1 Hydroxypropyl methylcellulose production technology
As the non-ionic cellulose ether with the largest amount, the most varieties and the widest application field, with the continuous expansion of the market application scope and the continuous improvement of product quality requirements in the application field, the production technology and process of HPMC have also experienced a long and tortuous process. The diversity, complexity and endless pursuit of quality in the production and application process of HPMC also show the development process of my country's cellulose ether industry from the side.
1.1 Foreign Hydroxypropyl Methylcellulose Production Technology
Cellulose ether was first reported in 1905. It was prepared by Suida by reacting alkali-swollen cellulose with dimethyl sulfate, but completely separated cellulose ether could not be obtained at that time. In 1912, the first patent for the preparation of cellulose ethers appeared. In 1927, hydroxypropyl methylcellulose was successfully synthesized and separated. In 1938, Dow Chem. realized the industrial production of methylcellulose and created the well-known trademark "Methocel". In 1948, large-scale industrial production of hydroxypropyl methylcellulose was carried out in the United States, and the production process reached maturity in 1960-1970 [1, 2]. At present, the more advanced HPMC production companies in the world are mainly Dow Chemical Co., Hercules Co., and Clariant Co. of Germany. Japan's Shinetsu Chemical Co. and South Korea's Samsung Fine Chemical Co. introduced technology and equipment from Dow Chemical Co. and Germany's Loedige Co. respectively .
The production process of HPMC can be divided into two categories: gas phase method and liquid phase method. At present, developed countries such as Europe, the United States and Japan are more likely to use the gas phase process, using wood pulp as raw material (cotton pulp is used when producing high-viscosity products), alkalization and etherification are carried out in the same reaction equipment, and the main reaction is a horizontal reactor with a central horizontal stirring shaft and side rotating flying knives designed specifically for cellulose ether production, which can achieve a good mixing effect. The reaction process uses advanced automatic control means to accurately control temperature and pressure. After the reaction is completed, the excess methyl chloride and by-product dimethyl ether enter the recovery system in the form of gas and are recycled and reused separately. Refining and purification are carried out in a continuous rotary filter press. Crushing is carried out in a high-efficiency finished product crusher, while drying and removing excess moisture. Auxiliary processes such as mixing and packaging are also completed under an automatic control system.
The gas phase process has the following advantages: compact equipment, high single batch output; lower reaction temperature and shorter reaction time than the liquid phase process; more precise reaction control than the liquid phase process; no need for complex solvent recovery system; low labor cost and low labor intensity.
However, the process also has the following disadvantages: large investment in equipment and automatic control, high technical content, and high investment and construction costs; due to the high degree of automation, the quality of operators is particularly high, and once a failure occurs, it is easy to cause a major accident. Usually, a problem in one place will lead to the shutdown of the entire line.
1.2 Development and current status of domestic hydroxypropyl methylcellulose production technology and process
At present, the production of HPMC in my country is mainly based on the liquid phase process. This process was promoted by research units represented by Wuxi Chemical Research and Design Institute Co., Ltd. on the basis of research results in the 1970s. It was originally a gas phase etherification reaction. Because my country's equipment is not suitable, liquid phase etherification reaction was later developed. To date, the high bath ratio liquid phase etherification reaction process route is still the main HPMC production process for cellulose ether production companies such as Shandong Heda, Shandong Ruitai, and Shandong Yiteng [1, 4].
Domestic HPMC production generally uses refined cotton as raw material (some manufacturers have also begun to try to use wood pulp), and uses domestic pulverizers to crush or directly use refined cotton for alkalization. Etherification uses binary mixed organic solvents and reacts in a vertical reactor. The refining and purification process adopts an intermittent process, that is, the organic solvent is removed in the reactor, and then the crude product is purified through several washing and dehydration processes in a scrubber and centrifuge. The finished product is processed intermittently, granulation is carried out under elevated temperature (some manufacturers do not perform granulation), drying and crushing are carried out in a conventional manner, and most of the special treatments are only to delay the hydration time of the product (rapid dissolution) without anti-mildew and compounding treatments. Packaging is done manually [5]. The liquid phase process has the following advantages: the internal pressure of the equipment during the reaction process is low, the pressure bearing capacity of the equipment is low, and the danger is low; the cellulose is fully swollen and evenly alkalized after being immersed in the alkali solution, and the alkali solution has good penetration and swelling of the cellulose; the etherification reactor is small, and the alkali cellulose can be evenly swollen, so the product quality is easy to control, and products with relatively uniform substitution degree and viscosity can be obtained, and the variety is easy to change.
However, this process also has the following disadvantages: the reactor is usually not too large (less than 15m3), and the production capacity is also small due to statistical limitations. If the output is to be increased, more reactors must be added; the refining and purification of crude products requires more equipment, complicated operations, and high labor intensity; because there is no mildew prevention and compounding treatment, the viscosity stability and production cost of the product are affected; the packaging is manual, with high labor intensity and high labor costs; the degree of automation of the reaction control is lower than that of the gas phase process, so the control accuracy is relatively low; compared with the gas phase process, a complex solvent recovery system is required.
With the improvement of domestic HPMC production technology, some enterprises have made great progress in the large-scale slurry method through continuous independent innovation and have their own technical characteristics. Shandong Heda Co., Ltd. adopts the original "one-step" HPMC production process, which not only has reasonable production process flow, accurate and reliable operation control parameters, and full and reasonable use of raw materials, but also produces products with uniform substitution degree, complete reaction, good solution transparency, and stable product quality [6]. In addition, the HPMC production lines of some enterprises, including Heda, have completed automation transformation and achieved the DCS automation control requirements of the device. Materials, including liquid and solid raw materials, can be accurately measured and added using the DCS system. The temperature and pressure control during the reaction process are also fully realized by DCS automatic control and remote monitoring. Compared with traditional production methods, there are obvious improvements in the feasibility, reliability, stability and safety of product production. It not only saves manpower and reduces labor intensity, but also improves the on-site operating environment.
