Key reactants

Section 15 5 Osmium tetraoxide is a key reactant m the conversion of alkenes to vie mal diols... 

A number of chemiluminescent reactions have been studied by producing key reactants through pulsed electric discharge, by microwave dissociation, or by observing the reactions of atoms and free radicals produced in the inner cone of a laminar flame as they diffuse into the flame s cool outer cone (182,183). These are either combination reactions or atom-transfer reactions involving transfer of chlorine (184) or oxygen atoms (181,185—187), the latter giving excited oxides. 

Blocked urethane adhesives are termed blocked by virtue of the fact that one of the key reactants is chemically blocked to prevent reaction. The concept of a blocked isocyanate has already been discussed. 

For a high-pressure non-ideal gas behavior, the term (TqTi/TtIo) is replaced by (ZqTqTi/ZTtIq), where Z is the compressiblity factor. To change to another key reactant B, then... 

Consider a well-mixed batch reactor with a key reactant A, during time t to time t -i- 6t, where 6t is very small. For a well-mixed batch system, assume the following ... 

Ca concentration of key reactant species with stoichiometric coefficient, mol/m ... 

In section 2.5 we have examined the effect of promoters and poisons on the chemisorption of some key reactants on catalyst surfaces.We saw that despite the individual geometric and electronic complexities of each system there are some simple rules, presented at the beginning of section 2.5 which are always obeyed. These rules enable us to make some predictions on the effect of electropositive or electronegative promoters on the coverage of catalytic reactants during a catalytic reaction. 

Polysulfides are the key reactants in the high-density sodium-sulfur and Hthium-sulfur batteries [4] which are based on the following reversible redox reaction taking place in the polysulfide melt ... 

The very basis of the kinetic model is the reaction network, i.e. the stoichiometry of the system. Identification of the reaction network for complex systems may require extensive laboratory investigation. Although complex stoichiometric models, describing elementary steps in detail, are the most appropriate for kinetic modelling, the development of such models is time-consuming and may prove uneconomical. Moreover, in fine chemicals manufacture, very often some components cannot be analysed or not with sufficient accuracy. In most cases, only data for key reactants, major products and some by-products are available. Some components of the reaction mixture must be lumped into pseudocomponents, sometimes with an ill-defined chemical formula. Obviously, methods are needed that allow the development of simple... 

Selectivity the ratio of the amount of a desired product obtained to the amount of a key reactant converted. 

In addition to the physiological reaction of N2 reduction, nitrogenase catalyzes a wide variety of reactions involving small unsaturated molecules(56). Table III lists key reactants and products for FeMo nltrogenases. All substrate reductions involve minimally the transfer of two electrons. Multielectron substrate reductions may involve the accretion of such two-electron... 

In cases where an acceptable kinetic mechanism can be established, it may be possible to obtain expressions, such as those in Sect. 2, which predict concentration changes with time when the values for the rate coefficients are known. However, the use of these expressions to evaluate rate coefficients from experimental data is not always straightforward, particularly with coupled reaction systems where a key reactant participants in a reversible step. Initial rate measurements are often of insufficient accuracy and, with very complex sj stems, it becomes necessary to obtain a great deal of data from experiments in which initial concentrations can be varied. 

Pick one reactant as the basis for determining the conversion. We call this the key reactant. Let A be the key. Then for ideal gas behavior. 

For multiple reactions we need-a variable to describe each reaction. Further, we caimot in general find a single key reactant to caU species in the definition of X. However, it is straightforward to use the number of moles extent Xi for each of the R reactions. Thus we can define the change in the number of moles of species j through the relation... 

It is also important to estimate the temperature increase or decrease in a reactor in which no heat is added or removed, which is called an adiabatic reactor. From the First Law of Thermocfynamics, we can construct a thermocfynamic cycle to estimate the AT in going from reactants at temperature Ti to products at temperature Ti, as shown in Figure 2-10. Assume that reaction occurs at Tq with heat of reaction A Hr per mole of a key reactant that... 

We always use Cj in moles per liter (or in moles per cubic decimeter or 1 kilomole/m for the SI purist) as the only unit of concentration. The subscript j always signifies species, while the subscript i always signifies reaction. We use j as the species designation and species A as the key reactant. For gases the natural concentration unit is partial pressure Pj, but we always convert this to concentration, Cj = Pj RT, before writing the mass-balance equations. Conversion X means the fiaction of this reactant that is consumed in the reactor, Ca = Cao( 1 — X), but we prefer to use C i rather than X and find the conversion after we have solved the equation in terms of G. We cannot use this unit of density of a species when the density of the fluid varies with conversion, but we prefer to do so whenever possible because the equations are simpler to write and solve. 

Cj concentration of species j, usually in moles/Uter Vj stoichiometric coefficient of species j in the reaction Y U Ca concentration of key reactant species with stoichiometric coefficient Va = 1 Pj partial pressure of species j, Pj/RT = Cj for ideal gases... 

Another quantity we need to define is the rate of production of a given product. By this we mean the amount of the desired product produced by the reactor per unit time Fj, usually in moles/hme. If species A is the key reactant and species B is the desired product, then... 

The following, well-acceptable assumptions are applied in the presented models of automobile exhaust gas converters Ideal gas behavior and constant pressure are considered (system open to ambient atmosphere, very low pressure drop). Relatively low concentration of key reactants enables to approximate diffusion processes by the Fick s law and to assume negligible change in the number of moles caused by the reactions. Axial dispersion and heat conduction effects in the flowing gas can be neglected due to short residence times ( 0.1 s). The description of heat and mass transfer between bulk of flowing gas and catalytic washcoat is approximated by distributed transfer coefficients, calculated from suitable correlations (cf. Section III.C). All physical properties of gas (cp, p, p, X, Z>k) and solid phase heat capacity are evaluated in dependence on temperature. Effective heat conductivity, density and heat capacity are used for the entire solid phase, which consists of catalytic washcoat layer and monolith substrate (wall). 

In conclusion hexafluoroacetone is a key reactant for the preparation of fluorinated diols and was involved in many investigations. The other interesting method concerns the oligomerization of fluorinated monomers the end-groups of such obtained telomers are changed by different pathways to produce difunctional compounds. 

The fractional extent-of-reaction is the fraction of the key reactant, ethylbenzene, in the reactor feed that reacts while passing through the reactor. In this example, 0.5 mol of ethylbenzene are consumed per 0.47 mol of styrene produced. 

Selectivity is another catalyst attribute that is often considered when ranking catalysts. Selectivity may be defined as the ratio of the molar amount of key reactant converted to the desired product to the total molar amount of the key reactant converted. As such, selectivity is a measure of the efficiency of the catalyst in promoting the formation of the desired product as compared to other products. Since selectivity is a function of the relative rates of reaction with a given reaction system, selectivity will be a function of reaction temperature, space velocity, feed composition, reactor geometry, and degree of conversion. Comparing selec-tivities of different catalysts is therefore only meaningful when all the latter parameters are constant. 

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