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Column Diameter and collection rate


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Alright you are all free to laugh at me for this question.  But I have been trying to figure this out for a while and it is driving me mad.  So i'm looking at different pot stills and they all say a certain diameter column, yes these are pot stills with like 12" x 2 ft columns.  Now they all say upgrade to the 16" column for yadayada dollars.  Now it seems plain and simple bigger column diameter higher collection rate.  But here's where my  brain goes into cart wheels, almost every still on the market(for craft distilling) has a column diameter and at the top of the column the pipe going to the condenser is 2".  So if everything has to squeeze into 2 inches does the previous column diameter really make a difference? And i'm just talking pot stills, vodka stills and bubble plates makes sense to me why a wider diameter makes a difference. Also i'm not talking about the shape of the Onion or neck I understand those make a difference in reflux for say whiskey vs. rum.   But pot stills with a round column I don't see it.  Eventually it all has to go essentially into a 2" diameter column.  My thought is that in these stills the only thing that would effect collection rates would be the amount of energy you put into the system.

Now the only thing that I can think of that would make a difference is the wider column allows more vapor and that the vapor velocity in the 2" pipe is just increased and the larger column really is faster.  But couldn't you just put a 2" column on the top of the still and the vapor velocity in that 2" column would match the 2" piping at the top of a 16" column.   

I'm sure there is some obvious answer to this, please anyone explain this to me.  You'll be saving my sanity.

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For a still with plates and bubble caps you are quite correct,  if column too narrow the vapor velocity is too high and liquid will get carried up the column reducing effectiveness of the plates.  With no plates as in an alembic still there is no need for a column. It will work more efficiently at separating ethanol without one. The main reason for the column or neck is for reaction with copper especially when making whisky to remove sulphur. The wider and longer the neck the more surface and also a cooler surface that allows more reflux and hence reaction with the copper. 

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I posted this on another forum discussing whether or not whiskey helmets make any sense at all.  What is the difference between a stainless cylinder atop a pot still and a beautifully bulbous copper helmet atop the same pot still?  They are, practically, more similar than they are different.   Here was my theory and opinion on it.


Remember that helmets and other capita on stills were invented and used before plates were.

Helmets rely on an old theory of rectification called dephlegmation, as compared to the fractional distillation that we talk about today. Coincidentally, this is where the dephlegmator (partial condenser, reflux condenser) inherited it's name from.

Dephlegmation is rectification through passive contact between liquid and vapor passing each other, versus fractional distillation which utilizes active contact (think plates and trays). If you are saying to yourself right now, so ... a packed column is a dephlegmator - you would be correct based on the original definition and use.

But, move backwards in time, and forget everything you know about distillation today. Those old dead guys couldn't make shell and tube condensers, they probably didn't have any way to include active cooling to a reflux condenser either (at least not for a few hundred years).

So, now think of the funny shape of a helmet. Why? Why go through all that trouble for the funny shape? What the hell does the funny shape mean? Well, it has significantly more surface area than a pipe, no? With more surface area comes more passive heat transfer, and with more passive heat transfer (on the outside), we get more condensation (reflux) on the inside.

Now we have the liquid side of dephlegmation taken care of, also consider that the funny shape induces turbulence in the head, and thus increases vapor-liquid contact.

In addition, all this increased surface area increased copper contact of the vapor.

So put yourselves in their shoes, 800 years ago. You ain't making trays or a reflux condenser, but what you can do it put a big bubble at the top of the still to increase passive reflux and induce more vapor/liquid interaction (dephlegmation) - and you would have a better performing still. Especially so running after harvest, in the cooler fall air - passive reflux would be maximized. Heck even Fischer esterification would be amplified.

But, in terms of distillation, we're not even talking about providing a full plate worth of distillation - a helmet alone will not result in higher purity than two runs would.

So why bother?

Flavor, primarily.

Keep in mind one thing, one very important thing, the impact produced by adding additional distillation stages (plates, runs, whatever) - is NOT LINEAR. THIS IS HUGELY IMPORTANT. Imagine rectification as being like distance.

The distance between Zero plates and 1 plate is a football field long. The distance between 1 plate and 2 plates is significantly less, probably half the distance of a football field. The distance between plate 10 and 11 is now the size of the football. The distance between plate 20 and 21 is a human hair. And so on.

Now, think of this in terms of flavor (purity) output. The difference between Zero plates and 1 plates is huge.

This is why helmets, passive reflux, and things like thumpers can make a big difference, a noticeable difference. You have a whole football field of flavor to play with. If the "distance" between distillation stages was linear/fixed, these methods would have never been invented, as their impacts would be largely unnoticeable, like trying to determine the difference between running 20 and 21 plates only using taste and smell.

This is the reason why we see variations in helmet shapes, and sizes, and lyne arms that skew up and down, and all of those crazy 1700s and early 1800s still designs. When you are playing on the football field between pot still distillation and adding one additional theoretical distillation stage, you have tons of variations that absolutely will have an impact.


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its not just a matter of column diameter, but also plate design.

a poorly designed plate will need to be bigger in order to get the throughput, but then the vapor velocity suffers.

a perforated plate column, should be slightly smaller in diameter in order to drive the velocity higher, or else you wont achieve balance in the column and the fluid will fall out.

it is a balancing act to make one of these systems work, there are some rules of thumb, tricks and workarounds, but it all comes back to the math.

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Thank you silk city distillers that was very insightful.  I never really thought about "the football field" of different flavor.  There really is a lot that can happen in that distance.  But, I still have a question.  From what I am gathering from your information is that the flavor of the different size columns will be different.  The larger column has more surface area and will have more dephlegamation(in the old sense of the word).  So If anything the larger column diameter would actually have a slower collection(output rate) than the smaller column?  Since it has a higher reflux ratio.

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15 minutes ago, flyhigher87 said:

The larger column has more surface area and will have more dephlegamation(in the old sense of the word).  So If anything the larger column diameter would actually have a slower collection(output rate) than the smaller column?  Since it has a higher reflux ratio.

Not if you use more power... With more surface area you get better separation when using the same amount of power, or you get the same separation with more speed when using more power.

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