Properties of saturated fluorocarbons: Experimental data and modeling using perturbed-chain-SAFT

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Abstract

We present experimental speed of sound data for the saturated fluorocarbons used most frequently in particle detector cooling systems. Data were collected over the moderate range of pressure (≤0.4 MPa) and temperature 250–340 K. A commercial sound/density analyzer operating on the vibrating tube principle was employed for the measurements in liquid state. A dedicated sonar analyser – described in detail – was recently redesigned and developed for the speed of sound measurements in gas phase. Data for speed of sound in saturated fluorocarbons presented in previous papers [V. Vacek, G. Hallewell, S. Ilie, S. Lindsay, Fluid Phase Equilib. 174 (2000) 191–201; V. Vacek, G. Hallewell, S. Lindsay, Fluid Phase Equilib. 185 (2001) 305–314] were reanalyzed and corrected and some new measured data were added. Our experimental data were then used to validate a theoretically based equation of state application rather than the empirical or semi-empirical equations of state mostly used up to now in fluorocarbon applications.

Introduction

Equations of state (EoSs) based on statistical associating fluid theory (SAFT) have been successfully used for modeling the properties of various substances with small, large, associating, and polydisperse molecules. The main advantage of these EoSs is their quite accurate approximation of complex multicomponent mixtures. We therefore decided to use the perturbed-chain (PC) SAFT model, introduced by Gross and Sadowski [1], to predict the thermophysical properties of several binary mixtures of saturated fluorocarbons.

Saturated fluorocarbons of the form CnF(2n+2) have long been employed in liquid and gas phases in the field of particle physics as high density Cherenkov radiation-generating fluids. More recently they have found use as refrigerants for liquid phase or bi-phase (evaporative) cooling of large-area silicon trackers for charged particles at the new Large Hadron Collider (LHC) at the CERN laboratory in Geneva. Their high radiation resistance, non-flammability, low viscosity, low toxicity and excellent dielectric properties make them very valuable in this application despite their enthalpy of evaporation being less than that of less radiation-stable refrigerants.

In the application under study, the evaporation of C3F8 (R218) is foreseen for the cooling of the silicon inner tracker of the ATLAS experiment at LHC [2], which must be maintained at a temperature lower than −7 °C at full power dissipation of around 60 kW in the very high radiation environment near the collision point of the LHC proton beams. Future applications will require a lower operating temperature while using the same installed plant; an approach using an admixture of C2F6 (R116) in the range 10–20% is expected to be advantageous, and has helped motivate this study of SAFT equations of state for mixtures saturated fluorocarbon fluids. The verifications of predictions of sound velocity in C3F8 and C4F10 (R610) mixtures was carried out using a simple sonar instrument described in the following section. Future studies with this instrument are envisaged for mixtures of R218 and R116.

Section snippets

The sonar gas analyzer

The sonar gas analyzer is based on an instrument initially developed for the analysis of gas mixtures used in a Ring Imaging Cherenkov Detector [3], and makes use of the phenomenon that in a binary gas mixture of constant known temperature and pressure, the speed of sound (SoS) is an unambiguous function of the concentration of the two components. The device (Fig. 1) is based on a pair of 43 mm diameter Polaroid/SensComp series 600 capacitative instrument-grade ultrasonic transducers1

Experimental data on sound velocity and density of liquid fluorocarbons

The density and the SoS in subcooled liquid phase of three heavy fluorocarbons were measured using a density and sound analyser.2 These fluorocarbons were chosen due to their low saturation vapor pressure at 0.1013 MPa to avoid rapid evaporation loss during the measurements. The precision of the measurements was ±5 × 10−5 g cm−3. All three heavy fluorocarbons with six to eight carbons were manufactured by 3-M Corp. Specialty Chemicals Division,

Updated data from previous studies

After detailed inspection, the apparatus for measurement of the SoS in vapor phase, utilized in our previous tests [4], [5], was found to have slight problems with the temperature measurement. Two temperature probes installed inside the sonar analyzer worked accurately at the temperatures close to ambient conditions. However, for temperatures further from room temperature, the probes showed certain deviations. Both temperature sensors have been therefore dismounted from the sonar analyzer, and

Comparison of sound velocity measurements with PC-SAFT predictions

Equations of state based on statistical associating fluid theory evaluate in principle the residual Helmholtz free energy, AR, as the main quantity. For more detail about how the PC-SAFT EoS evaluates the residual Helmholtz free energy see, e.g. Gross and Sadowski [1]. In addition to the primary variables, i.e. temperature, pressure, density and composition, the following properties of various fluorocarbon mixtures have to be determined from the PC-SAFT EoS: the enthalpy, h, entropy, s,

PC-SAFT predictions of thermophysical properties for mixtures of R116/R218

As has been shown, predictions of the PC-SAFT EoS approximate the experimental data on the SoS both for pure fluorocarbons and for fluorocarbon mixtures with quite good accuracy. The PC-SAFT model can therefore also be used for predicting other thermophysical properties of various saturated fluorocarbon mixtures in the next step.

The saturation properties predicted by the PC-SAFT model for saturated R116/R218 mixtures with mass ratios of 10%/90% and 20%/80%, respectively, are summarized in two

Conclusion

A dedicated sonar binary gas mixture analyzer was developed and built for this study. The potential of the instrument exceeds simple measurements of sound velocity in gases, since it can also be used as a fast on-line composition analyzer for binary refrigerant mixtures. In addition, it can also serve as precise flow meter with its electronics adapted accordingly.

We have collected sound velocity data in the liquid phase fluorocarbons C6F14, C7F16 and C8F18 together with relevant densities at

Acknowledgements

The authors are grateful to colleagues from the CERN laboratory and the Department of Physics at the CTU in Prague, especially to S. Lindsay (who took part in gas phase measurements), S. Ilie (who performed gas chromatography analyses) and to M. Doubrava (who made liquid phase measurements CTU), for their help with experimental work. The study was supported by grants of the Ministry of Education, Youth and Sports of the Czech Republic Gr. No.: LA08015 and LA08032.

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