Use of the surface methodology in the thermodynamic evaluation of glycerol supercritical water gasification systems

Abstract

There is strong interest in the use of glycerol, a by-product of the biodiesel production, as feedstock for hydrogen production through reforming processes such as the supercritical water gasification reaction (SCWG). In this work, the response surface methodology was applied to the results of glycerol SCWG simulations, with the aim of analysing the conditions which enhance hydrogen formation. The simulations were carried out with the aid of the software GAMS 23.9.5, through a non-stoichiometric thermodynamic model based on Gibbs energy minimization. Those simulations presented the final compositions of the gas phase for a range of operational conditions. The three variables whose influence was analysed were: temperature (from 617.16 K to 1182.84 K), pressure (from 231.72 to 288.28 atm) and initial quantity of glycerol (from 0.0189 to 0.2311 mol). Initially, three central composite rotational designs (CCRD) were done, each one with two independent variables and two levels for each of those variables. The statistical analysis of the results was done with the aid of the software TIBCO® STATISTICA™, which presented the response surfaces, in addition to the mathematical expressions of the obtained models and their respective coefficients of determination. It was verified that the influence of the pressure on the quantity of hydrogen produced was not statistically significant. On the other hand, the temperature and the initial amount of glycerol had strong influence on the hydrogen formation. There was continuous increase in the hydrogen production as the temperature was risen, and higher amounts of that product were obtained when greater initial quantities of glycerol were used. Those tendencies were present in the results of the three 2² designs. The maximum quantity of hydrogen (0.9365 mol) was obtained in the simulation in which the following values were used for the independent variables: 1100 K for the temperature, 260 atm for the pressure and 0.2 mol for the initial amount of glycerol.