What is activated carbon? How does it work?
Activated carbon mainly consists of elementary carbon in a graphite like structure. It differs from graphite by having a random imperfect structure which is highly porous over a broad range of pore sizes which can be divided into three groups:
- Macropores (above 50 nm diameter)
- Mesopores (2-50 nm diameter)
- Micropores (below 2 nm diameter)
The varied pore structure gives the carbon its very large surface area (approximatley 2500 m2 /g) and also allows the carbon to adsorb a wide range of compounds.
Activated carbon works by adsorbing the compounds it is removing. While particles may be removed, there is no significant filtration action. Adsorption is caused by London dispersion forces, a type of Van der Waals Force which exists between molecules. The force is very strong, but works over short distances only and is sensitive to the distance between the carbon‘s surface and the adsorbate molecule.
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How to use Activated Carbon
Typical uses of activated carbon in distilled spirts are:
- Color removal
- Aroma and flavor adjustments
To successfully apply activated carbon to your process, it is important to carefully define the desired specification for the final treated product.
Sometimes the specification will be a measurable change like color, but in other cases the specification maybe based on a taste profile. In either case, the process must be very well defined so that the results are repeatable. If the process is not well defined, the final product might be under treated resulting in uneccessary rework. If over treatment occurs, the product may be irreversibly stripped of desirable color and/or aromas.
Once the specification is established, careful small scale or laboratory trials are useful to determine which grade of activated carbon works most effectively and how much carbon media is needed per volume of product.
Pall can assist customers to select a suitable product from its range of activated carbon products.
Bulk Powdered Activated Carbon (PAC) vs Sheet Based Activated Carbon
Traditionally PAC is used in its loose/bulk form. The process generally involves 4 operations:
- Handling of the bulk powder and dosing into a mixing vessel
- Mixing of bulk powder with the product being treated
- Removal of PAC from the treated product after adsorption
- Cleaning of carbon from process equipment
The use of bulk PAC has several significant drawbacks:
- Handling and mixing of bulk PAC is labor intensive and requires capital expenditure for specialized equipment
- Cleaning of process equipment, after the product has been treated, is labor intensive
- Removal of residual PAC from the treated product generally requires additional filtration steps which adds cost to the overall process
Below is a typical decolorization process involving addition of bulk activated carbon (cleaning steps not shown).
Seitz® AKS immobilized carbon filter media alleviates these concerns by incorporating activated carbon within a matrix of cellulosic fibers. This immobilized carbon media is then coupled with a downstream filter paper that eliminates any possible carbon particle shedding downstream of the filter. Additionally, the adsorption efficiency of Seitz AKS immobilized carbon filter media is greater than an equivalent amount of bulk PAC, further reducing overall process time and increasing product yield.
Advantages of Sheet-Based Activated Carbon Products
There are two reasons for the improved efficiency of sheet-based adsorption compared to bulk mixing adsorption processes.
- At an optimized flow rate, the probability of contact between the impurities and carbon particles is greater in carbon-impregnated sheets. This is due to process fluids more efficiently contacting carbon particles immobilized into a sheet matrix.
- Due to the depth (thickness) of the sheet, it is possible to consider the structure as being made up of a series of layers containing PAC. The initial layers make first contact with the fluid and as time and increasing fluid volume is passed through this layer they are the first to become saturated with impurity. Subsequent layers, however, still have adsorbing sites and a capacity to adsorb impurity, which increases through the depth of the media. In essence, having a depth of PAC and passing the fluid at an optimal flow rate through that depth enables the carbon to be fully utilized in adsorbing impurities.
Activated carbon has a finite number of adsorptive sites and these are used up during the process of treating the customer‘s product. The downstream effluent should be monitored to determine when sheet or module adsorptive capacity is exhausted