1081
Ind. Eng. Chem. Res. 1992,31,1081-1085
Thermal Conductivity of Aqueous Salt Solutions at High Temperatures and High Concentrations Ralph M. DiGuilio and Amyn S. Teja* School of Chemical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0100
The thermal conductivity-temperature behavior of aqueous salt solutions is examined with particular emphasis on the high-concentration and high-temperature region. We have recently measured the thermal conductivity of LiBr-water solutions at temperatures up to 473 K and concentrations up to 65 w t ?& LiBr and found that the thermal conductivity surface for aqueous electrolytes is different from the surface obtained by previous investigators. Previous investigators have shown that the thermal conductivity-temperature curvea at constant composition are parallel to those for pure water. Our work, on the other hand, indicates that the curves cannot be parallel over the entire composition range. Justification for this behavior is presented, and a new correlation incorporating the correct temperature dependence is proposed.
Introduction Recent measurements by DiGuilio et al. (1990) of the thermal conductivity of LiBr-water solutions up to 470 K and 65 wt % LiBr have revealed unusual behavior which is different from that previously found for other aqueous electrolyte solutions. It has been reported previously by El’darov (1980) and Yusufova et al. (1975a,b, 1978) that the thermal conductivity-temperature curves of aqueous salt solutions at constant composition are parallel to those of pure water. Our experimental results for the LiBmater system, however, indicate that the constant-composition curves are not parallel at high concentrations. This contradiction led us to examine further the thermal conductivity surface of aqueous salt solutions. This examination included a review of literature data and models for the high-temperature-high-concentration region of the thermal conductivity surface. In addition, a new correlation which yields the correct temperature behavior is proposed. Experimental Work A variant of the transient hot-water technique was used to measure the thermal conductivity of LiBr-water solutions at temperatures ranging from 298 to 470 K and concentrations ranging from 30 to 65 wt % LiBr. The hot-wire technique uses a line source of heat, a very fine wire in practice, immersed in the liquid under study. At the start of the experiment, the line source is made to dissipate heat at a constant rate per unit length for several seconds. The thermal conductivity of the liquid is determined from the temperature rise obtained. In the case of electrically conducting fluids, it is necessary to electrically insulate the wire from the fluid. A novel way of achieving this was pioneered by Omotani et al. (Hoshi et al., 1981; Omotani et al., 1982; Omotani and Nagashima, 19841,who used a glass capillary filled with liquid metal to serve as an insulated hot wire. They measured the thermal conductivity of molten salts using this approach. The thermal conductivity cell constructed for our work was based on the design of Omotani et al. and consisted of a fine quartz capillary (0.47-pm i.d. by 0.80-pm 0.d.) filled with liquid mercury. The liquid mercury thread served as the insulated hot wire in our measurement of LiBrwater solutions. The other parts of the apparatus were a Wheatstone bridge, a data acquisition system, and a constant-temperature bath. The mercury thread was used as a resistor in one arm of the Wheatstone bridge, and the bridge circuit was used to monitor its change in resistance during the measurement. From a previous calibration of the resistance with temperature, the temperature of the thread could be determined. Complete details of the ex-
Table I. Thermal Conductivity of LiBr-Water Solutions wt%
LiBr 30.2
44.3
49.1
T,K 292.9 296.9 326.1 329.1 359.5 365.0 385.2 388.9 404.7 434.0 435.7 461.3 295.1 321.4 353.5 378.6 407.2 439.2 463.3 298.0 328.9 371.6 401.2 430.0 460.0
A,
W/(m K) 0.5040 0.5080 0.5404 0.5417 0.5664 0.5751 0.5876 0.5884 0.5932 0.5871 0.5862 0.5702 0.4645 0.4922 0.5180 0.5319 0.5471 0.5534 0.5497 0.4438 0.4750 0.5007 0.5103 0.5189 0.5202
w LiBr t%
T,K
W/(m A, K)
56.3
294.1 329.4 362.3 397.6 430.1 461.1 299.6 329.2 369.7 402.5 430.8 460.6 339.8 371.0 400.4 430.7 460.9 343.4 370.5 400.7 428.8 461.0
0.4164 0.4506 0.4655 0.4812 0.4906 0.4986 0.4064 0.4303 0.4548 0.4706 0.4737 0.4830 0.4264 0.4441 0.4543 0.4626 0.4733 0.4178 0.4291 0.4392 0.4502 0.4557
60.0
62.9
64.9
periment are given elsewhere in DiGuilio et al. (1990). The analysis of the data reported there, however, contains a small error because end effects were apparently ignored in our previous work. Correction for end effects was done by calibrating the thermal conductivity cell with water. The corrected data appear in Table I. It should be added here that the difference between the data given in DiGuilio et al. (1990) and those listed in Table I is less than 0.6% on average and 0.8% in the worst case. This is well within the claimed accuracy of *2% for the data.
Behavior of the Thermal Conductivity of Aqueous Salt Solutions Figure 1presents the thermal conductivity surface for pure water and shows that, for pure fluids, the thermal conductivity is a function of temperature and pressure. In the case of salt solutions, however, the concentration and the nature of the particular salt also affect the thermal conductivity. The effect of each of these variables is examined below. The concentration dependence of the thermal conductivity at constant temperature and pressure has been studied by many investigators. The most important study of this type is by Reidel (1951),who measured the thermal
0888-5885/92/2631-1081$03.00/0 1992 American Chemical Society
1082 Ind. Eng. Chem. Res., Vol. 31, No. 4, 1992
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