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

Abstract

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), CO 2 –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 Δ 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 T g min can be calculated from the Clausius–Clapeyron and Vant-Hoff equations.