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Reactant-Gas Concentration

As the gases flow down the tube, they become gradually depleted as tungsten is deposited and the amount of the by-product gas, HF, increases in the boundary layer. This means that, at some point downstream, deposition will cease altogether when WFg is no longer present. The reactant concentration is illustrated in Fig. 2.7. [Pg.50]

The boundary layers for these three variables (gas velocity, temperature, and concentration) may sometimes coincide, although in slow reactions, the profiles of velocity and temperature may be fully developed at an early stage while the deposition reaction is spread far downstream the tube. [Pg.50]

As can be seen, conditions in a flowing reactor, even the simplest such as a tube, may be far from the thermodynamic equilibrium conditions predicted by the equilibrium computer programs. However, in the diffusion controlled range, it is possible to use as the driving force for diffusion, the difference between an assumed equi- [Pg.50]

What is the rate limiting step of a CVD reaction In other words, what factor control s the growth rate of the deposit The answer to this question is critical since it will help to optimize the deposition reaction, obtain the fastest growth rate and, to some degree, control the nature of the deposit. [Pg.51]


Figure 4 Equilibrium CVD phase diagram for the Nb-Ge-H-CI system. The diagram was constructed from thermodynamic calculation results and depicts the condensed phases which form as a function of experimental variables. The Nb/(Nb+Ge) values are reactant gas concentrations. After Wan.9... Figure 4 Equilibrium CVD phase diagram for the Nb-Ge-H-CI system. The diagram was constructed from thermodynamic calculation results and depicts the condensed phases which form as a function of experimental variables. The Nb/(Nb+Ge) values are reactant gas concentrations. After Wan.9...
In Eq. (3) the initial reactivity is given by the parameter A . In Eqs. (3-5) the label o refers to the initial charcoal structure which is characterised by the reaction surface area per unit volume, S, the total length of the pore per unit solid volume, Lq, the particle radius, Rg, and the porosity, Cg. The surface reaction is characterised by the reaction rate constant K, and the reaction order n with respect to the reactant gas concentration C, Differentiating Eq. (2) with respect to t for o oo (i.e., the reaction on the outer particle surface is neglected) one obtains... [Pg.78]

Fig. 1.64. Production concentrations as a function of the reaction time for three different reaction temperatures and different reactant gas concentrations, (a) T = 300 K p(02) = 0.12Pa p(CO) = 0.12 Pa p(He) = 1.2Pa. (b) T = 150K p(02) = 0.04 Pa p(CO) = 0.04 Pa p(He) = 1.0 Pa. (c) T = 100K (Oa) = 0.02 Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data (squares), Aua (circles), AuaOa (triangles), Au2(C0)02 ). The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (1.65) to the experimental data... Fig. 1.64. Production concentrations as a function of the reaction time for three different reaction temperatures and different reactant gas concentrations, (a) T = 300 K p(02) = 0.12Pa p(CO) = 0.12 Pa p(He) = 1.2Pa. (b) T = 150K p(02) = 0.04 Pa p(CO) = 0.04 Pa p(He) = 1.0 Pa. (c) T = 100K (Oa) = 0.02 Pa p(CO) = 0.03 Pa p(He) = 1.0 Pa. Open symbols represent the normalized experimental data (squares), Aua (circles), AuaOa (triangles), Au2(C0)02 ). The solid lines are obtained by fitting the integrated rate equations of the catalytic reaction cycle (1.65) to the experimental data...
Careful adjustment of the reactant gas concentration and the rf power input to the coil will minimize the gas phase reaction, sidewall deposition, and fine-particle formation. [Pg.199]

This chapter is a practical summary of how to create CFD models, and how to interpret results. A review of recent literature on PEM fuel cell modeling was presented. A fiill three-dimensional computational fluid d5mamics model of a PEM fuel cell with straight flow channels has been developed. This model provides valuable information about the transport phenomena inside the fuel eell such as reactant gas concentration distribution, liquid water saturation distribution, temperature distribution, potential distribution in the membrane and gas diffusion layers, activation overpotential distribution, diffusion overpotential distribution, and local current density distribution. In addition, the hygro and thermal stresses in membrane, which developed during the cell operation, were modeled and investigated. [Pg.376]

Fluid flow and pressure variation in a fuel cell play a critical role in the distribution of reactant gas concentration at electrochemical reaction sites and, hence, in the distribution of local current densities and cause mass transfer loss. The governing equations for reactant gas flows in gas flow channels and in porous electrode-gas diffusion layers are given by conservation of mass and momentum equations. Solutions to these equations result in the distribution of pressure, P, and velocity field, which is also referred to as the bulk motion in the gas flow channels and porous electrode-gas diffusion layers. [Pg.215]

Figure 6.15 shows a typical reactant gas concentration distribution in the electrode-gas diffusion layers and gas flow channels of a tri-layer fuel cell for a typical operating current density. [Pg.247]

In the transport of reactant gases from the gas flow channel through an electrode composed of a GDL and CL, there are a number of mass transfer resistances that influence the mass transport as shown in Figure 6.17. These resistances are (i) convective mass transfer resistance in the gas flow channel, l conv,mi (ii) diffusion mass resistance in the gas diffusion layer, Rdijf and (iii) diffusion and reaction resistance owing to reaction kinetics. Figure 6.17 shows a typical reactant gas concentration distribution across the gas channel, the gas diffusion layer, and the active CL. [Pg.253]

As we have mentioned earlier, one of the fuel cell voltage losses is the mass transfer loss or concentration loss caused by lower reactant gas concentration distribution at the reaction sites. Mass transport establishes reactant gas concentration distributions in gas supply channels and in the electrodes of a fuel cell, and hence in the distribution of local current densities. The gas supply rates to the anode-membrane and cathode-membrane interface must be sufficient enough to meet the gas consumption rate given by the electrochemical reaction rates. Any insufficient supply of gas to reaction sites may cause sluggishness in the reactions and cause mass transfer loss and reduction in fuel cell output voltage. [Pg.268]

Equation 6.127 can also be used to estimate the reactant gas concentration... [Pg.269]

Notice that in situations where net mass transfer resistance is controlled by the diffusion resistance in the electrode-gas diffusion layer only, the reactant gas concentration at the reaction surface is given as... [Pg.269]

Equation 6.80 shows that the current density depends directly on the reactant gas concentration in the channel as well as the reactant gas concentration at the electrode-electrolyte interface. The current density increases with higher gas concentration, Q in the channel and lower concentration, Q i, at the reaction surface. For a given electrode-gas diffusion layer and gas channel design with a fixed Q value, the maximum current density or the... [Pg.269]

Concentration overpotential or loss is estimated as the change in Nernst voltage loss owing to the variation of reactant gas concentration from the bulk gas flow stream to the gas concentration at the electrode reaction surface as follows ... [Pg.271]

One of the major losses in PEM fuel cells is the mass transfer loss, which is caused by the lack of reactant gas concentration distribution at the electrode-catalyst reaction surface. In order to reduce this resistance, a high-performance gas flow channel design has to be developed. [Pg.434]

The catalyst layer and the reaction region are very thin and reactant gas concentration within this layer can be neglected. Electrochemical reaction is assumed to take place at the electrode-electrolyte interface as a surface reaction. [Pg.466]

Within a narrow reactant gas concentration range Eq. (4.3.82) may be approximated by the empirical relationship... [Pg.152]

The initial and boundary conditions for these equations would typically specify the reactant gas concentration at the inlet, the initial gaseous reactant profile within the bed, and the initial conversion of the solid reactant. Thus we have... [Pg.269]

This case may be handled by applying a slight modification to the procedure discussed in the preceding section. Consider that the reactant gas concentrations at the inlet and exit to the bed are given by C o and respectively. [Pg.312]


See other pages where Reactant-Gas Concentration is mentioned: [Pg.50]    [Pg.464]    [Pg.180]    [Pg.745]    [Pg.167]    [Pg.169]    [Pg.1156]    [Pg.116]    [Pg.285]    [Pg.241]    [Pg.572]    [Pg.268]    [Pg.417]    [Pg.465]    [Pg.128]   


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Gas concentration

Reactant concentrations

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