Mixing device production

Aggregation stirringAggregation reaction stirring device
For industrial polymerization production, the quality and production cost of polymers not only depend on the polymerization reaction process, but also on the structural type of the polymerization reactor. As an important polymerization reaction equipment, stirred tank reactors require professional selection and design to solve the problems of quality and energy transfer, ensure uniform product performance, and reduce production costs.

The design and engineering scaling up of stirred tank polymerization reactors have almost concentrated all the difficulties of stirred reactors, mainly including:

(1) The quality requirements for the product are high. For example, there are high requirements for product molecular weight, molecular weight distribution, mechanical properties, heat resistance, average particle size, particle size distribution, particle morphology, etc;

(2) The complexity and variability of polymer systems. For example, heterogeneous systems, high viscosity non Newtonian systems, variable viscosity systems, etc;

(3) The complexity of flow in a stirred tank. For example, the randomness of three-dimensional flow, the complex shape of the agitator, and the difficulty in determining boundary conditions;

(4) The polymerization process has a large thermal effect, and the reaction temperature requirements are strict. The heat transfer elements and stirrers often require special design.

(5) Large scale aggregation equipment.

Yuanzheng Company has had many years of industry university research cooperation with Zhejiang University, and has gained valuable experience and theoretical guidance in polymerization reaction technology and equipment. They have successively developed various stirring equipment for polymerization processes and successfully applied them in industrial production.

◇Intermittent solution polymerization reactor

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.

Aggregation stirringAggregation reaction stirring device

◇Continuous solution polymerization reactor

In addition to considering mixing and heat transfer, the design of continuous solution polymerization reactors also needs to pay special attention to the residence time distribution. In order to obtain a narrower residence time distribution, using multiple fully mixed reactors in series is one method, or a laminar flow reactor can be used to reduce the number of reactors and equipment investment.

When multiple fully mixed kettles are connected in series, the number of kettles generally ranges from 3 to 8. The more there are, the narrower the residence time distribution and the greater the investment. Therefore, various factors need to be balanced in the design. The agitator type of each reactor can be different. The polymerization first kettle is a low viscosity system, and using a turbine or blade agitator in combination with a baffle in turbulent flow can achieve good mixing, allowing the catalyst to be fully mixed with the monomer. The viscosity gradually increases, and when entering the transition basin operation, the agitator can use multi-stage counter current impellers, wide blade curved surface axial flow impellers, and turn on turbine impellers. In the later stage of the reaction, the viscosity of the system is higher, and it enters the laminar flow zone for operation. The agitator adopts screw belt, broken screw belt and other types, and the agitator adopts near wall design or scraper design according to the viscosity of the kettle material.

In order to narrow the residence time distribution, make the flow closer to laminar flow, and suppress axial backmixing, the equipment can be designed with a shape with a large aspect ratio. The agitator can be designed with a structure with good radial mixing and weak axial backmixing, such as a frame agitator or a skewed frame agitator. However, due to low viscosity, the first kettle is generally still designed as a full mixing kettle.

◇Batch lotion polymerization reactor

Most lotion polymerization reactors are produced intermittently. Due to the low viscosity of the system, the agitators are generally impeller type, three leaf swept back type, Brumakin type, etc. The design and amplification of the agitator reactor mainly consider the impact of mixing on the polymerization process:

(1) Liquid liquid critical dispersion. For lotion polymerization, the main components are monomer (oil phase) and continuous phase (water phase). The stirring intensity in the polymerization kettle must reach the critical point of liquid-liquid dispersion. If the stirring intensity is too low, the monomer will be separated from the water phase, leading to the reduction of the reaction rate of lotion polymerization.

(2) Temperature and concentration distribution. To stabilize the quality of polymer products, stirring must ensure that the materials in the polymerization kettle have sufficient flipping times, so as to achieve uniform temperature and concentration distribution inside the kettle.

(3) The shear rate and its distribution have an important influence on the consistency between the stability of polymer lotion and the product quality.

(4) Evacuation of aggregation heat. The heat transfer capacity of the polymerization kettle must meet the heat withdrawal requirements during the peak reaction period. Heat transfer mainly relies on the jacket and internal cooling tube. The setting of the internal cooling tube must consider the influence on the flow and mixing of materials inside the kettle and heat transfer. The flow pattern of the agitator should also try to improve the turbulence level of the fluid on the outer wall of the internal cooling tube and eliminate the mixing dead zone between dense internal cooling tubes.

PVDF polymerization reactor

◇Polyester polymerization reactor

Polyester polymerization reactors are generally composed of esterification reactors, pre polymerization reactors, and final polymerization reactors connected in series, with the final polymerization reactor being a laminar flow reactor and the rest being fully mixed reactors.

The esterification kettle is a low viscosity heterogeneous system that can use a combination of turbine blades and axial flow blades. The dissolution rate of the polyacid is related to the shear force of the turbine blades, and the heat input is also related to the circulation capacity and heating area of the agitator.

The pre condensation kettle is a homogeneous viscous system, and heat transfer is the main purpose of stirring. The axial flow propeller provides a larger displacement with the cooperation of the guide tube, and the large-area heating tube is also a necessary condition for heat transfer.

The key to polyester polymerization lies in the later stage of polymerization, where mass transfer becomes the controlling factor of the reaction. In order to improve the degree of polymerization, the design of the continuous polymerization kettle should not only avoid backmixing, but also provide a large mass transfer surface and a short diffusion path. The horizontal single axis disc reactor is a typical condensation equipment with strong surface renewal ability, which has been applied in many PET devices. However, the viscosity of certain polyesters far exceeds that of PET, such as PBT. If a horizontal uniaxial disk reactor is still used, the material cannot form a uniform liquid film on the disk, and the clustered material prevents the diffusion of small molecules and also affects heat transfer.

Based on the process characteristics of ultra-high viscosity polyester, the final polymerization kettle adopts a single axis disc equipment with a scraper or a dual axis self-cleaning mixing equipment, providing strong surface renewal and heat transfer capabilities, and providing a reliable device for the production of high polymerization degree and ultra-high viscosity polyester. At present, the original dual axis self-cleaning mixing equipment has been successfully applied in PBS, PBAT and other devices.

STC-6000 PBS Condensation Reactor

partial performance