Efficient hydrogen generation from organic chemical hydrides by using catalytic reactor on the basis of superheated liquid-film concept

• Published in: Chemical engineering science 2008-10, Vol.63 (20), p.4935-4941
• Main Authors: Saito, Yasukazu, Aramaki, Kiyoshi, Hodoshima, Shinya, Saito, Morihiro, Shono, Atsushi, Kuwano, Jun, Otake, Katsuto
• Format: Article
• Language: English
• Subjects: Applied sciences; Catalysis; Catalytic reactions; Chemical engineering; Chemistry; Dehydrogenation; Distillation; Energy; Exact sciences and technology; General and physical chemistry; Heat and mass transfer. Packings, plates; Kinetics; Organic chemical hydrides; Reactors; Theory of reactions, general kinetics. Catalysis. Nomenclature, chemical documentation, computer chemistry; Thermodynamic process
• ISSN: 0009-2509; 1873-4405
• DOI: 10.1016/j.ces.2007.11.036

Under boiling and refluxing conditions for catalytic dehydrogenation of organic chemical hydrides (decalin, methylcyclohexane and others) in a batch-wise reactor, either suspended states with excess amounts of substrate or sand-bath states with its scarce amounts were found to be inferior generally to the so-called “liquid-film states” with adequate amount ratios of substrate to catalyst, where the catalyst-layer temperatures were superheated or raised higher than the boiling point, and, consequently, reactivities became more favorable at higher heating temperatures in contrast to the boiling suspended states. Equilibrium shifts due to reactive distillation were well demonstrated under boiling and refluxing conditions in naphthene dehydrogenation. Moreover, desorption of hydrogen from the active sites to the bubble space was enhanced in the superheated liquid-film states, with large translational entropy endowed. Provided the extents of equilibrium deviation were large enough (Prigogine's approach), thermodynamic couplings among irreversible processes would be realized between heat transfer and mass transfer as an example of the extended De Donder's equation. Restriction of chemical equilibrium could be removed under temperature gradient conditions, as the consequence that its Gibbs energy change became more negative than that under iso-temperature conditions. Within the framework of irreversible thermodynamics, the decreased retardation constant K in the superheated liquid-film states was interpreted in terms of a vector-level coupling between temperature gradient and desorption. Moreover, vigorous bubble formation would give additional favor to the reaction rates owing to enlarged repeating frequencies of sequential non-microreversible processes in dehydrogenation catalysis. Organic chemical hydrides are attractive from the viewpoints of safe, economical, exergy-saving and large hydrogen contents for hydrogen storage and distribution. Their main defects have been pointed out hitherto that the endothermic reaction temperatures are too high. In this paper, a new concept on superheated liquid-film catalysis is explored for dehydrogenation temperatures to decrease, which would result in not only saving exergy for external heating but also avoiding catalyst deactivation due to carbon deposit.