The production process of polyvinyl alcohol can be divided into ethylene method and acetylene method according to raw materials.
Acetylene synthesis can be divided into calcium carbide acetylene synthesis and natural gas cracking acetylene synthesis according to the different sources of raw materials. Calcium carbide acetylene synthesis, the first to achieve industrial production. Due to the high energy consumption, low quality and high cost of the products of this process route, the impurities generated by the production process pollute the environment is also more serious, and the lack of market competitiveness is gradually eliminated. In areas rich in natural gas, coal and electric power, natural gas acetylene synthesis is still vital.
The process of synthesis of polyvinyl alcohol by ethylene method includes five steps: obtaining ethylene and synthesis of vinyl acetate, rectification, polymerization, alcoholysis of polyvinyl acetate (PVAc), recovery of acetic acid and methanol.
Vinyl acetate polymerization:
After preheating, ethylene acetate was mixed with solvent methanol and initiator azodiisobutyronitrile and sent to two tandem polymerization reactors for polymerization at 66 ~ 68℃ and atmospheric pressure. After polymerization for 4 ~ 6H, about 2/3 of vinyl acetate was polymerized into polyvinyl acetate. The heat generated by the polymerization reaction can be carried away by the evaporation of methanol, and the methanol vapor is condensed and returned to the polymerization kettle. The polymerization liquid is sent to the monomer to blow out the tower, and the unpolymerized vinyl acetate is blown out with methanol vapor. The ethylene acetate and methanol blown by the monomer blowout tower are separated and distilled, recovered and recycled. The polymerization solution was sent to the alcoholysis section for alcoholysis with methanol adjusted to 33% polyvinyl acetate.
Polyvinyl acetate alcoholysis:
Polyvinyl acetate and sodium hydroxide methanol solution were mixed into a high-speed mixer at the ratio of polyvinyl acetate: methanol: sodium hydroxide: water at 1∶2∶0.01∶0.002 at the same time, and then entered the belt alcoholysis machine for alcoholysis at 50℃. The belt was moved at a speed of 1.1 ~ 1.2m/min, and the alcoholysis was completed in about 4 minutes. To obtain cured polyvinyl alcohol. After the solvent was removed by grinding, pressing and drying, the finished polyvinyl alcohol was obtained.
Recovery of methyl acetate:
The extruded liquid contains a lot of methyl acetate and methanol. First, the azeotrope of methyl acetate and methanol is steamed in the azeotrope distillation column, and the bottom of the column is methanol aqueous solution. Methyl acetate and methanol azeotrope into the water extraction separation column and mixed with water, the top of the column separated methyl acetate, the bottom of the column is methanol aqueous solution. Methyl acetate was catalyzed by ion exchange resin in a hydrolyzer to obtain a mixture of acetic acid and methanol. The mixture is sent to a hydrolysate distillation column, where methanol and unhydrolyzed methyl acetate are steamed out and fed into a water extraction separation column. The bottom of the hydrolysate distillation column is dilute acetic acid, which is obtained after being sent to the dilute acetic acid concentration column to remove the water. The methanol aqueous solution obtained from the bottom of the azeotropic distillation tower and the water extraction separation tower can be reused by steaming the methanol in the methanol distillation tower.
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