Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.1 Absorptive solvent recovery 1535
Key plants with all the above mentioned processes are offered by special engineering
companies. 31,32 The investment cost for a typical recovery plant with hot gas regeneration
can be estimated by the following equation:
COST (in 1000 Euro) = 200 + 0.4 x A 0.7 +15xS 0.7
where:
A feed air rate in Nm 3 /h
S solvent flow rate in kg/h
22.1.5.2.2 Solvent recovery with adsorption wheels
Adsorption wheels 33 are used for the continuous purification of large volumes of exhaust air
containing relatively low solvent concentrations.
The adsorption wheel consists of a number of identical chambers arranged axially
around a vertical axis. All chambers contain adsorbent. The wheel rotates and each chamber
passes in sequence over an exhaust air duct and solvent molecules are adsorbed. As wheel
continues to rotate the chamber in which adsorption had occurred now moves into a
desorption position in which hot air is passed through the chamber and over the adsorbent.
This removes the adsorbed solvent. The hot air flow in the desorption sector is at relatively
low flow rate compared to that of contaminated gas stream. The number of chambers in the
desorption zone is much greater than the number in the absorption zone so the desorption
stream is many times more concentrated in solvent than was the exhaust stream. At this
higher concentration, the desorption stream can now be economically treated in one of the
ways:
• The adsorption stream is purified by means of recuperative or regenerative
oxidation. This approach is advantageous in painting applications with solvent
mixtures that cannot be reused in production.
• For solvent recovery by condensation, the desorption stream is cooled down in a
cooling aggregate and the liquid solvent is recovered for reuse. Water contents
below 1% can be achieved without further purification.
Example:
A manufacturer who specializes in flexible packaging, i.e., for confectionary, always
adds the same solvent mixture (ethanol, ethyl acetate, ethoxypropanol) to his printing ink.
Adsorptive solvent removal of the solvent-mixture by use of an adsorption wheel (Figure
22.1.20) and solvent recovery via condensation proved the technically and cost effective.
10,000 to 65,000 Nm 3 /h of exhaust air from printing machines and washing plants at a
temperature maximum of 45°C and a maximum solvent loading of 4.6 g/m 3 is to be cleaned.
Depending on the air volume two adsorption wheels with a capacity of 26,000 Nm 3 /h and
39,000 Nm 3 /h and separate desorption circuits are used either alternatively or together. The
total desorption air of max. 11,000 Nm 3 /h is being concentrated to max. 27 g/m 3 , which corresponds
to 50% of the lower explosion limit. The condensation unit for the solvent recovery
process is gradually adjusted to small, medium or large exhaust air volumes. The
recovered solvents are stored and returned to the production process.
22.1.5.3 Viscose industry
In plants which produce viscose fibre (stable fibre), viscose filament yarn (rayon) and
viscose film (cellophane), large volumes of exhaust air contaminated with carbon disulfide