A theoretical doubt about equilibrium

[Birrofilo]

When a column reaches equilibrium, we can imagine every fractional alcohol "sitting" on his plate together with other similar molecules. I get that.
Let's imagine a still is in thermal equilibrium: the heat which is applied at the base equals the thermal dissipation of the still. The temperature remains stable in every spot of the still. The kind of alcohol that we find at any spot doesn't change. At any given "level" we find, theoretically and ideally, a certain fraction of alcohol.

What I don't get is why, when alcohol is withdrawn from the upper plate, the lower alcohol fractions should climb up the column, if not because of a higher heat applied to the alcohol itself.
In my mind, if a certain fraction is heavier than the higher one, e.g. hearts being heavier than heads, thus condensing lower in the column, in a point (or points) of the column with a higher temperature, when heads are withdrawn I don't understand why the lower hearts should condense at a different temperature. They should go on condensing and refluxing at the same height of the column, if the column temperatures at the various heights remains the same.
Or to say it in another way, a certain fraction condenses at a certain level - corresponding to a certain temperature - of the column, "without knowing" whether above it the alcohol has been whitdrawn or not.
If the fractions sitting lower in the still "climb up" that should happen only because more energy was applied to the alcohol (maybe, the same energy is applied to a minor quantity of alcohol, so more energy is applied to the alcohol in the column).
If, instead, the various "layers" of alcohol remain each at the same temperature, I don't see why they should climb up the still. Nor I see how can their temperature raise if there is no additional heat added.

Said differently, I imagine a column in equilibrium as a column where it is possible to withdraw each fraction at each own height, or it is possible to have each fraction "climb" the column only by applying more heat.

Other than that, the climbing of the fractions toward the top seems "magic" to me.

Any clarification?

[LWTCS]

Because the operator continues to transfer material in the form of vapor into the apparatus. During 100% reflux mode, there is only so much available space with which the transferred constituents can occupy.
Couple that with the idea that the constituents with the lowest boiling point must occupy the highest available space.
Once space is not available, then those lighter constituents will displace any higher boiling point constituents. This is column enrichment.

This behavior is easily under stood during 100% reflux mode.

It gets a bit more nuanced when the operator begins to draw off product.

[LWTCS]

Said differently, I imagine a column in equilibrium as a column where it is possible to withdraw each fraction at each own height, or it is possible to have each fraction "climb" the column only by applying more heat.

Other than that, the climbing of the fractions toward the top seems "magic" to me.

Any clarification?

Sorry, I re read my answer and realize it was not so great.
But yes your final thought is quite good for basic understanding imo.

[acfixer69]

I moved this because there were so many assumed facts that just made answering the post with a good answer not possible with out some editing. That said as I started to answer line by line my answers would contradict the paragraph before it.


I am moving this back to the forum in hopes that one of us will be much better at tech writing then I am.

[DAD300]

There are failures or limitations in a "hobby" still that make equilibrium a misnomer.

At 100% reflux we are transferring the top most element as a liquid down to the next or possibly to the next three fractions as reflux. We are attempting to separate them as best a particular still will. At 100% reflux we are dropping liquid back into the boiler.

To reach equilibrium, you would have be able to draw off the element desired (heads) and replace it with new heads ascending the column. If your column could hold all the heads your boiler could produce you could stop reflux and draw them off . But, liquid to vapor, the heads in a keg let's say a half gallon of condensed liquid, the vapor is hundreds of times that. We don't have columns near big enough to hold it.

Even still, Truth is, we get a pretty good separation when you look at the limits of hobby equipment. And we do better than most any commercial distillery.

The other problem is that we can't make a hobby still release each fraction individually, at any given time, no matter how precisely you control the power/temp. At any given power/temp in the boiler, the liquid components will stratify or separate, forming layers. Even when it is boiling there is a mixing of the components.

An example of this would be a commercial still with an agitator. If I start the agitator in a 200 gallon boiler, while taking off hearts, I will get a very large smear of heads. But you say, " The heads are already gone." NOPE, just most of the heads were gone. Those remaining will suddenly be released and flood the column smearing the hearts.

[HDNB]

At 100% reflux we are transferring the top most element as a liquid down to the next or possibly to the next three fractions as reflux. We are attempting to separate them as best a particular still will. At 100% reflux we are dropping liquid back into the boiler.

exactly. the reflux pushes the work load down, the heat doesn't push it up.

An example of this would be a commercial still with an agitator. If I start the agitator in a 200 gallon boiler, while taking off hearts, I will get a very large smear of heads. But you say, " The heads are already gone." NOPE, just most of the heads were gone. Those remaining will suddenly be released and flood the column smearing the hearts.

exactly again. the problem with the concept of equalization is thinking you are going to collect all the heads and they're gone. you can stack a higher load initially but you won't get them all. in theory if you pulled from the plate verrryyy slowly at verrryyyy precise controlled temp, for a very long time...you may be able to pull a higher % of nasties.

what i mean is: if you could stack all the acetone on the top plate and then release into a "fores cut" ...by that thinking you should also be able to stack all the ethanol at 173* and it would all come pouring out the spout at once...but experience tells us we have to distill for hours to get the ethanol out.

equalizing as a practice is more about smoothing out the run for me, so my transitions are smooth and i don't get a big heads bump in the middle of hearts like DAD suggests from the big change of an agitator turning on mid run.

[MoonBreath]

We are all students, we all may create and answer threads, but we are all students..I know I am.
Pros are rate driven, extreme distillers that have to find better ways stay competitive, not no slow drip there, they stay on the cuttn edge on a larger, sometimes less efficient scale.
With bigger, come bigger hurdles..
Home stuff is different..Inefficient Efficiency is what it is..Come a long ways.
Squeezin shots/heads bout best you can do for cleaning.
Maintaining equal pooling without flooding while adjusting for your stream is the goal.
I believe in the stack theory, temp based..
I also believe a riser of up to 12" above your plates enhance the 'stack'..I've tried 3", 6", 10", and 12" risers and found the longer two just does something.. Better separation, more stable reflux, enhance temp control, and predictable run temps, basically fractions with the inefficiency of alcohol runnin backwards.
Aka: proper stacking.

[Big Stogie]

Moon Breath where are you putting the riser above or below the RC?

[MoonBreath]

Below..Always keepn the reflux at highest point before top elbow.
Just works better for me than direct reflux basically right on top of your plate..Chimney effect maybe?
A little copper mesh goes along way to, lookn for proof.

[zapata]

What I don't get is why, when alcohol is withdrawn from the upper plate, the lower alcohol fractions should climb up the column, if not because of a higher heat applied to the alcohol itself.

It climbs because you made the withdraw from the upper plate. Before the takeoff, it was the balance of heat from the boiler and cooling reflux from the upper plate (technically the latent heat of evaporation needed to vaporize that liquid reflux, not just the temperature of the liquid) that was keeping the lower fractions where they were, by keeping the temperature steady. Remove the upper fraction, you remove it's cooler reflux, and everything moves up a stage.

In my mind, if a certain fraction is heavier than the higher one, e.g. hearts being heavier than heads, thus condensing lower in the column, in a point (or points) of the column with a higher temperature, when heads are withdrawn I don't understand why the lower hearts should condense at a different temperature.

First off, "heavier" has little to nothing to do with it, it is all driven by temperature, energy, and partial vapor pressure. And no pure compound or specific mixture ever condenses or boils at a higher or lower temperature in a hobby still. So if by "hearts" you mean either pure azeotropic etoh, or some other specified composition, whatever temp it vaporizes or condenses at once, it will always be at that temperature unless you change the components of the mixture (or the pressure). So if I understand what you mean by "I don't understand why the lower hearts should condense at a different temperature" then the answer is simple. They don't.

They should go on condensing and refluxing at the same height of the column, if the column temperatures at the various heights remains the same.

But they won't stay the same. The top of a column is cooler before fores are taken than heads, and cooler with heads than hearts. Likewise there is a temperature gradient down the column that is stable under full reflux, but the gradient changes throughout the run.

Or to say it in another way, a certain fraction condenses at a certain level - corresponding to a certain temperature - of the column, "without knowing" whether above it the alcohol has been whitdrawn or not.

Not a complete picture here. The temperature at any given height of the still is a direct result of the balance of reflux coming down and vapor coming up. The reflux is how a fraction "knows" if there is an upper fraction or not. Remove the upper fraction, you remove the influence it has on all lower fractions.

If the fractions sitting lower in the still "climb up" that should happen only because more energy was applied to the alcohol (maybe, the same energy is applied to a minor quantity of alcohol, so more energy is applied to the alcohol in the column).

They climb up when there is nothing to prevent them from doing so. Do you know how a dephlegmator works? A partial condenser? Think of every drop of liquid reflux in a still as it's own dephlegmator. But instead of a stillman deciding how to run the dephlegmator, the liquid reflux is pre-programmed by it's boiling point, it's vapor pressure, and it's latent heat of vaporization. As a drop of liquid dephlegmator, it falls through the column absorbing energy and thus condensing vapor until it reaches it's programmed set point to vaporize and stop being a partial condenser. But it's like a game of tag, every molecule of vapor that it condensed is now it's own dephlegmator dropping down the column absorbing energy and condensing vapor it contacts. So if a fraction at any level doesn't have sufficient liquid reflux "dephlegmators" interacting with it, it rises.

If, instead, the various "layers" of alcohol remain each at the same temperature, I don't see why they should climb up the still. Nor I see how can their temperature raise if there is no additional heat added.

I think I hit both of these already. They climb because they are being "pushed up" from the boiler more than they are being "pushed down" by liquid reflux. The temperature of any given component or mixture never rises above it's boiling point.

Other than that, the climbing of the fractions toward the top seems "magic" to me.

Any sufficiently advanced technology appears to be magic to the uninitiated. I personally like to both understand science while not devaluing the magic I'm witnessing. Keep in mind that the history of distillation goes much further back in time than the history of science, usually being carried out by priests and alchemists. So yes, distillation is magic. But no worries, for we are magicians.

Hope I helped.

[bluefish_dist]

This may be a simplified version, but as I see it, by running a batch process we always push the lightest component to the top. This is how a column works, we have a temperature gradient even if it's small that stacks the components by boiling point. As we deplete one component the next lightest rises to take its place. In a continuous column this doesn't happen since you are adding new material with all components. So a continuous column stacks and stays that way. But a batch column slowly depletes the components by weight

I can see this on my vodka column with the fores starting about .5 deg F lower than the hearts. Then when the hearts are depleted the temp raises again even with increased reflux. The temp changes are small, maybe .1 deg F.
You can measure the change if you have accurate enough equipment. But at that point you also need to account for pressure changes during the run. Probably more work that it's worth. With enough runs and accurate enough equipment you could probably do cuts by temperature alone. But I bet the accuracy required is well beyond anything but a laboratory.

[Birrofilo]

Thanks to all for the answers.

I think I saw some light. I'm not sure I grasped it entirely correctly.

Yes, the way I imagined a theoretical column could have worked with a very, very, very tall column with a decreasing temperature along the height of the quasi-infinite column. I would imagine the various kind of vapours to condense at various heights, and not being able to move upword because the temperature in the "upper floors" is too cold to allow them to climb up as vapours.

Now I think I see my mistake. The column is of finite height. The dephlegmator is the factor that pushes down the condensing vapours, all condensing vapours, the factor is not the "diminishing temperature" at "higher height" of the column.
Heat coming from the kettle is what would keep the vapour going up and up, not only to a certain height of the column. Vapours, once they are vapours, would climb higher and higher and higher, and would condense somewhere in the sky, instead they will condense at most in the dephlegmator, or by the contact of some cold drop falling from above.

The "working" of the column - dephlegmator, and all the condensing-evaporation that happens in the column - will slowly "sort" the components in proper order within the column. At equilibrium, a certain fraction doesn't "sit" in a certain spot because that's the spot with the temperature corrisponding to its point of condensation. It will sit there because, in this queue, that's their place in the queue, their place in the set of cards.

What stops them from reaching the top of the column in the equilibrium state is the fact that the column is full of more volatile components, which fill the upper floors. If you remove the higher content of the column, the upper column has an "empty" and the vapours coming from below will climb to the top and will reach the dephlegmator, where they will be condensed, bringing a great disorder in the process. It's as if I were mixing a sorted set of cards with a reshuffled set of cards. I will have to re-sort all the cards anew.
So the lower vapours reach the top simply because the top is now "freed" from the previous occupier, not because there is an increase in heat. The vapours below "push" up the column, but if the column is filled, they will be stacked and form an ideal queue.

Another factor that I did not consider is that the boiler contains more than the column. The boiler continuously sends all the fractions to the column, and the column continuously works to separate them, through the continuous evaporation-condensation in layers.

In an ideal state of equilibrium all the foreheads in the column are on top of the column. Before bleeding, all the foreheads of the column are ideally stacked on top. But the column doesn't contain all the vapours of the kettle, and so it is able to "sort", after great labour, only a small quantity of vapour in the column at a time.

What happens is that a certain quantity of mash, a certain "batch" of vapour, vapourised along the column, is separated and sorted along the column within the capacity of the column itself. When a part of the content is withdrawn, some quantity (with mixed components) will climb up the column in its original mixed state, will create a chaos where it was order, and the column will begin again to slowly work to "sort" it into the proper order.

<< the reflux pushes the work load down, the heat doesn't push it up >>

I think I understand the answer here, but it explains to me more the "working" of the column in order to "sort" the various vapours in this column stack, before the ideal equilibrium is reached, rather than the state of equilibrium itself. In my understanding, ideally, when the equilibrium is reached, there shouldn't conceptually be any "up and down" motion any more, because every fraction should - if I get it right, which is not certain - ideally be "floating" in its own place in the column (a very ideal column, and a very ideal situation). If we imagine a continued "whirling" in the layers, in a state of equilibrium this "whirling" should be imagined as very tight, with very small volutes.

In an ideal very tall column with an ideal very small quantity of mash, I would expect the fractions, after some hours, to reach a state of theoretically motionless stacking, a bit like one can see wine sitting on top of water after hours of letting it still. If I insert a new mix of water and wine, that will alter the equilibrium and it will take hours to have again wine sitting on top of water.

I think I understand correctly how in a reflux column every single spot is ideally like a micro-potstill where a separation (evaporation and re-condensation of only a part of the drop) happens, and the distilling effect that vapours "coming from below" have on each single drop "dropping from above" and separating into the more volatile part of the drop evaporating and the less volatile part of the drop dropping. But after a long-enough time, this continuous separation work should theoretically come to and end. I see it as being like sorting a card deck. Once it's sorted, it's sorted.

But I don't have an ideally tall column with a large enough volume to contain the entire mash.
Instead what I have is a very small column with a very large mash. That means each time the column is filled with a vapour, that vapour is only a part of the total mash. After a long time the quantity of vapour in the column is "sorted", all neatly ordered, but If I withdraw some product from any height of it, the new "mixed vapours" from the boiler will invade the column and a great reshuffle will happen, and all the vapour will climb up to the dephlegmator, and fall for condensation, and climb again by the vapourization from heat from below, and fall again from condensation from cold they meet, until, after again some time, the state of equilibrium is reached anew for this new small quantity of vapours.

That's my understanding so far. I am not sure it's all correct, but I think it's better than when I asked the question.

[Single Malt Yinzer]

Said differently, I imagine a column in equilibrium as a column where it is possible to withdraw each fraction at each own height, or it is possible to have each fraction "climb" the column only by applying more heat.

As a side note, this is how a continuous column works. The fractions are drawn off at a specific height/plate. There's a couple variations on that but it's the general idea.

[Bushman]

Said differently, I imagine a column in equilibrium as a column where it is possible to withdraw each fraction at each own height, or it is possible to have each fraction "climb" the column only by applying more heat.

As a side note, this is how a continuous column works. The fractions are drawn off at a specific height/plate. There's a couple variations on that but it's the general idea.

One of our craft distilleries in my area has this set-up on their big still. Would love to stick my finger under the different take-offs to see how well it works.

[zapata]

At equilibrium, a certain fraction doesn't "sit" in a certain spot because that's the spot with the temperature corrisponding to its point of condensation. It will sit there because, in this queue, that's their place in the queue, their place in the set of cards.

Thats one way to look at it. Another slight variation is that it is the composition of the fraction at any one place that determines the temperature of where it is sitting. At equilibrium the fraction will maintain it's boiling point where it "sits". Which should also make it obvious that when room opens up above and a fraction moves up the still, it takes it's higher boiling point up with it.

But keep in mind that distillation is a process of energy and motion. Even at the most ideal equilibrium, every molecule is in a dance of vaporizing and moving up before condensing and moving down. It is the motion that transfers the energy through the system, without the fractions interacting it doesn't work. Which is why column design from packing to bubble caps or sieve plates is designed for maximum interaction of vapor and liquid. The better the interaction, the smaller the distance the fractions move up and down. This is reflected in the HETP (height of equivalent theoretical plate).