MASS DIFFUSION TRANSPORT STUDIES OF LITHIUM-SULFATE IN AQUEOUS-SOLUTIONS USING RAMAN-SPECTROSCOPY

The diffusivity coefficients of lithium sulfate solutions in water and water and silica gels over the concentration range 0.2 to 2.5 molal h ave been measured for the first time with micro-Raman spectroscopy for a horizontal-tube capillary, conventional 90 degrees Raman scattering for a narrow-bore vertical-tube system, and FT-Raman spectroscopy for the gels. Data were obtained for axial concentration profiles as func tion of the distance and for time elapse at fixed positions on the tub e axis with the use of integrated intensity measurements of the nu(1) (SO4) symmetric stretching band at 980 cm(-1). The use of computer mod el calculations to fit the experimental data yielded values of D-A = 0 .4 x 10(-5) cm(2) s(-1) for the vertical system and 0.3 x 10(-5) cm(-2 ) s(-1) for the gel system. Discrepancies observed for the horizontal system have been ascribed to the presence of convective flow. A convec tive flow rate of 5.10(-2) cm h(-1) was calculated in this case with t he use of computer simulation and used in the fitting of the experimen tal data through a general form of the second Fick's law. Under these conditions, a D-A value of 0.2 x 10(-5) cm(2) s(-1) was obtained.