Reflux Reboiling

Vapor, Liquid or Cooling Management. Flutes, plates, etc.

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skow69
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Reflux Reboiling

Post by skow69 »

The video shows the top of my column coming up to temperature under full reflux. I am certainly no expert, but I have played with it enough now that I think I can speak somewhat intelligently about it. All comments, discussion, disagreements and alternate theories are welcome. My only purpose is the advancement of our marvelous craft.



The column is 1 1/2 inch i.d. packed with stainless scrubbies. The head is a combination VM/LM Nixon-Stone offset. Power applied is about 1725 watts. The charge is 40% ABV. I try to adjust the heat to maintain a vigorous boiling action above the packing. This seems to be the sweet spot. I take it as an indication that the most volatile components are short cycling around the head, rising as vapor to the reflux condenser, pouring through the return tube as liquid onto the packing, immediately boiling and rising to the RC again.

The boiling action always exists only in the top 1 inch of packing. I can see bubbles developing inside the packing and being pushed out. I attribute this to the slight pressure created by the packing, the same resistance you feel when you breathe into your column to check the packing density. I think we'd be talking about a fraction of an inch of water column, but enough to prevent boiling. At the top the pressure diminishes and the same mixture is able to boil.

If I turn the heat down to 1700 the activity sinks into the packing like in reverse of the video. If I increase the heat enough I will hear a gurgling boiling sound coming out of the top of the reflux condenser. The thermometer in the VM arm goes up at the same time. I suspect this happens as the boiling liquid blocks the opening of the reflux return tube (sound) and licks the thermowell. 1825 watts is enough to do it if it's allowed to stabilize, so the sweet spot window is only about 100 watts wide.

Taking off product requires increased heat input, as you would expect, but very little, like 25 to 50 watts. As the alcohol is depleted from the wash the power requirement goes up, but not in a linear manner. It starts slowly and accelerates over time. By the end I can barely keep up with it. It'll be up to 2200 or 2300 when the tails arrive.

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Red Rim
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Re: Reflux Reboiling

Post by Red Rim »

Nice write up!
I have found the power width to be very narrow as well, but I only monitor it by sound, the sound of my marbles bouncing.
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Re: Reflux Reboiling

Post by bearriver »

skow69 wrote: so the sweet spot window is only about 100 watts wide.
For weeks I have pondered on this. My still also has a narrow sweet spot where there is a fluidized bed above the packing, just prior to flooding. (Lava rocks giggling and making noise) "Sweet spot", just about nails it, as the performance I'm getting from running in that fashion are absolutely astounding.

I'm thinking that the HD community needs to coin a term for this configuration...
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Re: Reflux Reboiling

Post by Kegg_jam »

From this description, I believe that I need to try applying more power. I've been inching my way to higher power settings then get nervous and back it off never quite achieving the boil you refer to.
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Re: Reflux Reboiling

Post by thatguy1313 »

I've had similar results in my column. Running just below flooding, when I can here the lava rocks doin the salsa I know I'm running her right.
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Re: Reflux Reboiling

Post by Kegg_jam »

Hope this isn't a stupid question, but are you guys taking fores and heads then going for the aggressive boil? Or just going for it straight away.
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Re: Reflux Reboiling

Post by DAD300 »

Previously the term for the phenom videoed was that the cold reflux was "flashing" on top of the hot packing.

If the flashing makes too large a pool, it will just fill to the takeoff and over flow. In a VM that means you loose control of the takeoff. In an LM, even though the ABV may go up, you'd be smearing.

In the SPP world, the partial solution is to provide a less dense packing on top to allow the reflux to penetrate a little before flashing. This usually means a SS Scrubbie fluffed on top of the denser SPP.

Good video Skow...

Most report a higher ABV when walking this line...but product might be "cleaner" at a lower power setting that allows for deeper reflux penetration and more redistillations. It seems the easier it is to make azeo, the less I want too, as I find better tastes and flavors with blending in the 90-94% range.

This sure plays in with googe's post about vapor speed and column feeding at the bottom.
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Re: Reflux Reboiling

Post by BigSwede »

Fascinating, and thank you for posting this. It makes me happy I bought a 2" glass sight tube to put at the top of my reflux column. You get a lot of information from watching the action in that way.

One thing that strikes me, and Dad mentioned it, the height of that "flash reboil" is surprising, and reinforces the notion that for certain processes, especially VM, there needs to be head space between the top of the packing and the take-off arm, or yes, mega-smearing will result. Looks like about 5" or so would be appropriate.

It's hard to see from the video, but is the SS scrubbie below that active boil area completely devoid of liquid? That's surprising to me. Obviously, vapor is pushing upwards vigorously, but the whole theory of reflux would seem to require a liquid/vapor interface throughout the packed column, certainly not just at the top. Maybe it's on such a micro-scale that we cannot see it? Don't know.

Again, nice video! :clap:
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Re: Reflux Reboiling

Post by DAD300 »

The video (I think) also makes a perfect argument against centering devices. Imagine the increased vapor speed and reflux in a centering device half the size of the column. The liquid would never make it past. I don't care if reflux goes down the column wall, as long as it goes down.

Temps are higher at the bottom because of proximity to boiler, but ABV is higher at the top, so it takes X degrees more to boil. Bottom ABV may be 10%-40%, but at the top it is 55%-95%. So the boiling temp within column with 10% wash varies by ~15 deg F bottom to top.

So, in order to get that reflux down the column further, causing more liquid/vapor interaction...you can try to manipulate the temp from top to bottom.

I insulate the bottom of my column and not the top, to purposely create a temperature variation. Hoping it helps the reflux penetrate.

Also, diameter can be used for same...a 4" column dia can have more temp differential from center to outside wall than a 1". Especially if the reflux IS channeling. Almost like using an exterior reflux return but more energy efficient and easier to build. I think this is why the rule of thumb DIA VS COLUMN HEIGHT favors larger dia. I need a taller 2" column than a 4" column to get the same HETP.

Or- A 2" column 39" high packed with SS Mesh gives 8.5 plates. A 4" x 39" column with same gives 11.4. You can horse that around by changing power, but 30% more HETP for same height... I think it is vapor speed that makes the dif. With all the other equipment the same, keg, 2" connection, the 4" dia column slows the vapor speed allowing for a lot of good things to happen.

Another point I tend to harp on is supercooled reflux. First glance would say the cooler the reflux, the deeper it will penetrate. But if the reflux is too cold it cools the packing enough to flood the top. So, there is a dance there also.
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Re: Reflux Reboiling

Post by rad14701 »

DAD300 wrote:Another point I tend to harp on is supercooled reflux. First glance would say the cooler the reflux, the deeper it will penetrate. But if the reflux is too cold it cools the packing enough to flood the top. So, there is a dance there also.
I think there have been several instances of members not reaching optimal performance due to supercooling... All that is required is enough cooling to adequately knock down 100% of the vapor and nothing more... Supercooling is just as bad for a reflux column as shock cooling is for a pot still... They're the same only different... Both cause major performance problems related to huffing... With a reflux column the reflux floods the top of the packed column until boiler pressure blows the flooded spirits out the top - a very dangerous situation... With a pot still the vapor in the boiler collapses momentarily and then builds again, causing huffing and surging which is far less dangerous but problematic nonetheless...
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Re: Reflux Reboiling

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BigSwede wrote: It's hard to see from the video, but is the SS scrubbie below that active boil area completely devoid of liquid? That's surprising to me. Obviously, vapor is pushing upwards vigorously, but the whole theory of reflux would seem to require a liquid/vapor interface throughout the packed column, certainly not just at the top. Maybe it's on such a micro-scale that we cannot see it? Don't know. :clap:
I think this gets to the heart of the whole issue, and I have spent a lot of time staring at that tube trying to understand it. The packing below the top is not devoid of liquid, but the packing is so dense that it hides in the middle and is impossible to see. The reason we can see it in the top inch is because there is so much vapor being created that the bubbles forming there are held down by pressure from bubbles forming above them and so they get forced out to the walls. Remember a volume of liquid expands like 1500 times when it vaporizes.

I intentionally made two voids in the packing down lower in the column for observation. At all times I can see liquid moving down through them. However, I find it interesting that it is just a small fraction of the total reflux, like maybe 10%. My reflux return tube goes inside the fitting and turns straight down in the center of the column, so without the boiling I can see almost all of the reflux in sort of a single stream. Not like a faucet, it twists all over in the turbulence. I'd say it's not as big as a whole pencil, but it is many times bigger than a pencil lead. 18" below there, at the first void, it's about one drip every 2 seconds, plus a little film on the wall sometimes. It is small enough that if you just glance at it, you could think it is dry. You have to stare at it and wait for something to happen. So apparently the majority of the reflux doesn't get that far. It also makes me wonder if maybe the lion's share of the column's work isn't being done in the uppermost section.

More later. I gotta set up the potstill. :ebiggrin:
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Re: Reflux Reboiling

Post by BigSwede »

Very cool, thank you for explaining this. It's surprising that the volume of downflowing liquid is apparently so small. Maybe the bulk actually wicks, something hard to see.

And something else to take away, vapor velocity in small columns, when you have too much heat, is absolutely going to inhibit a proper reflux. Liquid is simply not going to travel downwards when bombarded with high velocity vapor. This is intuitive but we tend to forget just what is going on in there in a desire to increase throughput. It's like using a packed column in pot still mode... blow enough heat in there, get the vapor velocity and pressure up, and the entire packed column becomes isothermal, no gradient, no reflux at all.
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Re: Reflux Reboiling

Post by skow69 »

Kegg_jam wrote:Hope this isn't a stupid question, but are you guys taking fores and heads then going for the aggressive boil? Or just going for it straight away.
Not a stupid question at all. Some guys do "sneak" the heads out before equilibrating. I use the column still mainly for neutral which I want as hi-test and clean as possible. I have no interest in vodka, I use it for absinthe. Everclear is pretty cheap. If it was acceptable, I wouldn't need a column. Anyhoo, that is why I stabilize the column and let the volatiles stack up before collecting anything. I think I get the heads compressed better that way.
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Re: Reflux Reboiling

Post by Kegg_jam »

Thanks skow.

I've been taking a smallish fores collection upon heat up. Then equalizing, taking another fores collection just to be safe, then collect as usual and make cuts after airing.

still developing my technique so l'm trying different things and checking out what others are doing.
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Re: Reflux Reboiling

Post by skow69 »

I hear ya. There are so many aspects that can receive a totally different treatment depending on perspective or emphasis. Not long ago marbles were universally panned for packing. Cross tubes were state of the art. You can't make whiskey in a reflux still. You can't make a VM without a valve. Is flooding good or bad? Top or bottom? Is it conditional? It's easy to make a very convincing argument for equilibrating at minimum power. Etc. Etc.

I think this just means that we are a long way from having this all figured out, and when we look back in ten years we are gonna laugh at what we're doing now. Maybe five years.

Did you see this thread? I had all but forgotten about it until somebody bumped it a couple days ago.
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