Equilibrium-Restricted Reactions

Beside their use in equilibrium-restricted reactions, CMRs have been also proposed for very different applications [6], like selective oxidation and oxidative dehydrogenation of hydrocarbons they may also act as active contactor in gas or gas-liquid reactions. 

As pointed out in Section 9.3.2.1, the most common application of CMRs concerns equilibrium-restricted reactions [1-12]. The selective removal of a reaction product from the reaction zone through a membrane will shift the equilibrium, leading to higher conversions when compared to conventional (nonmembrane) reactors. 

Due to its complexity (conversion and separation in the same unit) and because this system has been most widely studied experimentally, CMRs for dehydrogenation (or more generally for equilibrium-restricted reactions) have been the subject of modeling approaches [6, 54-59]. The modeling of CMRs requires mass and energy balances in both feed and permeate sides of the reactor (plug-flow behavior is always assumed) and appropriate boundary conditions. Generally these models fit the experimental data well. 

C Other Equilibrium-Restricted Reactions Besides dehydrogenation, other reactions producing hydrogen have been studied in CMRs. Among these are the water-gas shift [61-63], steam reforming [64], and H2S decomposition [65], Positive effects of the membrane-catalyst association have been reported. The recovery of tritium in a CMR through gas shift of tritiated water has also been studied within the framework of the fusion reactor project [66]. 

Equilibrium-restricted reactions (Section A9.3.3.1) have until now been the main field of research on CMRs. Other types of application, such as the controlled addition of reactants (Section A9.3.3.2) or the use of CMRs as active contactors (Section A9.3.3.3), seem however very promising, as they do not require permselective membranes and often operate at moderate temperatures. Especially attractive is the concept of active contactors where the membrane being the catalyst support becomes an active interface between two non-miscible reactants. Indeed this concept, initially developed for gas-liquid reaction [79] has been recently extended to aqueous-organic reactants [82], In both cases the contact between catalyst and limiting reactant which restricts the performance of conventional reactors is favored by the membrane. 

Most of the studies on IMRs focused on equilibrium-restricted reactions, where selective permeation of reactants (mostly H2, in some cases O2) led in any case to improvements compared to conventional fixed-bed reactors. However, it has to be admitted... 

All of the above reactions are reversible, with the exception of hydrocracking, so that thermodynamic equilibrium limitations are important considerations. To the extent possible, therefore, operating conditions are selected which will minimize equilibrium restrictions on conversion to aromatics. This conversion is favored at higher temperatures and lower operating pressures. 

An important crosscheck is the media restriction. Paths that form reactive cations almost exclusively occur in acidic media. Likewise, paths that form reactive anions are the domain of basic media. No medium can be both a strong acid and a strong base it would neutralize itself. The reactive species in equilibrium-controlled reactions have a limited range of acidities. For example, in neutral water the hydronium ion concentration and the hydroxide ion concentration are both 10 mol/L. Their relative concentrations are defined by = [H ][OH ] = Their p Ta values span 17.4 p fa... 

Our discussion of mineral surfaces will be restricted to simple oxide and hydroxide minerals that are widely used in adsorption studies (Table 9-1). Under strong weathering conditions, these minerals may comprise a substantial fraction of the available surface area in soils and aquifers. More complex minerals, including parent material and partially weathered products (especially aluminosilicates), are of equal or greater importance in most other subsurface environments. Excellent reviews of the equilibrium and reaction chemistry of aluminosilicate surfaces are available (Voudrias and Reinhard, 1986 Mortland, 1970). 

To avoid the equilibrium restrictions, the idea was to use highly selective adsorbent to remove the product in situ, as soon as it is formed. This could be achieved introducing a third, flowing solids phase (selective adsorbent) in the catalytic reactor, countercurrently to the gas phase. Moving the equilibrium would lead to higher conversion, and consequently, all negative effects caused by recirculation would be reduced. If the product could be taken away completely, in principle, the reaction could go to completion. 

This handbook is dedicated in particular to those readers interested in emerging applications of membrane reactors in the field of energy and environment. The main motivation for this handbook is to give to the reader a panorama of the various aspects of research related to membrane reactors and their applications. The utilization of membrane reactor technology on a larger scale could constitute a relevant enhancement of conventional systems already in existence. For example, in the field of reforming processes, the main benefit of a membrane reactor is the selective removal of a compound such as hydrogen from the reaction side, which may allow the thermodynamic equilibrium restrictions of the conventional fixed bed reactors to be overcome. 

When a steam reforming reaction is performed in a CR, conversion and hydrogen yield are limited by thermodynamics. By using Pd-based MRs, the selective removal of hydrogen from the reaction side allows one to overcome the thermodynamic equilibrium restrictions. This potentiality is called shift effect , which allows both higher hydrogen yields and superior conversions than the CRs exercised at the same MR conditions (Basile, lulianelli, et al., 2011, chap. 2). Meanwhile, this effect can allow reforming operations at milder conditions in terms of temperature and pressure than the traditional systems (Zaman Chakma, 1994). 

Cyclohexane dehydrogenation, similar to other dehydrogenation reactions, is an endothermic and equilibrium-limited reaction, which means that its conversion is restricted to thermodynamics and enhances with temperature. An increase in temperature means higher energy consumption and an enhancement in side reactions and coke formation. Considering the fact that hydrogen removal from the reaction side brings about an increase in conversion, a membrane reactor is a potential candidate for this reaction. 

One mole each of CO2 and O2 are placed in an evacuated container. A catalyst causes equilibrium in Reaction 15.C to occur. Only the gas phase is present. Is there a stoichiometric restriction in the application of the phase mle to this system Are there more than one If there is (are) write the appropriate equation(s) of the restriction(s), and sketch it (them) on an appropriate triangular composition diagram. 

Kotas [3] has drawn a distinction between the environmental state, called the dead state by Haywood [1], in which reactants and products (each at po. To) are in restricted thermal and mechanical equilibrium with the environment and the truly or completely dead state , in which they are also in chemical equilibrium, with partial pressures (/)j) the same as those of the atmosphere. Kotas defines the chemical exergy as the sum of the maximum work obtained from the reaction with components atpo. To, [—AGo], and work extraction and delivery terms. The delivery work term is Yk k kJo ln(fo/pt), where Pii is a partial pressure, and is positive. The extraction work is also Yk kRkTo n(po/Pk) but is negative. 

The sufficient and necessary condition is therefore Cb iCa. As a consequence of imposing the more restrictive condition, which is obviously not correct throughout most of the reaction, it is possible for mathematical inconsistencies to arise in kinetic treatments based on the steady-state approximation. (The condition Cb = 0 is exact only at the moment when Cb passes through an extremum and at equilibrium.)... 

In a discussion of these results, Bertrand et al. [596,1258] point out that S—T behaviour is not a specific feature of any restricted group of hydrates and is not determined by the nature of the residual phase, since it occurs in dehydrations which yield products that are amorphous or crystalline and anhydrous or lower hydrates. Reactions may be controlled by interface or diffusion processes. The magnitudes of S—T effects observed in different systems are not markedly different, which indicates that the controlling factor is relatively insensitive to the chemical properties of the reactant. From these observations, it is concluded that S—T behaviour is determined by heat and gas diffusion at the microdomain level, the highly localized departures from equilibrium are not, however, readily investigated experimentally. 

For catalytic reactions with AG<0 there is no thermodynamic restriction on the magnitude of A. Electrochemical promotion simply makes a catalyst more efficient for bringing the reactive mixture to equilibrium, i.e. minimum G at fixed T and P. 

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