Handbook of Solvents - George Wypych - ChemTech - Ventech!

1532 Klaus-Dirk Henning
Figure 22.1.18. Supersorbon � process for toluene recovery (After reference 30).
Solvent-laden air is exhausted at the three rotogravure printing presses by several fans
operating in parallel and is routed in an upward flow through four adsorbers packed with
Supersorbon � activated carbon. The solvent contained in the air is adsorbed on the activated
carbon bed. Adsorption continues until breakthrough, when the full retentive capacity of the
adsorbent for solvent vapors is used up.
The purity of the clean exhaust air complies with the regulation for emission level of
less than 50 mg/m 3 . Regeneration of the adsorbent takes place by desorbing the solvent with
a countercurrent flow of steam. The mixture of water and toluene vapors is condensed and
the toluene, being almost insoluble in water, is separated by reason of its different density in
a gravity separator. The recovered toluene can be re-used in the printing presses without further
treatment. Very small amounts of toluene dissolved in the wastewater are removed by
stripping with air and returned to the adsorption system inlet together with the stripping air
flow. The purified condensate from the steam is employed as make-up water in the cooling
towers.
Operating experience
The adsorption system is distinguished by an extremely economical operation with a
good ratio of energy consumption to toluene recovery. Solvent recovery rate is about 99.5
%.
22.1.5.2 Packaging printing industry
In the packaging printing industry solvents like ethyl acetate, ethanol, ketones,
tetrahydrofuran (THF), hexane and toluene are used in printing. The solvent laden air generally
contains between 2 and 15 g/m 3 of solvent and, depending on the season, 5-18 g/m 3 of
water.
Adsorptive recovery with steam desorption was widely used for many years but its
disadvantage is that the solvents are recovered in various mixtures containing large amounts
of water. The recovery of solvents is complicated because most solvents produce azeotropes
with water which are not easily separated and, consequently, the waste water fails to
meet the more stringent environmental regulations. In the last 10 years the adsorbent regen-