Selectivity, diffusional effects

The treatment of the two-phase SECM problem applicable to immiscible liquid-liquid systems, requires a consideration of mass transfer in both liquid phases, unless conditions are selected so that the phase that does not contain the tip (denoted as phase 2 throughout this chapter) can be assumed to be maintained at a constant composition. Many SECM experiments on liquid-liquid interfaces have therefore employed much higher concentrations of the reactant of interest in phase 2 compared to the phase containing the tip (phase 1), so that depletion and diffusional effects in phase 2 can be eliminated [18,47,48]. This has the advantage that simpler theoretical treatments can be used, but places obvious limitations on the range of conditions under which reactions can be studied. In this section we review SECM theory appropriate to liquid-liquid interfaces at the full level where there are no restrictions on either the concentrations or diffusion coefficients of the reactants in the two phases. Specific attention is given to SECM feedback [49] and SECMIT [9], which represent the most widely used modes of operation. The extension of the models described to other techniques, such as DPSC, is relatively straightforward. 

A -4 B — C). Determine the outlet concentrations of A, B, and C, and the selectivity to the desired product B. Compare your results to those in which diffusional effects are ignored. 

Since diffusional effects are most important, we wish to emphasize these processes in the gas phase. For the control volume selected in Figure 9.7, the bold assumption is made that transport processes across the lateral faces in the x direction do not change - or change very slowly. Thus we only consider changes in the y direction. This approximation is known as the stagnant layer model since the direct effect of the main flow velocity (it) is not expressed. A differential control volume Ay x Ax x unity is selected. 

Quantitative Interpretation of Intracrystalline Diffusional Effects. Since a qualitative effect of crystallite size upon selectivity was observed, the next step was to extract some quantitative values for the intracrystalline diffusional parameters. To do this, we must either know the intrinsic or diffusion-free kinetics or be able to make a simplifying assumption so that the diffusional parameters can be extracted from the available data. 

Temperature Dependence of the Activity and Selectivity of Xylene Isomerization over AP Catalyst. Based upon our analysis of the intracrystalline diffusional resistance in AP catalyst, we would expect that when the reaction temperature is increased, the selectivity would shift toward p-xylene since the diffusional effects are increased as the activity increases. A shift in selectivity toward p-xylene as the reaction temperature was increased was observed and is shown in Figure 6. The role of diffusion in changing the selectivity can be seen in the Arrhenius plot of Figure 7. The reaction rate constant for the o-xylene - p-xylene path, fc+3i, goes from an almost negligible value at 300°F to a substantial value at 600°F. Furthermore, the diffusional effects are also demonstrated by the changing... 

The second scale which determines the relation between the selectivity and conversion is the diffusion of the reactants through the catalyst poes. Model calculations conducted by McCarty indicated that at 10 atm die coupling of methyl radicals occurs preferentially inside the pores in a particle of 25 mm in diameter. The effect of this time scale is shown in Figure 10(a) in terms of the intraphase and interphase profiles for methane and ethane inside a catalyst pore. Clearly, higha C2 selectivities are obtained on catalysts with an open pore structure and low surface area. A majority of the literature results have been obtained using powdered catalysts in which diffusional effects are not in rtant however, such effects could be relevant at high pressure in fixed-bed reactors requiring the use of catalysts in a pelletized form. 

As is true for many industrial situations, the question of diffusional effects on multiple reaction selectivity is equally as important as the effectiveness of conversion considerations. The basic concepts were provided by Wheeler [133], through consideration of three categories of situations. 

Further, selective Pd-catalyzed hydrogenation of 3-methyl-l- pentyn-3-ol to PI without over-hydrogenation to P2 was used to prove the advantageous characteristics of segmented flow in eliminating diffusional effects and axial dispersion. 

The elFect of catalyst deactivation on selectivity can be easily assessed for very small catalyst particles such as those used in fluidized beds, for which the diffusional effect is absent and therefore, deactivation is uniform. A good example is provided by Weekman and Nace (1970) for the catalytic cracking of gas oil (A) into gasoline (G), and dry gas and coke (5) which deactivate the catalyst ... 

Recent literature has pointed out the importance of accessibility in cracking catalysts [22-28], Accessibility is a parameter related to the mass-transfer characteristics of an FCC catalyst, which may be a selectivity determining step. Indeed, the geometric disposition of the components in the final catalyst does affect the selectivity in the cracking process. In fact, reducing diffusional effects improves the gasoline selectivity. 

In Section III, emphasis was placed on flux kinetics across the cultured monolayer-filter support system where the passage of hydrophilic molecular species differing in molecular size and charge by the paracellular route was transmonolayer-controlled. In this situation, the mass transport barriers of the ABLs on the donor and receiver sides of the Transwell inserts were inconsequential, as evidenced by the lack of stirring effects on the flux kinetics. In this present section, the objective is to give quantitative insights into the permeability of the ABL as a function of hydrodynamic conditions imposed by stirring. The objective is accomplished with selected corticosteroid permeants which have been useful in rat intestinal absorption studies to demonstrate the interplay of membrane and ABL diffusional kinetics (Ho et al., 1977 Komiya et al., 1980). 

This equation gives the differential yield of V for a porous catalyst at a point in a reactor. For equal combined diffusivities and the case where hT approaches zero (no diffusional limitations on the reaction rate), this equation reduces to equation 9.3.8, since the ratio of the hyperbolic tangent terms becomes y/k2 A/ki v As hT increases from about 0.3 to about 2.0, the selectivity of the catalyst falls off continuously. The selectivity remains essentially constant when both hyperbolic tangent terms approach unity. This situation corresponds td low effectiveness factors and, in tliis case, equation 12.3.149 becomes... 

Modeling of pore diffusion phenomena can be a helpful tool mainly in terms of catalyst design considerations but also in terms of understanding the effects caused by diffusional restrictions. For example, a modeling study by Wang et al.7 demonstrated a negative impact on selectivity by particle diffusion limitations. 

No experiments with variation in particle size of the silica gel have been done to study intraparticle diffusion effects. In silica gel such diffusion would be only through the pores (analogous to the macropores of a polystyrene) since the active sites lie on the internal surface. The silica gel used by Tundo had a surface area of 500 m2/g and average pore diameter of 60 A.116). Phosphonium ion catalyst 28 gave rates of iodide displacements that decreased as the 1-bromoalkane chain length increased from C4 to Cg to C16, The selectivity of 28 was slightly less than that observed with soluble catalyst hexadecyltri-n-butylphosphonium bromide U8). Consequently the selectivity cannot be attributed to intraparticle diffusional limitations. 

Crystallite Size Effects upon AP Catalyst Selectivity. Previous studies have shown that with the pellet sizes investigated, gross particle size does not affect activity or selectivity. If there are diffusional limitations, they must be intracrystalline and therefore a function of the crystallite size of the zeolite component. 

Nitrogen physisorption of the Ge-ZSM-5 sample revealed a considerable contribution of mesopores to the total pore volume, accompanied by a drop in micropore volume of 20%. In a study of the catalytic activity of these materials it was found that the increased mesoporosity of Ge-ZSM-5 had a beneficial effect on the catalytic activity in a series of acid-catalysed reactions.1771 It was observed that the presence of germanium in the framework does not change the strength of the acid sites but, instead, decreases the extent of deactivation from coke residues formed upon reaction. The microporous domains only have short diffusional lengths, but the shape selectivity ascribed to the zeolitic channels is still fully... 

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