Ammonium Chloride (NH₄Cl)—Ammonia (NH₃): Sorption Characteristics for Heat Pump Applications

• Main Authors: George H. Atkinson, Samuel Hinmers, Robert E. Critoph, Michel van der Pal
• Format: Article
• Language: English
• Published: MDPI AG 2021-09-01
• Series: Energies
• Subjects: adsorption; ammonia; desorption; heat; heat pump; resorption
• Online Access: https://www.mdpi.com/1996-1073/14/18/6002

In a resorption heat pump, the adsorption and desorption reaction of ammonium chloride (NH₄Cl) with ammonia (NH₃) is of interest as a Low Temperature Salt (LTS). Reviewing previously published NH₄Cl-NH₃ equilibrium lines, ammonium chloride appears to offer useable working temperatures (50–70 °C) in the 10–15 bar pressure range during the adsorption reaction, and provides beneficial working conditions for the desorption reaction, when compared with alternative LTS candidates at atmospheric pressure. The NH₄Cl-NH₃ adsorption and desorption reactions, using a NH₄Cl composite salt, have been evaluated under dynamic ‘real-world’ conditions in a Large Temperature Jump (LTJ) experimental testing rig; although there are concerns with mass transfer characteristics, the salt exhibits no hysteresis between the adsorption and desorption reactions, contrary to previous literature. The experimentally obtained equilibrium line values for the reaction enthalpy and entropy are 29,835 J/mol and 207 J/(mol∙K), respectively. Using a semi-empirical model, the NH₄Cl composite salt has been successfully characterised, enabling the prediction of salt reaction behaviour. The model constants, <i>A</i> and <i>n</i>, identified are 4.5 and 5 for adsorption and 5 and 4 for desorption, with an overall salt active fraction (applicable to both reactions) of 0.98. Overall, the working equilibrium line and the dynamic performance of ammonium chloride has been investigated and the applicability of NH₄Cl as a LTS for a resorption heat pump determined.