Solid–gas equilibrium in chemical heat pumps: the NH 3–CoCl 2 system

Abstract

The design of heat transfer equipment used in chemical heat pumps requires a knowledge of the temperature at which heat is produced during the synthesis reaction and the temperature at which it is used during the decomposition reaction. The existing equilibrium data in the literature for the reaction, CoCl 2·2NH 3+4NH 3=CoCl 2·6NH 3 was obtained at pressures of 1 atm or less. Data at greater pressures were measured in this investigation. The measurements reported here were more complex than a single van’t Hoff equilibrium line. Pseudo-equilibrium hysteresis behaviour (equilibrium lines were a function of solid concentration) was observed between the synthesis reaction and the decomposition reaction. The widths of the hysteresis loops were found to be a function of heating rate. When heating rate was eliminated as a variable by maintaining the temperature constant for several hours, the hysteresis loops became narrower. Extrapolation of the data suggests that the hysteresis might be eliminated entirely by maintaining the temperature for extremely long times. Nevertheless, in all circumstances it appears that the conversion of the salt would be a function of temperature. The experimentally measured bivariant behaviour was contrary to monovariant behaviour which had been anticipated and has been discussed in the literature. Fixed bed chemical heat pumps normally operate as batch reactors which have different operating conditions in the synthesis and decomposition cycles. As the temperatures of the batch reactors are cycled relatively rapidly in real chemical heat pumps, true equilibrium conditions are not established. Therefore, when the rate of temperature change in the batch reactors is known, the pseudo-equilibrium hysteresis loop data corresponding to the same heating rate can be used. This hysteresis phenomena can make a 15°C temperature difference in the specifications of the heat transfer equipment.