Contribución al estudio de la ignición térmica en sistemas gaseosos dinámicos

[spa] La investigación objeto del presente trabajo ha perseguido, como fin primordial, adaptar un método continuo, ya existente en este Departamento, para la determinación de condiciones de ignición de combustibles liquidos. También desea ampliar y desarrollar un modelo matemático que describe teóricamente el fenómeno de la combustión tal y como se produce en el método experimental utilizado. La aplicación de las ecuaciones de conservación de la energia y materia, junto con la de conservación de cantidad de movimiento y con la ecuación cinética correspondiente conduce, en el caso de un reactor tubular con distribución radial de temperatures y concentraciomes, en regimen laminar y condiciones estacionarias, al modelo matemático del sistema, constituido finalmente por dos ecuaciones diferenciales en derivadas parciales. El sistema de ecuaciones, con sus adecuadas condiciones de contorno, no tiene solución analitica obteniéndola numéricamente solamente cuando los parámetros del sistema toman ciertos valores. Se han determinado, mediante empleo de un ordenador, las valores críticos del sistema correspondientes al modelo planteado. Del mismo modo se ha realizado el estudio de la influencia de las condiciones internas del sistema, como es la consumición de los reactantes y de condiciones externas al mismo, como son las condiciones de la pared que lo confina. En general, las soluciones se han obtenido considerando la temperatura de pared como constante e igual a la inicial de la mezcla gaseosa, si bien se ha deducido la influencia de la variación de dicha temperatura sobre los valores de los parámetros críticos. La comprobación experimental del modelo es simplemente orientativa ya que se carece de los datos cinéticos lo suficientemente precisos requeridos por el modelo planteado. La idea inicial, base del método experimental utilizado, fue dada por Costa Novella y col. (1) y posteriormente utilizada par Storch de Gracia [2]. Una mejora del conocimiento y control de la temperatura en la pared fue realizada por Torra (3], quien construyó el reactor utilizado , en el que existen dos lechos fluidizados coaxiales. El interno tiene como misión mezclar, calentar, controlar térmicamente la mezcla gaseosa y actuar de cierre aguas arriba al producirse las explosiones de la misma. El externo controla la temperatura de pared. La innovación primordial del presente trabajo ha consistido en adoptar el métod·o al estudio de combustibles líquidos, alimentados directamente al reactor en dicho estado, mientras que en las anteriores trabajos las combustibles utilizados eran gases (metano, etano, propano y butano). Se ha ceñido el estudio a los hidrocarburos n-heptano e isooctano, ya que se toman como referencia de los combustibles utilizados en los motores de combustión interna, y de sus rnezclas, con el fin de establecer la influencia de la concentración de uno de ellos. La investigación experimental se ha llevado a cabo determinando las lapsos de ignición a distintas temperaturas. Para el n-haptano las ensayos se han efectuado a 2, 3, 4, 5, 6 y 7 veces la velocidad mínima requerida para la fluidización del lecho interno y para el isooctano y mezclas a unas 4 veces dicha velocidad minima

Study of the oligomerization of 1-octene catalyzed by macroreticular ion-exchange resins

Oligomerization of 1-octene is a feasible alternative to improve the properties and the quality of fuels and lubricants oils. The performance of macroreticular ion-exchange resins as catalysts for such reaction has been evaluated. The experimental setup consisted of a batch stirred-tank reactor that operated at 353-393 K and 2 MPa. 1-Octene conversion was practically complete and the selectivity to double-bond isomerization and dimers after 6 h at 373 K was up to 95% and 12%, respectively. The accessible acidity of the catalysts was the most important structural parameter for these reactions. Selectivity to dimers and branched isomers increased with increasing temperature. Cracking compounds were not detected at all. The theoretical equations derived from the proposed kinetic model fit well the experimental results

Kinetics of 1-hexanol etherification on Amberlyst 70

The kinetics of the liquid-phase etherification of 1-hexanol to di-n-ethyl ether and water on the ion-exchange resin Amberlyst 70 in the temperature range 423-463 K is studied. The strong inhibition effect of water is considered following two approaches. First, a model stemming from a Langmuir-Hinshelwood-Hougen-Watson (LHHW) mechanism was used, wherein the inhibitor effect of water was explained by the competitive adsorption of water and hexanol. Secondly, a modified Eley-Rideal (ER) model that includes an inhibition factor, in which a Freundlich-like function is used to explain the inhibitor effect of water by blocking the access of hexanol to the active centers. Both models fitted data quite well, although the best fitting results were obtained with the modified ER model. The activation energy was 125 ± 3 kJ/mol for the LHHW model and 121 ± 3 kJ/mol for the modified ER on

Influence of acid ion-exchange resins morphology in swollen state on the synthesis of ethyl octyl ether from ethanol and 1-octanol

Ethyl-octyl ether (EOE) liquid phase synthesis from ethanol and 1-octanol over ion-exchange resins is feasible at 423K, though di-ethyl ether and di-n-octyl ether were also formed. The influence of the catalyst morphology on the reaction was checked by testing twenty-two acidic resins. Gel-type resins of low crosslinking degree yielded the higher amounts of EOE, whereas macroreticular ones of high crosslinking degree gave mainly di-ethyl ether. Ethanol conversion highly depends on the resin acid capacity, [H+], whereas 1-octanol conversion and selectivity to EOE depends on the specific volume of swollen polymer, Vsp, and porosity. The variation of ethanol and 1-octanol conversion, selectivity to EOE with respect to both alcohols as well as ethers TOF as a function of [H+]/Vsp suggests that a part of the active sites does not take part in the EOE synthesis reaction on highly cross-linked resins. Amberlyst 70 could be interesting in industry due to its selectivity to EOE and higher thermal stabilit

Liquid-phase synthesis of butyl levulinate with simultaneous water removal catalyzed by ion exchange resins

The liquid-phase synthesis of butyl levulinate by esterification of levulinic with an excess of 1-butanol (initial molar ratio 1:3) and simultaneous water removal has been studied at atmospheric pressure and at the boiling point of the reacting medium. The catalytic performance of ten commercial sulfonic resins has been compared: four gel-type and six macroreticular. For both type of resins the levulinic acid conversion was complete after 4-6h and no byproducts derived from the acid were detected. Selectivity of 1-butanol towards the ester was about 95%, di-butyl ether and butenes being the detected 1-butanol-derived byproducts. Among the tested catalysts, gel-type resins with low crosslinking degree showed the highest activity, what could be attributed to a higher accessibility to active centers in polar medium. In additional experiments where initial reactants content was at stoichiometric relation, the levulinic acid conversion was lower (82-85%), while selectivity of 1-butanol towards the ester was slightly higher, because of the lower 1-butanol concentration

Equilibrium conversion, selectivity and yield optimization of the simultaneous liquid-phase etherification of isobutene and isoamylenes with ethanol over Amberlyst 35

A prospective study on the product distribution at chemical equilibrium for the simultaneous liquid-phase etherification of isobutene and isoamylenes with ethanol over Amberlyst¿ 35 is presented. Experiments were performed isothermally in a 200 cm3 stirred tank batch reactor operating at 2.0 MPa. Initial molar ratios of alcohol/olefins and isobutene/isoamylenes ranged both from 0.5 to 2, and temperature from 323 to 353 K. Reactants equilibrium conversion, selectivities and yields toward products were clearly affected by the experimental conditions. Experimental etherification yields have been modeled using the response surface methodology (RSM), combined with the stepwise regression method to include only the statistically significant variables into the model. The multiobjective optimization (MOO) of etherification yields has been carried out numerically, by means of the desirability function approach, and graphically, by using the overlaid contour plots (OCP). Optimal conditions for the simultaneous production of ethyl tert-butyl ether (ETBE) and tert-amyl ethyl ether (TAEE) have been found to be at low temperatures (323 to 337 K) and initial molar ratio alcohol/olefins close to 0.9 and isobutene/isoamylenes close to 0.5

Dehydration of 1-octanol to di-n-octyl ether in liquid phase with simultaneous water removal over ion exchange resins: Effect of working state morphologies

The influence of the concentration of polar reactants and products on the working-state morphology of ion exchange catalysts has been investigated over different acidic ion-exchange resins for di-n-octyl ether (DNOE) synthesis from 1-octanol dehydration at 423-448 K and atmospheric pressure in a batch reactor equipped with a Dean & Stark device. By removing water formed 1-octanol conversion was practically complete; the olefin formation being the main secondary reaction. When 1-octanol is completely consumed the working-state morphology of ion exchange resins changes, which influences the selectivities towards products. At this point, for microporous resins all reactions stop while with macroreticular ones DNOE decomposes and significant amounts of olefin dimers appears. The best selectivity to DNOE was found in gel-type and macroreticular resins with low crosslinking degree. Macroreticular resins with medium or high crosslinking give good results in olefin formation

Kinetic modeling of the simultaneous etherification of ethanol with C4 and C5 olefins over Amberlyst 35 using model averaging

A kinetic study on the simultaneous liquid-phase etherification of ethanol with isobutene (IB), 2-methyl-1-butene (2M1B) and 2-methyl-2-butene (2M2B) catalyzed by Amberlyst¿ 35 to form ethyl tert-butyl ether (ETBE) and tert-amyl ethyl ether (TAEE) is presented. Isothermal experimental runs were carried out in a stirred tank batch reactor in the temperature range 323-353 K at 2.0 MPa, starting from different initial concentrations. Obtained reaction rates were free of catalyst load, internal, and external mass transfer effects. Mathematical fitting of a series of systematically originated models, model selection, and model averaging procedures were applied to find the best model and to draw conclusions about the reaction mechanism. The selected model involves a saturated catalytic surface with the participation of two active sites in etherification reactions and one active site in isoamylenes isomerization. Apparent activation energies for ETBE formation from IB and EtOH, TAEE formation from 2M1B and EtOH, TAEE formation from 2M2B and EtOH, and double bond isomerization between 2M1B and 2M2B were 72.8±1.4, 74.9±2.8, 81.2±2.2 and, 76.5±7.2 kJ/mol, respectively. The alkenes with the double bond in terminal position were more reactive towards EtOH than 2M2B, with the double bond in internal position

Ion exchange resins as catalysts for the liquid-phase dehydration of 1-butanol to di-n-butyl ether

This work reports the production of di-n-butyl ether (DNBE) by means of 1-butanol dehydration in the liquid phase on acidic ion-exchange resins. Dehydration experiments were performed at 150 ºC and 40 bar on 13 styrene-codivinylbenzene ion exchangers of different morphology. By comparing 1-butanol conversions to DNBE and initial reaction rates it is concluded that oversulfonated resins are the most active catalysts for 1-butanol dehydration reaction whereas gel-type resins that swell significantly in the reaction medium as well as the macroreticular thermostable resin Amberlyst-70 are the most selective to DNBE. The highest DNBE yield was achieved on Amberlyst 36. The influence of typical 1-butanol impurities on the dehydration reaction were also investigated showing that the presence of 2-methyl-1-propanol (isobutanol) enhances the formation of branched ethers such as 1-(1-methylpropoxy) butane and 1-(2-methylpropoxy) butane, whereas the presence of ethanol and acetone yields ethyl butyl ether and, to a much lesser extent, diethyl ether

Alkylation of toluene with 1-hexene over macroreticular ion-exchange resins

The macroreticular acidic ion-exchange resins Amberlyst 35, Amberlyst 46 and Purolite CT 275 were investigated as catalysts for the alkylation of toluene with 1-hexene and simultaneous dimerization and isomerization of the olefin at 373 K. After six hours of reaction, 1-hexene conversion was complete. At low toluene concentration double-bound isomerization of 1-hexene was the main reaction. As toluene concentration increased, double-bond isomerization decreased and toluene alkylation and olefin dimerization reactions increased. By using Purolite CT 275 resin and for an almost equimolar toluene to 1-hexene ratio, the selectivity to dimers was 22%, that of mono- and di-alkylated compounds were 42% and 3.5%, respectively, while that of double-bound isomerization was 32.5%. These reactions catalyzed by macroreticular ion-exchange resins of high acid capacity and degree of crosslinking can be useful to boost naphtha streams in the refining industry by reducing volatile compounds and aromatics of low boiling point