Constant inlet flow rate of oxygen

In many situations, the PEFC is run at a constant inlet flow rate of oxygen rather than at a constant stoichiometry. In that case, it is convenient to rewrite Eq. (6.37) as... 

As before, z — zjL, f) = ri/b°, Eq = Eo/U. Note that here we assume a constant inlet flow rate the case of constant oxygen stoichiometry will be considered in Section 4.3.7. 

Measurements Using Liquid-Phase Reactions. Liquid-phase reactions, and the oxidation of sodium sulfite to sodium sulfate in particular, are sometimes used to determine kiAi. As for the transient method, the system is batch with respect to the liquid phase. Pure oxygen is sparged into the vessel. A pseudo-steady-state results. There is no gas outlet, and the inlet flow rate is adjusted so that the vessel pressure remains constant. Under these circumstances, the inlet flow rate equals the mass transfer rate. Equations (11.5) and (11.12) are combined to give a particularly simple result ... 

The reaction takes place in a trickle bed of 4.1 cm diameter and 1 m height with liquid flow rate of 2.17 kg/s m2, gas flow rate of 0.017 kg/s m2, and inlet ethanol concentration of 0.006 kmol/rn3. Horowitz et al. (1999) noted that pure oxygen was used as the gas feed, and under the experimental conditions the conversion of 02 was less than 5%. Thus, the gas-phase concentration of 02 can be considered constant. Furthermore, the liquid phase entered the reactor saturated with 02. Under these conditions, the conversion of the ethanol was about 14%. 

One device of this type was discussed by Hickam and Witkowski(23). In the diffusion limited devices that have been discussed, the flow rate of the gas is not normally an important factor. In the Hickam device, however, the gas flow is of paramount importance and new possibilities or complications arise. The structure consists of pump (upstream) and sensor (downstream) cells cylindrically surrounding a flowing stream of gas containing oxygen. The sensor cell EMF is fedback to the pump so that oxygen is either added to or subtracted from the stream in the amount required to keep the sensor EMF at a constant value. For a calibrated device, the amount of pump current required measures the oxygen content of the gas at the inlet of the structure provided the flow rate is held constant. Alternately, if a gas of constant composition were employed, the structure could be used to measure flow rate. 

In the third set of experiments, inlet flow rates are varied and temperature is held constant at temperature 883°C and at methane to oxygen ration 1.86, As shown in table 2 and figures 6, 7, 8, and 9, CO and H2 rates increase then decreased slightly. Also selectivity and yield decrease at high and low flow rates. Methane conversion also decreased with the increase in the flow rate. At low flow rate (< 400 cm /min.), carbon deposition is detected. At high flow rate, lower CO and H2 yields are recorded. Therefore, flow rate is an important parameter controlling the selectivity of synthesis gas. 

The slowest timescale, by several orders of magnitude, is for the diffusive motion of liquid in the lateral direction within the membrane, Tmem = Idy = 8.3 X 10 s. On this slow timescale, we take the gas mole fractions Cy,Co, and Ay at steady-state, driven adiabatically by the changing water flux into the plenums. The well-stirred gas plenums are assumed to have constant pressure and inlet flow rates. On the cathode side the inlet gas is pure oxygen. The total molar concentration Cr, nondimensionalized by the saturation pressure Cs t T), is a constant of space and time and equals the molar concentration of the inlet gases. This yields the molar balance... 

The results of the section Impedance of the Cathode Side of a PEM Fuel Cell, have been obtained assuming constant oxygen stoichiometry X of the flow. However, impedance experiments are usually performed at a constant oxygen flow rate, rather than at a constant X. Indeed, keeping constant X means that the inlet flow rate must be changed in phase with the mean current density perturbation, which is hardly possible. 

Validation is performed in two steps first an experimental polarization curve, obtained with a fixed inlet gas flow rate, is compared with the calculated values, thus allowing the determination of some unknown parameter values (model calibration). Afterwards, three polarization curves, obtained at constant fuel and oxygen utiliza-... 

As recommended in Chapter 4, the inventory of reactants in the plant is maintained by fixing the reactor-inlet flows. Acetic acid is taken with constant rate from a storage tank, and the fresh feed is added on level control. The gas rate going to the evaporator is a good estimation of the ethylene inventory. Therefore, this flow is kept constant by adjusting the fresh ethylene feed. The fresh oxygen rate is manipulated by a concentration control loop, as previously explained. 

Similar experiments were carried out to determine the effect of oxygen con-cenbation on the fast SCR reaction. For these experiments, 500 ppm NO, 500 ppm NO2, and 1,000 ppm NH3 were kept constant in the inlet feed and Ar was used as a balance gas with 1,000 seem as the total flow rate. The inlet O2 concentration was varied in the range of 0-5 % for temperatures of 185,220, and 245 °C. The O2 was found to have no effect on the fast SCR reaction. Hence the apparent reaction order with respect to O2 can be considered to be zero. 

The oxygen excess is large, and this is why reactions (1) and (2) can be considered to follow the first-order kinetics in terms of ethanol and acetaldehyde, respectively. Since the ethanol concentration in the process is low (0.1% A at the reactor inlet), the volumetric flow rate can be assumed to be constant. 

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