Granular activated carbon reactivation.
The main mechanism by which organic contaminants are retained on the surface of the GAC (Granular Activated Carbon) is that of physical adsorption, and as such, it is reversible. The theory of adsorption indicates that by changing the conditions in which the carbon is found, the desorption or detachment of the retained adsorbates can be achieved, leaving its surface free.
However, desorption can be very slow and it may not be possible to restore all or almost all of the original capacity of the carbon. On the other hand, chemisorption is not reversible, so the molecules retained by this other mechanism will not be detached. And finally, there are inorganic molecules that have not been adsorbed, but that are deposited on the surface of the carbon, and whose elimination will not respond to the methods of desorption of physically adsorbed molecules either.
Fortunately, the graphical structure of activated carbon (AC) makes this solid very stable under very different conditions. It resists high temperatures, is hard and resistant to abrasion, is not affected by acids, alkalis or many different solvents, although it reacts with strong oxidants. Based on these properties, there are so-called “reactivation” or “regeneration” methods, with which adsorbates of various kinds and organic and inorganic substances of the carbon can be removed. Depending on the adsorbate or pollutant in question, the appropriate method must be chosen.
An AC that is removed from the process in which it was used, is called “spent carbon”, regardless of whether it is discarded or is to be reactivated.
When a GAC is to be reactivated, the support system should be made up of nozzles and not of gravel or sand beds, to prevent the coal from being mixed with a certain amount of particles of these materials.
Types of coal reactivation.
Reactivation with steam.
It consists of circulating water vapor through the carbon bed, without allowing it to condense, as in the case of steam sanitization. In this way, organic molecules with a volatility less than or approximately equal to that of water -that is, with a boiling temperature less than 100 °C (212 °F) at sea level-, and that had been retained by physical adsorption, are desorbed.
It is a widely used method for recovering solvents in air currents, since the molecules released maintain their original structure. Carbon is subjected to alternative adsorption-desorption cycles. In the first cycle, the solvent is retained until the carbon is saturated. In the second, the solvent is desorbed, and the mixture of solvent and water vapor is separated by decantation or distillation.
In the case of water treatment, reactivation with steam may be practical if the contaminant consists basically of odor or low molecular weight, and therefore, volatile compounds. It is not an effective method for the case of carbon that has been used for dechlorination, since the surface oxides generated in such a process are strongly bound.
The higher the pressure of the steam used, the higher it’s the temperature, and therefore it will be able to desorb heavier compounds. The maximum pressure at which seam is handled in a practical way is 6 Kg / cm2 (abs.), which corresponds to a temperature of 160 °C (323.6 °F).
Reactivation with hot gases.
It is the same as the previous one, but the combustion gases are used. In the case of some scientific studies at laboratory level, in which it is required to reactivate without the interference of some oxidizing gas, the desorption is carried out by means of an inert gas that is heated in an indirect way. A faster or more efficient result can also be achieved by using a vacuum.
It is the most widely used method, as it removes practically all organic contaminants retained by physical or chemical adsorption. It also removes some inorganic compounds, and destroys oxides and surface groups. Therefore, it reactivates carbons used in dechlorination or in the elimination of chloramines, potassium permanganate, ozone and other oxidizing agents.
Reactivation with acid.
When the particles of a carbon are white to light gray, they are most likely poisoned or blocked with carbonate or calcium hydroxide. In these cases, thermal reactivation fails to remove these compounds. However, a wash in an acidic solution does.
Any type of strong acid can be used, but hydrochloric is the most common. The solution of about 5% by weight is prepared, and the carbon is flooded into it. The process of dissolving the calcium salts is slow. The exact time varies depending on how poisoned the carbon is, but it can be between 10 and 40 hours. To speed up the process, the solution is heated to 60 – 70 °C (140 – 158 °F). With this, the time can be reduced to a couple of hours.
Once reactivation is complete, the carbon should have recovered its black colour. This is not noticeable while the charcoal is wet, so a small sample should be taken and dried in the lighter – or with a lighter.
Reactivation by modifying the pH in aqueous solution.
When the retention capacity of a specific adsorbate depends on the pH value this condition can be used to desorbate it, thus regenerating the carbon. For example, phenol is adsorbed in relatively high quantities at low pH values, and the opposite at high pH values. Therefore, if the GAC saturated with this compound is washed with 4% solution of soda, it is possible to dissolve a good percentage of it.
This method has no application in the water treatment industry, as it is not applicable for most of the contaminants normally retained by the GAC. Its use is reduced to very specific processes in which the carbon is adsorbed to a singles compound, such as phenol.
Biological reactivation of AC occurs in water treatment, since as described, bacteria that develop on the surface of the carbon feed on the adsorbed biodegradable matter. This benefit can also be achieved in the AC once it has been removed from the adsorption process. To do this, the carbon is placed in a column through which an aerated nutrient-rich solution is recirculated as an expanded bed. This method has found application in the wastewater treatment industry, achieving 80% regeneration in 96 hours. However, up to now its use is not frequent.
Thermal reactivation and conditions to make it profitable.
Thermal reactivation consists of removing the adsorbates from an exhausted carbon using a furnace equal to that used for thermal activation of AC, but at a lower temperature and with a lower concentration of water vapor. The less the process approaches the conditions under which a carbon made from the same raw material is activated, the less carbon is lost through oxidation.
The gases resulting from the reactivation process may contain air pollutants. The most common technology to avoid this consists of an afterburner followed by a scrubber. The afterburner oxidizes organic compounds, and the scrubber retains solid particles and soluble chemicals. Normally water is used in the scrubber, but if acid fumes are expected to be released, a diluted soda solution is used.
The thermal reactivation of a coal that is not hard enough, is not profitable because the process subjects the carbon to a series of movements and eroding actions – flow of hot gases and knocking – that break it down and reduce its size. On the other hand, the less hard carbon is the most reactive in the presence of oxidizing gases, and therefore convert to CO2 more easily.
In the reactivation furnace, the following phenomena occur sequentially:
- In a first stage, the temperature of the carbon increases until it reaches that of the boiling water. The most volatile adsorbates are released and the water evaporates. The wetter the carbon to be reactivated, the more energy, furnace space and time are used to evaporate the water. This is why it is advisable to decant the carbon, and if possible, pre-dry it, before reactivating it.
- The carbon continues to heat up to a temperature of between 300 and 450 °C (572 to 752 °F). During this time, other organic molecules that are less volatile than water, are desorbed.
- Organic compounds that have not been desorbed, begin to decompose. This decomposition is called pyrolysis, and as a result amorphous carbon is formed and remains deposited on the graphite surface of the AC.
- The temperature continues to rise and when it exceeds approximately 500 °C (932 °F), the amorphous carbon that resulted from the previous stage begins to react with the water vapor, oxygen, monoxide, and carbon dioxide in the gas stream. As a result, other gaseous molecules of water vapor and carbon monoxide and dioxide are formed. The graphic plate that form the structure of AC are less reactive than amorphous carbon atoms, and therefore do not, or not significantly, react.
It should be noted that it is essential to maintain the proper proportion of water vapor in the reactive gas mixture, as well as a limited concentration of oxygen. Otherwise, the high temperatures cause the graphite carbon to gasify as well. Experience with granular bituminous and coconut shell activated carbon used in industrial and municipal wastewater treatment indicates that 8 – 15% of carbon is lost per reactivation cycle. These losses include those due to breakage during handling and transport. The harder the carbon, the lower the losses.
Commonly, natural gas or LP gas burners are operated with 10 to 20% excess air volume, looking for 1 to 2% oxygen in the gases coming out of the furnace. In relation to water vapor, the atmosphere in the reactivation furnace does not require more than 30% in mole of that element. For cost reasons, the minimum ratio of injected steam to reactivated carbon should be sought.
The action of water vapor in the reactivation process is only effective at temperatures above 600 °C (1112 °F). Therefore, if wet carbon is fed into the kiln, the steam generated by heating the kiln has no effect on reactivation – it finishes forming when the carbon has not reached 150 °C (302 °F) –.
To restore the adsorption capacity of a carbon, the aim is for it to have the same apparent density at the exist of the furnace as it had when it was a virgin. A reactivated carbon can have a surface area of between 90 and 110% with respect to that of virgin carbon. In the second case, this is due to the fact that the process conditions went beyond reactivation, and produced new pores.
When a GAC poisoned with calcium carbonate is to be thermally reactivated, it is important to start by washing the carbon with 5% hydrochloric acid at 60 °C (140 °F) to remove this compound. In other words, you have to start by reactivating it with acid. The reason for this is that the calcium carbonate deposited on the carbon entering the furnace with it will act as a catalyst for the gasification reaction of the graphite carbon. This results not only in higher AC losses, but also in larger pores. That is, when some of the graphic plates react, there are larger spaces between them, and therefore the resulting pores are larger. As a consequence, the reactivated GAC will have a preference for larger molecules.
If a carbon has an excess of sodium chloride salts in its pores, it is advisable to wash it before putting it in the oven. This is because this compound forms an eutectic with the alumina of the refractory, which melts at 760 °C (1400 °F). In other words, the life time of the refractory decreases. To avoid this problem, it is recommended to wash the carbon in water before feeding it into the kiln.
Another typical problem in reactivation furnaces is the formation of incrustations, which are due to the presence of sodium and potassium salts and to sudden temperature changes in the furnace. One of the most common reasons for such changes is the variation in the flow of coal feed into the kiln. When incrustation forms, it grows until the kiln has to be stopped to remove it manually. If the kiln is of the continuous type, stopping it has a very high cost.
With regard to the cost-effectiveness of the thermal reactivation process, current technology can make it attractive for the user to install his own furnace when his coal consumption exceeds 250 – 500 Kg / day, provided that there is no GAC manufacturer nearby to provide the activation service.
Although many GAC producers offer the reactivation service, the latter represents a specialty as it is required:
- Adjustment of the operating variables of the furnace that is different from the activation process.
- Possibility of substantially varying the time of residence in the oven. This is because the time needed to reactivate carbons is very different depending on the application they had. For example, a GAC used in industrial wastewater treatment requires 4 times more time than one used in water purification.
- Production controls that prevent mixing between carbons from different users. If the kiln is of the continuous type, such as the rotary or multi-stage kiln, special care must be taken since the residence time in the kiln is long and it is difficult to separate one batch from another.
- Have the acid reactivation service to remove calcium carbonate if it is present.
- Possibility of screening to obtain the particle size specified by the user´s process.
Carbon manufacturers charge for the reactivation service based on the weight of product leaving the kiln, and the price is usually between 30 and 50% of the price of a virgin carbon. There is a minimum accepted quantity, which is usually more than 3 – 10 tons. Freight charges are not included, so the distance between the user´s plant and the reactivation plant may be the factor that makes the operation profitable or not.
Currently, environmental regulations in many countries do not allow for the disposal of spent carbon along with municipal solid waste. Normally a higher price has to be paid to companies that collect and receive non-hazardous industrial solid waste, so this factor has to be considered in the cost analysis.
Technical and legal requirements to thermally reactivate a granular activated carbon (AGC) when it is considered a hazardous waste.
In those countries with advanced environmental legislation, there are regulations that determine which materials are considered hazardous waste. These must be treated in a special way, or they must be sent to landfills that have the technical elements to ensure that they will not pollute the environment.
A waste can be considered hazardous according to various criteria, such as: corrosiveness, reactivity, explosiveness, toxicity, flammability or biological activity. An exhausted carbon will be defined as hazardous, depending on the amount and type of pollutants it has retained.
Standardized test are available to analyze the various hazard characteristics. If any of these test results in a carbon being considered a hazardous waste, it cannot be reactivated or sent for reactivation unless the company that will carry out the treatment has the permits to do so.
These permits are granted when it is demonstrated that the facilities and technical personnel, as well as the methodology, are available to ensure appropriate treatment of the waste.
In the case of exhausted carbon considered as hazardous, and for which the thermal reactivation method must be applied, in the USA it is required that the afterburner operates at a temperature of between 1000 and 1100 °C (1832 and 2012 °F) and that the gases have a residence time in it of at least two seconds.
The washing tower is also required as well as an analysis report with which it can be demonstrated that no hazardous emissions are generated during reactivation. Finally, it must be demonstrated that the carbon is free of that which made it dangerous before the treatment.
The regulations for hazardous waste are usually strict in terms of reports and permits required from both the user or generator of the waste and the companies that transport it, process it or confine it. Among other things, the authorities are scrupulous about the final destination of the processed material. In the case of the GAC, the final destination that it will have must be reported, and in most cases, it will be limited to the same application that it originally had.
(2010) © Engr. Germán Groso Cruzado.