I've been doing a bit of reading on plated columns, trying to suss out which I'd build if and when I get around to building one. The reading has been on industrial column design, and some of the issues that occur at industrial scale may not be experienced in a hobby sized stills. Nevertheless, the industrial columns research gives you a sense of what these columns are actually doing.
So, to the point: the following diagram, showing the performance range of a sieve tray, gives a sense of the importance of balancing vapour and liquid rates in plated columns. (I've reproduced the diagram from an article that discusses what happens inside a trayed distillation column. You should be able to access a copy at your local library. ;-) )
Sieve-tray-performance.gif
The F, E, M, W, L & D points on the gray line depict changes in column performance when you hold the liquid rate constant but change the vapour rate.
So, at M your column's operating normally, although the amount of liquid on your tray or passing across your tray is low to moderate. If you keep the liquid rate where it is but increase the vapour rate — turn up the boiler heat — you'll eventually push the tray to point E, where excessive entrainment (or carry over of liquid-rich vapour from a lower to a higher plate) occurs. Apparently it's here that the column becomes less efficient as the plate loses sufficient head room to allow the proper interaction between rising vapour and downcoming and cross flowing liquid. Pump more vapour into the column and you'll reach point F, where the column (or a couple of trays in the column) flood.
Now, what happens when you go the other way and reduce the vapour rate below M? Well, in a sieve tray there's insufficient upcoming vapour to keep the downcoming/crossflowing liquid on the tray. At point W the tray begins to weep onto the lower tray(s). Tray efficiency drops (marginally) as the amount of time the liquid spends on the tray interacting with vapour is reduced. Reduce the vapour rate further and you'll reach the lower operating limit of the tray, where the upcoming vapour is sheared off by the crossflowing liquid: point L. (This 'shearing' only seems to be a problem for industrial sized columns that are run at higher pressures than hobby columns and with greater tray crossflow.) Reduce the vapour rate even further and a sieve tray will dump its liquid load onto the tray below: Point D.
This, hopefully, gives a sense of the effects of vapour and liquid rates on sieve column performance.
But so what? Well, one thing seems to come from all this, if the analysis of industrial column performance applies to hobby columns. Bubble cap trays should avoid the lower vapour rate problems that you may get in a sieve tray. Each bubble cap acts as a weir, holding liquid on an unperforated tray. Unlike sieve trays, they don't rely on upcoming vapour to keep liquid on the tray. Therefore, a sieve tray will drop out of its normal operating range at a higher vapour rate than a bubble cap tray will. That is, you could keep a bubble cap column functioning at much lower heat than a sieve tray column. Why you would want to be able to do this, I haven't the faintest idea. One of the perks of a trayed column seems to be the speed at which you can collect distillate, and you're going to need to keep the vapour rate up to do that.
Anyway, I thought this might interest others.
Cheers. :handgestures-thumbupleft:
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