Addressing air in the bed of dry GAC
Ever since granular adsorbent media such as bone char and granular activated carbon (GAC) replaced powdered activated carbon (PAC), the amount of air in a bed of dry GAC has adversely affected the performance of many adsorption systems. Too many operators of liquid phase adsorption systems are unaware of this property.
In a typical bed of dry activated carbon, the carbon skeleton only occupies 20 volume percent of the bed. The remainder is air. Fifty percent of the air is in the voids and the other 50% is in the pores of the carbon. This air must be removed from the bed in all GAC liquid adsorption applications. In order to get rid of the air in the pores, the carbon must be properly wetted.
Unfortunately, wetting the carbon, which occurs when the liquid being treated enters the pores of the carbon and displaces the air under normal operating temperatures, is not instantaneous and is a function of temperature and time.
As the air is displaced from the pores, it forms air pockets in the bed. This air does not leave the bed as the liquid flows down through the bed. When possible, measures must be taken to remove it prior to putting the adsorber onstream. The technique to most effectively remove the air from the adsorber depends on the system.
When a pilot test is conducted and the air has not been removed from the carbon, a wrong conclusion about the efficacy of carbon performance is reached and the recommendation is to not use carbon for the particular application. This is not an unusual occurrence.
When the pilot system consists of using less than 10 lb of carbon, it is practical to put the carbon in boiling water for two to three hours prior to putting it in the columns.
For larger pilot systems, the carbon should be placed in a container of water or the liquid to be treated several days prior to putting it into the pilot columns.
Considering closed 55-gal drums: When possible, the water or liquid to be treated should be introduced into the drum several days prior to startup. Several times per day, the drum should be tilted and shaken in order to cause the air to rise to the top of the carbon and exit out an open nozzle.
If the drum is placed onstream without proper prewetting, the following procedure should be used. After 24 and 48 hours of operation, shake the drum and open the inlet in order to allow the displaced air to leave the drum. Another option, if compressed air is available, is to pressure the liquid out of the drum. When liquid is reintroduced into the drum, the air will be pushed to the top as the liquid fills the drum. This air then should be vented, allowing the liquid to completely fill the drum, including the GAC internal spaces.
Large Non-Backwashable Drums & Vessels
When these vessels are filled with dry virgin or reactivated granular activated carbon, one of the following procedures will ensure that all of the air is removed from the adsorber:
In many situations this is not possible because there is no place to put the untreated liquid. In these situations it would be desirable to use prewetted reactivated carbon. If this is not a desired option, the following procedure is recommended even though it is time consuming and not as easy as the previously described procedures and not a guarantee that all of the air is removed:
Backwashwable Systems Treating Water
After the adsorber is filled and allowed to stand as long as possible, a backwash operation for 30 to 45 minutes must be performed. This accomplishes three things:
Systems Treating Pure Hydrocarbons
When liquid hydrocarbon is introduced into activated carbon, the temperature increases due to the heat of adsorption.