Effective heat transfer in a metal-hydride-based hydrogen separation process

This paper presents the results of experimental and analytical study of the thermal cycling absorption process (TCAP); which is a metal-hydride-based hydrogen separation system configured as a helical shell-and-tube heat exchanger. The column (tube side) is packed with Palladium deposited on kieselguhr (Pd/k). This packed column is thermally cycled by a hot and cold nitrogen gas on its exterior surface (shell side), while a stream of hydrogen mixed with other inert gases are passed through the packed column. Hydrogen gas is absorbed and desorbed from the Palladium, causing a separation from the gas stream. The rate at which the hydrogen is separated depends only on how quickly the Pd/k can be thermally cycled. In this paper we present a transient heat transfer analysis to model the heat transfer in the Pd/k packed column. To improve the efficiency of the TPAC, metallic foam was added in the Pd/k packed column. It was observed that adding metallic foam significantly improved the separation rate of hydrogen. Thermal cycling times for varying packed column diameters, materials, and compositions are also determined. Comparison of performance is made between an existing 1.25 in (31.75 mm) column versus a 2 in (50.8 mm) column tube. A parametric argument is presented to optimize the material selection and geometric design of a TCAP heat exchanger. (C) 2001 Published by Elsevier Science Ltd. on behalf of the International Association for Hydrogen Energy.