Estimating reusability of organic air-purifying respirator cartridges.

Reuse of organic vapor air-purifying respirator cartridges after a job or shift can provide economy and energy savings. However, standards and manufacturers' guidance discourage reuse, presumably due to a lack of quantitative objective exposure and use information. Storage and simulated reuse laboratory studies and modeling have been done to provide such information. Two important parameters of breakthrough curves, midpoint time (related to adsorption capacity) and midpoint slope (related to adsorption rate), have been shown to be unchanged during storage for reuse. Extrapolations to smaller breakthrough concentrations and times can be made from this reference breakthrough and time. Significant step increases in breakthrough concentration upon cartridge reuse have been observed in some cases. Values of immediate breakthrough concentrations upon reuse (IBURs) have been measured and correlated. The Dubinin/Radushkevich adsorption isotherm equation has been used to estimate maximum IBURs, which depend on many factors, including conditions and duration of first use. An empirical equation describing rate of approach to maximum IBUR as a function of storage time has been developed to provide intermediate IBUR estimates, which are also very dependent on the vapor identity and extent of first-use loading. Using these equations, IBUR estimates with appropriate safety factors can be compared with the allowable breakthrough concentration to help the Industrial Hygienist make reusability decisions.

Estimating service lives of organic vapor cartridges III: multiple vapors at all humidities.

A published model for estimating service lives of organic vapor (OV) air-purifying respirator cartridges has been extended to include multiple organic vapors at all humidities. Equilibria among the OVs are calculated using Ideal Adsorbed Solution Theory, whereas the effects of adsorbed water are considered as due to micropore volume exclusion. Solubilities of OVs in water must also be taken into account. Adsorption kinetics of components of mixtures are based on published correlations of the effects of covapors and water vapor. The dynamics of adsorption and competition are incorporated using expanding zones within the carbon bed, taking into account vapor and water displacements. Measurements of breakthrough curves for two ternary OV mixtures at high humidities have been done for a single cartridge type. The service life estimation model, implemented as a spreadsheet and a computer program, has been tested against these data as well as data for OV mixtures from literature sources. Good agreements were obtained between model predictions and experimental breakthrough times at dry conditions and humid conditions.

Estimating service lives of air-purifying respirator cartridges for reactive gas removal.

A mathematical model has been developed to estimate service lives of air-purifying respirator cartridges that remove gases reactively from flowing air. Most gases, because of their high volatility and low polarizability, are not effectively removed by physical adsorption on activated carbon. Models previously developed for toxic organic vapors cannot estimate service lives of cartridges for toxic gases. Often, an activated carbon is impregnated with a chemical to enhance gas removal by chemical reaction(s). The kinds of reactions, types and amounts of impregnants, and effects of the presence of water vary; therefore, the model requires user inputs of gas capacity and water effect parameters. Ideally, these should be available from manufacturers of the cartridges. If they are not, they can be extracted from measured breakthrough times using this model. The key to this model is the observation that adsorption rates of gases can be adequately quantified by the same correlations that have been reported for organic vapors. The resulting model has been used to correlate and predict breakthrough times for several common toxic gases.

Estimating service lives of organic vapor cartridges II: a single vapor at all humidities.

A widely used equation model for estimating service lives of organic vapor air-purifying respirator cartridges has been updated with more recent research results. It has been expanded to account for effects of high relative humidities. Adsorption capacity competition between water vapor and organic vapor is largely explained by mutual exclusion of adsorption volume of the activated carbon. The Dubinin/Radushkevich equation is used to describe the adsorption isotherms of both water and organic vapors. Effects of relative humidity and adsorbed water on adsorption rates are described by an empirical correlation with breakthrough times. The dynamic natures of adsorption and competition are incorporated using an expanding zone model with displaced water rollup. The complete model has been tested and verified with published and unpublished data from many sources.