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4.3 Management of Spent Materials 347

4.3 MANAGEMENT OF SPENT MATERIALS

4.3.1 Activated carbon regeneration and reactivation

v
,
The adsorptie capacity of carbons is obviously finite. Consequently it is gradually
decreased and finally exhausted after the adsorption of the maximum possible
amount of a substance. The exhausted carbon is then characterized as “spent” and
ated,
v
has to be regenerated, reacti or properly disposed of The regeneration of
f.
v
f
spent adsorbents is the most dificult and expensie part of adsorption technology. It
accounts for about 75% of total operating and maintenance cost for a fed-bed GA ix C
operation.
generation” Although the terms “re and “reacti are used interchangeably “re , gen-
ation”
v
eration” means remo ving the contaminants from the carbon without destroying them while
“reactivation” means destroying the contaminants and reacti which usu- ating the carbon, v
ally occurs at very high temperatures.
Regeneration involves the removal of the adsorbed contaminants from carbon by means
of processes that destroy neither the contaminants nor the carbon. A common process is
pressure swing adsorption, where lo w pressure is applied to remove the contaminants from
the solid phase. Other common processes ine the use of steam (volatilization of
v
olv
adsorbed contaminants) or the use of a hot inert gas, such as nitrogen. The contaminants
v are recoered as liquid after a condensation step.
During the regeneration process, the contaminants are desorbed and a waste stream is
produced. For instance, during steam regeneration a mixture of water and organics from
the condensed desorbed vapor is produced. Thus, a re generation process should be accom-
panied by a waste treatment apparatus.
In most cases, the spent carbon w generated is treated using thermal destruc- aste to be re
tion/scrubbing systems. Under these conditions, the organic contaminants are destro yed
and the carbon is re generated.
olv
v
Spent carbon thermal reactiation ines the remoal of the adsorbed contaminants v
v
v
from the spent actiated carbon and their destruction at a high temperature (typically in
v
excess of 800°C). Carbon losses during reactiation processes can be held at 3–15%
(Zanitsch, 1997; F 1987). aust,
v There are seeral cases where regeneration or reactiation of the spent carbon is either v
technically or economically not viable. In these cases, the common practice is the proper
, disposal of the spent carbon. Generally we proceed to the disposal if
• the carbon is irreersibly contaminated by the adsorbed substance, v
,
• the regeneration or reactiation process is too costly mainly when dangerous sub-
v
v stances like radioactie particles are the adsorbed species.
v
xample, For e if the carbon has been used for the remoal of PCBs, dioxins, or hea vy
metals, and generally in the case of powdered acti the disposal of the spent ated carbons, v
material is the most appropriate method.