Frequently Asked Questions About Carbon

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Activated Carbon is a highly porous substance with large surface area. Activated Carbon is created by first burning a carbonaceous substance without oxygen which makes a carbon ‘char’, which is then treated physically or chemically to develop an interconnected series of internal pores. Because of this huge internal pore network, activated carbon has a great surface area that facilitates to attract and hold organic chemicals.

Organic chemicals are present in vapor or liquid streams in various applications or industry production processes and hence the need to clean the unwanted chemicals. Activated Carbon attracts the unwanted organic chemicals from vapor and liquid streams and it is proven that it is highly cost effective in treating large volumes of air or water.

Activated Carbon attracts the unwanted organic chemicals from vapor and liquid streams and it is proven that it is highly cost effective in treating large volumes of air or water. The molecular weight, polarity, solubility in water, temperature of the fluid stream and concentration in the stream are factors to be considered in planning to use Activated Carbon in the removal of contaminations. Volatile organic compounds (VOCs) are organic chemicals that have a high vapor pressure at ordinary room temperature. Their high vapor pressure results from a low boiling point, which causes large numbers of molecules to evaporate or sublimate from the liquid or solid form of the compound and enter the surrounding air, a trait known as volatility. VOCs are numerous, varied, and ubiquitous. They include both human-made and naturally occurring chemical compounds. Most scents or odors are of VOCs. Some VOCs are dangerous to human health or cause harm to the environment. Anthropogenic VOCs are regulated by law, especially indoors, where concentrations are the highest. Harmful VOCs typically are not acutely toxic, but have compounding long-term health effects. VOCs such as Benzene, Toluene, Xylene, oils and some chlorinated compounds are common target chemicals removed through use of carbon.

 

VOCs such as Benzene, Toluene, Xylene, oils and some chlorinated compounds are common target chemicals removed through use of carbon.

 

Carbon is the most abundant element on earth and the most common forms of carbon are coal, coconut shell, wood, peat and lignite.

Physisorption of contaminants is favoured by low temperature and reduces as process stream temperatures increase. Chemisorption of contaminants may be favoured by increased temperature reflecting the increased rate of reaction at elevated temperatures. However, this effect may be offset by resultant instability of chemisorption agents on the carbon and potential desorption of reaction products.

Contact Time (or EBCT – Empty Bed Contact Time) is the time required for the liquid or vapour to pass through a carbon column assuming that all the liquid or vapour passes through at the same velocity. It is equal to the volume of the empty bed divided by the flow rate.

Let us consider an example:

With a liquid flow rate of 60 cubic metres per hour, and a carbon bed containing 9000kgs of activated carbon with a density of 0.45cubic metres per 1000kg.

9000kg activated carbon will occupy a volume of 20 cubic metres.

The contact time will be 20/60 hours, i.e. 0.33 hours or 20 minutes.

No, the terms ‘Activated Carbon’ and ‘Activated Charcoal’ are used interchangeably. Also the term ‘Active Carbon’ is used for activated carbon and activated charcoal.

The first step is to subject carefully selected raw materials at low temperatures to remove natural volatile components and residual moisture levels, which is called as carbonisation. The second step is to pass the carbonised raw material through high temperature activation kilns in the presence of a controlled flow of steam which is used as the oxidizing medium.

The resulting product is Activated Carbon which is a powerful adsorbent with a range of pores of molecular dimensions. Under a scanning electron microscope, the pore development is clearly visible, appearing like a porous sponge. This high concentration of pores within a relatively small volume produces a material with a phenomenal internal surface area (800-1600 m2/g BET N2). This vast internal surface area that gives activated carbon its unique ability to adsorb a wide range of compounds from both the liquid and gas phase. The target compound is contacted with the activated carbon and subsequently diffuses into the internal pore structure. The internal surface area of the activated carbon exhibits weak Van der Waals forces which lock the compound into the pore structure. The process of transferring molecules from the gas or liquid phase onto a solid surface is defined as adsorption.

Contaminant removal can be done through Physisorption (physical adsorption) or Chemisorption (chemical adsorption) or a combination of these two.

In Physisorption, the contaminant enters the carbon granule through transport pores (meso and macropores), it diffuses into the carbon matrix until it enters the smaller pores (micropores) where the adsorptive forces begin to take effect. Once it reaches a higher-energy area, the adsorptive forces become greater than the diffusion forces and the contaminant becomes trapped in the micropore. In Physisorption, no reaction occurs and the contaminant remains unchanged. It can be desorbed and recovered by increased temperature or reduced pressure which is the basis of all solvent recovery operations.

In Chemisorption, the contaminant enters the carbon by diffusion as above however the adsorbent is specially prepared to promote chemical reactions in which the contaminant is consumed. Specialist carbons can be tailor made by Source Carbon in which additional chemicals are added to the carbon surface that react with a specific contaminant e.g. Mercury, or group of contaminants, e.g. acid gases. The contaminant reacts with these chemicals and is transformed and retained in the adsorbent.

First decide whether you are treating liquid or vapor stream. Air is best treated using large particles of carbon to reduce the pressure drop through the bed. Smaller particles are used with liquid applications to reduce the distance the chemicals have to travel to be adsorbed inside the carbon. Whether your project treats vapor or liquid, there are different sized carbon particles available. Different substrates such as coal or coconut shell base carbon to consider. Contact Us to get the best product for your job.

Activated Carbon is typically used in a column contactor. The columns are called adsorbers and are designed specifically for air and water. The design is engineered for loading (amount of fluid per area cross section), contact time (a minimum contact time is needed to insure required removal) and pressure drop through the adsorber (needed to size container pressure rating and fan/pump design rating).

Carbon’s capacity to adsorb chemicals depends on many things. The molecular weight of the chemical being removed, the concentration of the chemical in the stream being treated, other chemicals in the treated stream, operating temperature of the system and polarity of the chemicals being removed all affect the life of a carbon bed. With the flow rate, contaminant details and inlet concentrations, one can estimate on the activated carbon consumption rate.

Test the outlet of the carbon adsorber for the contaminant being removed. Once the concentration of the contaminant is above the acceptable emission or discharge limits, the activated carbon is considered spent and needs to be replaced. In situations where emission measurement is difficult or impossible, a sample of carbon taken from the proper zone of the adsorber may be sent to a specialist chemical lab for residual life analysis. Based on predictions and comparisons from the original activity level of the virgin material, the residual life can be predicted theoretically.

Contact time should be as long as is economically possible such that a contaminant approaches its saturation capacity upon carbon at any particular concentration. It would be helpful if some general indication of “typical” contact times are available to customers in the preliminary design stage of adsorbers. For liquid contaminants, 10 to 20 minute empty bed contact times are typical with the longer times required for contaminants present at low concentration. For gaseous contaminants, where diffusion of the contaminants into the carbon particles is much more rapid, typical contact times are reduced to seconds. Suggested contact times for gas phase adsorption are typically 0.1-1 second for contaminants subject to treatment by physisorption and 1–4 seconds for contaminants subject to treatment by chemisorption.

Activated carbon adsorbs. The chemical process of absorption is commonly compared to a sponge soaking up water. The water is fully integrated into the sponge, not being limited to the surface area. Differently, adsorption is a process whereby molecules stick to the surface area only. As mentioned above, activated carbon has a large surface area due to being a porous material. The unwanted substance sticks to the surface area of the carbon particles.

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