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This column is based on “Polyacrylonitrile Production – Cost Analysis,” a report published by Intratec. It can be found at: www.intratec.us/analysis/polyacrylonitrile-production-cost.
Polyacrylonitrile (PAN) is a polymer resin mainly composed of acrylonitrile. This important acrylic resin is used in the production of a wide range of products, from ultra-filtration membranes to high-quality carbon fibers.
The process discussed here comprises two main phases: first, acrylonitrile is polymerized with methyl acrylate comonomer and additives in an aqueous solution of demineralized water; then the copolymer is passed through transformation steps to become a fiber. Figure 1 presents a simplified flow diagram.
Polymerization. Initially, a mixture of acrylonitrile, methyl acrylate and itaconic acid is fed continuously to a continuous stirred-tank reactor (CSTR) where the polymerization takes place. The reaction is initiated by feeding aqueous solutions of potassium persulfate (oxidizer), sulfur dioxide (reducing agent), ferrous iron (promoter), and sodium bicarbonate (buffering agent). The heat of polymerization is removed by water circulated in a reactor jacket.
Next, an iron-chelating agent is added to the reactor overflow, where it acts as a chain stopper for the polymerization. The reactor overflow, an aqueous slurry of polymer particles, is fed to a monomer-separation column, in which unreacted monomer is steam-stripped from the slurry. The monomers recovered are returned to the reactor.
Pelletizing. The polymer is pelletized, dried, ground and then converted to fiber through subsequent steps involving physicochemical transformations, as briefly described below.
Wet spinning. The polymer is dissolved in dimethylformamide (DMF), forming a dope. The dope is pumped through a spinneret into a precipitating bath, for the coagulation of the solution thread line into a highly swollen gel yarn.
Washing, stretching and finish. The fiber is washed countercurrently by hot water, for the removal of the solvent. The fiber is then stretched while passing over rollers with adjustable speed. The stretching imparts fiber strength. Subsequently, silicone is applied as an aqueous emulsion to the fiber, to act as lubricant and antistatic agent.
Drying, crimping and relaxation. The fiber passes through drum dryers for water removal. Tows are crimped by means of a stuffer box crimper. Crimping provides interfilament cohesion across the tow and facilitates subsequent handling. Then the fiber is fed to an autoclave for a relaxation process that reduces the tendency for fibrillation and increases the fiber dimensional stability.
PAN production consists of the copolymerization of acrylonitrile — the main component — and other comonomers, in such a way that different manufacturing routes are related to different sources of such raw materials. In this context, typical PAN production routes are based on acrylonitrile manufacturing. Different pathways for polyacrylonitrile (using methyl acrylate comonomer) production are presented in Figure 2.
The total operating cost (raw materials, utilities, fixed costs and depreciation costs) estimated to produce polyacrylonitrile was about $4,600 per metric ton in the first quarter of 2015. The analysis was based on a plant constructed in the U.S. with capacity to produce 9,000 metric ton per year of polyacrylonitrile.
Edited by Scott Jenkins
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