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

Abstract

The hydrogenation of the triglyceride oil, soya bean oil, has been studied in the temperature range 130±160°C and in the pressure range 100±600 kPa using (i) a 5% w/w Pd/C slurry catalyst and (ii) a 3% w/w Pd/Al 2 O 3 Raschig ring catalyst in a cocurrent down¯ow contactor (CDC) reactor. Separate studies of residence time distribution (RTD) were carried out in a modi®ed CDC device in order to determine dispersion numbers and dispersion coef®cients. The RTD measurements indicated that the overall¯ow was a mixture of well-mixed and plug¯ow for the unpacked CDC, so that the entry section (0±30 cm from entrance) was perfectly mixed and the remainder of the column (30±130 cm) gave predominantly plug¯ow behaviour. The introduction of random packing in the form of 13 mm Raschig rings gave rise to increased back mixing in the lower part of the CDC and the overall dispersion number increased due to liquid and gas circulation around the packing elements. Kinetic studies revealed an initial rate reaction order of 1.24±1.26 with respect to hydrogen concentration both in slurry and ®xed bed CDC reactors and is interpreted as a combination of a parallel pair of ®rst and second order reactions during the initial stages of reaction. Mass transfer coef®cients for gas absorption (k L a) and liquid±solid mass transport (k s ) were determined for both types of reactor. The k L a values lay in the range 1.0±3.33 s À1 and the liquid±solid transport resistances (X LS ) were all`1%, so that the reaction was almost totally surface reaction rate controlled. Apparent energy of activation measurements gave values of E A =49 AE6 kJ mol À1 , which is strongly indicative of surface reaction rate control involving the hydrogenation of an ole®nic double bond. The selectivity in respect of linolenate (three double bonds) removal and linoleate (two double bonds) retention was high with, for palladium, relatively low trans-isomer production (`30%). The overall selectivity was slightly, but signi®cantly, better for the ®xed bed CDC reactor and this is attributed to the greater degree of plug¯ow 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 ®xed bed CDC reactor as there is in the slurry CDC reactor and this is likely to improve selectivity in a consecutive reaction sequence. NOTATION aGas±liquid interfacial area (m À1 ) a p External particle area (m À1 ) A Column cross-sectional area (m 2 ) C Aig Gas-side concentration of A at gas±liquid interface (mol m À3 ) C Ae Equilibrium concentration of A in solution (mol m À3 ) C As Surface concentration of A (mol m À3 ) d c Column diameter (m) d p Particle diameter (m) D Dispersion coef®cient (cm 2 s À1 ) D A Diffusion coef®cients of A (H 2 ) in solvent (m 2 s À1 ) E l Liquid energy dissipation term (kgf m À2 s À ) H dispPFR Height of PFR section (m) H p Height of packed bed (m) H s Height of single phase section (m) IV Iodine value k L Gas±liquid mass transfer coef®cients (m s À1 ) k r Reaction rate constant (m...

Improved biodiesel manufacture at low temperature and short reaction time

“…When the DGCR is used as a chemical reactor for liquideliquid reactions it provided a high degree of liquide liquid mass transfer. The highest mass transfer would have occurred in the entry section just after the specially designed inlet of the column [12,16].…”

“…Intense turbulence and mixing occurs in the upper section part of the DLCR column, which gives a very large interfacial area for reaction, rapid surface renewal and high mass transfer between the liquid phases. The inlet design is critical in order to obtain the appropriate high velocity [12,16]. The liquide liquid dispersion mixture slowly moved down the fully flooded column and its level, and therefore the reaction residence time inside the DLCR column, can be controlled by manipulating the operating conditions of the system, such as the liquid inlet flowrate and the dimensions of the orifice.…”

“…The DGCR has been used for many applications where gases have been contacted with liquids, such as hydrogenation [10,12,16], the treatment of industrial waste effluent [17,18] and oxidation reactions. Previous research using the DGCR for gaseliquid systems has shown that when used as a chemical reactor it led to reaction rates which were almost entirely reaction rate controlled [16].…”

Analysis of the performance of single capillary and multiple capillary (monolith) reactors for the multiphase Pd-catalyzed hydrogenation of 2-Butyne-1,4-Diol

“…The existing length between the reactor inlet and the end of the bubble dispersion is denominated bubble dispersion height (H D ). A more detailed operation of the CDC has been reported elsewhere (Winterbottom et al, 2000). The total volume of the system was 0.01 m 3 .…”

The selective hydrogenation of butyne-1,4-diol by supported palladiums: a comparative study on slurry, fixed bed, and monolith downflow bubble column reactors

“…The selectivity towards cis-butene-1,4-diol in the monolith DBC indicates the values which were similar to those in the fixed bed DBC, but significantly better than those in the slurry DBC. Although the slurry mode DBC also has the element of plug flow behaviour [12], it seems that the characteristics of a close approach to plug flow in the monolith DBC [6], and also in the fixed bed DBC, have enhanced the selectivity of this reaction further compared with the slurry reactions.…”