Low-pressure gas-phase reaction

As an introduction to the application of Eqs. 13.1-1 and 13.1-2 to chemical equilibrium problems, consider the prediction of the equilibrium state for the low-pressure, gas-phase reaction... 

For operation in a constant pressure nonisothermal mode with a relatively low pressure gas phase reaction, it is convenient to approximate the reactor volume by... 

On the other hand, when a reactant B Is frequently scattered by the solvent, its trajectory is entangled (case (b)) and It sweeps the same place repeatedly. Consequently the effective volume swept per unit time becomes smaller than ra2v. This is the reason the rate of reaction or the rate constant decreases with decreasing mean free path of reactant. Case (a) corresponds to low-pressure gas phase reactions, while case (b) corresponds to (narrowly defined) diffusion-controlled reactions. 

As it has appeared in recent years that many hmdamental aspects of elementary chemical reactions in solution can be understood on the basis of the dependence of reaction rate coefficients on solvent density [2, 3, 4 and 5], increasing attention is paid to reaction kinetics in the gas-to-liquid transition range and supercritical fluids under varying pressure. In this way, the essential differences between the regime of binary collisions in the low-pressure gas phase and tliat of a dense enviromnent with typical many-body interactions become apparent. An extremely useful approach in this respect is the investigation of rate coefficients, reaction yields and concentration-time profiles of some typical model reactions over as wide a pressure range as possible, which pemiits the continuous and well controlled variation of the physical properties of the solvent. Among these the most important are density, polarity and viscosity in a contimiiim description or collision frequency. 

Finlayson, B. J., J. N. Pitts, Jr., and R. Atkinson. Low-pressure gas-phase ozone-olefin reactions. Chemiluminescence, kinetics, and mechanisms. J. Amer. Chem. Soc. % 5356-5357, 1974. 

Any chemical reaction that yields polymeric material can be considered polymerization. However, polymerization in the conventional sense, i.e., yielding high enough molecular weight materials, does not occur in the low-pressure gas phase (without a heterogeneous catalyst). With a heterogeneous catalyst, polymerization is not a gas phase reaction. Therefore, the process of material deposition from luminous gas phase in the low-pressure domain might be better represented by the term luminous chemical vapor deposition (LCVD). Plasma polymerization and LCVD (terms explained in Chapter 2) are used synonymously in this book, and the former... 

Undiluted ozone at a pressure somewhat below atmospheric (depending on the silica gel loading) may be obtained by pumping ozone off the silica gel. By keeping the ozone gas at a low pressure, gas phase explosions may be avoided. If the ozone is to be used in a chemical reaction, the reactor might be placed between the silica gel beds and the vacuum pump. 

There have recently been a number of measurements of unimolecular reaction rates as a function of molecule-solvent interaction over a large range of such interactions—from the low-pressure gas phase (where damping is... 

T o end this section we note again that the assumption of fast intramolecular energy redistribution in the reactant well introduced the number n of molecular modes as an important parameter of the theory. In a broader context n is taken to be the number of strongly coupled molecular modes, and it is assumed that the reaction coordinate is part of this set. In liquid solvents n is exp>ected to be equal to the total number 3AT — 7 of modes. In the low-pressure gas phase n can be smaller, and the possible slow energy transfer between different... 

This example emphasizes an important difference between reactions in low-pressure gas phase and in solution, which has been considered in the naive pictures introduced in Section 1. A close contact between two solute molecule, eventually leading to a chemical reaction, is always a substitution process, in which a portion of the first solvation shell molecules is replaced by the second solute. This point is of particular relevance in the study of reaction mechanisms, but it must be taken into account even when this study is limited to the assessment of equilibrium constants. 

Few reactions have been studied over the enormous range indicated in Figure 5.1. Even so, they will often show curvature in an Arrhenius plot of n k) versus T. The usual reason for curvature is that the reaction is complex with several elementary steps and with different values of E for each step. The overall temperature behavior may be quite different from the simple Arrhenius behavior expected for an elementary reaction. However, a linear Arrhenius plot is neither necessary nor sufficient to prove that a reaction is elementary. It is not sufficient because complex reactions may have one dominant activation energy or several steps with similar activation energies that lead to an overall temperature dependence of the Arrhenius sort. Arrhenius behavior is not necessary since some low-pressure, gas phase, bimolecular reactions exhibit distinctly non-Arrhenius behavior even though the reactions are believed to... 

Standard computations treat isolated molecules (i.e., model the low-pressure gas phase situation). Chemistry in the condensed phase, however, can be significantly different because ionic and polar species are specifically stabilized. Computations can consider solvation explicitly by including the solvent molecules or as a continuous medium effect (reaction field methods). 

AGgjj > 0) UPD H is adsorbed in the hollow sites as H adsorbed from H2 in low-pressure gas-phase adsorption, whereas OPD H is probably adsorbed at the on-top sites, as the labile and highly reactive H involved at atmospheric or high pressure in the catalytic reactions [53,74]. 

Univation Technologies Unipol process, low-pressure, gas-phase, fluidised bed reactor, proprietary solid and slurry catalysts, low investment and operating costs, low level of environmental pollution, potential fire and explosion hazard 89 reaction lines with capacity ranging from 40,000 to 450,000 tonnes/y. LLDPE to HDPE (density 915-970 kg/m MFI = 0.1-200 g/10 min) for film, blow moulding pipes, rotomoulding, and extrusion applications. 

Thus, a joint analysis of experiments in the gas phase and in aqueous electrolyte gives evidence of the existence of fwo main kinds of H adsorption sites on a Pt surface high coordination sites (hollow sites) with a strong M-H js bond (AGh js < 0) and low coordination sites (on-top sites) with a weak M-H js bond (AGnads > 0) UPD H is adsorbed in the hollow sites as H adsorbed from H2 in low-pressure gas-phase adsorption, whereas OPD H is adsorbed at the on-top sites, as the labile and highly reactive H involved at atmospheric or high pressure in the catalytic reactions [60,78]. 

At the limit of extremely low particle densities, for example under the conditions prevalent in interstellar space, ion-molecule reactions become important (see chapter A3.51. At very high pressures gas-phase kinetics approach the limit of condensed phase kinetics where elementary reactions are less clearly defined due to the large number of particles involved (see chapter A3.6). 

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