Development of a validated 2D model for flow, moisture and heat transport in a packed bed reactor using MRI experiment and a lab-scale reactor setup

Abstract

Sorption materials such as zeolite are intensively investigated for thermochemical heat storage applications. The heat storage process is based on a reversible adsorption-desorption reaction, which is exothermic in one direction (hydration) and endothermic in the reverse direction (dehydration). For evaluating the transport phenomena occurring in a heat storage reactor, a detailed model is needed, considering also the transversal terms. In a cylindrical reactor, these terms appear as radial effects that disturb the plug flow assumption in the packed bed, and hence, a model with only axial terms is insufficient to simulate the bed. The radial effects in a porous medium, created by presence of the wall surrounding it, can be caused by: (i) heat losses to the ambient through the wall, (ii) a higher bed void fraction in the wall region, resulting in flow channelling, and (iii) non-uniform initial state of charge near the wall for the subsequent re/de-hydration (e.g. due to heat loss during dehydration). A 2D model is developed for transport phenomena in a packed bed by doing a literature survey on representative models. The model is validated by experimental results measured in a lab-scale setup by comparing the pressure drop over the bed, velocity profile below the bed and temperature profile inside the bed. In addition, the concentration of adsorbed water is compared with experimental results from MRI (Magnetic Resonance Imaging) experiments. The validated numerical model is employed to understand the significance of the above mentioned effects on the thermal performance of the reactor. An accurate model for the thermal dynamics of an adsorption bed on reactor scale is obtained, which is used to present suggestions to optimize the charging and discharging process times, hence, to improve the performance of the reactor.