Elsevier

Fluid Phase Equilibria

Volume 483, 15 March 2019, Pages 175-181
Fluid Phase Equilibria

The experimental study on the influence of crown ethers and glycols on the mutual solubility of lithium bromide in water

https://doi.org/10.1016/j.fluid.2018.11.036Get rights and content

Highlights

  • Crown ethers and glycols have been investigated as an anti-crystallization additive for (LiBr + water) system.

  • The experimental data on solubility of {LiBr (1) + additive (2) + water (3)} at a wide temperature and composition range were presented.

  • Based on the SLE data anty-crystallization additive was chosen.

Abstract

In this paper, the continuation of our work on searching for anti-crystallization additives to the aqueous solution of lithium bromide is presented. This type of research is important from the viewpoint of absorption refrigeration technology to improve the performance of the efficiency of refrigeration equipment. In this study, three crown ethers: 12-crown-4, 15-crown-5, and 18-crown-6 as well as the glycols: ethylene glycol, diethylene glycol, triethylene glycol and glycerol were investigated as anti-crystallization additives for {LiBr (1) + water (2)} system, conventionally used as a working pair in absorption refrigeration technology. For this purpose, the solubility of lithium bromide in water has been determined in the presence of the organic additive. The solubility measurements have been carried out using a dynamic method at a wide temperature and composition range for different (additive to LiBr) initial mass fraction from w20 = 0.1, 0.2 and 0.3. From experimental SLE data, the comparison range of the liquid state for tested systems at the absorber's working temperature was determined and compared to those from conventional (LiBr + water) system. Further measurements of vapor – liquid equilibria will be performed to select the best anti-crystallization additive.

Introduction

The need to save energy resources and protect the natural environment are some of the 21st century greatest civilization problems. One of the energy intensive process in question is cooling. The technology that meets these challenges and enjoys considerable interest is absorption cooling. Due to environmental problems related to emissions of freon this technology is an attractive compared to conventional systems using these undesirable substances. Substances used as refrigerants in adsorption devices (e.g. water) are environmentally benign and do not damage the ozone layer. Additionally, absorption cycles become of great interest because the electrical energy can be replaced with low grade or waste heat allowing both energy savings and energy efficiency improvements [1]. The use of absorption devices makes it possible to reduce greenhouse gas emissions, thus improving atmospheric conditions [2]. It is well known that the performances of an absorption heat pump largely depends on the thermophysical properties of working fluids. Nowadays, the most commonly used binary systems in the absorption heat cycle are {water + lithium bromide (H2O + LiBr)} and {ammonia + water (NH3+H2O)}, however, the lithium bromide aqueous solution is the most successful working mixture utilized in absorption cycles all over the world [2]. Nevertheless, an aqueous solution of lithium bromide has drawbacks, amongst them crystallization at high LiBr concentration, which makes it necessary to use high vacuum conditions should be preserved for suitable operation of the {H2O + LiBr} system, otherwise, the performance of the absorption cycle would be greatly reduced [3]. Due to numerous disadvantages, there exists a need for the development of new working pairs [4] thus searching for new beneficial and reliable binary systems to overcome technical limitations has become of great importance lately [[5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23], [24]]. In parallel to the search for alternative systems for (LiBr + water), work is underway to improve the properties of this working fluid. As mentioned, due to the crystallization of the salt inside the storage tank, it is essential to prevent this from occurring during the process. In the literature, a variety of information regarding the solubility of lithium bromide in water are available [[25], [26], [27], [28], [29], [30], [31]]. It has been shown that (LiBr + water) mixture can be improved using additives and adding a small amount of additive having nonvolatile and hygroscopic properties is one of the common methods to reduce the crystallization temperature of working fluids. In the available literature, many organic compounds are proposed as anti-crystallization additives [26,[31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41], [42], [43], [44], [45], [46], [47], [48]]. Among them, it was shown that organic glycols can be used in absorption systems, and mixtures containing both glycols and aqueous lithium bromide have been studied [32,33,42,47,48].

Since crown ethers form complexes with metal ions at molar proportion 1:1 we expect the system LiBr + LiBr·(crown ether) to be an eutectic system. As a result small additions of crown ether should, in principle, significantly increase the solubility of the mixture.

This work is a continuation of our research on searching for anti-crystallization additives to improve the properties of conventional working fluid [31,45,46]. Here crown ethers: 12-crown-4, 15-crown-5, and 18-crown-6 and glycols: ethylene glycol, diethylene glycol, triethylene glycol and glycerol have been tested as an anti-crystallization additive. World literature on crown ethers presents them as macromolecular compounds which exhibit advantageous host-guest properties, making them capable of forming selective complexes with selected cations [49,50]. These compounds find several applications [[51], [52], [53]], but no literature data on potential using of crown ethers in absorption refrigeration technology was found. The solubility of lithium bromide in water in presence of glycol as an additive obtained in this work is compared to the literature data. The addition of organic compound to (LiBr + water) solution would allow the absorber to operate in a wider composition range compared to a conventional system (LiBr + water) system. The higher concentration of LiBr in solution (in the absorber) gives a significant reduction in the vapor pressure, which will have a positive effect on COP growth. Based on (solid + liquid) phase equilibria measurements the best anti-crystallization additive will be selected and the further work on (LiBr + additive + water) system including (vapor + liquid) phase equilibria measurements and physicochemical characterization will be carried out.

Section snippets

Materials

Lithium bromide (CAS No. 7550-35-8) with nominal mass fraction purity greater than 0.99 was purchased from Fluka, crown ethers: 12-crown-4 (CAS No. 294-93-9); 15-crown-5 (CAS No. 33100-27-5) and 18-crown-6 (CAS No. 17455-13-9) with nominal mass fraction purity greater than 0.980 were supplied from Aldrich; glycols: ethylene glycol (CAS No. 107-21-1) with mass fraction purity greater than 0.995; diethylene glycol (CAS No. 111-46-6) with mass fraction purity 0.99; triethylene glycol (CAS No.

Experimental results and discussion

The main objective of this research is to present the solubility of lithium bromide in water in presence of the organic additive. The solubility in {LiBr + crown ether, glycol, or glycerol + H2O} system, where the (additive to LiBr) initial mass fractions, w20 are equal from (0.1–0.3) were measured. The experiment was performed at a wide temperature and concentration range using a dynamic method. The experimental results are listed in Table 2 and in Fig. 1, Fig. 2.

The experiment shows that the

Conclusion

The (solid + liquid) phase equilibria of lithium bromide in water in the presence of crown ethers, glycols and glycerol as anti-crystallization additives were presented. The experiment shows, that the greatest increase in solubility of LiBr in water was observed when 12-crown-4, or ethylene glycol were added (the additive to LiBr initial mass fraction = 0.3). In the case of 12-crown-4, the solution did not crystallize and maintained in liquid form even after prolonged cooling. As for ethylene

Acknowledgement

Funding for this research was provided by the Ministry of Science and Higher Education in the years 2016−2019 within the framework of the project “Iuventus Plus” No. 0379/IP3/2016/74.

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