Polyoxymethylene Production from Methanol and Acetic Anhydride

The process under analysis comprises three major sections: (1) Partial Oxidation; (2) Formaldehyde Purification; and (3) Polymerization.

* Partial oxidation. Fresh methanol and air are fed to an evaporator, and the resulting vapor is mixed with steam and sent to a reactor, where the methanol is partially oxidized in the presence of a catalyst bed of silver crystals, yielding a formaldehyde solution and hydrogen. The hydrogen generated is also burned by the oxygen, forming water.

* Formaldehyde Purification. The reactor effluent is fed to the absorption column. Methanol and formaldehyde are absorbed and sent to the methanol stripper. This stripper separates unreacted methanol, which is recycled to the reactor, from formaldehyde, which is routed to the ion exchanger and then to a concentration column. The concentrated formaldehyde is sent to the extraction column, forming hemiformal by contact with cyclohexanol. A cyclohexanol-water mixture is routed to a decanter to recover cyclohexanol. The hemiformal is sent to the pyrolysis area, where it is cracked to split off the formaldehyde monomer, which is directed to the polymerization reactor.

* Polymerization. Formaldehyde is fed to a reactor and polymerized into POM, which precipitates as it is formed. The solid particles are separated from the solvent in a vacuum filter, are dried to remove volatile materials, and routed to the acetylation reactor, where hydroxyl groups on the polymer chain are converted to acetate groups. The product from acetylation reactor is routed to a hot nitrogen dryer to remove residual acetic anhydride, which is recycled to anhydride recovery. The acetic anhydride stream is recovered, refined, and recycled to the acetylation reactor. The dried product is then sent to an extruder, to blending silos, and it is then packed and stored.