Reaction selectivity mass transport effect

It must be emphasized that the above considerations were made in the absence of reaction. Interfacial mass transfer followed by reaction requires further consideration. The Hatta regimes classify transfer-reaction situations into infinitely slow transport compared to reaction (Hatta category A) to infinitely fast transport compared to reaction (Hatta category H) [42]. In the first case all reaction occurs at the interface and in the second all reaction occurs in the bulk fluid. Homogenous catalytic hydrogenations, carbonylations etc. require consideration of this issue. An extreme example of the severity of mass transport effects on reactivity and selectivity in hydroformylation has been provided by Chaudari [43]. Further general discussions for homogeneous catalysis can be found elsewhere [39[. 

The manner in which mass transport can affect reaction selectivity depends on the kind of reactions involved.24 Three general types of selectivity have been defined and mass transport effects each in a different way. 

The results of a parametric study exploring the effect of mass transport limitations on this reaction gave the results shown in Figure 3. For this study, the reactor was assumed to be isothermal, and the conversion data of Figure 3 show the final composition exiting the reactor. The ideal (kinetics only) result gives the expected high conversion and selectivity due to the selective CO reaction. As mass transport limitation is added, the maximum CO conversion drops (Table 2) due to CO depletion near the surface since O2 is in excess. 

One of the most important parameters is the temperature, the starting temperature, and the initial reaction rate. One can determine experimentally the initial reaction rate after elimination of diffusion and mass transport effects and then determine the Arrhenius constants, which depend on the temperature. The collision factor (ko) and activation energy (E) parameters influence significantly the activity pattern and selectivity. Figure 3.1 illustrates the influence of the temperature on these parameters for different reactions and metallic catalysts. This effect is known as compensation effect, although empirically there are attempts on theoretical interpretations for different heterogeneous systems [1, 2]. 

Reaction and Transport Interactions. The importance of the various design and operating variables largely depends on relative rates of reaction and transport of reactants to the reaction sites. If transport rates to and from reaction sites are substantially greater than the specific reaction rate at meso-scale reactant concentrations, the overall reaction rate is uncoupled from the transport rates and increasing reactor size has no effect on the apparent reaction rate, the macro-scale reaction rate. When these rates are comparable, they are coupled, that is they affect each other. In these situations, increasing reactor size alters mass- and heat-transport rates and changes the apparent reaction rate. Conversions are underestimated in small reactors and selectivity is affected. Selectivity does not exhibit such consistent impacts and any effects of size on selectivity must be deterrnined experimentally. 

Intraparticle mass transport resistance can lead to disguises in selectivity. If a series reaction A — B — C takes place in a porous catalyst particle with a small effectiveness factor, the observed conversion to the intermediate B is less than what would be observed in the absence of a significant mass transport influence. This happens because as the resistance to transport of B in the pores increases, B is more likely to be converted to C rather than to be transported from the catalyst interior to the external surface. This result has important consequences in processes such as selective oxidations, in which the desired product is an intermediate and not the total oxidation product CO2. 

Rates and selectivities of soHd catalyzed reactions can also be influenced by mass transport resistance in the external fluid phase. Most reactions are not influenced by external-phase transport, but the rates of some very fast reactions, eg, ammonia oxidation, are deterrnined solely by the resistance to this transport. As the resistance to mass transport within the catalyst pores is larger than that in the external fluid phase, the effectiveness factor of a porous catalyst is expected to be less than unity whenever the external-phase mass transport resistance is significant, A practical catalyst that is used under such circumstances is the ammonia oxidation catalyst. It is a nonporous metal and consists of layers of wire woven into a mesh. 

The objectives of using solvents are diverse, e.g., to dissolve a solid substrate, to limit catalyst deactivation, to improve selectivity, or to enhance mass-transport. The solvents are selected depending on the substrate and the desired effect. Hence, they range from water, alcohols, ethers, or low alkanes, to CO2. The effects of the solvent on phase-behaviour, viscosity, and density at different concentrations, temperatures and pressures can explain much about the effect of the solvent on the reaction. 

Several quantitative analyses of the effect of intraparticle heat and mass transport have been carried out for parallel, irreversible reactions [1]. Roberts and Lamb [2] have worked on the effect of reversibility on the selectivity of parallel reactions in a porous catalyst. The reaction selectivity of a kinetic model of two parallel, first order, irreversible reactions with a second order inhibition kinetic term in one of them has also been investigated [3]. 

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. 

Type II selectivity involves the differentiation between two parallel reactions in which different products are formed by separate paths from the same starting material.33 This type of selectivity is encountered in the hydrogenation of crotonaldehyde to either butyraldehyde or 2 buten-l-ol (Eqn. 5.8). When both reactions are of the same kinetic order changes in mass transport will influence them both to the same extent and there will be no effect on reaction selectivity. When the reactions are of different kinetic orders, that one with the higher order will be more affected by mass transport limitation. 

The main difference between the cases discussed in this section on selectivity and the previously discussed case of a simple polystep reaction resides in the fact that here the single component which generates the species that becomes the intermediate in the polyfunctional composite can itself generate a distinct product species with appreciable yield. Since the coupling between the Y- and the X-system occurs in any event through mass-transport of intermediates between X-sites and T-sites, the diffusion criteria already discussed must apply or the kinetic schemes which accomplish interception or selectivity control will not be physically and effectively accomplished. The criterion, of formula (15) should be satisfied. ... 

Encapsulation of enzymes in LMs offers further improvements for immobilization of complex enzyme systems, as the enzymes / cofactors, etc. are situated in aqueous droplets surrounded by a stable liquid hydrocarbon film (Figure 1). Instead of the physical pores present in microcapsules, the HC barrier, which has a diffusion thickness of about 0.1-1.0 p, effectively blocks all molecules except those which are oil-soluble or transportable by the selected carriers. Encapsulation of enzymes in LMs is accomplished simply by emulsifying aqueous enzyme solutions. Hence, LMs offer many advantages over other systems used for separation and eirzyme immobilization they are inexpensive and easy to prepare they promote rapid mass transport they are selective for various chemical species they can be disrupted (demulsified) for recovery of internal aqueous solutions gradients of pH and concentration (even of small molecules) can be maintained across the HC barrier multiple enzyme / cofactor systems can be coencapsulated and enzymatic reaction and separation can be combined. Some of the potential disadvantages of LMs for enzyme encapsulation have been discussed earlier. 

An example would be the dehydration of ethanol to ethylene and its dehydrogenation to acetaldehyde. If both reactions are first order, selectivity is unaffected by internal mass transport the ratio of the rates of reactions, 1 and 2 is k jkj at any position within the pellet. Equation (11-89) cannot be applied separately to the two reactions because of the common reactant A. The development of the effectiveness-factor function would require writing a differential equation analogous to Eq. (11-45) for the total consumption of A by both reactions. Hence k in Eq. (11-89) would be k- + k2 and Fp would be (Tp) -1- (rp)2- Such a development would shed no light on selectivity. 

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