How much power?

[jim81147]

I have an offset head still and am wondering about how much heat you should input or if it even makes a difference . If you are controlling output by how fast or slow you pull off the distillate , then as long as you have a boil going is that enough? Would you benefit any by having the boil more vigorous ? My thoughts would be a more vigorous boil might keep the column temps more stable , but also drain the propane tank more quickly .A less vigorous boil may be a bit harder for a newb to control because changes from one stage to the next would be a bit sluggish . What do you guys with way more experience than me think(or know)? Thanks

[linw]

There is no question on this. Reading the theory section of this site would be good. A slow boil is the only way to go for high separation/purity. The vapour must not go too fast up the column.

[jim81147]

Lindsay , I have read the theory portion as well as most of the site . I guess I am not understanding what I am reading though. Here is a quote from thoery :
The collection rate is directly proportional to the element size, so if a 1500W element with reflux ratio of 4 takes 3 hours to distill, then 1000W will take 4.5 hours, or a 2000W 2.25 hours.
This quote refers to electric powered stills where my is propane so I do have a bit more flexibility . My thoughts are that if I am cooking with the equivalent of a 2K element but pulling off like it was a 1.5 K element then would I not be getting more reflux , hence a purer product?My thinking is that if I send it up the column faster , condense it and send it back down , while pulling off at a set rate , I should be getting more refluxing and that should equal a more pure product . It would be the same principal as turning up the heat and cutting back the amount pulled off when you start nearing the tails wouldnt it?Is my thinking or understanding wrong? Thanks

[linw]

You are overlooking a couple of points. The packed column is a delicate hothouse where liquid and vapour are changing states continuously at a high rate. But for this process to become stable the vapour speed up the column has to be kept relatively low. If it is really fast, the excessive amount of vapour will choke the column as the large amount of vapour stops the condensate dropping. You know when this occurs as strange worrying noises are apparent and the still may take on a like of its own! Dangerous. If vapour speed is too low, nothing happens! So, what you are aiming for is a vapour speed somewhere in between but at the lower end of the band. Simple fact is that too much vapour stops the re-cycling action to such an extent that purity is poor. The vapour has to take time to traverse the column not fire through like a bullet.

So, in answer to your question, no, you certainly can't operate your still for high separation/purity by pumping the vapour up with high power even though you apply high reflux ratios.

But, it ain't that hard to get it right! You just adjust the heat to maintain a slow roiling boil. This will ensure that the vapour spends long enough in the column for the packing to work its magic. With this happy state, you will be able to get good separation between heads, main and tails. If you push it too hard, the product will not be separated well.

Good luck.