Selectivity, catalyst instantaneous

In developing mathematical expressions for selectivities, knowledge of the rate equations are required. This is because the instantaneous selectivity is defined in terms of the rate ratios. The parameters that affect the instantaneous and the overall selectivities are exactly the same as those influencing the reaction rates, namely, the concentration, temperature, activation energy, time of reaction (residence time in flow reactors), catalysts, and the fluid mechanics. 

In scaling-up this process, appropriate reactor design was conducted elaborately by the results of several tests. The following two points are characteristic of the scale-up. One is the development of catalyst suitable for fluid beds. Selectivity should not decrease by the increase of hydrocarbon concentration. Also, reaction rate of the secondary reaction should be small. These problems were solved by the development of rather simple P-V catalyst (K2, K3). Another device is the instantaneous mixing inlet mechanism for hydrocarbon and air. By this, activity decline of the catalyst can be prevented (T12). Thus a process has been developed which is equally as profitable as the benzene process. 

The data analysis along the lines given above leads to a substantial insight into the relative importance of various phenomena and parameters. The surface area of the catalyst rendered inaccessible by pore blockage is about 10 % with instantaneous growth of pore or 13 % with finite rate of growth. With the site density selected from literature data, the blockage of the mesopores only takes a few seconds and it is caused by only 0,091 % of the total coke content. 

If transport resistances can be neglected, the concentration inside the catalyst pellet corresponds to the bulk concentration Cj = c,-. The instantaneous selectivity decreases with increasing conversion, X. In a catalytic fixed bed reactor with... 

The efficient instantaneous catalyst selectivity is given by the ratio of the efficient production rate of and the rate of reactant A disappearance (see also Equation 2.172). 

The oscillatory behavior may be explained in the following manner. As CO builds up on the anode catalyst, the anode overpotential increases to maintain the drawn current, eventually reaching a level where H2O dissociation to Ru—OH occurs (Eqn (15.24)), with the resultant instantaneous CO oxidation to CO2 (Eqn (15.24)), so that the surface is rapidly cleansed of CO and the overpotential drops precipitously. As CO begins to buUd up again on the catalyst surface, the cycle repeats. These oscillations, in fact, significantly enhance the CO oxidation rate as compared to steady-state CO electrochemical oxidation. Thus, the cPrOx is an excellent example of cogeneration of electricity and effective and selective chemical conversion under mild conditions in a compact unit. 

The dusty-gas model offers an accurate way to estimate the effectiveness factor, concentration profiles, rates and instantaneous selectivities in a porous catalyst particle. The assumption of considering only one effective diffusion in nitrogen leads to an important overestimation in the reaction selectivity. Under the conditions studied here, high temperatures and low pressures favor the production of styrene. 

Precatalytic Reactions and Xpre. The catalyst precursor must transform under reaction conditions into intermediates to obtain an active system. This transformation may involve, in a small number of cases, only a single elementary step, for example, the dissociation of a ligand from a transition-metal complex. However, a series of elementary reaction steps are usually required to convert the catalyst precursor. Useful examples include (1) the degradation of a polynuclear precursor to mononuclear intermediates, (2) the modification of a precursor with a ligand L which is used to control selectivity, and (3) the transformation of finely divided metal. The characteristic time scale for the precatalytic reaction will be denoted tpre, and the instantaneous reaction rate will be denoted Ppre- Precatalytic phenomena and the associated induction periods have been directly monitored in a number of in situ spectroscopic studies using a variety of mononuclear, dinuclear, polynuclear, and metallic precursors (11). 

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