The gas-solid trickle-flow reactor for the catalytic oxidation of hydrogen sulphide: a trickle-phase model

The oxidation of H2S by O2 producing elemental sulphur has been studied at temperatures of 100–300°C and at atmospheric pressure in a laboratory-scale gas-solid trickle-flow reactor. In this reactor one of the reaction products, i.e. sulphur, is removed continuously by flowing solids. A porous, free-flowing catalyst carrier has been used which contains a NaX zeolite acting as a catalyst as well as a sulphur adsorbent. In order to describe mass transfer in the trickle-flow reactor, a reactor model has been developed in which a particle-free, upflowing gas phase and a dense, downflowing gas-solids suspension, the so-called trickle phase, are distinguished. Within the trickle phase, diffusion of the reactants parallel to reaction in the catalyst particles takes place. The mass transfer rate from the gas phase to the trickle phase has been evaluated by the reaction of H2S with SO2, which is a much faster reaction than the reaction with O2. From the experiments and from the reactor model calculations it appears that for the H2S-O2 reaction no mass transfer limitations occur at temperatures up to about 200°C, whereas at 300°C gas-phase mass transfer and diffusion within the dense solids suspension offer resistance to reaction

Selective hydrogenation in trickle-bed reactor. Experimental and modelling including partial wetting.

A steady state model of a trickle bed reactor is developed for the consecutive hydrogenation of 1,5,9-cyclododecatriene on a Pd/Al2O3 catalyst. Various experiments have shown that the selectivity of this reaction towards the product of interest is much lower in co-current down-flow (trickle-bed) than in up-flow. This is due to uneven liquid distribution and to partial wetting of the catalyst surface at low liquid flow rates. The non-isothermal heterogeneous model proposed here takes into account the partial wetting of the catalyst, as well as the resistances to heat and mass transfer at the gas-liquid, liquid-solid and solid-gas interfaces. It assumes that the catalyst particles can be divided into two distinct concentration zones corresponding to the wetted and dry catalyst surfaces; mass transfer between these two zones is described by a simplified diffusion mechanism. Compared to previous models assuming a uniform concentration of liquid-phase components inside the catalyst particles, this model improves the prediction of the outlet concentrations of hydrogenation products

Mass transfer in a gas-solid packed column at trickle flow

The height of an overall transfer unit has been evaluated in a gas—solid packed column at trickle flow by measuring column performance during steady state adsorption experiments. Results have been interpreted with an extraction model: mass transfer and axial dispersion in both phases. Using Bodenstein numbers for the gas and solid phases from a previous investigation the height of a true transfer unit has been calculated.\ud \ud The column was filled with dumped Pall rings, the solid phase was a freely flowing catalyst carrier, and the gas phase was air at ambient conditions containing freon-12 as adsorbing component.\ud \ud At low gas velocities column performance is entirely determined by axial dispersion but at higher gas velocities mass transfer limitations become important. For conditions of practical importance the height of a true transfer unit corresponds to 4 – 9 Pall ring layers

ZnO/ionic liquid catalyzed biodiesel production from renewable and waste lipids as feedstocks

A new protocol for biodiesel production is proposed, based on a binary ZnO/TBAI (TBAI = tetrabutylammonium iodide) catalytic system. Zinc oxide acts as a heterogeneous, bifunctional Lewis acid/base catalyst, while TBAI plays the role of phase transfer agent. Being composed by the bulk form powders, the whole catalyst system proved to be easy to use, without requiring nano-structuration or tedious and costly preparation or pre-activation procedures. In addition, due to the amphoteric properties of ZnO, the catalyst can simultaneously promote transesterification and esterification processes, thus becoming applicable to common vegetable oils (e.g., soybean, jatropha, linseed, etc.) and animal fats (lard and fish oil), but also to waste lipids such as cooking oils (WCOs), highly acidic lipids from oil industry processing, and lipid fractions of municipal sewage sludge. Reusability of the catalyst system together with kinetic (Ea) and thermodynamic parameters of activation (∆G‡ and ∆H‡) are also studied for transesterification reaction

Green process for adipic acid synthesis: oxidation by hydrogen peroxide in water micromelusions using Benzalkonium Chloride C12-14 surfactant

Adipic acid was synthesized by the oxidation of cyclohexene using 30% hydrogen peroxide in a microemulsion in the presence of sodium tungstate as catalyst. The proposed green process is environmentally friendly since catalyst and surfactant are recycled and pure adipic acid is produced in high yield (70% to 79%). Microemulsions are used as a “green solvent” and give a better contact between the phases. Alkyldimethylbenzylammonium chloride (C12-C14) was used as a surfactant for the generation of the microemulsion since it enables the use of harmful organic solvents and phase-transfer catalysts to be avoided. Optimised operating conditions (temperature, reaction time, separation process) have been defined and applied to evaluate the industrial practicability. The main interest of the present work is the easy recovery of pure adipic acid and the reuse of the reaction media (surfactant and catalyst). This shows promise for developing a future green industrial process that will enable greenhouse gas emissions (N2O), among others, to be reduced

Determination of mass transfer resistances of fast reactions in three-phase mechanically agitated slurry reactors

A methodology for the determination of mass transfer resistances of fast reactions in three-phase mechanically agitated slurry reactors under the reaction conditions is presented. The mass transfer resistances affect significantly the overall mass transfer rate, the design equation and consequently the scale up of the reactor. There is not established methodology to separate the mass transfer resistances under reaction conditions by changing catalyst loading and manipulating the process variables, pressure and agitation speed. This allows to avoid the use of different catalyst particles and give the chance to calculate the mass transfer resistances without caring about the type of catalyst. We calculate each mass transfer resistance under conditions which do not allow to neglect any of the resistances. It is shown that the level off of mass transfer rate which is developed in the plot of mass transfer rate against agitation speed plots is not enough to determine the limiting regime. The hydrogenation of styrene over Pd/C (5% catalyst content) is used as case study to demonstrate the methodology

Rapid syntheses of difluorinated dihydropyrans

A very short reaction sequence opens with metal-mediated addition of commercial bromodifluoropropene to aldehydes; allylation under phase transfer catalysed conditions sets the stage for a ring closing metathesis (RCM) in the presence of commercial Grubbs’ catalyst to afford potentially useful difluorinated dihydropyrans

Transfer dehydrogenation of 1-Phenylethanol over Pd/C under mild conditions: effect of reaction conditions and optimization of catalytic performance

The catalytic activity of 5 wt% Pd/C has been evaluated using the liquid phase transfer dehydrogenation of 1-phenylethanol as a model reaction. The reaction parameters such as catalyst loading and stirring rate have been optimized to determine the required conditions to carry out the reaction under kinetic regime control. By performing the reaction under different temperatures, the value of apparent activation energy has been determined as being 61 kJ/mol. Furthermore, the influence of thermal treatment of 5 wt% Pd/C catalyst on its catalytic performance for the liquid phase transfer dehydrogenation of 1-phenylethanol has been investigated in a temperature range of 110–200 °C. The results reveal that the catalyst activity is strongly dependent on the ratio between Pd/PdO species. The fresh and used catalysts were characterized using a range of characterization techniques (XRPD, XPS, TEM, SEM-EDX, and BET) in order to investigate structure–activity relationships. The 5 wt% Pd/C exhibit high conversion (90%) and selectivity (91%) toward acetophenone under mild conditions. Moreover, the reusability tests have been carried out, and the results show that the catalyst preserves 80% of its initial catalytic activity after five cycles indicating the high stability of the 5 wt% Pd/C catalyst in the liquid phase transfer dehydrogenation of 1-phenylethanol. The influence of reaction conditions on the catalytic activity is also discussed