The difficulty of designing an intermittent solution polymerization reactor depends on the viscosity of the system in the later stage of the polymerization reaction. If the viscosity of the system does not change significantly during the polymerization process, the agitator design is relatively simple. However, most intermittent solution polymerization reactions start with low monomer viscosity, which rapidly increases as the polymerization proceeds. Conventional agitators are difficult to adapt to large range changes in viscosity, and some semi continuous polymerization processes continuously add monomers during the reaction process, resulting in significant changes in liquid level. If multiple impellers are used, there will be sudden changes in energy input and splashing of liquid level materials. In addition, some reactions produce sticky substances in the later stage, resulting in a significant decrease in heat transfer coefficient.
For this type of aggregation, Yuan Zheng has accumulated rich design and manufacturing experience and can take effective measures according to different processes:
(1) Developed a wide viscosity range agitator - SP304 large blade propeller, which is suitable for processes with significant changes in material viscosity. During the initial low viscosity state of polymerization, it is an axial flow propeller. As the viscosity increases, the flow pattern gradually tends towards radial flow, which is beneficial for improving the heat transfer coefficient of the built-in heat exchange element and jacket.
(2) Adopting coaxial stirring technology can adapt to a wider viscosity range. The inner layer is a high-speed multi-layer turbine blade, suitable for the initial stage of polymerization. When the viscosity increases, the outer low-speed frame agitator is activated, suitable for mixing high viscosity systems. Various scrapers can also be installed on the frame agitator to remove sticking materials and enhance heat transfer. The scraper structure needs to be specially selected and designed according to the physical properties and degree of sticking.
(3) By using a screw stirrer and coordinating with a guide tube, a high circulation capacity and mixing efficiency can be maintained within the viscosity range of 0.5~100000mPa. s, effectively eliminating the unevenness of concentration and temperature inside the stirred reactor. The wall of the guide tube can also be designed as a hollow structure with a cooling medium inside, and double-sided heat transfer has high heat transfer efficiency. Due to its strong circulation ability, wide adaptability to viscosity range, and high heat transfer efficiency, the screw guide tube has become a typical agitator for intermittent solution polymerization. Successful applications include acrylonitrile solution polymerization for producing carbon fibers, DMC ring opening polymerization for producing silicone rubber, etc.
(4) Part of the agitator can be designed as a hollow structure, with both the agitator and the agitator shaft being hollow. The cooling medium flows through the hollow channel, and due to the agitator being in motion, its heat transfer coefficient is five times greater than that of the built-in coil. Hollow mixers can also be used for heating or cooling processes with high solid content, and are more suitable for glass lined equipment where it is difficult to set baffles and coils.
(5) The mirror polishing of the mixer and the inner wall of the container can reduce the generation of sticking materials.
(6) When no sticking material is generated, using a near wall stirrer can improve the heat transfer coefficient;
(7) For large polymerization reactors, when the jacket heat transfer surface alone is insufficient, various internal components can be used to increase the heat transfer surface, or external circulation heat transfer can be used to supplement the heat transfer surface, or low boiling point solvents or monomers can be evaporated to remove heat.