Near-critical measurements of enthalpy increments for benzene: a new flow calorimeter for the measurement of near-critical enthalpy increments

A new design of flow calorimeter for the measurement of enthalpy increments in the near critical region is described. Fluid was pumped through a heat exchange coil immersed in a bath of heated silicone oil and then through a second heat exchange coil immersed in a bath of ethanol cooled to T = 298.15 K by an array of Peltier cooling modules. A platinum resistance thermometer in the bath controlled the power supplied to the cooling modules so that heat transferred to the ethanol was pumped continuously away and the temperature remained close to T = 298.15 K. The efficiency of the cooling system was measured using a calibration heater immersed in the ethanol. A feature of the calorimeter is that the flow of fluid from the hot bath to the cold bath is in a horizontal plane, and that the transducer used for the pressure measurements is in this same plane. This eliminates density gradients and uncertainties in the pressure which arise when a vertical configuration is used. Heat leaks were reduced to a minimum, and were carefully controlled. Enthalpy increments for benzene have been measured over the range T = 533.15 K to T = 573.15 K at pressures up to 10.0 MPa. These include a set of near critical region measurements extending from the critical temperature T = 561.75 K to T = 566.75 K at 1 K intervals. Extrapolation of the measurements to zero pressure yielded ideal gas enthalpies in agreement with values calculated from spectroscopic measurements to within +/-1 per cent. To within the combined experimental error the measurements are in agreement with earlier measurements made with a water-cooled heat-exchange calorimeter mounted in a vertical configuration. Comparison is made with the equation of state for benzene proposed by Goodwin, and the equation of state proposed by Bender and fitted to thermodynamic measurements on benzene by Polt, A.; Platter, B.; Maurer, G. Chem. Technik 1992, 6, 216-224. (C) 2000 Academic Press.