"tapered" reflux column

[12345]

Hi all,

I'm brand-spankin' new at all this, and I'm trying to wrap my head around some of the concepts. It's easier for me to grasp theory if there's a practical application to illustrate it, so I'm going to ask some questions along the lines of "what happens if you do this," not because I necessarily intend to try it, but just to help understand what's going on in that magic machine. Starting with this:

My current column is about 3ft/.9m of 2in/50mm stainless pipe. I have access to another 3.5ft/1.06m of 2in/50mm pipe, 3.5ft/1.06m of 4in/100mm pipe, and 2.5ft/.76m of 6in/150mm pipe, all stainless steel tri clamp. I understand that the taller the better when going for neutral (which I am). I have basically unlimited ceiling space (23 ft/7m in my greenhouse), so height isn't an issue. If I bought some conical reducers, what would be the effect of stacking all this pipe together? I understand that necking down increases vapor velocity, but I don't really understand how that plays into the process. I've seen just a smattering of discussion of tapering the column--mostly a few posts in a thread called "continuous still brainstorming"--but it seems like no one actually does it.

So, what happens? Is there a reason no one does this, other than no one's ever done it that way?

[bluefish_dist]

There is an optimal range of vapor speed. Usually considered 15-20 ft/sec. Beyond that you tend to get smearing of tails. So for a 2" you are limited to about 1800w to stay in optimal vapor speed. You can run faster, but the column won't be as efficient.
Taller is much better. The taller you go the less reflux you have to run to hold the same abv. From what Odin has said, up to 40 plates for vodka with a minimum of 15. You can mix the tubing sizes, but your smallest size will flood first. I currently run 4" packed over 6" plates. Works well.

[12345]

Thanks, Blue!

That leads me to a follow-up question. How would my theoretical 6-4-2 column compare to an all 2" column of the same height?

[bluefish_dist]

Way more expensive if you didn't already have the equipment. Cost for packing will be a lot more for the large sections. Output will be limited by the 2" although it might work better since the velocity in the large diameter sections will be lower. Personally I would skip the 6" and just run the two 2" and the 4". That should get you over 9 ft which is 27 plates. That would be a fine column.

[12345]

Thanks again!

I'm surmising that output is limited by the 2" because, if I turn the power up too much, there will be too much vapor going up, resulting in too much reflux coming down, filling in the open spaces in the packing, ie, flooding, regardless of whether the lower, wider sections can handle it.

Does lower velocity in wider columns mean that the "cool" parts have more time to mingle with the vapor and condense it? Does the lower velocity mean that the column has more time to condense-evap-condense-evap? It's been my understanding that increased diameter doesn't mean increased purity, just greater throughput volume. Do I have that wrong?

With a stepped-down column, is the vapor more rich in ethanol when it enters the smaller segment than it would be at that same height in an all-2"er?

I clearly need to do more research on the role of vapor speed. Am I on the right track here?

[bluefish_dist]

The larger diameter and slower vapor speed will result in more dwell time, so more time to transfer energy from raising vapor to falling liquid. This extra energy exchange should help purity. The gains will be small, so if starting from scratch it is probably cheaper to just add an extra foot of the smaller diameter.
Height helps purity and speed. As there are more plates, you need less reflux to produce the same abv. So take off is faster as well as more pure. Eventually most people run into a height limit. I know I have. 12 ft is my limit and I have 8ft of packed and a 35 gallon boiler.
Diameter still plays a big role in speed as a 4" can handle almost 3x the power as a 2". Then if you step up to 6" it can handle twice what a 4" can do.

[zapata]

Keep in mind bluefish is commercial so runs at a bigger scale than hobbyists. I've seen several reports of people having trouble with 4" columns due to insufficient alcohol to load the column in a typical keg boiler of wash. Maybe would work with a full charge of low wines though?
Since you'd be limited by the 2" anyway, why not sell the 4" and buy more 2" if 2"x6.5' is insufficient? Or switch to all 3" if you were hoping for more speed by using the 4"?

[12345]

Bluefish, can you tell me what imposes the height limit?

Zapata, more speed would be good. Not that playing with the still isn't fun, but there's only so much time. The 4" isn't actually mine (nor the 6" or most of the 2"), it's just resting in my shed for a friend who's in the middle of moving and doesn't have anywhere to put it yet. I don't have the $ for reducers right now anyway, so I'm not really planning of using anything but the 2". But looking at that big stuff certainly sparks the imagination...

Asking the question seemed like a good way to get a better understanding of how a column works. So far it's working.

[12345]

I suppose I should add that, although this particular gear isn't mine, my questions aren't purely for the intellectual stimulation. There will most definitely be some still-building in my near future. A friend is giving me a keg, so it's good to know that 4" might cause trouble, especially since I don't know wtf I'm doing. I'll be sticking with the 2" for now and keeping 3" in mind for when funds for building are available.

[bluefish_dist]

4" runs fine on low wines over a keg. I ran that setup for about a year until I got a bigger boiler. I have also run 6ft of packed 4" over low wines as well. That runs ok as well. I would not be afraid of running 3 or 4" over a keg.