Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.1 Absorptive solvent recovery 1531
22.1.4.3.6 Activated carbon service life
Activated carbon in solvent recovery service will have a useful service life of 1 to 10 years,
depending on the attrition rate and reduction in adsorption capacity.
Attrition rates are usually less than 1-3% per year. The actual rate will depend on carbon
hardness. Particle abrasion and the resulting bed compaction leads to an increased pressure
loss after several years of service. After 3-5 years of service screening to its original
size is necessary.
Adsorption capacity can be reduced by traces of certain high-boiling materials (resins,
volatile organosilicone compounds) in the waste air which are not removed during the
desorption cycle. Some solvents may decompose, react or polymerize when in contact with
activated carbon and steam (Section 22.1.4.3.5).
This gradual loading will decrease the carbon’s activity. While carbon can remain in
service in a reduced capacity state, it represents a non-optimal operation of the system that
results in
• reduced amount of solvent removed per cycle
• increased steam consumption
• higher emissions
• shorter adsorption cycles
Eventually, recovering capacity will diminish; it is more economical to replace carbon
than continue its use in a deteriorated state. In replacing spent activated carbon there are two
options:
• Replacing with virgin carbon and disposal of the spent carbon.
• Off-site reactivation of the spent carbon to about 95% of its virgin activity for about
one-half of the cost of new carbon
Some activated carbon producers offer a complete service including carbon testing,
off-site reactivation, transportation, adsorber-filling and carbon make-up. 13
22.1.5 EXAMPLES FROM DIFFERENT INDUSTRIES
Some examples of solvent recovery systems in different industries show that reliable and
trouble-free systems are available.
22.1.5.1 Rotogravure printing shops
Process principle
Adsorptive solvent recovery with steam desorption and condensation units with gravity
separator and a stripper have become a standard practice in modern production plants.
The solvents-laden air (toluene, xylene) is collected from emission points, e.g., rotogravure
printing machines, drying ducts by means of a blower and passed through the recovery
plant.
Design example 30
In 1995, one of Europe’s most modern printing shops in Dresden was equipped with
an adsorptive solvent recovery system (Supersorbon � process (Figure 22.1.18.)).
Design data
(First stage of completion)
Exhaust air flow rate 240,000 m 3 /h
(expandable to 400,000 m 3 /h)
Solvent capacity 2,400 kg/h (toluene)
Exhaust air temperature 40°C
Toluene concentration in clean air max. 50 mg/m 3
(half-hour mean)