Guaiacol and its mixtures: New data and predictive models part 1: Phase equilibrium
Introduction
The increase in the energy demand coupled with the gradual depletion of fossil fuels and the political and environmental requirements have motivated the search of new energy sources. Recently, the biomass conversion into hydrocarbon fuels has gained special attention due to its attractive characteristics in terms of availability and sustainability.
The use of bioresources in the production of energy and chemicals involves major environmental, economic and geostrategic implications. Indeed, creating new opportunities for bio-products in the field of energy, chemicals and materials should lead to a reduction of greenhouse gas emissions. The general objective behind this strategy is to ensure the protection of human beings and their environment while ensuring also the continuity of the economic activities with a reduced dependence on fossil energies. Unlike fossil fuels, bioresources are renewable, widely available and better distributed throughout the world, and their valorisation will certainly have a very positive impact at the regional level on the preservation of jobs and development of rural areas [1].
Chemical and petrochemical industries are now aiming at developing new green chemistry-based processes [2] [3], and lignocellulosic biomass (LCB) is called to play a major role as an alternative raw material. Indeed, the lignocellulosic material may be conditioned to obtain already “functionalised” molecules for specific applications, unlike hydrocarbons that require additional processing steps for the synthesis of specific chemicals.
The mixtures associated with the conversion of LCB include a large number of oxygenated molecules belonging to different families produced through the decomposition of the main constituents of LCB: cellulose, hemicellulose (polymers of sugars) and lignin (phenolic-derivatives polymers). The fast pyrolysis of LCB, for instance, produces solid, liquid (bio-oil) and gaseous fractions, in which the bio-oil is a source of several attractive compounds, including lighter components from different chemical families such as carboxylic acids, ethers or ketones and heavier compounds such as phenolic or furan derivatives. Recent studies have used guaiacol (or 2-methoxyphenol) as a model compound of bio-oil due to its molecular structure: it contains two oxygen-containing functional groups (phenolic OH- and methoxy -OCH3- groups) that facilitate its repolymerization [[4], [5], [6]].
The processing and refining of these products is needed to improve their quality and to promote their use as a fuel. Several treatments can be considered: cracking, hydrocracking, decarbonylation, decarboxylation, hydrodeoxygenation, catalytic hydrotreatment, hydrogenation, others. However, in order to carry out studies on the technological and economic potential of LCB conversion process, it is necessary to develop predictive thermodynamic models with strong theoretical basis able to deal with multifunctional molecules where complex polar and associating interactions take place.
There have been several previous studies aiming at modeling systems with phenol derivatives. Lee et al. [7] evaluated the phase equilibria of CO2 + guaiacol and the data was correlated by different equations of state (EoS): the Soave–Redlich–Kwong, the Peng–Robinson, and the Patel–Teja EoS with the van der Waals mixing rules. They all require two binary interaction parameters for a good modeling and can therefore not be called predictive. Hwang et al. [8] evaluated the phase equilibria of guaiacol + 1,2-dimethoxybenzene and guaiacol + diphenylmethane with several versions of the UNIFAC model. They were unable to predict the phase behavior of these systems, indicating that the interaction parameters of AC–OCH3 binaries had to be re-evaluated. On the other hand, good data correlation was obtained using the Wilson, the NRTL, and the UNIQUAC models (not predictive). Espinosa et al. [9] applied the Group Contribution Associating Equation of State (GCA-EoS) to predict the phase equilibria of different phenol derivative systems; however, this work did not involve the mixtures evaluated here.
The GC-PPC-SAFT EoS has been used with some success on various mixtures of oxygenated compounds [11]. The objective of this study is to evaluate the GC-PPC-SAFT model and molecular simulations using the AUA4 force field, to predict the phase equilibria of systems with guaiacol. The behavior of its mixtures with associative, non-associative, polar and apolar compounds was evaluated. Phase equilibria of mixtures of guaiacol with methane, carbon dioxide, ethanol, octanol, water, acetone, butyl acetate, n-hexadecane and other light components such as H2, CO, H2S and NH3 were evaluated in the present work. In order to evaluate the complexity of guaiacol, an analysis of its structure was also performed.
Section snippets
Materials
For carrying out the phase equilibrium experiments, the following reagents were used: methane from Air Liquide (purity ≥ 99.998%), and guaiacol from SAFC (purity ≥ 98%).
Apparatus and procedure
Experimental measurements were carried out using the static method. In an equilibrium cell, the solvent was first degassed by cryometry. The measurements were taken at a set temperature. Then known quantities of gas are introduced into the cell and the equilibrium pressure is measured. Mass and volume balances have been used in
Models used
In this work, the predictions of phase envelope were done using the molecular-based Group Contribution Polar Perturbed-Chain Statistical Associating Fluid Theory (GC-PPC-SAFT) based equation of state as well as Monte Carlo Molecular Simulation with the AUA4 force field.
Experimental results
For the system methane + guaiacol there are no experimental data from literature. Thus, measurements were carried out within this work (see Table 6), and the results are compared with models in section 5.3 below.
Molecular description
Several researchers debate on the strong inter and intramolecular hydrogen bonds (HB) of phenolic compounds such as with guaiacol (2-methoxyphenol) [[51], [52], [53], [54], [55], [56], [57]]. This is because HBs affect the total enthalpy of specific interactions between solute and
Discussion: mixtures computations
The difficulty to work with guaiacol is related to its molecular structure and strong and complex interactions (inter- and intramolecular hydrogen bonds - HBs), which can occur in antagonistic manner: aiding or opposing each other [51,52] (more details are discussed in section 4.2). We will investigate systems that could help us understand the phenomena at hand. Therefore, we shall focus on systems with water (extremely HB), with ethanol and n-octanol (moderately HB), acetone and, butyl acetate
Conclusions
In this work, new experimental data of the system guaiacol-methane were presented and compared with predictive models (GC-PPC-SAFT and Molecular Simulation).
Despite the lack of experimental data, an examination of the different systems makes it possible to apprehend the predictive capacity of the model.
The use of a SAFT-type model implies the definition of an association scheme. The molecular structure of guaiacol was discussed in that perspective: it appears that intra-molecular association
Acknowledgements
The authors wish to acknowledge the financial support of the Tuck foundation chair for thermodynamics for biofuels, and to CNPQ (246524/2012-0) for financial support.
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