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

Activated carbon is the tradename for highly porous products, made of carbonaceous raw materials, with a large internal surface of 400-1600 m²/g and a large pore volume of more than 30 cm³/100 g.

Above all, the large internal surface of more than 1000 m²/g is significant. The total surface of 200 mt of activated carbon - the content of a larger solvent recovery plant corresponds to the surface of the Federal Republic of Germany.

All activated carbon qualities are characterised by their widely ramified pore system(Figure 2) giving access to what are called macropores (diam = >25 nm), a large number of mesopores (diam = 2-25 nm), and micropores (diam = <2 nm). The pore volumedistribution is an important variable that affects carbon performance.

Figure 2: Activated carbon model

Due to the mainly hydrophobic surface properties, activated carbon adsorbs preferably organic substances and other non-polar compounds from the gaseous and liquid phase . For solvent recovery in Europe, mainly cylindrically-shaped activated carbons with a diameter of 4 mm are used, because they assure a low pressure drop across the adsorber system. Some physio-chemical properties of three typical activated carbons for solvent recovery can be taken from Figure 3.

In the years 1900 to 1902 the Ostreijko patents were issued. These patents are the basis for the manufacture of activated carbon according to present-day definitions.

Ostreijko discovered the two basic principles of activated carbon production which are still valid today, namely:

• chemical activation and
• gas activation.

By chemical activation, predominantly powdered activated carbons are made. And these qualities are often used for waste water treatment. Granular products and pellets for gas purification are predominantly made by gas activation. Commercially, wood, peat, lignite, hardcoal, oil products, charcoal, and coconut shells serve as feedstock.

Carbo-Tech GmbH produces activated carbon pellets by gas activation from specially de-ashed hardcoal. May I just shortly describe our manufacturing processes:

A simplified flow sheet of the process is given in Figure 4. Initially, the hardcoal feed is finely ground and then subjected to partial air-oxidation; subsequently, the coal dust is mixed with a binder. The binder is added to achieve a plastic coal/binder mixture in order to produce shaped extrudates of the desired diameter. These extrudates are then subjected to a carbonization step in a rotary kiln where they are heated to about 900°C. The extrudates are transformed to activated coke. In a subsequent activation step the carbon skeleton of the activated coke is partially gasified by steam activation in a multistep fluidized-bed oven.

Figure 4: Flow sheet of the manufacturing processes

It has become standard to term the resulting loss of weight "degree of activation". A lower degree of activation yields an activated carbon with a high proportion of micro pores, whereas with increasing degree of activation the portion of mesopores and macropores increases.

It is now left to the skill of the activated carbon manufacturers to control the steamgasification in a way to arrived exactly at the required pore system of the activated carbon.

Now a short glance on the activated carbon market:

As results from Figure 5, worldwide

• 15O,000 t/a of powdered,
• 170,000 t/a granulates, and
• 30,000 t/a of pellets are produced.

Figure 5: Activated Carbon World market