Optimization of adsorption dynamics in adsorptive chillers: Loose grains configuration

Dynamic performance of adsorption chillers (ACs) with adsorbent beds of loose grains is addressed in this paper. Temporal evolution of quasi-isobaric ad-/desorption curves for simple bed configuration of n-layers of loose grains (n = 1, 2, 4, and 8) located on a heat transfer metal support is studied. The support is subjected to fast temperature drop (50 degrees C -> 30 degrees C) or jump (58 degrees C -> 80 degrees C) typical for AC cycles. The size d of Fuji silica RD grains is varied from 0.2 to 1.8 mm. At n = 1-4, the experimental kinetic curves are nearly exponential, so that the major part of water ad-/desorption process can be described by a single characteristic time. It is found that the dynamics is invariant with respect to the ratio (S/m) = "heat transfer surface"/"adsorbent mass" which can be used for assessing the degree of dynamic perfection of an "adsorbent bed heat exchanger" (Ad-HEx) unit. For small grains (d < 0.5-0.8 mm) and (S/m) >= 2 m(2)/kg, the "grain size insensitive" regime is established, for which loose silica grains of different size result in the same dynamic performance if the ratio (S/m) is constant. The "grain size sensitive" regime is realized for large adsorbent grains (d >= 0.8 mm): the specific power reduces with increasing the grain size. Practical recommendations on proper organization of the Ad-HEx unit as well as of the AC cycle durations are made. Analysis of the dynamic performance of four selected AC prototypes described in literature shows that their cycle powers are 2-6 times lower than those measured in our LTJ (Large Temperature Jump) tests. We discuss this difference as caused by several imperfections and pitfalls of either hardware or process organization rather than by poor adsorbent properties. (C) 2012 Elsevier Ltd. All rights reserved.