Triangular phase diagrams to predict the fractionation of free fatty acid mixtures via urea complex formation

The authors previously developed a rapid and inexpensive procedure to fractionate free fatty acid (FFA) mixtures via urea complex (UC) formation. The process yields two phases in equilibrium, a solvent-rich extract phase containing unsaturated FFA and a solid raffinate phase consisting of UCs of saturated and monounsaturated FFA. It effectively removed saturated FFA, that is, palmitic and stearic acid, from an FFA mixture derived from low erucic acid rapeseed (LEAR) oil. In the present study, triangular phase diagrams were determined for the urea/ethanol/water/LEAR-FFA system at various temperatures, which delineated the phase boundary between the 1-phase and 2-phase regions. Calculations of the amounts and distribution of urea and LEAR-FFA between the two phases based on tie-lines from the phase diagram strongly agreed with experimental data. The diagram accurately predicted the temperature required to cosolubilize urea and LEAR-FFA in solvent. A plot of the percent extraction of an individual FFA species versus percent extraction of total FFA for all major FFA species yielded a universal curve describing results related to various combinations of urea, water, ethanol, and LEAR-FFA. Equations derived from the phase diagram, distribution data for the FFA species, and mass balances were combined to create a mathematical model, which accurately predicted the FFA composition of both phases. The methodology used and results obtained should aid critical evaluation of large scale UC-based FFA fractionation procedures.