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Development of porous stainless steel supported Pd-membranes for H2 production in membrane reactors

pubblicazioni - Presentazione

Development of porous stainless steel supported Pd-membranes for H2 production in membrane reactors

In questo lavoro viene descritta la preparazione di membrane al palladio a barriera ossidativa e ceramica e vengono ripotate le relative prestazioni in diverse condizioni operative.

Palladium based membrane reactors are widely studied for production of high purity hydrogen in several processes, such as natural gas steam reforming, ethanol stem reforming and WGS of synthesis gas. Due to the considerable cost of palladium, research effort are focused in obtain low thickness Pd membranes, prepared by deposition on cheaper materials, such as macro-porous metals and ceramics, by various techniques, including magnetron sputtering, chemical vapor deposition and electroless plating. Electroless plating, in particular, is an autocatalytic process which allows obtaining thin films with good adhesion characteristics; further it does not require expensive set-up and is relatively easy to scale-up from laboratory to industrial scale. As much as the thickness of Pd layer is lowered, however, obtaining defect-free coatings with adequate adhesion and stability is becoming a critical issue in order to produce high purity hydrogen suitable for feeding PEM fuel cells. Porous metallic supports, in deed, are very promising candidates having numerous advantages over the widely studied ceramic supports, but their application is limited by temperature due to the occurrence of intermetallic diffusion phenomena from about 400-450°C. The intermetallic diffusion of elements from the support to the Pd-layer can be prevented by deposition of a ceramic barrier layer between the support and the Pd-layer. Moreover, it can improve the morphological characteristics of the support surface, which usually is still very poor. In this frame RSE is developing palladium composite membranes prepared by electroless plating on metallic tubular macroporous supports, such as stainless steel. More in detail two types of membranes are developed:

• Pd- membranes which can operate up to 400-450°C (without anti- diffusion barrier) and can be suitable for operation in WGS and ethanol steam reforming .
• Pd membranes with a ceramic barrier layer which, as perspective, can be used for higher temperature applications such as natural gas steam reforming.

In this paper preparation process and performances of these membranes are presented. Specifically the first type of membranes are 10 to 20 micron thick and exhibit a selectivityH2/He up to 8000 at a temperature of 400 °C . They have been tested for more than 6000 hours in a laboratory pilot loop, in various operating conditions, including WGS tests [1] and reforming of bioethanol at 390°C in a membrane reactor [2]. The second type of membrane are obtained by deposition of a ceramic alumina barrier layer on porous stainless steel supports having different level of roughness by using the dip-coating technique and boehmite sols. The Palladium growth carried out on the PSS supports coated by the Alumina barrier layer lead to a dense uniform Palladium layer, 11-14 micron thick, that exhibits a Helium permeance of the order of about 10-9 mol*Pa-1m-2s-1 at room temperature. Membrane performances have been also tested at 500°C up to 1500 h.

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