J. of Supercritical Fluids 58 (2011) 189–197 Contents lists available at ScienceDirect The Journal of Supercritical Fluids jou rn al h om epage: www.elsevier.com/locate/supflu High pressure densities of carbon dioxide + dipentaerythritol hexaheptanoate: New experimental setup and volumetric behavior O. Fandi ˜ no a , L. Lugo a,∗ , J.J. Segovia b , E.R. López a , M.J.P. Comu ˜ nas a , J. Fernández a a Laboratorio de Propiedades Termofísicas, Departamento de Física Aplicada, Facultade de Física, Universidade de Santiago de Compostela, E-15782 Santiago de Compostela, Spain b Grupo de Investigación TERMOCAL – Termodinámica y Calibración Universidad de Valladolid, Paseo del Cauce s/n E-47011, Valladolid, Spain a r t i c l e i n f o Article history: Received 12 April 2011 Received in revised form 9 June 2011 Accepted 10 June 2011 Keywords: Density High pressure densimeter Lubricant Dipentaerythritol hexaheptanoate Carbon dioxide Supercritical fluids a b s t r a c t To perform an appropriate selection of the lubricants in air conditioned systems working with carbon dioxide as refrigerant, the thermodynamic behavior of the CO 2 + lubricant systems must be well known. In this work we present a new setup to prepare compressed gas–liquid mixtures and to determine the high pressure density by using an automated densimeter HPM and two syringe pumps. To analyze the reliability of the procedure proposed, we have determined the densities and mixing volumes of four CO 2 + n-decane mixtures. We have found a good agreement with previous literature data. In addition new density values are reported for the binary system CO 2 + dipentaerythritol hexaheptanoate (DiPEC7) at several temperatures and pressures from 10 MPa to 120 MPa. © 2011 Elsevier B.V. All rights reserved. 1. Introduction The selection of refrigerant for new friendly space-conditioning systems varies globally due to local legislations, but it is regulated by the requirements of the Montreal and Kyoto Protocols [1,2]. Car- bon dioxide (CO 2 ) is one of the most promising natural refrigerants that could be employed to replace the current HFC refrigerants that are used in air conditioning systems [3,4]. CO 2 is readily avail- able, inexpensive, nontoxic, nonflammable, it has a mild critical temperature (304.2 K) and critical pressure (7.38 MPa), and super- critical CO 2 , has been recognized as an environmentally benign solvent and has also attracted much attention in many chemical processes [5]. It has excellent chemical compatibility with common materials and relatively good solubility in several types of oils [4,6]. In the framework of the air conditioned and refrigeration industry, CO 2 has no ozone depletion potential, has negligible global warm- ing potential and does not provoke other serious environmental problems. Thus, transcritical CO 2 refrigeration cycles are consid- ered the logical step forward that addresses the environmental, safety and thermodynamic performance criteria [7,8]. ∗ Corresponding author. Current address: Departamento de Física Aplicada, Fac- ultade de Ciencias, Universidade de Vigo, E-36310 Vigo, Spain. Tel.: +34 986812295; fax: +34 986814069. E-mail address: luis.lugo@uvigo.es (L. Lugo). However, CO 2 gives rise to several technical problems when employed as working fluid in refrigeration systems [6,9,10], con- sequently the thermodynamic behavior of the CO 2 + lubricant system must be well known for a correct oil selection [11,12]. CO 2 + lubricant mixtures with a low content of lubricant are of high interest because practically a small amount of compressor lubricant circulates with the refrigerant. Hence, the working fluid is changed from a pure refrigerant with well-defined properties, to a mixture with properties that are poorly understood and dependent on the lubricant concentration. In addition, mixtures with high content of lubricant are present in the compressor, because of the refrigerant is dissolved in the lubricant, provoking changes in the lubricant properties. Among others, one of the properties needed to evaluate the refrigeration cycle performance are the densities of the mixtures refrigerant + lubricant. Especially when CO 2 is proposed as work- ing fluid, we must taking into account one of the peculiarities of supercritical fluids [13] is that it behaves as a hyper compressible media in the vicinity of its critical point and their densities and density-dependent properties are very sensitive to pressure. As a consequence, small changes in pressure yield large changes in density and the properties can be adjusted effectively by chang- ing pressure. Hence, at a fixed temperature, supercritical CO 2 can change from a supercritical state to a subcritical mixture with addition of a cosolvent/lubricant, and the properties of supercriti- cal CO 2 -cosolvent/lubricant mixtures can be significantly different from those of pure supercritical CO 2 [14]. 0896-8446/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.supflu.2011.06.004