Mechanical drying, by vertical or horizontal centrifuge, at the end of the line should deliver a flake with a moisture content below 1% and after this it will be a dryer unit job to decrease it to nothing. 

PET is an Hygroscopic polymer, meaning it absorbs moisture, and this is why drying is indispensable also for virgin pellets before processing. 

PET will be staying in the dryer for sometimes (few hours) with or without vacuum and this because the moisture that's inside the flakes, or pellets  should be released.

If the polymer is not completely dry, during extrusion IV (Intrinsic Viscosity) will decrease a lot and polymer will lose its characteristics.

Care must be taken to process the material immediately after drying, meaning  if you dry and leave it in a storage place, it will get moisture from the air and the energy used to dry is just thrown away.