Iodine number as a variable to assess the operational capacity of granular (or granulated) activated carbon.
In most applications, activated carbon is used to absorb molecules formed with a high percentage of covalent bonds, such as carbon-carbon bonds or carbon-hydrogen bonds in organic compounds. Activated carbon is also used as a reagent or catalyst, as in the dechlorination of water, one of the most common uses of activated carbon in Mexico. The process of dechlorination (free dechlorination to convert it into chloride ions) is carried out as follows:
HOCl(l) + C(s) ➜ CO* + H+(l) + Cl–(l)
where HOCl(l) is hypochlorous acid, C(s) is activated carbon, and CO* represents carbon oxide that may bind to the surface of activated carbon or dissolve in solution as carbonic acid.
above reaction is not the only one that occurs when water is discharged through activated carbon. There are other cases where carbon is involved as a reaction agent or as a catalyst.
Suppose the “operational capacity” of activated carbon is the amount of liquid (gas or liquid) it can withstand to achieve a certain quality. This quality can be measured in terms of chemical oxygen demand (COD), color, odor, free chlorine, or certain specific compounds or family of compounds. A “breaking point” occurs when the wastewater quality of equipment containing activated carbon is unacceptable. At this stage, it is necessary to replace carbon because its shelf life is over. Must be discarded Used carbon or reactivated.
only way to accurately assess the operational capacity of granular activated carbon is in the field.
Granular activated carbon manufacturers and users are looking for physical and chemical variables that are easy to measure and reflect the operational capacity of carbon. These variables include the adsorption capacity of iodine, phenol, methylene blue, molasses, carbon tetrachloride, butane, etc.
The most commercially accepted variable is the iodine number (iodine index), which is the number of milligrams of iodine that will be absorbed by one gram of carbon (on a dry basis) when the concentration of iodine remaining in solution is 0.02 N.
Why is the price of coal with a high iodine content higher?
The value of iodine has become so widely accepted that the price of most standard activated carbon on the market depends largely on its value. There are two reasons why the iodine value is accepted as a variable for carbon evaluation: (a) For certain types of activated carbon under certain conditions, the iodine value has been shown to be relatively proportional to the carbon surface area. (the area determined by nitrogen absorption); (b) The materials and reagents used to measure iodine value are inexpensive and the time required to perform the analysis is relatively short.
Unfortunately, the reality is that the iodine value is too low to reflect the surface area and operational capacity of activated carbon. To the extent that in science, the iodine number is not used as a variable to assess carbon.
Does the value of iodine determine the ability of activated carbon to be absorbed?
1. The iodine value is only proportional to the surface area of activated carbon.
To determine the iodine number, the molecule absorbed on carbon is a triiodide ion: I 3 -1. Since it is an anion, the surface oxides of activated carbon repel it. Therefore, the more surface oxides activated carbon contains, the lower the iodine value in it, although its surface area will not change.
2. Although the iodine number is proportional to the surface area of each active carbon, the adsorption capacity of iodine is not proportional to the absorption capacity of other molecules.
This is due to two reasons:
2.1 The adsorption capacity of activated carbon depends on the relationship between the molecular shape size to be retained and the carbon pore size distribution.
Particles larger than the diameter of the pores do not enter. And molecules much smaller than the diameter of the pores absorb less strongly and are therefore less efficient.
2.2 The adsorption capacity of activated carbon depends on the chemical composition of the molecule to be retained and the carbon surface chemistry.
For this reason, a higher iodine index does not necessarily correspond to the higher absorption capacity of the molecule (or family of molecules) we want to absorb. It is common to find graphs such as the following in the literature:
3. In most granular activated carbon applications, its operational capacity depends not only on its adsorption capacity (or more precisely “balanced adsorption capacity” or “adsorption capacity at equilibrium”), but also on its kinetics (rate). They work.
The kinetics of granular activated carbon action depend on the following:
3.1 Pore size distribution as well as predominant pore diameter. The larger the pore diameter, the greater the mobility with which activated carbon operates.
3.2 Particle size distribution of granular activated carbon. The smaller the particle size, the greater the mobility of granular activated carbon.
One way to illustrate this is through the following diagram, where two columns of granular activated carbon are fed with the same solution, the same instantaneous flow rate and the same type and amount of activated carbon. The only difference between the two columns is that the average size of the carbon molecules in the right column is smaller. If you measure the depth of the carbon layer where the absorption takes place (a depth called the “mass transfer zone,” MTZ), you will find that the smaller the particle size, the smaller the height of the MTZ. Since MTZ Less for carbon with a smaller particle size, the point of failure occurs later and this carbon will have a higher operational capacity.
Result:
From the above, it can be concluded that the iodine index is not a variable by which the ability of carbon to absorb a molecule other than the triiodide ion can be directly measured. Less than that it cannot be a variable by which the operational capacity of granular activated carbon can be predicted.
What can be useful in predicting the residual operational capacity of activated carbon in use is to measure the iodine number at the time of failure and use this data to predict the approximate time of failure for future operating cycles. As long as the same commercial granular activated carbon is used and as long as the composition of the solution treated with activated carbon is not significantly modified.