Development of dimensionless mass transfer correlations for packed bed liquid desiccant contactors

A lithium bromide based solar cooling unit (nominal 3 ton) was studied experimentally to develop a better understanding of the process of mass transfer in the system. Initially, experimental runs were carried out to obtain an accurate energy balance and to identify reliably measured parameters. Precautions suggested by these runs were observed during the subsequent experiments conducted to obtain the relative magnitudes of mass transfer resistances. Random (Polypropylene Tripack) and structured (Munters's CELDEK) packings were studied with varying bed depths in two packed towers, which are a central part of a liquid desiccant cooling system. The slopes of the log-log plots of mass transfer rate versus solution flow rate were found to be close to 0.8, which indicated that the conditions for the liquid phase were turbulent for the operating conditions in the regenerator and the dehumidifier. The small intercepts obtained for the Wilson plots, constructed using the log-log plots, suggested that the gas phase mass transfer resistance was negligible compared to the liquid phase mass transfer resistance.; It was then necessary to obtain a dimensionless mass transfer correlation to estimate the liquid phase mass transfer coefficient. However, to convert the data obtained from the mass transfer experiments into dimensionless form it was essential to know the diffusivity of water in lithium bromide solutions. A diffusivity apparatus was next developed by suitably combining the twin bulb and the diaphragm cell methods. The diffusivity of water in aqueous lithium bromide and aqueous lithium chloride solutions was measured using tritiated water as tracer to provide the necessary data.; Liquid phase mass transfer coefficients for the packed bed alone were obtained by separating the contributions of the other mass transfer regions in the contactors. The random packing mass transfer coefficients varied from 0.48 g-moles/s m{dollar}sp2{dollar} to 2 g-moles/s m{dollar}sp2{dollar}, while the double-layer, structured packing mass transfer coefficients varied from 0.018 g-moles/s m{dollar}sp2{dollar} to 0.035 g-moles/s m{dollar}sp2{dollar}. The experimentally measured diffusivity values were then utilized to convert these coefficients into a dimensionless form to obtain non-equipment specific correlations.