5663Re: [Distillers] Activated Carbon
- Aug 6, 2002Grant Dunn wrote:
Subject: Re: [Distillers] Activated Carbon
Its nice to meet somewhere who does some research and doesn't just follow the pack.
We don't entirely disagree. From what I have read, activated carbon acts in three ways:
Adsorption - relying on electrostatic Van der Walls forces. This attractive force forms relatively weak bonds between the carbon and adsorbate. In theory activated carbon could release or desorb what it removed at some point, but from practical experience desorption rarely occurs.
Absorption - refers to the diffusion of a gas or compound into the porous network where a chemical reaction or physical entrapment take place. Ozone for example is absorbed into activated carbon where it oxidizes a portion of the carbons surface. Ozone (O3) is reduced to oxygen (O2) thus detoxified. Ozone does not accumulate or build-up in the carbon structure.
Chemisorption - an irreversible chemical bond between the carbon surface and the adsorbate. Pollutants are tightly bound to the sorbent.
Chemisorption is associated with the removal of inorganic chemicals and the carbon used to remove these compounds is generally not regenerated. The filtering of alcohol primarily involves the removal of organics via adsorption. After the activated carbon has reached exhaustion and all the adsorptive sites are filled, it can be regenerated.
Where we differ is on what is required to regenerate it. While I don't have the equipment to prove it either way, everything I have read suggests that this requires temperatures above 800 degrees. I would be very interested to see any documented reports that indicate otherwise.
The surface area of activated carbon is calculated from measuring its adsorption of nitrogen gas. This clearly demonstrates the need to keep activated carbon in an airtight container. How then does rinsing it in water affect its reactivity? What are the impurities you are trying to remove? Do you know that rinsing removes them? Activated carbon is available in food grade so why don't you just use that?
I really didnt want to confuse matters on the List by rabbiting on about the difference between 'physiosorption' and 'chemisorption', as the only activity we are really interested in getting clean booze. Specifically, we are interested in removing compounds that have taste or odor and which, like those that contribute color, tend to bind strongly. With these, the only bonding is by weak Van der Waals forces. There is no significant redistribution of electron density in either the molecule or at the substrate surface, and subsequent release of the molecules by heating is easy. Heating in an oven to 160 deg C is quite sufficient to clean used carbon to the extent that it can be used again, particularly when the carbon is first soaked in water to provide active flushing with steam as it boils. Of course, this will not release all the adsorbed molecules heating to a much higher temperature in an inert atmosphere is needed to do that thoroughly but 90+% efficiency is good enough for all practical purposes, and has been used as a cost-effective recovery process by sugar refineries ever since white sugar was processed.
Wet activated carbon primarily removes oxygen from air, not nitrogen. Thus, an asphyxiation hazard exists inside enclosed spaces containing wet activated carbon. Dry activated carbon requires no special precautions. Rinsing activated carbon before first use is a sensible procedure as many carbons, particularly the stone carbons, are produced using chemical etchants which may still linger in the final product (eg. zinc chloride and phosphoric acid). Those are the impurities you want to remove before letting newly procured carbon anywhere near liquids that you are later going to drink.
As an aside, not that it matters much, knowledge of all this was gained during my time at Farnborough where I was engaged in work relating to purification of liquid coolants. Bottom line however, for anyone wanting to check on all this, is to leave aside all theory and the boring ravings of idjits like me, and to simply go ahead and try it. If it works for you, then wear a happy smile. If it doesnt, then just blame me and still wear a happy smile!!
All the best,
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