Selectivity mass transfer effect

One goal of catalyst designers is to constmct bench-scale reactors that allow determination of performance data truly indicative of performance in a full-scale commercial reactor. This has been accompHshed in a number of areas, but in general, larger pilot-scale reactors are preferred because they can be more fully instmmented and can provide better engineering data for ultimate scale-up. In reactor selection thought must be given to parameters such as space velocity, linear velocity, and the number of catalyst bodies per reactor diameter in order to properly model heat- and mass-transfer effects. 

For Rh-catalysed hydroformylation the role of the ionic liquid as an innocent solvent is by far the most important. To our knowledge, none of the published research in this area claims special chemistry. The selectivity found with the different Rh-ligand complexes corresponds in most cases to the values obtained in traditional organic solvent or water (with the surprisingly low selectivity of TPPTS ligands in ionic liquids being a remarkable exception). Overall activities were found to be very comparable if mass transfer effects between the gas phase and the two immiscible liquid phases were overcome by proper stirring. 

Mass transfer effects are very important for the selectivity in the Fischer-Tropsch synthesis. Even though the reactants are in the gas phase, the catalyst pores will be filled with liquid products. Diffusion in the liquid phase is about 3 orders of magnitude slower than in the gas phase and even slow reactions may become diffusion limited. Diffusion limitations may occur through limitation on the arrival of CO to the active points or through the limited removal of reactive products.8,9... 

Reactions described by other kinetic routes may be treated in similar fashions. Although, for reasons already explained in Sect. 4.1, mass transfer effects will not influence the selectivity of two concurrent reactions arising from the same reactant, heat transfer between fluid and solid does have an affect. Thus for the first-order reactions... 

We also note that, even though mass transfer effects are significant, the kinetics of individual steps strongly influence selectivities. On Rh surfaces, the H2 selectivity is much higher than on Pt, (Sh2 0.9 vs. 0.7), and this is due primarily to the slower rate of hydrogen oxidation on Rh than on Pt. 

A useful empirical approach to the design of heterogeneous chemical reactors often consists of selecting a suitable equation, such as one in Table 3.3 which, with numerical values substituted for the kinetic and equilibrium constants, represents the chemical reaction in the absence of mass transfer effects. Graphical methods are often employed to aid the selection of an appropriate equation140 and the constants determined by a least squares approach<40). It is important to stress, however, that while the equation selected may well represent the experimental data, it does not... 

As demonstrated by means of residue curve analysis, selective mass transfer through a membrane has a significant effect on the location of the singular points of a batch reactive separation process. The singular points are shifted, and thereby the topology of the residue curve maps can change dramatically. Depending on the structure of the matrix of effective membrane mass transfer coefficients, the attainable product compositions are shifted to a desired or to an undesired direction. 

While the intrinsic activity and selectivity of a catalyst establish its performance in the absence of mass transfer effects, it is well known that the placement of the active components and access to these components by reactants can play a major role in the performance of practical catalysts. One of the challenges for reaction engineers is to develop models for predicting the distribution of active components in a catalyst and the effects of this distribution, together with the pore size distribution and particle size and shape, on the performance of a catalyst. 

The contents of the present contribution may be outlined as follows. Section 6.2.2 introduces the basic principles of coupled heat and mass transfer and chemical reaction. Section 6.2.3 covers the classical mathematical treatment of the problem by example of simple reactions and some of the analytical solutions which can be derived for different experimental situations. Section 6.2.4 is devoted to the point that heat and mass transfer may alter the characteristic dependence of the overall reaction rate on the operating conditions. Section 6.2.S contains a collection of useful diagnostic criteria available to estimate the influence of transport effects on the apparent kinetics of single reactions. Section 6.2.6 deals with the effects of heat and mass transfer on the selectivity of basic types of multiple reactions. Finally, Section 6.2.7 focuses on a practical example, namely the control of selectivity by utilizing mass transfer effects in zeolite catalyzed reactions. 

Basically, reactant and product selectivities are mass transfer effects, where the diffusivities of the various species in practice frequently do not differ that extremely as indicated above. Instead, in most cases only a preferred diffusion of certain species is observed, a fact which often hinders a clear understanding of product shape selectivity. This is because the various products, during their way through the pore system, may be reacted when contacting the catalytically active surface of the wall. This combined effect of diffusion and reaction will be discussed in detail in the following, as it is of great importance for the product distribution in zeolite-catalyzed reactions. 

Selectivity may be determined in the integral or differential mode. Integral selectivity depends on the overall extent of the reaction (degree of conversion) and on the type of reactor used even if heat and mass transfer effects are eliminated. It may be called reactor selectivity for the formation of product P, from the set of reactants B when it is calculated as the mole fraction of P, in the products (exluding unconverted feed) at the exit of the reactor ... 

The selective hydrogenation of crotonaldehyde to n-butyraldehyde was studied using Pd/C catalyst. The initial rate of hydrogenation was analysed mainly to assess the importance of various mass transfer effects from which it was found that all the rate data under the conditions of the present work were in the kinetic regime. A Langmuir - Hinshelwood type rate model has been derived and the rate parameters were evaluated by using concentration-time data. The agreement of the predicted results with the experimental data was found to be excellent. 

Milligan, B., Isomer distribution in mixed-acid nitration of toluene Evidence for mass-transfer effects on selectivity, Ind. Eng. Chem. Fund., 25,783-789 (1986). 

This chapter first explains enzyme nomenclature, describes enzymatic, supercritical reactor configurations, and gives a compilation of published experimental results. The- most important topics concerning enzymatic reactions in SCFs are then covered. These are factors affecting enzyme stability, the role of water in enzymatic catalysis, and the effect of pressure on reaction rates. Studies on mass transfer effects are also reviewed as are factors that have an effect on reaction selectivities. Finally, a rough cost calculation for a hypothetical industrial process is given. 

When consecutive or parallel reactions are carried out between a gas and a liquid, the concentration gradients near the interface may influence the selectivity as well as the overall rate of reaction. For chlorination or partial oxidation of hydrocarbons, several workers have reported that the yield of intermediate products was influenced by agitation variables [6,7] and was less than predicted from the kinetic constants. Rigorous analysis of multiple reactions is complex, but film theory can be used to show when mass transfer effects are likely to change the selectivity [8]. 

For parallel reactions, the selectivity may be altered by mass transfer effects if the reactions are of different order. For ... 

In either case, mass transfer effects for both steps are restricted to the film and are unlikely to influence selectivity in a significant way. An interesting possibility is that an increase in the value of can lead to a situation where the first step continues in regime 3 but the second shifts to regime 2 and thus can occur only in the bulk. Because most of A would have been consumed in the film, its concentration in the bulk would be very low, thus severely restricting the second step. This will clearly result in an enhancement of selectivity for R. 

The chief conclusion from this study is that except at very high rates of agitation, the reaction tends to be greatly influenced by mass transfer to the magnesium surface. Another important conclusion is that the selectivity of the reaction is also greatly influenced by mass transfer limitations. In the absence of mass transfer effects the selectivity is higher, thus making it important to eliminate these effects. 

The next section addresses preliminaries such as the structure of homogeneous catalytic systems, reaction configurations, and physicochemical issues. The third section reviews use of liquid, gas-liquid, liquid-liquid, and gas-liquid-liquid reaction systems from a mixing and mass-transfer viewpoint and provides some working criteria. The fourth section provides an checklist for the experimentalist to minimize mass-transfer effects in transition-metal homogeneous catalysis research and hence achieve more vmiform and reproducible rate and selectivity results. 

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