Pellet cyclohexane, dehydrogenation

Yang, H.-S., Chou, C.-T. (2008). Non-isothermal simulation of cyclohexane dehydrogenation in an inert membrane reactor with catalytic pellets in the feed-side chamber. Journal of the Chinese Institute of Chemical Engineers, 39, 227—235. 

In 2000, Itoh and Haraya constructed the first CMR and experimentally examined the performance of a dehydrogenation reaction. Asymmetric polyimide hollow fibers were pyrolyzed in a vacuum oven at 1023 K in order to obtain hollow fiber carbon membranes. Their CMR consisted of SS in which 20 carbonized hollow fibers (0.295 mm diameter and 128 mm long) and catalyst pellets (0.5 wt% Pt/Al203) were allocated. The reactor, used for cyclohexane dehydrogenation to benzene at 468 K, showed a fair improvement over equilibrium conversions. In detail, the temperature dependency of the permeation rates showed that the carbon membrane had micropores with an average diameter close to those of the gas molecules and therefore the permeation process was molecular-sieving controlled. The ideal H2/Ar... 

Barnett et al. [AIChE J., 7 (211), 1961] have studied the catalytic dehydrogenation of cyclohexane to benzene over a platinum-on-alumina catalyst. A 4 to 1 mole ratio of hydrogen to cyclohexane was used to minimize carbon formation on the catalyst. Studies were made in an isothermal, continuous flow reactor. The results of one run on 0.32 cm diameter catalyst pellets are given below. 

Dehydrogenation of cyclohexane (1) Pt catalyst deposited within the pores of the membrane (Pt 34 wt.%) (2) Pt/Vycor glass pellets packed on tube side (Pt 34wt.%). 

Dehydrogenation of cyclohexane Pt/Al20j catalytic pellets packed on tube side. 

Dehydrogenation of cyclohexane Pt/Al203 catalytic pellets (0.5 wt.% Pt) packed on lube side. 

The conversion of cyclohexanes to aromatics is a highly endothermic reaction (AH 50 kcal./mole) and occurs very readily over platinum-alumina catalyst at temperatures above about 350°C. At temperatures in the range 450-500°C., common in catalytic reforming, it is extremely difficult to avoid diffusional limitations and to maintain isothermal conditions. The importance of pore diffusion effects in the dehydrogenation of cyclohexane to benzene at temperatures above about 372°C. has been shown by Barnett et al. (B2). However, at temperatures below 372°C. these investigators concluded that pore diffusion did not limit the rate when using in, catalyst pellets. 

Senkan and Ozturk (52) reported the synthesis and screening of a 66-member discrete heterogeneous catalyst library L7, containing Pt, Pd, and In, for the dehydrogenation of cyclohexane to benzene at 300 °C. The structure of L7 and its synthesis are reported in Fig. 11.8. Sixty-six jxrrous alumina pellets (30 mg each) were shaped into 0.3-cm-diameter, 0.1-cm-high cylinders (step a), then immersed in aqueous HCl solutions containing the 66 appropriate mixtures of InCh, PdCli, and HiPtCle (step... 

The cyclohexane and methylcyclohexane (Wako Pure Chemical Industries, Ltd.) used were 99.5% and 98.0% pure, respectively. The 0.5 wt%-Pt/Al203 cylindrical pellets (3.3 mm in diameter and 3.6 mm high, N.E. Chemcat Corporation, Tokyo) were used as the dehydrogenation catalyst. 

The majority of zeolite MR applications reported in the literature to date fall into the category of PBMRs. The reactor consists of a zeolite membrane with a conventional catalyst present in the form of a packed bed of pellets. The reaction takes place in the catalyst bed while the zeolite membrane serves mainly as a product separator (for H2 or H2O separation) [27] or a reactant distributor (for O2 distribution) [28]. Figure 3.5 illustrates a FAU-type zeolite PBMR combined with a packed bed reactor for dehydrogenation of cyclohexane [29]. Half of the catalyst is packed in the area upstream of the permeation portion to enhance the conversion, otherwise cyclohexane will preferentially permeate at the front end of the zeolite membrane, resulting in a decrease in conversion. 

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