Regeneration of activated carbon using hot vapors under reduced pressure
Chlorinated solvents are widely used in cleaning metal parts in manufacturing industries. Tetrachloroethylene (PCE), for instance, is used in eighty percent of all dry cleaning operations in the U.S.42. Most of these chlorinated solvents are known carcinogens. The advent of the Clean Water Act and Clean Air Act amendments of 1990, and other local strict regulations have led to extensive use of activated carbon beds for end of pipe pollution control. The growing cost of virgin activated carbon and stricter landfill regulations have made carbon regeneration a viable means to keep cleaning costs within reach. ^ Until today all of the regeneration processes are accomplished by heating the bed with an inert gas or steam (thermal swing) and then in some cases subjecting it to reduced pressure (pressure swing) in a later step. In either case a substantial amount of adsorbate is often left on the activated carbon. Moreover steam stripping tends to transfer the solid waste problem to water and ironically air pollution problems due to the need to dry the bed. ^ According to the present proposed model, both modes of regeneration are utilized in a single step. Wet activated carbon containing adsorbed volatile organic ingredient (PCE) is effectively regenerated using solvent saturated air which is heated under vacuum, which serve to drive off the adsorbed PCE. Heated air vapor mixture preferably 250 to 300°F, is introduced into the regeneration bed. The sensible heat of the heated vapors heats the activated carbon to promote devolatilization of adsorbed PCE. After a significant portion of the adsorbate is desorbed in the heating stage and the carbon is above 250°F, a partial vacuum of 250 torr is gradually created in the regeneration column by throttling the incoming regenerating stream. The reduced pressure within the vessel causes further desorption as the vapor pressure of the adsorbate approaches or exceeds the reduced pressure within the vessel. Once a significant amount of adsorbate is desorbed by a combination of temperature and pressure swings, and the bed is still at high temperature i.e. 250°F; the vessel is sealed and evacuated to a pressure of 25 torr. This exclusive pressure swing stage recovers the last traces of the recoverable adsorbate present in activated carbon. The regenerating stream and desorbed PCE are withdrawn from the bed. It is chilled, and PCE is recovered as a liquid. ^ It is found that this method is highly successful in regenerating polluted activated carbon. Activated carbon having 0.6 lb. PCE/lb.-carbon adsorbed to it is regenerated to a value of 0.25 lb. PCE/lb. carbon. The regenerated carbon when put into an adsorption cycle is exposed to a heavily polluted air-vapor stream of 12000 PPM. It is seen that the exiting stream has literally 0 PPM PCE in it. On the other hand there is not much change in the physical and chemical properties of the regenerated PCE and it is reused in metal cleaning operations. ^
"Regeneration of activated carbon using hot vapors under reduced pressure"
Dissertations and Master's Theses (Campus Access).