Fundamental equation of state correlation for hexamethyldisiloxane based on experimental and molecular simulation data

Equations of state in terms of the Helmholtz energy are presented for octamethyltrisiloxane and decamethyltetrasiloxane. The restricted databases in the literature are augmented by speed of sound measurements, which are carried out by a pulse-echo method. The equations of state are valid in the fluid region up to approximately 600 K and 130 MPa and can be used to calculate all thermodynamic properties by combining the Helmholtz energy and its derivatives with respect to the natural variables. The accuracy of the equation is validated by comparison to experimental data and correct extrapolation behavior is ensured.

Speed of Sound Measurements and Fundamental Equations of State for Octamethyltrisiloxane and Decamethyltetrasiloxane

An equation of state for octamethylcyclotetrasiloxane is presented. The available experimental data from the literature are supplemented by new measurements on the speed of sound. Furthermore, a new force field is proposed, allowing the generation of a comprehensive thermodynamic data set by means of molecular simulation. Both the experimental and molecular simulation data are applied to develop a fundamental equation of state in terms of the Helmholtz energy. On the basis of the experimental data, the equation of state is valid from the triple-point temperature to T = 590 K for pressures up to p = 180 MPa and can be used to calculate any thermodynamic equilibrium state property. Its accuracy is assessed by a comprehensive analysis of the underlying experimental data. Finally, the range of validity was extended to Tmax = 1200 K and pmax = 520 MPa by means of molecular simulation data.

Thermodynamic Properties of Octamethylcyclotetrasiloxane

The pulse-echo technique determines the propagation time of acoustic wave bursts in a fluid over a known propagation distance. It is limited by the signal quality of the received echoes of the acoustic wave bursts, which degrades with decreasing density of the fluid due to acoustic impedance and attenuation effects. Signal sampling is significantly improved in this work by burst design and signal processing such that a wider range of thermodynamic states can be investigated. Applying a Fourier transformation based digital filter on acoustic wave signals increases their signal-to-noise ratio and enhances their time and amplitude resolutions, improving the overall measurement accuracy. In addition, burst design leads to technical advantages for determining the propagation time due to the associated conditioning of the echo. It is shown that the according operation procedure enlarges the measuring range of the pulse-echo technique for supercritical argon and nitrogen at 300 K down to 5 MPa, where it was limited to around 20 MPa before.

Burst design and signal processing for the speed of sound measurement of fluids with the pulse-echo technique

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.

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Thermodynamic Properties of Dodecamethylpentasiloxane, Tetradecamethylhexasiloxane, and Decamethylcyclopentasiloxane

An apparatus for the measurement of the speed of sound based on the pulse-echo technique is presented. It operates up to a temperature of 480 K and a pressure of 125 MPa. After referencing and validating the apparatus with water, it is applied to liquid ammonia between 230 and 410 K up to a pressure of 124 MPa. Speed of sound data are presented with an uncertainty between 0.02% and 0.1%.

Apparatus for the measurement of the speed of sound of ammonia up to high temperatures and pressures.

For gas phase nanoparticle production, hot wall reactors are widely used. In this article, we will describe the fundamental design considerations for a hot wall reactor system able to produce oxide nanoparticles. The system is outstanding in its ability to produce mostly spherical nanoparticles at particle sizes of up to 100 nm and even larger at mass outputs in the order of grams per hour by being able to rapidly quench the aerosol. While high production rates or larger particle sizes are already easily obtained with hot wall reactors, it is very challenging to produce these spherical particles at high mass rates. We will show in this research that the temperature and the particle number concentration are the major aspects influencing the particle morphology at the end of the process. Investigation on the performance of the setup shows good control over the temperature and the particle production stability. A representative particle characterization using SEM and scanning mobility particle sizer showed that particles are mostly spherical, while the particle size distribution had a geometric standard deviation close to 1.5. In addition to the aspects mentioned above, a possibility to manipulate the aggregation downstream of the reactor is to be presented as well. We found that applying electrical charges to the aerosol particles (in opposite polarity) can significantly foster aggregation.

David B. Rasche

Papers

Apparatus for the measurement of the speed of sound of ammonia up to high temperatures and pressures.

An apparatus to synthesize ceramic nanoparticles with a precisely adjusted temperature history and a significant mass output.