Modeling effective diffusivity of volatile organic compounds in activated carbon fiber

Volatile organic compounds (VOCs) comprise 67% of total hazardous air pollu tants (HAPs) that are emitted by major industrial point sources into the U. S. atmosphere (1). Adsorption by activated carbon fiber (ACF) has been reco gnized as one of the feasible regenerative control processes to separate an d recover VOCs for reuse. Characteristics of VOCs transport in ACFs are req uired to efficiently design ACF sorption systems. However, extensive resour ces are spent experimentally obtaining transient sorption data to design ad sorption systems. As an alternative, this work develops a new model that pr edicts effective diffusivities of VOCs into ACFs. The diffusion process is modeled as Knudsen transport into the ACF open pore spaces coupled with act ivated surface diffusion on the ACF's internal surface area. Temperature an d Darken's factors are included in the surface diffusion model to provide c orrections for thermodynamic state and deviation from Fick's Law, respectiv ely. Depth of the adsorption potential well is considered as the product of the heat of adsorption of a reference VOC, an adsorption similarity factor , and a surface diffusion energy factor. Introduction of the adsorption sim ilarity factor in the effective diffusivity model is a new concept providin g a means to predict effective diffusivity of similar adsorption systems fr om a reference system. Experimental data from a short length column are use d to determine effective diffusivity of acetone in ACF, Results from this d iffusivity model are compared to experimental values for the acetone/ACF sy stem to describe the degree of closure between modeled and experimental res ults.