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348 4. Adsorption and Ion Exchange

Starting with the presentation of the regeneration techniques, we could classify them
into three categories (EPA, 1999):

• generation thermal swing re
• vacuum regeneration
• pressure swing adsorption.
Thermal swing rgeneration e
The spent carbon is treated with steam so that the adsorbed OC is v olatilized and forced V
to leave the carbon. The vapors containing the desorbed VOCs flow through another adsor-
ber. After the desorption is complete, the treated carbon is cooled and dried by means of
cooling air. Following this procedure, the spent carbons regain their initial capacity (EP A,
1999). Besides steam, other sources of heat can be also employed in the re generation
process, such as micro embedded heaters, or heated nitrogen. es, a w v gen- Thermal swing re
eration is most conenient when the adsorbed species are v apor pressures. OCs of lo w v V
Vacuum regeneration
v
If the recoery of the adsorbed VOCs is highly desirable, then instead of steam, a v acuum
regeneration system may be used. According to this method, the VOCs are forced to
,
volatilize not by temperature, but by means of pressure. Specif a vacuum pump is ically
employed to decrease the pressure in the carbon below the vapor pressure of the V OCs,
which leads to their boiling at ambient temperature. This method is generally used with
carbons, polymers, and zeolite adsorbents (EP 1999). A,
Pressure swing adsorber
This system is used for the separation of gases or vapors from air (EP 1999). Ho A, , we v er
the concentration of the vapors has to be of the order of tens of thousands of parts per mil-
lion for this technique to be applied. So, it is not suitable for the control of emissions b ut
for removal of vapors from vents on VOC storage tanks. On exposure to a mixture of gases,
the adsorbent shows a preference for some specific gases. After admitting the gas mixture,
the rest of the gases plus those that are not strongly adsorbed are vThen, the pres- ented.
sure is decreased so that the remaining gases can be desorbed. This method has been used
for the separation of oxygen and carbon dioxide from flue gas (EP 1999). A,

4.3.2 Ion-exchange regeneration

v ersible process, Since ion exchange is a re most of the ion exchangers can be re generated
by means of the appropriate substance: strong acid for cation media or alkali for anion
media. So, the operation life of the exchanger may be prolonged and thus money can be
saved. However, the capacity of the e xchanger is gradually e xhausted and the medium has
to be replaced after some time of operation, een with optimal re v generation.
Generally, regeneration has to be conducted in different vessels, a cation and an anion ves-
sel; otherwise the application of acidic and caustic media to the combined bed (coexistence of
cation and anion beads) will lead to total deactivation of the exchange media. So, the mixed