PURIFICATION OF AIR, WATER AND OFF GAS · SOLVENT RECOVERY
Activated Carbon for Solvent Recovery
K. -D. Henning J. Degel
Paper presented at the Meeting of the European Rotogravure Association Engineers GroupMulhouse/France. 20/21 March 1990
Activated Carbon for Solvent Recovery
4. Adsorption systems for solvent recoveryAfter our excursion into the fundamentals of adsorption and desorption I`d like to explain the basic arrangement of the process. A schematic flow sheet is shown on Figure 11.
Figure 11: Flow sheet of a solvent recovery system
The adsorber feed is pretreated to remove solids (dust), liquids (droplets or aerosols) or high-boiling components as they can hamper performance. To prevent an excessive temperature increase across the bed due to the adsorption heat inlet solvent concen trations are usually limited to about 50 g/m3. Most of the systems pass the solvent laden air stream upwards through a fixed carbon bed.
Spent carbon is usually regenerated by downwards flowing low-pressure steam. This removes the adsorbed solvent which is recovered by condensing the vapours and separating the solvent from water by either decantation or distillation. The counter-current pattern of adsorption and desorption favours high removal efficiencies.
After steam regeneration, the hot and wet carbon bed will not remove organics from air effectively, because high temperature and humidity do not favour complete adsorption. This is therefore dried by a hot air stream. Before starting the next adsorption cycle the activated carbon bed is cooled by an air stream down to ambient temperature. Typical operating data for solvent recovery plants and design ranges are given on Figure 12.
| Typical operating data | |
|---|---|
| Air velocity (m/s) | 0.2-0.4 |
| Air temperature (°C) | 20-30 |
| Bed height (m) | 0.8-1.5 |
| Steam velocity (m/s) | 0.1-0.2 |
| Time cycle for each adsorber | |
| Adsorption (h) | 2-6 |
| Steaming (h) | 0.1-5 |
| Drying (hot air, h) | 0.2-0.5 |
| Cooling (cold air, h) | 0.2-0.5 |
| Design Criteria | |
|---|---|
| Solvent concentration (g/m3) | 1-10 |
| Solvent adsorbed by activated carbon per cycle (%weight) | 10-20 |
| Steam/solvent ratio | 3-5:1 |
| Energy (kWh/mt solvent) | 50-600 |
| Cooling water(m3/mt solvent) | 30-100 |
| Activated carbon (kg/mt solvent) | 0.5-1 |
Figure 12: Typical operating data for solvent recovery plants
Recovery units have at least two, but more usually three or four adsorbers which passsuccessively through the stages of the operation cycle. Whilst adsorption takes place inone of them, desorption, drying and cooling take place in the remaining ones.
Gentlemen, many of you are operators of solvent recovery plants and are quite familiar with this field of activity. For this reason, I should like to introduce to you only briefly the published example of a toluene recovery plants design.
| Previous page | Next page | Contents page Index |