Monsanto Low-Pressure Process

In the Monsanto process methanol and CO react continuously in the liquid phase at 180 °C and 30 bar in the presence of a Rhl2 catalyst. Since the intermediates are anionic complexes and the reaction rate is enhanced by protic solvents, the reaction is carried out in polar solvents (e.g., acetic add/water medium). The main byproducts are CO2 and H2 formed with water [(6.15.4), water-gas-shift-reaction]. 

Example 16.6.2 shows the detailed catalytic cycle of the Rhl2-catalyzed carbonylation. 

Example 6.15.2 Catalytic cycle and mechanism for the methanol carbonylation. 

The rate-determining step is the oxidative addition of methyl iodide to the rhodium complex. The reaction rate is, therefore, independent of the two reactants methanol and carbon monoxide (6.15.7)  

Carbon monoxide and methanol from the scrubber unit are fed to the reactor containing the catalyst and the solvent. The liquid product stream is depressurized in a flash vessel and divided in a gas and liquid phase. In this step the rhodium catalyst is separated from the crude acetic acid and is recycled to the reactor. It is critical for the process economics to avoid any rhodium metal loss in this step that could occur by precipitation and vaporization. The gas stream is scrubbed with methanol to recover the toxic and volatile methyl iodide and the off-gas of the scrubber system is flared. The liquid phase is sent to the light ends column (separation of light compounds, e.g., methyl iodide and methyl acetate). The bottom of this column (catalyst, water, and acetic acid) is recycled to the reactor. Wet acetic acid is sent to the drying column and dried acetic acid is obtained as bottom product. The overhead (mixture of acetic acid and water) is recycled back to the reactor. In the product column heavy by-products (major liquid by-product propionic acid) are removed from the dried acetic acid. The overhead is sent to the finishing column, where ultrapure acetic acid is obtained as a side-stream. 

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Acetic Acid. Carbonylation of methanol is the most important reaction in the production of acetic acid.189-192 BASF developed a process applying C0I2 in the liquid phase under extreme reaction conditions (250°C, 650 atm).122 193 The Monsanto low-pressure process, in contrast, uses a more active catalyst combining a rhodium compound, a phosphine, and an iodine compound (in the form of HI, Mel, or T2).122 194—196 Methanol diluted with water to suppress the formation of methyl acetate is reacted under mild conditions (150-200°C, 33-65 atm) to produce acetic acid with 99% selectivity at 100% conversion. 

As seen from the above, conventional uses of methanol cover a wide range of products which in turn find application in a very broad cross-section of industrial and consumer goods. New end uses have continued to develop and spur the growth of methanol production. One such development is the Monsanto low pressure process that carbonylates methanol to acetic acid (6). Essentially all new acetic acid capacity now being installed is based on Monsanto technology. By 1981, eleven plants converting methanol to acetic acid are scheduled to be on stream. At capacity they will consume over 300 million gallons of methanol. 

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