Dehydrogenation conversion enhancement

Dehydrogenation of aliphatic hydrocarbons. A number of aliphatic hydrocarbons experience enhanced dehydrogenation conversions by carrying out the reactions in porous inorganic membranes. Most of the studies use porous alumina membrane tubes as the reactors. 

Mohan and Govind [1988c] applied their isothermal packed-bed porous membrane reactor model to the same equilibrium-limited reaction and found that the reactor conversion easily exceeds the equilibrium value. The HI conversion ratio (reactor conversion to equilibrium conversion) exhibits a maximum as a function of the ratio of the permeation rate to the reaction rate. This trend, which also occurs with other reactions such as cyclohexane dehydrogenation and propylene disproportionation, is the result of significant loss of reactant due to increased permeation rate. This loss of reactant eventually negates the equilibrium displacement and consequently the conversion enhancement effects. 

Ferreira, P., Rodriguez, I., Guerrero, A. (2002b). On the performance of porous vycor membranes for conversion enhancement in the dehydrogenation of methylcyclohexane to toluene. Journal of Catalysis, 212, 182—192. 

Jeong, Sotowa, and Kusakabe (2004) simulated the catalytic dehydrogenation of cyclohexane in an FAU-type zeolite membrane reactor. The cyclohexane conversion enhanced in the zeohte membrane reactor, which was more dependent on the permeance than the separation factor. Table 21.3 presents a summary of some of the membrane reactors used for cyclohexane dehydrogenation. 

There have also been several theoretical studies comparing oxidative dehydrogenation MRs with traditional packed bed reactors (Rodriguez et al., 2010). These studies show that, as well as reaction selectivity and conversion enhancement, the use of PBMRs can prevent oxygen accumulation, reduce the occurrence of hot spots within the reaction bed (Assabumrungrat et al, 2002) and improve the inherent production rates and safety of the process vessel. 

AXB) shows time courees of amounts of evolved hydrogen and decalin conversions with caibon-supported platinum-based catalysts unda" supeiheated liquid-film conditions. Enhancement of dehydrogenation activities for decalin was realized by using fiiese composite catalysts. The Pt-W / C composite catalyst exhibited the hipest reaction rate at the initial stage, whereas the Pt-Re / C composite catalyst showed the second highest reaction rate in addition to low in sensitivity to retardation due to naphthaloie adsorbed on catalytic active sites [1-5], as indicated in Fig. 2(A) ). 

Thus, the role of zinc in the dehydrogenation reaction is to promote deprotonation of the alcohol, thereby enhancing hydride transfer from the zinc alkoxide intermediate. Conversely, in the reverse hydrogenation reaction, its role is to enhance the electrophilicity of the carbonyl carbon atom. Alcohol dehydrogenases are exquisitely stereo specific and by binding their substrate via a three-point attachment site (Figure 12.7), they can distinguish between the two-methylene protons of the prochiral ethanol molecule. 

Similar improvements of activity and selectivity were reported in the oxidative dehydrogenation by C02 of ethane over Ga2C>3 (18.6% ethylene yield and 94.5% selectivity)391 and that of propane over rare earth vanadates.392 Cr203 shows medium activity in the oxidative dehydrogenation of ethane, but support on Si02 enhances the catalytic performance (55.5% ethylene yield at 61% conversion at 650°C).393... 

After the absorber/stripper unit, in conventional operations the pure H2S is fed to a Claus unit where the H2S is converted to elemental sulphur and H2O. The Claus unit can be equipped with an after-treatment to enhance conversions. Another method to decompose H2S to less harmful compounds is the thermal dehydrogenation of H2S to hydrogen and sulphur. Both processes will be treated in detail in the remainder of this chapter. 

Conversion of n-butane into isobutene over theta-1 and ferrierite zeolites was studied in a continuous flow microreactor at 530°C and 100% n-butane as a feed. The zeolites were used as catalysts in the H- and Ga-forms. Insertion of Ga into the zeolites resulted in improved isobutene selectivities due (i) to an increase in the dehydrogenation activities and (ii) to a decrease in the cracking activities of the catalysts. The highest selectivities to isobutene (-27%) and butenes (-70%) were obtained with the Ga-theta-1 catalyst at n-butane conversions around 10%. These selectivities decreased with increasing conversion due to olefin aromatisation, which was enhanced considerably by the Ga species present in the catalysts. 

In a base-free medium (dry MeCN), Fe Ch activates HOOH to form a reactive intermediate that oxygenates alkanes, alkenes, and thioethers, and dehydrogenates alcohols and aldehydes. Table 11 summarizes the conversion efficiencies and product distributions for a series of alkene substrates subjected to the Fe Cfi/HOOH/MeCN system. The extent of the Fe Cb-induced monooxygenations is enhanced by higher reaction temperatures and increased concentrations of the reactants (substrate, Fe Cls, and HOOH). For 1-hexene (representative of all of the alkenes), a substantial fraction of the product is the dimer of 1-hexene oxide, a disubstituted dioxane. With other organic substrates (RH), Fe Cb activates HOOH for their monooxygenation the reaction efficiencies and product distributions are summarized in Tables 11(b). In the case of alcohols, ethers, and cyclohexane, a snbstantial fraction of the product is the alkyl chloride, and with aldehydes, for example, PhCHO, the acid chloride represents one-half of the product. In the absence of snbstrate the Fe Cls/MeCN system catalyzes the rapid disproportionation of HOOH to O2 and H2O. 

Enhancement in conversion by the usage of a membrane reactor has been demonstrated for many dehydrogenation reactions. Product selectivity of some hydrogenation and other reactions arc found to improve with a permselective membrane as part of the reactor. Several dense metal as well as solid elecu olyte membranes and porous metal as well as various oxide membranes have been discovered to be effective for the reaction performance. 

Enhancement of reaction conversion by employing a permselective membrane often has the implication that, for a given conversion, it is possible to run the reaction at a lower temperature in the membrane reactor than in a conventional reactor. Catalyst deactivation due to coke formation generally becomes more severe as the reaction temperature increases. Therefore, the use of a membrane reactor to replace a conventional one should, in principle, reduce the propensity for coke formation because for the same conversion the membrane reactor configuration may be operated at a lower temperature than a conventional reactor. This is particularly true for such reactions as dehydrogenation. 

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