Adsorption with a LIQUID PHASE carbon is useful to remove organic compounds that originate unwanted color, smell or taste. This technique is, in most cases, the simplest and less expensive option as compared to other techniques, for example: distillation, crystallization, etc. Although most of the compounds that carbon adsorbs are organic in nature, there are some important inorganic exceptions. Adsorption in the LIQUID PHASE is the result of two phenomena:
- Physical Adsorption: caused by Van Der Waals forces.
- Chemical Adsorption: caused by chemical links
Adsorption in the LIQUID PHASE is the result of equilibrium between adsorption and desorption, consequently, it is a complex phenomenon that can be influence by many variables.
There is an empirical equation that is useful to predict the behavior of activated carbon in most of the liquid phase applications.
- X = amount of adsorbed impurities.
- M = carbon dosage
- C = residual impurity concentration.
- KC 1/n = constants
If we draw this equation in logarithm paper, we will get a straight line.
This graph is known as Freundlich Isotherm and is very useful when evaluating the behavior of activated carbon for a certain application, and to find the appropriate dosage.
When the data of carbon dosage vs. the average of removed impurities for a certain application are drawn in a graph, a graph like this one is obtained:
can be noted that there is a range where the efficiency of the activated carbon continually improves, but there comes a point when although more carbon is added, the rate of improvement in removal is reduced.
An activated carbon generally adsorbs from 10% to 60% of its weight in impurities. Unfortunately, in the LIQUID PHASE the impurities that have to be removed are usually a mixture of compounds whose precise composition is rarely known.
Therefore, producing isotherms is highly important. The isotherm will only apply to the same conditions under which it was produced, and by changing any one of the variables, the isotherm may change significantly.
For a compound to be adsorbed by the activated carbon, its molecules must penetrate the carbon pores, thus, their diameter must be bigger than the impurity molecules themselves.
In LIQUID PHASE most of the molecules are medium size or large, and that they require carbon with a significant mesopores.
Considering that adsorption is an equilibrium process, any impurity that has a similarity with the product in which it is present, will make adsorption difficult. For example, a highly soluble pollutant in the medium it is present will be more difficult to adsorb than one with a medium or low solubility.
During the adsorption process one of the generally more critical steps is the diffusion of the impurities to be removed towards the external surface of the carbon, consequently any variable that affects diffusivity can also affect adsorption.
In general, the most important variables that affect adsorption are:
A higher temperature generally makes reaching equilibrium faster; yet, the amount of adsorbed impurity is lower. That means that if time were not important, more adsorption could be achieved at a lower temperature; rarely practical at an industrial level, so increasing the temperature -when possible- is generally beneficial but needs to be evaluated on a case by case basis.
Many compounds that have color, vary in structure and color when the pH changes. In most of the cases discoloration at a lower pH is more efficient because of two reasons:
- Compounds that generate color are generally highly dependent on the pH, becoming less intense at a lower pH.
- Adsorption is possibly more efficient at a lower pH.
When you are not sure about the behavior, it is better to modify the pH and find an activated carbon with a pH that is close to that of the process.
Carbon particle size
The carbon surface area is largely internal and consequently in most applications the size of the particle does not have an appreciable effect on the adsorption capacity of the carbon itself. Yet, it does affect the velocity to reach the equilibrium.
For example: On a certain application, several hours of contact might be needed when using granular carbon in order to get the same results that might have been obtained using powdered carbon during a 30 minute contact. The disadvantages of using a smaller particle are:
- With granular carbon: A higher pressure drop
- With powdered carbon: Less filtration capacity
Carbon purity: ash
Ash is typically composed of inorganic compounds present in the raw material from which the activated carbon was produced and that did not volatilize during the activation process.
The ash content is often correlated to the quality of a carbon. This may be due to purity concerns for soluble ash components, but at the same time, ash can play a positive role in adsorption in certain applications.
There are many applications in which ash is not important; nevertheless, there are some processes in which the presence of inorganic compounds, for example, calcium, magnesium, and iron can cause an unwanted reaction.
USING ACTIVATED CARBON
There are two ways of using activated carbon during the LIQUID PHASE.
Using powdered activated carbon or granular activated carbon.
Both carbons have specific characteristics that make them more or less convenient for a specific case. Selecting one of them also means selecting the way it will be applied.
USING ACTIVATED CARBON
As mentioned before, the only difference between a granular and a powdered carbon is the size of the particle.
Accordingly, the time needed to get the same result is much longer when using granular carbon packed in a column through which the liquid flows. This kind of operation is advisable for continuous processes to achieve a stable quality of the liquor that is being purified and when working with large flows. It has an advantage; the carbon can be regenerated and therefore the consumption is lower; but the needed investment is high and is not always justified.
The column operation has the same principle as the countercurrent contact; moreover, it could be considered a multiple stage contact. Between the influent and the effluent, there is an impurity concentration gradient and the incoming carbon is exhausted faster that the outgoing carbon.
At the beginning of the operation there is a point in the column where the impurity concentration is the same as that of the outlet. This zone is known as the mass transference zone (MTZ). The rest of the carbon is still virgin. As the operation continues, part of the carbon is exhausted and the mass transference zone begins moving towards the outlet. Finally, there comes a time when the MTZ reaches the outlet. A moment later, the concentration of impurities in the effluent will begin to increase and it can be considered that the column is exhausted. How long this takes depends on several factors:
- Flow Rate (the larger the flow the larger the MTZ)
- Size of the carbon particle; the smallest carbon gives smaller MTZ’s, but a higher-pressure drop.
- Temperature; due to the reduction of viscosity, an increase in temperature generally reduces the MTZ height.
- Characteristics of the carbon being used (pore size) and of the product that is being purified (diffusion coefficient).
It should be emphasized that the capacity of a Granular Carbon is the same as that of a Powdered Carbon, the way to determine the easiness to adsorb a certain impurity is grinding the carbon and running Freundlich Isotherms in the laboratory.
In order to get the design parameters for a column (diameter, carbon height, flow, etc.) a pilot test must be carried out. A simple way to do so is using several 3.5” or 4″ columns in a series.
As a general rule, we can say that the ratio of carbon within the diameter of the columns varies from 2 to 1; and 5 to 1 and the average flow measured as the number of bed volumes that flow through the carbon in an hour (VCH) ranges between:
- Discoloration 0.2 – 0.6
- Deodorization 1.0 – 2.0
- Treatment 1.0 – 4.0
There are two system variations for granular carbon:
- Pulse bed system
- Fixed bed system
PULSE BED SYSTEM
Pulse bed systems operate in upflow mode and part of the carbon is extracted periodically. Such is replaced by virgin or regenerated carbon through the top of the column.
This is an efficient arrangement that provides a continuous operation, but it has some disadvantages, for example:
The liquor must be free of any suspended solids. If there are any, the carbon bed will act as a filter and create pressure.
The flow must be accurately controlled. If there are any important variations, the carbon bed might liquefy and the liquor might drag some carbon.
FIXED BED SYSTEM
This system operates in downflow, the carbon slowly is spent from top to bottom and when the outgoing concentration is higher than the allowed maximum, the column is no longer operated and the carbon is replaced. This arrangement is less efficient because when the column is no longer operated, part of the carbon still has a certain degree of activity, yet, it is more versatile. In the event the liquor had suspended solids that were caught by the carbon bed, the operation is stopped and it is backwashed to remove the solids to waste.
In addition, if the flow increases considerably there would be no difficulties. A variation for this arrangement to render it more efficient and to better exhaust the existing carbon is to use two columns in a series. When the first column is exhausted, No. 2 column becomes No. 1, and a recently regenerated column enters the system as column No. 2.
In the fixed bed systems, it is always necessary to have an empty 40 – 50% space. Such space must be available to backwash the carbon bed. When backwashing, a flow that allows from 20% to 30% bed expansion must be used.
When using powdered carbon, this is added to the product that is being purified in a tank that has enough agitation in order to get an homogeneous suspension. After adequate contact time, the carbon is removed either by sedimentation or filtration. When possible, it is recommended to have a small mix tank where the activated carbon suspension is prepared -with water or with a clean liquid- and the powder is allowed to wet efficiently.
The suspension is later added to the treatment tank.
The powdered carbon operation is more versatile because the dosage can be modified depending on the quality of the liquid that is being purified, so reducing variations in the process. In addition, the equipment needed is very simple and conventional, therefore the required investment is low.
The dosage range can be determined by using Freundlich isotherms described before. In general, the carbon dosage in most of the applications is lower than 2%. Sometimes, the isotherm for a certain application shows a steep slope that indicates that a high dosage of activated carbon is needed to achieve high purification levels. Such being the case, to improve the use of the carbon and to reduce its dosage, the double countercurrent contact is a good option. This process consists in a first treatment with activated carbon that has been through the process once. Then, the liquor obtained is treated again, but now with virgin carbon.
The final liquor would have a better quality than the one that only had one contact. The carbon that is collected from this second filtration is precisely the one that is used again for the first treatment.
The result of the double contact means important savings of carbon of 50% or more. Yet, before making a decision, the need for more equipment and more labor must be considered. An approximation to double contact is achieved by taking one of the filters used to filtering liquor with carbon, and instead of throwing away the cake and washing it, use it to filter the untreated liquor before sending it to the carbon treatment tanks. This double contact, although not as efficient as the one mentioned above, requires less additional equipment.