Monomer production technology
Acrylamide monomer is produced by taking acrylonitrile as raw material and hydration under the action of catalyst. The refined acrylamide monomer is obtained after flash refining, which is the raw material for producing polyacrylamide.
Acrylonitrile+(water catalyst/water) → synthesis → crude acrylamide → flash evaporation → refining → refined acrylamide.
According to the development history of catalysts, monomer technology has experienced three generations:
The first generation is sulfuric acid catalytic hydration technology, which has low acrylonitrile conversion rate, low acrylamide product yield and few by-products, which brings great burden to oil refining. In addition, due to the strong corrosiveness of sulfuric acid catalyst, the equipment cost is high, which increases the production cost.
The second generation of binary or ternary skeleton copper catalytic production technology, the disadvantage of this technology is that copper ions which affect polymerization are introduced into the final product, thus increasing the cost of post-treatment refining; The third generation microbial nitrile hydratase catalytic production technology has the characteristics of mild reaction conditions, high selectivity, high yield and high activity. The conversion rate of acrylonitrile can reach 100%, and the reaction is complete without by-products and impurities. The product acrylamide does not contain copper ions, so there is no need for ion exchange to remove copper ions produced in the production process, which simplifies the technological process. In addition, gas chromatography analysis shows that acrylamide products contain almost no free acrylonitrile and have high purity, which is especially suitable for preparing ultra-high relative molecular weight polyacrylamide and non-toxic polyacrylamide needed by food industry.
The production technology of acrylamide monomer catalyzed by microorganism, first, Japan established a 6000t/a acrylamide plant in 1985, and then Russia also mastered this technology. In 1990s, Japan and Russia successively built 10,000 tons of microbial catalytic acrylamide devices. China is the third country in the world with this technology after Japanese and Russian. The activity of microbial catalyst is 2857 international biochemical units, reaching the international level. The production technology of acrylamide monomer catalyzed by microorganisms in China was developed by Shanghai Pesticide Research Institute through three five-year plans, namely the Seventh Five-Year Plan, the Eighth Five-Year Plan and the Ninth Five-Year Plan. The microbial catalyst nitrile hydratase was screened in 1990, and nitrile was isolated from the soil at the foot of Mount Tai 163 strain and Wuxi soil 145 strain. The technology has been put into production in Rugao, Jiangsu, Nanchang, Jiangxi, Shengli Oilfield and Wanquan, Hebei, with excellent quality, reaching the quality index of producing ultra-high relative molecular weight polyacrylamide.
It indicates that China's microbial catalytic acrylamide technology has reached the international advanced level.
Polymerization technology
Polyacrylamide production takes acrylamide aqueous solution as raw material and carries out polymerization reaction under the action of initiator. After the reaction is completed, the polyacrylamide gel block is cut, granulated, dried and crushed, and finally the polyacrylamide product is prepared. The key process is polymerization, and the subsequent treatment should pay attention to mechanical cooling, thermal degradation and crosslinking to ensure the relative molecular weight and water solubility of polyacrylamide.
Acrylamide+water (initiator/polymerization) → polyacrylamide gel block → granulation → drying → crushing → polyacrylamide product.
The production technology of polyacrylamide in China has probably gone through three stages:
The first stage is the earliest disc polymerization, that is, the mixed polymerization solution is put into stainless steel discs, and then these stainless steel discs are pushed into the insulation drying room. After several hours of polymerization, they are pushed out of the drying chamber, polyacrylamide is cut into strips with a straw cutter, granulated in a meat grinder, dried in the drying chamber and crushed to obtain the finished product. This process is completely handmade.
In the second stage, a kneader is adopted, that is, the mixed polymerization reaction solution is heated in the kneader, and after polymerization, the kneader is started for kneading; after polymerization, granulation is basically completed, and the poured materials are dried and crushed to obtain the finished product.
In the third stage, at the end of 1980s, a conical kettle polymerization process was developed and successfully put into trial operation in Jiangdu Chemical Plant, Jiangsu Province. In this process, the lower part of the conical kettle is equipped with a rotary cutter for making materials. When the polymer is extruded, it is granulated, dried with a drum dryer, and crushed to obtain a product.
In order to prevent polyacrylamide gel from sticking to the wall of polymerization kettle, some technologies use fluorine or silicon polymer compounds to coat the inner wall of polymerization kettle, but this coating is easy to fall off in the production process and pollute polyacrylamide products.
There are also rotatable conical kettles, which are inverted to pour out polyacrylamide gel blocks after the polymerization reaction is completed), granulation methods (mechanical granulation, cutting granulation and wet granulation, that is, granulation in dispersion), drying methods (through-flow rotary drying and vibrating fluidized bed drying) and crushing methods. Some of these differences are differences in equipment quality and some are differences in specific methods, but generally speaking, the polymerization technology tends to be fixed conical kettle polymerization and vibrating fluidized bed drying technology.
In addition to the above-mentioned unit operations, the technological formula of polyacrylamide production process is also obviously different. Initiation can be divided into two processes: pre-alkali hydrolysis process and post-alkali hydrolysis process. These two methods have their own advantages and disadvantages. Pre-alkali hydrolysis process is simple, but there are some problems such as easy crosslinking of hydrolysis heat transfer and large relative molecular weight loss. Although the process is increased after adding alkali, it is not easy to cross-link and also causes the relative molecular weight loss of the product.
At present, the initiators used for polyacrylamide polymerization in China include inorganic initiators, organic initiators and inorganic-organic mixed systems.
(1) peroxide
Peroxides are roughly divided into inorganic peroxides and organic peroxides. Inorganic peroxides such as potassium persulfate, ammonium persulfate, sodium perbromide and hydrogen peroxide. Organic peroxides such as benzoyl peroxide, lauroyl peroxide and tert-butyl hydroxyl peroxide. The reducing agents they use are ferrous sulfate, ferrous chloride, sodium pyrosulfite and sodium thiosulfate.
(2) Azo compounds
Such as azodiisobutyronitrile, azodimethylvaleronitrile, sodium azodicyanovalerate and azoamidine salt series developed in 1980s, such as azosubstituted amidine propane hydrochloride, are all competitive products. Their addition concentration is 0.005- 1, which has high catalytic efficiency, helps to generate high molecular weight products, is soluble in water and is convenient to use.
Inverse suspension polymerization
Polyacrylamide is one of the most important organic polymer flocculants in industry at present. In industry, polyacrylamide is usually produced by aqueous solution method and inverse suspension polymerization method. Next, the process of producing polyacrylamide by inverse suspension polymerization is introduced.
Inverse suspension polymerization is the most extensive and relatively mature method to prepare polyacrylamide (PAM) microspheres. The monomer or monomer mixture is dispersed in a medium (the medium is an organic solvent) by strong stirring to become particles, and then the monomer, initiator, organic solvent and dispersion stabilizer are polymerized. When the polymerization reaction is completed, granular products can be obtained by boiling dehydration, separation and drying. The product obtained by inverse suspension polymerization has a solid mass fraction >; 90%, polymerization rate >; 95%, monomer residue
The method has the advantages of simple process, convenient operation and control, easy removal of polymerization heat, easy separation, washing and drying of polymers, pure, uniform and stable products and easy industrialization. However, inverse suspension polymerization also has some problems in industrial production. First, it is greatly influenced by the stirring speed, which is easy to coalesce and gel, and the system is unstable when boiling, and the water outlet time is long. There are also a series of reasons, such as wide particle size distribution, using a lot of organic solvents, safe production and operation, and high polymerization cost, which lead to the fact that inverse suspension polymerization is rarely used to produce polyacrylamide in China.