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Bubble Plate - Downcomer Height-Theory/Practice?

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Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Tue May 19, 2015 1:07 am

Hi all - I recently thought I'd try the effect of increasing the height of the downcomer entry in my Dash 2

I cut a section of 19mm rigid copper tube equal to the height from the plate to the top of the downcomer - filed off the sharp edges, buffed the outside and it's an interference fit in the downcomer tube - so it gives a height to the 'weir' of about twice the original

I've done one run with this on the lowest of 4 plates - it was like the Atlantic in a storm - much more furious activity - So it seems to easily handle about double the depth without drowning

I'm now about to try it on the bottom 2 plates

I'm thinking that the effect of this is:
1/ more fluid contact per plate - more separation
2/ higher stacking volume at each plate
3/ marginally more pressure in the column

I've had a look for references to the bath depth design but haven't found anything specific

So I'm wondering:
a/ if anyone has experimented with this
b/ how the weir height in the Dash 2 and others was/is established as being optimal
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Tracker » Tue May 19, 2015 7:47 am

There has been quite a bit of experimentation along this path.
If you break things down to their simplest form, a bubble plate is really a re-designed thumper where vapour gets condensed before re-vaporised. Each thumper or bubble plate serves as an extra plate in the distillation process but the depth of the liquid in the plate also determines the efficiency of that plate as in the percentage of the vapour that gets condensed then re-vaporised.
Creating a higher liquid level as well as the efficiency of the plate to distribute the vapour within the liquid above the plate, determines if more condensing occurs because of the longer vapour contact within the cooler liquid. More condensing allows for more of the lower volatiles to return to a lower level in the column via the overflow system.
My opinion is that you should be able to create a similar product with 4 shallow bubble trays as you can do with 2 deep bubble trays (or similar).
Others opinion of course may vary as I have not attempted to prove this. Just thinking on what I have worked out within my own head.


Cheers.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Harry » Tue May 19, 2015 6:56 pm

I agree with Tracker's take on it. I'd also like to add the following...

According to Douglas L. Erwin in Fractionation Tray Design and Rating, the desired weir height is less than 15% of the tray spacing in the column.

Source: Industrial-Chemical Process Design by Douglas L. Erwin pp101
(see an extract in this thread)


This would mean that a column with trays spaced say 5 inches apart have a weir height of...

5 x 0.15 = 0.75 which is three quarters of an inch (or less says Douglas).



More about good design...

The object of distillation is to separate a mix of fluids (water and ethanol) into near-pure substances.

Volatiles (ethanol, water etc.) are liberated or vaporized (boiled off) from a liquid mixture (wash). The resulting fluid mix (vapors) are injected into another fluid mixture (liquid) flowing across a tray or plate. Three major things happen.
1. Heat is exchanged (always from hot substance to cool substance),
2. Steam in the vapor mix condenses to water and joins the tray liquid (enrichment),
3. Ethanol in the liquid mix on the tray vaporizes and joins the rising vapor mix (enrichment).

Greater exposure of a thin liquid sheet to injected vapors is the way to go.

So, to allow this enrichment to be as near complete as possible, longer fluids-marrying time is necessary. Trays should be designed with a maximum possible liquid travel path between tray entry and exit points. Why?...
It follows then that the liquid held on the tray has to stay on the tray for a longer period before it can make its way off the tray. The longer hold-up time allows more liquid/vapor contact and thus more enrichment of the process fluids (water and ethanol). The thinner sheet of liquid on the tray ensures that ALL the liquid on the tray will be married with the injected vapors. There won't be any liquids avoiding or escaping the contact process. Thus separation of that tray's liquid mix approaches maximum, and less trays (stages) are required to achieve the object i.e. separation.

It follows also that distillation designs that allow some tray liquid to escape or bypass the injection process prematurely will be less efficient at saparation. This may or may not be a good thing, depending on what product one is making. Some alcohol products depend on separation inefficiency to achieve a certain desired flavour profile.

Hope this helps.
.


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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Tue May 19, 2015 8:42 pm

Thanks Guys
I had a very instructive run last night

I had the previously lengthened D/C plate on the bottom and the newer on above it - 2 more unaltered plates above that - The lower/first extension was just cut without measuring as a rough trial, the second one was made the same length as the existing weir height, so after pushing it in it made the overall height a bit less than twice the original - the lower one was visibly shorter.

So plates loading - from the top down, first 2 plates OK - plate 3 filling BUT because plate 4 (bottom) wasn't yet filled, and the run had just started, the vapour speed/pressure was high and the bath depth in plate 3 made the vapour path somewhat easier thru the downcomer - so I had an 'ol faithful' geyser shooting up almost to the plate above. Switched off - let plate 3 drain to plate 4 - switched on - all OK - plate 3 furious activity, plate 4 a bit less

Then as the run went on, and the vapour speed dropped, plate 3 ceased bubbling except for a few strays - so the bath depth and the pressure from the caps attained equilibrium with some drainage to plate 4, and the vapour again went through the downcomer - other plates continued unchanged - so it worked as a 3 plater

I could get it back to bubbling for a short while by shaking the column which spilt alc into the downcomer, temporarily blocking it enough for the bubbling to start up - it would last maybe a 30 seconds and then in a blink was still again

My take on this is that in the Dash - the downcomer entry point for vapour is hard against the bottom of the plate and in this 'extreme' case there needs to be more pressure differential between the cap inlets and the downcomer - ie - a much longer downcomer discharge tube

If the activity in plate 3 could be sustained I feel it is worth more experimentation

Also it would appear that the drop in temps up the column and the decreasing pressure (?) would make higher plates more sensitive to weir heights than lower ones- ie stepwise changes to weir heights at each plate (unless longer downcomers work for all heights)

Attached photos show the boiling in plate 3 compared to others, and the quiescent state later on. It's hard not to come to the conclusion that such 'boiling' must increase separation.....
Plate 3 Active.png.png
Plate 3 in full song

Plate 3 still.png.png
Plate 3 quiet

Plate 4.png.png
Plate 4


In terms of travel length and thin sheets - in this case I'm stuck with what I have - the downcomers are staggered but the path is quite short, and the fact that the cap slots are quite long (I believe lengthened over the first design for capacity reasons?) means that intuitively only a portion of the liquid depth is fully used as the bubbles will escape towards the top of the slots. So a longer vertical path through a deeper bath would seem to make some sense..

I'll mess around some more, but the proof is in the pudding and it remains to be seen if there will be any discernible product difference

And yes - it may not be desirable for brown spirits - but I have an itch which must be scratched :roll:
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Tue May 19, 2015 8:46 pm

And a pic of the unaltered top plate for comparison
Plate 1.png.png
Plate 1 (Top) - normal D/C
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby zedzedtop » Tue May 19, 2015 10:48 pm

In Distillation Design (Kister, I think) it is mentioned that the most exchange occurs in the 'flying-droplet' (my phrase) region above the bulk of the fluid. I would imagine that this is the primary reason that industrial trays maintain a relatively low level of fluid.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Wed May 20, 2015 1:24 am

I don't think I had any shortage of 'flying droplets' (good term btw) :D
https://www.dropbox.com/s/xl8pphd7a9ztr ... 7.mp4?dl=0

I've been thinking about the downcomer some more - and wondering why the SD one is so short - I guess it works but maybe it could work better (Punkin??)

I know there are heaps of details of these using extended tubes and I can now see why they would be better than the shortie in mine - A longer one would allow a deeper liquid trap level in the tube preventing vapour blowby - I can't see any downside to this and I'm wondering if the SD D/C has blowby which I/we just don't notice in normal use ??
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby punkin » Wed May 20, 2015 3:58 pm

We've had a few revisions of the caps and dc's in the standard caps and have settled on the current one as being pretty good. There is always room for experimentation and we love to hear about it, there's a lot going on at the stilldragon.org forum all over the world and we keep a keen eye on it if it offers us a chance to improve our products.
The new Procaps have optional down comer extensions that a couple of people are currently playing with on our forum, this may also be of some interest to you?

I don't think there is any blowby at the moment as you can see pretty well exactly what's happening in the Crystal Dragons compared to the Dash.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Thu May 21, 2015 6:38 am

Thanks Punkin
I'd like to experiment a bit more - need to find a copper fitting which will push fit in the 16mm dia hole---- will be a challenge....maybe I'll have to sacrifice one or two of my d/c's :shock:
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby googe » Fri May 22, 2015 8:14 am

In my testing with different plates, caps, flap, dual flow, no downcomers, I've found the ones that have a rolling type bubble activity instead of a active splashing activity have been superior.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Sun May 24, 2015 2:54 am

So in the red corner weighing in at not much at all - we have the kungfu champ 'flying droplets' AKA 'active splash'
and in the blue corner - looking slippery but a bit chubby - we have Mr Sumo himself 'rolling bubbles'

Who will win this 10 round titanic separation struggle ??

Place you bets gentlemen

Sorry guys -twisted sense of humour :lol: - thanks for the input
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Tracker » Sun May 24, 2015 3:36 am

Put my money on the rolling bubbles please and place it early because I think the odds are going to shorten quickly.


Cheers.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Zymurgy Bob » Sun May 24, 2015 9:22 am

Didn't "Rolling Bubbles" used to dance at that strip joint out by the Interstate? I hear it was quite an act.
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Eucyblues » Mon May 25, 2015 3:39 am

Puts a smile on the dial :lol:
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Re: Bubble Plate - Downcomer Height-Theory/Practice?

Postby Kapea » Sun May 31, 2015 9:28 am

Zymurgy Bob wrote:Didn't "Rolling Bubbles" used to dance at that strip joint out by the Interstate? I hear it was quite an act.

Holy whirling-tassel pasties Batman! This forum is getting more risque by the second! :lol:

My experimentation seems to indicate that adjusting the downcomers for a deeper fluid depth makes for a less "hot" product. Adjusted too high leads to flooding. I use a 6mm thick feeler gage to set my downcomers. That is the distance between the top of the procap and the bottom of the funnel-shaped part of the downcomer extension.

The procap downcomer extensions fit loose, so I tighten up the fit with a turn or two of PTFE plumbers tape.
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