Dehydrogenation reaction 269 equilibrium constant

Catalytic reformers are normally designed to have a series of catalyst beds (typically three beds). The first bed usually contains less catalyst than the other beds. This arrangement is important because the dehydrogenation of naphthenes to aromatics can reach equilibrium faster than the other reforming reactions. Dehydrocyclization is a slower reaction and may only reach equilibrium at the exit of the third reactor. Isomerization and hydrocracking reactions are slow. They have low equilibrium constants and may not reach equilibrium before exiting the reactor. 

K (277°C) and 650°K are 0.63 and 103 atm,3 respectively. Above about 350°C the equilibrium constants for this type of reaction are such that the aromatic is always highly favored thermodynamically over the corresponding cycloalkane. Moreover, olefin which is itself capable of further dehydrogenation to an aromatic (e.g., cyclohexene) is never observed in significant amounts under isomerization conditions. 

Corma and co-workers152 have performed a detailed theoretical study (B3PW91/6-31G level) of the mechanism of the reactions between carbenium ions and alkanes (ethyl cation with ethane and propane and isopropyl cation with ethane, propane, and isopentane) including complete geometry optimization and characterization of the reactants, products, reaction intermediates, and transition states involved. Reaction enthalpies and activation energies for the various elemental steps and the equilibrium constants and reaction rate constants were also calculated. It was concluded that the interaction of a carbenium ion and an alkane always results in the formation of a carbonium cation, which is the intermediate not only in alkylation but also in other hydrocarbon transformations (hydride transfer, disproportionation, dehydrogenation). 

For endothermic reactions, e.g. dehydrogenation, cracking reactions, etc., the equilibrium constant and the equilibrium conversion increase with temperature. 

The combination of Eqs. (150) and (151) provides a rate expression for the dehydrogenation/hydrogenation reactions that is dependent on the values of k, and Kx (as well as the overall equilibrium constant, Kcq). Estimates of these kinetic parameters can be made in terms of physically meaningful quantities such as entropies and enthalpy changes. Transition state theory gives the following expression for ky. 

As an example, a hydrocarbon reaction might be carried out in such a manner that the first reaction step be a dehydrogenation to the respective olefin. Such a reaction is characterized by a thermodynamic equilibrium constant for the dehydrogenation step,... 

Write a reaction for the dehydrogenation of gaseous ethane (C2Fdg) to acetylene (C2Fd2). Calculate AG° and the equilibrium constant for this reaction at 25°C, using data from Appendix D. 

Kg = equilibrium constant for dehydrogenation of MCH on surface. [MCH] = methylcyclohexane concentration, m is subscript pertaining to a deactivation reaction, n is subscript pertaining to a main reaction. 

Dehydrogenation reactions are difficult. High temperatures and low pressures are required to achieve reasonable per-pass conversions. Hydrocarbon dehydrogenation reactions are endothermic to the tune of about 30-35 kcal/mol and thus have large heat input requirements. The equilibrium constant, Kp, of a typical dehydrogenation reaction, A B + aH2, starting with 1 mol of A, mmol of B, and nmol of an... 

Because the dehydrogenation reaction is endothermic, the reaction mixture temperature decreases as the reaction proceeds. The reaction rate slows because of the closer approach to equilibrium and the decrease in kinetic reaction rate with the decreasing temperature. Furthermore, the equilibrium constant is less favorable at lower temperature. Therefore, in a normal design, about 80% of the temperature drop occurs in approximately the first third of the catalyst bed. 

The proper choice of catalysts for the vapor phase hydration of olefins under pressure to form alcohols is a very important factor. Apparently, catalysts active in promoting the hydration reaction are likewise active toward promotion of the undesirable polymerization reactions since this latter reaction often proceeds at a more rapid rate than that of alcohol formation as evidenced by the high yields of polymers and low yields of alcohols. The use of catalysts to lower the temperature for the reaction is necessitated by the fact that as the temperature is increased to obtain more favorable rates, the equilibrium conversion to alcohol becomes lower, and the tendency to polymerize is increased. Also, the catalyst must not promote dehydrogenation of the alcohol to form hydrogen and aldehyde since at the temperature of operation the equilibrium is very favorable for this reaction as has been pointed out in a previous chapter. Thus, the reaction, isobutanol = isobutyl aldehyde -f hydrogen has an equilibrium constant corresponding to about 72 per cent decomposition at 450° C even with 100 atmospheres of hydrogen pressure. 

For the dehydrogenation of n-butane our determined value of the thermodynamic equilibrium constant Kwas 0.0133 atm at our reaction temperature of 450°C. Hence we have... 

If the reduction is conducted with benzylviologen (E = -330 to -360 mV (6)) as the mediator at pH 6 the 2-oxo carboxylates will be completely reduced since the equilibrium constant for the reduction of pyruvate as an example is 8.10. At pH 8.5 with anthraquinone-2,6-disulphonate (E" = -184 mV) the equilibriiun constant of this reaction is about 6 orders of magnitude smaller than with benzylviologen and dehydrogenations of (R)-2-hydroxy carboxylates can be conducted quantitatively when the reduced quinone (AQ-2,5-DS-H2) is effectively reoxidized for instance by Reaction [22] ... 

In Table 2 it can be seen that, as expected, an increase in space velocity results in a decrease in the overall conversion. However the yield of butenes remains approximately constant at the equilibrium conversion. Therefore the main reaction affected by the increase in space velocity is not the dehydrogenation reaction but the carbon deposition reactions. As carbon laydown is one of the major other reactions occurring we analysed... 

If the side reactions are ignored, the overall dehydrogenation rate of ethane can be described by Eq. 17.1. This equation is derived from a simple formula for the equilibrium constant given in Eq. 17.2 that predicts a beneficial effect of hydrogen removal [12] for dehydrogenation processes. 

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