Convection reactant transport

Evaporated film catalysts are virtually always used with a static gas phase, and with reactant gas pressures less than about 100 Torr. One thus relies upon gaseous diffusion and convection for transport to the catalyst surface. However, provided one is dealing with reaction times of the order of minutes to tens of minutes, gas phase transport has but a negligible effect on the reaction, provided none of the reaction volume is separated from the film by small bore tubulation. Beeck et al. (77) in fact originally used an all-glass magnetically coupled turbine for gas circulation, but this is only... 

Available reaction-transport models describe the second regime (reactant transport), which only requires material balances for CO and H2. Recently, we reported preliminary results on a transport-reaction model of hydrocarbon synthesis selectivity that describes intraparticle (diffusion) and interparticle (convection) transport processes (4, 5). The model clearly demonstrates how diffusive and convective restrictions dramatically affect the rate of primary and secondary reactions during Fischer-Tropsch synthesis. Here, we use an extended version of this model to illustrate its use in the design of catalyst pellets for the synthesis of various desired products and for the tailoring of product functionality and molecular weight distribution. 

Convection Reactants can also be transferred to or from an electrode by mechanical means. Forced convection, such as stirring or agitation, tends to decrease the thickness of the diffusion layer at the surface of an electrode and thus decrease concentration polarization. Natural convection resulting from temperature or density differences also contributes to the transport of molecules and ions to and from an electrode. 

RRDE is significantly simpler than with conventional cyclic voltammetry data in quiescent solutions [88, 89]. As such, these forced convection systems have been widely used in the study of electrocatalysis in general. Of special interest are situations where the rate determining step is chemical (a) or electrochemical (B) (Scheme 3.7) [60], In particular, for an RDE at steady state, the rate at which the reactant is depleted at the interface must be equal to the rate at which it is replenished from the solution via convective mass transport. For a reaction first order in dioxygen this relationship reads ... 

Three Types of Reactant Transport in Electrolyte (Diffusion, Convection, and Migration) 45... 

Actually, the reactant transport by these three processes (diffusion, migration, and convection) occurs at the same time along the x direction. The total oxidant transport current density... 

Electrode reactions are heterogeneous processes, so that their rates will be influenced (a) by the concentration of the reactant at the surface which is related to the bulk concentration by an adsorption isotherm, (b) by the presence of any intermediates and/or products which are adsorbed on the surface, and (c) by the nature of the electrode material and solvent, also, (d) depending on concentration and/or solution agitation or flow rate, by diffusional or convective mass transport of reagents and/or products to or... 

Thus, in contrast to a conventional electrolyte with a diffusion layer at the electrodes, in the case of SPE technology, a convective mass transport occurs directly at the electrode surface due to EOF. It delivers reactants and removes products as indicated by the arrows in the lower part of Fig. 2. This is presumably the cause of the frequent observation of higher selectivities of electrode reactions using SPE technology compared... 

Convective mass transport of reactants and products to/from the surface of the electrodes... 

Reactant transport from the bulk flow to the electrode surface takes place primarily by convection and diffusion in the absence of significant elecffomigration, provided a strong supporting electrolyte is used. In this case, species conservation takes the general form ... 

As shown in Figure 9.1, flow-over designs generally provide streaming of fuel and oxidant over planar electrodes. Only a fraction of fuel and oxidant streams adjacent to the catalyst layer participate in electrocatalytic reactions. Due to the lack of effective convective mass transport, a depletion boundary layer with low concentration of reactant grows over both electrodes. To enhance fuel utilization in flow-over designs, an improved design of electrodes was implemented in the... 

Because of the ongoing electrooxidation of fuel and electroreduction of oxidant over anode and cathode, fuel and oxidant concentrations through the channel decrease. Due to the lack of convective mass transport to replenish fresh reactants to the catalytic active area, a depletion boimdary layer over the catalyst-covered electrodes is formed [40], as shown in Figure 9.3. 

Both share more or less the same merits but also the same disadvantages. The beneficial properties are high OCV (2.12 and 1.85 V respectively) flexibility in design (because the active chemicals are mainly stored in tanks outside the (usually bipolar) cell stack) no problems with zinc deposition in the charging cycle because it works under nearly ideal conditions (perfect mass transport by electrolyte convection, carbon substrates [52]) self-discharge by chemical attack of the acid on the deposited zinc may be ignored because the stack runs dry in the standby mode and use of relatively cheap construction materials (polymers) and reactants. 

In the film-penetration model (H19), it is assumed that the reactant A penetrates through the surface element by one-dimensional unsteady-state molecular diffusion. Convective transport is assumed to be insignificant. The diffusing stream of the reactant A is depleted along the path of diffusion by its reversible reaction with the reactant B, which is an existing component of the liquid surface element. If such a reaction can be represented as... 

Solution We suppose that the mass transfer and diffusion steps are fast compared with bulk transport by convection. This is the design intent for ion-exchange columns. The reaction front moves through the bed at a speed dependent only on the supply of fluid-phase reactants. Assuming piston... 

In electrochemical cells we often find convective transport of reaction components toward (or away from) the electrode surface. In this case the balance equation describing the supply and escape of the components should be written in the general form (1.38). However, this equation needs further explanation. At any current density during current flow, the migration and diffusion fluxes (or field strength and concentration gradients) will spontaneously settle at values such that condition (4.14) is satisfied. The convective flux, on the other hand, depends on the arbitrary values selected for the flow velocity v and for the component concentrations (i.e., is determined by factors independent of the values selected for the current density). Hence, in the balance equation (1.38), it is not the total convective flux that should appear, only the part that corresponds to the true consumption of reactants from the flux or true product release into the flux. This fraction is defined as tfie difference between the fluxes away from and to the electrode ... 

In the pores of the electrodes, practically no natural convection of the liquid takes place. Reactants dissolved in the liquid can be supplied in two ways from the external surface to the internal reaction zones (and reaction products transported away in the opposite direction) (1) by diffusion in the motionless liquid diffusion electrode),... 

The mass-transfer correlation obtained by Bohm et al. (B9), Eq. (33), in Table VII, is conspicuous for its remarkably high exponent (0.85) on the GrSc product. Since the current is almost independent of diffusivity, this must mean that the reacting ion is depleted at the downstream end of the narrow slit between the cathode and diaphragm. The total current then is determined largely by the convective transport of reactant into the slit, which, in turn, depends on the density difference but not on diffusivity. 

The flow terms represent the convective and diffusive transport of reactant into and out of the volume element. The third term is the product of the size of the volume element and the reaction rate per unit volume evaluated using the properties appropriate for this element. Note that the reaction rate per unit volume is equal to the intrinsic rate of the chemical reaction only if the volume element is uniform in temperature and concentration (i.e., there are no heat or mass transfer limitations on the rate of conversion of reactants to products). The final term represents the rate of change in inventory resulting from the effects of the other three terms. 

The deposition of metals has also been studied by a large number of electrochemical techniques. For the deposition of Cu2+, for example, it is reasonable to ask whether both electrons are transported essentially simultaneously or whether an intermediate such as Cu+ is formed in solution. Such questions, like those of the ECE problem discussed above, have usually been investigated by forced convection techniques, since the rate of flow of reactant to and away from the electrode surface gives us an important additional kinetic handle. In addition, by using a second separate electrode placed downstream from the main working electrode, reasonably long-lived intermediates can be transported by the convection flow of the electrolyte to this second electrode and detected electrochemically. 

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