Mike argues that you should use quite a lot less power Tony reported that a 25mm diameter column at W gave very poor results, but a 36mm diameter column handled that power well. A not-to-scale schematic has been posted in the Photos section of Distillers. So using W with a 25mm column would result in full separation occurring at a point right at the top of the column, with little or no leeway. Adding imposed reflux with a compound column did little to change the temperature gradient up the column, but what it did do was add that touch more separation in the region above the top of the packing.
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Mike argues that you should use quite a lot less power Tony reported that a 25mm diameter column at W gave very poor results, but a 36mm diameter column handled that power well.
A not-to-scale schematic has been posted in the Photos section of Distillers. So using W with a 25mm column would result in full separation occurring at a point right at the top of the column, with little or no leeway. Adding imposed reflux with a compound column did little to change the temperature gradient up the column, but what it did do was add that touch more separation in the region above the top of the packing. The straight line went down to the top of the packing, then quickly jumped to meet the temp in the top section of packing.
This indicated to me that the composition of the cycled vapour in the void between the top of the packing and the top condenser was the result of further separation imposed by the imposed reflux operation in that region. In effect, I had two stills one on top of the other, the bottom being a simple reflux still relying on internal reflux, and a recycling still that took what the reflux still gave it and used that as its starting point.
Essentially, it is that the figures in the table are good for indicating the maximum you can push a simple reflux column to and attain full separation If consistent results are wanted, then the aim should surely be to allow some leeway and try to get that curve settling down before the top of the packing is reached.
That way, the reflux column has a chance to do its job as fully as it can before either taking off product, as in a simple reflux still, or passing on the results to a secondary top section that operates with imposed reflux for that final touch of separation. Generally, a 2" 50mm diameter is an ideal size to use. This will happily run from W up to W without any trouble. If in doubt, go for 2".
Its this amount of energy that you put in which will determine the rate at which you make and collect the distillate. If you run a reflux ratio of 4 e.
Many homes only run 10 amp fuses in their fuseboxes. The risk of making the column diameter too small is that the column will "flood", as discussed in "Chemical Engineering - June " pp by Simon Xu and Lowell Pless about flooding in distillation columns. These guys have been using "gamma scanning" to work out where abouts various distillation columns are flooding, and why. For a given packed column, at the high end of liquid and vapour rates we encounter flooding as liquid backs up the column and fills all the void space in the packing bed.
Poor disengagement between vapour and liquid back mixing reduces the separation efficiency, and the high liquid hold-up in the bed increases the pressure drop. The traditional approach to analysing flooding in packed columns relies on measuring pressure drop.
At low liquid rates, the open area of the packing is practically the same as for dry packing. In this regime the pressure drop is proportional to the square of the vapour flowrate.
As the vapour rate continues to increase, eventually a point is reached when the vapour begins to interfere with the downward liquid flow, holding up liquid in the packing. The increase in the pressure drop is proportional to a power greater than 2.
At this point, the pressure drop starts to increase rapidly because the accumulation of liquid in the packing reduces the void area available for the vapour flow.
This area is called the "loading region". As the liquid accumulation increases, a condition is reached where the liquid phase becomes continuous The problem with this traditional approach is the difficulty in differentiating between the transition points of the loading or flooding in the pressure drop curve.
There are two forms of liquid hold-up in packed columns. One is referred to as static hold-up. Static hold-up is the amount of liquid that is held onto the packing after it has been wetted, then drained - the film of liquid or droplets of liquid that adhere to the packing.
This amount jointly depends upon the physical properties of the liquid and the type and material of the packing. The second aspect is the operating or dynamic hold-up. Dynamic hold-up is the amount of liquid held in the packing by the interaction of the vapour and liquid flows.
Dynamic hold-up must be measured experimentally. To measure this amount, instantaneously stop the liquid and vapour flows, then collect and measure the volume of liquid that drains from the packing. The total liquid hold-up in packing is the sum of these two forms of hold-up The void fractions in a packed bed may change across the bed due to fouling or damage, and vapour-liquid loads may be different along the bed for different operating conditions.
The peak loading could occur anywhere in a packed bed, or a liquid distributor could initiate the flooding An interesting phenomenon for random packing and most corrugated sheet packing is that the separation efficiency of an "initial flooding" bed could be better than a "normal" bed, because of high liquid hold-up and intimate vapour-liquid contact in the "frothing" regime But at the high-efficiency state it is difficult to keep the column stable, and the column could go out of control as a result of any slight process turbulence.
For this reason it is always recommended to avoid designing a packed column close to the initial flooding point. In operation we would not then be overly concerned with some liquid accumulation or hold-up, as long as the column could be kept stable and under control Using these as the best type of packing will allow you to use a smaller column or a lower reflux ratio to get the same purity.
Are you happy with the existing purity, or do you want cleaner alcohol? The stainless steel scrubbers are probably only good however up to about inch diameter columns. So for columns up to inches in diameter mm , you might as well go for the better performing, cheaper option of scrubbers.
Bigger than this though, and you might need to start using what commercial units do. David comments Use a good quality one preferably. Even less if you prefer. I tend to work in the vicinity of each filling somewhere between 55 and 63mm.
At 55mm on a 36" column this equates to almost 17 from which I deduct 1 to allow for space at the top ie. Allow at least 2" to 2. Do not unravel but tease them out by hand a bit so they fill the whole column diameter rather than just a part of it.
Most of the ones I have seen in NZ do not have rubber bands around them. Place them into the column from the bottom one at a time using some sort of restriction at the top and bottom to prevent them going further or dropping out back into the boiler..
I use a 2" pall ring which works well. You dont want them too loose or too tight. If too tight they will compact more. The main thing is to have an even constant heat so you dont get surging. Surging causes compaction.
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