Polypropylene carbonate (PPC) is a new type of aliphatic polycarbonate synthesized by copolymerization of carbon dioxide and epoxide monomers. It has good biodegradability and is an environmentally friendly material with great development prospects. Materials are attracting more and more attention and attention from people from all walks of life around the world.
The product of this project, polypropylene carbonate (PPC), is a non-crystalline resin. It is a viscous paste-like mixture of gas, solid and liquid. It is easily volatile and has a viscosity of more than 2 million cp. The molecular chain is highly flexible and the molecular weight distribution is very narrow, ranging from 1.05 to 1.05. Between 1.5 and 1.5, the molecular weight can range from tens of thousands to hundreds of thousands.
Due to its special material characteristics and process requirements, high-viscosity PPC must use equipment with small shear force, strong self-priming ability, large transportation volume and high outlet pressure to transport materials during the transportation process. The twin-screw pump can fully meet the above requirements among similar conveying equipment.
This article combines the author’s work practice to propose the working principle and working characteristics of the twin-screw pump, as well as its working situation in production and application when replacing other conveying equipment (gear pumps). It aims to illustrate the low shear of this equipment in the process of conveying PPC high-viscosity materials. , high reliability and other advantages to better serve safe production.
2. Conveying principle and conveying process conditions
1) Working principle
The transmission principle of the twin-screw pump is that through the mutual meshing of the main and driven screws in the pump body, and the cooperation between the screws and the pump body holes, sealing cavities are formed in the pump body. When the screw rotates, these sealing cavities continuously move toward the pump body. Move forward to push the liquid material in the sealed cavity to the outlet for discharge to achieve the purpose of transportation.
The product delivery equipment of this project uses a twin-screw pump with external bearings. From the delivery principle of the twin-screw pump, it can be seen that for a twin-screw pump with external bearings, the two screws do not contact each other in the bushing through the bearing positioning. Maintain a constant gap (the gap value is determined by the working conditions and the specifications of the pump itself), and the screw outer circle and the bushing inner circle also maintain a constant gap. The transmission of the two screws is completed by synchronized gears. There is independent lubrication in the gearbox, separated from the pump working chamber. This structure greatly broadens the application range of the twin-screw pump and can transport a large amount of high-viscosity media (the highest viscosity can reach 3*106mm/s).
2) Conveying process
The polypropylene carbonate (PPC) material that first enters the twin-screw pump is stored in the storage tank. The storage tank is under normal pressure. Under production and operation conditions, sufficient supply is ensured by the material’s own gravity and the strong self-priming capacity of the twin-screw pump. quantity. Then it is transported to the downstream process through the twin-screw pump. The twin-screw pump and the downstream extruder equipment are connected by pipelines. The pipelines between them are long. Due to the constraints of the site conditions and the high viscosity of the material itself, the twin-screw pump is used during operation. The generated higher outlet pressure is transported to the downstream extrusion equipment to achieve the purpose of continuous operation of the process.
3. Structural features
Due to the existence of a constant gap and the characteristics of the profile, the twin-screw pump is a sealed positive displacement pump. Therefore, in addition to transporting pure liquid, it can also transport a mixture of gas and liquid, that is, the transport of a vapor-liquid mixture;
Due to the unique design of the structure, the twin-screw pump can self-prime without the need for a special self-priming device, and due to the small axial flow speed of axial transportation, it has a strong suction capacity, that is, a small NPSHr value;
The twin-screw pump has no shearing or emulsification during the transportation process, so it will not destroy the molecular chain structure and the specific fluid properties formed during the working process. And because the transmission relies on synchronous gears, the pump operates with low noise and small vibration. Work smoothly;
The twin-screw pump can also run dry. Since the moving parts do not contact each other during operation, short-term dry rotation will not damage the pump components. This feature provides great convenience for the automatic control process. In addition, with the correct selection of equipment materials, twin-screw pumps can also transport many corrosive media (acid, alkali, etc.).
The user of this project has conducted many tests on the selection of polypropylene carbonate (PPC) conveying equipment, which has become a key process and issue in streamlining the production process. Internal gears, cam pumps and gear pumps have been used successively. Problems such as the bending of the transmission shaft and the large difference between the actual conveying flow rate and the theoretical conveying amount occurred. Moreover, this type of conveying equipment produces a large shear force on the material, destroying the molecular chain structure of the material, reducing the quality index of the product, and affecting the quality of the product.
Based on the characteristics of polypropylene carbonate (PPC) and combined with the transportation principle and characteristics of the twin-screw pump, a twin-screw pump is used to transport PPC materials. After many tests, many problems encountered in production have been solved.
Problem 1: Utilizing the strong self-priming ability of the twin-screw pump solves the problem of normal and continuous supply of high-viscosity mixed materials when the upstream process is at normal pressure, and meets the problem of mixed transmission of gas, solid and liquid during the production process;
Problem 2: Because the polypropylene carbonate (PPC) mixed material is corrosive and volatile, it must be carried out in a sealed environment. The twin-screw pump is solved by taking advantage of its good sealing properties and ability to transport corrosive media. The problem of easy gasification and volatilization, when the conveying equipment transports materials, reduces material loss and does not cause environmental pollution due to gas volatilization;
Problem 3: Affected by the performance of polypropylene carbonate (PPC), it is necessary to ensure that the quality and performance of the material after entering the conveying equipment are not damaged. The twin-screw pump is used to ensure that there is no shearing, no emulsification, and no damage during the conveying process. The molecular chain structure and specific fluid properties formed during the working process ensure the quality of subsequent products.
Nowadays, the twin-screw pump can meet the production needs, solve all the problems caused by the performance of the material itself and the special requirements of the process conditions, realize the continuous operation of production, and further improve the quality of the product.