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Still design


Troy A-Z

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Hey all, 

I am new here and being a Mechanical Designer I am curious about some of the design features of a still. So on a column still, is there any type of formula or rule of thumb to go by with the diameter of the column in relation to the diameter of the base/pot? I have read were people looking for an R&D still recommend using a 1/2 barrel (beer) and a 2" pipe for a column. I have seen larger stills with what look like 8" columns. Would it be detrimental to the over all process if you were to put too large of diameter on a smaller base?

 

Thanks for any info., I am kind of geeking out on the whole distillation process.

 

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This subject was discussed in

Unfortunately that thread got a bit messy with some irrelevant side-issues causing a bit of bickering and hair-splitting.

Basically the situation is that the diameter of the column and the size of the pot are determined by different factors, so there is no fixed ratio between them.

The column diameter is mainly determined by the vapor velocity up the column. In a small R&D column of around 2" diameter the vapor velocity will be in the region of 6 to 10 inches per second, but on a large vodka column of say 10 ft diameter you can get vapor velocities of 6 to 10 feet per second.  So the column diameter is determined by the rate at which you want to run.

In a pot still the pot size is determined by the heating method and the size of the batch you are working with.  Let us imagine your fermenters produce 100 gallons per batch.  You will probably want a pot of around 150 gallons to be able to boil this safely.  If you want to process this in 12 hours you will need a column roughly double the diameter (4 x the area) than if you want to process it in 48 hours.  And of course you need to put heat into the pot at 4x the rate for the 12 hour scenario.

Distillers generally want to be able to process a batch in 8 to 12 hours (one shift) so it turns out that in practice there is an approximately consistent ratio between the pot size and the column diameter (or more correctly the column cross sectional area) but this is a coincidence - as explained above there are different drivers in determining the pot and column sizes.

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What he said.

And it's not just a pure function of diameter, area, and vapor speed.  The type of column will also have an impact.  For example, dual-flow perforated plates are going to have a very different set of operating conditions than bubble caps, which will be different from a packed column.  Not only that, but each of these subtypes of column are going to have numerous variations that impact operating parameters, for example, the % open area on a dual flow tray, or the type of packing in the packed column.

Not to mention that theory will only get you into the ballpark, from there it's based on practical operating experience, as the real world is kind of stubborn in the fact that it doesn't always adhere to theory.

I always thought this was a pretty approachable read:

http://kolmetz.com/pdf/EDG/ENGINEERING DESIGN GUIDELINES - distillation column - Rev 04 web.pdf

 

 

 

 

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There are some rules of thumb on colum diameter vs output and heat required. IMHO for a column still the base size is effectively irrelevant other than can it input enough heat and product.  Think continuous still as one extreme.  I think it's more matching a batch size to output rate so that sill runs happen in a reasonable amount of time.  It would be hard to run 100 gallons of wash through a 2" column, but that would be a shift with a 6" column.  

A 2" colum will do 1-2l per hr, 4" about 1 gallon, 6" 2 gallon. That is provided there is enough input of heat and alcohol. Where this falls apart is if you try and run a 6" column on a 1/2 barrel boiler. From wash/wort there is hardly enough alcohol to load the still.  Yet from low wines it will run fine.  If you really want to discuss still design I would check out homedistiller.org 

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One part of your question which I did not address is whether it is detrimental to have a column that has a larger diameter than necessary.

Unfortunately columns that are larger in diameter than necessary do not work as well as correctly designed columns.  Depending on the type of column, there will be a narrower or wider range of flows over which it works well.

With packed columns if they are too large the liquid will not wet all the packing and there will be zones where the vapor can rise up through the packing without contacting the liquid at all.  WIth sieve (perforated plate) columns a minimum vapor velocity is required to prevent the liquid from weeping through the holes.  A small amount of weeping does not affect the efficiency, but too much will cause the plates to run dry and in larger columns can lead to mechanical problems like vibration.

Correctly designed bubble cap trays will not weep and these columns have the widest range of capacity. But even these trays will fall off in efficiency if they are grossly oversized because the intensity of the bubbling will decrease. This reduces the mixing and the vapor-liquid contact - reducing the separating efficiency of the column.

Of course, for all types of columns a detrimental impact of over sizing is the cost of the column. The quantity of liquid held up on each tray also gets to be significant with very large columns and this can affect the recovery of alcohol in batch systems, but is not really a problem for continuous columns.

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You can have a column that is too large for a still boiler, but the column would have to be really large.  A 16" diameter 2 plate bubble plate column will work fine on a 65 gallon still running low wines and it will do the job really fast if you have enough heat transfer going on.

That little 15.5 gallon keg will work great with a 3" pot still column with copper packing and no dephlegmator, to do brandy, or whiskey in 2 runs.  The output for the stripping run should be around 25%.   Its not safe to put high proof low wines in a direct fire still.  On the spirit run, you will be able to hit 60% or a little better.  If you have enough heat, each run will take around 3 hrs, once operating temp is reached.  If you want higher proofs, you will need a reflux column with deflegmater.  So as to have a faster run time at higher proofs, I would go with a 4" bubble plate column:  https://shop.distillery-equipment.com/collections/stills/products/4-bubble-plate-copper-stainless-moonshine-still-column-vodka-w-cooling-kit With 10% mash, she will put out 85% no problem, and with a 12 gallon charge you can do a 170 proof run in around 3 hrs.  If you want 190 proof and above, you could add 48" of 4" packed column along with the bubble plate column.  2 bubble plates on the bottom then the 4" raschig ring packed section and then 2 bubble plates on top with the correctly sized dephlegmator.  We have built several 10 and 20 gallon R&D vodka stills as described above, with 10 or 20 gallon jacketed boilers with our electric Baine Marie heating systems. The vodka columns are modular on ours, so that you can remove the 4" packed section and just run 4 plates, or you can remove all of the plates from the inside and run as a pot still..  There is an extension tube for the final condenser when the still; is in Vodka mode.  Here is one our very versatile little 45 gallon R&D stills that can be configured several different ways.  https://distillery-equipment.com/45 gallonStill.htm  It has very fast run times with our 6" columns.  

You will need a A Sanitary tri clamp reducer to connect your column to your keg.  A 2" tri clamp ferrule with a silicone gasket will clamp directly to the connection on top of the keg.  The keg connection is where the keg is tapped, so you will need to pull the guts out of the keg ferrule.  The keg ferrule is of a different design than a tri clamp ferrule, but they are close enough, so that they will work together fine and they will seal with a food grade silicone gasket.   If your column is a 4" reflux column, to be safe the column should have extra support.  With the keg, the Vodka configuration will be well over 10' tall and very top heavy empty.   You will need guide wires to support the column.  Beer kegs will work but cleaning them sucks.  The only way to scrub the inside is with a carboy brush through that tiny hole.  If you have to use it then you should cut out the ferrule on top and weld in a 6" tri clamp ferrule so that you can clean the thing properly.  I have all of the parts that you need to build your own still on all 3 of these web sites.

 https://triclamp.co/    

http://distillery-equipment.com

http://moonshine-still.co

Good luck and have fun.

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All,

Thank you VERY much for the incredible amount of information that you supplied, that .PDF was a great read too.  Again, I am still geeking out on the whole process of distillation and the different types of stills as well as how each design contributes to the process.

As I have mentioned in my first post, I am a Mechanical Designer and in the past have developed Fuel mixing systems for Cummins, Oil recovery units for test cells at Harley Davidson, Dyno test beds, Hydraulic lifting systems, just to name a few, so once I started visiting Distilleries I had to know more about the whole process and what needs to be taken in consideration for Still design, who knows maybe that will be my next job.

Again, thank you all. I plan to keep researching the topic and would love to hear more about design ideas as they come along.

 

 

 

Hey Pete..... 

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Fundamentals of Distillery Practice by Willkie and Prochaska. its an old book, but as a Mechanical Engineer, if you can read between the lines it will open up a whole world to you.

there is very little published on the math of distilling, anyone in the know keeps this close to the chest. If you have any questions I'd talk with you. I might not give you all the marbles but I'll answer yes or no questions, and let you know if you are on the right track. this is all hard won information, I'll share with anyone who will put in the hard work.

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  • 4 years later...

I have worked with packed columns of up to about 5 foot diameter, but I wouldn't be surprised if columns of 3 times that diameter are used.  Structured packings are often used in vacuum columns and these tend to have very large diameters to keep the velocities down.  A rule of thumb with packed columns is that the packing pieces should be not more than 8-10% of the column diameter.  Rings are available up to 3.5 inches, so a column using that type of packing should be at least 40 inches diameter.

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Any sprit distiller works with such a big diameter?

according to what i'v observed people with columns use packing only till about 3 inches in diameter, so whats with 4 inch and up?

For what size and on do you recommend an inner distributor?

Thanks in advanced

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I agree with your observations in terms of columns used in the craft distilling world.  Packed columns have a narrower range of (efficient) operation than bubble cap trays.  This means that if you are running a batch still where the rates and compositions are constantly changing then it is better to use trays. But if you are venturing into continuous operation where the column can be designed for a specific and constant capacity then it would be quite feasible to use a packed column.  I think columns up to about 12" diameter have been mentioned on this forum?.  A disadvantage of packed columns in this application is that it is not so easy to take off side draws.  It is also difficult to predict at the design stage where the side draws should be located - I'm sure you have seen the trayed columns that look like hedgehogs with off-takes on every tray.  Sometimes you will see a continuous column with trays in the bottom half where most of the side draws would be, and then a packed upper section.

I am currently involved in an expansion at a large vodka producer where some of the columns are 2500 mm (8 ft) diameter, but these are valve tray columns.

You should probably get a recommendation on the distributor requirements from your packing vendor.  I would guess that a single center mounted inlet is OK up to about 4". If it is a clean feed you can put a solid cone spray nozzle on it.  For larger than this, but less than 2 ft a simple spider made of standard piping components should be OK.  Above this it would be best to use a proprietary design.

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I have worked with a column stills running raschig rings that was 24" in diameter.

I can point you towards the vendor we used, or depending on how much you need we may have some left over.

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ViolentBlue, that's nice of you. I have raschig rings (the 8's smallest size) it gives me 95.2 just in the beginning and falls down to ~94.85 so i must slow down the column so it continues 95+ and its painfully slow.

meerkat, i realized that packed column could be much shorter then trays and the vodka may be much cleaner though plates carry over some flavor if i'm correct. 

I'm working with an 8" and 4"

just recently i was thinking to go with propak whats your take?

thanks  to all.

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I don't think that you can generalize that packed columns are shorter than trayed ones. There are so many factors that go into determining the Height Equivalent to a Theoretical Plate for packings.  And with trays there is always a trade-off between diameter and height.  You can minimize the carry over from a tray to the one above either by spacing the trays more widely (i.e. making the column taller) or by increasing the column diameter (i.e. decreasing the vapor velocity).

I have no personal experience with ProPak, but with all those perforations I would expect it to be good.  The price that came up when I searched for it was rather high, but maybe that includes shipping to South Africa?

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We worked with propak to do some testing right before the pandemic as we were interested in using their packing in our vodka still but wanted to find out if it was worth the extreme cost or not.

the results were that it was an extremely good packing for achieving neutral, but it does require that you run the still considerably slower to avoid blow-through/flooding as it’s perforations are incredibly good at holding liquid.

We tested their smaller size stainless variant, can’t remember which off the top of my head, maybe 5mm? in our old electric vodka still with a 5” column. We tried many variations of pure propak, propak in just the bottom of the column, and propak in just the final 8” of column. Bottom only showed no improvement. Top only was a slight improvement, of course best results being the entire column.

Our final verdict was that for laboratory distillations where the utmost purity of whatever is being distilled is the goal, it’s a great packing. However, for a beverage alcohol distillery that needs to make money it’s probably not worth the $40,000 cost to fill a small column.  I want to say it’s around $350 to $450 per liter of product, again just trying to remember off the top of my head. If you want easy math, an 8” diameter column is about 1 liter for every 1.25” of height, so about $3350 for every foot of pro-pak.

 

again, full disclaimer on the cost and numbers, I’m going purely off memory from over a year ago.

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9 hours ago, Bronfen said:

yes, its very expensive but much less. So i understand that you discontinued using it so what did you do with it?

The thread actually reminded me that it’s boxed up in the warehouse. I was going to send it back right as the pandemic started up, then 2020 went into full effect and I completely forgot until now.

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