Reaction-kinetic effects

The only interest in increasing the maximal loop temperature is increasing the reaction kinetics. Effect of HTR inlet temperature... 

Each mechanism exposes the particles to a range of gas atmospheres, the atmosphere varying rapidly in time. Either of the two mechanisms can give rise to reaction kinetic effects, which at times can be hurtful (Squires, 1982), and at other times, helpful (Squires, 1961, 1973). 

I caution, indeed, that even Mobil s analysis cannot faithfully reflect reaction-kinetic effects, if any, arising from top-to-bottom mixing of catalyst in a turbulent- or fast-bed design in question. The analysis may mislead if a given reaction is particularly sensitive to quasi-cyclic extremes in gas atmosphere. 

Khim. Akad. Nauk S.S.S.R. 2, 1202-10 (1953). Reaction kinetics effect of H bond. 

E. A. Moelwyn-Hughes and A. Sherman. J. Chem. Soc. 1936, 101-10. Reaction kinetics effect of complex formation. 

Our calculations using an equilibrium stage model agreed well with the experiments based on separation performance measurements using catalytic rings as internals (Fig. 2.6). On the basis of these results, we have to contradict various publications that attribute the decrease to reaction-kinetic effects. The primary effect is the saddle-point character of the products and the associated choice of a second feed point. The options for calculation and especially interpretation of RD lines, which have merely been outlined here are implemented in the program SYNTHESISER, a software product from the first EU-project. 

One advantage of such physicochemical models is that they can easily be extended to include effects of reaction kinetics. This is shown in Fig. 4.13 where results from a case study on reactions kinetic effects on separations of mixtures of formaldehyde + water + methanol are shown. Whereas the equilibrium model... 

Figure 10.19 The a-effects occur when a reiatively weak stereoeiectronic interaction in the reactant is transformed to a stronger anomeric effect during the reaction. Kinetic effects correlate with nucleophilicity, while thermodynamic effects correlate with basicity. The "excessive" a-effects associated with the positive deviations from the Bronsted correlation result from stronger interactions in the TS than in the product.

In the past, impedance measurements using reactively substituted Wheatstone bridges at audio frequencies have been the easiest to accomplish. Consequently, great emphasis has been placed historically on electrochemical processes having characteristic impedance spectra in the audio frequency range 20-20,000 Hz, namely, double-layer capacitive and moderately fast reaction kinetic effects at plane parallel electrodes. 

It is critical to determine and control the steam-to-carbon (S/C) and/or oxygen-tointernal reforming) to avoid carbon deposition. Thermodynamic analysis is commonly used to estimate the minimum ratios. For example. Figure 33.18 shows the equilibrium number of moles of carbon per mole of methane introduced into an ATR as a function of S/C and O/C at two reformer inlet temperatures of 150 and 400 °C [8]. It can be seen that for aU values of O/C between 0 and 1.5, carbon deposition should not be a concern if an S/C > 1.2 is maintained in the fuel gas mixture entering the ATR (fiiUy mixed inlet stream). It should be noted that many thermodynamic calculations (as in this example) assume adiabatic equilibrium reactions and do not take into account reaction kinetic effects. The inclusion of reaction kinetics in the analysis may lead to different results. 

Finally, it should be noted that the highest conversion that can be achieved in reversible reactions is the equilibrium conversion (which takes an infinite period of time to achieve). For endothermic (heat absorbed) reactions, the equilibrium conversion increases with increasing temperature up to a maximum of 1.0 for exothermic (heat liberated) reactions the equilibrium conversion decreases with increasing temperature. The reader is cautioned that these equilibrium concentration calculations are, for most intents and purposes, a set of fake or artificial values. They almost always represent an upper limit on the expected concentration at the temperature in question. Other chemical reactions, kinetic effects, and temperature variations in the system may render these calculations valueless. Nonetheless, these calculations serve a useful purpose since they do provide a reasonable estimate of these concentrations. 

Adsorption is invariably an exothermic process, so that, provided equilibrium has been established, the amount adsorbed at a given relative pressure must diminish as the temperature increases. It not infrequently happens, however, that the isotherm at a given temperature Tj actually lies above the isotherm for a lower temperature Ti. Anomalous behaviour of this kind is characteristic of a system which is not in equilibrium, and represents the combined effects of temperature on the rate of approach to equilibrium and on the position of equilibrium itself. It points to a process which is activated in the reaction-kinetic sense and which therefore occurs more rapidly as temperature is increased. 

Finally, a consideration of equilibrium chemistry can only help us decide what reactions are favorable. Knowing that a reaction is favorable does not guarantee that the reaction will occur. How fast a reaction approaches its equilibrium position does not depend on the magnitude of the equilibrium constant. The rate of a chemical reaction is a kinetic, not a thermodynamic, phenomenon. Kinetic effects and their application in analytical chemistry are discussed in Chapter 13. 

These pioneers understood the interplay between chemical equiUbrium and reaction kinetics indeed, Haber s research, motivated by the development of a commercial process, helped to spur the development of the principles of physical chemistry that account for the effects of temperature and pressure on chemical equiUbrium and kinetics. The ammonia synthesis reaction is strongly equiUbrium limited. The equiUbrium conversion to ammonia is favored by high pressure and low temperature. Haber therefore recognized that the key to a successful process for making ammonia from hydrogen and nitrogen was a catalyst with a high activity to allow operation at low temperatures where the equiUbrium is relatively favorable. 

Most of the chemical properties of tritium are common to those of the other hydrogen isotopes. However, notable deviations in chemical behavior result from isotope effects and from enhanced reaction kinetics induced by the ( -emission in tritium systems. Isotope exchange between tritium and other hydrogen isotopes is an interesting manifestation of the special chemical properties of tritium. 

Enhanced Reaction Kinetics. For reactions involving tritium, the reaction rates are frequendy larger than expected because of the ionising effects of the tritium P-decay. For example, the uncataly2ed reaction 2T2+O2 — 2X20 can be observed under conditions (25°C) for which the analogous reaction of H2 or D2 would be too slow for detection (30). 

That the specific rate is affected by extremes of pressure—sometimes upward, sometimes downward—is well known. A review of this subject is by Kohnstam ( The Kinetic Effects of Pressure, in Pi ogi e.s.s in Reaction Kinetics, Pergamon, 1970). Three examples follow ... 

A substantial body of data, including reaction kinetics, isotope effects, and structure-reactivity relationships, has permitted a thorough understanding of the steps in aromatic nitration. As anticipated from the general mechanism for electrophilic substitution, there are three distinct steps ... 

Scheme 10. Mechanislic possibililies for PF condensalion. Mechanism a involves an SN2-like attack of a phenolic ring on a methylol. This attack would be face-on. Such a mechanism is necessarily second-order. Mechanism b involves formation of a quinone methide intermediate and should be Hrst-order. The quinone methide should react with any nucleophile and should show ethers through both the phenolic and hydroxymethyl oxygens. Reaction c would not be likely in an alkaline solution and is probably illustrative of the mechanism for novolac condensation. The slow step should be formation of the benzyl carbocation. Therefore, this should be a first-order reaction also. Though carbocation formation responds to proton concentration, the effects of acidity will not usually be seen in the reaction kinetics in a given experiment because proton concentration will not vary.

Chemical themiodynamics provides tlie answer to tlie first question however, it provides information about tlie second. Reaction rates fall witliin tlie domain of chemical kinetics and are treated later in tliis section. Both equilibrium and kinetic effects must be considered in an overall engineering analysis of a chemical reaction. 

The dependence of reaction rates on pH and on the relative and absolute concentrations of reacting species, coupled with the possibility of autocatalysis and induction periods, has led to the discovery of some spectacular kinetic effects such as H. Landolt s chemical clock (1885) an acidified solution of Na2S03 is reacted with an excess of iodic acid solution in the presence of starch indicator — the induction period before the appearance of the deep-blue starch-iodine colour can be increased systematically from seconds to minutes by appropriate dilution of the solutions before mixing. With an excess of sulfite, free iodine may appear and then disappear as a single pulse due to the following sequence of reactions ... 

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