High performance vacuum system with dry screw/booster, scrubber system, and knockout pot.
More than 300 million tons of plastic is produced every year, and much of it ends up causing environmental problems. In response to these challenges, there are widespread global efforts to increase recycling capacity and innovate for more efficient and effective recycling solutions. Vacuum plays a critical role in achieving these improvements.
Typically, the purpose of the vacuum in these processes can essentially be generalized to removal of contaminants. The contaminants can be anything (from chemicals and residues to even water or air in some cases) other than the plastic polymer or byproduct that the recycling process intends to refine and produce. Additionally, because the source of material is often mixed waste trash, there can of course be heavy loads of contaminants mixed in with the input material.
The performance requirements and demands on the vacuum systems for plastic recycling are significantly increasing, due to several factors - extra efforts required to remove the contaminants inherent to recycling, the increasing sophistication of materials and higher purity demands.
These factors are all contributing to the need for higher performance vacuum technologies.
There are two primary targets of plastic recycling:
- Reproducing the plastic polymer back to its original “virgin” form
- Converting the plastic waste material into other forms, such as petrochemicals, synthetic fuels, or oils
For these processes, here are the primary applications where vacuum is applied:
When the target is to recycle back to the original basic polymer, typically, the process involves extrusion. The extrusion process entails melting the input plastic material, then forcing it through the extruder where it mixes, forms, and cools back to solid form. Vacuum is used along the extruder to remove contaminants – through degassing/devolitization (removing air, water vapor, unreacted monomers, other volatile compounds).
In any plastic extrusion process this can get tricky because inevitably, polymer material gets pulled into the vacuum system leading to contamination that can become a waste handling issue and/or detrimental buildup in the vacuum system. Add to that the contaminants inherent with recycled material, and the trouble can be compounded.
A extruder degassing module system.
Prior to processing, the incoming waste plastic material is shredded and washed with water and/or solvents. The water/solvents are removed in a drying process. Performing this process under vacuum pressures accelerates the drying and allows it to happen at a lower temperature, below 212 ℉ (100 ℃). This is more efficient, requires less heat and therefore less fuel, lowering financial and environmental cost.
Pyrolysis is commonly used for recycling applications for converting plastics into other petrochemicals. Pyrolysis is a thermal process using high heat for thermal decomposition in the absence of oxygen. Vacuum systems can be used to remove the air (therefore oxygen) prior to the thermal decomposition.
Although most versions of Pyrolysis are performed at atmospheric pressure or higher, one version called Flash Vacuum Pyrolysis is performed at less than atmospheric pressure, requiring a vacuum system.
Vacuum is also sometimes used to remove vapors (which can be hazardous) or suck out liquid materials after a pyrolysis reaction step, before opening the reactor to atmosphere.
The pyrolysis process results in various liquid chemicals depending on the source material and type of thermal decomposition. These materials require further refining to create the final desired product. Distillation is a common process used for separation and purification of the various chemicals. Vacuum distillation can be performed at lower temperatures, which is desirable for materials that are temperature sensitive and require less heat therefore reducing (or eliminating) fuel requirements.
Typical Vacuum Technologies Used for Plastic Recycling Applications
- Liquid Ring
- Rotary vane (oil sealed)
- Dry claw
- Dry screw
- Mechanical vacuum boosters
- Equipment to handle contamination, etc.
Liquid Ring Vacuum Technology
Liquid ring vacuum pumps (LRVP’s) are the front-line soldiers for plastic extrusion and recycling applications. These typically work well on less demanding applications where the level of vacuum is not as critical for performance. LRVP’s reasonably tolerate process upsets and extra junk sent to the pumps.
However, since LRVP’s use a liquid (typically water) as the mechanism for creating the vacuum, whatever contaminants that get into the system inevitably get entrained into the liquid/water. This compromises the performance of the vacuum pump, reducing the depth of vacuum that can be achieved, and can lead to other problems such as cavitation and mechanical issues.
In order to avoid the contamination issues, an LRVP can be operated with a once-through (no recirculation) setup for the seal fluid. This has a heavy environmental impact, however, as the pump requires a high volume of clean source water, that must then be flushed to a waste treatment handling facility.
Liquid ring vacuum pumps (LRVP’s) are the front-line soldiers for plastic extrusion and recycling applications.
Oil Sealed Rotary Vane Technology
Rotary vane pumps use a thin film of oil as a vacuum seal on vanes that slide in and out of an eccentric rotor. RV pumps are available in capacity ranges to cover the vast majority of plastics/recycling applications and can achieve deep vacuum levels with dual stage versions.
However, this technology is only suitable for less demanding applications with minimal process carryover and contamination, which readily mixes with the oil and significantly impacts performance.
Dry Claw Technology
Dry claw vacuum pumps perform without the need for any sealing fluid, therefore performance is not impacted by such contamination, nor is there a waste fluid handling concern as with wet pumps. However, the carryover polymers, monomers, and other contamination can tend to build up and interfere directly with the pump mechanisms. Filtration and other strategies such as solvent flushing are often included to prevent or remove the contamination buildup.
The claw type pumps used in plastics/recycling are usually single stage versions, which can typically achieve medium vacuum levels (around 28”HgV or 50 TorrA), therefore not intended for higher performance or deeper vacuum requirements.
Dry Screw Technology
Dry screw vacuum pumps perform without the need for any sealing fluid, therefore performance is not impacted by such contamination, nor is there a waste fluid handling concern as with wet pumps. However, the carryover polymers, monomers, and other contamination can tend to build up and interfere directly with the pump mechanisms. Filtration and other strategies such as solvent flushing are often included to prevent or remove the contamination buildup.
Dry screw pumps can achieve deeper vacuum levels to >29.8” HgV (<0.01 TorrA), and are available in high throughput capacities. This is a very efficient and effective choice for higher demand and higher performance requirements.
Dry screw pumps can achieve deeper vacuum levels to >29.8” HgV (<0.01 TorrA), and are available in high throughput capacities.
Mechanical Vacuum Booster Technology
Rotary lobe vacuum boosters are used, along with any of the previously mentioned technologies when higher throughput is needed at deeper vacuum levels - especially on the higher performance requirement applications.
Traps, Filters, Knockout Pots, etc.
As already highlighted, the tricky polymers and various contaminants can have detrimental effects on the vacuum system such as compromising the seal fluid in wet (oil sealed and liquid ring) pumps, and can even lead to damaging or locking up pump mechanisms. To limit these performance issues, often various arrangements of traps, filters, knockout pots, etc. are designed into the vacuum system. The selection of this ancillary equipment depends on the specifics of the application, the type of polymer, the contaminants involved, and the type of vacuum pump employed.
For instance, Edwards has created a unique vacuum scrubber system that traps and inerts the carryover polymer and other impurities before they get to the vacuum pump. This scrubber does a tremendous job of keeping the contaminants and harmful materials out of the vacuum pumps and downstream, benefitting the vacuum system and the environment.
The levels of contaminant, and tricky nature of the materials related to plastics/polymers and especially recycling, pose extra demands for the vacuum systems. This is especially true for the recycling applications with higher performance requirements. There are many variables that factor into determining the ideal vacuum solution for plastic recycling applications. Edwards can help achieve the highest performance for these challenging plastic recycling applications, with the most complete portfolio of vacuum technology, and the application expertise to engineer ideal solutions. Visit https://www.edwardsvacuum.com/en.
About the Author
Tracy Mosshart joined Edwards in 2004 and has performed various roles in addition to prior industrial systems experience. He holds a BS degree in Mechanical Engineering from Virginia Tech, and MBA from Georgia State. Tracy is now serving as Business Development Manager for Engineered Systems in the Edwards Industrial Vacuum business.
With over 100 years of rich heritage, Edwards is the partner of choice for tens of thousands of customers in critical applications around the world. Vacuum is required in diverse sectors, from the generation of power to the production of steel, to the challenging environments of space simulation and high energy physics research. Everywhere you find vacuum requirements, you will find Edwards leading the way. From medicines to mobile phones, from computers to coffee beans, to cars and chemicals, we pride ourselves in making a difference to people’s lives. And we do it responsibly, ensuring that we innovate sustainably, whilst helping our customers to maintain their competitive advantage and operational excellence. Edwards is part of the Atlas Copco Group (NASDAQ OMX Stockholm: ATCO A, ATCO B), a Sweden-based provider of industrial productivity solutions.
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