Revisiting the Liquid–Liquid Phase Behavior of n-Alkanes and Ethanol

Abstract

Mixtures of alkanes and ethanol are important in many areas, for example, as fuel blends. This paper describes new experimental data obtained for the liquid–liquid equilibrium phase behavior of normal alkanes (n-alkanes; C n H2n+2; 9 ≤ n ≤ 24) with ethanol. The results were obtained by applying the cloud point method in a temperature range of T = 230–423 K at ambient pressure. All systems are partially miscible with an upper critical solution point. The two phase regions of the phase diagrams show no indication of any obvious optical irregularities, like birefringence, coloring, formation of schlieren, or remarkable turbidity, except critical opalescence. With increasing length of the molecular chain of the n-alkanes, the (liquid–liquid) critical point is shifted to higher temperatures and higher ethanol content. The data are analyzed numerically implying Ising criticality. The nonsymmetric shape of the phase body is considered in different approaches for describing the diameter by presuming (a) the validity of the rectilinear diameter rule, (b) a nonlinear diameter predicted in the theory of complete scaling, and (c) combining both concepts. The numerical analysis yields the critical temperature, the critical composition, the width, and the diameter of the phase diagrams. The results are compared with literature data sets from similar mixtures; these data are also evaluated in terms of the models applied here. Phase diagrams of 13 different sets of mixtures are measured and analyzed to extract general aspects of the behavior of the normal alkane–ethanol mixtures. A simple Flory–Huggins-like approach allows a semiquantitative description of the experimental results of the critical temperatures. Therefore, it confirms the picture of molecular ordering within the solutions.

Liquid–Liquid Phase Behavior of Diesel/Biofuel Blends: Ternary Mixtures of n-Hexadecane, 2,2,4,4,6,8,8-Heptamethylnonane, and Ethanol and the Binary Subsystems

“…We emphasize, as in prior work ,,,, that the observation of the phase transition with the bare eyes seems to be more reliable to us than automatic methods that, e.g., monitor the turbidity. In the present study, no signs of decomposition or chemical reaction of the components were observed.…”

“…The synthetic method was applied for the preparation of the samples, followed by characterization with the cloud point method. As the protocol of the sample preparation was improved during the last years, ,, the reader is referred to the most recent papers on this topic, , which describes the actual technique in sample preparation for LLE mixtures and the specific handling of n -alkane–ethanol mixtures . In brief, sample sizes of 0.5–2 g end up in overall expanded uncertainties ( k = 2) in the mole fraction of U ( x C16 ) = 2 × 10 –4 or an even 1 order of magnitude higher precision for larger sample sizes of 10–30 g. The influence of unknown and uncontrollable traces of impurities in the components cannot be estimated.…”

“…The samples were prepared by weighing in Duran glass tubes and sealed with poly­(tetrafluoroethylene) (PTFE)-coated silicon rubber and temperature-resistant poly­(butylene terephthalate) (PBT)-caps (Duran Group, Mainz, Germany). They allow for convenient handling in a temperature range of T = 230–400 K so that the standard procedure of cloud point detection described in the previous studies can be applied here. ,, For a detailed description of the liquid baths and thermostats, consider refs , , .…”

“…To provide reference data and to gain a full understanding of the thermodynamics of surrogate fuels, we have recently started a systematic study on the characterization of binary alkane–ethanol mixtures . The present report also is not simply a continuation of this work but an extension toward realistic fuels and their blends.…”

“…As a first step, we have reinvestigated the liquid–liquid behavior of mixtures of n -hexadecane and its isomer 2,2,4,4,6,8,8-heptamethylnonane, each with ethanol. The ternary system formed by these substances serves as a test-fuel for processes on the basis of diesel or biodiesel (American Society for Testing and Materials (ASTM) D613). ,, In the literature, some studies have been reported; they focused on the miscibility of either the binary n -hexadecane–ethanol mixtures, ,− including the isomeric 2,2,4,4,6,8,8-heptamethylnonane or the ternary mixture . Some of the reports consider both the liquid–liquid and the solid–liquid behavior and its description by sophisticated thermodynamic or theoretical approaches. , …”