In this paper, the influence of carbon and silica particle slurry concentration up to 20 g/l (4 vol%) on regime transition, gas hold-up, and volumetric mass transfer coefficient is studied in a 2-dimensional slurry bubble column. From high speed video image analysis, the average large bubble diameter, the frequency of occurrence of large bubbles, the gas-liquid interfacial area, and the large bubble hold-up are obtained. The liquid side mass transfer coefficient is calculated from the volumetric mass transfer coefficient and the gas-liquid interfacial area. The lyophilic silica particles are rendered lyophobic by a methylation process to study the influence of particle wettability. The influence of organic electrolyte (sodium gluconate) and the combination of electrolyte and particles is also studied. It is found that lyophilic silica, lyophobic silica, and lyophobic carbon particles at concentrations larger than 2 g/l (0.4 vol%) decrease the gas hold-up and shift the regime transition point (where the first large bubbles appear) to a lower gas velocity. The volumetric mass transfer coefficient increases with gas velocity, increases with electrolyte concentration, decreases with slurry concentration, and is higher for lyophobic particles. The liquid side mass transfer coefficient increases with gas velocity, bubble diameter, and is higher for lyophobic particles. A correlation for the mass transfer coefficient based on dimensionless numbers is proposed for the heterogeneous regime.

Solid foam materials combine high voidage and high surface area. These two properties are advantageous for use in chemical reactors due to the low frictional pressure drop and relatively high surface area that may be used for catalyst deposition. Hydrodynamic parameters such as liquid holdup, pressure drop, and flow regimes similar to those for packed beds, have been obtained for the gas and liquid flows through these solid foam packings. The open-celled solid foam packings used were in the range of 5–40 pores per linear inch (ppi). The regimes studied are two high liquid holdup regimes and a low liquid holdup regime (trickle flow regime). Also the flooding points for counter-current flow have been determined.

This work aims at describing quantitatively the catalytic decarboxylation of malonic acid over a 5.0wt.% Pt/graphite catalyst. The study was carried out using a slurry phase continuous flow stirred slurry reactor (CSTR) at a temperature range of 120–160°C and at a reactor pressure of 1.8MPa. The conversion of malonic acid during catalytic oxidation was found to proceed via decarboxylation to CO2 and acetic acid, and also oxidation to CO2 and H2O. No indication of deactivation of the platinum catalyst was observed at a maximum residual oxygen pressure in the reactor up to 150kPa. A reaction mechanism involving elementary steps has been suggested to explain the decarboxylation and oxidation of malonic acid. A kinetic model that accounts for both non-catalysed and catalysed decarboxylation of malonic acid has been developed and validated. The non-catalysed reaction is first order in malonic acid. The activation energies and adsorption enthalpies have been determined. The model is able to describe the experimental data adequately.

The influence of the reaction start-up procedure on the oxidation of a polyol, methyl α-D-glucopyranoside, was investigated. Results were obtained from semi-batch experiments with Pt catalysts and molecular oxygen as oxidant. Three types of reaction start-up procedures were applied with respect to the pretreatment of the catalyst slurry: reductive, oxidative, and inert. The experimental results are described by a recently developed dynamic electrochemical kinetic model. The reductive start-up results in the highest initial catalyst activity, compared with the other start-up procedures. It was found that the catalyst needs pretreatment before the reaction is started, as inert start-up resulted in no catalytic activity at all. The formation of inactive platinum oxides (i.e., overoxidation) is the main cause of catalyst deactivation under oxygen-rich conditions, for a weak reducing compound, and is independent of the start-up procedure. It also appeared that the rate of overoxidation is lower in the absence of reaction, which could be modelled with the assumption that overoxidation needs free sites to take place. The mechanism of catalyst deactivation has been verified through intermediate catalyst reactivation. The model adequately describes this reactivation step.

A model is presented to describe mass-transfer enhancement in slurry reactors by catalyst particles adhering to the gas-liquid interface. This model is a combination of the particleinterface adhesion-dehesion (PIAD) model and the gas-to-liquid-to-solid (GLS)-gas-to-solid (GS) model. The PIAD model is a dynamic description of the equilibrium between the catalyst particle adhesion and dehesion rates at the gas-liquid interface. These rates determine the average residence time of the particles at the gas-liquid interface. The GLS-GS model is a combination of the classical, resistances-in-series, GLS mass-transfer model and a direct GS mass-transfer model. The average particle residence time at the gas-liquid interface, the solid-liquid partition coefficient, and the reaction rate determine the mass-transfer rate by shuttling of the particles between the gas-liquid interface and the bulk liquid. The model parameters are determined from mass-transfer and reactivity experiments, performed with two different slurry systems and two Pd-catalyzed reactions, i.e., oxidation of glucose (aqueous liquid) and hydrogenation of alpha-methylstyrene (organic liquid), with carbon and silica catalysts in a laboratory-scale surface-aeration stirred-slurry reactor with a known flat gas-liquid interfacial area. The mass-transfer coefficient under reactive absorption conditions is higher than that under nonreactive, physical absorption conditions. Experimental and theoretical mass-transfer enhancement factors under physical and reactive absorption conditions agree well. The GS mass-transfer coefficient increases with the mixing intensity, but the GLS mass-transfer coefficient increases more, finally leading to a decrease of the mass-transfer enhancement factor with the mixing intensity. The mass-transfer model is able to predict physical and reactive mass-transfer rates as a function of the mixing intensity and catalyst concentration.

This paper investigates the influence of the catalyst support type on mass transport and reaction rate for the case of hydrogenation of α-methylstyrene to cumene in a gas inducing stirred slurry reactor and in a slurry bubble column. The reaction is carried out in the presence of 3% Pd/carbon and 3% Pd/silica catalyst particles. The lyophobicity of the two catalyst supports in the cumene slurry is found to be similar. The overall rate of the hydrogenation reaction is described by the classical transport and reaction resistances-in-series model. The rate of gas-to-liquid mass transfer is somewhat larger during reaction than without reaction. This enhanced mass transfer points to particle-to-bubble adhesion as a result of the relative affinity of both catalyst supports to the gas phase. The observed reaction enhancements are similar for both Pd/carbon and Pd/silica catalyst/cumene slurries.

Changes in the coherent standard deviation and in the average frequency of measured pressure time series with gas velocity, are proposed, as unique and unambiguous criteria to mark flow regime transitions in slurry bubble columns. In a 2-dimensional (2-D) slurry bubble column, pressure time series are measured at different gas velocities simultaneously with high-speed video recording of the gas-liquid flow. The frequency of occurrence and the average diameter of the large bubbles are determined from video image analysis. The gas velocity where the first large bubbles are detected, with an average diameter of 1.5 cm, and with a frequency of occurrence of one bubble per s, is designated as the first regime transition point (transition from the homogeneous regime to the transition regime). At this point, the coherent standard deviation of the measured pressure fluctuations clearly increases from zero. The gas velocity where the average diameter and the frequency of occurrence of the large bubbles become constant, is designated as the second regime transition point (transition front the transition regime to the heterogeneous regime). From this point onward, the slope of the coherent standard deviation of the measured pressure fluctuations clearly decreases with gas velocity, while the average frequency becomes constant. These clear changes with gas velocity in the coherent standard deviation, and in the average frequency are also demonstrated in a 3-D slurry bubble column. (c) 2005 American Institute of Chemical Engineers.

The average large gas bubble size in slurry bubble columns is estimated using a spectral analysis method applied to measured pressure time series. A pressure time series measured in a bubble column consists of local pressure fluctuations and global pressure fluctuations. The local pressure fluctuations arise from the liquid velocity fluctuations induced by the large gas bubbles and by the changes in gas holdup. The standard deviation of these local pressure fluctuations is a measure of the average large bubble size. The coherence between the pressure time series measured at the sparger and at any other location in the column is used to separate the local pressure fluctuations from the global pressure fluctuations. The global pressure fluctuations are measured instantaneously throughout the column and form the coherent part of the pressure time series. The local pressure fluctuations, which are absent at the sparger, form the incoherent part of the pressure time series. In a 2-D bubble column, a clear correlation is demonstrated between the incoherent standard deviation of the pressure time series and the average large gas bubble size obtained from video imaging. A good agreement is also found between this correlation and the 3-D model proposed by Krishna et al. for predicting the average large bubble size in 3-D (slurry) bubble columns. (c) 2005 American Institute of Chemical Engineers.

The Pt-catalysed oxidation of the alcohol methyt alpha-D-glucopyranoside to 1-O-methyl alpha-D-glucuronic acid has been studied under varying pH conditions. Two types of catalysts with different metal dispersion are used, that is, Pt on active carbon support and Pt on graphite support. The pH of the reaction medium is varied between 6 and 10. The initial reaction rate increases with an increase in pH, and so does the rate of catalyst deactivation, because of overoxidation, for both catalysts. The carbon-supported Pt catalyst gives higher initial reaction rates compared with the graphite-supported Pt catalyst.A dynamic, pH-dependent, electrochemical kinetic model is presented based on a detailed investigation of the mechanism of alcohol oxidation. The model adequately describes the observed pH effect by considering the alcohol dehydrogenation occurring in two parallel reaction steps, one playing a role under acidic conditions, which is independent of the hydroxyl concentration, and a second one playing a role under alkaline conditions, which is linearly dependent on the hydroxyl concentration. (C) 2004 Elsevier Inc. All rights reserved.

A reaction-engineering model is presented, which describes catalyst performance as a function of the catalyst activity profile, the reaction kinetics, and the degree of catalyst deactivation. With this model, the catalyst activity profile can be optimised for Pt catalysed methyl α-d-glucopyranoside (slowly-reactive) and glucose (highly-reactive) oxidations. This is done by comparing modelling results with experimentally obtained data for catalysts of different activity distributions. Experiments in a semi-batch stirred reactor showed that for methyl α-d-glucopyranoside (MGP) oxidation at oxygen partial pressures below 40kPa, egg shell catalytic activity distribution gives a higher rate of oxidation than a uniform distribution. It was also observed that with increase in oxygen concentration from 10 to 40kPa, the rate of deactivation due to catalyst over-oxidation increased dramatically. For glucose oxidation, both catalyst activity distributions give the same oxidation rate for all investigated oxygen partial pressures (5–100kPa). The developed model adequately describes the observed experimental results of both reactions. It was found that the active metal particle size has a significant influence on the catalyst deactivation for MGP oxidation; the uniform catalyst with higher dispersion shows a higher deactivation rate than the egg shell catalyst. For modelling glucose oxidation, the effect of catalyst particle-to-bubble adhesion and higher diffusivity or partition coefficient for oxygen have to be taken into account.

The kinetic rate parameters of a reaction rate model are usually estimated using reaction rate measurements obtained at intrinsic kinetic conditions, viz., without limitation of the reaction rate by mass transport, irrespective of the order of the reaction kinetics. For positive order kinetics, this is good practice and the kinetic rate parameters can be safely applied both at intrinsic and mass transport limited conditions. However, for negative order kinetics, erroneous results are obtained because the kinetic rate parameters estimated at intrinsic kinetic conditions fail to predict the reaction rate when mass transport limitation also plays a role. This is demonstrated for the simple case of Langmuir–Hinshelwood (LH) negative-order kinetics and for the, more complex, kinetics of the oxidation of methyl α-D-glucopyranoside on noble metal catalysts. It is concluded that for a negative-order reaction rate model, which is valid for a wide range of reaction conditions, the kinetic rate parameters need to be estimated both at intrinsic and mass transport limited conditions.

The adhesion of catalyst particles to the gas–liquid interface significantly influences the rate of reaction in a three-phase gas-inducing stirred slurry reactor. For the Pd-catalyzed glucose oxidation reaction at mass transport-limited conditions, the experimental reaction rate is higher for lyophobic 3% Pd/C catalyst than for lyophilic 3% Pd/SiO2 catalyst. This is attributed to a higher particle-to-bubble adhesion (PBA) of the Pd/C catalyst. The interfacial catalyst concentration is quantified by a PBA equilibrium parameter in a PBA isotherm. The classical resistances-in-series “GLS model” cannot describe the overall reaction rate. An additional gas-to-solid “GS model” is presented with a gas-to-solid mass transfer coefficient to describe the increased rate of reaction by the catalyst particles adhered to the gas–liquid interface. The PBA equilibrium parameter and the gas-to-solid mass transfer coefficient during reaction are estimated as a function of mixing intensity, oxygen partial pressure, and catalyst concentration. The combined GLS–GS model adequately describes the experimentally observed reaction rates.

This study further evaluates four mechanisms for the enhancement of gas–liquid (G–L) mass transfer [Can. J. Chem. Eng. 81 (2003) 632–639]: (1) boundary layer mixing, (2) shuttling, (3) coalescence inhibition, and (4) boundary layer reaction. The present work focuses on G–L mass transfer enhancement in a gas inducing stirred slurry reactor (GIR) in a range of mixing intensities (0.5–30 kW ml−3). Physical enhancement (mechanisms 1–3) and reaction enhancement (mechanism 4) are investigated separately by dynamic gas absorption experiments without reaction and pseudo-steady-state gas absorption experiments with reaction. Two Pd-catalysed reactions are studied: oxidation of glucose (aqueous phase) and hydrogenation of α-methyl styrene (AMS) (organic phase). The influence of lyophobic carbon particles, lyophilic silica particles, and of electrolyte on G–L mass transfer is studied. Mechanism 1 is predominant at low mixing intensity, whereas the contribution of mechanism 2 is insignificant. Carbon/silica particles and electrolyte individually increase the volumetric G–L mass transfer coefficient, which is mainly attributed to mechanism 3. Especially a combination of particles and electrolyte strongly increases G–L mass transfer. Mechanism 3 also holds at higher mixing intensity. Mechanism 4 magnifies the impact of mechanisms 1 and 3. The carbon/silica particle lyophobicity strongly influences the interaction with the G–L interface. In aqueous glucose slurry, physical enhancement (mechanisms 1 and 3) and reaction enhancement (mechanism 4) are observed. In organic AMS–cumene slurry, lyophobicity/lyophilicity affects reaction enhancement only.

Mass transfer in multiphase systems is one of the most studied topics in chemical engineering. However, in three-phase systems containing small particles, the mechanisms playing a role in the increased rate of mass transfer compared to two-phase systems without particles, are still not clear. Therefore, mass transfer measurements were carried out in a 2D slurry bubble column reactor (0.015×0.30×2.00m3), a stirred tank reactor with a flat gas–liquid interface, and in a stirred tank reactor with a gas inducing impeller. The rate of mass transfer in these reactors was investigated with various concentrations of active carbon particles (average particle size of 30μm), with electrolyte (sodium gluconate), and with combinations of these. In the bubble column, high-speed video recordings were captured from which the bubble size distribution and the specific bubble area were determined. In this way, the specific mass transfer area agl was determined separately from the mass transfer coefficient kl. Mechanisms proposed in literature to describe mass transfer and mass transfer enhancement in stirred tank reactors and bubble columns are compared. It is shown that the increased rates of mass transfer in the 2D bubble column and in the stirred tank reactor with the gas inducing impeller are completely caused by an increased gas–liquid interfacial area upon addition of carbon particles and electrolyte. It is suggested that an increased level of turbulence at the gas–liquid interface caused by carbon particles accounts for a smaller effective boundary layer thickness and an enhancement of mass transfer in the flat gas–liquid surface stirred tank reactor. However, for the carbon particles used in this study, it is rather unlikely that mass transfer enhancement takes place due to the well-known shuttle or grazing effect.

The catalytic wet oxidation of phenol was studied in a slurry phase continuous stirred tank reactor (CSTR) using platinum on graphite support as a catalyst. The investigation was carried out in the temperature range 120–180°C and at total pressure of 1.8MPa, while the phenol feed concentration was varied between 5 and 70mol/m3, and oxygen partial pressures between 0.01 and 0.8MPa. It was found that both the oxygen load and the stoichiometric oxygen excess determine the extent of oxygen coverage on the platinum surface, which influences the reaction pathways and selectivity to CO2 and H2O. A fully oxidised platinum surface resulted into catalyst deactivation (over-oxidation), which favoured the formation of p-benzoquinone and polymeric products. Whereas free platinum surface was vulnerable to poisoning by carbonaceous compounds, a fully reduced platinum surface favoured the formation of acetic and succinic acids which are difficult to oxidise. A reaction scheme for platinum catalysed phenol oxidation in liquid phase is proposed.

The catalytic performance of graphite supported platinum (5 wt.%) catalyst in liquid phase oxidation has been studied using a continuous flow stirred tank slurry reactor (CSTR) in order to determine the proper operation window. The study was carried out in a temperature range of 120–180 °C and in a total pressure range of 1.5–2.0 MPa. Other operational variables employed were oxygen partial pressure (0.01–0.8 MPa), initial phenol feed concentration (0.005–0.07 M), and catalyst concentration from 1 to 10 kg m−3. It was found that the extent of oxygen coverage on the platinum surface determines the reaction pathway and selectivity to CO2 and H2O. Complete oxidation of phenol to CO2 and H2O could be achieved at 150 °C when the reaction proceeds within the range of weight specific oxygen loads of 0.15–0.35 mol s−1 kgPt−1 and at stoichiometric oxygen excess in the range of 0–80%. The activity of the platinum catalyst remained high when the residual partial pressure of oxygen in the reactor was kept below 150 kPa. Higher residual oxygen partial pressure resulted into deactivation of the platinum catalyst (over-oxidation), which was temporary and could be reversed at reducing conditions. The formation of p-benzoquinone, followed by the formation of polymeric products was also favoured at higher oxygen load, which resulted into permanent deactivation of the platinum catalyst (poisoning). While the platinum surface was vulnerable to poisoning by carbonaceous compounds when insufficient oxygen was used, a fully reduced platinum surface favoured the formation of acetic and succinic acids which are difficult to oxidize. Higher temperatures can enhance the activity of the platinum catalyst, while at lower temperatures catalyst deactivation occurs with increased formation of polymeric products and lower selectivity to CO2 and H2O. In order to maintain the catalyst within the proper operation window, a CSTR is the preferred reactor.