OCR Text |
Show TREATMENT OF DATA The sensor output may be considered as a sample of a continuous distribution of 0 mole fractions in the gas passing the probe tip. The recorder traces such as that shown in Fig. 2 are analyzed by counting the frequencies with which the cell potential falls in small increments, AV. Far from the distributor (z = 0.70 m) the distributions of these frequencies typically show two peaks: one in the range 0 - 0.06 V and another in the vicinity of 0.8 V, such that the entire histogram is qualitatively similar in shape to the probability density distribution for a sine wave, as might be expected by inspection of Fig. 2. A cell potential of 0.8 V corresponds to an 0 mole fraction of 4 ,~-16 2 x 10 at 1100 K and 100 kPa. The magnitude of this 0 concentration suggests that its high probability is the result of equilibration or partial equilibration of Op. One possibility is the system C0/0p/C0?: CO + 1 02 = C02 (R1) This idea cannot be tested quantitatively since instantaneous CO and COp concentrations were not determined. An estimate can be based, however, on the time-averaged CO mole fraction of 0.002, determined by gas sampling (Dutta, 1983). Using this value and a CO mole fraction of 0.18 in the equilibrium constant expression for Reaction R1 at the bed temperature (1100 K) one expects an equilibrium 0 mole fraction of -15 1.6 x 10 . This corresponds to a cell potential of 0.77 V, which is in the neighborhood of the peak in the cell output distribution corresponding to low Op. It is not known whether these low values are attained with or without the intervention of the Pt electrode. We must consider the possibility that the measured 0 mole fractions have been shifted to lower values by reaction with CO, hydrocarbons, SO , etc. at the Pt surface. The maximum shift, based on the average mole fractions of CO and C to C hydrocarbons, is 0.0017. A change of this magnitude has a small effect on the sensor output distribution in the region of the peak at 0 to 0.06 V. Cell potentials in this range correspond to 0? mole fractions of 0.21 to 0.017, at least ten times greater than the estimated shift. 4 |