Heat capacity measurement by flow calorimetry: An exact analysis

Abstract

in Wiley InterScience (www.interscience.wiley.com).The principal unsolved problem in flow calorimetry for liquid heat capacity measurement accurate accounting for heat loss from the heater lead-in wires as a function of system properties is analyzed by exact procedures for a five-zone calorimeter model. Temperature distributions in the fluid, and bi-metal wire are obtained from solutions of the governing third-order ODE in the fluid temperature for realistic boundary conditions. Conductive heat losses at the fluid exit q HL /q˙are large (up to 20% of energy input), and physical property and flow rate dependent. A new correlating equation for (q HL /q˙) gives separately and explicitly, for the first time, its dependence on calorimeter characteristics, flow rates and fluid properties. Experiments on five pure liquids confirmed the predictions of the theoretical model and produced Cp values in close agreement with literature data. Fluid friction and small convection heat losses (U i A i (DT) lm ) were accounted for experimentally. 2008 American Institute of Chemical Engineers AIChE J, 55: 206-216, 2009 Keywords: heat capacity measurement, flow, exact analysis IntroductionDesign procedures for heat-transfer equipment generally require values for the Prandtl number Prð¼ Cpl=kÞ as a function of temperature for pure liquids and liquid mixtures. Calculation of the evaporation ratio by new techniques (for exact determination of limiting activity coefficients by differential ebulliometry), requires precise values of solvent Cp, and latent heat as functions of temperature (Raal et al. 1 ). Large collections of liquid heat capacity data (e.g., Zabransky et al. 2 and Domalski and Hearing 3 ) are available in the literature, but for many liquids reliable data will not be available or may be uncertain due to discrepancies in reported values. For mixtures no reliable prediction procedures in terms of pure component values appear to be available. Prediction procedures for pure liquids are proposed and evaluated by Poling et al. 4 based on group contribution methods or corresponding states procedures. These procedures can give considerable errors for certain groups of compounds or at high-reduced temperatures. A sophisticated C L P prediction procedure has recently been developed by Diedrichs and Gmehling 5 based on a modified group contribution (VTPR) Peng -Robinson EOS. C p is related, from its definition as an enthalpy derivative, to PVT properties and their derivatives through easily derived exact equations. Very good predictions were obtained for a substantial number of components.Heat capacities are now frequently measured with commercial instruments (e.g., TA. Instruments, Tian-Calvet calorimeter (Settaram)), based on differential scanning (DSC) techniques. Recent heat-capacity measurements on ionic liquids by Diedrichs and Gmehling 6 using DSC and the more rapid ''modulated-temperature DSC'' methods, showed differences of up to 5% for the two techniques. Fairly extensive calibration procedures are required. Flow calo...

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