Reactivity enhancement of chemical materials used in packed bed reactor of chemical heat pump

In this study, a mixture of expanded graphite (EG) and magnesium hydroxide (Mg(OH)₂) was used to enhance the thermal conductivity and reactivity of a magnesium oxide/water (MgO/H₂O) chemical heat pump, because EG is chemically stable and has high thermal conductivity and high moldability to form the heat exchange structure. Calcium chloride (CaCl₂) was also introduced into the mixture of EG and Mg(OH)₂ to ensure smooth diffusion of vapor in materials and enhance the fittability between EG and Mg(OH)₂. The reaction kinetics of pure Mg(OH)₂, a mixed material containing Mg(OH)₂ and CaCl₂ (termed MC), and a mixed material containing EG, Mg(OH)₂, and CaCl₂ (termed EMC) were examined under the same reaction conditions by performing thermobalance measurements. EMC exhibited a higher dehydration rate than the other materials. It also exhibited hydration reactivity at temperatures of up to 200 °C; at this temperature, pure Mg(OH)₂ exhibited low reactivity. The addition of CaCl₂ also enhanced the hydration reactivity of MgO because of the high water adsorption ability of CaCl₂ in EMC. A reaction rate equation for the hydration of EMC was proposed on the basis of an assumed reaction model. The thermal performance of a MgO/H₂O chemical heat pump manufactured using EMC was evaluated from this equation. EMC was concluded to have good potential for use as a packed bed material in the MgO/water chemical heat pump owing to its low cost, high hydration reactivity, high thermal conductivity, and high moldability to form the heat exchange structure.