Pot Still Vapor Temperature

[ParrotHead]

I'm very much a novice at home distilling, and I'm just about to start my first brandy wash with a Concord grape wine base. But I'm also a chemistry geek and I have access to a GC in my job as a lab tech for an industrial ethanol producer. As it happens, I just did a set of tests using very pure ethanol mixed with other compounds, diluted with water down to 15% ABV and distilled on a heating mantle using a glass retort and a Graham condenser. This simulates a pot still. I've seen some stuff you might find interesting, and I'll post my data in its own thread soon.

The first post that NZChris linked to talks about how methanol concentrates in the tails more than in the heads, compared to ethanol. If you look at the third image, you'll see something that is the opposite of conventional wisdom about which compounds end up in which cuts in a single pot still run: at ethanol concentrations that occur in any fermentation (<25%), the most volatile alcohols are the fusel alcohols isobutyl alcohol and isoamyl alcohol (aka 3-methyl-1-butanol) while the least volatile is methanol. Never mind the fact that isobutyl alcohol boils at 108 C and isoamyl at 131 C, while methanol boils at 65 C and ethanol at 78. This is true: I have seen the data and will post it in a thread soon.

In a single pot still run, the longer-chain alcohols like isobutyl and isoamyl end up disproportionately in the heads/foreshots (along with acetaldehyde and ethyl acetate, which make up most of the headsy smell) while methanol ends up disproportionately in the tails. This is because water pushes out the more hydrophobic compounds including the fusel alcohols while pulling in the most hydrophilic ones like methanol, and this effect is more powerful than the difference in boiling points for pot still runs. The hydrophilicity/hydrophobicity effect diminishes so that the opposite is true for high ABVs, as you'll see if there is lots of reflux or if you are distilling something above about 50% ABV. But for a pot still, the order is the opposite of what you would think from the boiling points.

The flame test is wrong not just in that it's an unreliable guide to methanol, it's the opposite of correct: methanol burns with a clean blue flame, while the more hydrophobic stuff burns orange because it contains more carbon to generate soot which glows orange at typical flame temperatures. People have learned to associate orange flames with fores and heads because there is lots of carbon-heavy stuff in those fractions. But that's not where the methanol is - it appears throughout the run but is somewhat more concentrated in the tails.

Thank you for such a wonderful explanation. So if methanol boils at 65 C, is it safe to say that under the hydrophilic conditions, its coming off in the water/ethanol tails portion? early tails or late tails (<40% ABV) ? So these high temp boiling fusel alcohols, isobutyl and isoamyl, actually are not boiling, they are just extremely volatile and come off early in the run? So then it makes sense that this is why its not safe to do a single pot still run and take tails cuts. Is this also why you should do a high ABV (35-40%) spirit run after your stripping run? - so that you get better "cut placement" of these fusel oils and methanol? You would expect methanol to come off in the foreshots/heads in less hydrophilic conditions of higher ABV spirit runs.

[WithOrWithoutU2]

I believe still head temperatures are important to controlling the process, but they are not useful for cuts.

+1

[NZChris]

I believe still head temperatures are important to controlling the process, but they are not useful for cuts.

+1

Don't go taking that out of context. Bluefish wrote about that in regards to a reflux column, not about pot stills.

[WithOrWithoutU2]

I believe still head temperatures are important to controlling the process, but they are not useful for cuts.

+1

Don't go taking that out of context. Bluefish wrote about that in regards to a reflux column, not about pot stills.

I believe he was referring to column distilling overall. Temps, along with other data points, can be useful to know what is going on with your still even with column pot distilling. An easy example on my column I know once I get to around 120F at the head, it will be mere moments before product starts to come out. I also know, if I come up to this 120F to fast, my collection rate will be too fast(to much smearing) from the outset. I appreciate others who have developed the skills to read these things without head temp. While this has nothing to do with making cuts, it does have to do with controlling the process.

[Brew bama]

.... Temps, along with other data points, can be useful to know what is going on with your still even with column pot distilling. ...

+1. I believe complete temp control — slow even heating and even the capability to hold at a given temp, if required — can be a very useful tool in the toolbox regardless of equipment or method used. It’s not the only tool, just one of many.

[Tummydoc]

+1. I believe complete temp control — slow even heating and even the capability to hold at a given temp, if required — can be a very useful tool in the toolbox regardless of equipment or method used. It’s not the only tool, just one of many.

Spoken like a PID user. In pot distillation you can control power input and therefore vapor speed. But you cannot control the temperature of the boil or the vapor temp. Those are determined by the components of the liquid. With a PID you can underpower your boiler, let the vapor speed slow to a halt, and the head temp will drop, but only because you aren't pushing vapor at that point. Its why many of us say a PID is not the best tool for power control in distillation.

[Brew bama]

Spoken like a PID user.

Love ‘em for precision process controls.

.... In pot distillation you can control power input and therefore vapor speed. But you cannot control the temperature of the boil ...

? That depends on where your temp probe is. If it’s installed at a location to sense the temp of the liquid in the pot you can absolutely control the temp of that liquid at any set point, boiling or otherwise.

With a PID you can underpower you boiler, let the vapor speed slow to a halt, and the head temp will drop, but only because you aren't pushing vapor at that point.

Exactly. Which is why it’s a very useful tool

... Its why many of us say a PID is not the best tool for power control in distillation.

I guess there are many ways to skin a cat. To each his own I’d say.

[jonnys_spirit]

In distilling a PID usefulness is really limited to bringing the temp up to somewhere below the boiling point of the still charge in which case it just technically just heating - not distilling. This can be useful in a mash environment but if you actually intend on distilling the boiler charge then a PID is the wrong tool because the vapor temp in a pot still is controlled (ir limited) by abv of the boiler charge not by varying the power input. This has been covered in detail already.

Cheers!
-jonny

[Tummydoc]

You aren't controlling the temp of the boil. Put water in your boiler and make it boil at something other than 100 deg C at 1 atmosphere pressure with your PID.

[Saltbush Bill]

Spoken like a PID user. In pot distillation you can control power input and therefore vapor speed. But you cannot control the temperature of the boil or the vapor temp. Those are determined by the components of the liquid.

100% correct Tummy it takes a while for that concept to sink in for a lot of new distillers.

[Brew bama]

I guess we simply agree to disagree.

Not sure how bumping up a PID controller is any different than turning up heat any other way, manual or otherwise, but OK.

[jonnys_spirit]

If you’re not sure how it’s different then you should read up on it and do your homework. It’s been discussed ad infinitum around here. Simply stated - you can’t change the temperature that a mix if alcohol and water (wash/low-wines) boils at. This is a property of physics. Input power determines amount of vapor not boiling point. Using a PID to attempt to control the boiling point is an amateur mistake no matter how much you want to believe that physics somehow behave differently than they do.

Good luck!
-jonny

EDIT:
It’s not a matter of agreeing or disagreeing. It’s simple physics.

[Saltbush Bill]

Beer brewers are always right......it's that simple.....even if they started distilling last week.

[WithOrWithoutU2]

You aren't controlling the temp of the boil. Put water in your boiler and make it boil at something other than 100 deg C at 1 atmosphere pressure with your PID.

+1

People get confused about "boiling point" with how vigorous you can get a boil.

[LWTCS]

That depends on where your temp probe is. If it’s installed at a location to sense the temp of the liquid in the pot you can absolutely control the temp of that liquid at any set point, boiling or otherwise.

Not of you want it to boil, as it will only boil at one temperature until you start taking product.
For our purposes the boiling point of the kettle charge will change as you render out the alcohol. Therefore setting a target temperature will ultimately always leave alcohol in the kettle unless you simply adjust your target temp to overshoot and at that point your "controller" is rendered as effective as an on/off toggle.

[Brew bama]

Physics?

A pot of liquid doesn’t care how the heating energy is applied. Whether that’s with a wood fire, propane, an induction cooktop, electric stove, or water heater element inside the pot.

A temperature output device registers temperature regardless of whether that temp is provided by a mercury ‘stick’ thermometer, an analog gauge, or pt100 probe with remove readout.

When I plug a temp probe into a box that provides a temperature readout, and based on that perceived temperature, calculates power required to raise the liquid from that temp to a higher set value by providing power to a heat source, I disagree that all of a sudden I am breaking the laws of physics.

Whether I am turning a dial or pushing a button with an up arrow on it likewise should not matter.

It is exactly the same as me manually looking at an analog temperature device and turning up the stove or adjusting a propane burner.

I understand that at any one point in time the PID control can set the steady state, but as soon as the alcohol in the wash drops, the steady state, and therefor the controlling characteristics of the system change. As the run continues using several cues such as the parrot, the temp, and sensory analysis (sight, smell, taste), I bump up the SV.

I’m sorry but I don’t see how I’ve broken any laws of physics.

[Brew bama]

Beer brewers are always right......it's that simple.....even if they started distilling last week.

Finally, we agree.

[Brew bama]

That depends on where your temp probe is. If it’s installed at a location to sense the temp of the liquid in the pot you can absolutely control the temp of that liquid at any set point, boiling or otherwise.

Not of you want it to boil, as it will only boil at one temperature until you start taking product.
For our purposes the boiling point of the kettle charge will change as you render out the alcohol. Therefore setting a target temperature will ultimately always leave alcohol in the kettle unless you simply adjust your target temp to overshoot and at that point your "controller" is rendered as effective as an on/off toggle.

Agreed. I understand the dynamics taking place. It was simply an example to an absurd comment.

[Tummydoc]

Bama:
"Agreed. I understand the dynamics taking place. It was simply an example to an absurd comment."

"you can absolutely control the temp of that liquid at any set point, boiling or otherwise. "

Bama, this is the absurd comment. You are going to control the boiling temperature? Only if you can walk on water first.

[Brew bama]

Bama:
"Agreed. I understand the dynamics taking place. It was simply an example to an absurd comment."

"you can absolutely control the temp of that liquid at any set point, boiling or otherwise. "

Bama, this is the absurd comment. You are going to control the boiling temperature? Only if you can walk on water first.

Do you increase heat as the run continues?

So do I. ...but I push a button vs turn a knob.

Not sure how that creates such animosity but I guess that’s ok.

[jedneck]

Adding power is not adding heat

[LWTCS]

Bama, explain the circumstances that an operator would choose to increase the input during a run.

[Brew bama]

Adding power is not adding heat

???? [emoji1021] I give up.

Running power thru a heating element produces heat. A heating element converts electrical energy into heat through the process of resistive (otherwise known as Joule heating). The electric current passing through the element encounters resistance, which produces heat.

[Tummydoc]

Adding power is not adding heat

Jed, heat is energy, so adding power (kilowatts or BTUs) is adding heat. But its not adding temperature above boiling

[LWTCS]

Sorry Bama. Jed is not being cryptic on purpose.
He is leaving you an opening for you to explain that you understand how distilling actually works beyond the simplicity of evaporation.

[Stibnut]

Here is the thread I promised.

[NZChris]

I believe still head temperatures are important to controlling the process, but they are not useful for cuts.

+1

Don't go taking that out of context. Bluefish wrote about that in regards to a reflux column, not about pot stills.

I believe he was referring to column distilling overall. Temps, along with other data points, can be useful to know what is going on with your still even with column pot distilling. An easy example on my column I know once I get to around 120F at the head, it will be mere moments before product starts to come out. I also know, if I come up to this 120F to fast, my collection rate will be too fast(to much smearing) from the outset. I appreciate others who have developed the skills to read these things without head temp. While this has nothing to do with making cuts, it does have to do with controlling the process.

I do something similar, but by using my hand to feel how far from the top the vapor is getting. The actual temperature anywhere, isn't important.

[WithOrWithoutU2]

+1

Don't go taking that out of context. Bluefish wrote about that in regards to a reflux column, not about pot stills.

I believe he was referring to column distilling overall. Temps, along with other data points, can be useful to know what is going on with your still even with column pot distilling. An easy example on my column I know once I get to around 120F at the head, it will be mere moments before product starts to come out. I also know, if I come up to this 120F to fast, my collection rate will be too fast(to much smearing) from the outset. I appreciate others who have developed the skills to read these things without head temp. While this has nothing to do with making cuts, it does have to do with controlling the process.

I do something similar, but by using my hand to feel how far from the top the vapor is getting. The actual temperature anywhere, isn't important.

I use to do the same until I insulated my column and boiler.

[NZChris]

A pot of liquid doesn’t care how the heating energy is applied. Whether that’s with a wood fire, propane, an induction cooktop, electric stove, or water heater element inside the pot.

That's exactly our point

You don't need a PID interfering with the amount of heat input to successfully run a still on any of those heat sources.

I don't know if you've ever fired your still on wood, but the changes in heat input as the fire dies down then gets stoked can be a bit of a PITA. A PID has a similar effect, raising the % on when you raise the SV, then dropping the % on as the temperature get closer to the SV. I'd rather run with a steady heat input any day. If I'm running off grid, I don't have a choice, but at home I do, so my PIDs never get used on my stills.

[NZChris]

Don't go taking that out of context. Bluefish wrote about that in regards to a reflux column, not about pot stills.

I believe he was referring to column distilling overall. Temps, along with other data points, can be useful to know what is going on with your still even with column pot distilling. An easy example on my column I know once I get to around 120F at the head, it will be mere moments before product starts to come out. I also know, if I come up to this 120F to fast, my collection rate will be too fast(to much smearing) from the outset. I appreciate others who have developed the skills to read these things without head temp. While this has nothing to do with making cuts, it does have to do with controlling the process.

I do something similar, but by using my hand to feel how far from the top the vapor is getting. The actual temperature anywhere, isn't important.

I use to do the same until I insulated my column and boiler.

I can feel where the vapor is up to through an inch of fiberglass insulation.