How to turn plastic waste into diesel fuel cheaply

[Excalibur]

Col
Good on you for trying things.
I hope the preheat/centrufuge experiment works well. I can see the motivation for wanting to reduce carbon buildup. That's horrible stuff and a real dirty job.
After initially struggling with suspended water I started preheating the feedstock to 110°C+ and held it for sufficient time to drive water off. My strainer was about 100 - 200µ though the oil was pre-settled so that takes care of much of the sediment.
Depending on how much preheat you use, there may be some steam evaporation. Perhaps a way of water vapor removal would save it recondensing and dripping back into the feedstock. The worry is, can you achieve water removal and satisfactory solids removal on a single pass? Of course you can slow the feed down to the to the centrifuge but to go slower than the reactor wants to process it, slows down the whole system. It will be interesting..

[Col]

Pressure / Vacuum Relief

Great. Thanks for the input guys.
I have a large pressure gauge plumbed directly from the retort via a pipe-in-pipe jacketed cooler (maybe 1500mm long, been a while since I looked at it) as the gauge is not rated to 500 C. The smallest vapour pipe I have on the system (before a relief port made from a bath plug in the top of the bubbler) is 1 1/2" (2" pipes all the way to the chilled condensing tank). I will be experimenting with oil. So, big pipes, no plastic at this stage so no blockages expected for quite a while. Will need to check pipes later of course.
I'll give both the spring loaded relief valve and the oil column a miss and just rely on the bath plug. It is a long way down stream in the system but with 2" pipes all the way to that point I think I am safe from blockages using oil feed stock.
Am just putting together a pre-heating tank and centrifuge at the moment to get rid of the water and most of the suspended solids before feeding it to the reactor. I don't want to have to clean out that reactor very often as the design I have used is not very clean-out friendly . The commercial system I saw just filtered the oil through probably a 500 micron strainer then boiled / cracked it from there in a batching system so water content in the feed stock didn't matter. He only ended up with maybe 1 x IBC of dry carbon waste after his first 100,000L of feed oil. I think that is good going. But the less the better for me.
Col

[wheels]

Hi Guys
What is the best mechanism to relieve pressure from the retort in an emergency?
A spring loaded pressure relief valve seems to be the best as it relieves the pressurised gas then snaps shut while there is still some pressure in the retort which means there is no chance for air to enter the retort while the valve is open, avoiding an oxygen-fuel mixture leading to an explosion.
I am looking for a stainless steel pressure relief valve, hopefully not too expensive. I want to avoid any brass/bronze/copper in the retort.
I was wondering if some other mechanism could be used - perhaps a column of heavy oil generating the pressure (far enough away from the main heat of the retort so it doesn't distill away). If retort pressure spikes, the excess pressure pushes the oil column up into a mushroom-plate/splash plate/perforated plate/scrubs within a compartment larger than the volume of oil used which allows the pressurised gas to bypass the oil and the oil to fall back down into the column re-creating the 'plug'. The oil-plug should be back in place while there is still pressure in the retort, similar to the spring-loaded pressure relief valves.
This could be used as a pressure relief valve but it is also susceptible to vacuum pulling air into the retort during shut-down. Anyone know what level of vacuum can be expected at shut down if not managed with purge gas?
Thoughts?
Col

I have similar experience to excalibur. Relief valves are inherently problematic, because they are complex inside them with spring and ball etc, so easily block. I found two ways of doing this. First one was to use the water bubbler with a second exit pipe, but the outlet was deeper, thus more water pressure to overcome. However, if the tank became energetic, this also bubbled. Not that this is a problem and it still provided two a second source of relief incase the first blocked. In the end, I went with one very large exit pipe of 60mm and have never had a blockage. Plus I had a pressure gauge and the Vessel was capable of taking a high pressure. So if the gauge ever started reading high, then I could turn off the heat and let it all cool. Which I have never had to do.

[Excalibur]

More thoughts..

Placement of the relief valve is something to consider. If placed on/near the retort vessel, the additional piping will behave as extra reflux capacity. Similarly if it runs off the reflux. It will be like having a larger reflux and possibly it will be cooler than the reflux vessel itself.
Positioning it downstream of the reflux has the benefit of not interfering with the reflux function. A downside to this location is if a blockage occurred upstream then the relief valve would have nothing to release.

Give some thought to the autoignition point of the vapor. For example diesel with ignite above 210°C if sufficient oxygen is added even without an ignition source. The oil column idea may cool the vapor enough but it's still highly flammable if it happens to "find" an ignition source.

Hope this helps. Be great to hear how you get on.

[Excalibur]

I never fitted a relief valves to any of my prototype but I did seriously consider the pros and cons.

I recall an incident where a relief valve was fitted on a plant cracking plastic. It may have taken several runs but the valve was discovered to be plugged with solid plastic. Possibly vapor was continually condensing on the valve gradually building up reconstituted plastic.

The thought of a column of oil acting as a valve is an interesting idea. It would need to exceed the backpressure of any bubbler (if so fitted). Perhaps with a bit of thought it could be double acting, a relief for over pressure and vacuum for a cooling retort.

More a bit later as I recall the thoughts..

[Col]

Pressure relief valve

Hi Guys
What is the best mechanism to relieve pressure from the retort in an emergency?
A spring loaded pressure relief valve seems to be the best as it relieves the pressurised gas then snaps shut while there is still some pressure in the retort which means there is no chance for air to enter the retort while the valve is open, avoiding an oxygen-fuel mixture leading to an explosion.
I am looking for a stainless steel pressure relief valve, hopefully not too expensive. I want to avoid any brass/bronze/copper in the retort.
I was wondering if some other mechanism could be used - perhaps a column of heavy oil generating the pressure (far enough away from the main heat of the retort so it doesn't distill away). If retort pressure spikes, the excess pressure pushes the oil column up into a mushroom-plate/splash plate/perforated plate/scrubs within a compartment larger than the volume of oil used which allows the pressurised gas to bypass the oil and the oil to fall back down into the column re-creating the 'plug'. The oil-plug should be back in place while there is still pressure in the retort, similar to the spring-loaded pressure relief valves.
This could be used as a pressure relief valve but it is also susceptible to vacuum pulling air into the retort during shut-down. Anyone know what level of vacuum can be expected at shut down if not managed with purge gas?
Thoughts?
Col

[cornishscrimper]

Methane lovely ! Was thinking of taking Methane from a Bio- digester too. As for storage gas bags as opposed too cylinders. At least a gas bag will just flair and not send shrapnel everywhere. l wonder what the free cubic metres of gas (at atmospheric pressure) would be produced from a Kilo of plastic ? I guess the specific energy would be the same also ?

[wheels]

Is it possible to convert most of the output into gas rather than liquid fuels ? Say with the correct catalysts etc ? Have read through cannot see a mention of it.Thinking about off grid set up and using gas (not gasoline) to run generators, gas fridges , cooking and gas water boilers etc.

Simply raise the Retort temperature. The Hotter the process, the more cracking to Gasses.
The bigger issue is storing the Gas. And this is where some caution comes in. There is a range of gasses that come off. The mix varies with the process. Methane tends to be a common one.

[cornishscrimper]

Perfect ! Thanks

[Excalibur]

Is it possible to convert most of the output into gas rather than liquid fuels ? Say with the correct catalysts etc ? Have read through cannot see a mention of it.Thinking about off grid set up and using gas (not gasoline) to run generators, gas fridges , cooking and gas water boilers etc.

Not that I've tried but yes, I'm convinced you could do this and without any catalyst. The method would be to reduce reflux chamber temperature thereby only allowing non-condensable gas to escape downstream. The product would be something along the lines of LPG/propane. Settling on a reflux temperature setting would be a case of trial and error but I'd start with water cooling perhaps thermo-syphoning to an overhead radiator.

[cornishscrimper]

Is it possible to convert most of the output into gas rather than liquid fuels ? Say with the correct catalysts etc ? Have read through cannot see a mention of it.Thinking about off grid set up and using gas (not gasoline) to run generators, gas fridges , cooking and gas water boilers etc.

[Hoddamdg11]

Copper catalyst?

Another interesting snippet

Renewable energy: Scientists accidentally turn carbon dioxide into ethanol - Science News - ABC News (Australian Broadcasting Corporation)

[wheels]

FYI
see article at:
Fuel from Mixed Plastic to Be Tested in Marine Engines | Engineering360

While that is good, I am not sure if it will be practical. The reason being is the volume of Fuel Ships consume and the limited amount of waste plastic available to produce fuel from. For instance, one Commercial European Plant is unable to keep up with demand and is having to import waste plastic from the UK. But the UK has the same issue with three Commercial Plants and are importing waste plastic from China. As more and more of these commercial Plants are built, the more Plastic is needed and even though we think there is a colossal volume of Plastic produced, it is actually not the case when compared to the Volume of Fuel a ship uses.
Just for comparison, the Queen Mary consumes fuel at a rate of 1 US gallon per foot.

[Hoddamdg11]

Fuel from Mixed Plastic to Be Tested in Marine Engines

FYI
see article at:
Fuel from Mixed Plastic to Be Tested in Marine Engines | Engineering360

[wheels]

In the interests of safety, the question has to be asked..
If (for example) 304 stainless was used for the retort vessel, does the applied flame temperature have to be limited to 870°C at the point of contact?

No. That temperature is still well below any form of Failure. Metling point is ruffly twice the operating temp. Simply, 870 is a temp at which corrosion will take place. However, SST is only Stainless because of the presence of oxygen. So depending on the type of heat source, the way it is applied and the oxygen within the heating box, Corrosion rates will vary slightly between peoples designs.