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# Bubble Cap Vapor Speed

edited June 2014

Do the same basic principles of wattage for a given column size for a reflux still apply to bubble stills?

I'm asking because a 4" column with 5 caps that have 1/2" riser holes only has 0.98sqin, not 12.56sqin. This would cause a an astronomical vapor speed, right? So, how do I figure out how many squat inches if riser space I need for a given column diameter or wattage?

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• The game is changing mate. I'd wait twelve days before giving a solid answer on that one.... :-B

StillDragon Australia & New Zealand - Your StillDragon® Distributor for Australia & New Zealand

• Sorry I didn't use metric units also. I've been brainwashed.

• edited June 2014

Similar, but not quite identical, there are too many variables.

Overall vapor speed in the column would be similar in the space between plates, as the high vapor speed would only exist at the cap openings.

Don't know if this is entirely correct, but it's been my experience:

``````More plates          -> more power
Larger diameter      -> more power
Deeper liquid levels -> more power
Taller column        -> more power
More reflux          -> more power (to keep the same product output speed)
More speed           -> more power
Lower heat up time?  -> More Power!
``````

By their nature, bubble cap designs (vs sieve plates or theoretical plates via packing) have a much wider operating range (called turndown ratio). Because of this, they can still run effectively at slower speeds, using less power. Reason is simple, the downcomers/drains keep the liquid level set, unlike sieve plates for example, that have a minimum vapor speed to keep liquid suspended, or packing, which requires some level of vapor flow to keep liquid dynamically suspended on the packing. So, with bubble caps you could get away with less power than an equivalent packed column.

I'd wager a bet that for a given column with a specific number of theoretical plates, a specific reflux ratio, and a specific product output speed, for a comparable plated column, the input power requirements would be similar.

Usually though, if you size your heating to minimize boiler heat up time, you'll usually find you have more than enough power to run the appropriately sized column. It's easy to turn down the power, difficult to add more.

• edited June 2014

@Grim

No matter. Your response is insightful.

And in 10 days @punkin's cryptic response will be far more insightful.

StillDragon North America - Your StillDragon® Distributor for North America

• edited June 2014

Actually, @Grim, you did a fine job explaining it. It now makes sense that the vapor speed would be the same between plates. Now, do you get better efficiency from having a few smaller diameter riser vs one large one? I imagine the higher velocity would help your bubble cap work better?

Oh, and your explanation of more power -----> more power was just exemplary as well ;)

Thanks for breaking that down.

• One big DC or many small ones should be about the same, more DC diameter (whether single or combined) allows for faster running plates because it can pass more reflux to the plate below it. More power = more vapor = more reflux = increased need for bigger DC area.

• @Lloyd said: One big DC or many small ones should be about the same, more DC diameter (whether single or combined) allows for faster running plates because it can pass more reflux to the plate below it. More power = more vapor = more reflux = increased need for bigger DC area.

So, what is the limit of how large you can make the caps before you don't have enough room for the bubbles to travel across the plate? 50% of the plate area?

• I view the area of the plate that is not covered by a cap as the 'active area'. This is a misnomer of course as everything inside the chamber is an active area. The SS walls or the glass of a Crystal Dragon, the fluid that weeps into the bottom of the bubble cap, the walls of the downcomer, the bottom of the plate above... everything is an active surface.
The typical rule of thumb is to have about 50% of the plate covered by caps. That rule is VERY flexible and folks report good results with both more AND less active plate area.