Static gas phase

Evaporated film catalysts are virtually always used with a static gas phase, and with reactant gas pressures less than about 100 Torr. One thus relies upon gaseous diffusion and convection for transport to the catalyst surface. However, provided one is dealing with reaction times of the order of minutes to tens of minutes, gas phase transport has but a negligible effect on the reaction, provided none of the reaction volume is separated from the film by small bore tubulation. Beeck et al. (77) in fact originally used an all-glass magnetically coupled turbine for gas circulation, but this is only... 

In order to advance the usefulness of theoretical calculations in broader applications, the computations should be carried out for a system that describes experimental conditions as accurately as possible. To do this, consider the following topics for a description of a system beyond using a single static structure at 0 K conformational averaging of static gas phase structures (this has been partially addressed already in Sect. 3), solvent effects on static structures, zero-point and finite temperature vibrational averaging, and molecular dynamics (MD) or Monte-Carlo (MC) sampling without and with solvation. 

Conformational Averaging mainly of Static Gas Phase Structures... 

Quantification is thus based on the distribution of the volatile compounds between the static gas phase and the sample matrix under defined discontinuous gas extraction conditions, keeping the volumes of sample phase and of the gas phase constant. Because this method is very dependent on the nature of the sample matrix, the identity and quantity of each component are determined by comparison with known pure standards added in the same matrix as the sample analysed. 

Kinetic studies on the static gas-phase thermal decomposition of the halogenomethanes CHFgCI (temperature range 434—S23 °C, initial reactant pressure 100—500 Torr) and CHF (437—515 C, 20—250 Torr) have established that the initial mechanistic step is a-eliraination of HX (X = Cl or Br) leading to difluorocarbene release, a reaction that has been put to preparative use in the synthesis of gem-difiuorocyclopropylsilanes [via flow copyrolysis of vinylchlorosilanes with chlorodifluoromethane at 485— 505 C (see p. 197)]. Fluoro-carbenes, -carbenoids, or -carbenoid complexes have also featured in several other pieces of work on the synthesis of halogenocyclopropanes [for example, see Schemes 29, 30 (olefins used... 

The hydrocarbon is carried in a stream of H2 or Ar, and P-SiC is formed or P-SiC is formed by reaction in the gas phase, under static conditions, of compounds such as SiO or CO formed in situ during the process. In this latter case the important reaction appears to be SiO + 3CO — SiC + 2CO2. This... 

It is noteworthy that it is the lower cross-over temperature T 2 that is usually measured. The above simple analysis shows that this temperature is determined by the intermolecular vibration frequencies rather than by the properties of the gas-phase reaction complex or by the static barrier. It is not surprising then, that in most solid state reactions the observed value of T 2 is of order of the Debye temperature of the crystal. Although the result (2.77a) has been obtained in the approximation < ojo, the leading exponential term turns out to be exact for arbitrary cu [Benderskii et al. 1990, 1991a]. It is instructive to compare (2.77a) with (2.27) and see that friction slows tunneling down, while the q mode promotes it. 

Reactors with a packed bed of catalyst are identical to those for gas-liquid reactions filled with inert packing. Trickle-bed reactors are probably the most commonly used reactors with a fixed bed of catalyst. A draft-tube reactor (loop reactor) can contain a catalytic packing (see Fig. 5.4-9) inside the central tube. Stmctured catalysts similar to structural packings in distillation and absorption columns or in static mixers, which are characterized by a low pressure drop, can also be inserted into the draft tube. Recently, a monolithic reactor (Fig. 5.4-11) has been developed, which is an alternative to the trickle-bed reactor. The monolith catalyst has the shape of a block with straight narrow channels on the walls of which catalytic species are deposited. The already extremely low pressure drop by friction is compensated by gravity forces. Consequently, the pressure in the gas phase is constant over the whole height of the reactor. If needed, the gas can be recirculated internally without the necessity of using an external pump. 

Headspace methods provide an indirect method of sample analysis suitable for the determination of organic volatiles [11,318-323]. The gas diase in contact with the sample and not the sample matrix itself is taken for analysis. If the sample is in thermodynamic equilibrium with the gas phase in a closed thermostated vessel, then this method of analysis is referred to as static headspace. If a carrier gas is passed over the sample and the sample volatiles accumulated in a cryogenic or sorbent trap, then the method is generally referred to as dynamic headspace. If the carrier gas is introduced below the surface of... 

The principles behind MAP liquid-phase and gas-phase extractions are fundamentally similar and rely on the use of microwaves to selectively apply energy to a matrix rather than to the environment surrounding it. MAP gas-phase extractions (MAP-HS) give better sensitivity than the conventional static headspace extraction method. MAP-HS may also be applied in dynamic applications. This allows the application of a prolonged, low-power irradiation, or of a multi-pulse irradiation of the sample, thus providing a means to extract all of the volatile analytes from the matrix [477]. 

Static headspace may also be carried out by substituting the heating step by a microwave treatment. In this procedure the material is immersed in a solvent that is transparent to microwaves relative to the sample in order to impart most, if not all, of the microwave energy to the sample [208]. Another configuration of MAP gas-phase extraction relates to dynamic headspace sampling. 

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