Page 122 - Adsorbents fundamentals and applications

ACTIVATED CARBON FIBERS 107

The diameters are retained after activation. Most commercial activated carbon
fibers have a diameter of nearly 10 µm, although other sizes in the range of
8–20 µm are also available. Figure 5.18 shows a SEM image of a commercial
ACF. The fiber diameters of ACFs derived from various precursors are shown
in Figure 5.19.
The small and uniform fiber diameter has a direct and important implication
on the mass-transfer rates for both adsorption and desorption. The uptake and
2
desorption rates are directly related to the diffusion time constant, D/R (where
2
D is diffusivity and R is radius). The value of R /D is approximately the time
for accomplishing 99% diffusion upon a step change for a spherical particle with
clean initial condition. The value decreases rapidly with decreasing R. ACF has
only micropores of a diffusion length less than R (i.e., fiber radius), whereas
GAC has both micropores and meso/macropores. Hence two diffusion time con-
stants are involved. The interplay of these two diffusion time constants has been
delineated from the analysis of diffusion in a bidisperse porous structure by Ruck-
enstein et al. (1971). In all commercial sorbents, the resistance by the macropore
(in pellet) diffusion is as important as the micropore diffusion because of the
much larger diffusion distance in the macropore.
Numerous studies have been reported on the breakthrough curves of VOCs
in packed beds of ACF and were compared with that from beds of GAC (Lin
and Chen, 1995; Schmidt et al., 1997; Lordgooei et al., 1998). Schmidt et al.
(1997) showed that the adsorption of methylene blue from aqueous solution in a
rayon-based ACF was 2 orders of magnitude faster than in a granulated activated
carbon, and 1 order of magnitude faster than in a powdered activated carbon. The
work of Suzuki (1990) best illustrates the comparison between ACF and GAC,
and is shown in Figure 5.20. The breakthrough behavior of trichloroethylene
(TCE) from a contaminated ground water in a packed bed of ACF is compared
directly with that in a GAC bed. Not only the amount of TCE adsorbed in the
GAC bed is less, but the breakthrough curve in the ACF bed is much sharper.
The sharper breakthrough curve is due to the higher mass-transfer rates, rather
than less axial dispersion (Suzuki, 1994). The role of mass transfer resistance in
spreading the wavefront is well understood (Yang, 1987).
Axial dispersion in the beds of ACF was studied by Suzuki. The axial disper-
sion coefficient was proportional to flow velocity, and the proportionality con-
stants for different beds could be correlated (increased) with the bed densities (in
g/cc) (Suzuki, 1994). Using the dispersion coefficient and a Freundlich isotherm,
Suzuki could predict the breakthrough curves in ACF beds (Suzuki, 1994).
(3) Graphitic structure, high conductivity, and high strength: It has been
pointed out early in the development of ACF that it has the advantages of greater
rates as well as possible in situ electrical regeneration of the sorbent due to
its high electrical conductivity (Lin and Economy, 1973; Economy and Lin,
1976). The high conductivity of ACF (or low electrical resistivity) derives from
its graphitic structure. The electrical resistivity (as defined by Ohm’s law) of
isotropic ACFs are in the range of 4–6 m -cm (manufacturers’ data quoted from
Burchell et al., 1997). These values are only about 3–5 times higher than that