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Abstract
To guide the selection of gas phase filtration media in the air cleaning devices, it is important to understand and estimate the media performance under usage concentrations. Filters for improving indoor air quality are typically subject to low volatile organic compounds (VOCs) concentration levels (e.g., ∼50 ppb), while the current standard tests per ASHRAE 145.1 (ANSI/ASHRAE 2008). are performed at relatively high challenge concentrations (∼1–100 ppm level). The primary objective of this study was to determine if media that perform well at the high concentration test condition would also perform well under the low concentration. The secondary objective was to investigate if and how existing models of filtration by media bed can be applied to extrapolate the results from the high concentration tests to the low concentration condition. Experiments and simulations were carried out at both high concentrations (100 ppm for toluene and 1 ppm for formaldehyde) and low concentrations (0.05 ppm for toluene and formaldehyde) for six selected filtration media. The results show that (1) the high concentration test data were able to differentiate the relative performance among the media at the low concentration properly, confirming the validity of using ASHRAE 145.1 (ANSI/ASHRAE 2008) for relative performance comparison; (2) significant initial breakthrough observed at high concentration tests of large pellet media was not present at the low concentration tests, indicating the dependency of the adsorption capability of the sorbent media on the concentration level as well as the possible “by-pass” effects (i.e., not all the VOC molecules in the air stream had the same chance to contact with the sorbent media); and (3) existing models need to be improved by incorporating the concentration dependency of the partition coefficient and the by-pass effect in order to predict the breakthrough curve at low concentrations properly. Such an improved model was proposed, evaluated with the measured data, and was found to be promising for physical sorbent, but requires further development for chemical, catalytic sorbent and large pellet sorbent. The study provides previously unavailable experimental data and new insight into the behavior of the filtration media for volatile organic compounds as well as evidence in support of the application of ASHRAE Standard 145.1 (ANSI/ASHRAE 2008) for media performance evaluation.
Nomenclature
| As | = | pellet surface area, m2 |
| Cg | = | Gas phase concentration in the fixed bed, ppm or mg/m3(air) |
| Cp | = | Gas phase concentration in the pore air, ppm or mg/m3(air) |
| Cr | = | Removal capacity of the sorbent media,% |
| Cs | = | Sorbed phase concentration, ppm or mg/m3 (matrix) |
| d | = | pellet diameter, m |
| Dapp | = | Apparent diffusion coefficient, m2/s |
| Dm | = | Molecular diffusion coefficient, m2/s |
| Dp | = | Effective pore diffusion coefficient inside pellet, m2/s |
| Ds | = | Surface diffusion coefficient inside pellet, m2/s |
| Dx | = | Axial diffusion coefficient through the sorbent bed, m2/s |
| Er | = | Removal efficiency of the sorbent media,% |
| f | = | sorbed phase concentration as a function of gas phase concentration at equilibrium condition |
| km | = | Convective mass transfer coefficient, m/s |
| ksa | = | VOC partition coefficient between concentration in solid matrix and in gas, kg/m3(solid)/(kg/m3(gas)) |
| kr | = | Reaction rate constant, s−1 |
| M0 | = | Maximum chemisorption capacity, kg |
| Mr | = | Mass of pollutant removed, kg |
| N | = | Update rate by each pellet, ppm/s or mg/s |
| Q | = | airflow rate, m3/h |
| R | = | Radius of the sorbent pellet, m |
| Sc | = | ν/Dm |
| Sh | = | Sherwood number, Sh = kmd/Dm |
| t | = | Time, h or s |
| u | = | Mean air velocity over pellet, m/s |
| us | = | Superficial velocity (= flow rate/ bed cross section area), m/s |
| ν | = | Kinematic viscosity of air, m2/s |
| REV | = | Representative elementary volume, m3 |
| W | = | Weight of media sample, g |
| ρVOC, gREV | = | Gas phase VOC mass density per REV, kg/m3(REV) |
| ρVOC, sREV | = | Sorbed phase VOC (in sorbent material) mass density per REV, kg/m3(REV) |
| ρVOC, ggas | = | Intrinsic gas phase VOC mass density in gas, kg/m3(gas) |
| ρVOC, ppor | = | Gas phase VOC mass density in pore air, kg/m3(gas) |
| ρVOC, ssld | = | Sorbed phase VOC (in solid matrix) mass density per REV kg/m3(solid) |
| = | Constant intrinsic inlet gas phase VOC mass density in gas kg/m3(gas) | |
| ρs | = | Density of sorbent material, kg/m3 |
| ρa | = | Density of air, kg/m3 |
| ϵp | = | Sorbent pellet porosity, m3(pore)/m3(sorbent) |
| ϵb | = | Sorbent bed porosity, m3(gas)/m3(REV) |
| σ | = | Adsorption flux into sorbent pellet, kg/s |
| α | = | Reaction activity of chemisorbent |
| αc | = | Reaction activity of catalytic media |