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Plastic is used in more than 300 million tonnes every year. This plastic can cause environmental damage. There are many global efforts to improve recycling capacity and invent more effective recycling solutions. These improvements are possible because vacuum plays an important role.
The vacuum used in these processes is usually used to remove contaminants. Any contaminants, including chemicals, residues and even water, can be removed from the process. Due to the fact that the material source is often made from mixed waste, contaminants can be added to the input material.
Due to several factors, performance requirements and demands for vacuum systems for plastic recycling are increasing. These include extra effort required to remove contaminants, increasing sophistication of materials, and higher purity demands. All these factors contribute to the demand for vacuum technology that is more efficient.
1. Extrusion: Extrusion is a common method of recycling back to the basic polymer. Extrusion involves melting the input material and forcing it through an extruder. It then mixes, forms, cools, and returns to its solid form. Vacuum is used along the extruder to remove contaminants – through degassing/devolitization (removing air, water vapor, unreacted monomers, other volatile compounds).
This can be a problem in any plastic extrusion process. Inevitably, polymer material is pulled into the vacuum system. This can lead to contamination. The problem can get worse if you consider the contaminants that are inherent in recycled material.
2. Drying: The incoming plastic waste is shredged and washed with water or solvents before being processed. After drying, the water and solvents are removed. This drying process can be done under vacuum pressures to accelerate it and allow it to take place at a lower temperature below 212 (100). This drying process is more efficient and requires less heat, which in turn means less fuel. It also lowers financial and environmental costs.
3. Pyrolysis: Pyrolysis is used to recycle plastics and convert them into other petrochemicals. Pyrolysis, a thermal process that uses high heat to thermally decompose in the absence or oxygen, is called. Vacuum systems are able to remove air (therefore oxygen), prior to thermal decomposition.
While most Pyrolysis versions are performed at atmospheric pressure or greater, Flash Vacuum Pyrolysis, which is performed at lower atmospheric pressure, requires a vacuum system.
Vacuum can also be used to remove hazardous vapors or to extract liquids after a pyrolysis reaction, before opening the reactor up to the atmosphere.
4. Distillation: Pyrolysis produces a variety of liquid chemicals, depending on the source material or type of thermal degradation. These materials need further refinement to produce the final product. Distillation is used to separate and purify various chemicals. Vacuum distillation can also be done at lower temperatures. This is useful for materials sensitive to heat and requires less heat, thus reducing or eliminating fuel consumption.
The front-line soldiers in plastic extrusion or recycling are liquid ring vacuum pumps (LRVP). These pumps are able to handle less demanding applications, where vacuum levels are not as important. LRVP's can tolerate minor process disturbances and junk that is sent to the pumps.
LRVPs create vacuum using liquid (typically water). Any contaminants that get in the system will eventually get absorbed into the liquid/water. This can reduce the vacuum pump's performance and lead to problems like cavitation or mechanical problems.
An LRVP can be operated using a one-through setup (no recirculation), to seal the fluid. This will avoid contamination. The pump has a significant environmental impact. However, it requires high volumes of clean water. This must be flushed to a wastewater treatment facility.
A thin oil film is used to seal vanes that slide into and out of an eccentric rotating rotor. Rotary vane pumps are made from a thin layer of oil. There are many RV pumps that can be used to recycle plastics and other materials. They also have dual-stage versions that can reach deep vacuum levels.
This technology is not suitable for all applications. It requires minimal process contamination and carryover.
Dry claw vacuum pumps work without any sealing fluid. This means that performance is not affected by contamination. The pump mechanism can be affected by contamination from carryover polymers, monomers and other contaminants. To prevent or eliminate contamination buildup, filtration and other strategies like solvent flushing are often used.
Plastics/recycling pumps are typically single-stage versions of claw type pumps. They can achieve moderate vacuum levels (around 28'HgV or 50 TorrA), and are therefore not designed for greater performance or deeper vacuum requirements.
Dry screw vacuum pumps work without any sealing fluid. This means that performance is not affected by contamination. The pump mechanism can be affected by contamination from carryover polymers, monomers and other contaminants. To prevent or eliminate contamination buildup, filtration and other strategies like solvent flushing are often used.
Dry screw pumps are capable of reaching deeper vacuum levels than 29.8" HgV (0.01 TorrA) and can be supplied with high throughput capacities. This pump is an excellent choice for high performance and higher demand.
When higher throughput is required at deeper vacuum levels, rotary lobe vacuum boosters can be used. This is especially true for higher performance requirements applications.
As we have already mentioned, tricky polymers and other contaminants can cause problems in the vacuum system. They can compromise the seal fluid of oil sealed or liquid ring pumps and can even damage or lock up pump mechanisms. Many different arrangements of traps and filters are used to limit performance issues. These are built into the vacuum system. This ancillary equipment is selected based on the particular application, the type and contaminants involved, as well as the type of vacuum system used.
Everest Vacuum, for example, has developed a vacuum scrubber system that captures and inserts carryover polymer and other impurities prior to reaching the vacuum pump. The scrubber is a great tool for keeping harmful substances and contaminants out of vacuum pumps. It also benefits the vacuum system as well as the environment.
Vacuum systems are required to handle higher levels of contaminants and the tricky nature of plastics/polymers, especially recycling. This is particularly true for recycling applications that have higher performance requirements. There are many factors that go into choosing the right vacuum solution for plastic recycling applications. Everest Vacuum can assist with these difficult plastic recycling applications by providing the best vacuum technology and application expertise.
