data Technical Note Determination of Concentration of the Aqueous Lithium–Bromide Solution in a Vapour Absorption Refrigeration System by Measurement of Electrical Conductivity and Temperature Salem M. Osta-Omar * and Christopher Micallef Mechanical Engineering Department, University of Malta, MSD 2080 Msida, Malta; [email protected] * Correspondence: [email protected]; Tel.: +35-699-357-005 Academic Editor: Jamal Jokar Arsanjani Received: 24 November 2016; Accepted: 11 January 2017; Published: 19 January 2017 Abstract: Lithium–bromide/water (LiBr/water) pairs are widely used as working medium in vapour absorption refrigeration systems where the maximum expected temperature and LiBr mass concentration in solution are usually 95 °C and 65%, respectively. Unfortunately, published data on the electrical conductivity of aqueous lithium–bromide solution are few and contradictory. The objective of this paper is to develop an empirical equation for the determination of the concentration of the aqueous lithium–bromide solution during the operation of the vapour absorption refrigeration system when the electrical conductivity and temperature of solution are known. The present study experimentally investigated the electrical conductivity of aqueous lithium–bromide solution at temperatures in the range from 25 °C to 95 °C and concentrations in the range from 45% to 65% by mass using a submersion toroidal conductivity sensor connected to a conductivity meter. The results of the tests have shown this method to be an accurate and efficient way to determine the concentration of aqueous lithium–bromide solution in the vapour absorption refrigeration system. Keywords: electrical conductivity measurements; LiBr/water solution; vapour absorption refrigeration system; regression analysis 1. Introduction The basic principle of the vapour absorption refrigeration technology is the phenomenon of sorption. This process occurs as a result of the chemical affinity of one substance (sorbent) for holding another one (sorbate). In LiBr/water vapour absorption refrigeration systems, the sorbate is the water vapour and the sorbent is the aqueous lithium–bromide solution, which is responsible for the sorption and desorption of the sorbate. The desorption process, which consists of separating the sorbate from the sorbent, takes place in the generator by means of heat input. The sorption process, where the sorbate is absorbed again by the sorbent, takes place in the absorber via an exothermic reaction [1]. Aqueous lithium–bromide is a salt solution substance. The maximum amount of salt which can dissolve in water is highly dependent on the water temperature. The higher the water temperature, the more salt can be dissolved in water, and it is said that the solubility of the salt increases with temperature. When no more salt can be dissolved in water the solution is said to be saturated. If the temperature of a saturated solution drops, precipitation/crystallization of the salt takes place [2]. Thus, the sorption and desorption processes of the LiBr/water solution in the vapour absorption refrigeration system should be designed properly to avoid the precipitation and crystallization of the salt in the solution, which can lead to blockage of solution passage ways. Data 2017, 2, 6; doi:10.3390/data2010006 www.mdpi.com/journal/data Data 2017, 2, 6 2 of 11 Data 2017, 2, 6 2 of 12 When the crystallization of the LiBr/water solution, which leads to the shutting down of the vapour absorptionWhen the crystallization refrigeration of system, the LiBr/water appears solu duringtion, which the operationleads to the of shutting the vapour down absorptionof the refrigerationvapour absorption system due refrigeration to a significant system, temperature appears during drop orthe increase operation in LiBrof the mass vapour concentration absorption of the solution,refrigeration the flow system of the due LiBr/water to a significant solution temperature in the system drop isor blocked.increase in To LiBr avoid mass this, concentration the concentration of of thethe solution solution, has the to flow be monitored of the LiBr/water at critical solution positions in the prone system tocrystallization, is blocked. To inavoid order this, to the heat up the LiBr/waterconcentration solution of the solution before has the to saturation be monitored limit at critical is reached. positions The prone crystallization to crystallization, problem in order tends to to heat up the LiBr/water solution before the saturation limit is reached. The crystallization problem occur in the weak LiBr/water solution between the throttling valve and the absorber as indicated in tends to occur in the weak LiBr/water solution between the throttling valve and the absorber as Figureindicated1, at which in Figure point 1, theat which temperature point the temperature of the LiBr/water of the LiBr/water solution solution is at its is relative at its relative lowest lowest and LiBr massand concentration LiBr mass concentration of the solution of the is solution at its highest is at its in highest the system in the [system3]. [3]. Figure 1. Schematic diagram of a LiBr/water (lithium–bromide/water) absorption refrigeration system Figure 1. Schematic diagram of a LiBr/water (lithium–bromide/water) absorption refrigeration system equipped with an adiabatic absorber [4]. equipped with an adiabatic absorber [4]. A number of feasible methods for determining the concentration of aqueous lithium–bromide Asolution number are ofimperative feasible methodsto review forhere. determining The most two the common concentration and well-known of aqueous methods lithium–bromide are: the solutiondetermination are imperative of the LiBr/water to review concentration here. The by most measuring two common the specific and gravity well-known of the solution methods with are: the determinationa hydrometer of[5,6] the and LiBr/water by use of concentrationtitration with bysilver measuring nitrate [2]. the Both specific methods gravity give of accurate the solution measurements but are necessarily time-consuming, as they require the extraction and manipulation with a hydrometer [5,6] and by use of titration with silver nitrate [2]. Both methods give accurate of a sample from the system in a laboratory [2,5]. However, an alternative method which could be measurements but are necessarily time-consuming, as they require the extraction and manipulation of used to determine the concentration of LiBr/water solution is that of measuring the secondary a sampleproperties from thethat system are associated in a laboratory with and [2 ,5affected]. However, by the an change alternative in concentration. method which An couldexample be usedof to determinesecondary the concentrationproperties which of can LiBr/water be measured solution properly is thatby using of measuring a sensor inside the secondarythe vapour absorption properties that are associatedrefrigeration with system and affectedis the electrical by the changeconductivi inty concentration. through which An the example concentration of secondary of LiBr/water properties whichsolution can be can measured be determined properly easily by usingin the system a sensor duri insideng the the operation vapour without absorption extracting refrigeration a sample system [6]. is the electrical conductivity through which the concentration of LiBr/water solution can be determined easily in the system during the operation without extracting a sample [6]. Data 2017, 2, 6 3 of 11 The electrical conductivity is the ability of the solution to conduct an electric current. This depends on such factors as concentration and temperature. An increase in the temperature of solution increases the mobility and the number of the ions in solution due to ionisation of molecules, which in turn leads to an increase in the electrical conductivity of the solution. Moreover, an increase in the concentration of solution leads to increase its electrical conductivity due to an increase in the number of ions per unit volume [7,8]. It is therefore always important to associate the electrical conductivity measurement of a solution with a reference concentration and temperature. The electrical conductance of the aqueous lithium–bromide solution for concentrations in the range of 40.7% to 63.55% was studied by Fried and Segal [5]. Their data shows that the electrical conductance increased from 162 mmho at 15 ◦C to 419 mmho at 80 ◦C for a concentration of 45% and from 115 mmho at 15 ◦C to 333 mmho at 80 ◦C for a concentration of 50%. Unfortunately their study gave measurements of the electrical conductance rather than electrical conductivity. Since electrical conductance measurement is a function of both the distance between the electrodes and the effective area of the electrodes, their results depends on the equipment being used and thus cannot be considered as reference data. Sun et al. [9] investigated the electrical resistivity (ohm × cm) of aqueous lithium–bromide solution for concentration and temperature ranges of 35% to 70% and of 10 ◦C to 100 °C respectively. Their results can be used to establish the concentration of aqueous lithium–bromide solutions when the electrical resistivity and the temperature of the solutions are known. Nowadays the majority of equipment which are readily available on the market measures the electrical conductivity (mS/cm) of solutions. Thus, the existing literature is no longer useful since it gives data based on electrical conductance and electrical resistivity. The purpose