Handbook of Solvents - George Wypych - ChemTech - Ventech!

22.1 Absorptive solvent recovery 1539
22.1.5.4 Refrigerator recycling
Condensation process
Condensation processes are especially suitable for the cleaning of low flow highly
concentrated streams of exhaust gas. 36 The entire waste gas stream is cooled below the dew
point of the vapors contained therein, so that these can condense on the surface of the heat
exchanger (partial condensation). Theoretically, the achievable recovery rates depend only
on the initial concentration, the purification temperature and the vapor pressure of the
condensables at that temperature. In practice however, flow velocities, temperature profiles,
the geometry of the equipment, etc. play decisive roles, as effects such as mist formation
(aerosols), uneven flow in the condensers and uncontrolled ice formation interfere with
the process of condensation and prevent an equilibrium concentration from being reached at
the low temperatures.
The Rekusolv Process 37 uses liquid nitrogen to liquefy or freeze vapors contained in
the exhaust gas stream. In order to reduce the residual concentrations in the exhaust to the
legally required limits, it is often necessary to resort to temperatures below minus 100°C.
The Rekusolv process is quite commonly used in the chemical and pharmaceutical industry
and at recycling plants for solvent recovery.
Example of solvent recovery in refrigerator recycling 36,37
In refrigerator recycling plants R11 or pentane is released from the insulating foam of
the refrigerator during shredding. Figure 22.1.22. shows the Rekusolv process as it is used
by a refrigerator recycling company. At this plant around 25 refrigerators per hour are recycled
thereby generating 8 kg/h of polluting gases. The Rekusolv plant is capable of condensing
almost all of this. The unit is designed to operate for 10 to 12 hours before it has to be
defrosted. The plant is operated during the day and is automatically defrosted at night. The
concentration of pollutants in the exhaust gas is reduced from 20 to 40 g/m 3 to 0.1 g/m 3 -a
recovery rate of more than 99.5%.
22.1.5.5 Petrochemical industry and tank farms
Vapor recovery units are installed at petrochemical plants, tank farms and distribution terminals
of refineries. Tank venting gases are normally small in volume, discharged intermittently
at ambient temperature and pressure and loaded with high concentrations of organic
vapors. 2
Processes used for the cleanup of such waste air streams with organic vapor up to saturation
point are often combined processes: 2,38,39
• absorption and pressure-swing adsorption
• membrane permeation and pressure-swing adsorption
• condensation and adsorption
Adsorption on activated carbon and vacuum regeneration
Figure 22.1.23 represents the basic principle of the systems which are used to recover
vapors displaced from tank farms and loading stations and blend them back into the liquids
being loaded. The plant consists of two fixed-bed adsorbers packed with activated carbon
which operate alternatively. The vapor pressure gradients required as a driving force for
desorption is generated by vacuum pumps. As the raw gas entering the adsorber is saturated
with hydrocarbon vapors, the heat of adsorption causes local heating of the well insulated
adsorbent bed. The higher temperature of the spent activated carbon supports subsequent
desorption as the pressure is being lowered by means of a vacuum pump.