Causes and preventive measures of wax formation during methanol synthesis

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Summary of causes of wax formation during methanol synthesis and preventive measures: The wax formation phenomenon that occurs during the synthesis of methanol was studied, its causes were analyzed, and corresponding preventive measures were discussed. keywords: Causes, Preventive Measures and Treatment of Wax Produced in Methanol Synthesis Process HUANG jin-qian, LIU jin-hui, Qiu Dong (Research Institute of Nanjing Chemical Industry Group, Jiangsu Nanjing 210048, China) Abstract: The phenomenon of wax produced in methanol synthesis process is studied and the causes are analyzed. The corresponding measures and treatments are discussed. Key words: wax; methanol synthesis; causes; preventive measure 1 Introduction Methanol is one of the important basic organic chemical raw materials. It is widely used as a solvent and in the production of formaldehyde, acetic acid, methyl tert-butyl ether, methylamine, esters, medicines, pesticides and other products, and has a huge potential market for fuel applications. At present, in the process of using natural gas, petroleum gas or coal gas to synthesize crude methanol, whether it is a mono-alcohol plant or a combined alcohol plant, varying degrees of high-carbon chain hydrocarbons and paraffin hydrocarbons are found to be generated. Especially the joint alcohol plant is restricted by the conditions of joint production with synthetic ammonia.: The scale is relatively small, some equipment is crude, the raw material gas purification conditions are poor, and high-pressure operation is required, which has long caused serious problems of wax formation in production. Once there is paraffin in the crude methanol product, it cannot be removed in the subsequent process. Although paraffin wax has little impact on the quality of refined methanol products, it has a greater impact on methanol synthesis, distillation and other processes. It is deposited on the outlet end of the heat exchanger in the lower part of the synthesis tower internals, on the water cooler tube wall and connecting pipes, and in the methanol separator. When the situation is serious, it will cause blockage of the water cooler, methanol separator and other equipment, pipes, and valves. Even the outlet pipe of the methanol synthesis tower with higher temperature will be blocked by paraffin, which will directly affect the normal production, and sometimes the production may be forced to stop for wax removal. Therefore, it is very important to analyze the causes of paraffin production during methanol production and avoid or reduce wax formation. 2 Properties of paraffin wax Paraffin wax is a mixture of higher alkanes, that is, aliphatic hydrocarbons, with a molecular formula of CnH2n+2, a specific gravity of 0.786~0.800g/ml, a melting point above 37°C, and a boiling point above 343°C. 3. Reasons for the formation of paraffin in methanol synthesis. During the methanol synthesis process, reactions under the following conditions may contribute to the production of paraffin. At lower reaction temperatures and when the copper-based catalyst bed contains very small amounts of Group VIII elements such as iron, cobalt, and nickel, hydrocarbons may be generated. ； When a small amount of thorium is mixed in the catalyst bed, a considerable amount of hydrocarbons may be generated.: nCO＋2nH2 (CH2)n＋nH2O＋Q When there is water vapor in the feed gas, carbon monoxide will also react as follows on the catalyst bed containing iron.: 3CO + H2O CH2 + 2CO2 Combining the above two reactions, in the production process of methanol, the reasons for the production of paraffin are relatively complex, which are not only related to the structure and materials of the catalyst, feed gas, and methanol tower, but also to the process conditions and operating methods of the synthesis reaction. 3.1 Causes of Catalysts 3.1.1 Causes in the Catalyst Production Process Some of the equipment and pipes used in the catalyst production process are made of carbon steel. When the catalyst comes into contact with them, a small amount of iron and nickel elements may be brought in. During the precipitation process, iron may also be brought in by the steam used. ； In addition, when washing the precipitate, due to insufficient washing, sodium salt is likely to be brought into the catalyst. Iron, nickel, and sodium alkaline metals are catalysts for generating wax. Similarly, when new catalysts are in contact with iron containers during storage, transportation and filling, the rust on the containers will adhere to the surface of the catalyst or be mixed into the catalyst. If it is not cleaned when the catalyst is filled in the synthesis tower, it will also enter the synthesis tower. In addition, when the catalyst contains a small amount of SiO2 or other acidic oxides, the formation of paraffin will also be promoted. 3.1.2 Reasons during the use of catalysts In the process of using copper-based catalysts to produce methanol, as the use time increases, especially in the middle and later stages of catalyst use, wax will accumulate in the crude methanol products, equipment, and pipelines. The reason is that due to the limitation of catalyst selectivity, methanol production will inevitably be accompanied by the generation of a small amount of formic acid and other organic acids, and the equipment used to produce methanol is almost all carbon steel, which causes corrosion of equipment and pipelines. There is a certain concentration of CO in the feed gas. The corroded iron and nickel, as well as the iron and nickel brought in by the catalyst itself, and the CO in the feed gas, may generate carbonyl iron Fe (CO) 5 and carbonyl nickel Ni (CO) 4 under appropriate pressure and temperature. The products are deposited on the catalyst surface through volatilization, decomposition, and air flow entrainment, resulting in a decrease in its activity. This gives the catalyst the conditions to generate aliphatic hydrocarbons. Production practice has proved that the longer the continuous operation time of the catalyst, the more carbonyl iron and carbonyl nickel accumulate on the catalyst surface, the amount of aliphatic hydrocarbons generated also increases, and the wax formation phenomenon also increases. George W. Roberts believes that iron carbonyl and nickel carbonyl have a much more serious impact on methanol catalysts than sulfur, and even a content of one part per million is harmful to methanol catalysts. G∙Natta confirmed in experiments that iron reduced by carbonyl iron is the most active catalyst for the reaction of CO and H2 to produce paraffin. 3.2 Reasons for methanol tower internals The structure of the internals of the methanol synthesis tower is crucial to the synthesis of methanol. If the design of the internals does not match the actual production conditions, it will cause a large temperature difference in the catalyst bed and difficulty in controlling the temperature, which will lead to the production of paraffin. In addition, if the design is improper, the lubricating oil will easily form wax when it enters the copper-based catalyst during the production process. The lubricating oil is an organic alkane, and when it encounters the copper-based catalyst, it can generate high-carbon chain aliphatic hydrocarbon wax. 3.3 Reasons for operating conditions 3.3.1 Reasons for synthesis reaction temperature The copper-based catalysts currently used by various factories are mainly composed of copper, zinc, aluminum (or add a small amount of chromium, vanadium, etc.). In the methanol synthesis reaction, at moderate temperatures and pressures, they will have very good activity and have extremely high selectivity for the synthesis of methanol. Under certain temperatures and pressures, Al and Na in copper-based catalysts will also promote the reaction of CO and H2 to generate paraffin. Production practice proves: During the methanol production process, the reaction temperature of the synthesis tower is controlled too low, causing the gas entering the tower to enter the catalyst layer at a temperature lower than the normal activity temperature of the catalyst. The generated crude methanol will contain a large amount of paraffin. This is mainly due to the fact that during the start-up and feeding stage, the reaction temperature of the synthesis tower has not reached the normal operating value and during the shutdown process, the reaction temperature of the synthesis tower drops, and the raw material gas is not completely replaced. These two stages produce the most paraffin. Other data show that the bed temperature of the synthesis tower is between 185 and 215°C, and the wax formation phenomenon is serious. The temperature fluctuates greatly during operation, and wax is easy to form. In the methanol synthesis reaction, the side reaction has a higher activation energy and is more sensitive to the reaction temperature. Increasing the temperature is more conducive to the progress of the side reaction. For example, for copper-based catalysts, when the reaction temperature exceeds 300°C, methanation reaction easily occurs. The increase in methane content indirectly accelerates the production of paraffin. 3.3.2 Reasons for synthesis reaction pressure When synthesizing methanol, the higher the pressure, the synthesis reaction will move in the direction of generating higher alkanes, and the greater the probability of wax formation. Because the hydrocarbon generation reaction is a volume reduction reaction, the volume of the reactants is much larger than the volume of the product. Increasing the pressure is conducive to the progress of side reactions. When these side reactions occur, the volume shrinkage before and after the reaction is more obvious than that of the methanol synthesis reaction. The longer the carbon chain of the hydrocarbons generated, the greater the change in reaction volume. Therefore, the length of the carbon chain of hydrocarbons generated is related to the reaction pressure. The higher the pressure, the longer the carbon chain of hydrocarbons generated. Details are as shown in Table 1: Table 1 Wax formation components (%) when synthesizing methanol under different pressures Synthesis pressure MPa ~ C15 C15 ~ C20 C20 ~ C25 C25 ~ C30 C30 ~ C35 C35 ~ C40 C40 ~ 5 1.75 16.84 50.04 29.25 2.12 15 35.60 41.87 19.59 2.94 30 0.88 25.22 44.54 27.61 1.75 3.3.3 Cause of space velocity During the methanol synthesis process, the control of the contact time between the synthesis gas and the catalyst is achieved by adjusting the space velocity of the synthesis cycle gas. High space velocity, short contact time between synthesis raw material gas and catalyst, high product purity and few by-products ； The space velocity is low, the contact time between the synthesis tower feed gas and the catalyst is long, and the conversion rate is high, but the purity decreases and by-products such as paraffin increase. 3.3.4 Causes of Raw Material Gas The CO content in the raw gas entering the tower is too high, which increases side reactions, increases the chance of generating aliphatic hydrocarbons, and increases wax formation. The inlet temperature of the feed gas to the synthesis tower will also affect the formation of paraffin. If the temperature entering the tower is controlled too high, the water vapor in the synthesis feed gas will react with CO under the action of the iron-containing catalyst to produce paraffin, and ethanol will also be generated. 3.4 Reasons for starting and stopping Frequent starting and stopping is also a cause of wax accumulation. During the start-up and feeding stage, the reaction temperature of the synthesis tower has not yet reached the normal operating value, and paraffin is easily produced when the reaction gas passes through the low-temperature reaction zone of the synthesis tower. ； During the shutdown stage, the system replacement is not complete, and the unreplaced CO and H2 easily produce paraffin when passing through the low-temperature reaction zone of the synthesis tower. 4 Impact of wax formation on methanol production 4.1 Wax formation will affect the yield of methanol. Since the melting point of paraffin in the by-product is about 37°C, when the temperature of the synthetic circulating gas leaving the methanol water cooler drops to about 40°C, the paraffin in it will be condensed to form a viscous liquid, which will adhere to the pipe walls of the pipe, methanol water cooler, and methanol separator respectively. This increases the thickness of the pipe wall and affects the cooling effect of the methanol water cooler. The gas leaving the tower is in the methanol water cooler. The alcohol water cooler is not sufficiently cooled, causing the temperature of the gas exiting the methanol water cooler to rise. As a result, the methanol in the gas phase cannot be completely condensed, so that the synthetic circulating gas after separation contains methanol. When the synthetic circulating gas returns to the synthesis tower, the inlet gas contains methanol, which affects the conversion rate of methanol, causing the single-pass synthesis rate of methanol to decrease, affecting methanol output and increasing production costs. 4.2 Wax formation causes increased resistance and increases power consumption. The production of paraffin will inevitably increase the resistance of the pipeline, thereby increasing power consumption. When wax formation is severe, it will cause the pipes of the methanol water cooler to be blocked, and even the outlet pipes of the methanol separator and synthesis tower to be blocked, causing shutdowns and production shutdowns, bringing instability to production and causing great waste and losses. 5 Prevention and Treatment of Waxing Phenomenon 5.1 Improving the Quality of the Catalyst Improving the quality of the catalyst is not only to improve the yield and quality of methanol, but is also very important for the waxing phenomenon that occurs during the synthesis of methanol. During the process of methanol synthesis, there is competition between the methanol synthesis reaction and many side reactions on the catalyst surface. If the catalyst has good selectivity for the methanol synthesis reaction, the occurrence of side reactions will be relatively suppressed. Reducing the content of harmful impurities in the catalyst, especially iron, nickel, sodium, silicon and other elements, can reduce the production of paraffin during production. 5.2 The internal parts of the methanol tower must be designed rationally. During the methanol synthesis reaction, a large amount of heat is released and a lot of reaction heat needs to be removed. ； Catalyst has a narrow effective activity temperature range ； The fluctuation of effective gas components such as CO has a great influence on the bed temperature, so the design requirements for the internal parts of the methanol synthesis tower are relatively high. Choosing a methanol synthesis tower internal component that conforms to the production process conditions and whose process parameters are easy to control is crucial to reducing the temperature fluctuation, thereby reducing the occurrence of wax formation. 5.3 The raw gas must be purified. The raw gas must be purified. The normal temperature desulfurization tank must be heated up and a carbon-based iron-nickel adsorbent must be installed. By increasing the amount of pre-washing methanol, the removal of carbon-based iron and nickel can be enhanced. The CO content in the raw gas cannot be too high, and the CO2 content can be appropriately increased, because the presence of CO2 can reduce the formation of by-products such as paraffin. 5.4 Select reasonable operating conditions. During the synthesis of methanol, try to avoid the synthesis reaction temperature area where paraffin is easily produced. In the early stage, low-temperature operations are inevitable and carbonyl iron is easily produced, so low-temperature operations should be avoided as much as possible in later production. Since large temperature fluctuations and over-temperature can also lead to the production of paraffin, temperature fluctuations should be strictly controlled during the production process. According to some data, when the high temperature exceeds 260°C and the low temperature is lower than 210°C, paraffin is easily generated, so this temperature zone should be avoided as much as possible in actual production. During normal operation, selecting appropriate synthetic pressure and airspeed can reduce the occurrence of wax formation. 5.5 Reduce the number of starts and stops. Try to reduce the number of starts and stops. Due to large temperature fluctuations, the chance of wax formation will increase. After each shutdown, high-purity nitrogen must be used to replace the reactants in the system and maintain positive pressure to prevent air from entering. 5.6 Regular cleaning attaches great importance to system maintenance. Every large and medium system maintenance should thoroughly clean the methanol tower system to remove all impurities such as wax and metal iron in the system. Focus on cleaning the water cooler, separator, oil filter and collection tank. Cleaning method: It is best to use hot water of 80-100℃, steam can also be used. 5.7 Treatment of wax formation Normally, in the pipelines and equipment of the methanol synthesis system, the wax formation phenomenon in the methanol water cooler is more serious and has a greater impact on production. Therefore, the method of increasing the operating temperature of the methanol water cooler can be used to melt the paraffin in the methanol water cooler and use the synthetic circulating gas to bring it into the methanol separator. The "tropical" operation method. In this way, normal production can be temporarily maintained for a long time, and the wax removal work on the equipment pipelines can be carried out when the equipment is shut down for maintenance. The specific operation method is as follows: During normal production and operation, when the operating temperature of the methanol water cooler rises significantly, indicating that the waxing phenomenon in the pipeline is serious, the cooling water inlet and outlet valves of the methanol water cooler can be closed to artificially increase the operating temperature of the methanol water cooler (note that the inlet and outlet valves cannot be closed for too long, and the specific time is subject to the inlet and outlet temperatures of the synthetic cycle compressor not exceeding the normal control temperature). The paraffin in the methanol water cooler is melted and brought into the methanol separator through the synthetic circulating gas. Since the methanol separator is large in size, it will not affect production when the wax formation is not serious. After the "tropical" wax removal, the operating temperature of the methanol water cooler will drop significantly. The paraffin brought into the methanol separator will be cleaned when there is an opportunity to shut down the machine for maintenance. 6 Conclusion During the methanol production process, the phenomenon of wax formation is common, which also has certain adverse effects on methanol production. The causes of paraffin production are relatively complex, including causes during catalyst production and use, synthesis tower internals, and operating conditions (temperature, pressure, air velocity, feed gas, startup and shutdown, etc.). It is currently difficult to make an accurate judgment as to which factors are the main factors. Currently, effective measures should be taken as much as possible to prevent and reduce the formation of paraffin. It is key to reduce the content of harmful impurities in the catalyst, avoid fluctuations in operating conditions during the production process, reduce the number of shutdowns, and achieve stable production. References: Chen Ziqi. 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Discussion on the phenomenon of wax formation during methanol synthesis. Nitrogen Fertilizer Information, 1994(1): 18-20. This post was last edited by snowdfr on 2009-4-13 18:44 ]