Abstract
Adsorption phenomena are encountered in several engineering applications. One of its uses is in the storage and transport of gas in the form of adsorption tanks. The exothermic nature of the adsorption process decreases adsorption capacity presenting an impetus to understand the thermal characteristics of the gas storage process. Studies using mixtures of phase change materials and adsorbents in adsorption tanks demonstrate potential improvements in the adsorption capacity of the tanks. They also show that the distribution of phase change materials and adsorbent are important. Thus, this work presents two approaches for optimising the adsorbent domain. The first is to use a semi-analytic model to determine the best homogeneous material concentration for the adsorbent and phase change material for the vessel composition. The other is to use a 2D axisymmetric model to perform FGM optimisation to distribute material in the tank. Results for both models are presented and discussed for different conditions. The study shows that, for the cylindrical geometry, FGM optimisation is always, at least, marginally better than the homogeneous distribution from the semi-analytic model. However, FGM optimisation demands more computing time increases the complexity of implementation and results assembling. The semi-analytic approach is a possible alternative for optimising adsorption systems with phase change material mixed with adsorbents.
Highlights
Compressed natural gas (CNG) and liquefied natural gas (LNG) require high pressures or low temperatures, respectively
The gas may not be adsorbed by phase change material; it may flow through the spaces between spheres; the mass balance for the Phase change materials (PCMs) becomes:
The semi-analytic model describes a homogeneous PCM distribution that is suitable for vessels whose dimensions are large compared to the inlet and external convection effects because it does not consider local effects
Summary
Gas adsorption is exploited in many industrial applications; examples include gas transport and storage (adsorbed natural gas), in the industrial separation process (Cavalcante 2000), in the CCS process (Leung et al 2014) and in cooling application (Askalany et al 2012). The study shows that the phase change material amount and position inside the domain affect the results (Toledo et al 2013) This suggests a distribution problem that arises from the fact that adding PCM reduces the area available to absorb. The system studied in this paper is a cylindrical adsorption vessel with a mixture of adsorbent and phase change material mixed in its interior. An adsorption vessel performance may be improved, by changing its external surrounding, boundary conditions, and geometry (Prado et al 2018) Such vessels may be enhanced internally by changing material distribution, composition, and any apparatus that aids in thermal management inside the domain.
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