Carbon is the most abundant element on earth. Amongst the most common forms of carbon are coal, coconut shell, wood, peat and lignite.
Carefully selected raw materials are processed at low temperatures to remove natural volatile components and residual moisture levels. This is the initial carbonisation step. This is followed by passing the carbonised raw material through high temperature activation kilns in the presence of a stringently controlled flow of steam which is used as the oxidizing medium.
The resulting product 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). To put this into perspective, a tea spoon of activated carbon would exhibit a surface area equivalent to that of a football field. It is 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.
There are many methods of producing activated carbon. The most common method used by Jacobi Carbons is by Steam Activation in either rotary kilns (coconut shell) or vertical shaft furnaces (coal). Jacobi Carbons also produce wood based carbons by Chemical Activation using phosphoric acid.
Activated carbon can be manufactured from principally any carboneacous raw material. Jacobi Carbons materials are made from either wood sawdust, bituminous coal, anthracite coal or coconut shells. We select the correct raw material in order to provide the best activated carbon for a particular application.
Adsorption pores are the internal volume where the graphitic plates are very close together creating strong attractive forces (Van der Waals forces). These forces hold the contaminant in the carbon structure. This is known as adsorption. According to the International Union of Applied Chemistry, the pore sizes in activated carbon are divided into three groups: Micropores (r<1nm) Mesopores (r1-25nm) Macropores (r>25nm).
Contaminant removal can be via physical adsorption – Physisorption or by chemical reaction- Chemisorption, or a combination of these mechanisms. 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. 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 are available from Jacobi Carbons 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.
The only real way to tell when activated carbon is spent is to 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 Jacobi Carbons 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.
Jacobi Carbons can estimate the activated carbon consumption rate utilising data compiled over many years practical experience. If you provide the flow rate, contaminant details and inlet concentrations, we can provide an estimate on the activated carbon consumption rate.
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. For specific information on the effects of temperature etc., please contact Jacobi Carbons.
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.
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, ie 0.33 hours or 20 minutes.
Contact time should be as long as is economically possible such that a contaminant approaches its saturation capacity upon carbon at any particular concentration. However, Jacobi Carbons recognises that it is helpful if some general indication of “typical” contact times are available to customers in the preliminary design stage of adsorbers. For liquid contaminants 10-20 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. For specific information on contact times etc., please contact Jacobi Carbons.
If you have any further questions in regards to Jacobi carbon products then please get in touch and one of our team will be able to assist you.