1,721,046 research outputs found
Deterministic modeling and stochastic simulation of poly-alkoxylation reactions
No full textThe main scope of this work is to show the feasibility and the advantage of using a stochastic approach to describe the poly-alkoxylation kinetics of different substrates. For this purpose, the reactions of ethylene and propylene oxides with respectively ethylene glycol, 1-octanol, and 2-octanol were considered. Two kinetic models were used for interpreting all the kinetic runs available in the literature, one deterministic and another one stochastic, for a useful comparison between the two different approaches. As the adopted reaction mechanism, rate laws, and related kinetic parameters were the same for both the kinetic models, the obtained results for what concerns the substrate consumption, and the oligomers distribution profiles were the same in both cases. In the case of the stochastic kinetic approach, the calculations must be made on a small volume of the reacting mixture containing a sufficiently high number of molecules that is suitable for the statistical analysis but as small as possible for reducing the calculation time. The calculations made have allowed individuating this optimal volume. This study is propaedeutic to the application of a stochastic kinetic approach to the study of ethylene-propylene oxides copolymerization that cannot be faced with a deterministic model for the extremely long or impracticable calculation time due to the great number of material balance differential equations that must be integrated
Special Issue on “Industrial Chemistry Reactions: Kinetics, Mass Transfer and Industrial Reactor Design”
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Influence of the vapor-liquid equilibria (VLE) on the kinetics in gas-liquid and gas-liquid-solid systems
No full textGas-liquid and gas-liquid-solid reactors are frequently characterized by the presence of components, reagents or products, that are partitioned between liquid and vapor phase. In this case we have a difficulty in describing the kinetic behaviour of such type of reactors when it occurs that: (i) the amount of partitioned component in the two phases is never negligible in both phases; (ii) the vapor-liquid equilibrium could have a non-ideal behaviour. We need therefore, to introduce in the kinetic model the mass balance equations for describing the partition of the components affecting their concentration in liquid phase also considering, when necessary, the non-ideal behaviour of the involved phases. Vapor-liquid equilibria (VLE) in reactive systems are poorly treated in the literature, especially for reactions occurring at high pressures. In the present work we will examine the different possibilities occurring in practice and the methods to solve them. Some practical examples are reported for better explaining both the theoretical and practical approach. © 2003 Elsevier Science B.V. All rights reserved
Industrial Chemistry Reactions: Kinetics, Mass Transfer and Industrial Reactor Design (II)
No full textMonte Carlo Approach to the Simulation of Ethylene Oxide and Propylene Oxide Polymerization and Copolymerization
No full textThe simulation of block or random ethylene oxide (EO) and propylene oxide (PO) copolymerization with a deterministic model requires the solution of many thousands of differential and algebraic equations. This approach, therefore, is not practicable, requiring too long computer calculation time. The use of a stochastic model allows overcoming this drawback. In this work, a Monte Carlo based model has been developed considering, for a given substrate such as ethylene glycol, only the reaction of the overall consumption of the alkoxides. The addition of new units of EO or PO, respectively, to the hydroxyls of the substrate or the terminal hydroxyls of the growing chains is ruled by the stochastic probability of the two mentioned events. The developed model considers also the effect of the side reaction of PO conversion to allyl alcohol with the formation of new growing polymeric chains. To do the calculations a microscopic volume, assumed to be a well-mixed system, is considered containing about 5000 or a few more molecules, which is a number significant for statistical validity. Kinetic laws and related parameters have been estimated from data published in the literature. Examples of simulations of high-molecular-weight EO and PO homopolymers, of an EO-PO random copolymer, and a triblocks copolymer of the type (EO)a-(PO)b-(EO)a will be presented
Advances in catalysts for liquid biofuel: Production processes
No full textIn this chapter, the state-of-the-art catalysts used to produce the most common liquid biofuels are reviewed. In particular, the catalysts used to produce biofuels from either oils and fats or cellulose lignin feedstocks will be discussed. Attention will be paid to discussing the recent advances in the current catalytic technologies, showing for each the related drawbacks and possible perspectives
A kinetic biphasic approach to biodiesel process intensification
No full textansesterification of vegetable oil with methanol, promoted by alkaline catalysts, has been tested in many different reactors and the reaction time to obtain a complete conversion, in some of these reactors, resulted in the range of few seconds. By using static mixers, micro-reactors, oscillatory-flow reactors, cavitation-inducing reactors, microwave reactors or centrifugal contactors, it is possible to obtain high biodiesel yields in a much shorter time than in stirred tank reactors. As these reactors are characterized by a very high liquid-liquid interface we can conclude that higher is the interface area shorter is the reaction time. This behavior cannot be described with the monophasic kinetic model reported in the literature. Therefore, we have developed a biphasic kinetic model that is able to describe any type of reactor such as: more or less stirred batch reactors, more or less efficient continuous structured reactors, micro-reactors etc. We have also experimentally observed that it is possible to obtain a complete conversion in less than 10 seconds by using a very efficient micro-mixer followed by a void settling tube of an opportune volume size. This reactor has been simulated also with the mentioned liquid-liquid biphasic model. This model is based on a reliable transesterification mechanism and takes into account both the intrinsic kinetic laws, mass transfer and reactants and products two phases partition. Some of the unexplained kinetic aspects, observed in batch runs, like the difference in the final conversions for different catalyst concentrations have been clarified with the biphasic model. This final conversion is due to catalyst deactivation and not to chemical equilibrium as suggested in the monophasic kinetic models normally employed in the literature. In this work, the developed biphasic approach will be applied on different kinds of laboratory reactors characterized by a different micromixing efficiency. The kinetic parameters for the intrinsic occurring reactions will be given and discussed. Moreover, the role of the mass transfer effect will be considered and the consequence of the methanol and glycerol partition on the conversion and yield will be described. By concluding, the possibility to produce biodiesel in a micro-device with residence times of few seconds will be experimentally shown and theoretically interpreted
Skeletal isomerization of 1-butene to isobuten over acid catalysts obtained by grafting silicon alkoxide on γ-alumina
No full textA study was made of the preparation and properties of some acid catalysts obtained by grafting silicon alkoxides on γ-alumina. All the prepared catalysts were tested in the skeletal isomerization of 1-butene to isobutene. Isobutene is an intermediate in the production of MTBE, today the most important oxygenated compound used as an additive in lead-free gasoline. The discussion covers introduction; experimental (catalyst preparation and characterization); results (determination of the catalyst surface properties by potentiometric titrations and TGA analyses; TPD determination of acid surface properties; XPS determination of surface properties of silicated alumina catalysts; and catalytic activities and selectivities in skeletal isomerization of 1-butene to isobutene). The type of acidity, Lewis or Brönsted, and its strength affects selectivity, and by examining published data, the best catalysts seemed to be those with acid sites of medium strength because the selectivity toward isobutene is enhanced and catalyst deactivation prevented. Silicate aluminas have these requisites. Therefore, the acidities of the catalysts A1S1, A1S2, A1S3, A1S4, and A1SM were tested and, for comparison, those of silica, silica-alumina, and alumina support. The highest conversion and selectivity to isobutene was over A1S4. The conversion was 83.3% and the products distribution was isobutane 0.69, n-butane 0.18, 2-trans-butene 23.7, isobutene 36.7, 2-cis-butene 17.5, and 2-cis/2-trans butene 0.74. The catalysts A1S4 and A1SM also showed a remarkable stability
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