Pneumatic conveying systems use a combination of well-engineered components that work simultaneously to move bulk materials safely, efficiently, and economically. Pneumatic conveying systems are excellent for transporting dry food goods, chemicals, medical powders and more.
Pneumatic conveying systems have also proven very helpful in the plastics industry. It is the ideal solution for moving plastic pellets and resin quickly and reliably. Most plastic materials are lightweight and can easily be modified for transfer using a pneumatic conveying system.
What Is Pneumatic Conveying?
Pneumatic conveying is an efficient method for transferring bulk materials using a series of pipelines that move products by pumping air. These products typically consist of powders or granules. These systems function by moving materials through enclosed, airtight pipelines through a combination of airflow or another type of gas and pressure.
Integrating pneumatic conveying into your plastic material handling equipment helps streamline the routing of materials. The conveyor line requires a small amount of floor space and can flow in horizontal and vertical directions.
Plastic Process Industry Applications
In modern times, plastic materials are integrated into much of the equipment and tools we use on a daily basis. Whether it be plastic bags, household items, furniture, electric cables and much more; products are frequently manufactured from plastic raw materials.
The advantages of plastics include:
- It is lightweight
- It is resistant to corroding
- Plastic is electrically insulated
- It is easy to manufacture by molding/machining etc.
Typical Plastic Manufacturing Processes
Plastics (also known synthetic polymers) are manufactured from hydrocarbons such as ethylene, propylene and styrene. Polyethylene/polypropylene are homopolymers while ABS (Acrylonitrile, butadiene, styrene) are copolymers with more than one polymer involved in their production. Sometimes additives are used to increase abrasion resistance and machinability. A well-known example of this is Polyvinyl Chloride (PVC) which has a multitude of uses.
In the manufacturing process, one or more hydrocarbon groups are processed in a solvent catalytic reactor to produce a polymer product which is filtered, dried and cooled to form Plastic powders.
These powders are then pneumatically conveyed. Historically, conveyors used a transport system referred to as “dense phase transport” which carefully moved product along the bottom of a piped system. Nowadays, most industries are moving towards “dilute phase” pneumatic conveying systems, which use larger amounts of low pressure air to move product along and keep it suspended in the air while in transport through the piping system. These systems are easy to control and maintain.
Plastic powders are sold to retail manufacturers in tankers, or super sacks. Very small portions are moved in 50lb paper bags. To transport plastic powders, the pneumatic conveyors rely on dilute phase vacuums or pressure depending on the distance the material must travel. In a few cases where the plastic powders can get oxidized with air, Nitrogen is used as the conveying medium. Since Nitrogen is expensive, it is recycled back through the system multiple times. This is called a “closed loop conveying system”.
In all these applications, we use the “Central Powered Conveying System’ where the air (or Nitrogen) is processed through a remote mounted pressure blower or a vacuum pump and the product is separated from the conveying air through a filter receiver. Generally the capacities range above 10,00p lb/hr.
Most manufacturers of plastic items such as pipes, poly bags, plastic laminates and other products prefer to use pallets over plastic powders. The advantages of using plastic pellets as raw materials include:
- There is little to no dust created.
- There is no contamination from other products.
- With the larger pallet size, generally 1/8”, there is no exhaust air filtration required.
Generally, the producers of plastic powder also produce pellets using a special melting process and pelletizing equipment. Most operations also involve a blending step, where additional ingredients are added for specific functions like pigmentation or abrasion resistance to chemicals. This process also requires a Pneumatic Conveying/blending system to transport base plastic powders and minor ingredients required for blending to the pelletizing machines.
Bulk Handling of Plastic Pellets
Many plastic manufacturers use small plastic pellets rather than powders in their production.
Large Scale manufacturers obtain the pellets via:
- Rail Cars, if the requirement is more than 100 Tons/week
- Road Tankers If the requirement is 50-100 Tons/week
- Super Sacks if the requirement is less than 50 Tons/week
The above scales are of course, general guidelines as rail cars can bring about 125 Tons and a road tanker can bring up to 25 Tons, depending on its size.
Rail Car Unloading, Storage and Distribution
Industries that rely on huge quantities of plastic materials often have their inventory transported to their processing facility via train. Each rail car typically contains 125 Tons of pellets and will have 3 discharge points. These discharge points have a hinged hatch and some kind of manual gate valve to isolate the flow of product into a collection device.
The pellets are unloaded through the discharge points and poured into a “Day Bin” or other temporary storage device. The aim is to transfer the pellets to the storage silos as quickly as possible. Pellets are stored in Storage silos or bins which hold about 50 Tons. The storage capacity and number of silos depends on the production requirement.
Conveying Product from Trains to Processing Facilities
Generally it is not advisable to directly unload pellets from rail cars into silos using Vacuum conveying if the rail car is more than 300 feet away. This is because Vacuum Conveying to a silo requires a receiver and some kind of air lock at the silo to prevent the possibility of collapsing if the pressure is too high.
Because of this, the rail car unloading process is done in 2 steps:
1ST Step: Attach a rail car adapter to any of the rail car’s discharge points and convey the product to an intermediate filter receiver located next to the Rail Car tracks (no further than about 75-100 feet).
2nd Step: Use a pressure blower to blow pellets from the underside of the filter receiver through an air lock and convey them to the silo through a bin vent directly flanged to the silo top.
This pneumatic transport process is sometimes called a “Pull-Push System”. Care must be taken to maintain equilibrium between both steps and maintain a consistent unload rate. In the case of multiple silos, the conveying process utilizes a multiport diverter to transport product into a specific silo.
Distribution of Pellets from Day Bins to Production
If the silos are located outside, which is normally the case, the pellets are brought inside the plant to a “day bin”, which is a small bin storing about 10,000 lb or more or materials. This transport step relies on dilute phase vacuum conveying using a central powered vacuum blower and a pellet receiver. It is not uncommon to have more than one “day bin” to store different quality pellets or in some cases, powder ingredients as well.
After the “day bins”, the pellets are then transferred via dilute phase vacuum conveying to machines called “extruders”. Typically facilities set up this transport range to be less than 50 feet. Extruders are pieces of equipment that can shape and mold material as it passes through. In the plastics industry, this can include melting and shaping the pellets to form specific molds.
For smaller scale producers, using a large extruder may be unnecessary. To avoid a lot of manual labor and potential workplace injuries, smaller companies can rely on Self Contained Vacuum Loaders to convey materials, which we’ll describe next.
Self Contained Vacuum Loaders
Self-contained vacuum loaders, as created by Pneu-Con, are vacuum conveying systems that operate automatically to maintain a constant material level in a hopper. Unlike central powered systems that are dependent on a remote blower package, Pneu Con’s self-contained vacuum loaders incorporate a durable brush-type motor to pull material through the system.
Each self-contained system operates on a batch principle, using an adjustable timer for conveying, and with a proximity switch to initiate the process. Designed for simple “plug-and-play” installation, Pneu-Con’s self-contained vacuum loaders are equally suitable for a single “up and in ” installation or for complex multi-node operations, and all complexities of operations.
As you can tell, the plastics industry has many unique needs and challenges when it comes to pneumatic conveying. This is due, in large part, to the sheer breadth of plastic products available in all types of sizes and shapes, and serving a multitude of purposes. This post highlights the need-to-know information about conveying plastic materials whether it be off-loading them from a rail car, or conveying them successfully inside a plastic facility. If you have any questions or would like to speak to the experts in pneumatic conveying, connect with Pneu-Con today!