Gas Dispersion and Immobile Gas Volume in Solid and Porous Particle Biofilter Materials at Low Air Flow Velocities

Abstract

Gas-phase dispersion in granular biofilter materials with a wide range of particle sizes was investigated using atmospheric air and nitrogen as tracer gases. Two types of materials were used: (1) light extended clay aggregates (LECA), consisting of highly porous particles, and (2) gravel, consisting of solid particles. LECA is a commercial material that is used for insulation, as a soil conditioner, and as a carrier material in biofilters for air cleaning. These two materials were selected to have approximately the same particle shape. Column gas transport experiments were conducted for both materials using different mean particle diameters, different particle size ranges, and different gas flow velocities. Measured breakthrough curves were modeled using the advection-dispersion equation modified for mass transfer between mobile and immobile gas phases. The results showed that gas dispersivity increased with increasing mean particle diameter for LECA but was independent of mean particle diameter for gravel. Gas dispersivity also increased with increasing particle size range for both media. Dispersivities in LECA were generally higher than for gravel. The mobile gas content in both materials increased with increasing gas flow velocity but it did not show any strong dependency on mean particle diameter or particle size range. The relative fraction of mobile gas compared with total porosity was highest for gravel and lowest for LECA likely because of its high internal porosity.

Relating solute and gas dispersion in gravel at different transport velocities

“…Fig. 3 also shows that the slope (representing dispersivity, α) of the linear parts of the u -D mech relationship for both NaCl and O 2 increases with increasing R in agreement with earlier observations [32].…”

“…More regular and evident increase is exhibited from α g for increasing R, ranging between 3.8 and 25.5 mm. This is likely because increasing R causes increasing pore network tortuosity, and thus, increasing dispersion [32,33]. The α -D m relationship is somewhat more complex but α is generally largest for small values of D m , for both NaCl and O 2 .…”

“…Gas dispersion data were taken from Sharma [32], following the approach of Poulsen [28]. O 2 breakthrough curves were measured in duplicate at gas flow rates of 0.2, 0.5, 1.0, 1.5, and 2 l/min.…”

“…where C m and C im are the tracer concentrations in the mobile and immobile fluid phases (kg m -3 ), D is the overall dispersion-diffusion coefficient (m 2 s -1 ), is the pore fluid velocity (interstitial velocity) in the mobile phase (m s -1 ), is the tracer mass transfer coefficient (s -1 ) for mass transfer between the mobile and immobile fluid phases, and x and t are the space (m) and time variables (s) [32].…”

“…Assuming for a one-dimensional configuration that D mech increases linearly with increasing fluid flow pore velocity [28,32,39,40] we obtain:…”