HETP - How accurate are Ackland's calcs?

[bastardsquad]

Some fairly impressive maths on the parent site from Tony Ackland around column heights, theoretical plates and HETP. I've read through it a number of times and believe I have a good handle on it. I'm embarking on a 2" column keg boiler based rig build, with a boka head to start. So , again I looked at the info in determining the best column height with scrubbers as packing.

Now, I have a T500 - widely regarded as a journeyman's still. I have a pot and reflux head for it, but the factory reflux head output doesn't seem to match Ackland's calculations? It's a 24" height column at 2" in diameter and it will with a very slow takeoff rate produce 93% @ 20 degrees. Typically I run faster than that and get 91% but...

My interpretation of Ackland's calculations implies that 24" of ceramic raschigs/saddles is only 2-3 theoretical HETP, whereas the actual real world output is equivalent to 5-6 HETP. If this is so, wouldn't a say 36" column on the keg boka be sufficient, especially if using scrubbies with a better HETP than raschigs?

I realise there's a few more plates between a 91% and azeotrope output, and i'm happy with anything inbetween. Any reason to make the column higher than 36" though? I have approx. 80" of 2" stainless for the column so plenty of material, but burner, keg, 48" column, 12" head is getting pretty tall for domestic operation !

[pounsfos]

feel free to test the calcs yourself, a CM column isn't the best thing to test it on... but the theory should be close

if it's any consolation I have roughly 150cm (60inches) of packing and that will take me to about 90% with a boiler charge of 12% and it will take me to azeotrobe with a boiler charge of low wines at 40% with a reflux ratio of 1

[wv_cooker]

When I Built my LM I used the calculators to figure my height. It said at 36" I could reach 94.5 % with this height. After 9 month's of use I get consistent 95% runs and have since day 1. and yes my column has 36" of actual copper mesh packing. I attributed the extra results to the copper mesh instead of using scrubbies.
Though maybe not exact as all stills will be different it should give you a good base to build from. Hope this helps.

[rad14701]

Members have proven, through practical application, that 36" - 42" of packed column will produce 95% with very little effort... That's just the packed section... Less than 36" can be a bit trickier and the take off rate will need to be reduced even further than the already agonizingly slow 3 - 4 drips per second... I do stripping runs and then charge the boiler with 35% - 40% low wines and can take off at 95% at 1oz per minute stream rather than collect in drips...

You really don't need to focus so much on the actual math as much as you need to pay attention to what has been proven to work in the field (or basement, or shed, or kitchen, or garage)...