Fir fibers

Here, the sorption of viscous organics, heavy oils, different oils such as engine and cooking oils, and also biomedical fluids are discussed on macroporous carbon materials, mainly three kinds of materials, i.e., exfohated graphite, carbonized fir fibers, and carbon fiber felts. The recovery of sorbed heavy oils from macroporous carbon materials is also discussed. 

Figure 27.3 Scanning electron microscope (SEM) images of carbonized fir fibers.

Fir fibers carbonized at 900°C Fir fibers carbonized at 380°C Exfoliated graphite... 

Figure 27.8 Dependences of sorption capacity of exfoliated graphite and carbonized fir fibers for the C- and B-grade heavy oils on bulk density of carbon sorbents.

Kinetics of sorption into carbon materials for different oils was studied by using the so-called wicking method [31]. The system for the measurement is schematically shown in Fig. 27.11(a). The mass increase by capillary suction of oils from the bottom into carbon sorbents, either exfoliated graphite or carbonized fir fibers packed into a glass tube with a cross-sectional area of 314 mm with different densities or carbon fiber felts cut into similar cross-sectional area, was measured at room temperature as a function of time. The change in the mass of carbon sorbents due to the sorption of oils was plotted against time (sorption curve) as shown in Fig. 27.11(b). 

For the viscous C-grade heavy oil, the value of sorptivity was so small, about 0.2kg/m s, that its slight dependence on the bulk density of the two carbon sorbents — carbonized fir fibers and carbon fiber felts — was observed. 

So far, mats of some polymers, such as poly(propylene) and poly(urethane), have been used for the sorption of spilled oil. Their maximum sorption capacity is about 10-30 g of heavy oil per 1 g of polymer [35]. However, they sorb water, as well as heavy oil, and show no special selectivity for heavy oils. Therefore, the effective sorption capacity of the polymer mats for heavy oils floating on water must be lower than the figures mentioned above. Some natural sorbents prepared from cotton fibers, milkweed flosses, and kenaf plants were reported to have rather high sorption capacity and certain potential for oil recovery and sorbent reusability [35—41]. The sorption capacity of macroporous carbon materials, exfoliated graphite, and carbonized fir fibers, is very high in comparison with these materials. Preferential sorption of oils is an advantage of carbon materials in addition to their high sorption capacity. 

The same discussion on heavy oil sorption into carbonized fir fibers is reasonably assumed, which have similar pore structure, large spaces among the fibers of fir plants, small pores inside the fibers, and also rough surface of the fibers. In... 

Inagaki, M., Kawahara, A., and Konno, H. (2002). Sorption and recovery of heavy oils using carbonized fir fibers and recycle. Carbon, 40, 105-11. 

FIGURE 4.3 SEM images of fir fibers carbonized at 900°C. (a) Fibrous particles and large spaces between them, (b) rectangular pores along the fiber axis, and (c) small pores on the wall of the fibers. 

This strong dependence of sorption capacity on the bulk density of carbonized fir fibers suggests that the spaces formed between entangled fibrous particles with irregular surfaces, as shown in Figure 4.3, are primarily responsible for heavy oil sorption. 

FIGURE 4.10 Dependence of sorption capacity of carbonized fir fibers on their bulk density, (a) A-grade heavy oil and (b) C-grade heavy oil. 

Fibrous components of several plants, such as milkweed, kenaf, cotton, and sugi (don), have been applied for recovery of spilled heavy oils [15-21,70]. In Section III.B, carbonized fir fibers were described as useful sorbents for heavy oils. The hydrophobic (oleophilic) nature of most carbon materials is expected to be advantageous for oil sorption. It seemed interesting from the viewpoint of global environment to apply as oil sorbents porous charcoals with a low bulk density prepared from quickly growing woods and plants. 

The capacity of carbon sorbents for heavy oils was found to depend mainly on their bulk density, rapidly decreasing with increasing bulk density, because the macropores in the sorbent in the range of 1 to 600 pm are primarily responsible for heavy oil sorption. As both exfoliated graphite and carbonized fir fibers can have very low bulk density, for example, 6 to 7 kg/m, they have a very high sorption capacity of about 80 kg/kg. The sorption capacity of exfoliated graphite was also found to depend on the viscosity of the oil. Carbon fiber felts, however, do not have such low bulk density, and so their sorption capacity is correspondingly low. 

Water adsorption (g/g) by exfoliated graphite and carbonized fir fibers... 

In the case of carbonized hr fibers, water adsorption depends strongly on the type of water, as shown in Table 4.3 the fibers adsorb a relatively large amount of distilled water, but only a small amount of seawater adsorption is much more for distilled water and much less for seawater than for exfoliated graphite. Fir fibers carbonized at 900°C adsorbed a little less water than those carbonized at 380°C. After the fir fibers were saturated with distilled water, heavy oil was dropped... 

FIGURE 4.19 Plots of versus f 2 o" carbon sorbents with different bulk densities, (a) Exfoliated graphite, (b) carbonized fir fibers, and (c) carbon fiber felt. 

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