rendering the synthesis tower ineffective. Therefore, determining the appropriate insulation layer thickness is also important for the safe operation of the synthesis tower. To control the temperatures within the insulation layer and the reaction tube, two temperature measurement ports are installed on the head of the synthesis tower; each port contains a thermocouple sleeve, with 3 or 4 thermocouples inserted into each sleeve. These thermocouples are used to measure the temperatures at various heights within the insulation layer and the reaction tube, thereby enabling more accurate control of the temperature inside the tower. V. Material selection and structural design of the tube sheet: Since the methanol synthesis tower is a large-scale chemical process equipment, there are two possible options for the material of its tube sheet: one is an integral forged piece with butted ends on both sides (as shown in Figure 1), and the material that can be used is 20MnMo forged steel ； Another option is to use sheet metal, with a mosaic-style structure with the reinforcement section (as shown in Figure 2); 20MnMoNi55 sheet metal, which is the same as the material used for the housing, can be chosen. But butted forgings are mostly used in devices with smaller sizes; their advantage is that automatic welding is convenient, and the welds can be inspected using radiographic testing. For synthesis towers with larger dimensions and relatively thin tube sheets, from an economic perspective, it is advisable to use sheet metal as the tube sheet, which can significantly reduce costs. Moreover, the joint between the tube sheet and the reinforcement section features a double-sided, single-edge \"U\"-shaped groove, along with a fully penetrative welding structure that allows for 100% ultrasonic testing of the welds, ensuring their quality. Figures 1 and 2. VI. Design of the reinforcement sections and their connections to the nozzles: Six shell-side boiler water inlets and steam outlets are provided in the area of the shell close to the upper and lower tube sheets. For the purpose of overall reinforcement, the shell can be divided into three sections; the thickness of the middle section is determined based on the internal pressure in the shell side, while the thickness of the shell sections near the upper and lower tube sheets can be set to be the same as that of the tube sheet shells. First, consider overall reinforcement by increasing the wall thickness of the shell; second, consider eliminating local stresses in order to withstand the transverse shear stresses and moments present between the tube sheet and the shell, so that the reinforcement can provide effective support and strengthening for the tube sheet. Due to the relatively large wall thickness of the intensified section of the methanol synthesis tower, if plug-type nozzles are used to achieve a fully penetrated weld structure, the welding groove must be deep and more weld filler metal is required, resulting in higher welding residual stresses. To address this issue, a mounted structure is used to connect and strengthen the sections (as shown in Figure 3). To ensure full penetration of the weld, the inner diameter of the nozzle forging should have a certain amount of machining allowance during rough machining; after it is welded to the reinforcement section, the diameter is bored to the required design size in order to remove any unpenetrated areas at the root of the weld, thus ensuring full penetration at that location. For connections in other locations, an insert-type structure can be used (as shown in Figure 4). Figures 3 and 4. VII. Several requirements for processing and manufacturing: Since the cylinder, head, reinforcement sections, and tube sheets of the methanol synthesis reactor are all made of medium-thick or thick plate materials, the head is made of 15CrMoR, which is a Cr-Mo steel; the shell is made of 20MnMoNi55, a high-strength steel with an Rm value greater than 540 MPa; and the reaction tubes are made of S31803 duplex stainless steel. As a result, the material strength levels are high, making the processing and manufacturing processes complex. To ensure the manufacturing quality of the equipment, strict controls must be applied at every stage, from raw material procurement and re-inspection upon arrival at the factory to production organization and process control. Special attention should be paid to the following key points: First, effective measures must be taken to prevent deformation during the cladding of the tube sheet, the dilution rate of the transition layer must be controlled, and the composition and structure of the surface layer must be ensured ； Secondly, the dimensions of the hole bridges in the tube sheet drilling must remain within a controllable range of deviation, and the holes around the tube sheet need to be precision-machined ； Third, the shape of the head should be regular, and welding test plates should be provided for hot forming ； Fourth is the control of the roundness and straightness of the cylinder shape, as well as the assembly of the reinforcement sections with the cylinder and pipes ； Fifth, the inlaid welding of the tube sheet and reinforcement sections must ensure full penetration, with the welds free from defects such as inclusions, pores, and lack of fusion ； Sixth is the welding and expansion joining of the tubes to the tube sheet, as well as the inspection of sealing performance ； Seventh, it is necessary to strictly control pre-weld heating, post-weld dehydrogenation, and the interpass temperature of the weld seam in accordance with the process specifications ； Eighth, it is necessary to establish appropriate process specifications for overall and local heat treatment to ensure the effective removal of welding residual stresses ； Ninth, it is necessary to select appropriate non-destructive testing methods, testing ranges, proportions, and timing for such tests; for example, 100% magnetic particle inspection of the weld grooves prior to welding, and ultrasonic sampling inspections of Class A and B welds after heat treatment and hydrostatic testing of the equipment ； Ten are the requirements for voltage withstand tests, airtightness tests, and ammonia leakage tests, etc. In summary, in order to provide users with methanol synthesis towers of high quality that are safe and reliable, it is necessary not only to select appropriate materials and optimize the design structure during the design phase, but also to establish specific requirements for aspects such as material control, processing and manufacturing, welding, heat treatment, non-destructive testing, and overall assembly of these devices, in order to ensure the feasibility of the manufacturing process and the reliability of the product quality.