Pseudo-stable transitions and instability in chemical heat pumps:: the NH3-CoCl2 system

Research in our laboratory has been directed toward the development of a chemical heat pump? based on the NH3-CoCl2 system, to provide both refrigeration and heat for the agribusiness industry. In this work the stability of the salt in an inert atmosphere (nitrogen) and in the presence of ammonia was examined experimentally. For a given pressure, the stable composition of the CoCl2. xNH(3) salt was found to vary with temperature, when the gaseous atmosphere alternated between ammonia and nitrogen. Specifically the salt changed from CoCl2 . 6NH(3) to CoCl2. 2NH(3) (140 degreesC, 260 kPa), from CoCl2. 6NH(3) to CoCl2 (135 degreesC, 260 kPa), and from CoCl2. 2NH(3) to CoCl2 (170 degreesC, 260 kPa). During the conversion of the salt from one phase to another, pseudo-stable transitions occurred at some processing conditions. In each case they were stable for several minutes, but always less than 1 h. In a nitrogen atmosphere CoCl2 was found to be unstable above 300 degreesC for pressures from 100 to 600 kPa. In ammonia CoCl2. 2NH(3) stability was found to be a function of temperature and pressure. An explanation for its decomposition, which could lead to the formation of solid NH4Cl has been suggested. In summary, the CoCl2. 2NH(3) salt was stable at processing conditions close to the phase diagram equilibrium line for 100% decomposition to CoCl2. 2NH(3) and unstable at large departures from it. If commercial chemical heat pumps are to be technically viable, the salt used must be stable for many cycles of synthesis and decomposition, Regions of stability can be defined by plotting experimental results at different processing conditions on phase diagrams of the type developed here. Crown Copyright (C) 2001 Published by Elsevier Science Ltd. All rights reserved.