Process Flow for the Preparation of Polyvinyl Alcohol
Apr 24, 2026
The process involves the polymerization of vinyl acetate to produce polyvinyl acetate, followed by the alcoholysis of the polyvinyl acetate to yield polyvinyl alcohol (PVA), with the subsequent recovery of acetic acid and methanol.
Polymerization of Vinyl Acetate
Based on the method of execution, the polymerization reaction of vinyl acetate can be classified into bulk polymerization, solution polymerization, emulsion polymerization, and suspension polymerization. The polymerization process generally employed for the production of polyvinyl alcohol is solution polymerization; the solvent used is methanol, which constitutes 16% to 22% of the total mass of the vinyl acetate and methanol feed. Azobisisobutyronitrile (AIBN) is utilized as the initiator, and the reaction is conducted at a temperature of 65°C.
Numerous factors influence the vinyl acetate polymerization reaction and the quality of the final PVA product. In addition to the dosage of the initiator and the ratio of the methanol solvent, key influencing factors include the polymerization temperature, reaction duration, polymerization conversion rate, and the presence of impurities within the vinyl acetate—such as acetaldehyde, crotonaldehyde, benzene, acetone, and water. These factors exert a significant impact on both the polymerization reaction and the quality of the finished product.
Alcoholysis of Polyvinyl Acetate
Polyvinyl acetate reacts with methanol in the presence of a base to produce polyvinyl alcohol. The alcoholysis process can be broadly categorized into two methods: the high-alkali method and the low-alkali method. In the high-alkali alcoholysis method, the molar ratio of the base to the monomer units within the polyvinyl acetate chain is relatively high. Conversely, in the low-alkali alcoholysis method, the reaction mixture is essentially anhydrous; a very low molar ratio of base is employed—specifically, only one-seventh of the ratio used in the high-alkali method.
Both the saponification reaction and various side reactions occur in the presence of water, and they consume the base to generate sodium acetate. In the low-alkali alcoholysis process, the reaction system is essentially anhydrous, the quantity of base consumed is minimal, and consequently, very little sodium acetate is generated; thus, no recovery step is required for the sodium acetate. In contrast, the high-alkali alcoholysis process generates a substantial amount of sodium acetate as a by-product; therefore, a dedicated process step is incorporated to decompose the sodium acetate and recover the acetic acid.
The primary process parameters for both alcoholysis methods are presented in Table 5-2. Following the alcoholysis stage, the material undergoes subsequent steps—including crushing, extrusion, and drying—to yield the final PVA product.
Kuraray Co.
Denka Co.
Process Conditions
High Alkali
Low Alkali
Low Alkali
Polyvinyl Acetate Methanol Solution Concentration (%)
22-23
33
35
Water Content (%)
2
<0.1
<0.1
Alkali Addition Molar Ratio
0.12
0.016
0.016
Alcoholysis Reactor Type
Twin-Screw
Belt Conveyor
Belt Conveyor
Residence Time
50~80s
8~10min
15~20min
Prior to the 1960s, the global standard for alcoholysis primarily involved high-alkali continuous alcoholysis utilizing screw-type reactors; currently, however, most major manufacturers worldwide have adopted the low-alkali alcoholysis process utilizing belt-type reactors.
In addition to the two methods mentioned above, alcoholysis technology also encompasses a "low-alkali oil-phase granulation" method. This technique yields granular PVA directly during the low-alkali alcoholysis process, thereby eliminating the need for a subsequent pulverization step. The method involves introducing a liquid paraffinic hydrocarbon—which is immiscible with methanol—into the alcoholysis solution to facilitate the dispersion of the PVA. The final product is obtained through subsequent filtration, washing, and drying.
Recovery of Methanol and Acetic Acid
The waste liquid generated during the alcoholysis of polyvinyl acetate consists primarily of methanol and methyl acetate, along with minor quantities of water, sodium acetate, acetaldehyde, and acetone. Among these components, the recovery of methanol is essential. Furthermore, methyl acetate can be converted back into acetic acid and methanol; after purification, these recovered substances can be reused. This recycling process is a critical factor in reducing the specific consumption rate of raw materials in PVA production.
Comparison of Polyvinyl Alcohol Production Processes
There are typically two primary raw material routes for the production of PVA: The first route utilizes ethylene as the feedstock to synthesize vinyl acetate, which is then converted into PVA. The second route employs acetylene (derived from either calcium carbide or natural gas) as the feedstock to synthesize vinyl acetate, which is subsequently converted into polyvinyl alcohol. Currently, manufacturers in countries such as Japan and the United States predominantly utilize the ethylene-based route—specifically, the "petroleum ethylene method." Each of these three production methods possesses its own distinct advantages and disadvantages; a comparative analysis of their respective processes and characteristics is presented in Table 5-3.
Raw Material Route
Petroleum Ethylene
Natural Gas Acetylene
Calcium Carbide Acetylene
Reaction Mode
Fixed-bed Gas-phase
Fixed-bed Gas-phase
Fluidized-bed Gas-phase
Temperature (°C)
150-200
170-210
170-210
Pressure / MPa
0.49–0.98
Atmospheric
Atmospheric
Space Velocity (L/h)
2040~2100
250~280
110~150
Raw Material Ratio (Molar Ratio)
Ethylene:
Acetic Acid: Oxygen = 9:4:1.5
Acetylene:
Acetic Acid = 1:(7±1)
Acetylene:
Acetic Acid = 1:(3±1)
Catalyst Composition
Palladium, Gold (Precious Metals)
Zn(AcO)₂/Activated Carbon
Zn(AcO)₂/Activated Carbon
Catalyst Lifetime
5–6 months
3 months
5–6 months
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