What is the effective diffusion coefficient Transport and he

What is the effective diffusion coefficient? Transport and heat transfer problem

Given Activated carbon, an inexpensive, highly surface area, natural sorbent, is commonly applied in a variety of industrial and environmental processes. Making activated carbon requires combustion of a woody product (often coconut shells or coal) in a reducing environment followed by sulfuric acid treatment to open the pore structure. Varying the woody feedstock and modulating combustion time and sulfuric acid treatment can vary the pore size. In a typical activated carbon bed, 40% of the volume is void volume around the carbon particles, another 40% comprises the carbon\'s pores, and 20% is skeleton carbon. Transport of fluid through the primary flow path is rapid compared to within the pores, resulting in a constant bulk phase concentration outside the pore. Remember that diffusion is a molecular phenomenon. It is desirable to use activated carbon to remove trichloroethylene (TCE) from air. TCE has a formula of C_2HCl_3 and a formula weight of 131.4 g/mol, and normal boiling point of 87.2 degree C. Air temperature is 14 degree C. Assume infinite dilution for TCE in air. Pressure is 2 atm. Average pore diameter for activated carbon is 20 angstroms (2 times 10^-9m). Want: What is the effective diffusion coefficient of TCE in the activated carbon bed?

Solution

EFFECTIVE DIFFUSION COEFFICIENT

Effective diffusion coefficient is normally symbolised as a weighted average of lattice diffusion and grain boundary diffusion coefficient. Arrhenius equation is normally modelled diffusion along the grain boundary as well as in the lattice. As the activation energy ratio of grain boundary diffusion and lattice diffusion varies in between 0.4-0.6 it causes enhancement of grain boundary diffusion when temperature is low. Sometimes enhancement of grain size occurred due to high temperature.