Numerical modelling of free energy for methanol and water mixtures for biodiesel production

Abstract

As the demand for energy is ever increasing, biodiesel has become a major alternative to fossil fuels, primarily due to the latter’s depletion rate. In methanol-water rectification, a vital part of the biodiesel production process, distillation is the preferred technique to recover methanol from water. This study aims to investigate the excess Gibbs free energy (ΔG) behavior on methanol in a water mixture, for the binary system. The study was conducted using the GROningen MAchine for Chemical Simulations (GROMACS©). The effect of several important parameters, namely, sub-atmospheric pressure, temperature and methanol concentration, were investigated on the excess Gibbs free energy of methanol in water. The simulation results were partially validated, since the experimental data was limited. The results demonstrate that the excess Gibbs free energy of methanol in a water mixture positively increases with an increase in methanol concentration, until it reaches a threshold concentration of 0.5. The positive excess Gibbs free energy of the mixture system indicates that the process is not spontaneous and requires additional energy to occur. Increasing the temperature directly increases the excess Gibbs free energy of the system. Increasing the pressure does not have a significant increase on the excess Gibbs free energy. The simulation results are comparable with the experimental results; as validated by the Pearson correlation coefficient (ρ) = +0.9982. This study is beneficial to predict the thermodynamic behavior of methanol in a water system, in order to contribute to biodiesel production process optimization.