Originally By Tony Ackland
Types of CarbonFor heaps of information about Carbon, and how it works, go to the Norit site and read the section under Tech Info titled Introduction to the Porous Structure of Activated Carbon
There is also a free ebook about activated carbon at http://www.home-distillation.com/free_ebook.html
Reactivity of carbon is measured using iodine. If you can't purchase the spirit specific carbons, you may have to look around a bit for an alternative. You can see below that Charcoal is fairly useless; try to see what you can get by way of water filters etc.
If you can't source any highly reactive carbon, but want to make your own charcoal, see How to Make Charcoal at Home by Dan Gill.
If you can't find activated carbon in your local brew shop ...
How does the carbon work ? Mike's simple summary ...
The other thing about carbon is that it can be prepared so that its structure resembles a sponge, with millions of tiny passages and holes in it. This preparation is loosely called "activation". These passages and holes can control what molecules get deep inside the carbon, and which cannot ... a purely physical matter of size. It is this physical structure that primarily governs whether a particular 'activated' carbon can be used to 'target' molecules of a particular range of sizes. If you are making a respirator, then you will want to know all about that, and some respirator cartridges are best for one range of gases, but not others. In our case, we need not be so fussy as all we are dealing with are very small molecules of water and relatively huge hydrocarbon molecules ... the alcohols etc. The water can penetrate all the way into the average 'activated' carbon, but the large hydrocarbons can only penetrate by various amounts according to how big they are. Just a simple sieve thing.
The other thing about a sponge structure is that it presents a huge surface area for molecules to stick to. So 'activating' carbon not only greatly increases its ability to deal with quantity by having a huge surface area, but also offers a degree of selectivity by physically controlling access to this surface area.
The question of which hydrocarbons are attracted strongly to the carbon surfaces they encounter, and those which are attracted weakly, can get a bit complicated. It is not simply a matter of size. Some hydrocarbon molecules, which alone would be attracted weakly, can form a loose association with water and then be strongly attracted to carbon. Water has a boomerang shape, with the two hydrogen atoms at the tips of the boomerang and the oxygen atom in the middle. This gives it a strong negative charge on the hydrogen side and a strong positive charge on the oxygen side (egghead/cocktail party term #1: it's bi-polar). This enables water molecules to stick to parts of some hydrocarbons and give them a bunch of electrostatic anchors to hang onto a carbon surface.
Bottom line is that if a big hydrocarbon molecule can get to a carbon surface, and it is 'sticky' enough, then the carbon will hang onto it (egghead/cocktail party term #2: it adsorbs the hydrocarbon molecule).
Bottom line to bottom line ... it won't necessarily stay stuck! Other molecules can come charging in and knock that molecule off the carbon surface it was stuck to (egghead/cocktail party term #3: adsorption is subject to dynamic equilibrium).
So now you have all the basic buzzwords that will have all the girls hanging on your every word at your next party :-) In the meantime, back in your shed, you have a batch of booze to clean with carbon. What would be your best way of dealing with it?
If you pass the booze through a tube filled with carbon, will the molecules you want to trap got enough time to infiltrate their way into the carbon granules and get stuck? Alternatively, would it be better to dump both the booze and the carbon into a container and let it get on with it over a period of time? Your choice, but I know what mine would be.
It is pretty obvious that if you use a container and soak, then you will have to filter off the carbon in the end. If, however, you decide to pass it through a tube of carbon, will you need to do that? After all, lots of people call that carbon slug a 'filter'. Will that get rid of all the microscopic particles of carbon that are so small you cannot even see them? Not likely. So 'real' filtering is needed as well, for all those tiny particles will be laden with the hydrocarbons you have been trying to get rid of! One filtering enough? Not really ... keep filtering until the filter paper remains clean.
Should you start out with high %abv booze and process that, or would it be better to dilute it to 40-50% ? One school of thought says it doesn't matter, and another says that the agile wee water molecules help the lumbering big hydrocarbon molecules to move around ... like fast sheep dogs agitating a slow mob of sheep. My choice, based on graphs describing the dynamic equilibrium process, would be to dilute. Final results seem to bear this out.