PET recycling plants have a lot of important process equipment connected by pneumatic and mechanical conveying systems.Downtime due to poor transmission system design, incorrect application of components, or lack of maintenance should not be a reality.Ask for more.#Best Practices
Everyone agrees that producing products from recycled PET (rPET) is a good thing, but producing high-quality parts from relatively random raw materials, such as post-consumer PET bottles, is not easy.The complex process equipment (eg optical sorting, filtration, extrusion, etc.) used in rPET plants to achieve this has received a lot of attention – and rightfully so.Unfortunately, the transport systems that move material between this equipment are sometimes added as an afterthought, which can result in less than optimal overall plant performance.
In a PET recycling operation, it is the conveying system that ties all the process steps together – so it should be designed specifically for this material.
Keeping your plant running starts with quality plant design, and not all transfer equipment is created equal.The screw conveyors that have worked so well on chip lines over the past decade are likely to be undersized and fail quickly on flake lines.A pneumatic conveyor that can move 10,000 lb/hr chips may only be able to move 4000 lb/hr chips.A common pitfall is not following design guidelines specifically for handling recycled materials.
A pneumatic conveyor that can move 10,000 lb/hr chips may only be able to move 4000 lb/hr chips.
The most basic idea to consider is that the low bulk density of PET bottle flakes reduces the actual capacity of the transfer system compared to the higher bulk density of granular materials.The flakes are also more irregular in shape.This means that the equipment for processing the sheets is usually quite large.A screw conveyor for PET chips may be half the diameter and use two-thirds the motor power of a screw conveyor designed for flakes.A pneumatic transfer system that can move a 6000 lb/hr chip through 3 inches.Pipe needs to be 31/2 inches.segment.Solids to gas ratios of up to 15:1 can be used for chips, but it is best to operate flake systems with a maximum ratio of 5:1.
Can you use the same conveying air pickup speed for flakes to handle uniformly shaped particles?No, it’s too low to get irregular flake movement.In the storage box, the 60° cone that allows the particles to flow easily must be a tall 70° cone for flakes.Depending on the size of the storage container, it may be necessary to activate the silo to allow the flakes to flow.Most of these “rules” are developed through trial and error, so rely on engineers with experience designing processes specifically for rPET flakes.
Some traditional glidants for bulk solids are insufficient for bottle tablets.The silo outlet shown here is assisted by an inclined screw that breaks bridges and discharges the flakes into a rotating airlock for reliable and stable feeding into the pneumatic conveying system.
Good conveying system design does not guarantee system reliability.To achieve reliable performance, the components in the transport system must be specifically designed for rPET flakes.
Rotary valves that feed flakes into a pressure delivery system or any other part of the process must be heavy-duty to withstand years of abuse from irregular flakes and all other contaminants that pass through them.Heavy-duty cast stainless steel housing and rotors definitely cost more than thinner sheet metal designs, but the extra cost is offset by reduced downtime and reduced hardware replacement costs.
Recycled PET flakes differ from PET flakes in particle shape or bulk density.It is also abrasive.
Rotors in rotary valves designed for lamella should have a V-shaped rotor and a “plough” in the inlet to reduce shredding and clogging.Flexible tips are sometimes used to overcome shredding issues, but these require constant maintenance and also introduce small metal fragments into the process that can create problems downstream.
Due to the abrasive nature of flakes, elbows in pneumatic conveying systems are a common problem.The sheet transport system has a relatively high speed, and the sheet sliding along the outer surface of the elbow will pass through a grade 10 stainless steel tube.Various suppliers offer specialized elbows that minimize this problem, and can even be fabricated by mechanical contractors.
Wear occurs at regular long radius bends as abrasive solids slide along the outer surface at high speed.Consider using as few bends as possible, and possibly special bends designed to reduce this wear.
Developing and executing a maintenance plan for a plant’s conveyor system is the final step, as there are many moving parts that come into direct contact with irregular flakes and contamination.Unfortunately, planned maintenance is often overlooked.
Some rotary airlocks have shaft seals that need to be constantly tightened to avoid leaks.Look for valves with labyrinth shaft seals and outboard bearings that do not require regular maintenance.When these valves are used in sheet applications, it is often necessary to purge the shaft seal with clean instrument air.Make sure the shaft seal purge pressure is set correctly (usually about 5 psig above the maximum delivery pressure) and that the air is actually flowing.
Worn rotary valve rotors can cause excessive leakage in positive pressure delivery systems.This leakage reduces the amount of conveyed air in the duct, thereby reducing the overall capacity of the system.It can also cause bridging issues with the hopper above the rotary airlock, so check the gap between the rotor tip and the housing regularly.
Due to high dust loads, air filters can quickly clog rPET plants before releasing the conveying air back into the atmosphere.Make sure the differential pressure gauge is working properly and make sure the operator checks it regularly.Very light and fluffy PET dust can clog or bridge the outlet of the collector, but a high level transmitter in the discharge cone can help detect these blockages before they cause bigger problems.Make sure to regularly clear the dust buildup inside the baghouse.
This article cannot cover all the rules of thumb for the reliable design and maintenance of transfer systems in rPET plants, but hopefully you understand that there are many points to consider and that there is no substitute for experience.Consider following the recommendations of equipment suppliers who have handled rPET flakes in the past.These vendors have gone through all the trial and error, so you don’t have to go through them too.
About the Author: Joseph Lutz is Director of Sales and Marketing for Pelletron Corp.He has 15 years of technical experience in developing plastic bulk material handling solutions.His career at Pelletron began in R&D, where he learned the ins and outs of pneumatics in a testing lab.Lutz has commissioned numerous pneumatic conveying systems around the world and has been granted three new product patents.
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Post time: Jul-25-2022
