The production of polyacrylamide consists of two main steps:
Monomer Production Technology: The production of acrylamide monomer utilizes acrylonitrile as the raw material. Under the action of a catalyst, the acrylonitrile undergoes hydration to yield a crude acrylamide product. Following flash evaporation and purification, refined acrylamide monomer is obtained; this monomer serves as the primary raw material for the production of polyacrylamide.
Acrylonitrile + (Water/Catalyst) → Hydration → Crude Acrylamide → Flash Evaporation → Purification → Refined Acrylamide.
Based on the historical evolution of catalysts, monomer production technology has progressed through three distinct generations:
The first generation employed sulfuric acid-catalyzed hydration technology. The drawbacks of this method included low acrylonitrile conversion rates, low acrylamide product yields, and the generation of numerous by-products, which placed a significant burden on the subsequent purification process. Furthermore, due to the strong corrosiveness of the sulfuric acid catalyst, equipment costs were high, thereby increasing overall production costs. The second generation utilized binary or ternary skeletal copper catalysts. The disadvantage of this technology was the introduction of copper ions-which interfere with the polymerization process-into the final product, thereby increasing the costs associated with downstream purification and post-processing. The third generation employs microbial nitrile hydratase catalysis technology. This technology operates under mild reaction conditions-specifically at ambient temperature and atmospheric pressure-and is characterized by high selectivity, high yield, and high catalytic activity. The acrylonitrile conversion rate can reach 100%, ensuring complete reaction with no generation of by-products or impurities. The resulting acrylamide product contains no copper ions; thus, there is no need for ion exchange procedures to remove copper ions generated during production, which significantly simplifies the overall process flow. Moreover, gas chromatography analysis indicates that the acrylamide product contains virtually no residual free acrylonitrile, demonstrating a high degree of purity. This makes it particularly suitable for the preparation of ultra-high molecular weight polyacrylamide, as well as the non-toxic polyacrylamide required by the food industry.
Regarding the technology for producing acrylamide monomer via microbial catalysis, Japan was the first to establish a commercial facility-specifically a 6,000-ton-per-annum (t/a) plant-in 1985. Subsequently, Russia also mastered this technology; during the 1990s, both Japan and Russia successively commissioned large-scale, ten-thousand-ton-class facilities for the microbial-catalyzed production of acrylamide. Following Japan and Russia, our country is the third in the world to possess this technology. The activity of the microbial catalyst stands at 2,857 International Biochemical Units, a level that has reached global standards. Our country's technology for the microbial-catalyzed production of acrylamide monomers was developed and completed by the Shanghai Pesticide Research Institute over the course of three consecutive Five-Year Plans: the "Seventh," "Eighth," and "Ninth" Plans. The microbial catalyst-nitrile hydratase-was first screened in 1990; it was derived from nitrile hydratase obtained through seed culture, utilizing 163 bacterial strains isolated from soil at the foot of Mount Tai and 145 strains isolated from soil in Wuxi. This catalyst was designated by the code "Nocardia-163." This technology has since been successfully put into commercial operation in Rugao (Jiangsu), Nanchang (Jiangxi), the Shengli Oilfield, and Wanquan (Hebei). The resulting products are of superior quality, meeting the specific quality metrics required for the production of ultra-high relative molecular weight polyacrylamide. This achievement signifies that our country's technology for the microbial-catalyzed production of acrylamide has reached an advanced international level.
Polymerization Technology: The production of polyacrylamide utilizes an aqueous solution of acrylamide monomers as the raw material. Under the action of an initiator, a polymerization reaction takes place. Upon completion of the reaction, the resulting polyacrylamide gel blocks undergo a series of processing steps-cutting, granulation, drying, and pulverizing-to yield the final polyacrylamide product. The critical step in this process is the polymerization reaction itself; during the subsequent processing stages, particular attention must be paid to preventing mechanical degradation, thermal degradation, and cross-linking to ensure that the polyacrylamide retains its intended relative molecular weight and water solubility.
Acrylamide + Water (Initiator/Polymerization) → Polyacrylamide Gel Blocks → Granulation → Drying → Pulverizing → Polyacrylamide Product
Our country's polyacrylamide production technology has generally evolved through three distinct stages:
The first stage involved the earliest adoption of "pan polymerization." In this method, the thoroughly mixed polymerization reaction solution was poured into stainless steel trays. These trays were then pushed into a temperature-controlled drying oven. After polymerizing for several hours, the trays were removed from the oven; the polyacrylamide gel was cut into strips using a guillotine cutter, fed into a meat grinder for granulation, dried in an oven, and finally pulverized to produce the finished product. This entire process was conducted in the style of a traditional manual workshop. The second stage involves the use of a kneader: the pre-mixed polymerization reaction solution is placed into the kneader and heated. Once polymerization begins, the kneader is activated, allowing kneading and polymerization to proceed simultaneously. By the time polymerization is complete, granulation is also largely finished; the discharged material is then dried and pulverized to yield the final product.
The third stage emerged in the late 1980s with the development of the conical reactor polymerization process. This technology was successfully piloted by the Fifth Research Institute of the Ministry of Nuclear Industry at the Jiangdu Chemical Plant in Jiangsu Province. This process features a rotating granulation blade located at the bottom of the conical reactor; as the polymer is extruded, it is simultaneously formed into granules. The material is subsequently dried using a rotary drum dryer and pulverized to produce the final product.
To prevent polyacrylamide gel blocks from adhering to the inner walls of the polymerization reactor, some techniques employ coatings made of fluorine- or silicon-based polymer compounds applied to the reactor's interior. However, these coatings are prone to peeling off during the production process, thereby contaminating the polyacrylamide product.
There are also designs utilizing rotatable conical reactors; once the polymerization reaction is complete, the reactor is inverted to discharge the polyacrylamide gel blocks. Furthermore, variations exist regarding granulation methods (including mechanical granulation, cutting granulation, and wet granulation-i.e., granulation within a dispersion medium), drying methods (such as through-flow rotary drying or vibrating fluidized bed drying), and pulverization techniques. While some of these differences stem from variations in equipment quality, others reflect differences in the specific operational approaches adopted, broadly speaking, the prevailing trend in polymerization technology is shifting toward the use of fixed conical reactors combined with vibrating fluidized bed drying technology.
In addition to the aforementioned unit operations, polyacrylamide production technology exhibits significant variations in process formulation. Specifically regarding the initiation step, a distinction is made between the "pre-alkali co-hydrolysis" process and the "post-alkali post-hydrolysis" process. Each method has its own advantages and disadvantages: the pre-alkali co-hydrolysis process is procedurally simpler, but it presents challenges related to heat transfer during hydrolysis-specifically, a propensity for cross-linking to occur and a significant loss in relative molecular mass. Conversely, while the post-alkali post-hydrolysis process involves a more complex procedural sequence, it ensures uniform hydrolysis, minimizes the risk of cross-linking, and results in negligible loss of the product's relative molecular mass.
In my country, the initiators employed for polyacrylamide polymerization generally fall into three categories: inorganic initiators, organic initiators, and mixed inorganic-organic systems. (1) Peroxides
Peroxides are broadly classified into inorganic peroxides and organic peroxides. Inorganic peroxides include potassium peroxydisulfate, ammonium peroxydisulfate, sodium perbromate, and hydrogen peroxide. Organic peroxides include benzoyl peroxide, lauroyl peroxide, and tert-butyl hydroperoxide. The reducing agents typically paired with these peroxides include ferrous sulfate, ferrous chloride, sodium metabisulfite, and sodium thiosulfate.
(2) Azo Compounds
Examples include azobisisobutyronitrile (AIBN), azobis(dimethylvaleronitrile), sodium azobis(cyanovalerate), and the series of azoamidine salts developed in the 1980s-such as azo-N-substituted amidinopropane hydrochlorides. These represent a class of products that have been the subject of intense competitive development. They are typically added at concentrations ranging from 0.005 to 1 part per 10,000; they exhibit high catalytic efficiency, facilitate the production of polymers with high relative molecular masses, and are water-soluble, making them convenient to use.
Inverse Suspension Polymerization: Polyacrylamide is one of the most industrially significant organic polymeric flocculants. Industrially, polyacrylamide is typically produced using either the aqueous solution method or the inverse suspension polymerization method. The following section outlines the process for producing polyacrylamide via inverse suspension polymerization.
Inverse suspension polymerization is currently the most widely utilized and technologically mature method for manufacturing polyacrylamide (PAM) microspheres. The process involves using vigorous agitation to disperse a monomer (or a mixture of monomers) within a continuous medium (typically an organic solvent), forming fine droplets. Subsequently, polymerization is initiated among the monomers, initiators, organic solvent, and dispersion stabilizers. Upon completion of the polymerization reaction, the product undergoes azeotropic dehydration, separation, and drying to yield a particulate product. Products obtained via inverse suspension polymerization typically possess a solid content exceeding 90%, a polymerization conversion rate greater than 95%, and a residual monomer content of less than 0.5%; the product particle size ranges from 10 to 500 micrometers, and the product exhibits excellent water solubility.
This method is highly amenable to industrial-scale implementation due to its simple process, ease of operational control, facile removal of polymerization heat, and the ease with which the polymer can be separated, washed, and dried; furthermore, the resulting product is characterized by its purity, uniformity, and stability. However, inverse suspension polymerization faces several challenges in industrial production. Foremost among these is its high sensitivity to stirring speed, which frequently leads to particle coalescence and gel formation. Furthermore, the system tends to be unstable during azeotropic distillation, and the process is characterized by prolonged dewatering times. Additionally, factors such as a broad product particle size distribution, the extensive use of organic solvents, safety concerns regarding production operations, and excessively high polymerization costs have collectively resulted in the inverse suspension polymerization method being rarely utilized domestically for the production of polyacrylamide.
