Thermodynamic Properties of Lithium Chloride Ammonia Complexes For

Journal of Thermal Analysis and Calorimetry, Vol. 86 (2006) 3, 825–832 THERMODYNAMIC PROPERTIES OF LITHIUM CHLORIDE AMMONIA COMPLEXESFORAPPLICATIONINAHIGH-LIFTHIGH TEMPERATURE CHEMICAL HEAT PUMP E. R. T. Bevers1*, P. J. van Ekeren1, W. G. Haije2 and H. A. J. Oonk1 1Chemical Thermodynamics Group, Department of Chemistry, Utrecht University, Padualaan 8 3584 CH Utrecht, The Netherlands 2Energy research Centre of the Netherlands, Energy Efficiency in Industry (EEI), P.O. Box 1, 1755 ZG Petten, The Netherlands Ammonia absorption by and desorption from lithium chloride at different pressures has been studied using high-pressure differential scanning calorimetry, for application in a high-lift high temperature chemical heat pump. The measurements were performed under isobaric as well as under isothermal circumstances. Clausius–Clapeyron plots were constructed and used to calculate the thermodynamic parameters and to determine the stability regions of the different complexes. Controversies in literature as to the real existing phases are resolved. Keywords: ammonia, chemical heat pump, DSC, lithium chloride, thermodynamic properties Introduction industrial waste heat with a typical efficiency of about 35% thus leading to significant energy and CO2 Because the world’s oil and gas resources are limited emission savings. and because of environmental problems associated The working principle of a CHP has been de- with their use, one becomes more and more aware of scribed extensively by Spoelstra et al. [1], and by the fact that the use of these resources must be re- Wongsuwan et al.[2];inbrief,itisasfollows.ACHP duced. This may be achieved by (a combination of) always uses two carefully selected materials [1, 2], a the following options: low temperature salt (LTS, which occasionally may • make more use of sustainable energy resources, also be replaced by an evaporating fluid like water or such as solar energy, wind energy, bio energy ammonia) and a high temperature salt (HTS) that are • use current resources more efficiently contained in separate reactors. The reactors are con- nected to each other so that gas can flow between them That there still are significant possibilities to use in a cyclic process. The HTS complex is decomposed energy resources more efficiently can be learned from by using available waste heat at medium temperature. many recent studies. As an example, Spoelstra et al. The formed gas flows to the reactor containing the reported that in the Netherlands industrial waste heat 15 LTS. There, a complex of the LTS is formed under re- in the order of 110 PJ (i.e. 110⋅10 J) per year is ac- lease of heat at low temperature. When these reactions, tively cooled and dissipated into the environment [1]. both at a relatively low pressure, are more or less com- Unfortunately, this enormous amount of heat is plete, the temperature of the reactor containing the available at a temperature range of 50 to 150°C, complex of the LTS is increased from low temperature whereas heat that is useful for industrial purposes to medium temperature and available waste heat at me- must be available at a temperature of about 230°C dium temperature is used to decompose the complex. (middle pressure steam). Now the formed gas flows to the reactor containing the Lifting heat to a desired high temperature can be HTS, where it is used for the formation of a complex of done either with a conventional compression heat the HTS under the release of heat at high temperature. pump at the expense of a substantial amount of pri- Upon completion of these reactions, that both take mary energy or with a chemical heat pump (CHP). place at a relatively high pressure, the temperature of In a CHP the high temperature lift is realized by the reactor containing the complex of the HTS is de- chemical reactions, in particular the absorption of a creased from high temperature to medium temperature, vapour in a solid. For a specific application the which completes the cycle. solid-vapour pairs must be carefully selected and The work described in this paper is part of a re- characterized. The advantage of a CHP over a com- search project on the design of a CHP based on the pression HP lies in the fact that it can be ‘fuelled’ with * Author for correspondence: [email protected] 1388–6150/$20.00 Akadémiai Kiadó, Budapest, Hungary © 2006 Akadémiai Kiadó, Budapest Springer, Dordrecht, The Netherlands BEVERS et al. formation and decomposition of (solid) ammonia complexes, according to the general reaction: MZ(s)+nNH3(g) D M(NH3)nZ(s)+ΔH (1) From many different working couples, collected and tabulated by Bonnefoi [3], the couples of magne- sium chloride and lithium chloride with ammonia ap- pear to be promising, because their transitions are in the right p–T area and the potential temperature lift is sufficient. Fig. 1 Clausius–Clapeyron representation of the pres- In terms of equilibrium thermodynamics, the sure–temperature relationships under the process cir- equilibrium pressure of the monovariant heteroge- cumstances of our experiments, showing an area of neous equilibrium pseudo-equilibrium between formation (I) and decomposition (II) A(s) D B(s)+xC(g)(2) is the solution of the equation ture difference between the gas and the crystalline complex caused by the (exothermic/endothermic) ΔΔ=+0 pT() = heat effect of the reaction. rrGT() G () T xRT ln 0 (3) p0 The results presented in this paper pertain to the complexes formed between LiCl and NH .Thefor- where ΔG is the molar Gibbs energy change of the re- 3 mation of these complexes, for pressures up to action, p a reference pressure, ΔG0 the Gibbs energy 0 about 1 bar, has been studied by Bonnefoi [7], and by change at p ,andR the gas constant 0 Collins and Cameron [8]. Bonnefoi reported the for- (R=8.3144472 mol–1 K–1). From this equation, taking mation of a stable di-ammonia complex; a result that into account that Δ G0=Δ H0–TΔ S0,whereH stands r r r could not be confirmed by Collins. In our study, we for enthalpy and S for entropy, it follows that applied pressures from 1 to 5 bar, and used high-pres- ⎛ ΔΔ00⎞ ⎛ ⎞ sure differential scanning calorimetry (HPDSC) to pT()= ⎜ rrH ⎟ 1 + ST() ln –⎜ ⎟ ⎜ ⎟ (4) measure the heat effects and the pT relationships of p ⎝ xR⎠ ⎝ T ⎠ xR 0 the formation and decomposition reactions. In the Equilibrium pressures, as a rule, are represented CHP under development, LiCl is the low temperature in a Clausius–Clapeyron [3] diagram, where the loga- salt, and MgCl2 the high temperature salt. The results rithm of pressure is plotted vs. reciprocal temperature. obtained for the complexes formed by MgCl2 and The slope of the line in the diagram equals to minus ammonia are the subject of a forthcoming paper. the enthalpy effect of the reaction per mole of C di- vided by the gas constant. Under the process conditions used for our exper- Experimental iments, the pT relationships of the formation reaction and those of the decomposition reaction, as a rule, do Materials and methods not coincide. In the Clausius–Clapeyron representa- Sample preparation tion, as a result, there are two lines; one for the formation and one for the decomposition, Fig. 1. The width Lithium chloride (Merck, 99.0%) was used as starting of the area between the lines, referred to as the area of material. Because lithium chloride is hygroscopic, the pseudo-equilibrium [4–6], depends on the cool- sample was weighed and then dehydrated in a Mett- e ing/heating rate (β) of the experiment. The lower the ler-Toledo DSC821 . After dehydration the sample cooling or heating rate is taken, the smaller the was quickly weighed again to determine the amount of pseudo-equilibrium area becomes and the closer the anhydrous lithium chloride. Then the sample was in- real thermodynamic equilibrium is approached. troduced into the HPDSC cell and heated again be- Trudel et al. [6] adduce two possible reasons for cause between weighing and introducing it may have the explanation of this pseudo-equilibrium hysteresis. slightly hydrated. The atmosphere was nitrogen (ob- The first is related to the expansion and contraction of tained by the vaporization of liquid nitrogen, negligi- the crystal lattice as the ammonia content in the mate- ble impurity, Hoekloos) so that the small amount of rial changes: the hysteresis may be caused by an acti- water absorbed by the sample was able to evaporate. vation barrier necessary to expand the material (work The uncertainty in the sample mass (including the bal- of expansion that is not recovered on contraction). ance-error) is approximately 2.5%. After heating up, The second explanation of the observed hysteresis the atmosphere was changed from nitrogen to ammo- can be found in the fact that there could be a tempera- 826 J. Therm. Anal. Cal., 86, 2006 THERMODYNAMIC PROPERTIES OF LITHIUM CHLORIDE AMMONIA COMPLEXES nia (Hoekloos, ammonia purity higher than 99.98%). At this point the sample was ready to be measured. Thermal analysis The HPDSC used for the measurements was a Mett- ler-Toledo DSC27HP, which has a maximum working temperature of 600°C and a maximum working pressure of 70 bar. The cooling was vested by stream- ingcoldwater. The system was calibrated prior the measurements, using indium, tin and lead. It was interesting Fig. 2 Principle of experimental scanning temperature method that we found that the temperature calibration param- in Clausius–Clapeyron’s diagram eters depend on pressure [9]. It is quite well possible that, for a change involving gases and unlike the melt- ing of a metal, the heat regulation inside the sample container is such that the heat effect is not measured to its full extent.

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