Thread Content
Reducing by-products in MTBE and improving product purity Sun Peizhi, Shan Jin (Zhongyuan Petrochemical Co., Ltd., Puyang 457000, Henan) Abstract: This paper introduces the principle of the etherification reaction in MTBE/butene-1 plants, the main factors affecting this reaction, analyzes the reasons that impact the purity of MTBE products, and outlines effective measures to address these issues. Keywords: MTBE, product, purity, reasons, analysis, countermeasures. The 50,000-ton/year comprehensive utilization project of mixed C4 compounds at Zhongyuan Ethylene was put into operation in November 2004. The raw material used is a mixed C4 rich in 1,3-butadiene. This unit comprises three production units: hydrogenation, etherification, and 1-butene purification. The hydrogenation unit utilizes the C4 selective hydrogenation technology developed by the French company IFP; the etherification unit employs the three-stage cylindrical external circulation etherification technology as well as reaction distillation technology, both of which were developed and localized by Qilu Engineering Company. The butene-1 refining unit employs the ultra-fine distillation technology developed and localized by Qilu Engineering Company. The catalyst used in the etherification reaction is type D-006 etherification catalyst. 1. Overview of the unit process: The mixed C4 byproduct from the ethylene plant enters the hydrogenation unit of this facility, where 1,3-butadiene in the feed undergoes a selective hydrogenation reaction with hydrogen to produce butene-1. The tetrahydrocarbon product after hydrogenation, with a butadiene content of ≤10 PPm, proceeds to the etherification unit. The etherification unit consists of two sections: three external circulation cylindrical reactors (R-102) and reaction distillation towers (C-101A/B). In the tubular reactor, isobutylene and methanol react to produce methyl tert-butyl ether (MTBE), with an overall conversion rate of ≥90% (on a weight basis) in the three-stage reactor; the output from the tubular reactor is sent to a reactive distillation column. In the reactive distillation tower, the remaining isobutylene continues to undergo a deep conversion reaction with methanol, resulting in an isobutylene content in the C4 products of ≤0.29% (Wt). These products are sent to the methanol recovery unit, while the reaction product MTBE is continuously separated within the reactive distillation tower; ultimately, MTBE product with a purity of ≥98% (Wt) is obtained at the bottom of the tower. Since the raw material used in this unit is the product obtained from the hydrogenation of mixed C4 hydrocarbons, the average composition of isobutylene in the feed is around 22%, which differs significantly from the raw material composition in most domestic units (which is generally around 50%). Schematic diagram of the MTBE etherification process 2. Main problems: Since the C4 unit was put into operation in November 2004, the MTBE produced by our plant has been supplied as a gasoline additive to customers in the surrounding area. As MTBE used as a gasoline additive does not require extremely high purity, the total ether content in the MTBE product has remained above 98%, with the MTBE content itself being less than 98%. In recent years, the number of users who utilize MTBE to produce high-purity isobutylene as a chemical raw material for further manufacturing high-value-added products has been increasing, and these users have very strict requirements regarding the purity of MTBE products. To meet customer requirements and improve the purity of MTBE products as soon as possible, we made exploratory adjustments to the etherification unit since early 2007. 3. Analysis of the reasons for low purity of MTBE products: The inability to effectively prevent side reactions during the etherification process is the key factor contributing to the low purity of MTBE products. During the etherification reaction, a small number of side reactions occur, and the products include dimethyl ether (DME), isobutylene dimer (or oligomer), tert-butanol (TBA), methyl sec-butyl ether (MSBE), ethyl tert-butyl ether (ETBE), and others. Through data collection and practical exploration, we analyzed the reasons for the low purity of MTBE: 3.1, the alcohol-to-olefin ratio is inappropriate. At the initial stage of plant operation, as required by the design, the alcohol-to-olefin ratio was controlled at 1.02–1.1. Due to the low amount of methanol, the content of dimers resulting from the autopolymerization of isobutylene was high, leading to low product purity; therefore, we gradually adjusted the alcohol-to-olefin ratio to around 1.2. In May 2006, we replaced the methanol metering device with a mass flow meter. Once the amount of methanol could be measured accurately, we gradually adjusted the alcohol-to-olefin ratio to 1.16–1.18, allowing the overall purity of the MTBE product to be maintained at around 98%. At the current stage, the alcohol-to-olefin ratio is basically fixed. 3.2 High reaction temperature. Increasing the reaction temperature can boost the reaction rate, but the amounts of dimethyl ether (DME) and MSBE produced also increase as the reaction temperature rises. Under these conditions, methanol undergoes self-polymerization to form dimethyl ether, while part of the isobutylene also undergoes self-polymerization; this is an important factor affecting the MTBE product. Control the bed temperature of the cylindrical reactor by lowering the reactor inlet temperature, increasing the reactor circulation rate, and reducing the temperature between sections of the cylindrical reactor. 3.3 The bed temperature of the reactive distillation column is high. Unstable operating parameters of the reactive distillation column are also a significant reason for the low purity of MTBE. A high bed temperature promotes the formation of by-products such as MSBE and DME ； If the temperature at the bottom of the reactive distillation column is low, it will result in the presence of C4 in MTBE, affecting the purity of the MTBE product. By adjusting the operating pressure of the reactive distillation column to lower the temperature of its bed, the occurrence of side reactions can be effectively controlled, thereby improving the purity of the MTBE product. 3.4 Occurrence of the reverse reaction. Some of the detached catalyst particles fell into the tower. Due to the presence of these catalysts, the reverse reaction of etherification occurred in the lower part of the reactive distillation column. When operating parameters such as the temperature at the bottom of the reactive distillation column are normal, if both methanol and C4 levels in MTBE exceed the specified limits, it indicates that the reverse reaction of etherification is taking place in the lower part of the column. In addition, small amounts of catalyst particles were found in both the filter at the inlet of the intermediate reflux pump in the reactive distillation column and in the MTBE product. Based on the analysis results of the butane and methanol contents in MTBE, periodically adding diethylamine to the reactive distillation tower can deactivate the catalyst that falls into the tower, thereby effectively preventing the occurrence of reverse reactions. 3.5 The content of impurities such as C5 in the raw materials is high. 3.6 There is water present in the catalyst bed. Water present in the raw materials or methanol can result in a high content of tert-butanol in MTBE. 4. Treatment Measures 4.1. Control the quality of raw materials – A high water content in the raw materials increases the formation of the by-product tert-butanol, affecting the purity of the MTBE product ； If the raw material contains alkaline substances, it will deactivate the D-006 catalyst ； If the feedstock contains a large amount of C5, it will directly affect the purity of MTBE; meanwhile, C5 will adsorb on the surface of the catalyst, reducing its activity. During the production process, the pentane content in the raw materials is unstable, which directly affects the purity of the MTBE product. From August 7 to 9, 2007, the high C5 content in the tetra-carbon feedstock (up to 5.1%) severely affected the purity of MTBE; from August 7 to 13, the purity of MTBE remained below 98% (as low as 93%). Therefore, to ensure the proper operation of the device and avoid affecting the purity of the MTBE product, the presence of these substances in the raw materials should be minimized as much as possible. 4.2. Control the alcohol-to-olefin ratio – The value of the alcohol-to-olefin ratio is a key parameter that affects the etherification reaction. An excessive alcohol-to-olefin ratio, with excess methanol entering the MTBE product, affects the purity of the MTBE product; moreover, the excess methanol undergoes side reactions in the reactor, resulting in by-products such as dimethyl ether and tert-butanol ； When the alcohol-to-olefin ratio is too low, isobutylene polymerizes on its own to form oligomers, generating a lot of heat; high temperatures also promote side reactions such as the formation of dimethyl ether. Additionally, incomplete reaction of isobutylene affects the quality of butene-1 in the subsequent process. Through continuous experimentation, the alcohol-to-olefin ratio has now been adjusted to 1.16–1.18, which is an appropriate ratio resulting in good operational performance. 4.3. Control of key operational parameters in the etherification reactor: Parameters such as the reactor inlet temperature, bed temperature, circulation rate, and circulation temperature are crucial factors that affect the etherification reaction. After careful consideration, by adjusting the reactor inlet temperature appropriately and increasing the circulation volume in stages one and two accordingly, as well as reducing the circulation temperature, the reactor bed temperature was kept between 45 and 50°C. This reduced the reactor bed temperature compared to before; while ensuring the desired levels of MTBE, isobutylene, and methanol at the reactor outlet, it improved reaction selectivity and reduced the occurrence of side reactions. 4.4 Optimization of operating parameters in the reactive distillation column: The performance of the reactive distillation column has a direct impact on the quality of MTBE products. During the operation of a reactive distillation column, it is necessary to maintain normal values for various key parameters, such as column pressure, bed temperature, top temperature, bottom temperature, sensitivity plate temperature, top reflux rate, and intermediate reflux rate, in order to ensure that the purity of the MTBE product meets the required standards. In recent years, we have optimized many operating parameters. If the original design pressure for the tower was 0.64 MPa, it was gradually adjusted to 0.5 MPa ； The maximum bed temperature was adjusted from 70°C to 55°C℃ ； The reflux rate at the top of the tower was 12 m3/h at the beginning of operation, but it has now been adjusted to 8 m3/h. The temperature of the sensitive plate in the tower was 140°C at the start of operation, whereas it has now been reduced to 120°C. This not only reduces the occurrence of side reactions and increases the purity of MTBE (with its content now able to be maintained at at least 98%), thus meeting market demands, but it also reduces energy consumption and saves costs. 5.5 Preventing catalyst loss in the reactive distillation tower from causing MTBE to fail quality standards: If catalyst loses its position inside the reactive distillation tower, MTBE decomposes into isobutylene and methanol, which affects the purity of the MTBE product. To determine whether there is catalyst loss, one can check for the presence of catalyst at the lowest points in the bottom of the reactive distillation column (such as at the pump inlet filter), as well as by analyzing the levels of methanol and isobutylene in the MTBE product to see if they are above the specified limits. If catalyst shedding is confirmed, it can be resolved by injecting diethylamine into the tower. 4.6 Improve the drainage of the feed buffer tank in the etherification unit. In daily operations, it is necessary to control the liquid level in the raw material tank to ensure proper separation of the raw material from water, and to drain water regularly to prevent it from entering the etherification system. 4.7 Optimization of the operation of the methanol recovery tower: To prevent the water content in the recovered methanol from exceeding acceptable levels, we optimized the operation of the methanol recovery tower. Key parameters such as the top pressure, top temperature, and temperature of the sensitive plate were included in the performance assessments for the crew members. Strict monitoring ensured that these parameters remained stable, thereby keeping the water content in the recovered methanol below 0.5%. 5. Conclusion There are many factors that affect the purity of MTBE products. Through analysis of the problems, we identified one by one the factors affecting the purity of MTBE products, and took a series of targeted effective measures; as a result, the issue of low purity in MTBE products was resolved, with the purity of MTBE in the products remaining stable at over 98%. References: ⑴ Zhang Xuzhi et al. Engineering of C4 and C5 Olefins. Beijing Chemical Industry Press, April 1998