Formation of a “dead zone” in porous structures during processes that proceeding under steady-state and unsteady-state conditions

Abstract

The problem of the origination of a “dead zone” in porous materials during the proceeding of the arbitrary chemical reactions within them under steady- and unsteady-state conditions is investigated. As the “dead zone”, we understand the region in a porous medium that cannot be penetrated by the reagents that participate in the chemical transformations. In this study, the necessary conditions of the origination of the dead zone in the central region of the spherical, cylindrical, or lamellar porous catalyst granule in the course of the arbitrary catalytic reaction in it are found. The resistances to the mass-and-heat exchange in the outer surface of the porous material and the external reaction mixture are taken into account. It should be noted that the proposed algorithm for deriving the necessary conditions of the dead zone is also valid for porous materials of any other geometric shape. It is also shown that, under definite process conditions in the unsteady-state conditions (involving the artificially formed conditions), the phenomenon of pore locking is observed in certain time intervals. This phenomenon, which involves the duration of pore locking, can be controlled. The locking phenomenon cannot be used in technologies developed based on applying porous media and the repeated origination of the dead zone in the porous material. The analytical expression for determining the critical value of the Thiele parameter at which the dead zone originates is only derived for the simple cylindrical or lamellar shape in the steady- and quasi-steady-state conditions. For the catalytic reaction of the mentioned type, which proceeds under steady- and unsteady-state conditions, the nonisothermality of the porous granule on the critical value of the Thiele parameter is also investigated. Similar investigations are also performed for the catalytic processes occurring by the parallel, consecutive, and consecutive-parallel schemes. For example, it should be noted that catalysts with porous granules are widely used under the conditions of chemical technologies. Additionally, in connection with the rapid development of the microelectronics and nanoelectronics, the problem of the development and use of effective miniature devices for energy generation and storage also becomes very important. Currently, porous materials are widely used in various technologies of the development of accumulators as the electrodes. In recent years, the porous materials are of considerable interest in the projects associated with the development of the alternative power engineering, in particular hydrogen power engineering. In particular, the technologies of storing hydrogen that can successfully be used in the future bond hydrogen in metal hydrides, metals, alloys, intermetallic compounds of composites, and its storage in carbon nanostructures.