The precise measurement of the (vapour + liquid) equilibrium properties for (CO2 + isobutane) binary mixtures

doi:10.1016/j.jct.2010.09.004

The Journal of Chemical Thermodynamics

Volume 43, Issue 3, March 2011, Pages 244-247

https://doi.org/10.1016/j.jct.2010.09.004 Get rights and content

Abstract

Recently, it has been suggested that natural working fluids, such as CO₂, hydrocarbons, and their mixtures, could provide a long-term alternative to fluorocarbon refrigerants. (Vapour + liquid) equilibrium (VLE) data for these fluids are essential for the development of equations of state, and for industrial process such as separation and refinement. However, there are large inconsistencies among the available literature data for (CO₂ + isobutane) binary mixtures, and therefore provision of reliable and new measurements with expanded uncertainties is required. In this study, we determined precise VLE data using a new re-circulating type apparatus, which was mainly designed by Akico Co., Japan. An equilibrium cell with an inner volume of about 380 cm³ and two optical windows was used to observe the phase behaviour. The cell had re-circulating loops and expansion loops that were immersed in a thermostatted liquid bath and air bath, respectively. After establishment of a steady state in these loops, the compositions of the samples were measured by a gas chromatograph (GL Science, GC-3200). The VLE data were measured for CO₂/propane and CO₂/isobutane binary mixtures within the temperature range from 300 K to 330 K and at pressures up to 7 MPa. These data were compared with the available literature data and with values predicted by thermodynamic property models.

Introduction

Natural refrigerant mixtures with CO₂ as the main component are expected to be one of the most promising long-term alternative fluids for refrigeration and heat-pump systems. For successful developments in this area, new reliable thermodynamic property models with high accuracy are required for these mixtures. In addition, a database of various thermophysical properties for mixtures over a wide range of temperatures and pressures is needed for the generalisation of Helmholtz-type equations of state (EOS). These requirements dictate that several researchers conduct a number of these measurements in the near future. Recently, several sets of reliable data for natural refrigerant mixtures have been published by Prof. Uematsu’s group for PρTx [1], [2], [3], [4], saturation properties [5], and critical parameters [6]. Literature data have also been published relatively recently for the (CO₂ + propane) mixture [7], [8].

In this study, we attempted to measure the (vapour + liquid) equilibrium (VLE) properties, including bubble point pressures (P_bub) and dew point pressures (P_dew), for (CO₂ + hydrocarbon) mixtures with a wide range of compositions (CO₂ mole fraction x = 0 to x = 1). A new re-circulation type of apparatus, which is one of the most reliable methods available for VLE studies, was used for these measurements. Several sets of calibration measurements were conducted, followed by measurements for (CO₂ + propane). The data were systematically compared with literature data and the values predicted by the Peng–Robinson (PR) equation of state (EOS) with the newest binary interaction parameter [8]. This confirmed that the measurement method had sufficient reliability. Additionally, VLE data were also determined for (CO₂ + isobutane) binary mixtures and compared with REFPROP ver.8.0 [9].

Section snippets

Experimental

The recirculation type apparatus (figure 1) included an optical cell with four windows and was immersed in a liquid bath. A platinum resistance thermometer was inserted directly into the cell and the temperature of the sample was measured within ±0.26 K. The pressure was measured with a pressure gauge within ±0.43%. The overall uncertainties (95% confidence level) in temperature and pressure measurements were estimated as 0.56 K and 0.86%, respectively.

The sample in the cell was circulated by two

Results and discussion

For this method, reduction of fluctuation and establishment of high repeatability are the most important elements for precise and reliable measurements. After obtaining more than 100 points of unacceptable data, we established an appropriate measurement procedure with high repeatability. To confirm the overall reliability and repeatability, we then performed measurements for the CO₂/propane binary mixture.

As discussed in Section 1, this binary system is especially important due to the potential

Conclusions

We aimed to provide VLE data with more reliable behaviour for (CO₂ + isobutane) binary mixtures. After calibration of the gas chromatograph and estimation of the expanded uncertainties, we performed measurements for the CO₂/propane binary mixture. These results confirmed the reliability of the apparatus. Subsequent measurements for the (CO₂ + isobutane) binary mixtures provided reliable results for the P_bub and P_dew in comparison to literature data published by Weber [15] in 1989.

In future, we will

Acknowledgements

This work was supported through 2008–2011 by the Grant-in-Aid for Scientific Research Fund (Project No. 20686016) by the Ministry of Education, Culture, Sports, Science and Technology, Japan.

We would like to thank Dr. K. Fujii, Dr. Y. Kayukawa, Prof. Y. Higashi, Dr. K. Tanaka, Prof. T. Tsuji, Prof. Y. Iwai, Prof. K. Fukuchi, and Mr. S. Aizawa for their kind advice and assistance, all of which were indispensable throughout the study.

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^☆: Presented at the 21st IUPAC International Conference on Chemical Thermodynamics held in Tsukuba Science City, Japan, 31 July to 06 August 2010.

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The precise measurement of the (vapour + liquid) equilibrium properties for (CO2 + isobutane) binary mixtures☆

Abstract

Introduction

Section snippets

Experimental

Results and discussion

Conclusions

Acknowledgements

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

J. Chem. Thermodyn.

Fluid Phase Equilib.

The precise measurement of the (vapour + liquid) equilibrium properties for (CO₂ + isobutane) binary mixtures☆