Phase equilibrium modeling of the system CO2 + glycerol + light alcohols at high pressures

L. J. ROVETTO; A.R. VELEZ; E.A. BRIGNOLE

Eindhoven

Simposio; 27th European Symposium on Applied Thermodynamics (ESAT 2014); 2014

The capability of thermodynamic models in describing chemical reactions and phase behavior of the mixtures has become an indispensable engineering tool. The accurate interpretation of the phase behavior of mixtures and the knowledge of phase transitions enhance the understanding and optimization of chemical processes, in particular the efficiency of separation stages. The biodiesel industry has grown due to, not only environmental problems associated with global warming, but also economic issues related to the use of fossil fuels. Biodiesel is generally produced through the transesterification of vegetable oils or animal fats with (mostly) methanol by homogenous alkali-catalysis (NaOH); the reaction products are distributed in two immiscible liquid phases, the lighter phase contains mainly the fatty acids methyl esters (known as biodiesel), whereas the heavier phase contains mostly glycerol, the main byproduct of the process and the excess alcohol used in the reaction is distributed between the two phases[1]. The production of high quality biodiesel depends on the purification process of the resulting FAME. In that sense, lot of effort has been done in understanding the product distribution and the operating conditions affecting the partition coefficients of the main components, in order to obtain biodiesel within commercial specifications. As a consequence, many researchers have studied different ternary systems (fatty acid methyl esters + glycerol + methanol) to enhance the understanding in the liquid−liquid phase behavior [2,3] The large increase in biodiesel production, have led to glycerol overproduction, affecting the crude glycerol market and raising up some environmental concerns; this fact has promoted research studies proposing new applications for glycerol. Thereby, different alternatives has been developed to balance the production and demand of glycerol, contributing also to the biodiesel process become economically more feasible, by developing high value products. The purification of glycerol to appropriate levels for noblest applications seems to be an unavoidable step so far in the industry. In that sense, supercritical fluid technology is a potential interesting alternative for separation and purification. On the other hand, one novel application is the direct synthesis of 1,2-glycerol carbonate (which have several applications) from glycerol and CO2 [4]. Based on these interests, the Group Contribution Equation of State (GCA-EoS) thermodynamic model [5] is applied to represent the VLE and LLE for systems including glycerol and CO2, in presence of methanol and ethanol (which is used instead of methanol especially in Brazil for biodiesel production). The GCA−EoS model described the complex behavior of the mixtures at high pressures with satisfactory accuracy, showing a good predictive capability with a single set of updated interaction parameters.