Model toluene contamination

Mehdizadeh, S. N., Mehmia, M. R., Abdi, K. and Sarrafzadeh, M. H. 2011. Biological treatment of toluene contaminated wastewater by AlcaUgenes faecaUs in an extractive membrane bioreactor experiments and modeling. Water Science and Technology, 64,1239-1246. 

The class of chemicals designated as VOCs includes a wide range of carbon-based molecules of sufficient vapor pressure to be present in the air, such as aldehydes and ketones. The most common VOC is methane, the primary component of nafural gas. There are various sources, both natural and human, of VOCs. The response of the fuel cell to VOCs will vary significantly, depending on the molecules in question, but can be significant. For example, benzene and toluene at the ppm level have both been found to significantly affect performance, with the dominant effect believed to be due to adsorption on the catalyst surface resulting in kinetic losses. A semiem-pirical model for toluene contamination based on kinetic losses is described in chapter 3, section 3.8. 

Semiempirical Model for Fuel Cell Performance in the Presence of Toluene In the presence of toluene in the air stream, the fuel cell performance degraded. Figure 3.15 illustrates two sets of representative results of toluene contamination tests, conducted with various levels of toluene concentration at current densities of 0.75 and 1.0 A cm , respectively. The cell voltage experienced a transient period (nonsteady state) immediately after the introduction of toluene, then reached a plateau (steady state). The duration of the transient period and the magnitude of the cell voltage drop to the plateau were strongly dependent on toluene concentration and current density. 

Prediction of the Steady-State Polarization Curves Figure 3.18 shows the predicted and experimentally tested polarization curves for the test cell across the full range of toluene concentrations encompassed by the model. The top curve (solid black) is the baseline polarization curve (i.e., no toluene contamination). Curves below that indicate decreasing performance as the toluene concentration increases. 

A semiempirical model that considered only the effect of toluene on kinetics was constructed to describe cell voltage as a fimction of contamination time and current density. The parameters independent of the contamination process, i.e., open circuit voltage (E°), cell resistance (RJ, and Tafel slope (fc), were first estimated based on experimental data in the absence of toluene (giving a baseline). Then these parameters were used to empirically obtain the expressions of two other parameters, and Kcbc/ which accoimted for the effect of toluene contamination on transient and steady-state cell performance, using experimental data at various levels of toluene concentration imder four current densities. The model was validated by comparing the contamination testing results with model-predicted results. Several other definitions were also presented, based on the model, such as the threshold toluene concentration and the degradation rate. 

For the purpose of validation, the general model was applied to a PEM single cell in which toluene contamination was present [20]. 

One of the difficulties in fuel cell contamination modeling is estimating the unknown ORR parameters. In the case of no toluene being present, we needed to know the forward and backward reaction rates or their ratios for reaction (6.36) to reaction (6.38). To do this, we simulated experimental baseline data free of toluene contamination. The parameters used for the simulation are listed in Table 6.1, and the modeling results and experimental polarization curves are shown in Figure 6.3. 

By using the developed toluene contamination model and the ORR parameter relations obtained from experiments at ppm levels, we numerically studied the cell performance degradation when the toluene inlet concentration was at ppb levels, which closely resembled normal indoor and outdoor toluene levels [20]. One could also estimate the degree of cell performance degradation at a certain contaminant level and current density. 

Figure 6.6 demonstrates the effects of toluene contamination on steady-state cell performance at different inlet concentration levels. Thus, the extent to which toluene contamination affects cell performance depends on both toluene concentration and current density. Based on this model, we can estimate the maximum allowable toluene concentration in order to limit the cell voltage drop to a specified range. For instance, to limit the contamination... 

Currently, several air-side contamination models have been published in the literature, ranging from simple empirical and adsorption models to general kinetic models. These models have been applied to simulate and predict SO2, NO2, NH3, and toluene contamination. The kinetic model is a very general one based on the associative oxygen reduction mechanism. It takes into account contaminant reactions, such as surface adsorption, competitive adsorption, and electrochemical oxidation, and has the capability of simulating and predicting both transient and steady state cell performance. The model can be applied to other cathode contaminants, e.g., SO2 and NO2. 

A set of equations derived from the model assumptions complemented by Langmuir adsorption isotherm was used by Shi et al. (2009) to describe experimental steady-state and transient performance of a PEMFC in the presence of adsorbed toluene species in the cathode airstream. The model allowed the authors to quahtatively describe air-side toluene contamination observed for four different current densities (0.2,0.5,0.75, and 1.0 A/cm ) and three different concentrations (1,5, and 10 ppm). 

Toluene is a common solvent, and is used in model glues. Most toxicity in humans from exposure to toluene is from glue-sniffing or accidental exposure in the workplace. These incidents rarely provide information as to the amount to which someone was exposed. In this example, we are interested in ingestion of toluene. For example, take a case where toluene-contaminated groundwater was used as a drinking water supply and we want to be sure that the toluene levels are below those that could cause toxic effects in humans. There is not sufficient data for humans to establish a safe concentration or dose for this type of exposure. 

Two main components were used in the model catalysts described in this paper. One component was a europium exchanged ammonium Y zeolite (EuNH-Y). The other component was an amorphous aluminosilicate containing about 75% Si 0 and 25% Al203 (AAA-alumina). All materials were artificially V-contaminated by impregnation with vanadyl naphthenate solutions in benzene. Tetraphenyl tin (in hot toluene) was the passivating agent used. It was added either before or after loading vanadium on the zeolite (EuNH-Y), on the gel or on a gel-zeolite mixture. 

Chudyk et al. [IS] reported results obtained in a test of remote fluorescence analysis of groundwater contaminants using UV laser light sources and fiber optics. Several priority pollutants such as phenols, toluene, and xylenes and also naturally occuring humic acid, all of which display UV fluorescence, were readily detected over a distance of 20-25 m. Typical detection limits over this distance are 10 ppb for phenol, 1 ppb for o-cresol, and 0.07 ppb for xylenes. A prototype instrument for monitoring phenolic groundwater contaminants has been described, and its suitability demonstrated by using phenol as a model contaminant [16]. 

An experimental apparatus for continuously processing an aqueous stream containing an organic contaminant was designed and constructed. The criteria for choosing the contaminant/surfactant/ extraction solvent are discussed along with the system operating parameters such as the heptane/water ratio, the hydrodynamic conditions of the ultrafilter and the overall efficiency of the toluene separation. A mathematical model of the continuous system was also developed and evaluated. 

---