Elsevier

Fluid Phase Equilibria

Volume 355, 15 October 2013, Pages 46-51
Fluid Phase Equilibria

New device for measuring the solubility of inorganic salts in liquid ammonia

https://doi.org/10.1016/j.fluid.2013.06.042Get rights and content

Abstract

A variable-volume cell was designed, built and validated to measure the solubility of salts in liquid ammonia, using a polythermal method at high pressure. The apparatus is based on a cell made of stainless steel and fitted with a sapphire window to allow the solubility process to be visually controlled. The solubility of three systems was measured: lithium nitrate in water at atmospheric pressure, lithium nitrate in ammonia at 20 bar and sodium thiocyanate in ammonia, also at 20 bar. An image processing treatment was also tested as a new procedure for detecting the solubility temperature.

Introduction

Solubility data of salts in different solvents are continuously required for such chemical and engineering purposes as waste water treatment, crystallization processes, desalination of sea water, absorption refrigeration with salts as absorbents, and others [1].

In absorption refrigeration systems, the working fluid is formed by a pair of fluids: the refrigerant and the absorbent. The refrigerant is a volatile fluid and, in many cases, the absorbent is a solution in which the solute is a non-volatile inorganic salt and the solvent the same fluid as the refrigerant. One of the most commonly used working fluids is the pair water/lithium bromide, with water as the refrigerant and an aqueous solution of the salt as the absorbent. Lithium bromide is an excellent salt for absorbent fluids because it is practically non-volatile and very hygroscopic, and the aqueous solution has a low viscosity [2]. However, an important drawback is the limited solubility of lithium bromide in water and the consequent risk of crystallization at low temperature. Moreover, the freezing point of the refrigerant (water) limits the use of the water/lithium bromide pair to temperatures above 273.15 K. To overcome this problem, many other working fluids have been proposed with ammonia as refrigerant, because it has a freezing point of 195 K, which considerably increases the range of operation. Salts such as lithium nitrate [3], [4], [5], [6], [7] or sodium thiocyanate [4], [8], [9] have been proposed to form the absorbent. The solubility of the salts in ammonia must be known if the working conditions of these systems are to be determined and the machines are to perform appropriately.

Adequate experimental data of solubility of salts in liquid ammonia are scarce in the literature. Scherer [10] reported the solubility of eight salts at atmospheric pressure and low temperature, from 223 K to 243 K for keeping ammonia liquid. Hunt [11] measured the solubility of several salts in liquid ammonia at 298.15 K and atmospheric pressure. Blytas and Daniels [12] reported the solubility and other thermophysical properties for the sodium thiocyanate/ammonia mixture. Seidell and Linke [13] and Tsimbalist et al. [14] reported the solubility of lithium nitrate in liquid ammonia. Experimental data are also available on ternary systems with water: Peters et al. [15] measured the solubility of LiBr in NH3/H2O and Brass et al. [12] reported the solubility of two alkaline hydroxides (NaOH and KOH) in NH3/H2O at two temperatures.

In this study, a new experimental device was designed and built to measure the solubility of inorganic salts in liquids at moderate or high pressure, using the polythermal method. The device was validated at atmospheric pressure with the system water/lithium nitrate and at 20 bar with the system ammonia/lithium nitrate. The solubility of sodium thiocyanate in liquid ammonia was also measured and compared with data from the literature [12]. The quantities, units and symbols used are those recommended by IUPAC [16].

Also, an image reported processing treatment was proposed and compared with visual procedure for determining the solubility temperature, in an attempt to provide more objective criteria than visual detection.

Section snippets

Chemicals

Compressed ammonia (CAS number 7664-41-7, mass fraction purity 0.9998) was supplied by Carburos Metálicos, sodium thiocyanate (CAS number 540-72-7, mass fraction purity >0.98) was supplied by Panreac and lithium nitrate (CAS number 7439-93-2, mass fraction purity >0.98) was supplied by Fluka. Milli-Q water (resistivity lower than 18.2  cm) was used to prepare aqueous solutions. All chemicals were used without further purification, and the solid compounds were dried in an oven at 373 K for at

Validation

To test the validity of the measurement method and device, the solubility of LiNO3 in water at atmospheric pressure (Table 1) and the solubility of LiNO3 in NH3 at 20 bar (Table 2) were measured and compared to data from the literature [14], [22], [23], [24], [25], [26].

Fig. 5 shows the solubility of LiNO3 in water measured in the present study using both procedures: visual and image processing. The data reported in the literature [14], [22], [23], [24], [25], [26] have also been included. Very

Conclusions

A new experimental device has been designed and built for measuring the solubility of salts in liquid ammonia at high pressure. The device and the procedure was validated, first at atmospheric pressure with the H2O/LiNO3 mixture, and then at 20 bar with the NH3/LiNO3 system. Results were compared with the values reported in the literature, and very good agreement was found for both systems.

The solubility of NaSCN in ammonia was experimentally determined at 20 bar and several compositions. The

Acknowledgments

This research project was financially supported by the Spanish Ministry of Science and Innovation (ENE2009-14177). The authors thank Dr. María Soledad Larrechi, Mrs. Silvana C. Tourn and Mr. Luis A. Cagua for their collaboration in this work.

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