Activated charcoal is an auxiliary material which has been used by industry for decades. Activated charcoal is available in powder, granular or shaped block form, in addition, different types with different characteristics are manufactured. Unlike graphite or diamond, activated charcoal does not constitute an accurately defined form of the element carbon, but is a generic term for a group of porous charcoals. All activated charcoals share this structure consisting of a spongiform secondary grid of small graphite crystallites, three dimensionally cross-linked by amorphous carbon.
Figure 6.5.1
Whereas graphite forms relatively large crystals, containing carbon layers in strict orientation, there are only very small crystallites with approx. 7 - 11 Ä height in activated charcoal, with an idealised diameter of 20 - 25 Ä. These layers are linked through random shifts and displacements. The specific structure of activated charcoal leads to the formation of a multiplicity of cracks and crevices, which are called pores, an idealised description being that of cylinders. Depending on the open width of these pores, one distinguishes:
- Micro pores with radii smaller than 10 Ä
- Meso pores with radii from 10 to 250 Ä
- Macro pores with radii larger than 250 Ä
The distribution of differing pore radii is often represented in graphical form, in which the prevalent pore volume is allocated to that of the pore radii and entered appropriately. The wall surface of the pores is described as the internal surface and, with commercially activated charcoals, is of a value of 500 - 1500 m2/g. The micropores, particularly, make a very large contribution to the total surface, whereas activated charcoal with large pore sizes often possesses only a relatively small total surface in spite of high porosity. The manufacture of activated charcoal from non-porous carbon-containing starting materials, is known as activation. In the course of this activation, microcrystalline carbon is generated, and this should ideally be permeated as evenly as possible by a large number of statically distributed pores of varying size.
Diagram 6.5.1
Two processes for manufacturing activated charcoal have become the most prevalent ones :
- Gaseous activation using steam and carbon dioxide
- Chemical activation using phosphoric acid or zinc chloride
Following activation, particular types of activated charcoal are selectively separated, through grinding as well as crushing and sieving processes, in order to achieve the required grain size range. For many tasks, such as the chemical adsorption of toxic gases, impregnation with inorganic salts or organic compounds is necessary. This often calls for an additional heat or gas treatment, in order to activate impregnation and achieve suitable chemical transformation. Adsorption is the accumulation of substances on the surface of a solid. Such accumulation is effected mainly through physical forces, the so-called van der Waal forces. Adsorption processes are reversible and the opposite process is called desorption. Adsorption forces can act only across very small distances; thus pore size assumes a considerable significance in addition to the size of the internal surface. It is necessary to adapt pore distribution to the particular task in hand. For adsorbing relatively small gas or vapour molecules, fine pored activates are preferred. In order to achieve adsorption equilibrium, the charge materials are conveyed through the pore network by means of diffusion.
Figure 6.5.2
The phenomenon of diffusion has the effect that the matter to be adsorbed is not spontaneously adsorbed when flowing through activated charcoal beds. Adsorption takes place in the direction of flow and within a specified layer of activated charcoal, the so-called mass transfer zone. This applies to gaseous media at the usual linear flow velocities of 6 - 30 m/s. The length of the mass transfer zone forms an important parameter for dimensioning and economically operating an activated charcoal adsorber. The mass transfer zone is influenced by the following parameters : Linear approach flow velocity, exercising strong influence on the length of the adsorption zone. High velocities lead to long mass transfer zones and to rather elevated filter resistances. Particle size of the activated charcoal used as significant factor for the length of the mass transfer zone. Small grain sizes lead to a compact adsorption zone but also to high pressure loss. Suitable pore distribution favours the diffusion process, however, drawbacks as far as the mechanical hardness of the activated charcoal is concerned, may well arise. Higher temperature with more rapidly proceeding diffusion processes, as the viscosity of the gaseous medium is materially diminished. At the same time, higher temperature causes a clear reduction of the pressure loss in the charcoal bed. The mass transfer zone is constant throughout the total adsorption period only if one sole substance is targeted. In the case of mixtures of substances of varying affinities to be adsorbed, the weakly adsorbable compounds are displaced from the inlet side in favour of more readily adsorbable components and thus moved along in the direction of the outlet side. This leads to an elongation of the adsorption zone in the course of operating time.
