Universal relation between the boundary temperatures of a basic cycle of sorption heat machines

For analyzing a basic cycle of an adsorption heat machine (AHM) an empiric rule was suggested, which manifests that adsorption isosters and equilibrium line ln P(1/T) for the pure sorbate intersect at T approaching infinity. This rule prompts how to plot the cycle, gives a link between the boundary temperatures of the cycle and allows estimation of a minimal temperature T-g(min) of an external heat source that is necessary to drive the cycle. In this paper the validity of the T-g(min) estimation was justified for working pairs which are most commonly used for adsorption units: water-silica gel, water-zeolite 13X, water-zeolite 4A, water-selective water sorbents (SWSS), CO2-carbon, methanol-carbons (AC-35, TA90), methanol-hydrophobic zeolite CBV 901 Y and ammonia-carbon PX31. Four main working pairs for absorption heat machines-ammonia-water, water-LiBr, methanol-LiBr and R22-isobutylacetate-are also analyzed. This allowed the formulation of requirements to an optimal adsorbent to be used in a single-effect non-regenerative cycle of an AHM. The accuracy of the T-g(min) estimation was examined for each pair. Moreover, it was shown that Trouton's rule is always valid if sorption equilibrium obeys the Polanyi potential theory, i.e., the equilibrium sorption is a unique function of the sorption potential Delta F = -RT ln(P/P-0). For chemical reactions between various salts and sorbates this rule is violated because of a large difference between the standard changes of the entropy and enthalpy in the course of reaction and evaporation. In this case Tin can be calculated from the Clausius-Clapeyron and Vant-Hoff equations. (C) 2008 Elsevier Ltd. All rights reserved.