Dimensionless lumped formulation for performance assessment of adsorbed natural gas storage

Adsorbed natural gas (ANG) has been emerging as an attractive alternative to compressed natural gas or liquefied natural gas, on various circumstances. However, in spite of the advantages associated with ANG over other storage modes, there are some issues that need be properly addressed in order to ensure a viable employment of such alternative. One major problem is that the thermal effects associated with the sorption phenomena tend to diminish the storage capacity, thereby resulting in poorer performance. Hence, in order to design commercially viable storage vessels. the heat and mass transfer mechanisms that occur in these devices must be carefully understood and controlled. With the purpose of improving the understanding of mass and energy transport within ANG vessels, dimensionless groups associated with this problem have been developed in this study, resulting in an innovation to the ANG literature. Along with the dimensionless groups, a lumped-capacitance formulation has been also proposed. Although this type of formulation is limited compared to the multi-dimensional formulations present in the literature, its computational solution is remarkably faster. Numerical solution results using the proposed lumped formulation are compared with those of a previous study, suggesting that the simpler model can be applied to larger process times. The process of charging and discharging ANG vessels was then simulated employing the proposed formulation for different combinations of the developed dimensionless groups. In order to properly assess charge and discharge processes, a performance coefficient was employed. The results show that increasing the heat capacity ratio and dimensionless heat transfer coefficient tend to augment the performance coefficient, whereas an increase in the dimensionless heat of sorption worsens performance. The proposed normalization scheme is applicable to both multi-dimensional and spatially-lumped formulations, thereby facilitating the analysis of heat transfer enhancement in these storage vessels. (C) 2009 Elsevier Ltd. All rights reserved.