In search of a safe and inexpensive solvent that can dissolve depolymerized polypropylene waste plastic I work at a place where we depolymerize post consumer waste polypropylene waste plastic through a thermal catalytic conversion process. During process shutdown we have to clear up the vessels which becomes a really difficult job as the depolyermized material which we call wax hardens without heat and have to hammered and scraped out which takes a lot of time. Therefore to make this cleaning process easy I am looking for a chemical clean up technique. Is there a reasonable and economical solvent that can dissolve/soften this wax?
Short answer: Not really, unless you can do the cleaning still at high temperature. Solid hydrocarbons tend to only become soluble rather close to their melting point.

The only reasonable solvent is petrol ether, i.e. a mixture of alkanes. Polar solvents obviously don't work, chlorinated solvents are ecologically problematic and only have the advantage of an increased boiling point. Toluene or xylene might be alternatives.

So basically what you should do is to switch your feedstock from polyolefines to liquid alkane (petrol, basically) before shutdown. I would however be amazed if your process engineers don't do exactly that anyway. Maybe they could do that for a little longer, or maybe your process should be run (at least from time to time) at a slightly higher temperature so solidified (i.e. crystallised) residues in the reactor can melt and be washed out.

And whatever remains after shutdown and cooling, you will have to scrape off by hand, sorry.

(A sloppily designed or maintained reactor with bad flow profile, insufficient/broken insulation/heating devices of course might or might not be fixable.)