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:
- Step 1. Fill the vessel with liquid as soon as it is in place. The more time the liquid is in contact with the carbon prior to startup, the better. Assuming the carbon has had time to be wetted, the liquid should be removed in one of two ways: Drain the liquid - after the vessel is empty, introduce the liquid into the vessel while venting the displaced air - or displace the liquid from the vessel through the normal discharge line by using pressurized air. When all the liquid is out, reintroduce the liquid into the vessel while venting the air. When the vessel is full of liquid, the air has been removed.
- Step 2. If the carbon has not been adequately prewetted prior to being placed onstream, one of the previously discussed air removal steps should be performed after 24, 48 and 72 hours onstream. This may appear to be a nuisance, but if it is not done, the carbon usage will be greater than expected, making the treatment prohibitively expensive and the pressure drop could limit flow.
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:
- Step 1. Determine the volume, in gallons, occupied by the carbon by dividing pounds of carbon by 27 lb/ft3 and multiplying by 7.48 gal/ft3.
- Step 2. Introduce 20% of this volume of water into the bottom of the vessel at the rate of 5 to 10 gal per minute/ft2.
- Step 3. After two days minimum, add 20% more volume by repeating Step 2. The longer between the partial fillings, the better. Make sure a vent is open to allow air to escape.
- Step 4. Repeat Step 3 three more times. After the last filling, wait at least two days before going onstream.
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:
- Step 1. The carbon granules are segregated so that the smaller granules are on top of the bed and the larger granules are on the bottom. Fortunately, when the vessel is backwashed in the future, the relative position of the granules does not change so the mass transfer zone is not disturbed.
- Step 2. The air is removed from the bed. If the carbon was not sufficiently prewetted prior to going onstream, subsequent backwashes should be done after 24, 48 and 72 hours to remove the air.
- Step 3. The carbon fines are removed from the bed.
Systems Treating Pure Hydrocarbons
When liquid hydrocarbon is introduced into activated carbon, the temperature increases due to the heat of adsorption.
- Step 1. In order to minimize the temperature rise, the liquid should be introduced into the top of the vessel.
- Step 2. After a minimum of two days, drain or pressure the liquid out of the vessel. If it is not desirable to wait two days, Steps 2 and 3 should be repeated two or three days later to remove more air.
- Step 3. Then introduce the liquid to be treated while venting the air.
- Step 4. When the vessel is full, the system is ready to go onstream.