A comparison of triglyceride oil hydrogenation in a downflow bubble columnusing slurry and fixed bed catalysts

The hydrogenation of the triglyceride oil, soya bean oil, has been studied in the temperature range 130-160 OC and in the pressure range 100-600 kPa u sing (i) a 5% w/w Pd/C slurry catalyst and (ii) a 3% w/w Pd/Al2O3 Raschig r ing catalyst in a cocurrent downflow contactor (CDC) reactor. Separate stud ies of residence time distribution (RTD) were carried out in a modified CDC device in order to determine dispersion numbers and dispersion coefficient s. The RTD measurements indicated that the overall flow was a mixture of we ll-mixed and plug flow for the unpacked CDC, so that the entry section (0-3 0 cm from entrance) was perfectly mixed and the remainder of the column (30 -130 cm) gave predominantly plug flow behaviour. The introduction of random packing in the form of 13 mm Raschig rings gave rise to increased back mix ing in the lower part of the CDC and the overall dispersion number increase d due to liquid and gas circulation around the packing elements. Kinetic st udies revealed an initial rate reaction order of 1.24-1.26 with respect to hydrogen concentration both in slurry and fixed bed CDC reactors and is int erpreted as a combination of a parallel pair of first and second order reac tions during the initial stages of reaction. Mass transfer coefficients for gas absorption (k(L)a) and liquid-solid mass transport (k(s)) were determi ned for both types of reactor. The k(L)a values lay in the range 1.0-3.33 s (-1) and the liquid-solid transport resistances (X-LS) Were all <1%, so tha t the reaction was almost totally surface reaction rate controlled. Apparen t energy of activation measurements gave values of E-A = 49 +/- 6kJ mol(-1) , which is strongly indicative of surface reaction rate control involving t he hydrogenation of an olefinic double bond. The selectivity in respect of Linolenate (three double bonds) removal and Linoleate (two double bonds) re tention was high with, for palladium, relatively low trans-isomer productio n (<30%). The overall selectivity was slightly, but significantly, better f or the fixed bed CDC reactor and this is attributed to the greater degree o f plug flow behaviour in the latter, despite the bed causing an increase in dispersion number. However, there is no reaction in the well-mixed section of the fixed bed CDC reactor as there is in the slurry CDC reactor and thi s is likely to improve selectivity in a consecutive reaction sequence. (C) 2000 Society of Chemical Industry.