Carbon Link Activated Carbons: Activated Carbon for Solvent Recovery

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

6. Process conditions and activated carbon types

Contents

6.1 Pressure drop

The particle size of the activated carbon should not be too small in order to keep the pressure drop through the activated-carbon bed low. Figure 14 compares the pressure drop when using cylindrically-shaped activated carbon pellets with the one due to various activated-carbon granulates. The economic advantages of activated-carbon pellets become obvious from Figure 15 on which the energy consumption of the blowers is plotted for various products. On the other hand, the activated carbon particle diameter must not be excessively large, because the long diffusion distances would delay adsorption and desorption. Commercially, cylindrical pellets with a particle diameter of 3 to 4 mm turned out to be most efficient.

Figure 14: Typical pressure drop for various activated carbon types

Energy consumtion of the blower

Figure 15: Energy consumtion of the blower

6.2 Infiuence of humidity

Activated carbon is basically hydrophobic; it adsorbs preferably organic solvents. However, the water contained in the waste gases may affect the adsorption capacity of the activated carbon Figure 16 demonstrates with - (toluene taken as example) - how the relative humidity of the exhaust air influences the activated carbon loads.

Figure 16: Dynamic toluene adsorption from humidified air according to Börger

Dr. Börger, a German scientist, could prove that

relative humidity rates below 30 pct. will neither reduce the adsorption capacity nor the adsorption time,
relative humidity rates above 70 pct. substantially reduce the adsorption capacity,
the humidity of the gas will affect the adsorption capacity much more in case of low toluene concentrations than with high concentrations,
with toluene concentrations of 10 to 20 g/m³ the negative influence of the humidity is low.

The water content of the activated carbon after desorption may constitute another problem. The purification efficiency of each activated carbon is the better with less water present after desorption. Unfortunately, the desorbed activated carbon in the vicinity of the adsorber walls contains, in many cases, high proportions of water (approx. 10 to 20 pct.). These high water contents are difficult to remove even by hot air drying over longer periods.The wet and poorly-regenerated activated carbons in these zones frequently lead to higher toluene concentrations in the purified air, and this even at the beginning of the adsorption cycle. Some proposals for improvement:

good insulation of the adsorber walls,
sufficient pneumatic drying after each regeneration cycle,
not only the desorption steam but also the drying air shonld be cycledcountercurrently to the direction of the adsorptive stream.

6.3 Activated carbon type

And now some remarks as to the selection of the most suitable activated carbon type. We all know: the adsorption of solvents on activated carbon is controlled by the properties of both the carbon and the solvent. And of course also by the conditions in which they become contacted. Generally, the following factors are to be considered when selecting the best suited activated carbon quality:

kind of the solvent (aliphatic/aromatic/polar solvents)
molecular weight
boiling point

Furthermore, the solvent concentration and the adsorption temperature need to be con sidered. For the selection of the activated carbon:

the pore volume distribution curve,
the form of the adsorption isotherm,
the working capacity,
and the steam consumption.

As previously mentioned, adsorption takes place at the interface boundary. The surface area is one important factor in the adsorption process. In general, the bigger the sur face area is, the higher the adsorption capacity will be. However, that surface area has to be available within the activated carbon. At low concentration, the surface area in the smallest pores into which the solvent can enter, is the most efficient one. With higher concentrations the larger pores become more efficient. With higher concentrations, capillary condensation will take place within the pores and the total micropore volume will become the limiting factor.

These molecules are retained at the surface in the liquid state, because of intermolecular or Van der Waals forces. Figure 17 shows the relationship between maximum effective pore size and concentration for the adsorpion of toluene according to the Kelvin theory.

Figure 17: Relationship between pore size and toluene concentration

It is evident that the most valuable information concerning adsorption capacity of a given activated carbon is its surface area and its pore volume distribution curve.On Figure 18, idealized toluene adsorption isothermes are presented for three carbon types:

large pores predominant
medium pores predominant
small pores predominant

You can see that these adsorption lines intersect at different concentrations, depending on the pore size distribution of the carbon.

Figure 18: Idealized Toluene adsorption isotherms

The following applies to all types of activated carbon:

With increasing toluene concentration in the exhaust air, the activated carbon load increases.

From the viewpoint of adsoption technology, higher toluene concentration in the exhaust air is thus more economic than low concentration. For protecting the plant against explosions, however, the toluene concentration should not exceed 25-40 % pct. of the lower explosivity limit. Health and safety regulations frequently require a certain extent of air exchange in the zones of the printing machines, so that the toluene concentration in the exhaust air cannot be adjusted at will.

There is, however, an optimum activated carbon for each toluene concentration. The pore structure of the activated carbon must be matched to the solvent and the solvents concentration.

In our laboratories we ran - in a small plant - many experiments in view of adsorptive toluene removal. In one incident the exhaust air was fouled with 2 g of toluene/m³, and in another one with 8 g of toluene/m³. The purification tests were run each with a wide-pore and with a narrow-pore quality of activated carbon.

After three adsorption/desorption cycles we measured the toluene breakthrough curves in a fourth cycle (Figure 19).

Figure 19: Toluene breakthrough curves

With the low concentration of 2 g of toluene/m³ the narrow-pore activated carbon turned the best-suited quality with the longest useful life. With high toluene concentrations, however, the suitability of both activated carbon qualities changes. As ex pected, the narrow-pore activated carbon quality is less appropiete whereas the best results are obtained with the wide-pore quality.

With low concentrations, the working capacity as well as the steam consumption are more favourable when narrow-pore activated carbon qualities are used (Figure 20).

activated carbon	worklng capaclty in g/l	steam consumptlon in kg/kg
low toluene concentratlon [2g/m³]
small pores predominant	60	4,0
large pores predominant	43	4,1
hlgh toluene concentratlon (8g/m³]
small pores predominant	64	4,1
large pores predominant	92	3,5

Figure 20: Pore size distribution, working capacity and steam consumption

With higher toluene concentrations (e.g. 8 to 20 g/m³) a wide-pore activated carbon is recommended if low specific steam consumption and high working capacity are desirable.

These examples also show that the producer of activated carbon is in a position of giving useful instructions contributing to the technical and economic success of a solvent recovery plant. It is probably not a too bad idea to contact also the activated carbon producer when the construction of a new plant is considered, or if the purification efficiency and the recovery are not satisfactory.

Please do not hesitate to contact us via Carbon Link with any of your queries related to this subject matter or any other activated carbon issue.