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?

  • $\begingroup$ Economically and ecologically would be difficult as then you'll have to dispose out of a liquid waste. $\endgroup$ – Alchimista Feb 10 at 8:32
  • $\begingroup$ @Alchimista Na, this "waste" can directly be fed into a refinery after you have filtered out the solids (which you can feed into the OPs reactor a second time, where they are hopefully broken up a bit more). $\endgroup$ – Karl Feb 10 at 11:18
  • $\begingroup$ @Karl. I am speaking of a DIY scenario as for the question drives in that direction. Where is the refinery, for instance. This is unlikely a chemical plant diagram scenario. I see that your answer basically says the same. If there would be a scenario as you are thinking of, probably the Q wouldn't be asked. $\endgroup$ – Alchimista Feb 10 at 11:41
  • $\begingroup$ @Alchimista Well, yea. I don't exactly get that either. You can't sensibly do this process outside of a rather large-scale chemical plant, which is ideally located directly next to a refinery. I'm guessing OP is working there, but his employer doesn't tell his workforce why things are run the way they are? $\endgroup$ – Karl Feb 10 at 12:15
  • $\begingroup$ @Alchimista you've guessed that correctly. The plant isn't a large scale chemical process and we don't have any provision to utilize this 'waste'. If we are able to find a solvent that can dissolve this, we'll have to find a safe way to dispose that off. $\endgroup$ – Sasha Feb 14 at 1:23

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.)

  • $\begingroup$ Thanks for your recommendation. I'll give that a try by using petrol ether or toluene/xylene on a small sample of that waste wax to see how it goes. So do you suggest to raise the temperature of the hardened wax and then try these solvents. However raising the temperature and exposing it to oxygen is not safe, as this material (wax) catches fire at its autoignition temperature of 240 deg C. I am not sure if we can change the feedstock to liquid alkane prior to shutdown as it might affect our catalysts (not sure), but that's an interesting proposition. Will ask my process engineer. Thanks $\endgroup$ – Sasha Feb 14 at 3:12

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