Dryers extrusion process

In Figure 47.14, the gas supply to the column dryer is heated after dehumidification using a similar heat exchanger as in the fluid bed. This relatively small amount of gas mixes with the gas above the fluid bed and the same quantity is then vented off to atmosphere (air) or to a return line (nitrogen) after the supply fan, such that the whole system is kept in pressure balance. This small air loss is in fact the only energy loss in the system. Since the gas for the fluid bed is recycled, the heat input closely matches that required to heat the chips, which in any case are heated for the subsequent extrusion process. In many instances, the column dryer is positioned directly above the extruder. 

A company (Naitove, 2012) in Southern California uses gas turbines to generate in-house some of the electrical energy used to run its plastic processing operation. The waste heat from boilers was recovered and used in the crystallizer/dryer unit to condition the PET resin prior to sheet extrusion. No additional electric heating was therefore needed for the purpose, resulting in energy as well as cost savings. 

Every plastic processing operation produces scrap material either generated directly from the production operation, such as nmners and sprues or extrusion edge trim, or from reject finished parts. Whatever the source, economics dictate that the scrap be reintroduced into the material stream. The first step in the process is to grind the parts into a particle size small enough to be mixed with virgin pellets and flow through the various blenders, dryers, and loaders upstream of the injection press or extruder. 

Due to the modular design the drying process can be carried out in different ways. For example, two different types of dryers are described as they are specially adapted to the extrusion/granulation system. 

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