Experimental study of the thermal (PVT) and caloric (Cv) properties of aqueous ammonia solution in the critical, supercritical, and retrograde regions

P. Nikolay1, I. Abdulagatov2, B. Rabiyat1 and S. Gennadiy1

1Institute of Physics of the Dagestan Scientific Center of the Russian Academy of Sciences, Makhachkala, Dagestan, Russia
2National Institute of Standards and Technology, Boulder, Colorado, U.S.A.

Keywords: critical point, retrograde regions, heat capacities
property: heat capacities, PVT, (∂P/∂T)v
material: aqueous ammonia

The one- and two-phase isochoric heat capacities (Cv), saturation densities (ρ′s and ρ"s) and heat capacities (C′v1,C"v1,C′v2,C"v2), vapor pressures (Ps), and thermal-pressure coefficient (∂P/∂T)v of (0.7393 H2O+0.2607 NH3) in the near – and supercritical regions have been measured with a high–temperature and high–pressure nearly constant–volume adiabatic piezo-calorimeter. The measurements were carried out in the temperature range from 403 K to 633 K for 15 liquid and 25 vapor densities from 120 kg m-3 to 672 kg m-3 and at pressure up to 28 MPa. The calorimeter was additionally supplied with a calibrated extensometer to accurately and simultaneously measure the PVT,Cv , and thermal-pressure coefficient, (∂P/∂T)v. The quasi-static thermo- (T- plot) and barograms (P - plot) techniques were used to accurately measure the phase transition parameters (Pss,Ts) and (∂P/∂T)v. The total experimental uncertainty of density (ρs), pressure (Ps), temperature (Ts), isochoric heat capacities (C′v1,C"v1,C′v2,C"v2), and thermal-pressure coefficient, (∂P/∂T)v, were estimated to be 0.06 %, (0.02 to 0.09) %, 15 mK, (1 to 3) %, and 0.12 % to 1.5 %, respectively, in depending on temperature and pressure ranges. Detailed measurements were made in the immediate vicinity of the specific points (critical, maxcondentherm, and phase transition points) on the thermodynamic surface to accurately determine the values of Pc, ρc, Tc, Ps, ρs, Ts, Pmc, ρmc and Tmc.The measured values of saturated caloric (C′v1,C"v1,C′v2,C"v2), and saturated thermal (Ps,Ts,ρ′s,ρ"s,(∂P/∂T)v) properties were used to calculate other derived thermodynamic properties (Cp,Cs ,Pint), of the solution at the bubble- and dew pressure points curves. An unusual behavior of the isochoric, isobaric heat capacities, thermal-pressure coefficient (∂P/∂T)v, and internal pressure, Pint, of the mixture was found near the critical and maxcondetherm point (in the retrograde region).

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