Propylene Glycol Double Boiler

[myles]

For any of you that are interested I have been experimenting with a propylene glycol double boiler. Have more tests to run but these results are encouraging.

My outer bath is bigger than what i wanted but it is ok. A 27 litre aluminium pot with a 3kW element. The pot is encapsulated with fireproof expanding foam with a fibreglass outer skin for durability.





My inner boiler is a thin stainless steel 20 litre stock pot with a copper bowl fitted, ends up with a 2" slip coupling for whatever gets attached - column or pot still.





For this test I putt 22 litres of plain water - no alcohol - in the inner boiler with 15 litres of propylene glycol in the bath. No column fitted just a lid on the boiler. Am just testing thermal properties and did not wish to add in cooling from reflux etc. Heated up with the full 3kW untill the bath reached 120 degrees then turned off the power.




The boiler hit 120 degrees at 75 minutes into the run, at which time the wash was at 84.5 degrees. With no further energy input the wash peaked at 89 degrees some 20 minutes later.

Over the next 2 hours the wash temperature dropped by 9 degrees centigrade. Not bad insulation in my opinion.

Now the reason I am thinking about this is all to do with column stabilisation for heads concentration. What I think is that you could warm up at full power. Factor in the overshoot caused by the stored heat in the propylene glycol and then dial back the power input to just ballance the extra cooling introduced by your reflux.


The propylene glycol is a fairly efficient thermal buffer so you don't need much energy input. With really good insulation you could stabilise for an hour or two with zero energy input :8)


Then take off the heads and crank up the power. This could be usefull I think.

[Oracle]

myles,

Curious about your selection of propylene glycol. I have also been interested in trying alternatives to water (not ready for steam) in order to get the bath temperature up to around 225°F to 250°F. In the open bath (electrically heated), it doesn't seem to matter what I try I always end up with irritating vapors even though the fluids do not reach boiling temperature. I've tried glycerol, various mineral oils, silicon oil, propylene glycol (auto antifreeze grade), and they all do the same thing. Some are more irritating than others, but none seem very appealing. If I had a closed system (with a pressure relief valve like a steam kettle) or a way to pipe or "fan" the vapor outside it might not be so bad, but I just cannot see using this sort of thing in an enclosed environment. I have yet to try peanut oil, but expect the room will soon smell like a fried chicken restaurant if I go that route. Your assembly certainly looks like you have minimized the exposed bath fluid, but it does not appear you are sealing the inner pot over the outer pot sheath (or am I wrong?). If you do get a basic seal, do you have vapors escaping or any pressure build up?

I take it you are happy so far with the propylene glycol. Can you elaborate on how you chose it and how you are making it work for you? Have you completed a full-blown spirit run with this assembly?

[myles]

Well oracle I thought about it for some time.

Propylene glycol is safe for food use – I don’t really want anything toxic where it could potentially contaminate something I am going to drink.

In my younger years I was a chef and my wife still is. I am familiar with oil baths and steam jacketed boilers etc and decided early on that oil was not an option. Smell, flashpoint (which should not be allowed to become an issue) and autoignition point – which could possibly be an issue.

Most catering oils have an autoignition temperature of 285 to 385 degrees centigrade. For propylene glycol it is 371 degrees.

As you say I have minimised the exposure to bath fluid. How I did it was like this: I extended the aluminium pot that is the outer boiler with a 3mm thick aluminium collar that sticks up vertically to increase the height of the pot. This has a band at the top to increase the thickness to 6mm. The collar extends an inch down inside the pot and there is a double band to take the width to 9mm where it actually rests on the rim of the outer pot.

The outer pot has a curved over top section as it ORIGIONALLY was a double boiler anyway. I just replaced the inner pot with a stainless stock pot. The extension to the stock pot rests ON TOP of the collar. The underside of this extension has a thick layer of silicon as a gasket. The full weight of the contents of the boiler presses down to make an effective seal between the collar and the stock pot.

I have not got around to insulating the collar as when I built it, the intention was to use steam, and it was supposed to re-condense on the collar. I will be insulating it.

The bath is exposed to atmosphere – but in a sneaky way.
The access to atmosphere is via the refill port that I use for my temperature probe. This port reduces down to ½” and the end is BELOW the surface of the bath in the boiler. In effect the only surface exposed to atmosphere, is the surface area inside the refill port – this limits the odour etc. Expansion of the glycol can take place and the level changes both inside the boiler and inside the refill port.

Now propylene glycol boils at 188.2 degrees centigrade. So far I have never taken mine above 140 degrees. This helps to limit the odour release etc. Also if you boil it you can get localised burning on the element and then need to filter the glycol.

I am not sure how much of a temperature gradient is needed – but if you go too high you will risk burning the wash again – and avoiding that was the whole point of the double boiler in the first place.


I am still 'playing' with it to try and optimise the thermal storage properties of the glycol so NO I have not done a full spirit run yet. I keep recycling it back into the boiler for the next test. For pot still use it works fine, but I am trying to get it right for 2 hours 100% reflux on a column at the start of a Rum run to increase the esterification. I am testing with a basic sugar wash for now, so as not to waste a good molasses wash.

[myles]

Well for pot still use this is absolutely brilliant. With the element controller you can dial in any bath temperature you like and then stabilise it at that temperature for the entire run. I think it is so good that I am going to build a new inner pot. Am going to go for a design with a bigger opening so that I can fill it with thick mashes and get my hand inside for cleaning.

For reflux - well you can do it BUT the inherent time lag is a PITA. I do not think it worth it - the response times to heat input changes are buffered by the propylene glycol.

If you want to distill on the grain, then I would say do this instead of building a steam jacketed double boiler. This is very controleable indeed. No steam, no smell, and pick any temperature gradient you like. I have only taken my bath to 140 degrees but in theory you could go to about 170 degrees.

[Oracle]

Thanks for the update. I am likewise interested in thick mashes (and in gin/absinthe recipes). I am going to give this approach another shot. I picked up a couple of gallons of propylene glycol (veterinary grade) and am in the process of assembling a new outer boiler using an aluminum stock pot. I'll go with an electric element I was previously preparing for another application, just need to determine the best way to mount it through the wall. I am curious about the insulation you used. What is the source for the high temperature foam insulation? I know you constructed the outer shell using fiberglass, but the inner foam is what I am curious about. I am tempted to simply use a second, larger pot as an outer jacket and fill the space between with insulation (like your foam, or even something like perlite). The bigger challenge is that I am going to try and make the inner pot using a 5 liter copper alembic. I figure it is already available and if I can make it modular then it could be used for direct gas or electric heating and indirect (ala the glycol bath). I'll post pics when I have something worth sharing. Ought to be a sight to see: a pot in a pot in a pot...

[myles]

Sorry for the delay, have been away for a bit. On the insulating foam issue I used 'fire rated expanding foam', this is the stuff you use when you are installing fires or chimney flues into the brickwork to seal up the gaps.

A word of warning though. I used normal expanding foam on a metal shroud on my steam kettle which is heated with gas. The foam melted at the hot surface and peeled off. Now I suspect the exhaust temperature from the gas burner was much hotter than the temperature I will ever get the propylene glycol up to, but it is worth bearing in mind. Am not sure what the temperature limits for the fire rated foam are, but I have not seen any problems so far.

I have come across another alternative that might be of interest. You can insulate with a false rock made from a combination of fire cement and something like perlite or vermiculite. I am re-insulating my steam kettle shroud with fibreglass type material and am using a thinned down fire cement instead of the usual resin. I may finish off with a layer of the fire cement / vermiculite mixture if it is needed. Doing it that way you would not need the second aluminium pot on the outside of the insulation.

Having loose insulation like perlite between two pots should work fine also as an alternative.

One last point about the glycol though. You might want to ensure that there is a good seal between your inner alembic and the wall of the glycol bath. There is a bit of an odour to the glycol, and I gather some people are sensitive to the glycol fumes. The design I used virtualy eliminates any exposure to vapour, whilst still allowing for thermal expansion without a pressure build up in the bath.

[tas63426]

I use regular cooking oil from sams. It's cheap and works fine I heat with wood and the oil gets up to about 275F degrees and slowly rises as the alcohol content goes down never had it go above 300f. also never noticed any smell. it tends to oxidize over time and it needs to be topped up. I do all grain mashes only and strip with a quite a bit of solids in the wash. but never had a problem with burning. I would recomend it to every one.

YB

[myles]

I thought some of you might be interested in this. I recently reconfigured the pot still so that it now looks like this:



The odd multi angle section is there just to take in a 2 foot sideways shift. I tried to do it simpler but it would have meant cutting wierd angles. This uses standard couplings and works just fine. I modified the parot to fit 1 galon demijons.



I just ran a strip run with a basic sugar wash to test the parot. Heated at 3kw until I started to collect product. There were no cuts, everything was counted.

20 litres of basic sugar wash in the boiler, electric element set to 3kW, and the glycol bath ended up at 131 centigrade when the first drops of distilate appeared.

It started at 60% with a distilate temperature of 28 centigrade. Max flow rate on the condenser cooling. I collected to 30% by which time the glycol was at 136 centigrade. At a collection rate of just under 5.2 litres / hr.

Then turned off the element but left the condenser running and continued collecting on the residual heat in the boiler. I stopped with the bath temperature at 104 centigrade.

Total yield was 3.65 litres at 40% ( between 60% and 30% I got 3.2 litres at 45% )
Might be of interest if you are thinking about the glycol. Not sure how that compares to direct immersion heated, but even with thick washes I think you could run the glycol to 130 - 140 degrees and not get burning. To get the same temperature with steam, would need 40 to 45 psi.