Cyclic adsorption of glycerol over silica beds for biodiesel refining

J.C. YORI; S.A. D’IPPOLITO; V.M. BENITEZ; C.L. PIECK; C.R. VERA

Buenos Aires, Argentina

Congreso; Congreso Interamericano de Ingeniería Química y V Congreso Argentino de Ingeniería Quimica; 2006

Instituto Argentino de Petroquìmica

            A new method for the complete removal of glycerol from biodiesel streams coming out from industrial transesterification reactors is presented.  The method is posed as a “dry” alternative to the conventional “wet” methods involving water washing.  It is based on the use of silica beds and relies on the adsorption of glycerol at room temperature to retain the small amounts dissolved in the solutions of fatty acid methyl esters and adjust their content to the quality standards for biodiesel fuel.             Fresh silica has a great processing capacity and the breakthrough of the bed depends mainly on the feed rate, the concentration of glycerol and the mass of adsorbent.  In the case of the silica gel used the saturation capacity was found to be 0.13 g of glycerol per gram of silica.  If the particle diameter is 1-1.5 mm the breakthrough and saturation point almost coincide and the full capacity of the bed is used.  However industrial adsorption units should use 1/8-1/16” silica beads and they suffer from mass transfer limitations inside the pellet pores.  For a particle size of 1/8” the breakthrough point (C/C0=0.01) is located at about one half the time of full saturation.   For a glycerol concentration of 0.11-0.25% typical of biodiesel streams issuing from gravity settling tanks and an entrance velocity of 11 cm min-1, a 2 m high silica bed with 1/8” beads has a breakthrough point of 8 h and a net processing capacity of 0.01-0.02 m3biodiesel kgsilica-1.             The breakthrough curves were studied using approximate solutions to the set of differential equations.  Assuming a linear isotherm gives erroneous results; fitting the experimental breakthrough curves produces underestimated values of the Henry’s adsorption constant and of the mass transfer resistances.  Modeling the high dilution regime with the UNIFAC method gives more realistic values of the Henry’s constant (1.1 m3 kg-1).  The measured saturation capacity is close to the monolayer capacity (13-15% w/w).  These values give a Langmuir isotherm which can be well approximated by a square irreversible isotherm.  Accordingly breakthrough curves were fairly well predicted using an irreversible isotherm, a shrinking-core adsorption model and common correlations for the mass transfer coefficients.             The silica bed was succesfully regenerated eluting 2-4 bed volumes of methanol and drying with a nitrogen stream for 1 h.  TPO of fresh, regenerated and glycerol impregnated silica pellets indicates that desorption of glycerol is complete upon flushing with methanol.  In the industrial practice this volume can be recycled to the transesterification reactors with no waste of products or reactants.  Evaporation of the adsorbed methanol during regeneration/drying of the bed produced a decrease of the bed temperature.  About 200 kJ kgsilica-1 must be provided to keep the bed at a constant temperature.