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The poisoning resistance to sulfided and oxygenated compounds of some VIII Group PYGAS selective hydrogenation catalysts based on metals was assessed. Low content alumina supported Rh, Pd, Ru and Pt catalysts (0.35 wt%) were prepared from chlorided precursors. In the case of the palladium catalysts a nitrogenated precursor was also used. The catalysts were mainly assessed in the catalytic test of selective styrene hydrogenation in the presence or absence of known poisons. Model feedstocks spiked with thiophene, thiophane and tetrahydrofuran were used. The catalysts were further characterized by means of chemical analysis, XPS, TPR and chemisorption. The results indicate that chlorided precursors yield more sulfur resistant catalysts. The effect was attributed in part to the formation of oxychlorinated species, refractory to reduction, that leave the metal in an electron deficient state, thus inhibiting the formation of strong poison-metal bonds, the chloride species could also be a steric factor that can contribute to the sulfur resistance of the catalyst. Pd based catalyts had the highest activity and resistance to poisons of all the metals tested. This superior performance was attributed in part to the total occupancy of the 4d electronic levels of the Pd metal that was supposed to promote the rupture of the H 2 bond during the hydrogenation reaction. Keywords Low metal loading catalysts Á Selective hydrogenation Á Sulfided and oxygenated poisons List of Symbols PF Poison free condition TE Thiophene TA Thiophane THF Tetrahydrofuran D Metal dispersion S Metal specific surface (m 2 g met -1 ) d Average metal particle size (Å ) V HAds (CNPT) Volume of hydrogen chemisorbed at normal conditions (cm 3 ) PA Atomic weight of the metal (g mol -1 ) w Metal content per unit gram of catalyst M Mass of sample (g) m Stoichiometry of chemisorption of the gas on the surface metal atoms V m CNPT Volume of the chemisorbed monolayer of the adsorbate at normal conditions (cm 3 g -1 ) N Avogadro's number (6.023 9 10 23 ) V N Molar volume of the adsorbate at normal conditions r Number of metal atoms per square meter (atoms m -2 ) xAdsorption stoichiometry (number of hydrogen atoms adsorbed per unit of metal atom) qDensity of the metal (g m -3 ) M Red /M Tot Reduced metal amount and total metal amount ratio wt M% Nobel metal content (weight percentage) T Red Reduction temperature (K) r°E S Initial styrene hydrogenation rate in poison free condition (mol g cat -1 min -1 ) DESRelative deactivation per concentration of poison (ppm -1 )

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A simple method for the almost complete removal of glycerol from methanol-free 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 at room temperature to retain the small amounts of glycerol 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 with 1/8 in. silica beads suffer from mass-transfer limitations inside the pellet pores, and for this particle size, the breakthrough point (C/C 0 = 0.01) is located at about one-half of 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 in. beads has a breakthrough point of 8 h and a net processing capacity of 0.01−0.02 m3 biodiesel 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 experimentally measured saturation capacity is close to the monolayer capacity (13−15% w/w). These values give a Langmuir isotherm which can be fairly 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 4 bed volumes of methanol and drying in a nitrogen stream for 1 h. Temperature programmed oxidation tests of fresh, regenerated, and glycerol impregnated silica pellets indicated that desorption of glycerol was practically complete. In the industrial practice, the eluted volume can be recycled to the transesterification reactors with no waste of products or reactants. Evaporation of the adsorbed methanol during drying of the bed produced a decrease of the bed temperature and about 200 kJ kgsilica -1 should be provided in order to maintain the temperature.