MODELING OF THE PHASE EQUILIBRIA RELEVANT TO THE BIODIESEL PRODUCTION AND PURIFICATION PROCESSES WITH THE CUBIC-PLUS-ASSOCIATION (CPA) EQUATION OF STATE
Authors: Mariana B. Oliveira, António José Queimada, João A.P. Coutinho, CICECO, Chemistry Department, University of Aveiro, Aveiro, Portugal, and others
Abstract: As a consequence of a range of environmental, economical and political problems related to the use of conventional petroleum based fuels, several countries are now focusing their attention on alternative fuels. Biodiesel is at the forefront of the alternatives to petroleum based fuels in the transportation sector, being considered as an important short-time option as its prices can be similar and no motor changes are required. Among the various approaches to produce biodiesel, basic catalyzed transesterification is the preferable method. After the transesterification reactor purification steps are always necessary in order to provide the fuel with the quality levels required by the standards for alternative fuels, reducing engine problems and consequently increasing consumers‘ acceptance. Among the specifying minimums are the water, the alcohol and the glycerol contents. Being able to correctly describe the phase equilibria of systems of interest for the biodiesel purification processes in a broad range of thermodynamic conditions is a necessary condition for a correct simulation of the industrial process, in order to accomplish high productivity and low operating costs. The use of oxygenated molecules as fuels represents a significant change in terms of solution thermodynamics. While for petroleum-based fuels, cubic equations of state and classical activity coefficient models have proved to be appropriate, new fuels, such as biodiesel, add other type of intermolecular interactions not covered before, such as hydrogen bonding. To be able to represent such a larger set of interactions more complex engineering thermodynamics models are required. In this chapter it will be shown how the CPA EoS (Cubic – Plus – Association Equation of State) can be used for an adequate description of different phase equilibria of several binary and multicomponent systems containing water, fatty acids, fatty acid esters, glycerol and alcohols.