Grigore Bozga

The present study assesses, using a theoretical approach, the performances of two widely used fixed-bed reactors for the ethanol-to-butadiene (ETB) one-stage (Lebedev) process: multibed adiabatic reactors (MBAR) with inter-bed heating and a multitubular reactor (MTR). The mathematical model consists of mass and heat conservation equations at catalyst particle and particle bed scales, coupled with a published kinetic model specific for a modified MgO–SiO₂ catalyst. The process simulations at the level of catalyst particle have shown that the inter-phase concentration and temperature gradients are negligible, the only physical step with a limiting influence on process kinetics being the internal diffusion. A simplified version of the reactor model was also formulated, including empirical expressions developed in the study for the calculation of internal effectiveness factors. The reactor simulations highlighted that the MBAR with inter-bed heating by injection of hot reactants provides worse butadiene (BD) yields as a consequence of reduced reaction times (due to reactant by-pass of catalyst beds). The MBAR with inter-bed heating using heat exchangers provides better performances but inferior to those of MTR if the number of catalyst beds is less than six. A preliminary optimization study for the MBAR and MTR is also presented. The results showed that the maximization of BD yield has the drawback of a low BD production rate, whereas the maximization of BD production rate corresponds to a relatively low BD yield and high ethanol and inert recycles. A trade-off between the two extreme cases can be attained by maximizing the income generated by reactor operation.

Glycerol, the by-product of the Biodiesel manufacturing from vegetal and animal fats, is an important bio-resource, which can be transformed into a significant number of valuable products, by catalytic, enzymatic, or biological technologies. This paper presents a review of published studies on the kinetics of the main catalytic transformations of glycerol into valuable products, along with information regarding the catalytic reactors experienced or proposed for these transformations. In the kinetic description of the glycerol catalytic transformations, the most used are the rate expressions based on the Langmuir-Hinshelwood-Hougen-Watson (LHHW) theory and the empirical ones, of the power law type. The published results are evidencing that, in addition to the general technical and economic advantages, the liquid phase continuous processes of glycerol transformation are more efficient, ensuring, in many cases, a higher selectivity of the transformation and a slower deactivation of the catalyst.

The dimethyl ether (DME) is an environmentally friendly fuel, that can replace succesfully the petroleum Diesel in engines, or the LPG fuel. In the last period, the world production of DME was steadily rising, with annual rates of 5-8%. The actual commercial technologies producing DME are based on the methanol etherification (the two-step process). An important research effort was expensed in last decennia, in the development of technologies transforming directly the synthesis gas into DME, without separation of the intermediary methanol (the direct or one-step process). This work presents data related to the direct DME synthesis technologies from synthesis gas, thermodynamic particularities of the chemical process and design issues of multi-tubular fixed bed catalytic reactor for gas phase DME synthesis. It is proposed a new constructive solution of the synthesis reactor, with the reactor divided in multiple zones, having distinct catalyst compositions and cooling characteristics. The simulation calculations evidenced the important increase in DME production that can be achieved by this constructive solution, as consequence of an improved control of process evolution along the catalytic reactor.

In this study, we investigated the kinetics of chitosan hydrogel formation from aqueous chitosan solutions with sodium hydroxide (NaOH) as the coagulant. Two sets of experiments were performed, one in a parallelepiped cell and the other with cylindrical chitosan solution extrudates. The coagulation occurred by the neutralization of the protonated amino groups () present in the chitosan chains, with the kinetics being controlled by NaOH transport toward the gelification zone. In this study, we confirmed the appropriateness of Fick's second law to describe NaOH transport, considering the instantaneous reaction between the NaOH and groups. The experimental data were used to determine the NaOH diffusion coefficient in gels having different chitosan concentrations. The diffusion coefficient values obtained from the cylindrical coagulation data were lower than those determined for linear coagulation because of the influences of transport geometry and gel structure, respectively. Accordingly, in fiber coagulation calculations, it is recommended to use diffusion coefficient values determined from cylindrical coagulation studies. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci.2018,135, 46062.

The combustion of methyl methacrylate (MMA) over a commercial Pt/gamma-alumina catalyst was investigated, in the lean air mixtures specific for the depollution applications. The experiments were performed at temperatures between 150 and 360 degrees C, with MMA concentrations of 460 to 800 ppmv and the gas flow rates between 200 and 300 mL min(-1). The results evidenced a negative influence of MMA concentration on the combustion kinetics. A kinetic model of the combustion process was developed, based on the Langmuir-Hinshelwood mechanism, assuming the surface reaction between adsorbed oxygen atoms and adsorbed MMA molecules as the controlling step. The rate expression included the inhibition effects of MMA and water adsorption on the process kinetics. The MMA combustion process simulations evidenced the significant influences of the bulk gas to catalyst particle mass transfer, on the overall kinetics.

Catalytic combustion of methyl isobutyl ketone (MIBK) in diluted air mixtures over a commercial Pt/Al₂O₃ catalyst was experimentally investigated in a continuous laboratory setup. The experimental data were used to develop a kinetic model of the MIBK combustion over the Pt/Al₂O₃ catalyst. Among the available kinetic theories, the Langmuir‐Hinshelwood one proved to be the most appropriate for the obtained data. The kinetic parameters of the formulated rate expression were estimated using both differential and integral methods. The adequacy of the proposed kinetic model and the estimated parameter values were tested by statistical criteria and thermodynamic consistency. The computation results indicate that the integral method, based on a heterogeneous reactor model, is the recommended approach in the estimation of the kinetic parameters for catalytic combustion processes involving volatile organic compounds.

o‐Xylene combustion in air over a Pt/γ‐Al₂O₃ catalyst was investigated in a laboratory reactor at low concentrations typical for depollution applications. The results evidenced a slightly negative influence of o‐xylene concentration on combustion rate. A kinetic model of the combustion process was developed by using the Langmuir‐Hinshelwood mechanism, assuming the surface reaction between adsorbed oxygen atoms and adsorbed o‐xylene molecules as controlling step. The rate expression includes the influences of o‐xylene and water adsorption on the active centers of the catalyst. The estimation of rate expression parameters is based on o‐xylene conversion measurements obtained under conditions free of influences of physical steps.