Anhydride production technology

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Anhydride production technology: Maleic anhydride (MA), also known as succinic anhydride or anhydrous malic anhydride, is an important organic chemical raw material and fine chemical product. It is currently the third most widely used anhydride in the world, after phthalic anhydride and acetic anhydride. It is primarily used in the production of unsaturated polyester resins and alkyd resins, and finds applications in fields such as pesticides, pharmaceuticals, coatings, inks, lubricant additives, paper chemicals, textile treatment agents, food additives, and surfactants. Furthermore, using maleic anhydride as a raw material, a range of widely used fine chemical products such as 1,4-butanediol (BOD), γ-butyrolactone (GBL), tetrahydrofuran (THF), maleic acid, fumaric acid, and anhydrides can also be produced, offering great prospects for further development and utilization. Currently, the industrial production processes for maleic anhydride can be classified into four categories based on the raw materials used: the benzene oxidation method, the n-butane oxidation method, the C4 olefins method, and the method utilizing by-products of phthalic anhydride production. Among these, the benzene oxidation method is the most widely used. However, due to limited supplies of benzene, technologies for producing maleic anhydride using C4 olefins and n-butane as raw materials have emerged. Countries with abundant natural gas and associated gas from oil fields, which possess large amounts of n-butane, have seen rapid development in technologies for producing maleic anhydride via n-butane oxidation. This method has now become dominant in maleic anhydride production, accounting for approximately 80% of the world’s total production capacity. Benzene oxidation method: Benzene vapor and air (or oxygen) undergo a gas-phase oxidation reaction on a catalyst with active components such as V2O5-MnO3 and α-Al2O3 as the carrier to produce maleic anhydride. The benzene oxidation method is a traditional approach for producing maleic anhydride; it features mature and reliable process technologies. The main methods include the SD method from the United States, the Alusuisle/UCB method, and the catalyst chemistry method from Japan. Among these, the SD method is the most widely used, while the Alusuisle/UCB method requires the least amount of benzene as raw material, making it a more advanced production method. Olefin method: This method uses effective components in mixed C4 fractions such as n-butylene and butadiene as raw materials, along with air (or oxygen), and through a gas-phase oxidation reaction catalyzed by V2O5-P2O5 catalysts, maleic anhydride is produced. In this process, n-butylene is first dehydrogenated to form butadiene, which is then oxidized to yield maleic anhydride. During the reaction process, in addition to the main product, carbon monoxide, carbon dioxide, and water are produced as by-products, along with small amounts of acetaldehyde, acetic acid, acrolein, and furan. German companies BASF and Bayer have developed a fixed-bed oxidation process using mixed C4 fractions as raw material. The Japanese company Mitsubishi Kasei has developed a fluidized-bed oxidation process for the production of maleic anhydride using butadiene-containing C4 fractions as raw material. Since dehydrogenation is an endothermic reaction and produces many by-products, the prospects for oxidizing mixed C4 olefins to maleic anhydride are not very promising. Phthalic anhydride by-product method: When producing phthalic anhydride from o-xylene, a certain amount of maleic anhydride can be obtained as a by-product, with a yield of about 5% of that of phthalic anhydride. In the production of phthalic anhydride, the reaction off-gases are discharged into the atmosphere after organic substances are removed in a scrubber tower. The scrubbing liquid contains maleic anhydride along with small amounts of impurities such as benzoic acid and phthalic acid; maleic anhydride is obtained by concentrating and purifying this liquid and then heating it for dehydration. Butane oxidation method: The butane oxidation process uses butane as a raw material, and under the action of a V2O5-P2O5 catalyst, a gas-phase oxidation reaction takes place to produce maleic anhydride. Since its industrialization in 1974 by companies such as Monsanto in the United States, this process has seen rapid development due to factors such as low raw material costs, minimal environmental pollution, and abundant butane resources in regions like Europe and the United States; it represents the trend in the development of maleic anhydride production processes. Currently, the relatively typical and advanced process technologies for producing maleic anhydride from n-butane abroad include the n-butane fluidized-bed solvent absorption process developed jointly by the U.S. company Lummus and the Italian company Alusuisle, namely the ALMA process ； The butane fluidized bed water absorption process developed by the British company BP, namely the BP process ； The n-butane fixed-bed water absorption process developed by the US company SD, namely the SD process ； The butane fixed-bed solvent absorption process used by the Italian company SISAS Chemicals, namely the Conser-pan tonochim process, involves a fixed-bed configuration. In this process, since the selectivity for butane oxidation and the reaction rate are lower compared to the benzene-based method, the butane concentration in the butane-air mixture can reach 1.6%-1.8% (on a molar basis). The yield of maleic anhydride is approximately 50% based on the amount of butane used; therefore, larger reactors and compressors are required for plants of the same scale ； The use of a fluidized-bed reactor allows the concentration of n-butane in air to be increased to 3%-4% (mole fraction). Fluidized bed reactors offer good heat transfer performance and lower capital costs, but the catalysts used in them wear out quickly. For large succinic anhydride production facilities (with a capacity of 20,000 tons per year or more), if inexpensive and reliably available n-butane raw material can be obtained, fluidized bed reactors are a suitable choice. Compared with the traditional benzene method, the n-butane oxidation method has advantages such as lower raw material costs and less pollution. The n-butane method consumes 1.1–1.2 tons of n-butane per ton of maleic anhydride produced, whereas the benzene method uses 1.1–1.3 tons of benzene per ton of maleic anhydride produced. Furthermore, the theoretical yield of maleic anhydride produced by the n-butane method is 1:1.69, while that of the benzene method is 1:1.256; thus, the theoretical yield of maleic anhydride using n-butane is much higher than that using the benzene method. With the continuous advancement of technology, the consumption of n-butane as a raw material will be much lower compared to the benzene-based method. Not only is less n-butane required, but it is also less toxic than benzene. Additionally, the production of maleic anhydride using the n-butane method causes less environmental pollution. As environmental protection demands increase globally, the n-butane method offers greater advantages in terms of meeting environmental requirements and future prospects compared to the benzene-based method. For this reason, currently about 80% of the global maleic anhydride production capacity relies on the n-butane route, and this trend is continuing to increase. Advances in production technology: Currently, the production technology for maleic anhydride abroad is shifting from the benzene-based method to the n-butane oxidation method; no new production routes have emerged. The technological advancements mainly lie in process improvements for existing facilities and enhancements in catalyst performance.