Dimensional stability of adsorbents subjected to thermal cycling
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Dimensional stability of adsorbents subjected to thermal cycling Mark W. Ackley • Steven J. Pontonio
Received: 1 May 2013 / Accepted: 4 October 2013 / Published online: 15 October 2013 Ó Springer Science+Business Media New York 2013
Abstract The dimensional stability of adsorbent beads subjected to varying temperature conditions must be understood to assess the effect of thermal cycling on both the adsorbent and the structure that contains it. Most of the literature on the coefficient of thermal expansion (CTE) of adsorbents relates to zeolite crystals or clusters of crystals with application to membranes. Such crystals or powder materials have been shown to exhibit both positive and negative volume expansion coefficients depending upon the temperature range. This duality in the CTE with increasing temperature and the large variation in the CTE magnitude for a given zeolite structure suggest that the dimensional stability of zeolite crystals under varying thermal conditions is not likely a good indicator of the thermal stability of agglomerated zeolites. In this study, a method has been developed and applied to measure the CTE of activated alumina and 13X molecular sieve adsorbent beads. A McBain gravimetric microbalance was modified in a simple manner to be used as a dilatometer. The method was validated by measuring the CTE of a 316 stainless steel rod and showing that the measured CTE of this study agreed with the published CTE within 3.3 %. Average CTEs for alumina and 13X adsorbents were determined as 4.88 9 10-6 and 2.96 9 10-6 mm/mm/ °C, respectively for the range of temperature 20–400 °C. Keywords Coefficient of thermal expansion Activated alumina 13X molecular sieve Thermal stability of adsorbent beads
M. W. Ackley (&) S. J. Pontonio Praxair, Inc, 175 East Park Drive, Tonawanda, NY 14151, USA e-mail: [email protected]
1 Introduction Agglomerated adsorbents are commonly employed to purify gases on a large scale utilizing for example pressure swing adsorption (PSA) or temperature swing adsorption (TSA) processes. Adsorbent beds may be configured to conform to various vessel geometries within which the adsorbent is contained and/or constrained by not only the vessel walls but also by internal support structures inside the vessel. When subjected to significant changes in temperature (as in TSA processes), the vessel walls, internal structure and adsorbent expand and contract. The relative movement of these components results in non-negligible forces applied to both the adsorbent and any internal support structures within the vessel, e.g. screens, perforated plates, etc. If designed improperly, the support structure may fail or separate at the vessel interface and allow adsorbent particles to escape at the edges or through damaged parts of the support. Excessive forces may also result in crushing the adsorbent particles. The thermal expansion and contraction of a packed bed depends not only upon the CTE of the packing material but also upon the configuration of the packing and constraints impose
