Height for purity, diameter for speed, and HETP Calcs

[Odin]

This is the way we traditionally look at packed columns and their performance. If you want (need) more purity, make the column higher. If you want more liters per hour pushed out, you need a wider diameter column. Of course you need to fill that column with more gases and reflux, so a higher energy input is neede, but simply put: height for purity, diameter for speed.

When I make new column packing, I always want to know it's performance. Part of that performance is to calculate HETP: Height Equivalent of a Theoretical Plate. It tells us how many centimeters of packing equals one distillation.

The procedure I use is a sort of industry standard, that goes like this:
- Put 10% alcohol in the boiler;
- Add enough plates or packing to be able to take that 10% to 96.5%.

To take the 10% low wines or wash to 96.5% needs 13 distillation cycles. But we do not want to just test by top of column temperatures. Instead, we want to take product out for additional testing via a hydrometer. So it's not a closed system. And since we draw off, we need a 14th plate (the one we draw off) and because the action of drawing off liquids creates turbulence, we need one more additional plate or redistillation. 13 plus 1 plus 1 equals 15. Fifteen redistillations to take us from 10% to 96.5%.

This is not my method, this is how companies like Hollstein go about their business. I just apply the procedure to my columns to be able to compare results in, well, a comparable way.

All right, let's say that I take my 3 inch diameter column, that's 125 centimeters high, and pack it with 100 centimeters of my new column packing. I run it and create 96.5% with it. 96.5% means I got at least 15 redistillations (see above). Next thing I do is take out packing, so there is only 75 centimeters of packing in. If I can take the output to 96.5%, this means again 15 redistillations took place. Or more. A first calculation (75:15) teaches that the HETP is no more than 5 centimeters.

Next thing I do, is take out my 50 centimeter column section and fill it with 45 centimeters. All right, we can still draw 96.5%, so we can now calculate that HETP is actually much lower: no more than 3 centimeters.

Testing some more teaches me that I can still draw off 96.5% with just 30 centimeters of packing, but no less. Devide 30 by 15 and HETP is 2 centimeters.

Now here's the funny thing. The less packing I use (the shorter I stack my column), the more reflux I need to maintain 96.5%. The more reflux I need, the less product I can assemble.

There you have it: column height is not just related to purity, but also to speed. If I fill my 3-incher with 125 centimeters of packing, production speed goes up tremendously.

Regards, Odin.