Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.1 Absorptive solvent recovery 1529
• relative humidity rates below 30% will neither reduce the adsorption capacity nor
the adsorption time,
• relative humidity rates above 70% substantially reduce the adsorption capacity,
• the humidity of the gas will affect the adsorption capacity much more with low
toluene concentrations than with high concentrations,
• with toluene concentrations between 10 and 20 g/m 3 the negative influence of the
humidity is small.
The water content of the activated carbon after desorption may constitute another
problem. The purification efficiency of each activated carbon is the better the less water is
present after desorption. Unfortunately, the desorbed activated carbon in the vicinity of the
adsorber walls usually contains high proportions of water (approx. 10 to 20%). With such a
high water content it is difficult to remove all the water even when drying with hot-air over
longer periods. The wet and poorly-regenerated activated carbons in these zones frequently
lead to higher solvent concentrations in the purified air, and this is even at the beginning of
the adsorption cycle. Some proposals for improvement include:
• good insulation of the adsorber walls,
• sufficient drying after each regeneration cycle,
• cycle the desorption steam and the drying air counter-current to the direction of the
adsorptive stream.
22.1.4.3.5 Interactions between solvents and activated carbon
The majority of solvents are effectively recovered by adsorption on activated carbon and,
when this is the case, the operation of the plant is straightforward. But some solvents may
decompose, react or polymerize when in contact with activated carbon during the adsorption
step and the subsequent steam desorption. 19,25,26
Chlorinated hydrocarbon solvents can undergo hydrolysis to varying degrees on carbon
surfaces, resulting in the formation of hydrogen chloride. Each activated carbon particle
which is in contact with a metal screen or other constructional component may then act
as a potential galvanic cell in the presence of moisture and chloride ions.
Carbon disulfide can be catalytically oxidized to sulphur which remains on the internal
surface of the activated carbon.
Esters such as ethyl acetate are particularly corrosive because their hydrolysis results
in the formation of e.g. acetic acid.
Aldehydes and phenol or styrene will undergo some degree of polymerization when in
contact with hot activated carbon.
During the adsorptive removal of ketones such as methyl ethyl ketone or
cyclohexanone a reduced adsorption capacity has been measured. Corrosion problems were
also apparent and in some cases even spontaneous ignition of the activated carbon occurred.
Activated carbon acts, in the presence of oxygen, as a catalyst during adsorption and
even more frequently during desorption because of higher temperature. 25,26 Figure 22.1.16
shows that the catalytic reactions occurring with cyclohexanone are predominantly of the
oxidation type. Adipic acid is first formed from cyclohexanone. Adipic acid has a high boiling
point (213°C at a 13 mbar vacuum). Further products identified were: cyclopentanone
as a degradation product from adipic acid, phenol, toluene, dibenzofuran, aliphatic hydrocarbons
and carbon dioxide. The high-boiling adipic acid cannot be desorbed from the activated
carbon in a usual steam desorption. Consequently the life of the activated carbon
becomes further reduced after each adsorption/desorption cycle.