Fast reactions homogeneous

Fig. 6. Reaction zones for a first-order, fast irreversible homogeneous reaction, in reactants A and B with Ha > 2 and (a)

Due to existence of an isothermal region, temperature of both entrance and outlet was rather lower than that of intermediate section. Where temperature was high, the reaction was sever and fast. So, at site 200-300 mm away from entrance, the temperature was highest, the scale layer was thickest and the whisker column was longest there. The reaction route in this zone could be described as phase reaction homogeneous nucleation — coagulation. When... 

For fast reactions Da becomes large. Based on that assumption and standard correlations for mass transfer inside the micro channels, both the model for the micro-channel reactor and the model for the fixed bed can be reformulated in terms of pseudo-homogeneous reaction kinetics. Finally, the concentration profile along the axial direction can be obtained as the solution of an ordinary differential equation. 

It has been shown [90] that the homogeneous dissociation of methane is the only primary source of free radicals and it controls the rate of the overall process. This reaction is followed by a series of consecutive and parallel reactions with much lower activation energies. After the formation of acetylene (C2H2), a sequence of very fast reactions occurs, leading to the production of higher unsaturated and aromatic hydrocarbons and finally carbon ... 

The radicals undergo the usual reactions as dimerizations, disproportionations, atom-transfer reactions, or additions [3]. Compared to homogeneous radical reactions, bimolecular dimerizations and disproportionations are favored at the electrode. Stationary radical concentrations are higher in heterogeneous electrochemical conversions because the radicals are confined to a narrow reaction layer at the electrode surface. This layer arises from the slow diffusion of the radicals generated in high concentration at the electrode surface into the bulk of the solution and their fast reaction on this way. The more reactive the radical is, the narrower the reaction layer will be and thus the higher is the concentration of the radical. 

In the quest for suitable solvent systems the [Rh(CO)2(SULPHOS)] complex (SULPHOS = 31) was found to catalyze the hydroformylation of 1-hexene in water-methanol/isooctane (1/1/1, v/v/v) yielding heptanal and 2-methylhexanol in a ratio of 2.2 (80 °C, 30 bar syngas) [83]. An important point here is in that the biphasic micture becomes homogeneous above 60 °C, but phase separation occurs again upon cooling to room temperature. This kind of solvent behaviour may lead to fast reactions at higher... 

A brief reading of the literature would indicate that the liquid-phase reaction is what is technically known as an absorption with fast reaction and that gaseous CO2 physically dissolves in the liquid phase and reacts in a region close to the gas—liquid interface with dissolved NH3, according to second order in ammonia, first order in CO2 homogeneous liquid-phase kinetics... 

This behavior of multiple reaction could provide a powerful tool in the study of partial segregation in homogeneous systems. It has been used by Paul and Treybal (1971) who simply poured reactant B into a beaker of A and measured the amount of R formed for a very fast reaction of Eq. 14. 

Since homogeneous catalysts tend to offer fast reaction with high selectivity and heterogeneous catalysts offer ease of separation, it is not surprising that efforts have been made lo combine the advantageous properties of both. One way to effect this combination is to attach the "homogeneous catalysts to the surface of a polymer such as polystyrene. Wilkinson s catalyst, for example, can be treated as follows ... 

Comparison of eqns. (56) and (58) shows the analogy in the mass transport effects in electrode reaction kinetics and homogeneous second-order fast reactions in solution. 

Some reactions can be stopped by adding a suitable component. This is sometimes possible for catalytic reactions where a catalyst killer can be added in small amounts. For pH-sensihve reactions, a pH modification may also slow down or even stop the reaction. In these cases, the addition of only a small amount of a compound will suffice. Agitation is a critical factor, especially to ensure that a small amount of an inhibitor must be dispersed homogeneously in a large volume of reaction mixture. In order to achieve a fast and homogenous dispersion, often the vessel containing the inhibitor is pressurized, for example, nitrogen and nozzles are used to spray it into the reaction mass. 

There are several demands that must be more or less fulfilled by the mediator before a successfull amperometric detection of NADH with CMEs can be realized. Despite having a E° lower or comparable with the optimal working potential range for amperometric detection, the mediator should exhibit fast reaction rates both with the electrode proper and NADH, and also be chemically stable at any redox state. Furthermore, the redox reaction of the mediator should involve two electrons and at least one proton making possible, at least theoretically, a fast inner sphere hydride transfer in the homogeneous reaction with NADH. 

Among experimental studies of chemical reactions in turbulent media, fast reactions in tubular reactors with multijet injection of reactants are very popular, since the first experiments of Mao and Toor (34) and Vassiliatos and Toor (35). Their data have been (and are still) extensively exploited for testing theoretical models, although one may ask if homogeneous isotropic turbulence was perfectly controlled in these experiments. In order to rule out this objection, a new series of experiments was recently performed by Bennani et al. (28, 29, 30, 36) in a 0.29 m i.d. tube eliminating the influence of boundary layers. Turbulence was created by a grid and carefully controlled by velocity fluctuation measurements. Previous studies (2) had confirmed that the decrease of Ig with a non-reacting species (passive scalar) obeys Corrsin s equation ... 

Immobilized Since homogeneous catalysts tend to offer fast reaction with high seleclivity and... 

Cozzani et al. (36, 44) have evaluated the homogeneous (not-catalytic) tarl-cracking time (to convert 90 % of the primary tar at temperatures of 800-850 C) in 0.7 to 1.8 seconds (44), It is a very fast reaction then. Activation energy for this not catalytic tarl-cracking has been evaluated in 102 kJ/mol (44) or in 144 kJ/mol by Chan et al.(30). 

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