J. Chem. Eng. Data 1995,40, 267-270
267
Vapor Pressure of (Potassium Hydroxide + Ammonia Solutions
+ Water)
G. Cacciola,*9+G. Restuccia? and Yu. Aristov' National Council of Research, Institute of Transformation and Storage of Energy, S. Lucia sopra Contesse, 98126 Messina, Italy, and Boreskov Institute of Catalysis, Prospekt Lavrentieva 5, Novosibirsk 630090, Russia
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Experimental results on equilibrium pressure over KOH NH3 H20 solutions are reported. The temperature was varied between -6 and +30 "C, while the molalities of NH3 and KOH were varied in the range 1-12 and 1-10 mol-kg-I, respectively. An analytical equation of the equilibrium pressure as a function of temperature and concentrations was obtained. The partial molar enthalpy of solution of ammonia in the KOH HzO mixtures and the Henry coefficients for ammonia were also calculated. For the ternary mixture the Setschenow relation was considered and validated in the range of measured concentrations.
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Introduction Thermally driven heat pumps or cooling systems were proposed from the beginning of this century, and in particular the ammonia plus water refrigeration system was developed. Afterward, systems based on ternary mixtures were also suggested but not developed because of the subsequent wide use of vapor compression systems. In recent years, research activity on absorption systems has increased and some old proposals have been reviewed. Among these, a demixing-resorption machine utilizing potassium hydroxide ammonia water seems to have good characteristics. Aqueous KOH NH3 solutions represent an interesting alternative to the traditional vapor-liquid pairs such as ammonia water and water lithium bromide that are commonly used in absorption heat pumps and refrigeration systems (1,2). In fact, with the ternary system which uses KOH NH3 HzO the total pressure in the demixing-resorption machine can be lowered with respect to the absorption system in which only water and ammonia are used. The engineering design of these systems requires accurate data on the thermodynamic properties of potassium hydroxide ammonia water mixtures over a wide range of conditions. However, published results (3-8) cover a limited range of concentrations, usually at molalities less than 3 molokg-l of solvent for both KOH and NH3 and temperature ranges higher than 20 "C. The aim of the present paper is to extend the measurements on the vapor pressure over aqueous KOH NH3 solutions to temperatures as low as -6 "C and higher concentrations of both KOH and NH3, to m = 1012 mol-kg-'.
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Experimental Section The experimental arrangement shown in Figure 1 was used to measure the vapor-liquid equilibrium curves for KOH + NH3 + HzO mixtures. All experiments were performed in the stainless steel container (R), where the temperature was kept constant in the range -6 to +30 "C by the thermocryostat (T)with an accuracy of f0.02 "C. Before the start of each series of experiments, the container was evacuated by the pump (P), and then a fixed volume of KOH HzO solution of known
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+NationalCouncil of Research. Boreskov Institute of Catalysis.
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Figure 1. Scheme of the experimental setup: R, container; T, thermocryostat; V, vacuum gauge; P, pump; D, recorder; S, mixer; B, burette.
concentration was inserted by means of the burette (B). Once the constant temperature and pressure had been established, they were compared with the equilibrium data presented in ref 9 to verify the validity of the method. Subsequently, a measured volume of W O H solution of known concentration was added and the total pressure and the temperature were recorded. After equilibrium was reached, the temperature was changed. The measurements were performed in the temperature range t = -6 to +30 "C. The total pressure over the solution was measured by an absolute pressure vacuum gauge (V)that is composed of a sensor head, where the pressure to be measured acts directly on a fine membrane, and a control unit (Membranovac MV llOS2). The accuracy of the instrument was within 0.05 kPa. The temperature of the mixture was measured by three J-type thermocouples placed a t three different levels to verify the homogeneity of the temperature distribution that appeared to be within 0.3 "C. Commercially available potassium hydroxide (Rudi Point), ammonium hydroxide (Baker grade), and distilled water were used for the mixture preparation. The molalities of N H 3 (ml) and KOH (mz) were varied in the range ml = 1-12 mobkg-' and m2 = 1-10 mol-kg-'. The accuracy of the ammonia and potassium hydroxide concentration was calculated by taking into consideration all of the measurement errors involved in the solution preparation. From
0021-9568/95/1740-0267$09.00/00 1995 American Chemical Society
268 Journal of Chemical and Engineering Data, Vol. 40, No. 1, 1995 Table 1. Vapor Pressure p at Temperature t for KOH 1 1 1 m limol-kg-l 1 1 1 1 1 1 1 1 5 mdmol*kg-l -3.8 30.1 -3.6 3.2 11.7 20.6 t/"C 8 1.76 1.32 1.8 3.1 4.93 pkPa 5 5 5 5 5 5 mllmol-kg-' 1 1 1 1 1 5 mimol-kg-' 4.7 12.9 21.4 30.1 -4.4 tl"C -2.5 3.76 5.8 9.11 14.4 22.1 9.35 pkPa 10.7 10.7 10.7 10.7 10.7 mllmol-kg-l 10.7 1 1 1 1 1 5 mimobkg-' -2.4 4.4 12.5 21.2 30 -3.1 tPC pkPa 8.55 13 19.9 30.9 46 18.6 mlimobkg-' 12 12 12 12 12 12 mzimobkg-' 1 1 1 1 1 5 t/"C -4.6 4.7 12.7 20.9 28.8 -0.8 pkPa 9.5 15 21.6 31.2 46 24.7
+ N H 3 + HzO 1
5 3.2 2.71 5 5 3.5 15.3 10.7 5 3.9 28 12 5 12.1 47.5
1 5 12 4.64 5 5 12.2 24.6 10.7 5 12.4 42.8 12 5 20.9 70.4
"-
L-
1 130
1 5 21 7.91 5 5 21 37.2 10.7 5 21.3 64.3 12 5 29.1 101
1 1 5 10 30 -4.2 12.42 4.96 5 5 10 5 30 -5.4 55.58 18.5 10.7 10.7 5 10 30 -4.7 94.02 29.2 12 12 5 10 -5.6 33.11
1 10 2.9 6.93 5 10 2.7 28.5 10.7 10 3.1 46.2 12 10 2.9 49.9
1 10 11.9 10.96 5 10 11.7 43.01 10.7 10 11.9 69.46 12 10 11.6 75
1 10 20.8 16.35 5 10 21.1 64.06 10.7 10 21 103.7 12 10 20.7 110.3
1 10 30.2 24.24 5 10 30.1 91.67 10.7 10 29.7 151.4 12 10 29.4 161.6
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