Mechanical Properties of Carbon Fiber Composites for Environmental Applications

Activated carbon fiber composites show gnat promise as fixed-bed catalytic reactors for use in environmental applications such as flue gas clean-up and ground water decontamination. A novel manufacturing process produces low density composites h m chopped carbon fibers and binders. These composites have high permeability, can be activated to have high surface area, and have many potential environmental applications. This paper reports the mechanical and flow properties of these low density composites. Three point flexural strength tests were used to measure composite yield strength and flexural moduli. Composites containing over 10 pph binder had an adequate yield strength of about 200 psi at activations up to 40% weight loss. The composites were anisotropic, having along-fiber to cross-fiber yield strength ratios between 1.2 and 2.0. The pressure drop of air through the composites correlated with the gas velocity, and showed a dependence on sample density Introduction: The use of activated carbons for waste stream clean up is well known. However, the costs of using beds of adsorptive carbons are prohibitive in many cases. The uptake of chemicals into beds of granular carbon can be slow, and the pumping cost for beds of powdered carbons can be prohibitively high. The carbon fiber composites studied here have the advantages of high mass transfer rates inside the fibers, good mechanical seength, and high permeability to liquids and gases. are made &om amorphous carbon fibers which are ''glued'' together using a variety of binder systems. The binder works by cementing together fibers at their points of intersection. Binder may also adhere to the fiber surface, and can be activated as well. These materials, which can be described as a highly porous solids, can be machined or formed to fit in many existing systems, and will act as fixed bed catalytic reactors for chemical removal or adsorption. Specific applications of these materials will result in different static and dynamic loadings. The composite parts must be able to withstand the loads seen during installation as well as operation and exhibit low pressure drop characteristics. Mass Transfer Inside the Fibers Reactants need to move &om the bulk gas stream to the fiber surface, and then diffuse to reaction or sorption sites inside the fiber interior. One critical step in this sequence is the diffusion of the reactants inside the fibers. Because of the small diameter of the fiber, the rate at which this step takes place will be rapid compared to other facton in the process. The fibers have small pores through which gases and liquids can diffuse to the interior. 'the Fourier number fordiffusion can be used with various unsteady state solutions to the cylindrical diffusion equation (Newman, 1931) in order to estimate the time needed to accomplish 90% of a step change in the gas phase concentration at the fiber surface. The diffusion coefficient of the solute is taken to be appropriate values for gas or liquid solutes, and the average fiber diameter was less than 25 microns. The Fourier numbers for the two types of carbons show that the diffusion time for a fiber is several orders of magnitude shorter than for a granular carbon particle (Table 1). The fibers used in the ACFC's studied here have intrinsically rapid uptake rates for both gas and liquid systems. The composites were manufactured using a novel process developed at the University of Kentucky. ACFC's