How Perforated Plates Outperform Bubble Caps!
Introduction
Say what? Counter intuitive? Another opinion that's against the grain? Yes, another opinion against what the industry holds to be true. But for good reason, as I am going to explain to you in the text underneath. Perforated plates outperform bubble caps. How? Time to dive in deeper!
The commonly held belief
Industry-wide the commonly held belief is that bubble caps are a better option than perforated plates. Okay, they are more difficult to manufacture, and therefore more expensive, but they give better separation power. And they are more efficient, right? That's the thinking in the industry ...
And I understand where it comes from. I mean ... perforated plates don't hold their liquor. Instead, that liquor - or reflux, to be more precise - weeps down through their many perforated holes. That's inefficiency right there, isn't it?! And inefficiency translates into less separation power ... so more distillation cycles are needed ... so more perforated plates are needed. More than would be the case with bubble caps, right?
The industry strongly believes so. But today I am going to make a case that the industry is wrong and I am right: if you want to use a plated column for distillation, then the perforated plate beats the bubble cap hands down.
Some basics on plated columns
Both bubble caps plates and perforated plates let gasses through. These gasses travel upward: from the plate under investigation to the plate above. Both also let returning liquids through. This reflux travels downward: from the plate under investigation to the plate underneath.
The bubble cap plate and the perforated plate assemble reflux on top of the plate. Gasses travel through the reflux that sits on top of the plate. The actual fortification between the rising gasses (that come from under the plate) with the reflux (that comes from above the plate) takes therefore place in the liquid bath above the plate. This is important to understand: the act of redistillation, that a plate provides, takes place on top of that plate, and not under that plate.
Both types of plated columns have a structure that facilitates the upward movement of the gasses produced by the boiler (or the plate underneath). The bubble cap and the perforated plates also have structures that facilitate the movement of the reflux downward towards the boiler (or the plate underneath). But there are significant differences in how how each type of plate handels gasses and reflux.
The bubble cap has one big pipe where the gasses rise up, through the plate. By a roof or "mushroom"-like structure, over that pipe, these gasses are then bend downwards. Slots in the lower parts (skirt) of this mushroom-like structure allow the gasses to exit on top of the plate, where they meet, mingle, and redistill with the already collected reflux. The reflux is also handled by one big pipe. The so-called "downcommer". Reflux levels above the mushroom-like structure's height are syphoned of that plate and transported to a release point under that plate, where the reflux - now more depleted in alcohol - falls on the plate underneath.
Where the bubble cap can be summarized as having one big opening upwards for gas transport, that is capped to push gasses down into the liquid bath above the plate, and one big opening downwards for the reflux transport, that is capped at the bottom to prevent gasses using it to move upwards, the perforated plate's biggest distinction is that it has multiple holes both for gasses traveling upwards and for reflux moving downwards.
The perforated plate has many small perforations. The gas pressure from underneath keeps the liquid bath in place. So ... if you reduce the power input, the plate will empty. And even if you don't, well, still some reflux returns via those holes to the plate below. The process of reflux draining through the many small holes is called "weeping". The gasses travel up through the many holes and meet, mingle, and redistill with the already assembled reflux on top of the plate. The build-up of reflux on top of the plate either "weeps" downward via the perforations, and/or uses an additional downcommer to be tranported through the plate and released above the plate underneath.
The critical difference in results is that, when one powers down, the bubble cap maintains reflux on the plate. The perforated plate, without gas pressure coming from the boiler, drains its reflux. It drains and empties. Oh, and it weeps while under way; the perceived cause of inefficiency of the perforated plate versus the bubble cap!
Here are some new insights
The "weeping" of the perforated plate is actually beneficial! As liquids are heavier (denser) than gasses, the weeping creates two additional effects (that result in three huge benefits):
- It creates a (relatively) high pressure zone under the plate;
- While simultaneously establishing a low pressure zone above the plate!
How is this beneficial? Well, first of all, the high pressure zone under the plate reduces weeping and prevents the plate from draining too quickly. This is important, as a drained plate no longer facilitates that extra distillation cycle taking place on top of that plate.
But the high pressure zone under the plate has another benefit, and a much more important one at that. It dampens the reflux that is disturbed by the rising gasses of the plate underneath. The high pressure under the plate prevents reflux from the plate below (that is lower in ABV, remember) to travel upward and contaminate the plate above! The perforated plate's weeping is not an inefficiency, but the preventer of entrainment, where lower-grade reflux dilutes the reflux of the plate above.
That's the effect weeping has beneath the plate. But it also creates another, opposite, effect above the plate. There, the weeping creates a low pressure zone. And what is a low pressure zone above the plate good for? It dampens the liquid bath and creates a more stable reflux layer. The enhanced stability of the reflux above the plate (and remember: that's where the extra redistillation cycle takes place) allows for a better mingling of gasses and liquids ... and it dampens (again) entrainment from entering the plate above it!
Of course there are design options (more on that in a future post) to have bubble caps create these desired air pressure zones as well, but this comes at the cost of performance. It might well be the reason, why a packed 2.5 inch column easily handles 3 kW in power input, where a bubble cap plate needs a 4 inch wide column. A numerical analysis? Here you go. The surface area of a 2.5 inch packed column is (gross) 31 square centimeters. The gross surface area of a 4 inch diameter bubble cap plate is a whopping 79 square centimeters. The bubble cap plate needs 2.56 times more space to process the same amount of gasses or product as the packed column needs.
How's that for an eye opener? And here's another one: perforated plate performance (or at least the perforated power plate that I designed) is closer to the packed column performance than it is to the bubble cap plate's performance!
A comparison: visual and with more numbers
Please see the video underneath. It is a 4 inch bubble cap plated column, that displays the exact bubble caps our German, American, and Chinese "competitors" use. The video shows how the bubble cap plate gets overwhelmed and totally floods at a power input of 4.14 kW:
And here is a video of our iStill Perforated Power Plate at 9 kW power input. That is 9 kW instead of 4.14 kW. Okay, the gross column diameter is 5 inch instead of 4 inch, but we'll get to the exact comparison a bit later, in the text underneath. For now, do you see how stable the liquid bath is? No flooding, no entrainment, no drama, just great and utter performance.
All right, some numbers then! The surface area of the 4 inch bubble cap plate is 79 square centimeters. The surface area of the 5 inch perforated plate is 123 square centimeters. This bubble cap plate offers 64% of the surface area of the perforated plate, so it should easily handle 64% of 9 kW and run just as sweet as the perforated plate does at that power input. It should - for equal performance - run nicely and without drama at 5.76 kW. But - as we can all see - it does not! It floods instantly at that power setting. It already floods, as previously shown, at 4.14 kW. For the bubble cap to run as nicely and leisurely as the perforated plate 3 kW is the correct power input.
Conclusions
The bubble cap plates that our "competitors" use, are not even half as effective or efficient as iStill's Power Plates. Perforated plates jump bubble cap plates. iStill Perforated Power Plates jump bubble cap plates by a mile!
