Toluene contamination

The results of laboratory experiments with fluidized-bed reactors treating toluene-contaminated influent (Massol-Deya et al. 1997) came to important conclusions ... 

FIGURE 5.9 Co-current pilot plant for remediation of toluene-contaminated air. 

Various solvents were evaluated as methylene chloride replacements. Toluene was selected as best meeting all the needs. Thus, the R-amine was sufficiently soluble, water phases were readily separable at 25°C, and toluene contamination was of no consequence since the reaction of R-amine and 5-bromoacetylsalicylamide was already conducted in the presence of traces of toluene. 

Before injection into the GC system, a clean-up of the sample extract is necessary, since lipophile compounds, which also are extracted into benzene or toluene, contaminate the system. Westoo (1967 and 1968) used a clean-up procedure consisting of an intermediate extraction of MeHg into an aqueous cysteine solution. The applicability of the Westoo methods on fish samples was carefully tested and confirmed by Kamps and McMahon (1972). 

Rhodococcus sp. 124 was isolated fi-om a toluene-contaminated aquifer and was found to oxidize indene to 1,2-indandiol and several other products. The undesired products 1-indenol and 1-indanone were formed directly from indene while racemic l-keto-2-hydroxy-indan was formed from the indandiols. Based on product formation profiles and induction experiments, 124 was hypothesized... 

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. 

Reddy, K. R., R. Semer, ruid J. A. Adams. 1999. Air How Optimization and Surfactant Enhancement to Remediate Toluene-Contaminated Saturated Soils Using Air Sparging, Environmental Management Health, vol. 10, no. 1, pp. 52-63. 

Toluene contamination in air could affect fuel eell performanee, mainly resulting in performance degradation in the kinetic region (low current densities). This contamination also results in easy flooding at the cathode side, possibly changing the hydrophilicity of the cathode eatalyst layer. The mechanism of toluene contamination is not clear [56]. 

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. 

Hui et al. [29] conducted toluene contamination tests at different toluene concentrations. They also studied the effects of different operational conditions on toluene contamination, including the effects of fuel cell relative humidity (RH), of Pt loading in the cathode catalyst layer, of back pressure, and of air stoichiometry [30]. Figure 3.8 shows a set of representative results of contamination tests at 1.0 A cm with various levels of toluene concentration in the air. It can be seen that the cell voltage starts to decline immediately after the introduction of toluene, and then reaches a plateau (steady state). These plateau voltages indicate the saturated nature of the toluene contamination. For example, the cell voltage drops from 0.645 V to 0.522 V at 1.0 A cm-2 within 30 min of the cathode... 

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. 

As discussed eariier, the effect of toluene contamination on PEM fuel cell performance features a transient decay followed by a plateau in the cell performance. The performance plateau represents the maximum performance depression (MPD %) that toluene can cause to the cell performance at a given current density and toluene concentration. Based on the difference... 

The rate of cell performance degradation is another interesting measure of the impact of toluene contamination on cell performance. This can be estimated by differentiating equation (3.21) with respect to t to obtain dE n/dt, as shown by equation (3.27) ... 

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 6.5 shows the transient cell performance behaviors at different current densities and with different toluene inlet concentrations. On the one hand, the effect of toluene contamination becomes more severe with a higher toluene concentration at the same cell current density for example. Figure 6.5(d) indicates that at the same current density of 1.0 Acm, the cell voltage drops due to toluene concentrations of 250, 500, and 750 ppb are 37, 42, and 48 mV, respectively. On the other hand, the toluene contamination increases steadily with increasing cell current density for example, the voltage drops in response to 750 ppb toluene in the cathode flow channel are 9,16, 27, and 48 mV, corresponding to cell current densities of 0.5, 0.75, and 1.0 AcmV respectively, as shown in Figure 6.5. Furthermore, the time required for the cell voltage to reach steady state is also affected by both toluene concentration and current density, i.e., a larger toluene concentration and a lower current density result in a longer time before cell performance reaches steady state.

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... 

Effects of toluene contamination on transient fuel cell performance at different current densities (a) 1,. (d) 1,. = 1.0 Acm". (From Shi, Z. et al. Power Sources, 186 (2009) 435. With permission.)... 

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. 

Electrochemical impedance spectroscopy demonstrated that the kinetic and mass transfer resistances are significantly increased as a result of toluene contamination, while the membrane s resistance remained unchanged. Results of EIS measurement are presented as a bar chart in Figure 8.15. 

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