Thermodynamically limited reactions

Countercurrent flow has advantages in product and thermodynamically limited reactions. Catalytic packings (see Figure 9. Id) are commonly used in that mode of operation in catalytic distillation. Esterification (methyl acetate, ethyl acetate, and butyl acetate), acetalization, etherification (MTBE), and ester hydrolysis (methyl acetate) were implemented on an industrial scale. 

Membrane reactors can be used to shift the equilibrium in thermodynamically limited reactions. Several types of membrane reactors are currently under investigation, especially for dehydrogenation reactions such as the dehydrogenation of propane to propene [6] or of ethylbenzene to styrene [7], Also the dehydrogenation of H2S has been studied in membrane reactors [8,9],... 

In membrane reactors, the reaction and separation processes take place simultaneously. This coupling of processes can result in the conversion enhancement of the thermodynamically-limited reactions because one or more of the product species is/are continuously removed. The performance of such reactors depends strongly on the membrane selectivity as well as on the general operahng conditions which influence the membrane permeability. 

TTie two reactors with recycle shown in (i) and (j) can be used for highly exothermic reactions. Here the recycle stream is cooled and returned to the reactor to dilute and cool the inlet stream thereby avoiding hot spots and runaway reactions. The PFR with recycle is used for gas-phase reactions, and the CSTR is used for liquid-phase reactions. The last (wo reactors, (k) and (I), are used for thermodynamically limited reactions where the equilibrium lies far to the left (reactant side)... 

Reactor design for thermodynamically limited reactions Kinetics... 

Membrane reactors can be used to increase conversion when the reaction is thermodynamically limited, as well as to increase the selectivity when multiple reactions are occurring. Thermodynamically limited reactions are reactions where the equilibrium lies far to the left (i.e reactant side) and there is little conversion. If the reaction is exothermic, increasing the temperature will only drive the reaction further to the left, and decreasing the temperature will result in a reaction rate so slow that there is very little conversion. If the reaction is endothermic, increasing the temperature will move the reaction to the right to favor a higher conversion however, for many reactions these higher temperatures cause the catalyst to become deactivated. 

As mentioned above, MMRs benefit from fast and thermodynamically limited reactions, for which mass transfer limits the reaction processes to a large extent. Owing to their compact features and high degree of control, MMRs are especially suitable for portable chemical processing. Table 8.1 summarizes typical applications of MMRs for catalytic reactions. 

Other Technologies. As important as dehydrogenation of ethylbenzene is in the production of styrene, it suffers from two theoretical disadvantages it is endothermic and is limited by thermodynamic equiHbrium. The endothermicity requites heat input at high temperature, which is difficult. The thermodynamic limitation necessitates the separation of the unreacted ethylbenzene from styrene, which are close-boiling compounds. The obvious solution is to effect the reaction oxidatively ... 

Dehydrogenation of /i-Butane. Dehydrogenation of / -butane [106-97-8] via the Houdry process is carried out under partial vacuum, 35—75 kPa (5—11 psi), at about 535—650°C with a fixed-bed catalyst. The catalyst consists of aluminum oxide and chromium oxide as the principal components. The reaction is endothermic and the cycle life of the catalyst is about 10 minutes because of coke buildup. Several parallel reactors are needed in the plant to allow for continuous operation with catalyst regeneration. Thermodynamics limits the conversion to about 30—40% and the ultimate yield is 60—65 wt % (233). 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

A much more selective reaction is possible by using vapor-phase fluonnahon over a chromia" catalyst at 300 to 400 °C, but conversions are thermodynamically limited to 10-20% under acceptable operating conditions [fO 11 Despite this disadvantage, this process has been selected by ICI and Hoechst for their first plants... 

The series of reactors and exchangers which methanates a raw syngas without pretreatment other than desulfurization is collectively termed bulk methanation. The chemical reactions which occur in bulk methana-tion, including both shift conversion and methanation, are moderated by the addition of steam which establishes the thermodynamic limits for these reactions and thereby controls operating temperatures. The flow sequence through bulk methanation is shown in Figure 1. 

Membranes can be applied to catalysis in different ways. In most of the literature reports, the membrane is used on the reactor level (centimeter to meter scale) enclosing the reaction mixture (Figure 10.3). In most cases, the membrane is used as an inert permselective barrier in an equilibrium-limited reaction where at least one of the desired products is removed in situ to shift the extent of the reaction past the thermodynamic equilibrium. 

The chemical industry of the 20 century could not have developed to its present status on the basis of non-catalytic, stoichiometric reactions alone. Reactions can in general be controlled on the basis of temperature, concentration, pressure and contact time. Raising the temperature and pressure will enable stoichiometric reactions to proceed at a reasonable rate of production, but the reactors in which such conditions can be safely maintained become progressively more expensive and difficult to make. In addition, there are thermodynamic limitations to the conditions under which products can be formed, e.g. the conversion of N2 and H2 into ammonia is practically impossible above 600 °C. Nevertheless, higher temperatures are needed to break the very strong N=N bond in N2. Without catalysts, many reactions that are common in the chemical industry would not be possible, and many other processes would not be economical. 

The vast number of thermodynamically possible reactions obtained by permuting oxidants and reductants within the scope of this review present major problems of classification and selection. To only a limited extent is the modernity or detail of a paper indicative of its relevance, some of the definitive papers having been published before 1950. Discussion has been concentrated, therefore, at points where a kinetic investigation of a reaction has resulted in a real advance in our understanding both of its mechanism and of those of related reactions, and work which has been more of a confirmatory nature will not receive comparable consideration. Detailed reference to products, spectra, etc. will be made only when the kinetics produce real ambiguities. 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

In typical diesel exhaust gas, the N02/N0x fraction is only 5-10%. This fraction may be increased by passing the gas over a strong oxidation catalyst containing platinum as the active component. However, it is difficult to obtain useful fractions of N02 at temperatures below 200°C at high space velocities due to the strong temperature dependency of the NO oxidation over platinum [42], As expected the NO conversion rises exponentially with temperature, but declines at higher temperatures due to the thermodynamic limit, of the reaction (Figure 9.13). 

In the above manner, we may proceed downstream in the reactor until we either reach the desired conversion level, run into thermodynamic limitations on the reaction rate, or exceed the effluent temperature constraint (see Table... 

Goldberg, K., Edegger, K., Kroutil, W. and Liese, A. (2006) Overcoming the thermodynamic limitation in asymmetric hydrogen transfer reactions catalyzed by whole cells. Biotechnology and Bioengineering, 95,192-198. 

Kragl and coworkers investigated using organic-solvent-free systems to overcome the thermodynamic limitations in the synthesis of optically active ketone cyanohydrins. With organic-solvent-free systems under optimized reaction conditions, conversions up to 78% with > 99.0 enantiomeric excess (ee) (S) were obtained. Finally, 5 mL of (S)-acetophenone cyanohydrin with an ee of 98.5% was synthesized using MeHNL [52]. 

The performance of adsorption processes results in general from the combined effects of thermodynamic and rate factors. It is convenient to consider first thermodynamic factors. These determine the process performance in a limit where the system behaves ideally i.e. without mass transfer and kinetic limitations and with the fluid phase in perfect piston flow. Rate factors determine the efficiency of the real process in relation to the ideal process performance. Rate factors include heat-and mass-transfer limitations, reaction kinetic limitations, and hydro-dynamic dispersion resulting from the velocity distribution across the bed and from mixing and diffusion in the interparticle void space. 

Mathematical models based on physical and chemical laws (e.g., mass and energy balances, thermodynamics, chemical reaction kinetics) are frequently employed in optimization applications (refer to the examples in Chapters 11 through 16). These models are conceptually attractive because a general model for any system size can be developed even before the system is constructed. A detailed exposition of fundamental mathematical models in chemical engineering is beyond our scope here, although we present numerous examples of physiochemical models throughout the book, especially in Chapters 11 to 16. Empirical models, on the other hand, are attractive when a physical model cannot be developed due to limited time or resources. Input-output data are necessary in order to fit unknown coefficients in either type of the model. 

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