Hacia un mecanismo de oxidación de metanol válido para alta y baja presión

Como ejemplo más común de biocombustibles siempre se han destacado los alcoholes y han sido objeto de considerable interés. El metanol es el alcohol más simple y a pesar existir estudios sobre su oxidación, no hay ningún mecanismo capaz de reproducir los datos experimentales en un amplio rango de condiciones. Debido al desarrollo de modelos jerárquicos, el mecanismo del metanol forma parte de los mecanismos de oxidación de alcoholes superiores, por lo que su conocimiento resulta de gran importancia. Este proyecto se apoya en los resultados obtenidos en dos trabajos desarrollados anteriormente en los que se proponen mecanismos para la conversión del metanol en diferentes condiciones y válidos respectivamente para presión atmosférica y para elevada presión. Estos mecanismos de oxidación describen la oxidación de metanol y la interacción con óxidos de nitrógeno. Ambos mecanismos funcionan de manera correcta bajo las condiciones en las que se desarrollaron pero fallan en el rango de presiones opuesto. Por lo tanto, es necesario el desarrollo de un nuevo mecanismo para la simulación del metanol en todo el rango de presiones. En base a estos dos mecanismos existentes se creó una primera versión de un mecanismo de oxidación de metanol común para los dos rangos de presión. Los mecanismos de partida comparten la mayor parte de las reacciones y sus parámetros de velocidad, sin embargo existen 28 reacciones diferentes entre ellos cuyos parámetros se calcularon de nuevo para incluirlos en el mecanismo común. Los resultados de la simulación de los tres mecanismos muestran las diferentes concentraciones de metanol, monóxido de carbono y dióxido de carbono obtenidas en comparación con los resultados experimentales disponibles. El análisis global de sensibilidad es una herramienta ampliamente utilizada para investigar los mecanismos de combustión de manera detallada. Para cada parámetro de velocidad de reacción del mecanismo común se estimaron sus límites de incertidumbre y se llevaron a cabo simulaciones de Monte Carlo para predecir las concentraciones máximas y mínimas que es posible obtener a diferentes temperaturas. Con el fin de identificar los coeficientes globales de sensibilidad y obtener las reacciones más sensibles globalmente susceptibles de ser modificadas para mejorar el rendimiento general del mecanismo se utilizó el método de la Representación de Modelos de Alta Dimensionalidad (HDMR: High Dimensional Model Representation). La reacción más importante de este mecanismo en todas las circunstancias investigadas fue la Reacción 121, CH3OH + HO2 = CH2OH + H2O2, que controla la concentración de metanol, monóxido de carbono y dióxido de carbono. Para mejorar el mecanismo de reacción, se revisaron los parámetros de velocidad de la misma con lo que se propuso un nuevo mecanismo de reacción. Se procedió a investigar el mecanismo mejorado mediante un nuevo análisis de sensibilidad, cuyos resultados se mejoraron significativamente lo que sugiere que las recomendaciones de los nuevos parámetros deben estar presentes en la actualización del mecanismo para la conversión del metanol

Time scale and dimension analysis of a budding yeast cell cycle model

BACKGROUND: The progress through the eukaryotic cell division cycle is driven by an underlying molecular regulatory network. Cell cycle progression can be considered as a series of irreversible transitions from one steady state to another in the correct order. Although this view has been put forward some time ago, it has not been quantitatively proven yet. Bifurcation analysis of a model for the budding yeast cell cycle has identified only two different steady states (one for G1 and one for mitosis) using cell mass as a bifurcation parameter. By analyzing the same model, using different methods of dynamical systems theory, we provide evidence for transitions among several different steady states during the budding yeast cell cycle. RESULTS: By calculating the eigenvalues of the Jacobian of kinetic differential equations we have determined the stability of the cell cycle trajectories of the Chen model. Based on the sign of the real part of the eigenvalues, the cell cycle can be divided into excitation and relaxation periods. During an excitation period, the cell cycle control system leaves a formerly stable steady state and, accordingly, excitation periods can be associated with irreversible cell cycle transitions like START, entry into mitosis and exit from mitosis. During relaxation periods, the control system asymptotically approaches the new steady state. We also show that the dynamical dimension of the Chen's model fluctuates by increasing during excitation periods followed by decrease during relaxation periods. In each relaxation period the dynamical dimension of the model drops to one, indicating a period where kinetic processes are in steady state and all concentration changes are driven by the increase of cytoplasmic growth. CONCLUSION: We apply two numerical methods, which have not been used to analyze biological control systems. These methods are more sensitive than the bifurcation analysis used before because they identify those transitions between steady states that are not controlled by a bifurcation parameter (e.g. cell mass). Therefore by applying these tools for a cell cycle control model, we provide a deeper understanding of the dynamical transitions in the underlying molecular network

Comparison of the performance of several recent syngas combustion mechanisms

Journal articleA large set of experimental data was accumulated for syngas combustion: ignition studies in shock tubes (732 data points in 62 datasets) and in rapid compression machines (492/47), flame velocity determinations (2116/217) and species concentration measurements from flow reactors (1104/58), shock tubes (436/21) and jet-stirred reactors (90/3). In total, 4970 data points in 408 datasets from 52 publications were collected covering wide ranges of temperature T, pressure p, equivalence ratio phi, CO/H-2 ratio and diluent concentration X-dil. 16 recent syngas combustion mechanisms were tested against these experimental data, and the dependence of their predictions on the types of experiment and the experimental conditions was investigated. Several clear trends were found. Ignition delay times measured in rapid compression machines (RCM) and in shock tubes (ST) at temperatures below 1000 K could not be well-predicted. Particularly for shock tubes, facility effects at temperatures below 1000 K could not be excluded. The accuracy of the reproduction of ignition delay times did not change significantly with pressure. The agreement of measured and simulated laminar flame velocities is better at low initial (i.e. cold side) temperatures, at fuel-lean conditions, for CO-rich and highly diluted mixtures. The reproduction of the experimental flame velocities is better when these were measured using the heat flux method or the counterflow twin-flame technique, compared to the flame cone method and the outwardly propagating spherical flame approach. With respect to all data used in this comparison, five mechanisms were identified that reproduce the experimental data similarly well. These are the NUIG-NGM-2010, Keromnes-2013, Davis-2005, Li-2007 and USC-II-2007 mechanisms, in decreasing order of their overall performance. The influence of poorly reproduced experiments and weighting on the performance of the mechanisms was investigated. Furthermore, an analysis of local sensitivity coefficients was carried out to determine the influence of selected reactions at the given experimental conditions and to identify those reactions that require more attention in future development of syngas combustion models.OTKA (Hungarian Scientific Research Fund) grants # K84054 and # NN100523peer-reviewed2017-01-2

Comparison of the performance of several recent hydrogen combustion mechanisms

A large set of experimental data was accumulated for hydrogen combustion: ignition measurements in shock tubes (770 data points in 53 datasets) and rapid compression machines (229/20), concentra- tion–time profiles in flow reactors (389/17), outlet concentrations in jet-stirred reactors (152/9) and flame velocity measurements (631/73) covering wide ranges of temperature, pressure and equivalence ratio. The performance of 19 recently published hydrogen combustion mechanisms was tested against these experimental data, and the dependence of accuracy on the types of experiment and the experimen- tal conditions was investigated. The best mechanism for the reproduction of ignition delay times and flame velocities is Kéromnès-2013, while jet-stirred reactor (JSR) experiments and flow reactor profiles are reproduced best by GRI3.0-1999 and Starik-2009, respectively. According to the reproduction of all experimental data, the Kéromnès-2013 mechanism is currently the best, but the mechanisms NUIG- NGM-2010, ÓConaire-2004, Konnov-2008 and Li-2007 have similarly good overall performances. Several clear trends were found when the performance of the best mechanisms was investigated in various cat- egories of experimental data. Low-temperature ignition delay times measured in shock tubes (below 1000 K) and in RCMs (below 960 K) could not be well-predicted. The accuracy of the reproduction of an ignition delay time did not change significantly with pressure and equivalence ratio. Measured H 2 and O 2 concentrations in JSRs could be better reproduced than the corresponding H 2 O profiles. Large dif- ferences were found between the mechanisms in their capability to predict flow reactor data. The repro- duction of the measured laminar flame velocities improved with increasing pressure and total diluent concentration, and with decreasing equivalence ratio. Reproduction of the flame velocities measured using the flame cone method, the outwardly propagating spherical flame method, the counterflow twin-flame technique, and the heat flux burner method improved in this order. Flame cone method data were especially poorly reproduced. The investigation of the correlation of the simulation results revealed similarities of mechanisms that were published by the same research groups. Also, simulation results cal- culated by the best-performing mechanisms are more strongly correlated with each other than those of the weakly performing ones, indicating a convergence of mechanism development. An analysis of sensi- tivity coefficients was carried out to identify reactions and ranges of conditions that require more atten- tion in future development of hydrogen combustion models. The influence of poorly reproduced experiments on the overall performance was also investigated