(12e) Thermodynamic Properties of Saturated Liquid Dimethyl Ether from Adiabatic Calorimetry

A knowledge of thermodynamic properties at saturation can be useful for process design calculations. Prior to this work, however, reliable data for two key properties of dimethyl ether - saturated liquid heat capacity C_ó and vapor pressure p_ó, were not available in a wide range of temperature and different sources were shown to disagree by more than their estimated uncertainties. To help mitigate those problems, new heat capacity measurements, for both single phase liquid and two-phase liquid-vapor states, were carried out using an adiabatic calorimeter. A high purity sample of dimethyl ether, verified by chemical analysis, was used for these measurements. To derive values for C_ó and p_ó, our two-phase heat capacity data were treated in terms of internal energy and entropy surfaces. In this way, we derived thermodynamically consistent values of saturated liquid heat capacity and vapor pressure in a temperature range from the triple point of dimethyl ether near 132 K to the upper temperature limit of the calorimeter near 345 K. The expanded uncertainty (i.e., a coverage factor k=2 and thus a two-standard deviation estimate) for values of C_ó is estimated to be 0.7 %, while for for p_ó it ranges from 2 Pa near the triple point temperature to 50 Pa near the normal boiling temperature. The derived thermodynamic data at saturation were fitted by simple functions of temperature that cover wide ranges (150 to 200 °C) of temperature, which increases their utility for process design. These fitted functions were also used to make direct comparisons with the limited published data for C_ó and p_ó.