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Steam conversion process for methanol production from natural gas and key control points – Kang Yingying, Du Lun, Shuai Minggang Abstract: Currently, the steam conversion process using natural gas remains one of the main methods for producing methanol. In the actual production process of producing methanol from natural gas, the optimization of plant operation focuses mainly on the control of key indicators. The operating parameters of the steam conversion process include pressure, temperature, water-to-carbon ratio, and space velocity. The proper operation of the equipment, as well as maintenance activities on the plant, can all affect the quality of methanol synthesis gas. By properly controlling the appropriate operating parameters during the conversion process and optimizing the key operating parameters of the production process, it is possible to improve the quality of syngas and increase methanol production. Keywords: Key points of steam reforming control; Process parameter control. Methanol is a very important basic chemical raw material, and natural gas is a key ingredient in its production. Abroad, 90% of methanol is manufactured using natural gas as a raw material. Small and medium-sized methanol production facilities generally employ the steam reforming process; this reaction produces the raw gas needed for methanol synthesis (mainly CO, H2, and CO2), namely syngas. The quality of this syngas has a direct impact on the yield of methanol. The operational control parameters during the steam reforming process affect the quality of syngas; optimizing and properly controlling these key parameters in the production process is essential for increasing methanol production. 1 Brief description of the natural gas steam reforming process: Natural gas is a gaseous hydrocarbon whose main component is methane; it contains small amounts of alkanes such as ethane and propane, but no olefins. 1.1 Principle of natural gas steam reforming: The steam reforming reaction for producing methanol from natural gas involves the conversion of methane and water vapor into methanol synthesis gas under certain conditions; the specific reaction is as follows. Under certain conditions, side reactions occur during the steam conversion process, and the carbon deposition reaction is as follows. As can be seen from the above reactions, the overall methane steam conversion process is highly endothermic; therefore, in actual production, external heat supply is required to drive this reaction. According to the principles of steam reforming reactions, these reactions proceed more efficiently under low pressure and high temperature conditions to produce syngas. However, in actual production, the selection of process parameters also needs to take into account factors such as energy consumption and economic investment. Taking all factors into account, pressurized conversion is generally used. 1.2 Introduction to Natural Gas Steam Reforming Process (1) Steam Reforming Process. This process technology is mature, and many methanol conversion processes around the world use the steam conversion method. The steam conversion process involves the generation of a mixed conversion gas consisting of H2, CO, and CO2 at high temperatures with the participation of a catalyst. This process has a simple technical flow; it is the one with the shortest process route among various conversion processes, and it requires fewer pieces of equipment. The operating conditions during this process are mild, with the maximum temperature of the converted gas being around 900℃ ； The drawback of this process is that the reformate gas contains a high proportion of hydrogen and a low proportion of carbon, which results in larger equipment sizes for the reforming, compression, and synthesis processes ； High temperatures facilitate the steam conversion reaction in the direction of syngas synthesis, but excessively high temperatures reduce the service life of the converter tubes. Moreover, not all of the heat can be utilized, which increases energy consumption and also raises the cooling load. (2) Pre-conversion steam conversion process. The pre-conversion steam reforming process involves the adiabatic conversion of feedstock with the help of catalysts containing high levels of nickel; this process reduces pollutant emissions and aligns with green environmental protection principles. The process involves desulfurizing the feed gas; thereafter, the feed gas enters a saturator where it absorbs most of the steam required for the conversion reaction. It then goes into a pre-conversion reactor, where the mixed feed gas and the heavier components in the water vapor are broken down into lighter components, resulting in a gas mixture that is finally sent to the conversion furnace for further reaction. There is no air preheater in this process, thus reducing nitrogen oxide emissions. The design of the pre-conversion process reduces the workload on the conversion furnace, allows the furnace equipment to be smaller in size, and improves the preheating and recovery of flue gases from the conversion furnace. 1.3 Process flow of natural gas steam reforming The process of producing methanol from natural gas via steam reforming mainly includes the purification of the feed natural gas, catalytic reforming reaction, heat recovery, and gas-liquid separation. The raw natural gas contains sulfides, so sulfur must be removed to the levels required by the process to prevent catalyst poisoning in the conversion process. The purified natural gas, together with the added water vapor, achieves the water-to-carbon ratio required by the process. After being preheated in the convection section, it enters the conversion furnace where the conversion reaction takes place; the heat needed for this reaction is provided by the burners in the radiation section of the conversion furnace ； The high-temperature conversion gas exiting the converter has its heat recovered in the heat recovery section, and the recovered heat is used to generate steam to drive turbines, providing process heat ； The conversion gas is separated from the condensate through heat exchange to obtain methanol synthesis gas. 2 Key control points of the natural gas steam reforming process: In the actual production of methanol from natural gas, the optimization of plant operation focuses mainly on the control of key indicators. The control of critical parameters in the reforming process directly affects the quality of syngas, and thus influences the yield of methanol. The operating parameters that affect the conversion process for producing methanol synthesis gas include pressure, temperature, water-to-carbon ratio, and space velocity. 2.1 Process parameter control (1) Influence of conversion pressure. Based on the methane steam reforming reaction, it can be seen that this is a reaction in which the number of moles increases. From the perspective of chemical equilibrium, increasing the pressure of the reaction system shifts the equilibrium in the opposite direction, which is not conducive to the formation of syngas. The higher the pressure, the higher the methane content in the equilibrium composition of the gases exiting the converter, especially when the system temperature is low, and this effect is significant. If reducing the pressure of the system leads to an increase in energy consumption. In the process of synthesizing methanol, the lower the CH4 content, the more favorable it is for methanol production. Therefore, in order to reduce the methane content in the outlet gas, while increasing the pressure, it is necessary to raise the water-to-carbon ratio and the reaction temperature. Even if the pressure in the reaction system is high, by increasing the water-to-carbon ratio and temperature, the methane content in the equilibrium composition of the outlet gas can be reduced. Furthermore, increasing the pressure can effectively save energy, as the steam conversion reaction is one that results in an increase in the number of moles. The reaction pressure for methanol synthesis is higher than that for conversion; therefore, raising the conversion pressure reduces the compression work required for syngas. (2) Effect of conversion temperature. The conversion reaction is an endothermic reaction; increasing the temperature facilitates the shift of the chemical equilibrium in the direction of forming conversion gas, which in turn speeds up the reaction rate and increases the conversion rate of methanol. In the actual production process, the conversion temperature refers to the exit temperature of the bed layer, and this temperature affects the composition of the gas exiting the bed. Without carbon deposition, the higher the inlet temperature of the converter, the more heat can be supplied to the steam conversion reaction; this allows for less heat to be provided by the combustion of natural gas, resulting in lower natural gas consumption per ton of methanol produced. (3) Effect of water-to-carbon ratio. The water-to-carbon ratio refers to the ratio of the number of water vapor molecules entering the conversion furnace to the number of carbon atoms in the hydrocarbons added. The ideal carbon-to-water ratio for the converter feedwater is 2. From the perspective of chemical equilibrium, increasing the water-to-carbon ratio helps to raise the methane conversion rate, promotes the formation of reaction gases, and increases the reaction rate ； When the water-to-carbon ratio is too low, a carbon precipitation reaction occurs ； An increase in the water-to-carbon ratio indicates a greater amount of steam entering the conversion furnace, which promotes the steam conversion reaction and suppresses the occurrence of carbon deposition side reactions. However, when the water-to-carbon ratio is too high, the consumption of water vapor increases. The steam conversion reaction requires heat absorption, and the water vapor that does not participate fully in the reaction also absorbs heat upon entering the furnace tubes, which increases energy loss and likewise raises the thermal load on the furnace tubes. The disadvantage of increasing the water-to-carbon ratio is that it raises the pressure drop in the conversion system, as more natural gas needs to be burned in the converter, which leads to increased energy consumption. Therefore, the water-to-carbon ratio is determined such that it remains as low as possible without causing carbon deposition. In current methanol production processes, the water-to-carbon ratio is controlled within the range of 2.6–3.2. (4) Effect of air velocity. Air velocity refers to the amount of intake air that passes through per cubic meter of catalyst per hour. The practical significance of space velocity represents an indicator of the catalyst production intensity. The higher the air velocity, the greater the production intensity. The selection of space velocity in industry takes into account various factors comprehensively. Improving the pore structure of the conversion catalyst, increasing the utilization rate of the catalyst’s internal surface, changing the particle size of the catalyst, and enhancing the heating conditions of the catalyst all contribute to an increase in space velocity. As the air velocity increases, the resistance inside the tube rises; therefore, the resistance limit of the conversion unit is the factor that restricts an increase in air velocity. Reaction pressure affects the reaction rate as well as the actual residence time of the reactants on the catalyst; when the reaction pressure increases, it becomes possible to use a higher space velocity. The space velocity commonly used in conversion furnaces is 800-2500 h-1. (5) Carbon dissolution effect. In the conversion process, carbon deposition has a significant impact on the catalysts used in this process. Carbon will deposit on the surface of the catalyst particles, blocking the micropores within the catalyst and affecting the progress of the conversion reaction. This can lead to phenomena such as local overheating, hot spots, and thermal gradients in the furnace tubes, thereby reducing their service life. The catalyst breaks down into powder, increasing the resistance in the bed layer and forcing the conversion system to shut down, resulting in severe losses. Carbon deposition caused by hydrocarbons can be reversed by passing steam over the surface of the catalyst, thereby restoring its activity. This type of carbon deposition is easy to remove and does not cause permanent damage to the catalyst. Carbon deposition caused by the thermal decomposition of CO can damage the catalyst. When there is less water vapor, carbon deposits and accumulates in the catalyst ； When the water-to-carbon ratio is low, carbon is generated within the catalyst in a short time, and the reaction system maintains an appropriate water-to-carbon ratio ； Choosing catalysts that are stable, highly active, and resistant to carbon deposition can prevent carbon deposition. In summary, the product obtained from the conversion process is syngas, and the quality of this syngas is closely related to the methanol yield. For the synthesis of methanol, an excess of CO is present in the reaction mixture, with only trace amounts of CO2. The lower the concentration of CH4, the more favorable it is for methanol to be produced. Additionally, the higher the pressure of the syngas, the lower the energy consumption. Therefore, it is necessary to control the key operational parameters of the conversion process and further determine the appropriate process parameters. However, the determination of process parameters requires taking into account various influencing factors, such as the type of furnace, raw materials, furnace tube material, catalysts, and their impact on the key process parameters. Furthermore, the proper determination of process parameters requires considering not only the impact on the conversion process itself but also the impact on the compression and synthesis processes. Optimal process operating conditions should ultimately be reflected in the overall energy consumption and investment costs. 2.2 During normal operation of the equipment control unit, in addition to the influence of the operational parameters of the unit, whether the equipment is in good working condition also has a significant impact on the production process. For example, the higher the outlet temperature of the converter, the lower the remaining CH4 content in the conversion gas. However, excessively high temperatures can affect the service life of the converter tubes, reducing their lifespan and increasing the consumption of fuel natural gas, which in turn reduces the economic efficiency of the facility. Furthermore, during operation, it is inevitable that equipment and pipelines may experience leaks, seepages, or other such issues. These should be detected and addressed promptly to prevent the situation from worsening and affecting the normal production of the facility. 2.3 Maintenance Control: To ensure the long-term safe and stable operation of the facility and to achieve target production levels, reasonable maintenance can be scheduled based on the actual operating conditions of the facility as well as market conditions for the products. The purpose of this is to eliminate and resolve any potential problems or faults that may arise during operation, as well as to fix factors that could affect the safety and stability of the facility’s operations. If maintenance is carried out during periods when market demand for the products is low, it will not have a significant impact on the facility’s economic interests. The operating conditions of the main equipment in the conversion unit, such as the conversion furnace, pre-conversion furnace, and hydrogenation reactor, as well as the catalytic performance of the catalysts, all have an impact on the operation of the unit. During maintenance, it is necessary to properly arrange the inspection and upkeep of the critical parts of these main devices, to ensure that they are in optimal condition while the unit is in operation, thereby maximizing their efficiency and ensuring their safe and stable functioning. Furthermore, from an economic investment perspective, due to the large number of equipment units, their considerable size, and the many maintenance tasks involved, it is essential to plan the maintenance activities in a scientific and rational manner before carrying them out, in order to reduce costs. 3 Conclusion The process of converting natural gas vapor into methanol feed gas remains one of the main methods for producing methanol at present. The operating parameters of the steam conversion process, including pressure, temperature, water-to-carbon ratio, space velocity, the efficiency of the equipment, and maintenance activities, all affect the quality of methanol synthesis gas. It is crucial to properly control the appropriate operating parameters during the conversion process, optimize the key operating parameters of the production process, and improve the quality of syngas in order to increase methanol production.