Introduction to Advances in Vinyl Acetate Technology

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For the original text, please visit the news center of Instrument Information Network. Vinyl acetate is widely used in the production of polyvinyl acetate, polyvinyl alcohol, coatings, pastes, adhesives, vinylon, films, vinyl copolymer resins, acetal resins, and more. In terms of demand, North America is currently the largest consumer region in the world, accounting for 30.9% of the global total demand ； East Asia came in second, accounting for 21.3% ； Western Europe came in third, accounting for 21%. It is expected that by 2009, demand for vinyl acetate in South Asia and East Asia will grow at an average annual rate of 9.3%. In terms of applications, currently worldwide, vinyl acetate is used to produce 39% of polyvinyl alcohol, 34% of polyvinyl acetate, 9% of polyvinyl chloride, 6% of ethylene-vinyl acetate copolymers, 5% of ethylene-vinyl alcohol copolymers, and 7% for other purposes. Currently, there are two production methods for vinyl acetate: the ethylene method and the acetylene method. Among them, the ethylene method holds a dominant position due to its favorable processability and economics, accounting for over 72% of the total production capacity. The United States completed the transition from the acetylene process to the ethylene process in the 1970s. Currently, the acetylene method is still used in areas rich in natural gas and electricity resources. The acetylene method includes two types: the liquid-phase method and the gas-phase method. The liquid-phase method has low selectivity and produces many by-products, so it has been phased out. The gas-phase method uses calcium carbide acetylene and acetic acid as raw materials for desulfurization and dephosphine removal; the catalyst is zinc acetate–activated carbon (15:100), with bismuth subcarbonate used as a co-catalyst. The reaction temperature ranges from 170 to 200°C, and the pressure is at atmospheric level. The results of the gas-phase reaction are as follows: the one-pass conversion rate of acetic acid is 25%–40%, and the one-pass conversion rate of acetylene is 12%–16% ； The selectivity for vinyl acetate production is 92%–98% on an acetylene basis and 95%–98% on an acetic acid basis ； The overall yield of vinyl acetate is 97%–98% based on acetic acid, and 92%–96% based on acetylene ； The main by-products include acetaldehyde, ethylidene diacetate, etc. Due to the high cost of raw materials for the acetylene method, it has gradually been replaced by the ethylene method, but it still holds an important position in China. The catalysts used in the ethylene production process via the vapor phase ethylene method are mainly supported catalysts of Pd-An, Pd-Pt, and Pd-Cd, with silica gel and alumina serving as the carriers. The two main factors affecting the catalyst are the coordination ability of Pd and the ability of acetic acid or acetates to oxidize Pd. The process flow is as follows: Liquid acetic acid continuously enters a vaporizer where it is vaporized, and then mixed with ethylene, so that a certain ratio is maintained between acetic acid and ethylene in the gas stream. The resulting mixed gas is preheated before entering a tubular fixed-bed reactor for reaction. The gases exiting the reactor contain vinyl acetate, ethylene, acetic acid, water, carbon dioxide, and other by-products. After condensation and absorption via countercurrent washing with acetic acid, crude vinyl acetate is obtained. Unreacted ethylene is sent to a thermal carbonate absorption system to have carbon dioxide and some gases removed, after which it is returned to the reaction section for reuse. Vinegar acetic acid is distilled to remove acetic acid, water, light components, and heavy components, thereby obtaining the vinyl acetate product. In this process, the one-pass conversion rate of ethylene is 8%–10%, and the one-pass conversion rate of acetic acid is 8%–20% ； The selectivity for vinyl acetate, based on ethylene, is 90%–94% ； Catalyst life: 1–3 years ； The by-product yields, based on ethylene, are 5%–8% for CO, 0.5%–1% for acetaldehyde, and less than 0.1% for other by-products. In the vinyl acetate products produced by the ethylene method and the acetylene method, the vinyl acetate content exceeds 99.5% in both cases, although the impurities present differ slightly. The aldehyde content in vinyl acetate produced by the acetylene method is **higher than that in the ethylene method. Although the absolute amount of these aldehydes is not large, they have a significant impact on the polymerization reaction. Therefore, for vinyl acetate, whose ultimate product is vinylon, the quality obtained by the ethylene method is better than that obtained by the acetylene method. Although the ethylene process for producing vinyl acetate has seen significant technological improvements over the acetylene process, and the catalysts used in this process continue to be improved, resulting in higher selectivity and yields, the use of fixed-bed technology still presents many insurmountable drawbacks. As a result, scientists later developed a new method for producing vinyl acetate using a fluidized bed. In 1998, foreign companies announced a new process for producing vinyl acetate using the ethylene fluidized-bed gas-phase method. This process utilizes a newly designed fluidized bed reactor system and catalysts, which reduces the capital investment required for the installation. The yield of the product is 99% when measured in terms of acetic acid, and 92%–94% when measured in terms of ethylene, which is significantly higher than that achieved by fixed-bed technology. Compared with the fixed-bed process, the advantage of using a fluidized-bed process for the production of vinyl acetate is that the catalyst is continuously and uniformly mixed in the reactor, which significantly improves the uniform addition of the co-catalyst and maintains a constant concentration of the co-catalyst ； The operation of the fluidized bed allows for the continuous removal of deactivated catalyst and the replacement with new catalyst ； The fluidized bed operates in an isothermal environment, minimizing catalyst deactivation caused by overheating. To prevent catalyst failure caused by high temperatures and loss of control over reactor temperature, it is necessary to remove the heat generated during the reaction. In a vinyl acetate fluidized bed reactor, the aforementioned heat removal is achieved by introducing a certain liquid into the fluidized bed and using the latent heat of vaporization of this liquid to cool the reactor. This reduces or eliminates the large number of cooling tubes/coils required in the reactor, and also facilitates the use of smaller reactors. Furthermore, the fluidized bed process eliminates the liquid distillation tower and gas preheating exchanger required in the fixed bed process. Foreign companies have attempted to use acetic acid as the sole raw material in the production of vinyl acetate through a three-step liquid-phase reaction process: acetic acid is first cracked to produce ethylene, which is then hydrogenated to form acetaldehyde; acetaldehyde subsequently reacts with the added ethylene to yield vinyl acetate. It is said that the yield of vinyl acetate monomer produced by this method is as high as 95%. The production of vinyl acetate in our country began in the 1960s, with the Beijing Organic Chemical Factory being the first to introduce the acetylene-based fluidized bed technology from Japan. Later, Shanghai Petrochemical Complex and Sichuan Vinylon Plant introduced ethylene-based and natural gas acetylene-based plants respectively. China currently has 17 vinyl acetate production units, with an annual production capacity of nearly 800,000 tons. It is estimated that China’s demand for vinyl acetate will be 840,000 tons in 2005 and 1 million tons in 2010. China's vinyl acetate production has made many significant advances on the basis of absorbing and integrating introduced technologies. The CT series catalysts, developed in collaboration by the Shanghai Petrochemical Research Institute and the Shanghai Petrochemical Complex, have been used in the introduced plants, increasing their production capacity by 10%. Currently, the Technology Development Company of Shanghai Petrochemical Co., Ltd. is conducting single-tube tests on the CTV-III ethylene-based vinyl acetate catalyst. Furthermore, acetylene-process catalysts have been industrially applied, using bituminous coal as a raw material to replace the activated carbon carrier made from coconut shells.