Transport limitation, catalytical

C. Changes in the Liquid Composition Within Transport-Limited Catalytic Pellets... 

Catalytical Activity and Transport Limitation Catalytical lltficiency and Thiele Modulus- As is well known in heterogeneous catalysis, the relevance of transport limitations in such a reaction system can be evaluated if the Thiele modulus is known (23), ). For a set of assumptions (28), the Thiele modulus for s erical particles and a zero-order reaction can... 

In fuel-lean H2/air combustion, superadiabatic surface temperatures are attained as illustrated by three computations in Fig. 3.4 (Cases 1—3). Case 1 (marked T. i) was computed using an infinitely fast catalytic step H2 +1402 —H2O (i.e., transport-limited catalytic hydrogen conversion), without gas-phase chemistry and without inclusion of heat conduction in... 

It is not unusual for the full chemical potential of a reaction to be diminished by slower transport processes (i.e., to be transport limited). In fast liquid phase enzyme reactions, mechanical stirring rates can have a strong influence on the observed kinetics that may be limited by the rate of contacting of the reactants and enzymes. Most heterogeneous catalytic reactions take... 

Describe qualitatively the consequences of transport limitations on the apparent activation energy of a catalytic process by using an Arrhenius plot. What is the best temperature to run this reaction in an industrial application ... 

The second mechanism often invoked to explain the increase in n y of simple Fe porphyrins at potentials more reducing than that of the Fe couple (under anaerobic conditions) is based on the fact that at such potentials the fraction of the catalyst in the 5 -coordinate ferrous state is maximal because (i) the equilibrium (18.9) is shifted completely to the ferrous form and (ii) the concentration of O2 in the catalytic film is low owing to mass transport limitations. The higher the concentration of the 5-coor-dinate ferrous porphyrin in the catalytic film, the greater the probability that any released H2O2 will re-enter the catalytic cycle by coordinating to a molecule of ferrous porphyrin and decay according to (18.13b) instead of (18.17). 

However, with respect to ee, the same catalyst immobilized on amorphous silica performed even better (conversion 72%, ee 92%) than the one immobilized on MCM-41. This example illustrates an important issue, i.e., OMS-based catalysts have to be compared with those based on amorphous silica or silica-alumina. If the amorphous materials perform as well or even better than the OMS materials, then there is no advantage in using the significantly more expensive OMSs. However, in those cases where the catalytic reaction benefits from the regular and well-defined pore systems of the OMS materials, such materials can be very attractive, e.g., for the conversion of bulkier molecules or to overcome transport limitations in more narrow pores. 

It can be seen in the plot in Figure 11 that EA . shows a clear temperature dependence. For rising temperatures the mass transport limitation can be observed, which leads to a lowering of EAs by a factor of V2 in the pore diffusion regime down to 0, owing to the shift of the reaction from the interior of the pore system of the catalytic particle to the outer surface. In the final state, the diffusion through the boundary layer becomes the rate-limiting step of the reaction. 

The HTE characteristics that apply for gas-phase reactions (i.e., measurement under nondiffusion-limited conditions, equal distribution of gas flows and temperature, avoidance of crosscontamination, etc.) also apply for catalytic reactions in the liquid-phase. In addition, in liquid phase reactions mass-transport phenomena of the reactants are a vital point, especially if one of the reactants is a gas. It is worth spending some time to reflect on the topic of mass transfer related to liquid-gas-phase reactions. As we discussed before, for gas-phase catalysis, a crucial point is the measurement of catalysts under conditions where mass transport is not limiting the reaction and yields true microkinetic data. As an additional factor for mass transport in liquid-gas-phase reactions, the rate of reaction gas saturation of the liquid can also determine the kinetics of the reaction [81], In order to avoid mass-transport limitations with regard to gas/liquid mass transport, the transfer rate of the gas into the liquid (saturation of the liquid with gas) must be higher than the consumption of the reactant gas by the reaction. Otherwise, it is not possible to obtain true kinetic data of the catalytic reaction, which allow a comparison of the different catalyst candidates on a microkinetic basis, as only the gas uptake of the liquid will govern the result of the experiment (see Figure 11.32a). In three-phase reactions (gas-liquid-solid), the transport of the reactants to the surface of the solid (and the transport from the resulting products from this surface) will also... 

Supercritical fluids (SCFs) offer several advantages as reaction media for catalytic reactions. These advantages include the ability to manipulate the reaction environment through simple changes in pressure to enhance solubility of reactants and products, to eliminate interphase transport limitations, and to integrate reaction and separation unit operations. Benefits derived from the SCF phase Fischer-Tropsch synthesis (SCF-FTS) involve the gas-like diffusivities and liquid-like solubilities, which together combine the desirable features of the gas- and liquid-phase FT synthesis routes. 

The catalytic principle of micelles as depicted in Fig. 6.2, is based on the ability to solubilize hydrophobic compounds in the miceUar interior so the micelles can act as reaction vessels on a nanometer scale, as so-called nanoreactors [14, 15]. The catalytic complex is also solubihzed in the hydrophobic part of the micellar core or even bound to it Thus, the substrate (S) and the catalyst (C) are enclosed in an appropriate environment In contrast to biphasic catalysis no transport of the organic starting material to the active catalyst species is necessary and therefore no transport limitation of the reaction wiU be observed. As a consequence, the conversion of very hydrophobic substrates in pure water is feasible and aU the advantages mentioned above, which are associated with the use of water as medium, are given. Often there is an even higher reaction rate observed in miceUar catalysis than in conventional monophasic catalytic systems because of the smaller reaction volume of the miceUar reactor and the higher reactant concentration, respectively. This enhanced reactivity of encapsulated substrates is generally described as micellar catalysis [16, 17]. Due to the similarity to enzyme catalysis, micelle and enzyme catalysis have sometimes been correlated in literature [18]. 

These values can be correlated with the heat of adsorption of hydrogen on the catalytic metal since the oxidation mechanism, apart from diffusion and mass transport limitations, is controlled by an adsorption step in a two consecutive step mechanism ... 

The gelification of the biocatalyst on the membrane is based on one of the main drawbacks of membrane processes fouling. Disadvantages of this systems are the reduction of the catalytic efficiency, due to mass transport limitations and the possibility of preferential pathways in the enzyme gel layer [60],... 

Paul Weisz suggested in a lucid note published in 1973 that cells, and indeed even entire organisms, have evolved in a way that maintains unity effectiveness factor [24]. That is, the size of the catalytic assembly is increased in nature as the overall rate at which that assembly operates decreases, and the relationship between characteristic dimension and activity can be well approximated by the observable modulus criterion for reaction limitation. It is possible that Weisz s arguments may fail under process conditions, and internal gradients within a compartment or cell may be important. However, at present it appears that the most important transport limitations and activities in cells are those that operate across cellular membranes. Therefore, to understand and to manipulate key transport activities in cells, it is essential that biochemical engineers understand these membrane transport processes and the factors influencing their operation. A brief outline of some of the important systems and their implications in cell function and biotechnology follows. 

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