How to Dry Wet Polypropylene Plastic Power

I do not understand the desire to tempt fate by bringing the air temperature close to the Polypropylene melting point. Water will evaporate without having to boil the water. The critical factor is to use very dry air. I'd move a large volume of air through a dehumidifier. I prefer the cooling approach of a dehumidifier since it separates the water from the air. You might wish to then raise the air temperature no higher than 50° C to further dry the air before gently blowing this through or over the polypropylene. Infrared heating could also be done in the drying area but again to no higher than a safe temperature (50° C). Depending on how fine of a powder(?) and the volume of material to be dried, I would consider a mixing agitator action to the process.

Vacuum drying might be a good way.

I would suggest you click on the Search Globalspec link in the upper right corner of your screen and search for plastics drying equipment.

This is a mature industry and there are companies such as Nucon (not a recommendation, I'm just familiar with the name) readily available on Globalspec that can give answers for your specific needs. You will likely wind up needing to procure equipment from one of them anyways, and since you must be dealing with thousands of kilos to make it worthwile to recycle a juryrigged system will probably cost more than it's worth.