This column is based on “Polyoxymethylene from Methanol and Acetic Anhydride – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/polyoxymethylene-production-cost.
Polyoxymethylene (also known as POM and polyacetal resin) is a thermoplastic polymer fiber. Owing to its crystalline structure, consisting of organized stacking chains, POM has high strength, stability, stiffness and hardness, as well as low friction. These characteristics, combined with chemical resistance and excellent dimensional stability, make the polymer a common substitute for metal. It is widely used in the manufacturing of high-performance engineering materials and electronics.
The process examined here (Figure 1) is a conventional silver-catalyzed process with a methanol recycle, combined with a polymerization process. It consists of 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 gas generated is also burned, with water as the only product.
Formaldehyde purification. The reactor effluent is fed to an 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 an ion exchanger and then to a concentration column. The concentrated formaldehyde is sent to an 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 the 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.
Polyoxymethylene production can occur by the homo- or co-polymerization of formaldehyde, which can come from different raw materials. In this context, typical POM production routes are based on formaldehyde production (via catalytic oxidation of methanol). Different pathways for polyoxymethylene production are presented in Figure 2.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce polyoxymethylene was about $1,870 per ton of polyoxymethylene in the first quarter of 2016. The analysis was based on a plant constructed in the U.S. with capacity to produce 85,000 metric tons per year of POM.
Edited by Scott Jenkins
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