Article pubs.acs.org/IECR
Application of the Step Potential for Equilibria and Dynamics (SPEAD) Method to Bioderived Esters and Acetals Abu M. Hassan,† Dung T. Vu,† Damien A. Bernard-Brunel,† J. Richard Elliott,*,‡ Dennis J. Miller,† and Carl T. Lira*,† †
Department of Chemical Engineering and Materials Science, Michigan State University, East Lansing, Michigan 48824, United States Department of Chemical Engineering, University of Akron, Akron, Ohio 44325, United States
‡
S Supporting Information *
ABSTRACT: The Step Potential for Equilibria and Dynamics (SPEAD) model, which is a combination of discontinuous molecular dynamics simulation and thermodynamic perturbation theory, has been used to study the thermodynamic equilibrium properties of potential biofuel blending compounds. Step potentials and site sizes for predicting vapor pressures and liquid densities of secondary alcohols, esters, and cyclic ethers have been optimized. Fifty two (52) compounds were simulated for either parametrization or bench-marking. Twelve (12) new groups are parametrized in this work, which are present in secondary alcohols, esters, cyclic C5 compounds, and cyclic ethers. Errors in predicted vapor pressures are generally in the range of 10%, except in the case of multifunctional cyclic compounds, where errors of 30%−70% were found. Also, bubble points are measured for a mixture of 4-hydroxymethyl-1,3-dioxolane and 5-hydroxy-1,3-dioxane, which are superimposed on the literature data and do not suggest a significant difference in the vapor pressures of the two compounds.
1. INTRODUCTION Diminishing oil supplies and increasingly stringent environmental goals have boosted the search for alternative energy sources that can be less polluting and renewable. Biofuels are under study to help address both issues. Designing a biofuel blend with desirable cradle-to-grave fuel properties still remains a challenge. Currently, most biofuels are much more monodisperse than traditional petroleum fuels, and the formulation of more-complex blends requires pure-component properties. Measurements and predictions of primary properties can be leveraged to secondary properties by integration, differentiation, or other correlation. For example, vapor pressure governs the flash point of a fuel1 and is related to its surface tension and heat of vaporization,2 which, in turn, dictate fuel evaporation and combustion inside an engine.3 Ethanol has long been used as a blending agent to gasoline, and recently, higher alcohols have gained attention, because of their higher energy content and the lower volatility required for diesel-grade fuels.4 Esters, which are the main components of biodiesel, have also been under testing for use as blending agents for jet fuels, because of their desirable combustion properties.5 However, biodiesel derived from highly saturated natural oil exhibits poor cold flow properties that limits its use in both diesel6 and jet fuel.7 Cyclic acetals derived from glycerol, which is an inexpensive byproduct of biodiesel formation through trans-esterification, have been shown to impart desirable cold flow properties to biodiesel,8 and also reduce particulate emissions from biodiesel combustion,9 without modifying its combustion characteristics. The acetals of interest are 5-hydroxy-2-methyl-1,3-dioxane and 4-hydroxymethyl-2-methyl-1,3-dioxolane. However, property data for the acetals are very limited. Molecular simulation provides a valuable method to estimate vapor pressures when experimental data are limited or © 2012 American Chemical Society
unavailable. Recently, the Step Potential for Equilibria and Dynamics (SPEAD) method has been recognized as a rapid technique for predicting the vapor pressure and heat of vaporization ∼50 times faster than conventional molecular simulations.10 SPEAD estimates the vapor pressures of hydrocarbons including aromatics,11 low-molecular-weight ethers and primary alcohols,12 and acetates13 with errors of
