Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.1 Absorptive solvent recovery 1533
eration with hot inert gas has became the more favored process in the packaging printing industry.
22.1.5.2.1 Fixed bed adsorption with circulating hot gas desorption 31,32
Activated carbon preferentially adsorbs non-polar organic solvents, while adsorbing relatively
little water. Thus activated carbon allows the passage of 95-98 % of the water while
retaining the solvents from the gas stream. During the adsorption step solvent laden air or
gas is delivered to the bed through the appropriate valves. As adsorption starts on a fresh
bed, the effluent gas contains traces of solvent. Through time, the solvent level increases
and when it reaches a predetermined value, the adsorption is stopped by closing the feed gas
valves. The adsorption step usually lasts for 4-16 hours. An example of a solvent recovery
plant with 5 activated carbon beds and hot gas desorption is shown in Figure 22.1.19.
If necessary, feed air is cooled to 35°C and sent by the blower V-1 to the main heater 6
from which it enters 4 beds simultaneously, while one bed is being regenerated. The cleaned
air is collected in header 7 and vented.
After the adsorption step, the air is displaced from the bed by an inert gas such as nitrogen.
A circulating inert gas at 120-240°C serves for heating and stripping the solvent
from the carbon. The solvent is recovered by cooling and chilling the circulating gas. The
optimum chilling temperature depends on the boiling point of the solvent. Generally, chilling
temperatures are between +10 and -30°C.
During the heating step, the temperature of the effluent gas gradually increases until a
predetermined value (for example, 150°C) is reached, after which the heater is bypassed and
the bed is cooled down by cold gas. Typical solvent recovery plants will have from 2 to 8
beds. All beds go through the same adsorption, inertization, heating and cooling steps but
each at different time.
Referring to the flow sheet in Figure 22.1.19 the regeneration gas is circulated by
blower V-2 to the gas heater, the header 8 to the bed being regenerated. The effluent gas is
passed to the header 9, cooler, molecular sieves (or water condenser), to the chiller for sol-
Figure 22.1.19. Fixed bed adsorption with circulating hot gas desorption (After references 31,32).