Which structural type is suitable for the heat exchanger in methanol-to-hydrogen production?

For heat exchangers in methanol-to-hydrogen systems, floating-head or U-tube type shell-and-tube heat exchangers are recommended as the preferred choice. The leaks that occur in various types of structured heat exchangers you encounter are usually caused by the following factors: 1. Temperature differential stress and thermal expansion: The temperature changes during the methanol-to-hydrogen production process are significant, and fixed tube sheets or rigid structures are unable to accommodate thermal deformation, which leads to thermal stress and subsequent leaks. 2. Impact of process condition fluctuations: Frequent start-stop operations and large variations in operational load both exacerbate equipment fatigue damage. 3. Corrosivity of the medium: Small amounts of impurities or weakly acidic components such as CO₂ in the methanol-water solution used as raw material can cause gradual erosion of the equipment materials. 4. Manufacturing quality issues: such as welding defects, inappropriate material selection, and inadequate details in the structural design. Overall, process conditions (especially large temperature variations and thermal stress) are the dominant factors. It is recommended that the heat exchanger adopt a structural design with strong thermal expansion adaptation capabilities, such as a floating-head shell-and-tube type or a U-tube type. At the same time, by strengthening material selection and welding quality control, and optimizing process flows and operational controls to reduce fluctuations in operating conditions, it is possible to effectively prevent issues and extend the equipment’s service life. .

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Selection of Heat Exchanger Structures for Methanol-to-Hydrogen Production and Analysis of Leakage Causes I. Common Heat Exchanger Structures and Their Applicability in Methanol-to-Hydrogen Production In the methanol-to-hydrogen production process, the commonly used heat exchanger structures include fixed-tube-sheet type, coiled-tube type, and spiral-plate type; each of these types has its own characteristics and suitable applications. Advantages of fixed-tube-sheet heat exchangers: simple structure, low manufacturing cost; the maximum number of tubes can be installed within the same shell diameter, with the fewest bypasses; each heat transfer tube can be easily replaced, and cleaning inside the tubes is convenient. Disadvantage: The shell side cannot be mechanically cleaned ； When the temperature difference between the heat exchange tube and the shell exceeds 50°C, thermal stress is generated; therefore, expansion joints need to be installed on the shell, but this will limit the pressure in the shell side ； Suitable for scenarios with a temperature difference of no more than 70°C. Problems in methanol-to-hydrogen production: Due to the large temperature difference between the gas exiting the converter and the raw methanol aqueous solution, thermal stress is likely to occur, leading to deformation of the tube sheet and damage to the welds. Advantages of coiled tubular heat exchangers: High heat transfer efficiency (the heat transfer coefficient can reach 14,000 W/(m²·℃), which is 20%-40% higher than that of conventional devices) ； High-pressure resistance (up to 30 MPa) ； The spiral structure allows axial expansion and contraction of the free end of the tube bundle, reducing the stress impact on tube sheet welding and providing good vibration resistance. Features: It adopts a multi-layer spiral winding structure, with a heat transfer area per unit volume of 100–170㎡/m³, resulting in a small footprint ； Suitable for conditions with large temperature differences and high pressure. Advantages in methanol-to-hydrogen production: It can effectively address the temperature difference between the high-temperature gas at the converter outlet and the low-temperature methanol aqueous solution, thereby reducing the impact of thermal stress. Advantages of spiral plate heat exchangers: large heat transfer area and compact structure. Disadvantages: Narrow flow channels, prone to clogging, difficult to clean ； High requirements are placed on the cleanliness of the medium. Problems in methanol-based hydrogen production: Methanol aqueous solutions may contain impurities, which can lead to blockages in the flow channels and increase maintenance difficulties. II. Analysis of the causes of heat exchanger leakage According to the search results, the main reasons for heat exchanger leakage can be summarized as follows: Excessive thermal stress During the operation of shell-and-tube heat exchangers, the temperature difference between the cold and hot fluids causes different rates of thermal expansion in the shell and tube walls; when this temperature difference is large, it may cause the tubes to bend or become loose from their mounting brackets. Exceeding the specified rates of temperature rise and temperature drop during start-up and shutdown (for example, the temperature drop rate should be ≤2°C/min) can subject the tubes and tube sheets to significant thermal stress, leading to damage at the welds or expansion joints. When there is a sudden change in load, such as when steam supply to the steam side stops too quickly while water continues to flow into the water side, the pipes contract faster than the tube sheet, which often leads to damage at the joints. Tube sheet deformation: Deformation or insufficient thickness of the tube sheet can lead to such issues; the center of the tube sheet bulges toward the steam side, where the pressure is lower and the temperature is higher, while it dips on the water side, causing leakage at the tube ends. In methanol reforming units for hydrogen production, the pressure on the water side of the tube sheet is high while the temperature is low; conversely, the pressure on the steam side is low while the temperature is high. This large temperature difference increases the risk of deformation. Equipment manufacturing factors: Improper material selection – for example, the use of non-corrosion-resistant materials such as 316L stainless steel accelerates the rate of corrosion in a methanol environment. Poor welding quality: The welds have defects or residual internal stresses, making them prone to cracking during operation. The design of the compensation structure is unreasonable: for example, in the fixed tube sheet type, the impact of thermal expansion is not adequately considered. Operational control factors: Improper start-up and shutdown procedures: The rate of temperature change exceeds the allowed values (temperature rise rate of 2–5°C/min, temperature drop rate of 1.7–2.0°C/min). Too rapid load adjustment: leads to sharp changes in pressure and temperature, exacerbating thermal stress. Poor maintenance: Lack of regular cleaning or inspection allows minor issues to develop into leaks. Process flow factors: Medium properties: Methanol is corrosive and can cause equipment damage with long-term use. System design: Failure to adequately account for thermal expansion compensation leads to stress concentration. III. Analysis of Key Factors Based on search results and practical applications, the key factors responsible for leaks in methanol-to-hydrogen heat exchangers, ranked by importance, are as follows: Thermal stress: This is the most direct and common cause; a large temperature difference exists between the gas exiting the converter (at high temperature) and the methanol aqueous solution used as the feedstock (at low temperature), resulting in significant differences in thermal expansion. Fixed-tube-sheet heat exchangers are most sensitive to this, and even coiled-tube and spiral-plate types cannot completely avoid it. Equipment manufacturing quality: includes material selection (such as whether corrosion-resistant 316L stainless steel is used), welding techniques, and tube sheet thickness design. Manufacturing defects can significantly reduce the lifespan of equipment, turning stresses that were originally tolerable into direct causes of leakage. Operation control: Improper start-stop operations and load regulation can exacerbate the effects of thermal stress; in particular, an inadequate control of the temperature drop rate (exceeding 2°C/minute) can directly lead to damage to the welds. Process flow design: Although the impact is relatively indirect, failure to adequately consider thermal compensation or medium properties can accelerate equipment degradation. IV. Recommendations for Selection of Structural Types For applications in the methanol-to-hydrogen process where heat is exchanged between the gas exiting the converter and the methanol aqueous solution feedstock, and based on an analysis of the causes of leaks, the following recommendations are proposed: Coiled-tube heat exchangers should be given priority, as their spiral structure enables them to effectively absorb thermal expansion stresses. Under temperature differences of 100°C, the level of thermal stress is reduced by 60% compared to fixed-plate heat exchanger designs. Their ability to withstand high pressures (up to 30 MPa) and resistance to vibrations make them suitable for operating conditions with fluctuations. It features high heat transfer efficiency (the heat transfer coefficient increases by 20%-40%), which helps to reduce the generation of temperature difference stress. If a fixed tube sheet design is chosen, special attention must be paid to ensuring that the temperature difference does not exceed 70°C; otherwise, expansion joints must be installed. High-quality materials (such as 316L stainless steel) and strict welding processes are used. Strengthen operational control and strictly limit the rate of temperature change. The spiral plate type is generally not recommended, as its flow channels tend to get clogged, which increases maintenance difficulties and the risk of leaks. It requires high purity of the medium and is not suitable for methanol aqueous solutions that may contain impurities. V. Comprehensive measures to prevent leaks: Regardless of the structural design chosen, the following measures should be taken to prevent leaks: In terms of equipment manufacturing, corrosion-resistant materials (such as 316L stainless steel) should be used. Ensure welding quality and eliminate residual internal stresses. The tube sheet is designed with sufficient thickness to prevent deformation. In terms of operation control, strict limits are imposed on the rates of temperature change during start-up and shutdown (temperature rise rate of 2–5°C/min, temperature drop rate of ≤2°C/min). Avoid sudden load changes and adjust process parameters smoothly. Conduct regular maintenance inspections and cleaning. In terms of system design, the requirements for thermal compensation are fully taken into account 4 8. Set a reasonable range for the operating parameters. Consider using polymer composite materials to protect critical areas. VI. Conclusion In the methanol-to-hydrogen process, the leakage problem in the heat exchangers used to exchange heat between the gas exiting the converter and the raw methanol aqueous solution is driven by thermal stress and the quality of equipment manufacturing; among these, thermal stress is the most direct cause, while the quality of equipment manufacturing determines the capacity of the equipment to withstand thermal stress. Improper operation control can exacerbate the problem, while the impact of process flow design is relatively indirect. Coiled tubular heat exchangers are recommended, as they excel in terms of heat transfer efficiency, pressure resistance, and thermal compensation, thereby effectively reducing the risk of leaks. If a fixed tube sheet design must be used, the temperature difference must be strictly controlled and the manufacturing quality ensured. At the same time, the service life of the equipment can be further extended through optimized operational control and improved maintenance.

As far as I remember, fixed tube sheets are more common; plate heat exchangers tend to leak easily.