A solid traps a specific fluid (liquid or gas) molecule in the walls of GAC, as part of the adsorption process. The trapped molecule is referred to as adsorbate, the material as adsorbent, and the process as adsorption.
The majority of organic compounds have a covalent propensity, which is why it adsorbs them. Because, it is more porous and is composed of graphitic plates, where the carbon atoms are orderly bound together, it is different from other forms of carbon. They are less reactive (or more stable) as a result of this order, compared to the atoms that constitute amorphous carbons.
What is reactivation of granular activated carbon or GAC?
“Londo dispersion forces”, one of the forms of Van der Waals forces occur between a covalent adsorbent and an adsorbate, during the adsorption process in which activated carbon participates. It is known as physical adsorption.
The atoms and molecules involved in physical adsorption do not change in terms of their chemical make-up. It is hence reversible.
If the retained adsorbate is volatile, it will separate from the activated carbon if the temperature rises enough. The adsorbates volatility affects this temperature. Thus, reactivation is the process when both this and the carbon remain in their initial state.
What happens if adsorbate is not volatile?
If the adsorbate is not volatile, pyrolysis occurs when the temperature is raised to a specific point (often above 300 ° C). As a result, it stops containing hydrogen and oxygen atoms and turns into amorphous carbon.
The resulting amorphous coal reacts with the furnace gases when the temperature is raised further, turning into carbon dioxide. The graphitic plates' surface remains free while it gasifies. This results in a reactivation as well.
The graphitic plates that makes up activated carbon, are less reactive than amorphous carbon, therefore, they do not gasify as long as the temperature and length of the reactivation process do not rise, over what is reasonable.
Therefore, the removal of inorganic compounds from activated carbon cannot be accomplished by raising the temperature. However, there are various ways to get rid of them, which may involve dissolution in an acidic solution.
Granular activated carbon reactivation methods
Below are descriptions of various reactivation techniques. They are also made possible by the fact that activated carbon is unaffected, by aqueous solutions over the whole pH range, in addition to the stability of its graphitic plates.
Although, powdered activated carbons (CAP) and granular activated carbons (GAC) can both be reactivated, only very rarely has the latter proved lucrative due to the difficulty in handling powders, which are typically smaller than mesh 200.
The mechanical strength of your granules determines whether or not the GAC reactivation, procedure will be profitable. The percentage of material loss as a result of breakage increases as it becomes lower. It should be noted that activated carbon that has been consumed, and removed from a process is referred to as "spent carbon."
Let’s understand few reactivation methods for activated carbon.
It is the most popular reactivation technique. The same furnaces that are used to thermally activate coal are employed for this process. Rotary furnaces and direct heating multi-stage furnaces, are the most popular types of furnaces by introducing combustion gases.
Reactivation often takes place at 500–800 °C temperatures. Steam limits the atmosphere's ability to oxidise inside the furnace. The non-volatile chemicals pyrolyze and gasify while the volatile ones are released from the GAC.
· Water vapour reactivation
In order to prevent condensates from building up, it involves moving water vapour across the coal bed, typically in a downward direction. The volatile organic molecules that are attached to the carbon are released by the steam.
This reactivation technique is used to recover solvents from air currents. Alternating adsorption-desorption cycles occur in this. The solvent is kept in the first reaction up until the carbon is saturated.
In the second, the solvent is removed, and the water vapour and solvent combination are then either decanted or distillated, depending on whether or not they are miscible. Less volatile chemicals can be desorbed from the GAC, by increasing the temperature and pressure of the vapour used to revive it.
· Using hot gases to reactivate
It is identical to the last one but uses a different gas instead of steam. The desorption is accomplished using an inert gas that is heated indirectly in some laboratory scientific experiments, where it is necessary to reactivate without the involvement of any oxidising gas.
Reactivation can be done in a vacuum when attempting to do so at a lower temperature, so as not to harm the adsorbate.
· Acidic reactivation
The thermal reactivation process does not get rid of inorganic salts that precipitate in the GAC. They can, however, be eliminated by dissolving in an acidic solution.
The precipitation of lime, which is used to soften water before it passes through the GAC, is a typical example. The precipitation of carbonates and other salts found in fluids with a predisposition to foul is another. When there is a high salt precipitation, the coal dries with a grey or whitish hue.
Since, hydrochloric acid is the potent acid with the highest ability to dissolve inorganic materials, it is used for this type of reactivation. The coal is submerged in a solution that is about 5% by weight ready. The calcium salts take a long time to dissolve. Depending on how deeply ingrained the charcoal is, the process can take several hours. The solution is heated to 60–70 o C to increase process speed.
· Reactivation in aqueous solutions by adjusting the pH
This circumstance can be employed to desorb an adsorbate when its retention ability is pH-dependent. For instance, the pH of the solution in which phenol is present has a significant impact on phenol adsorption. Its adsorption reduces when the pH rises to the point where desorption is possible.
A 4% soda solution can revive charcoal that has been saturated with this substance. By submerging the carbon in a solution with a pH that causes inorganic chemicals, which have precipitated on activated carbon to dissolve, the inorganic compounds can also be removed.
There are very few uses for reactivating activated carbon by changing the pH of an aqueous solution.
· Biology reactivation
In the natural treatment of waters with a reasonably high level of biodegradable organic compounds, GAC undergoes biological reactivation. For instance, tertiary wastewater treatment falls under this category.
The biodegradable material that has been absorbed by the bacteria that live on the coal's surface, is used to produce carbon dioxide and new bacteria. The GAC is backwashed to get rid of the latter. Activated carbon is known as biological charcoal when it functions in this manner.
Although it is uncommon, a GAC can also be biologically reactivated after being withdrawn, from the adsorption process in which it was used.
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