Thermodynamic Properties of Dodecamethylpentasiloxane, Tetradecamethylhexasiloxane, and Decamethylcyclopentasiloxane

Siloxanes are widely used in the
chemical industry and in process
and power engineering. For example, they are used as working fluids
of organic Rankine cycle power plants since 20 years ago.
For the process design and optimization, thermodynamic properties,
such as enthalpy, entropy, speed of sound, density, and vapor–liquid
equilibrium, are required. While the properties of short-chained siloxanes,
such as hexamethyldisiloxane (MM) or octamethylcyclotetrasiloxane
(D4), have already been investigated comprehensively, information
on thermophysical properties of higher order siloxanes is limited.
Therefore, measurements on density and speed of sound in the liquid
state of dodecamethylpentasiloxane (MD3M), tetradecamethylhexasiloxane
(MD4M), and decamethylcyclopentasiloxane (D5) are presented here. On the basis of these measurements and other
experimental data from the literature, new fundamental equations of
state were developed for these three fluids. The equations are based
on the Helmholtz energy and, thus, allow for the calculation of any
thermodynamic state property by means of derivatives with respect
to the natural variables, namely temperature and density. The obtained
models also feature a correct extrapolation behavior in regions where
no data are available in order to ensure the applicability of the
equations to mixture models in the future. On the basis of the present
equations of state and recently published equations for other siloxanes,
the possibility of the siloxanes belonging to the group of Bethe–Zel’dovich–Thompson
fluids is also investigated.