How to turn plastic waste into diesel fuel cheaply

[Babataku]

cast iron

Thanks for the replies.

I was thinking of using Cast iron for the Firebox. I'm not sure though how this will affect time to heat up the retort and if this will solve steel oxidation problem. Has anyone tried this out? I don't have any local suppliers for stainless steel.

And also Excalibur, using refactories and creating a flame path like you did...how does this affect heating up time? Do you use more fuel to get to temperature and maintain that temp? And by the way on your runs how long does it take you to get to 400deg?

[Excalibur]

Agreed, overheating steel is going to decay it. The retort contents will try to reduce the temperature of the steel but extreme applied heat will mean the liquid won't be behaving in any kind of calm fashion. It's likely to be dancing on the hot spot/s.
Babataku, your 4mm steel plate shields will have nowhere to lose their heat so they will sit there and glow super hot, oxidize and eventually crumble.

When my steel refractory burnt out last year I moved away from direct flame on the retort vessel. Instead I heat the refractory bricks and give the flame somewhat of a 'torturous path'. I call this "softening the heat" and the firebox is now more of an oven than a blast furnace.
In addition I moved the burner head further away.
For some time I've had a TC on the flame inlet duct so the burner can be adjusted to give the required flame temperature to match retort conditions.

Suggestions:
Fit a TC on the flame so you know exactly how much heat you are dealing with.
Use refractory bricks as a type of heat storage and direct the flame front onto them.
Work out a way of controlling burner temperature, perhaps multiple or adjustable diesel jet/s.

[wheels]

Another question:

I am currently using a Diesel Burner for heating my retort. And the rate of metal fatigue is unbelievable! The Flame is not in direct contact with the retort...I have installed some 4mm mild steel plates in between the burners flame and the retort which can be easliy changed out but I'm just amazed at the rate at which the metal just degrades...

My question is: can I expect my retort (6mm Mild steel) to suffer same fatigue from the heat? Since its not in direct contact with the burner flame? If I was using electric heater bands would this even be a concern?

I am thinking safety here!

Any comments and suggestions are welcome please.

If you do not have direct flame on the Retort, it should be OK. But you need to keep an eye on the Retort, both inside and out. Outside damage if any, will be from the Burner. Internal damage can be caused by Acids, which some plastics create. So if by accident you get the main feedstock contaminated by a piece of PVC for instance, then Chlorine and Hydrogen Chloride will be created and with the retort at such a high temp, any Acid will work double time on steel.
So basically, the Flame is the rapid oxidation, via a chemical reaction, of the Diesel fuel and Oxygen. Rust/Corrosion or steels is exactly the same thing, except it happens very slowly. So now if you take the flame and blast it onto Steel, you dramatically speed up the Oxidation process. So steel does not last long.
Not all, bit most Stainless steels will handle heat without oxidation.
Most austenitic steels, with chromium contents of at least 18%, can be used at temperatures up to 870°C and Grades 309, 310 and 2111HTR (UNS S30815) even higher. Most martensitic and ferritic steels have lower resistance to oxidation and hence lower useful operating temperatures. An exception to this is the ferritic grade 446 - this has approximately 24% chromium, and can be used to resist scaling at temperatures up to 1100°C.

[Babataku]

metal fatigue

Another question:

I am currently using a Diesel Burner for heating my retort. And the rate of metal fatigue is unbelievable! The Flame is not in direct contact with the retort...I have installed some 4mm mild steel plates in between the burners flame and the retort which can be easliy changed out but I'm just amazed at the rate at which the metal just degrades...

My question is: can I expect my retort (6mm Mild steel) to suffer same fatigue from the heat? Since its not in direct contact with the burner flame? If I was using electric heater bands would this even be a concern?

I am thinking safety here!

Any comments and suggestions are welcome please.

[rozier56]

rozier56

Thks Excalibur.

[Excalibur]

Excalibur, just wondering why you connected your retort/reflux vessel's with a 3 inch pipe between them? Why not connect the reflux vessel directly to retort top?
Also noticed that you have a long pipe leading from the reflux vessel to the condenser! What is the point of this,is it to assist cooling ?
Your retort vessel is made of what thickness material?

I reasoned that the 3inch connecting pipe would give the best chance of creating and maintaining the two distinct vessels at their respective target temperatures. These vessels work in harmony and sympathy with each other and are finely balanced. Finding the balance that gives correct results is the trick and may require some trial and error. Directly connecting the vessels or indeed using the very same vessel with the lower part in the hot zone and the upper in a cooler zone has been the subject of earlier discussion on this forum. I see no reason why it would not work though I've not tested the theory. What's critical is the temperatures, not so much the shape, size or distance apart.
The long pipe leading from the reflux just serves as a bit of extra initial cooling before vapor/condensate gets to the heat exchanger. The pipe was used at the exact length it was picked up from the steel rack, not so much by design.
From memory my retort wall thickness is 8mm, retort floor is 10mm and flanges about 22mm each.

[rozier56]

rozier56

Excalibur, just wondering why you connected your retort/reflux vessel's with a 3 inch pipe between them? Why not connect the reflux vessel directly to retort top?
Also noticed that you have a long pipe leading from the reflux vessel to the condenser! What is the point of this,is it to assist cooling ?
Your retort vessel is made of what thickness material?

[wheels]

My understanding is that oil refineries use vacuum distillation when they want to avoid cracking. For us we need to reach cracking temperatures to achieve finished product viscosity. My last run had retort temperature mainly between 380°C & 420°C.
Shell Petroleum handbook says cracking commences at 350°C so a feedstock that boiled under vacuum below that marker would be below cracking threshold.
Col, my retort/reflux is tall and skinny so quite the opposite of yours. I think either design will have strengths and weaknesses, pros and cons.
I'm adding/removing reflux insulation to achieve an average target so I'll allow it to drift high or low but so long as it averages out then my fuel SG will be within spec.
Re cracking temperature, there will be a sweet spot where the feedstock cracks to diesel at a good rate but without causing unwanted problems from driving the process too fast (wax, gum, excess off-gas etc). It would be a mistake to think that more heat was always better.

Commercial refineries vary around the world, but the one common similarity is that they all have very complex and controlled means of cracking that we don't have. It also depends on what range of oil is being refined. Not all refineries take a full crude product ad not all Crude Oils are the same.
One of the reasons (just one of the many) that they crack in a vacuum is no oxygen present to oxidize chains till they can be cracked into what they require as a final product.
The temperature of cracking a chain depends on two things of which both come back to temperature. A long chain requires a higher temperature to release it from the base stock. Lets just say that is 360deg. A short chain may release earlier with a much lower temperature, lets day 150deg, but being more stable, needs far more heat to vibrate it enough so it breaks by temperature alone. Lets say that breaking temp is once again 360deg.
Very high temperatures can cause many problems. So being able to crack at lower temperatures allows a more stable product to be produced. under a vacuum, the temperature can be greatly reduced as I described earlier.
In commercial refineries, as far as I know, every stage is employing a catalyst for controlled cracking. Plus many refineries use high pressure high temperature Steam. Steam is a great way to control the temperature of the environment.
Boiling points of heavy chains can be very high. As much as 400+degC. WMO is very unlikely to have Chains of that weight. But you still want to have an operating temperature high enough to get rid of the very last of the heavy weight stuff from the bottom of the retort. If you don't, the carbon left behind will be spongy and oily when cold, and not dry hard carbon.
A reflux needs to be hot enough to allow the lighter fractions to pass on through, but cool enough to allow the very long chains to condense and be recycled back to the Retort, so they can be reheated and bounce around some more and hopefully crack. A Catalyst needs a working temperature and so you would want that part after the Reflux and heated to it;s requited temp.

[Excalibur]

My understanding is that oil refineries use vacuum distillation when they want to avoid cracking. For us we need to reach cracking temperatures to achieve finished product viscosity. My last run had retort temperature mainly between 380°C & 420°C.
Shell Petroleum handbook says cracking commences at 350°C so a feedstock that boiled under vacuum below that marker would be below cracking threshold.
Col, my retort/reflux is tall and skinny so quite the opposite of yours. I think either design will have strengths and weaknesses, pros and cons.
I'm adding/removing reflux insulation to achieve an average target so I'll allow it to drift high or low but so long as it averages out then my fuel SG will be within spec.
Re cracking temperature, there will be a sweet spot where the feedstock cracks to diesel at a good rate but without causing unwanted problems from driving the process too fast (wax, gum, excess off-gas etc). It would be a mistake to think that more heat was always better.

[Col]

Controlling the temperature of the reflux

Hi guys
Thanks for all the help so far.
My retort is 440mm diameter. My reflux is the same diameter, though much shorter. I am wondering if I will have sacrificed uniformity of temperature across the diameter of the reflux by going so wide. Oh well, will never know because I didn't put TC's in there, just one at the top of the exit pipe.
Is anyone actually heating / cooling their reflux to control the temperature or just insulating it and adding / removing insulation? Do you find there is a need to baby-sit the reflux temperature or once you get the physical dimensions and distances right (ie - the 'sweet spot') is looks after itself?

[wheels]

Am I right in saying vacuum would be of little benefit to us in cracking plastic or oil into diesel? We could boil out the diesel fraction that already exists in the oil under vacuum thereby reducing the amount of heat required to recover that part of the diesel fraction. But we are relying on heat and time in heat to crack the remaining long chains into shorter ones so if we applied vacuum we may reduce the temperature to the point where cracking does not occur. So even though the high temperatures can lead to destabilization we need those high temperatures to get the cracking done. Our alternative is catalysts right?

Correct in that you will not achieve what you want to achieve. Vacuum reduces Boiling point and that is all. So under a Vacuum, you can achieve getting the plastic to release the Gasses at a much lower temperature. But that is all. Nothing else really changes.
Col, if you read my comments in reply to Rozier56, I described that increasing heat causes the Molecules to bounce around faster and thus they take up more space which is what we see as expansion. So now if you were able to get your hands around a bunch of those molecules and squeeze them, you restrict there ability to move around. This is what pressure does. So you now need more heat to get them to overcome your squeezing and create space again. Air pressure is doing the same thing as our hand would be. It is squeezing the molecules with a force of 14.7 PSI or 101.3Kp. If we take all that air pressure away, the molecules can now bounce around much more freely.
Plastic and WMO work very differently in a Pyrolysis environment. You are not actually pyrolysing Oil. You are distilling it. Where as Plastic, being a solid, gives up it's hydrocarbons differently once it reaches a high enough temperature. With WMO, the very light fractions, of which have a low boiling point, will vaporize first. Each longer chain will require just that little more heat to vaporize it. So you will never get just the Diesel range without the lighter fractions. You can stop heavier chains by not allowing the heat to rise above 260degC.
When it comes to cracking, that is something very different and is done in two ways. By increasing heat, we are causing those Molecules to bounce around energetically again. The heavy weight fractions are more correctly called "Long Chain Hydrocarbons". If you make a long chain bounce around vigorously with heat energy, it will eventually break. The "eventually" part means it takes time and that time is not precise. Breaking a chain is what is called cracking. But heat does not cause the chain to break in a precise way each time. It tends to be random, so you can get chains of all types.
The other way is using a Catalyst. This causes a chain to crack by a reaction. Using a Catalyst will cause the chain to crack in a more precise place and produce more of what is wanted than unwanted. But getting that part right so you get exactly what you want is difficult or even impossible for us Amateurs.
I hope that helps.

[Col]

Vacuum distillation / cracking

Am I right in saying vacuum would be of little benefit to us in cracking plastic or oil into diesel? We could boil out the diesel fraction that already exists in the oil under vacuum thereby reducing the amount of heat required to recover that part of the diesel fraction. But we are relying on heat and time in heat to crack the remaining long chains into shorter ones so if we applied vacuum we may reduce the temperature to the point where cracking does not occur. So even though the high temperatures can lead to destabilization we need those high temperatures to get the cracking done. Our alternative is catalysts right?

[Excalibur]

An added benefit of a near horizontal heat exchanger is there will be no hold up of product after the run. If I want to accurately test my results from each batch I will need to end up with all process liquids split off into their respective holding tanks, not mixed up in the condenser.

Agreed. I think a condenser that freely drains into the diesel tank is the best compromise.

Snippet:
Interestingly while my diesel tank target temperature remains at 70°C which of course I control via condenser coolant flow, I had a dramatic drop in petrol production last run. I attribute this to the waste mineral oil feedstock having significantly less volatile content (solvents, etc.) Also boil-over episodes have disappeared. I noted that diesel product begins to darken in the sight glass with retort temperatures above 420°C. I hope to retest this next time out.
Also I aim to start out with a near empty retort and trial running the level much lower. My retort level indicator which I have now pictured on DiyDiesel is working much better than earlier iteration so am hoping to test various levels back-to-back for comparison.

[Col]

Condenser

An added benefit of a near horizontal heat exchanger is there will be no hold up of product after the run. If I want to accurately test my results from each batch I will need to end up with all process liquids split off into their respective holding tanks, not mixed up in the condenser.

[Excalibur]

Col, I pondered over similar thoughts when I looked at heat exchanger / condenser orientation. I wanted it vertical mounted but the downside was my shed wasn't high enough. I didn't favor a riser because of the back pressure issue. I elected to mount it inclined from horizontal. Any liquid product that reaches it will favor the 1 or 2 lower tubes. Vapor can choose any of the tubes.
I believe it will be mostly liquid because of the several meters pipe run to initially get to the h/e. More recently I elected to run this pipe closer to horizontal. The h/e was also re-oriented with shallower fall. The reason was to force the liquid to travel slower and have a longer residence time.