Oxygen Inlet Flow

In the case of Mgx = const. Equation 5.58 should be reformulated in terms of Max-Using Equation 5.177, and the definition of/x. Equations 5.57 and 5.58 transform to 

It is easy to verify that in the limit of Mgx - oo, the first term on the right-hand side of Equation 5.181 tends to 1//, while the third term tends to 1 /(/j — J)- Thus, the first term represents the total CCL charge transfer resistivity at a constant Mgx. By analogy to Equation 5.178, the last term in Equation 5.181 gives the combined resistivity, due to oxygen transport in the GDL and in the channel. 

FIGURE 5.28 The CCL charge transfer resistivity at a constant oxygen flow rate (the first term on the right-hand side of Equation 5.181). 

Comparing this to the first term in Equation 5.181, one can see that the following relation holds  

in the constant-flow case, the total charge transfer resistivity is given by the inverse local current at the channel outlet. Clearly, as y o(l) J, the eell charge transfer resistivity, at a constant flow, always exceeds this resistivity at a constant X (provided that the flow corresponds to X at the mean cell current). In other words, the charge transfer resistivity fromEIS measurements with constant oxygen flow rate is always larger than this resistivity calculated from the A. = const, polarization curve. 

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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 ... 

Technically realised SOFC systems operate with air instead of oxygen and with an excess air X > 1. The incoming air flow is the inlet flow cai of the cathode... 

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. 

In Chapter 4, two ways for achieving production-rate changes were presented. The first strategy, manipulating the reactor inlet flows, does not work here. The acetic acid does not influence the reaction rate, the per-pass conversion of ethylene is very low (10%), while the reactor-inlet oxygen concentration is restricted by the safety concerns. Therefore, the second strategy of manipulating the reaction conditions should be applied. 

In the first set of experiments, the inlet flow rate is fixed at 500 cm /min. and temperature at 883°C. It is observed that the outlet flow rate is usually higher than the inlet by 100 to 150 cm /min. As shown in table 1 and figures 3, 4, and 5, the rates of H2 and CO increased with the increase in the methane/oxygen ratios (R). 

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 glucose solution (72 g glucose in 400 ml water) was heated in the reactor to 50°C. Once the temperature was stabilized, the catalyst (meat = 54-200 mg) was added to the solution and the oxidation reaction started by introducing oxygen (flow rate 0.4 l.min 1) in the stirred (1000 rpm) slurry. Measurements performed with 50 to 100 mg catalyst at different stirring rates (in the range 1000-1800 rpm) confirmed the absence of diffusional limitations under these conditions. After 4 hours reaction, the oxygen inlet was turned off and the... 

Conversions were cdculated as the difference between the outlet and inlet molar flow rates of methane or oxygen, divided by their inlet flow rates. The CO and H2 selectivities were calculated as the molar flow rates of CO and H2 in the effluent divided by the total amount of carbon oxides and H2 and H2O in the effluent respectively. Water flow rates were calculated by mass balance. 

Temperature Program From 70°C to 280°Cat 8°/min Carrier Nitrogen, flow rate 20 mL/min Combustion Gases Oxygen inlet pressure, 50 psig... 

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... 

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... 

After each hydrogen sulfide treatment of the stainless steel reactor, oxygen was passed through the reactor and Initially most of the oxygen was adsorbed on the reactor surface. For example, the Inlet flow rate of oxygen was adjusted to 13 cc/mln. after the second hydrogen sulfide treatment Initially the outlet flow was almost zero for about 50 minutes. Subsequently, the exit flow Increased In amount during the next three hours, and the... 

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. 

Cp = specific heat of hydrogen, 14.307 J/g-K riio ij, = mass flow rate of oxygen inlet, g/s... 

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