Elsevier

Fluid Phase Equilibria

Volume 472, 25 September 2018, Pages 107-116
Fluid Phase Equilibria

Quaternary isothermal vapor-liquid equilibrium of the model biofuel 2-butanone + n-heptane + tetrahydrofuran + cyclohexane using Raman spectroscopic characterization

https://doi.org/10.1016/j.fluid.2018.04.009Get rights and content

Highlights

  • Phase behavior of 2nd generation model biofuel blend investigated.

  • Efficient, non-invasive analysis of quaternary VLE using Raman spectroscopy.

  • Modeling of the phase behavior using PCP-SAFT equation of state.

  • Good agreement of binary phase equilibrium data with literature.

  • Good prediction of quaternary VLE based on pure substance and binary mixture data.

Abstract

Vapor-liquid equilibrium data of fuel mixtures are of major importance for both fuel production as well as combustion. However, the measurement of vapor-liquid equilibrium data usually requires significant experimental effort. The experimental effort is particularly high if multicomponent mixtures are of interest, as experimental effort rises strongly with a rising number of components. In this work, we efficiently characterize the vapor-liquid equilibrium of a quaternary model biofuel and its binary subsystems. For this purpose, we employ the recently developed milliliter-scale Raman Spectroscopic Phase Equilibrium Characterization (RAMSPEQU)-setup. Vapor pressures are collected at T = 283–333 K, and isothermal pTx-data for mixtures at T = 303.2 K resulting in a pressure range of p = 2.8–81.9 kPa. The PCP-SAFT equation of state is used for thermodynamic modeling. Our binary data agrees well with experimental data from literature. The quaternary phase behavior is predicted with very good accuracy using PCP-SAFT with parameters adjusted to pure substance and binary mixture data only. The milliliter-scale setup allows us to characterize the phase equilibria with just 22 ml (binary) and less than 105 ml (quaternary) of the respective mixtures. The agreement of predicted and experimental quaternary phase equilibrium data indicate the reliability of the employed method for multicomponent vapor-liquid equilibrium measurements.

Introduction

To reduce greenhouse gas emissions and to overcome the depletion of fossil resources, 2nd generation biofuels are of major interest [1]. 2nd generation biofuels avoid the table-or-tank conflict by e.g. utilizing agricultural residues or waste instead of sugars or vegetable oils [2]. Furthermore, such tailor-made biofuels were shown to not only allow for the integration of a renewable carbon source in production but also to be beneficial in combustion engines by reducing soot and NOx emissions [[3], [4], [5], [6]]. To optimize fuel properties, several components are usually blended into a tailored fuel mixture. The suitability of a fuel mixture for combustion [7] as well as fuel production [8] depends on vapor-liquid equilibrium (VLE) behavior.

However, the characterization of VLE usually requires significant experimental effort, in particular, if multicomponent mixtures are of interest: with a rising number of components, the number of experiments increases strongly. As a result, the amount of substances increases as well. For characterization of compositions, sophisticated analytical techniques are necessary that often require sampling. However, sampling is invasive: as the equilibrium can be disturbed, sampling is a well-known error source [9]. To efficiently characterize VLE, we recently developed the spectroscopic milliliter-scale VLE-setup Raman Spectroscopic Phase Equilibrium Characterization (RAMSPEQU) [10]. RAMSPEQU uses Raman spectroscopy for the non-invasive quantitative characterization of the mixture composition. The non-invasive characterization of mixture compositions avoids sampling such that phase equilibria can reliably be characterized based on less than 3 ml of sample. The RAMSPEQU setup allows for fast VLE characterization using reduced sample size. Consequently, RAMSPEQU should be particularly suited for the characterization of multicomponent VLE. However, the method has only been applied to binary mixtures till now [10].

In this work, we now characterize the VLE of a quaternary model-fuel and its binary subsystems using the RAMSPEQU-setup. The model-fuel represents a blend of two bio-based 2-butanone and tetrahydrofuran (THF) with two fossil components, n-heptane and cyclohexane. 2-butanone has recently been identified as a promising biofuel-candidate due to favorable combustion characteristics [5]. THF is the simplest hydrofuran and is used here to represent the hydrofuran family. Several furans and hydrofurans have been identified as promising biofuel-candidates: 2-methylfuran was shown to reduce hydrocarbon emissions and increase efficiency in comparison to model gasolines [11]; 2-methyltetrahydrofuran was shown to reduce or even eliminate soot formation in certain blends [6]. THF has received attention as a model-fuel, e.g. in combustion kinetics [12].

To represent linear fossil hydrocarbons in fuels, we chose n-heptane. n-Heptane is often used as a model-fuel for several combustion modes, e.g. jet-fuel, homogeneous charge compression ignition (HCCI)-fuel, and diesel-fuel [13,14]. To represent cyclic fossil hydrocarbons in fuels, we chose cyclohexane which is also often used as model-fuel [[15], [16], [17]].

The RAMSPEQU-setup is employed to determine isothermal VLE data for 2-butanone + n-heptane + THF + cyclohexane and its binary subsystems. For thermodynamic modeling, we use the perturbed-chain polar statistical associating fluid theory (PCP-SAFT) equation of state (EoS) [[18], [19], [20]].

In Section 2, the experimental setup and data collection procedure are briefly described. Additionally, we show the essentials of the PCP-SAFT EoS. Experimental and modeling results are discussed in Section 3. Our study is summarized in Section 4.

Section snippets

Materials

Experiments were carried out with 2-butanone HiPerSolv CHROMANORM and cyclohexane SPECTRONORM purchased from VWR (Germany), n-heptane Uvasol and tetrahydrofuran (THF) Uvasol purchased from Merck Millipore (Germany) (Table 1). All chemicals were used as received and no additional purification steps were performed.

Density measurements

Density measurements were carried out according to the oscillating U-tube principle using the DMA 4500M apparatus (Anton Paar GmbH). Experimentally determined densities are presented in

Density measurements

We determine liquid densities at a temperature of T = 298.15 K and a pressure of p = 0.1 MPa using a commercial apparatus (DMA 4500M, Anton Paar GmbH). Density measurements are solely performed for the estimation of PCP-SAFT parameters (Table 4).

The experimental liquid density data are listed in Table 2. The densities measured in this work agree well with data from literature [[26], [27], [28]] (see Table 2).

Pure substance vapor pressures

Pure substance vapor pressures are measured using the RAMSPEQU-setup. The vapor

Conclusions

The suitability of the milliliter-scale Raman Spectroscopic Phase Equilibrium Characterization (RAMSPEQU)-setup [10] is demonstrated for the characterization of the VLE of a quaternary model biofuel. New isothermal VLE data of the quaternary system comprised of 2-butanone + n-heptane + THF + cyclohexane and its binary subsystems at 303.2 K are presented.

PCP-SAFT is used for thermodynamic modeling. We find a good quality of fit for the description of the binary subsystems. For comparison, the

Competing interests

The authors declare that they have no competing interests.

Acknowledgements

The authors thank Sebastian Kaminski for support with the in-house software used for all EoS calculations performed in this work and for proof-reading the manuscript.

This work was performed as part of the Cluster of Excellence “Tailor-Made Fuels from Biomass” funded by the Excellence Initiative by the German federal and state governments to promote science and research at German universities.

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