Chemical reaction, rates

Keck J 1960 Variational theory of chemical reaction rates applied to three-body recombinations J. Chem. Phys. 32 1035 Anderson J B 1973 Statistical theories of chemical reactions. Distributions in the transition region J. Chem. Phys. 58 4684... 

Miller W H 1998 Quantum and semiclassical theory of chemical reaction rates Faraday Disc. Chem. Soc. 110 1... 

The overall requirement is 1.0—2.0 s for low energy waste compared to typical design standards of 2.0 s for RCRA ha2ardous waste units. The most important, ie, rate limiting steps are droplet evaporation and chemical reaction. The calculated time requirements for these steps are only approximations and subject to error. For example, formation of a skin on the evaporating droplet may inhibit evaporation compared to the theory, whereas secondary atomization may accelerate it. Errors in estimates of the activation energy can significantly alter the chemical reaction rate constant, and the pre-exponential factor from equation 36 is only approximate. Also, interactions with free-radical species may accelerate the rate of chemical reaction over that estimated solely as a result of thermal excitation therefore, measurements of the time requirements are desirable. 

To analy2e premixed turbulent flames theoretically, two processes should be considered (/) the effects of combustion on the turbulence, and (2) the effects of turbulence on the average chemical reaction rates. In a turbulent flame, the peak time-averaged reaction rate can be orders of magnitude smaller than the corresponding rates in a laminar flame. The reason for this is the existence of turbulence-induced fluctuations in composition, temperature, density, and heat release rate within the flame, which are caused by large eddy stmctures and wrinkled laminar flame fronts. 

Mechanism The mechanism of leaching may involve simple physical solution or dissolution made possible by chemical reaction. The rate of transport of solvent into the mass to be leached, or of soluble fraction into the solvent, or of extract solution out of the insoluble material, or some combination of these rates may be significant. A membranous resistance may be involved. A chemical-reaction rate may also affec t the rate of leaching. 

For many laboratoiy studies, a suitable reactor is a cell with independent agitation of each phase and an undisturbed interface of known area, like the item shown in Fig. 23-29d, Whether a rate process is controlled by a mass-transfer rate or a chemical reaction rate sometimes can be identified by simple parameters. When agitation is sufficient to produce a homogeneous dispersion and the rate varies with further increases of agitation, mass-transfer rates are likely to be significant. The effect of change in temperature is a major criterion-, a rise of 10°C (18°F) normally raises the rate of a chemical reaction by a factor of 2 to 3, but the mass-transfer rate by much less. There may be instances, however, where the combined effect on chemical equilibrium, diffusivity, viscosity, and surface tension also may give a comparable enhancement. 

The above discussion relates to diffusion-controlled transport of material to and from a carrier gas. There will be some circumstances where the transfer of material is determined by a chemical reaction rate at the solid/gas interface. If this process determines the flux of matter between the phases, the rate of transport across the gas/solid interface can be represented by using a rate constant, h, so that... 

The catalyst volume is the same on both sides. It is assumed that no diffusional rate limitation exists even in the larger pellets. That is, the chemical reaction rate is controlling. Pressure drop must be the same for both sides, so the flow has to be less over the smaller pellets to maintain the AP (L/dp)(u /2g) = constant. 

K. J. Laidler, Theory of Chemical Reaction Rates, McGraw-Hill, New kbik, 1969. 

The chemical reaction rate is generally a function of a reactant concentration and temperature. In the case of an exothermic reaction, unless the heat of reaction is removed, an increase in temperature may result in a runaway reaction. For most homogeneous reaction, the rate is increased by a factor of 2 or 3 for every 10°C rise in temperature. This is represented by... 

The Chapman-Jongnet (CJ) theory is a one-dimensional model that treats the detonation shock wave as a discontinnity with infinite reaction rate. The conservation equations for mass, momentum, and energy across the one-dimensional wave gives a unique solution for the detonation velocity (CJ velocity) and the state of combustion products immediately behind the detonation wave. Based on the CJ theory it is possible to calculate detonation velocity, detonation pressure, etc. if the gas mixtnre composition is known. The CJ theory does not require any information about the chemical reaction rate (i.e., chemical kinetics). 

Laidler, K.J. Theories of Chemical Reaction Rates McGraw-Hill New York. 1969. 

Chemical reaction rates increase with an increase in temperature because at a higher temperature, a larger fraction of reactant molecules possesses energy in excess of the reaction energy barrier. Chapter 5 describes the theoretical development of this idea. As noted in Section 5.1, the relationship between the rate constant k of an elementary reaction and the absolute temperature T is the Arrhenius equation ... 

Generally, in an equation of a chemical reaction rate, the rate constant often does not change with temperature. There are many biochemical reactions that may be influenced by temperature and the rate constant depends on temperature as well. The effect of temperature on... 

The kinetics of culture media sterilisation describe the rate of destruction of microorganisms by steam using a fust-order chemical reaction rate model. As the population of microorganisms (N) decreases with time, the rate is defined by the following equation ... 

Increase in interfacial area. The total surface area for diffusion is increased because the bubble diameter is smaller than for the free-bubbling case at the same gas flow rate hence there is a resultant increase in the overall absorption rate. The overall absorption rate will also increase when the diffusion is accompanied by simultaneous chemical reaction in the liquid phase, but the increase in surface area only has an appreciable effect when the chemical reaction rate is high the absorption rate for this case is then controlled by physical diffusion rather than by the chemical reaction rate (G6). 

Mass-transfer rates have been determined by measuring the absorption rate of a pure gas or of a component of a gas mixture as a function of the several operating variables involved. The basic requirement of the evaluation method is that the rate step for the physical absorption should be controlling, not the chemical reaction rate. The experimental method that has gained the widest acceptance involves the oxidation of sodium sulfite, although in some of the more recent work, the rate of carbon dioxide absorption in various media has been used to determine mass-transfer rates and interfacial areas. 

In evaluating their results they assumed the film theory, and, because the oxygen is sparingly soluble and the chemical reaction rate high, they also assumed that the liquid film is the controlling resistance. The results were calculated as a volumetric mass-transfer coefficient based, however, on the gas film. They found that the volumetric mass-transfer coefficient increased with power input and superficial gas velocity. Their results can be expressed as follows ... 

An increase in bubble diameter will generally depress the total average absorption rate. This effect diminishes as the chemical reaction rate is decreased. 

For chemical reaction-rate constants greater than 10 sec-1, NT increases linearly with the total bubble surface area, i.e., linearly with the gas holdup. In other words, the agitation rate only affects the total bubble surface area and has almost no effect on the rate of absorption per unit area. This result is in accordance with the work of Calderbank and Moo-Young (C4), discussed in Section II. 

For low values of the chemical reaction-rate constant, the reaction... 

Fig. 13. Total average volumetric absorption rate in the dispersion, l9T/F, as a function of gas holdup ratio Qf)/V, chemical reaction rate k (in sec ), and the following values of volumetric gas flow late Q (in cm3/sec) A) 50, B) 150, C) 500, D) 2000 [after Gal-Or and Resnick (G2,G6)] (a = 0.1 cm.)...

Fig. 17. Influence of bubble size on NAj as affected by interaction between bubbles (as a function of a in subreactors and for constant gas holdup), chemical reaction rates, and contact times. NAJ was calculated from Eq. (176) with c, = 1.46 x 10-7 gr-mole/cm3 D= 2.3 x 10"5 cm2/sec G> =0.064 d = 0.1 cm 0 = 2.85 sec [after Gal-Or and Hoel-scher (G5)].

Chemical reaction rate, see Rate of reaction Chemical reactions condensed phases, 42-46 enzymatic, see Enzymatic reactions gas phase, see Gas-phase reactions heterolytic bond cleavage, 46, 47, 51,... 

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