Selective hydrogenation of 1,5,9-cyclododecatriene in up- and down-flow fixed-bed reactors: experimental observations and modeling

The performance of trickle and flooded-bed reactors has been investigated and compared for an exothermic multi-step catalytic reaction. Selective hydrogenation of cyclododecatriene over Pd/Al2O3 has been studied in both up- and down-flow modes of operation in the same pilot reactor. In the down-flow mode, hot spots and runaway could not be avoided without diluting both catalyst bed and liquid reactant. With this diluted system, the up-flow reactor leads to a higher productivity and a much better selectivity. A non-isothermal plug-flow reactor model predicts the performances of the up-flow reactor satisfactorily, but is found to be unsuitable to the case of a trickle-bed reactor. In the latter case, the productivity was underestimated, when complete wetting of catalyst particles was assumed. On the other hand, when partial wetting effect was incorporated, the calculated selectivity was always much higher than that observed actually in a trickle bed, due to heterogeneities of liquid velocity and partial wetting (poorly irrigated zones

Kinetic modeling of reductive alkylation of aniline with acetone using Pd/Al<SUB>2</SUB>O<SUB>3</SUB> catalyst in a batch Slurry reactor

The kinetics of reductive alkylation of aniline with acetone was studied in a slurry reactor under isothermal conditions in a temperature range of 378-408 K using 3% Pd/Al<SUB>2</SUB>O<SUB>3</SUB> catalyst. Experimental data on concentration-time as well as hydrogen consumption-time profiles were obtained to study the effect of concentration of aniline, catalyst loading, and partial pressure of hydrogen. Separate controlled experiments were performed to understand the nature of the condensation reaction between aniline and acetone, which forms the Shiff's base intermediate. From the concentration-time profiles and the effect of reaction conditions, it was found that the noncatalytic equilibrium formation of the Shiff base intermediate was the slowest step in the multistep reaction sequence. Several rate equations were considered to fit the batch slurry reactor data, and rate models based on competitive dissociative adsorption of hydrogen and the reactive substrates in the rate-limiting catalytic steps were found to represent the experimental data. The kinetic parameters were evaluated by fitting the integral batch reactor data at different temperatures. The activation energies, heat of adsorption, and entropy of adsorption of all the reactant species were also evaluated

A trickle-bed reactor model for hydrogenation of 2,4 dinitrotoluene: experimental verification

A trickle-bed reactor model has been developed for hydrogenation of 2,4 dinitrotoluene (DNT). This model incorporates the contributions of partial wetting and stagnant liquid hold-up effects in addition to external and intraparticle mass transfer resistances for a complex consecutive/parallel reaction scheme under consideration represented by L-H-type kinetics. As the reaction is highly exothermic, the heat effects have also been incorporated in the model. The reactor performance for complete wetting, partial wetting of catalyst particles and in the presence of stagnant liquid pockets has been compared and the significance of different parameters discussed. Experimental data were obtained for different particles sizes, different gas and liquid velocities in a temperature range 318-328 K. The model predictions were compared with experimental data and were found to agree very well for a wide range of operating conditions. The model proposed here also allowed prediction of maximum temperature rise in the catalyst bed and which was also found to agree well with the steady-state experimental data. Under certain conditions, hysteresis behaviour of the reactor performance has been observed

Hydrocarbonylation of methyl acetate using a homogeneous Rh(CO)Cl(PPh<SUB>3</SUB>)<SUB>2</SUB> Complex as a catalyst Precursor: kinetic modeling

The kinetics of hydrocarbonylation of methyl acetate to ethylidene diacetate (EDA) using the Rh(CO)Cl(PPh3)2/PPh3/MeI catalyst system was studied in a temperature range of 433-463 K. Concentration-time profiles were obtained for different reaction conditions such as concentrations of methyl acetate, methyl iodide, and catalyst and partial pressures of CO and hydrogen. Rate equations were proposed on the basis of a reaction mechanism with [Rh(CO)2I2]- as the catalytically active species. Activation energies for acetic anhydride (Ac2O) and EDA formation were found to be 80.72 and 91.79 kJ/mol, respectively. A semibatch reactor model was developed, and the effect of the carbon-monoxide-to-hydrogen ratio on the conversion of methyl acetate and selectivity to EDA is discussed

Catalytic hydrogenation of p-nitrocumene in a slurry reactor

The hydrogenation of p-nitrocumene to p-cumidine over supported palladium catalysts was investigated in a laboratory-scale slurry reactor. The primary objective was to demonstrate the methodology for development of a slurry reactor model that could predict either isothermal or nonisothermal performance using intrinsic kinetic and transport parameters that were determined from independent data and engineering correlations. Several catalysts were screened to identify a suitable one for kinetic and reaction engineering studies. Various catalyst supports, such as alumina, calcium carbonate, and activated carbon, as well as reducing agents used during the catalyst preparation, including hydrogen, sodium formate, and formaldehyde, were investigated. A 1 wt % palladium-on-alumina catalyst was identified as the preferred catalyst because it had both superior activity and selectivity. The effects of hydrogen pressure, catalyst loading, and the initial concentrations of p-nitrocumene, water, and p-cumidine on the initial rate of hydrogenation and the concentration-time profiles were also studied in a batch reactor. The initial rate data showed that both the kinetic and mass-transfer resistances were important for temperatures greater than 353 K, while the kinetic regime was controlling at lower temperatures. A Langmuir-Hinshelwood (L-H) model was proposed based on the rate data in the kinetic regime. The rate model was suitably modified to account for combined transport-kinetics resistances above 353 K. Using a basket reactor, intraparticle diffusion effects were also studied by transforming the catalyst powder used for the kinetic study into catalyst pellets. Equations for an overall catalyst effectiveness factor were derived for the L-H type rate model. The experimental data for different catalyst particles agreed well with the theoretical predictions. To verify the applicability of the kinetic model over a wide range of conditions, a slurry reactor model was also developed for both isothermal and nonisothermal conditions. The predicted concentration versus time profiles were in excellent agreement with the experimental results using model parameters that were independently determined from experiments or correlations

Modeling of hydrogenation of maleic acid in a bubble-column slurry reactor

A bubble-column slurry-reactor model has been developed for the hydrogenation of aqueous maleic acid (MAC) to tetrahydrofuran (THF). This particular reaction system has recent commercial relevance and represents a case where complex multistep catalytic hydrogenation reactions are conducted at high pressure (&gt;15 MPa) and high temperature (&gt;240&#176;C). It also has additional complexities associated with the reaction chemistry, since the THF reaction product is volatile and the reaction is highly exothermic. The proposed model is derived using the mixing cell approach and incorporates the contributions of gas-liquid and liquid-solid mass transfer, intraparticle diffusion effects, product volatility, heat effects, and complex multistep reaction kinetics. The effect of gas and liquid velocities, catalyst loading, inlet maleic acid concentration, and temperature on the conversion, selectivity, temperature rise, and productivity of the desired products (THF and &#947;-butyrolactone (GBL)) is also discussed. Since the reaction step involving the hydrogenation of GBL to THF is relatively slow, severe operating conditions are necessary to achieve high THF selectivity. The distribution pattern of THF in the gas and liquid phases is also discussed. The model proposed could be useful for simulation of existing pilot- or industrial-scale reactors, as well as the design and scale-up of new reactors for this particular reaction or one that has similar characteristics

Hydrogenation of 2,4-dinitrotoluene Using a Pd/Al<SUB>2</SUB>O<SUB>3</SUB> catalyst in a slurry reactor: a molecular level approach to kinetic modeling and nonisothermal effects

The kinetics of hydrogenation of 2,4-dinitrotoluene (2,4-DNT) using a 5% Pd/Al<SUB>2</SUB>O<SUB>3</SUB> catalyst was studied in a semibatch slurry reactor in a temperature range of 323-363 K. Experimental data on the concentration-time and H<SUB>2</SUB> consumption-time profiles were obtained, and the effects of 2,4-DNT concentration, H<SUB>2</SUB> pressure, and catalyst loading were studied under both isothermal and nonisothermal conditions. A fundamental approach based on a molecular level description of the catalytic cycle has been used to derive the rate models. Several rate forms were derived considering different types of interactions, but the rate equations derived assuming that the reaction between the transient molecular species formed due to the interactions of H<SUB>2</SUB> and liquid phase components on different sites of Pd catalyst were found to best represent experimental data. The overall hydrogenation rate was found to vary by approximately second order with respect to catalyst loading, and this trend is adequately explained by the kinetic model proposed. It was found that the intraparticle diffusional effects were important for particle sizes (d<SUB>p</SUB>) &gt; 3 &#215; 10<SUP>-4</SUP> m, but the external mass-transfer (gas-liquid and liquid-solid) effects were unimportant. For a complex rate equation observed, an approximate expression for the overall effectiveness factor was derived and the experimental data for different particle sizes were found to agree with the predictions of the model incorporating intraparticle diffusion effects. Under certain conditions, a significant temperature rise was observed and the increase in temperature was found to vary with time and the initial set of conditions. A mathematical model to predict the temperature and concentration profiles in a semibatch reactor under nonisothermal conditions has been proposed. A comparison of the experimental data with model predictions showed an excellent agreement