Carbon adsorption is an effective method for removing impurities from crude oils, such as heavy metals, chlorophyll, carotenoids, pesticides, and others. Contact time is critical when using this method. At an insufficient contact time, the adsorption of impurities may not be complete. Carbon has different rates of affinity to the many impurities in the oil. The impurities with the greatest affinity for the adsorbent would be adsorbed at a faster rate initially and followed by those with less affinity at a lesser rate. Additional time is needed for other particles to be adsorbed.
Research suggests optimal contact time for adsorption is between 20-45 minutes, with a longer time given to oils with greatest impurities. Agitation is critical in this process as intimate contact must occur for adsorption of impurities by the adsorbent.
In a column where fluid cascades over a column of carbon, residence time is usually inadequate, especially in short columns. As oil percolates down the column, the top layer of carbon pores quickly saturates with impurities and no longer performs any function. The impurities with less affinity may adsorb at the bottom of the column or not at all. The result is either a waste of carbon because it must all be replaced before it is all spent, or worse, oil with impurities still in it.
The tried-and-true method is carbon added to oil under agitation (Carbon in Leach or CIL), where the spent carbon is disposed of after a single-use. This method yields cleaner oil, uses less carbon, and yields fewer losses because of oil absorbed onto the carbon.
If your crude is low on impurities, a viable option is depth filtration using carbon-infused filter paper. If you must use carbon in column (CIC), consider a fluidized bed column using 6 x 12 mesh carbon. In this system, the fluid flows up the carbon column and the carbon freely moves in the solution instead of being compacted. There are issues with this system since you must use several properly sized columns and removal of the solvent is more cumbersome.