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Compounding Polyvinyl Chloride – PVC: Material Flow and Handling

Material handling is as much an art as it is a science. Certainly there are many aspects of the systems that lend themselves to scientific analysis and design. However, there is much to be said for the practical operating experience that comes after the system is installed, the expert “instinct” for each unique combination of system and product.

Powder characteristics are one major factor in the system design and component selection. Bulk density will be used for bin sizing and support structures. Particle size distribution must be factored into the design of flow aids and pneumatic conveying system design. The moisture content of a bulk powder will impact its bulk handling properties. Coatings on the powder, in conjunction with operating temperatures, will also have an impact on the overall system architecture.


Silos For PVC, Fillers and Compounds
Bulk silos are typically designed for funnel flow, which at a minimum requires a 60- to 70-degree hopper angle. Mass flow designs are specific to the product to be stored in the silo. These designs use various cone shapes, dimensions, and discharge openings to ensure mass flow. Mass flow minimizes segregation and yields first-in-first-out. This FIFO operation facilitates lot control. Funnel flow, which is a typical design, can result in some segregation. When the ratio of the largest to the smallest particles is relatively low, the risk of segregation is minimal.

The location of the silo, inside or outside, also needs consideration. Outside, the system is subject to weather variations. Silos are intended to be watertight. Diligent silo assembly and maintenance are required to maintain that watertight integrity. Maintenance also figures in the physical location of the silo. The majority of silos are located outside because floor space at most plants is at a premium. In addition, construction costs would increase for silos inside a building.

Most bulk powder silos have standard features that are critical to successful operation. Typically, the fill pipe is 4-inch pipe or tubing—the shorter the run, the faster the truck/railcar will unload. Each silo needs a means of relieving vent pressure during loading and relieving vacuum while unloading. Bin vents are located on top of the silo. Most are cleaned with high-pressure air and some are equipped with fans to maintain a slight vacuum within the silo to prevent dusting during loading. A pressure switch or timer should be used to control the cleaning duration. Typically, a dust collector operates efficiently when the pressure drop is operated between 3 and 6 inches of water. 

Silos can be equipped with numerous types of flow aids from simple air jets or vibrators to full fluidized or vibrating bin bottoms. Any plant air used as a flow aid that is introduced directly into the silo must be free of oil and water. This is essential because moisture in a powder will hinder its flow.


Conveying PVC, Fillers and Compounds
Generally, a low-pressure (15 psig or less) or low-vacuum (12 inch Hg) dilute phase conveying system is adequate for most applications. This system tends to have the least complex components: air supply, line charger, conveying line, and filter receiver. The air supply is typically a rotary lobe blower that can deliver air at a maximum pressure of 15 psig or pull a 12-inch Hg vacuum. A rotary feeder or air lock is the industry workhorse line charger for a pressure or vacuum system. Conveying lines typically consist of 3- or 4-inch pipe or tubing. At the end of the conveying line, filter receivers remove the powder from the conveying air and also inventory powder for the process. 

Depending on how the filter receiver is used, the flow aid can be aeration or vibration based, e.g. air pads or vibrators. Again, any air used as a flow aid should be free of oil and water. To completely empty a day bin, a vibrator works well. Aeration should be considered when the filter receiver is only going to be partially emptied during each withdrawal.


Adding Liquids During Compounding
Within the blend plant, several bulk powders and liquids are mixed together to make the final product or compound. The liquids are added at such a level that the physical moistening effect is minimal. However, there are some liquids and powders that can contribute to handling problems when heat is added to the mix. These materials become tacky at elevated temperatures and can impede the flow of the powders through the processing or handling equipment. In some cases, the tacky material starts to adhere to surfaces, which causes a number of further problems. 


Handling Coated Ingredients and Fatty Acids
When working with coated products, it is important to understand the melting point of the coatings. In addition, some powders, such as stearic acid, melt at low temperatures. Depending on the makeup of the acid, the melting point for the various components will differ. When operating coolers and the equipment downstream of the cooler, it is important to consider which ingredient has the lowest melting point.

This section of Specialty Minerals Inc.’s (SMI’s) web site contains a number of pages that deal with the technology of preblending and melt compounding rigid PVC. Click on one of these topics to go to that page:

 

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