[Truncated] Heat capacities and enthalpies are the basic thermodynamic quantities available through calorimetry. Accurate isobaric heat capacity, cp, enthalpy of fusion ΔfusH, and enthalpy of vaporisation ΔvapH data for hydrocarbon mixtures at low temperatures and high pressures are important to the design and operation of liquefied natural gas (LNG) plants. However relatively few experimental measurements of mixture heat capacities have been made at high pressure and low temperature due to the expensive and complicated equipment and procedures involved for determining accurate and reproducible data. The equations of state used to calculate the calorimetric properties of these mixtures are usually regressed only to pressure-volume-temperature (PVT) and vapour liquid equilibria (VLE) data, and their ability to provide accurate heat capacity data has been rarely tested. To illustrate this problem and highlight the need for such experimental data, substantial inconsistencies in the prediction of cp by two EOS of industrial importance: the GERG 2008 EOS1 as implemented in the software REFPROP 9.12 (GERG 2008) and the Peng Robinson EOS3 as implemented in the process simulation software Aspen HYSYS,4 (PR-HYSYS) for binary mixture of methane (1) + butane (4) with x1 = 0.60 have been demonstrated in this work.
To help address this problem, a commercial differential scanning calorimeter (DSC) Setaram BT2.15 was converted to a specialized high-pressure cryogenic calorimeter for isobaric heat capacity measurements of mixtures of light hydrocarbons. The optimised DSC was adapted to enable measurements of the cp of pure liquids, binary and multi-component mixtures of light hydrocarbons, such as those representatives of mixtures in an LNG plant. Three key modifications to the commercial DSC were required to enable these accurate cryogenic, high-pressure liquid cp measurements: (1) improved methods of transferring liquid from the DSC calorimeter to stabilise the instrument’s baseline; (2) incorporation of a ballast volume so that the liquid sample’s thermal expansion did not cause significant pressure changes; and (3) active heating of the tubing connecting the sample cell to the ballast volume to prevent convective heat transfer at low temperatures. These modifications were validated by measurements of cp for liquid methane, ethane and propane over the ranges (108 to 258) K, (1.1 to 6.4) MPa, with relative standard deviations of the measurements from the reference EOS values for these pure fluids of 0.5 %, 1.0 % and 1.5 %, respectively.
|Qualification||Doctor of Philosophy|
|Publication status||Unpublished - 18 Mar 2015|