Depth of Distillate Bed

richard

What is the ideal total depth of distillate bed ..??

Also does this depth vary between the first and last plate ..??

I am presuming that whether it is a 2" column or 12" column or whatever, this depth will always be the same.

jacksonbrown

Ideal for what outcome?
Under what conditions?
That's quite a complex if vague question.

richard

Where I am going with this question is;

During a stripping run or spirit run and irrespective whether you use the procap or standard cap I am of the opinion that the vapour needs to bubble through the distillate bed on top of the plate.

Now it is this bed depth that I am referring to.

Of all the info that I am presently reading it is apparent that the bubble caps are always / best submerged.

I am sure that it is an extremely complex discussion / question but I am trying to get a layman's answer to an ideal depth.

jacksonbrown

Well I don’t think I have the info you’re after.
It’ll be great if someone else does but to be honest I don’t think it exists.

I think you need to look at the mechanics of what is going on with a plate.
The falling cool reflux knocks some latent heat out of the hotter rising vapour, condensing it.
Conversely the hot vapour is heating and vaporising the cooler reflux. This is basically acting like another boiler, increasing the ABV.
As the ABV increases up the column with each plate (or HETP) the temp is dropping which allows the falling, cool reflux to continue the cycle. This is the column temp gradient that we sometimes read about.
A plate is basically a heat transfer operation and as with ‘normal’ heat exchangers they rely on contact area and residence time. The more of both you have the more efficient it will be but there is a law of diminishing returns as you get closer to ΔT=0. So tiny bubbles and a very deep bed will get you closer to η=1 but you’ll never get there and why would you bother? You’re better off with a less efficient plate and a few more of them.

But what is ideal??? Like I said in my first post, for what outcome and under what conditions?

That probably doesn't help you at all, sorry.

richard

Thanks for the reply. The in depth technical aspect of this having done thermodynamics approx 40 years back, is now above my paygrade and I stick more to the mechanical aspect.

It is certainly the case if you don't ask, you don't get.

Maybe some others can comment. They often talk about having downcomer extensions above procap top surface by an amount of between 3/16" to 1/4" .... and there are others whom apparently have none.

jacksonbrown

And what were the real, measurable outcomes?
Most R&D at this level is just suck it and see without even understanding the full processes at hand.
There's plenty of voodoo over at HD if that floats your boat :)

grim

Assuming that initial tray depth is adequate - Increased tray depths require increased reflux ratios to increase separation efficiency. Increasing depth while maintaining reflux ratios could actually result in no change in efficiency, or even possibly a lower overall efficiency.

What I mean by adequate is that you don't have half your vapor blowing straight through the tray with no liquid contact at all.

My worthless 2 cents says once tray depth hits a point at which you have good bubble action, stop increasing it. If you are trying to increase output abv using reflux ratio, and hit a wall, you may need increase tray depth slightly. The upper limit to how far you can push this is going to be driven by downcomer backup flooding (or usable output speed).

grim

Just playing around myself, it seems that the deeper the bed, the worse the bubble action. What happens is the vapor starts taking fewer paths to bubble through, and you end up with a fewer amount of larger bubbles coming though the same location on the caps. Glug..glug..glug.. Almost seems that deeper beds require a different cap design to force good distribution and small size - probably very fine teeth, razor thin, and a very precise surface/level. Totally redesigned for the higher column pressure drop. All it takes is one cap slightly off kilter - and all the vapor jets through the high point. The deeper the liquid level, the more this phenomenon becomes obvious.

You get to this point and your efficiency goes to shit, it's almost like operating the column in total flood mode. Not to mention dealing with odd pressure-related issues, like middle plate flooding or flooding that cycles through plates.

Realistically, tuning tray depth probably happens in intervals of 16ths or 32nds and not in inches, and I'd imagine that the end-result is going to be pretty minor.

jacksonbrown

If maxing the efficiency for a given column height is the aim then why bother stuffing around with plates at all. Packed columns win hands down in that regard.
Perhaps I was misguided but my reason for switching over to plates was for consistent separation, not efficient separation.
I use them for making flavoured spirits, through few a few plates, because they have a more predicable and repeatable vapour/liquid interaction which is less dependent on power in or RR.
It doesn't matter that my packed columns only have a narrow window of performance and they vary widely when not in that band (or not packed consistently after cleaning) as I only use them for one thing anyway, neutral.

grim

@CothermanDistilling probably has the best real-world insight on this, I recall a pretty good thread on the downcomer extensions and tuning output abv.

Making whiskey, I opt to not use the extensions, and to run the trays inefficiently, to give me slightly more flexibility in pushing a 4 plate column to run at a lower output abv. Using extensions, I have no doubt it would increase my lowest possible output abv.

richard

Many thanks. Its always good to get other views.

richard

Here's a photo from @CothermanDistilling what got me thinking as well as another.

CothermanDistilling

with the original procaps (I think later ones are taller?), the bed was a bit low, but the real problem is that the flat surface lets more downcomer liquid (splashes, etc..) go straight down to the next plate, and the next plate, and all the way down 24 plates... I could not get true, calibrated 190 even with a lot of reflux..

the one pic of the plate operating backwards, is due to the ratio of the height of the liquid on the plate and the liquid in the downcomer cup... filling the plates, then dumping them are part of my routine now, this ensures that the downcomer cups are full..

I did lower the extenders from the above pics, I made two of the black plastic spacer blocks, and ended up going with the shorter one after testing, I think it was 1/4" vs 5/16"

I had actually planned on also trying extenders with no funnel, just a nice rounded edge

punkin

We did indeed extend the height of the Procaps on Larry's design improvement after your testing.

Kapea

Is it me and my monitor, or does this photo look like penguins getting ready to dive into a flooded plate?

@jacksonbrown said: If maxing the efficiency for a given column height is the aim then why bother stuffing around with plates at all. Packed columns win hands down in that regard.
Perhaps I was misguided but my reason for switching over to plates was for consistent separation, not efficient separation.
I use them for making flavoured spirits, through few a few plates, because they have a more predicable and repeatable vapour/liquid interaction which is less dependent on power in or RR.

+1 on that.

My VM column is made for producing azeotrope. It is a lean, mean, high proof automaton when it comes to that.

I use the plated procap column when I want to get artsy fartsy with my spirits. It is a joy to run in that realm. I set the downcomer extensions at 1/4" and am happy with their performance at that fluid depth. Using a feeler gage to get uniform extension across the plates is important.

I have relegated my pot still head to being my stripper. I run it like a NASA SRB, and it rocks in the role.

CothermanDistilling

anyone has an 8" or larger VM column for making neutral they want to share a video of?

grim

I'm still trying to find a reasonable source for good stainless column packing for an 8' x 20' column. All the decent sources on Alibaba want to sell me a cubic meter, which is about 3x more than I think I would need. Even a half meter would be enough for 2 columns.

I have a line on the stainless piping locally, which is less expensive than I thought it would be.

Kapea

There's always that perennial favorite, scoria... B-)

jacksonbrown

At 8" have looked into structured packing

grim

Most of those guys do type "500Y" packing, which is only about 5 theoretical plates a meter.

The 2 or three that do 700Y - which is 8-10 theoretical plates a meter, only do a full meter order. I could resell column packing, I probably couldn't resell the structured packing. That's north of $2000 before freight.

jacksonbrown

I wonder how many effective HETP in a 10 plate column? What I mean is, what is the actual efficiency of the plates?

I Just looked through some of the Sulza and Koch promo stuff.
0.5HTPE/m?? does that mean inverse of what we normally use i.e. 2m per theoretical plate?
That's huge! I'm guessing that must be with high throughput. "Most economical load range : F factor 1.5 – 2 √Pa" Huh?? lots of numbers in there I've got no idea about.
Tiny little HETPs are great but if it all falls down when you start pushing volume through the plant its effective useless.
That's one reason I don't bother with SPP. Its great for pissing comps but a PITA to use.
I'm curious about life span of this gear too. If it cost twice as much but doesn't foul, CIP's better and lasts four time longer then it could be a goer.