Acetic anhydride Processes

Figure 14.8 Simplified fiowsheet for the acetic anhydride process. The composite curves show both distillation columns to be across the pinch. (From Smith and Linnhoff, Trans. IChemE, ChERD, 66 195, 1988 reproduxxd by permission of the Institution of Chemical Engineers.)...

The Eastman acetic anhydride [108-24-7] process provides an extension of carbonylation chemistry to carboxyUc acid esters. The process is based on technology developed independendy in the 1970s by Eastman and Halcon SD. The Eastman acetic anhydride process involves carbonylation of methyl acetate [79-20-9] produced from coal-derived methanol and acetic acid [64-19-7]. 

The cmde product from the gasifier contains CO2 and H2S, which must be removed before the gas can be used to produce chemicals. The Rectisol process is used to remove these contaminants from the gas. This is accompHshed by scmbbing the product with cold methanol which dissolves the CO2 and H2S and lets the H2 and CO pass through the scmbber. The H2S is sent to a Claus sulfur plant where over 99.7% of the sulfur in the coal feed is recovered in the form of elemental sulfur. A portion of the clean H2 and CO are separated in a cryogenic distillation process. The main product from the cryogenic distillation is a purified CO stream for use in the acetic anhydride process. The remaining CO and hydrogen are used in the methanol plant. 

The acetic anhydride process employs a homogeneous rhodium catalyst system for reaction of carbon monoxide with methyl acetate (36). The plant has capacity to coproduce approximately 545,000 t/yr of acetic anhydride, and 150,000 t/yr of acetic acid. One of the many challenges faced in operation of this plant is recovery of the expensive rhodium metal catalyst. Without a high recovery of the catalyst metal, the process would be uneconomical to operate. 

A related but distinct rhodium-catalyzed methyl acetate carbonylation to acetic anhydride (134) was commercialized by Eastman in 1983. Anhydrous conditions necessary to the Eastman acetic anhydride process require important modifications (24) to the process, including introduction of hydrogen to maintain the active [Rhl2(CO)2] catalyst and addition of lithium cation to activate the alkyl methyl group of methyl acetate toward nucleophilic attack by iodide. 

Some other processes are known for sulfoxidation but have no technical importance. The acetic anhydride process has attracted some interest because it does not need exposure to light and enables conversion rates up to 15% of paraffin feedstock. Once started by peroxide or UV light initiation, it propagates without further radical-forming initiation steps. The addition of some 2.5% acetic anhydride to the reacting alkane is crucial to form a mixed anhydride of par-... 

Other companies (e.g. Hoechst, now Celanese) have developed a slightly different process in which the water content is low in order to save CO feedstock [1], In the absence of water it turned out that the catalyst precipitates. Also, the regeneration of ihodium(III) is much slower. The formation of the trivalent rhodium species is also slower because the HI content is much lower when the water concentration is low. The water content is kept low by adding part of the methanol in the form of methyl acetate. Indeed, the shift reaction is now suppressed. Stabilisation of the rhodium species and lowering of the HI content can be achieved by the addition of iodide salts (Li, ammonium, phosphonium, etc). Later, we will see that this is also important in the acetic anhydride process. High reaction rates and low catalyst usage can be achieved at low reactor water concentration by the introduction of tertiary phosphine oxide additives [1]. 

Acidity (as % AcOH), when RDX is Type A(made by nitric acid process) Acidity (as % HN03), when RDX is Type B (made by acetic anhydride process)... 

The main difference between the Monsanto acetic acid process and Tennessee Eastman acetic anhydride process is the presence of water in the acetic acid process, which produces HI and acetic acid. In both reactions, a small amount of H2 is added to the CO stream to act as a reducing agent to keep the catalyst in the more active Rh oxidation state. An engineering problem with both processes is the highly corrosive nature of the Hl/iodide mixture, requiring the use of special chemically resistant alloys, pumps, and seals. 

USA. The requirements of RDX used by the Armed Forces are set forth in US Specification MIL-R-398C(1) dated 30 Occcber 1963- This specification covers two types of RDX and 8 classes based on uses Sc advisory granulations Type A - Made by the nitric acid process Type B - made by the acetic anhydride process Properties Type A Type B... 

The heavy end products of acetic anhydride processes are separated from catalyst streams and distillation residues. The high affinity of the heavy ends for rhodium components affords specified procedures to separate rhodium and recycle it to the reaction stage of the process. Different methods for rhodium recovery were tested during the process development, including extraction methods [65], precipitation [66], complexing, and electrochemical methods [67]. 

Eastman Chemical Company, together with Halcon, developed a commercial acetic anhydride process to an industrial scale [41b, 47]. This process starts with coal to make a hydrogen-rich synthesis gas, which is purified (Figure 4). A portion of the syn gas is separated to produce methanol from 2 1 H2/CO. Part of the methanol is used to scrub H2S from the coal-gasification step. The remainder of the methanol is combined with acetic acid to make methyl acetate. The methyl acetate is carbonylated to give acetic anhydride. The acetic anhydride is used to produce cellulose acetate in another process, and the resulting acetic acid is recycled to the esterification section. The acetic anhydride step of the pro-... 

Figure 5. The Eastman acetic anhydride process (simplified).

The major catalytic uses involve the complex cis-[Rh(CO)2l2] in the Monsanto acetic acid and Termessee-Eastman acetic anhydride processes, discussed in detail in Section 26.4. Application of iodine as a stabilizer includes its incorporation into nylon used in carpet and t5re manufacture. Iodized animal feed supplements are responsible for reduced instances of goitre (enlarged thyroid gland) which are otherwise prevalent in regions where the iodine content... 

The Tennessee-Eastman acetic anhydride process converts methyl acetate to acetic anhydride (equation 26.16) and has been in commercial use since 1983. 

Fig. 26.7 Catalytic cycle for the Tennessee-Eastman acetic anhydride process. 3. In Figure 26.7, which step is an oxidative addition ...

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