Exergy analysis of flow dehumidification by solid desiccants

Abstract

Equations for the temporal and spatial exergy values and changes in the humid air stream and the desiccant for flow of humid air over desiccants and in desiccant-lined channels were established, and solved based on a thorough transient conjugate numerical analysis of laminar and turbulent flow, heat, and mass transfer that yielded the full velocity, temperature, and species concentration in the humid air and the solid desiccant. The desiccant was silica gel, the Reynolds number ranged from 333 to 3333, and the turbulence intensity in the turbulent flows was varied from 1% to 10%. Some of the major findings are: (1) in laminar flow, a total of ~20% of the humid air exergy is reduced in its drying, (2) in the desiccant, practically all of the exergy reduction is due to the release of absorption heat, (3) most of the exergy reduction, following the dehumidification rates, takes place in the first 1.5 s and first centimeter, (4) for the same inlet velocity, a desiccant-lined channel is more effective for dehumidification than a flat bed, and proportionally ~20% more exergy is expended, (5) turbulent flow improves dehumidification and proportionally increases exergy expenditure by 27–30%. Conclusions from these results are drawn to increase the exergy efficiency of the process.