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| What is the capacity of adsorption of activated carbon? | |||||||||||||
| Why does the pH of water change when a virgin carbon is installed? | |||||||||||||
| Why should an ash-free carbon be used to recuperate solvents? | |||||||||||||
| What kind of carbon is the most adequate to discolor? | |||||||||||||
| What kind of activated carbon is more adequate to make water potable? | |||||||||||||
| What kind of activated carbon is more suitable to purify air and gases? | |||||||||||||
When carbon is exposed to free chlorine, reactions occur in which the acid HOCI or the OCI is reduced to chlorine ion. Such reduction is the result of different ways of reaction possible. In two of the most common, the granulated activated carbon acts as a reduction agent, according to the different reactions
The capacity of an activated carbon to retain a determined substance is not only given for its superficial area, but also due to pore proportion of appropriate size; this is from one to five times the diameter of the molecule of such substance. If this condition is met, the capacity of an activated carbon could be between 20% and 60% of its own weight . This means that 1 kg. of carbon will retain from 200 gr. to 600 gr. of the contaminant. ..
Contrary to common belief, the capacity of an activated carbon does not increase no matter how small its particle size is. This is because it doesn’t matter how small a carbon is grinded, its superficial area does not increase.( the superficial area happens at a molecular level unaffected by mechanical methods) Although , the kinetics ( adsorption particle velocity ) does substantially increase as the size of the carbon diminishes. This means that, the smaller the size of a particle of carbon, the smaller the beds that are needed to achieve a determined performance. On the other hand, the smaller a particle of granulated carbon is, the bigger the pressure fall and therefore increases the cost to obtain the flow from within. So it is concluded that in all cases the smaller carbon size should be used only if the benefit of requiring a smaller bed is higher than the cost of making fluid circulate through it. Note: The increase in kinetic adsorption can be estimated with the next ecuation:
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When a carbon is chemically activated, it’s unpractical and unnecessary for the manufacturer to eliminate all chemicals that were used. So, if the chemical was an acid, the P.H of the first liters of water that come in contact with the carbon will decrease. The contrary will occur if the chemical used was an alkaline. In the event of a thermically activated carbon ( without the presence of other chemicals that are not water vapor and combustion gases) there is a pH increase in the first liters of water treated. This is because all vegetables have important quantities of sodium and potassium that in the carbonization process remain in the carbon in the form of oxides. These oxides turn to hydroxides when in contact with water. They dissolve and increase their pH. When there is no change in the pH of the first liters of water in contact with a carbon; it could be because the carbon may be reactivated.
The recovery of solvents consists in adsorbing a volatile organic compound ( VOC) on a bed of granular activated carbon and to recover it with its desertion without taking the carbon away with water vapor that is set to circulate through the bed each time the carbon has been saturated. This application is done when the VOC is found in low concentrations but in a large quantity of air. The carbon concentrates the VOC in its surface and in the desertion process it is released with a small volume of water vapor. Furthermore, both the water vapor and the VOC are separated by decantation ( if they are immiscible ) or by distillation. At the desertion stage, the saturated carbon and the VOC are subject to a high temperature which could provoke an ignition on the bed. To decrease the possibility of ignition, its necessary to use a low ash carbon. Ashes are sodium, potassium and other metal oxides always present in a virgin carbon that could work as a spark provoking the ignition of the bed of the saturated carbon. ..
The colors that are manifested in liquids are big molecules that are adsorbed in big pores; so the best carbon to retain it is a the one with more macroporosity. The problem with this carbon is that it isn’t hard and so it’s little resistant to abrasion and must be used in powder. When it is required for the discoloring carbon to be granular the best alternative is the lignite carbon. This is the mineral carbon with the highest macroporosity. ..
The typical contaminants present in the water of wells are of low molecular weight, in this case the most adequate carbon is one with a high macroporosity. The carbons that are the most suitable are the ones from coconut shells and the second from bituminic minerals. ..
All contaminants in gas state have molecular diameters smaller to 2 nm. This means they are preferably adsorbed in micropores. The carbon from coconut shells is the one with the highest microporosity and so it is the most used in the purifying of air and gases. |
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