Half a croissant, on a plate, with a sign in front of it saying '50c'
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Heat Pump Pyrolysis

Expansion of my Pyrolysis in a Fire Syringe idea
 
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Some plastics are easier to recycle than others. Turning an old plastic bottle is easy. Turning old synthetic clothing into new synthetic clothing, not so much.

For plastic which cannot be easily recycled, our least bad choice is pyrolysis, where we take plastic, heat it in a low-O2 environment, and it turns into hydrocarbon gasses, hydrocarbon liquids, and ashes.

Both the gasses and liquids have more value than the (waste) plastic which we started with, and the amount of ash produced is relatively small. The only reason pyrolysis isn't more popular is due to the not-insignificant amount of heat required. This heat is typically produced through either combustion or resistive heating, both of which have their downsides.

This idea is to pyrolyze plastic a different heat source, namely an air source heat pump.

Our working fluid (refrigerant!) begins at a low temperature and low pressure, and might be a gas, a liquid, or a mix of the two.

Our refrigerant first passes through a heat exchanger warmed by the atmosphere, which heats it to a tepid 20C. If some or all of the refrigerant was liquid, some or all of it may evaporate.

Our refrigerant then goes through a heat/mass exchanger, where hot but low temperature pyrolysis oil give some of it's heat and some of it's substance to the refrigerant.

The now warmish refrigerant is then compressed, using power from an electric motor. This compression causes it to experience adiabatic heating, which raises it's temperature to about 500C.

The 500C refrigerant passes through the pressurized reaction chamber, where it comes into direct contact with the plastic.

The hot refrigerant heats the plastic by contact, causing it to pyrolyze.

Still inside the pressurized vessel, the gasses, liquids, and ash are separated.

The liquid passes through a hydraulic motor. This motor's speed is controlled to get liquids out of the reactor as quickly as they are created, without letting gasses enter the motor's intake.

The gases pass through a pneumatic motor. This motor's speed is controlled to ensure that neither the pressure nor temperature inside of the reactor become excessive. If either gets too high, the motor is allowed to spin faster, consuming more gas, and lowering both the pressure and temperature in the reactor.

The electricity produced by the hydraulic and pneumatic motors reduces how much mains electricity the electric motor needs.

The (hot!) liquid which had passed through the hydraulic motor passes through the heat/mass exchanger, which cools it down. The less-hot liquid which reaches the bottom of the exchanger becomes "product."

The gas which passed through the pneumatic motor will be low pressure and low temperature, and some of it may have condensed to a liquid. Separate the cold gas from any cold liquid.

If we have enough cold liquid, then 100% of the refrigerant is this liquid, the remainder of the liquid becomes "product," and all of the cold gas also becomes "product."

Otherwise, 100% of the cold liquid becomes some of the refrigerant, some of the cold gas becomes the rest of the refrigerant, and the remainder of the cold gas becomes "product."

During system startup, almost any low-O2 inert gas could be used as the refrigerant, as long as we don't mind it's presence in the system's output.

I picked 500C for two reasons -- first, some of the web sites talking about plastic pyrolysis say it takes place between 400C and 600C, and second, the air temp inside of a diesel engine before fuel is injected is between 500C and 600C.

However, that range of temps for pyrolysis is assuming atmospheric pressure, and some of what I've read suggests that at higher pressure, less heat is needed.

If you make this machine, be careful and stay safe; don't forget that the running reactor will be full of hot high pressure flammable hydrocarbons.

goldbb, May 20 2023

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       The problem with this is that your "liquid" is not "product". Pyrolysis is focused on the yield of propylene and ethylene gas to be used as raw materials for polymerization. Harvesting heat from the effluent streams to help power the reactor is basic chemical engineering. That's what your suggesting, yes?
daseva, May 20 2023
  

       //full of hot high pressure flammable hydrocarbons//
Perhaps encase the whole thing in a shell full of an inert gas (the same one used for start-up)?
Since compressing a gas increases it's temperature, could you do the whole thing just with compression? Put plastic + inert gas (I'm thinking a light noble, so you can liquify everything else but the gas to separate) into a piston arrangement, compress the hell out of it, (step 3), profit!
neutrinos_shadow, May 21 2023
  

       Daseva, It is harvesting heat from both the effluent and from the atmosphere.   

       Neither the liquid produced nor the gas produced will be pure anything; they will need to be distilled.   

       neutrinos_shadow, great minds think alike: that was my first thought, which I posted as "Pyrolytic Plastic Recycling in a Fire Syringe".   

       The problems are that pyrolysis takes several seconds, and heat recovery.   

       Also, the inert gas will be cold after you stop squishing it.   

       Also, the inert gas will come out mixed with the pyrolysis gas, which can be solved either by separating the inert gas by distillation to reuse it, or by not caring, and allowing the inert gas to be replaced by pyrolysis gas over time.
goldbb, May 22 2023
  

       How are you compressing this gas to get 500C? Like you say, diesel engines do this, but they do it to a few cubic centimetres for a fraction of a second. So either you run a small pump for a while - where your heat will leak away, or you have an impractically large pump.   

       Also, operating with large temperature swings and temperatures like 500C isn't easy. Pyrolysis also looks messy as all hell, you'll get ethylene etc. but also a whole mess of much larger contaminants, especially if you're operating with a waste stream.   

       Much easier to use a huge insulated vessel and heat it conventionally or maybe harvest the exhaust of an existing gas burner etc.
bs0u0155, May 24 2023
  
      
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