Measurement of Water Diffusivity in Aqueous Lithium Bromide and Lithium Chloride Solutions

Abstract

A relatively simple method was employed for measurement of water diffusivity in aqueous lithium bromide and lithium chloride solutions, The twin bulb apparatus used for these measurements was developed using an analogy between this apparatus and the conventional diaphragm cell apparatus. Tritiated water (TOH) was used as a tracer for these experiments because of its chemical similarity and proximity to the molecular weight of water. High tracer activity used at the beginning of the experiments allowed the use of relatively shorter time duration for each experiment ('" 20h) and a quasi-steady state equation to calculate the diffusivity from the observed tracer activity data.Initially, the water diffusivity in lithium bromide solutions for concentrations varying from 0.5 M to 3 M (22.1 weight percent) was measured to obtain a comparison with published values. The lithium bromide concentration was further varied from 3 M to II M (57.4 weight percent) to obtain data in the concentration range usually employed in absorption cooling applications, which is near the solubility limit. The water ditfusivity was found to vary with lithium bromide concentration, and to have a maximum of 16.7 x 10-10 m'ls at 4 M. These diffusivity values were also compared with those obtained using an equation proposed by Rosevaere et al. (1941) for non ideal solutions. The water diffusivity in lithium chloride solutions was measured at concentrations of 7 M (25.7 weight percent) and 8 M (28.87 weight percent) to obtain a comparison with published values. The lithium choride concentration was further varied from 8 M to 13 M (43.24) weight percent) to gather data beyond those of other researchers. Similar to the trend of water diffusivity in lithium bromide solutions, water diffusivity in lithium chloride solutions was also found to vary with concentration with a minimum of6.2 x 10-10 m 2/s near the solubility limit.

Dimensionless Mass-Transfer Correlations for Packed-Bed Liquid-Desiccant Contactors

“…A diffusivity apparatus was developed by suitably combining the twin-bulb method and the diaphragm cell method (Potnis et al, 1995). The diffusivity of water in aqueous lithium bromide solutions was measured, using tritiated water as the tracer, for concentrations ranging from 0.5 to 11 M. The value of diffusivity was found to increase from 13.2 × 10 -10 to 16.7 × 10 -10 m 2 /s for the concentration variation of 0.5 to 4 M and then to decrease to a steady value of approximately 6.5 × 10 -10 m 2 /s from 8 to 11 M.…”

“…To obtain Sherwood numbers for the regenerator, it is necessary to have the value of diffusivity of water in lithium bromide solution at the regenerator solution temperature (41 °C). The diffusivity data obtained earlier were at 25 °C (Potnis et al, 1995). The value of diffusivity at 41 °C was thus obtained from these values following the procedure discussed in the following paragraphs.…”