PET is a hygroscopic material and loses many of the desirable properties if the water content is above 50 ppm. High temperature dryers are needed to draw moisture out of the PET. These dryers need to be maintained very well because as much as 50% of all processing problems can be attributed to malfunctioning dryers.

A dryer works by moving hot air through a bed of desiccant beads. These beads absorb moisture out of the air and the dry air then absorbs moisture when moving through the PET in the dryer. After a set time or when the dewpoint of the air reaches a setpoint, the dryer switches from one desiccant bed to another. Some dryers have two, others have up to five desiccant beds. The bed that was drying until now is then heated with very hot air, in the 500 F area. This hot air makes the desiccant give up the moisture they absorbed before. The desiccant is then cooled or let to cool at ambient temperature.

To get PET to the moisture content below 50 ppm several temperature and drying times combinations can be used. 12 hours @ 325 F might work as well as 6 hours @ 340 F. The one to choose will depend on the job you are running and the hopper size you are using. Let's go through an example:

In our example, if the throat temperature is 305 F when drying @ 310 F and you need a low AA level, consider making the fill level on the hopper adjustable. You could cut out a long hole and cover it with Plexiglas. A sensor could be made adjustable in height and the on and off timing of the hopper loader could come from this sensor. You could then lower the sensor and run at the higher temperature because your resin stays less time in the hopper. If you are not sure how long the resin is actually in the hopper, add a few color pellets to the resin and time its arrival in the preforms or bottles you are making.To make things more complicated, study the graph below. It shows that the residence time is highly dependent on the position of the color pellets with respect to the diameter of the hopper.