Dehydrogenation thermodynamically-limited

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). 

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],... 

The main drawbacks of the non-oxidative dehydrogenation reaction can be summarized as, the thermodynamic limitation, the low conversion rate, the need for recovery of unreacted ethylbenzene, the high energy consumption, and deactivation of the catalyst. Thus in recent years several alternatives to overcome those problems have been investigated. 

Typically, this reaction is conducted at high temperature to achieve appreciable conversion in a reasonably short reaction time. However, under these conditions where the reaction kinetics are fast, this reaction tends to be thermodynamically limited. Therefore, it has been studied in a Pd membrane reactor [1, 3, 4] where the H2 is continuously removed. Since the rate of the dehydrogenation reaction is slow relative to the rate of hydrogen removal through the Pd membrane, these membrane systems remove the thermodynamic limitation and are instead kinetically limited [5]. 

These products (iC4Hg, CO, and H2) are more reactive than the iC4Hio, so they behave as intermediates and continue to react and approach thermodynamic equilibrium. Thus, the co-production of isobutylene and hydrogen in oxidative dehydrogenation causes this system to be thermodynamically limited. This reaction system is presented schematically in Figure 1. By using a membrane reactor, we continuously remove the co-produced H2 while still benefiting from the fast oxidation kinetics. 

Unlike the traditional process of catalytic dehydrogenation of ethylbenzene, catalytic oxidehydrogenation, does not present thermodynamic limitations. Its reaction ... 

Oxydehydrogenation of /i-Butenes. Normal butenes can be oxidatively dehydrogenated to butadiene in the presence of high concentration of steam with fairly high selectivity (234). The conversion is no longer limited by thermodynamics because of the oxidation of hydrogen to water. Reaction temperature is below about 600°C to minimise over oxidation. Pressure is about 34—103 kPa (5—15 psi). 

For a given dehydrogenation system, i.e., operating temperature and pressure, thermodynamic theory provides a limit to the per pass conversion that can he achieved. A general formula is... 

The dehydrogenation of paraffins to olefins, while it does not take place to a large extent at typical reforming conditions (equilibrium conversion of n-hexane to 1-hexene is about 0.3% at 510°C. and 17 atm. hydrogen partial pressure), is nevertheless of considerable importance, since olefins appear to be intermediates in some of the reactions. This matter will be discussed in more detail in a subsequent section. The formation of olefins from paraffins, similar to the formation of aromatics, is favored by the combination of high temperature and low hydrogen partial pressure. The thermodynamics of olefin formation can play an important role in determining the rates of those reactions which proceed via olefin intermediates, since thermodynamics sets an upper limit on the attainable concentration of olefin in the system. 

Preliminary results obtained in an effort to model the dehydrogenation of ethylbenzene to styrene in a "membrane reactor" are described below. The unique feature of this reactor is that the walls of the reactor are conprised of permselective membranes through which the various reactant and product species diffuse at different rates. This reaction is endothermic and the ultimate extent of conversion is limited by thermodynamic equilibrium constraints. In industrial practice steam is used not only to shift the ec[uilibrium extent of reaction towards the products but also to reduce the magnitude of the ten erature decrease which accon anies the reaction when it is carried our adiabatically. 

The paraffin dehydrogenation reaction scheme is shown in Fig. 2. Paraffins are dehydrogenated to form mono-olefins with the double bond distributed according to thermodynamics (less than 10% in the a position). The extent of the reaction is largely controlled by thermodynamic equilibrium, and typical paraffin conversion levels are limited to 10-15%. The reaction is typically carried out at low pressure to enhance the equilibrium in favor of olefin production. 

Examples of synergistic effects are now very numerous in catalysis. We shall restrict ourselves to metallic oxide-type catalysts for selective (amm)oxidation and oxidative dehydrogenation of hydrocarbons, and to supported metals, in the case of the three-way catalysts for abatement of automotive pollutants. A complementary example can be found with Ziegler-Natta polymerization of ethylene on transition metal chlorides [1]. To our opinion, an actual synergistic effect can be claimed only when the following conditions are filled (i), when the catalytic system is, thermodynamically speaking, biphasic (or multiphasic), (ii), when the catalytic properties are drastically enhanced for a particular composition, while they are (comparatively) poor for each single component. Therefore, neither promotors in solid solution in the main phase nor solid solutions themselves are directly concerned. Multicomponent catalysts, as the well known multimetallic molybdates used in ammoxidation of propene to acrylonitrile [2, 3], and supported oxide-type catalysts [4-10], provide the most numerous cases to be considered. Supported monolayer catalysts now widely used in selective oxidation can be considered as the limit of a two-phase system. 

Most feeds contain some olefin as an impurity moreover many sulfated zirconia catalysts contain traces of iron or other transition metal ions that are able to dehydrogenate hutane. In the presence of such sites, the olefin concentration is limited by thermodynamics, i.e a high pressure of H2 leads to a low olefin concentration. That aspect of the reaction mechanism has been proven in independent experiments. The isomerization rate over sulfated zirconia was dramatically lowered by H2. This effect is most pronounced when a small amount of platinum is deposited on the catalyst, so that thermodynamic equilibrium between butane, hydrogen and butene was established. In this way it was found that the isomerization reaction has a reaction order of +1.3 in -butane, hut -1.2 in hydrogen [40, 41]. The byproducts, propane and pentane, are additional evidence that a Cg intermediate is formed in this process. As expected, this kinetics is typical for butane isomerization only in contrast pentane isomerization is mainly a monomolecular process, because for this molecule the protonated cyclopropane ring can be opened without forming a primary carbenium ion [42]. 

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