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

Contents

To understand the adsorptive solvent recovery we have to consider some fundamentals of adsorption and desorption (Figure 6). Adsorption is the term for the enrichment of gaseous or dissolved substances (the adsorbate) on the boundary surface of a solid (the adsorbent). On their surfaces adsorbents have what we call active centers where the binding forces between the individual atoms of the solid structure are not completely saturated. At these active centers an adsorption of foreign molecules takes place.

Figure 6: Fundamentals of adsorption and desorption

The adsorption process generally is of an exothermal nature. With increasing temperature and decreasing adsorbate concentration the adsorption capacity decreases. For the design of adsorption processes it is important to know the adsorption capacity at constant temperature in relation to the adsorbate concentration.

Figure 7 shows the relation, termed adsorption isotherm, in the known representation according to Freundlich. The equilibrium loads of five typical solvents in relation to the solvent concentration were plotted. It is shown that various solvents are adsorbeddifferently depending on the strength of the interaction forces between adsorbate andadsorbent.

Figure 7: Adsorption isotherms of typical solvents

Please look carefully at solvent No. 2. It is toluene. We will come across toluene again when we speak about the plant examples.

To remove the solvent, the contaminated air flows through an activated carbon bed. Figure 8 shows the idealized breakthrough curve as a function of the gas volume to be treated. After short working time the activated carbon bed is subdivided into threezones:

• On the inlet side of the adsorber the equilibrium load corresponding tothe inlet concentration c_o is achieved.
• Next follows the adsorbate reduction in the so called adsorption ormass transfer zone (MTZ).
• The outlet zone of the adsorber filling is still unloaded at this time.

Figure 8: Idealized breakthrough curve of a fixed bed adsorber

To reverse the adsorption mechanism the equilibrium must be reversed by increasingthe temperature and decreasing the solvent concentration by purging (Figure. 9).

Direct steaming of the activated carbon bed is the most widely used regeneration technique, because it is cheap and simple. Steam is very effective in raising the bed tem perature quickly and is easily condensed to recover the solvent as a liquid. A certain flow is also required to reduce the partial pressure of the adsorbate and carry out the solvent of the activated carbon bed. The amount of steam required depends on the interaction between solvent and activated carbon.

Figure 9 shows schematically the principle of steam desorption.

Figure 9: Principle of steam desorption

First, the temperature of the activated carbon is increased to appprox. 100°C. This temperature increase reduces the equilibrium load of the activated carbon. Further reduction of the residual load is obtained by the flushing effect of the steam and the declining toluene partial pressure. The load difference between spent and regenerated activated carbon - we call it "working capacity" - will be available for the next adsorption cycle.

Desorption of the adsorbed solvents starts after a certain delay, i.e. after the activated carbon bed has heated up. The specific steam consumption increases strongly with decreasing residual load of the activated carbon (Figure 10). For reasons of cost effectiveness desorption is not run completely. The desorption time is optimized by consideration both of the acceptable residual load and the specific steam consumption.

When designing a recovery plant,the engineer now tries to consider as for as possible these bases of adsorption technology.

Figure 10: Desorption efficiency and steam consumption