Modelagem do ponto crítico de misturas multicomponentes presentes na produção de biodiesel

Abstract

Biodiesel is a mixture of alkyl esters and it is industrially obtained by transesterification of vegetable oils or animal fats with a short-chain alcohol usually in the presence of an alkali or acid catalyst. However, the presence of these catalysts causes various inconveniences to the process. The transesterification with alcohol in supercritical conditions has been successfully evaluated since it is able to minimize these limitations and provide high yields. The addition of various co-solvents has been reported aiming the increase of the solubility of the alcohol/oil mixture and the reduce of operational temperature and pressure. However, the large number of variables which influence the production of biodiesel in supercritical conditions makes the assessment of this process very complex. Although some pure substances involved in the transesterification do not have critical properties experimentally defined, modeling studies employing cubic equations are very important since experimental works require costly equipment, long time and high reagents costs. In view of the exposed, the present master thesis aimed to determine critical points of binary mixtures (alcohol + co-solvent, ester + co-solvent), pseudo-binary (oil + alcohol) pseudo-ternary (oil + alcohol + co-solvent), pseudo-quaternary (oil + alcohol + ester + glycerol) and pseudo-quinary (oil + alcohol + co-solvent + ester + glycerol) present in biodiesel production. Various co-solvents such as carbon dioxide, propane, hexane, cyclohexane and toluene were tested. The method of construction of the bubble point curve using the cubic equation of state Peng-Robinson, Soave-Redlich-Kwong and van der Waals mixing rules was used. In addition, binary interaction parameters were determined of a variety of mixtures and correlations were developed as function of the system temperature and critical properties of pure substances. According to the results, the addition of propane and carbon dioxide in the pseudo-ternary reaction mixture significantly reduced the critical temperature (Tc) of the system, however, a significant increase in critical pressure (Pc) was verified especially when CO2 was added. The addition of hexane, cyclo-hexane and toluene promote the simultaneously reduction of Tc and Pc. For the pseudo-quaternary and pseudo-quinary mixtures, it was verified in the evaluation of reactive systems reported in literature that the mixture do not need, necessarily, to be in the supercritical state to obtain high yields, although the system pressure must always be above the Pc of the final reaction mixture. Thus, one possible way to define the transesterification operating region is: 0. 88 < Tr < 0.95 and 1.1 < Pr < 1.5. Regarding the binary interaction parameters, the modeling was effective since the deviations in pressure and composition were reduced using the parameter values which were obtained by regression with experimental data and by the correlations.