PVTx measurements and other derived volumetric properties of the binary ((1-propanol + n-pentane)) mixtures in the critical and supercritical regions

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Highlights

  • PVT measurements of 1-propanol + n-pentane in the critical and supercritical regions.

  • Excess and partial molar volumes.

  • Krichevskii parameter and thermodynamic properties of dilute mixture near the critical point.

  • Isomorphic thermodynamic behaviour of the mixture near the critical curve.

Abstract

The PVTx properties of binary (1-propanol + n-pentane) mixture have been measured over wide temperature and pressure ranges from (306.15 to 573.15) K for densities between (15.52 and 663.69) kg·m−3 up to 49.09 MPa for four concentrations (0.2, 0.5, 0.8, and 0.9 mol fraction of n-pentane). The well-known high-temperature and high-pressure constant-volume piezometer technique was used for measurements of the PVTx relationship. The measurements were concentrated in the critical and supercritical regions in order to study the features of the mixture critical curve behaviour (critical phenomena in the (1-propanol + n-pentane) mixture and other derived thermodynamic properties near the liquid-gas critical points. The combined expanded uncertainty of the density (ρ), pressure (P), temperature (T), and concentration (x), measurements at the 95% confidence level with a coverage factor of k = 2 is estimated to be 0.2% (density in the liquid phase), 0.3% (density in the gas phase); 0.5% (density in the critical and supercritical regions); 0.05% (pressure), 15 mK, and 0.01 mol%. The critical curve values (TC-x), (PC-x), and (ρC-x), were determined from the measured PVTx results. The value of the Krichevskii parameter (P/x)TCVC = 7.985 MPa for (1-propanol + n-pentane) mixture was estimated from the present and reported critical curve and the directly measured P-x results along the critical isochore-isotherm of pure 1-propanol. The critical curve values were also used to study the critical phenomena (isomorphism in the binary mixture) in the (1-propanol + n-pentane) mixture. In particular, the characteristic reduced temperature and density differences, which determine the isomorphic thermodynamic behaviour (pure-like and mixture-like critical behaviour) of the mixture, were estimated using the critical curve values. The measured PVTx values were also used to calculate the derived volumetric properties such as excess and partial molar volumes of n-pentane near the critical point of pure 1-propanol.

Introduction

Knowledge of volumetric (PVTx, excess molar volumes, and partial molar volumes) properties is very important in various chemical industrial fields. Accurate thermodynamic properties for mixtures are needed for optimization, design, simulation, control, and improving the industrial processes taking place at supercritical conditions. Very limited data are available for the (1-propanol + n-pentane) mixture in the critical and supercritical regions. It is a challenge to predict accurately the properties of the mixture of the polar and non-polar components like (1-propanol + n-pentane) using the pure component properties. More accurate PVTx measurements near the critical point are needed to develop new thermodynamic models of the mixture and build their phase diagrams. Detailed measurements of volumetric properties of near- and supercritical fluids and fluid mixtures should bring an improved understanding of the fluid structure and molecular interactions between various components in supercritical fluid mixtures. Accurate volumetric data also give us an insight to the systems on an atomic scale, which can be used to establish a relationship between intermolecular potential and measured macroscopic thermodynamic behaviour.

Reliable PVTx results are required to develop an accurate equation of state (EOS) (determine molecular parameters of the theoretically based EOS) for supercritical fluids and fluid mixtures. It is problem to develop a reliable EOS for mixtures of molecules with a large differences in size, shapes, polarity as well as systems in the vicinity of their critical point. Anomalies of the thermodynamic properties of fluids and fluid mixtures in the critical and supercritical regions are very important for phase diagram modelling. For complex mixtures containing dissimilar structure of the molecules and chemical nature with polar and non-polar molecules such as (1-propanol + n-pentane), it is difficult to predict accurately their properties based on pure component properties due to the complexity of the intermolecular interaction (H-bonding effect in alcohol solutions) between their molecules. Most cases involving a molecular-based EOS fail to predict accurate properties of the supercritical fluids. The experimental study of PVTx relationships and other volumetric properties of fluid mixtures in the near- and supercritical regions are also interesting from the fundamental scientific point of view order to check and confirm the reliability and predictive capability of the various molecular-based fundamental EOS and accurately determine the molecular parameters of the EOS or to modify the structure of the models and their physical bases.

Accurate critical point values for mixtures can be used to test physical bases of the EOS. The shape of the critical curves for the mixtures provides very useful information on the thermodynamic behaviour of the mixture near the critical point. This allows one to study isomorphism or Fisher renormalization (pure-fluid and mixture –fluid behaviour) of the critical behaviour of binary mixtures. Other examples include the study of the thermodynamic behaviour of the dilute mixture near the critical point for one of the components or to correct the determination of the Krichevskii parameters that are required for reliable critical curve results. In addition, the qualitative shape of the PC-TC projection plot of the critical locus is a defining criterion for the type of mixture according to the classification of van Konynenburg and Scott [1] and Rowlinson and Swinton [2]. The objective of the present work is to determine accurately the PVTx properties for the binary mixture of (1-propanol + n-pentane) and to extract the critical curve results in various projections (TC-x,PC-x, ρC-x, and PC-TC). In the present work, we also report other derived volumetric properties (excess and partial molar volumes) of the mixture in the critical and supercritical regions.

Only two data sources [3], [4], [5] (authors in Refs. [4] and [5] report the same results) for the critical properties, PC(x) and TC(x) or PC-TC were found in the literature for the binary (1-propanol + n-pentane) mixture. The literature search was based on the TRC/NIST archive [6]. These sources are representing 8 critical temperatures and 3 critical pressures over the whole concentration range. Christou and Young [4] and Christou [5] employed direct experimental measurements (the meniscus disappearance and reappearance technique) of the critical temperatures of (1-propanol + n-pentane) mixtures over the whole concentration range. The uncertainty in concentration and critical temperature measurements are within 0.005 and <1 K, respectively. The measured critical temperatures were fitted to the correlation equationTC(x)=xTC2+(1-x)TC1+T1x(1-x)+T2x(1-x)(2x-1)+T3x(1-x)(2x-1)2,where TC1=536.76K and TC2=469.74K are the critical temperatures of 1-propanol and n-pentane, respectively, and Ti represents the fitting parameters determined from the measured critical temperatures. In total, five experimental data points were obtained for the concentrations (0.132, 0.399, 0.561, 0.717, and 0.894 mol fraction of n-pentane). Jung et al. [3] reported the critical properties data (TC,PC) for (1-propanol + n-pentane) mixture from VLE measurements with a circulation-type apparatus with a visual method. The measurements were made at four isotherms of (468.15, 483.15, 498.15, and 513.15) K. The values of the critical pressure for three temperatures of (483.15, 498.15, and 513.15) K were reported. Critical points were identified by critical opalescence and compared to the published results for (n-pentane + alkanols). The results were correlated with the Peng−Robinson−Stryjek−Vera (PRSV) equation of state with Wong−Sandler mixing rule. The critical lines for other representatives of (alcohol + n-pentane) mixtures were reported by other authors (methanol + n-pentane [7]; ethanol + n-pentane [8]; and butanol + n-pentane [9]). Oh et al. [10] reported isothermal VLE (for four temperatures) and critical properties data (TC,PC) for three 1-propanol + n-hexane mixtures (0.599, 0.786, and 0.319 mol fractions using visual method -critical opalescence observation). Gil et al. [11] used the same method to measure the liquid-gas critical locus, TC-x and PC-x, over in the whole range of mole fraction for (1-propanol + n-hexane) mixtures. The repeatability in the critical temperature and the critical pressure measurements for the mixture are <0.15 K and <0.013 MPa, respectively. Xin et al. [12] reported critical properties (TC-x and PC-x) for 1-propanol + n-alkane (from C6 to C10) binary mixture. The measurements were made using the critical opalescence observation method. The uncertainty of the critical temperature and the critical pressure measurements are 0.4 K and 0.01 MPa, respectively. The measured critical lines were fitted to Eq. (1). The critical properties of the mixture n-alkane (n-hexane) with other alcohols (methanol, ethanol) were measured by other authors [7], [11], [13], [14], [15]. The critical phenomena of methanol with n-hexane (and other n-alkane) were studied by de Loos et al. [7]. They studied the L-G critical curve shape changes in the (alcohol + n-alkane) mixtures. Zawisza [16] determined the high-pressure VLE, L-G critical curve, and PVTx properties for the (1-propanol + n-heptane) mixture. In another paper, Zawisza [13] found that the L-G critical PC-TC curve for (methanol + n-alkane) mixtures runs through a temperature minimum. Liu et al. [14] also reported TC-x and PC-x results for the (methanol + n-hexane) mixture. Sauermann et al. [15] reported critical properties for the (ethanol + n-hexane) mixture. All previously reported critical properties for (1-propanol + n-pentane) mixture were measured using the visual opalescence method. This method is less reliable than direct PVTx measurements due to the development of the critical opalescence which impedes the visual observations. In this method the temperature range within 1 K around the critical point becomes virtually unattainable for visual observation. Thus, the uncertainty of the method is higher than 1 K. Unfortunately, there are no reported critical density values for the (1-propanol + n-pentane) mixture. For the first time, critical density is reported in the present work.

A literature survey (NIST/TRC search) revealed that there are no reported one-phase density (PVTx) values for the (1-propanol + n-pentane) mixture under pressure. In the NIST/TRC database we found only two sources [17], [18] where density is reported at 298.15 K and atmospheric pressure. Most one-phase PVTx measurements reported in the literature were made in the liquid phase at low temperatures mostly at 298.15 K and at atmospheric pressure. The measured values of density as a function of concentration were used to calculate excess molar volumes at 298.15 K and 0.101 MPa. Thus, there are no reported PVTx results in the one-phase region, especially in the near- and supercritical regions, for the (1-propanol + n-pentane) mixture. In the present work, we have expanded the available thermodynamic database for the (1-propanol + n-pentane) mixture. We report accurate PVTx values data for the (1-propanol + n-pentane) mixture within the temperature range from (303.15 to 573.15) K for liquid, vapour, near- and supercritical regions for isochores between (15.52 and 663.69) kg·m−3 up to 49 MPa using a high-temperature and high-pressure constant-volume piezometer.

Section snippets

Materials

The chemicals used were n-pentane and 1-propanol. Their description is given in Table 1. The n-pentane (Moscow) with purity of 99.95 mass% (GC). 1-Propanol (CAS # 71-23-8) was supplied by “Reactive” with purity of 99.99 mass% (HPLC). The samples were used without additional purification.

Apparatus

The experimental details of the method (procedure of measurements and uncertainty assessment) and apparatus have been described in our earlier publications [19], [20], [21], [22], [23], [24], [25], [26], [27],

Results and discussion

Measurements of the PVTx relationship of (1-propanol + n-pentane) mixtures were performed at four concentrations (0.2, 0.5, 0.8, and 0.9 mol fraction of n-pentane) over the temperature range from (306 to 573) K in the liquid and vapour phases, including near- and supercritical regions. In total more than 1263 experimental data points were obtained for the isochores between (15.52 and 663.69) kg·m−3 and at pressures from (0.16 to 49.09) MPa. The accuracy and reliability of the method, procedure, and

Conclusions

In this work, we have successfully employed the well-known high-temperature and high-pressure constant-volume piezometer technique to study PVTx properties of (1-propanol + n-pentane) mixture over a wide temperature and pressure range including sub-, near-, and supercritical regions. New PVTx values over the temperature range from (306.15 to 573.15) K for densities between (15.52 and 663.69) kg·m−3 up to 49.09 MPa for four concentrations (0.2, 0.5, 0.8, and 0.9 mol fraction of n-pentane) for

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