Comparison of catalyst activity

Reactions of highly active catalysts are compared by allowing temperature of catalyst bed, initially at 77 K, to increase slowly; marked deviation in smooth warming curve denotes temperature at which detectable reaction occurs. The lower the temperature at which reaction commences, the more active the catalyst

Temperature modelling and model predictive control of a pilot-scale batch reaction system

The temperature control equipment on a pilot scale batch reaction system located at EAFIT University in Medelln, Colombia, is modeled and a new controller is designed aiming at using it in the reactor current PLC-based control system. Some mathematical models are developed from experimental data to describe the system behavior and using them several model based predictive controllers are designed. The simplest, yet reliable, model obtained is an ARX polynomial model of order (1,1,1) that yields a four states ane model for which an explicit MPC was calculated. This controller has a reduced mathematical complexity and can probably be used directly on the existing control system.Preprin

Benzonitrile as a Proxy for Benzene in the Cold ISM: Low-temperature Rate Coefficients for CN + C₆H₆

The low-temperature reaction between CN and benzene (C₆H₆) is of significant interest in the astrochemical community due to the recent detection of benzonitrile, the first aromatic molecule identified in the interstellar medium (ISM) using radio astronomy. Benzonitrile is suggested to be a low-temperature proxy for benzene, one of the simplest aromatic molecules, which may be a precursor to polycyclic aromatic hydrocarbons. In order to assess the robustness of benzonitrile as a proxy for benzene, low-temperature kinetics measurements are required to confirm whether the reaction remains rapid at the low gas temperatures found in cold dense clouds. Here, we study the C₆H₆ + CN reaction in the temperature range 15–295 K, using the well-established CRESU technique (a French acronym standing for Reaction Kinetics in Uniform Supersonic Flow) combined with pulsed-laser photolysis-laser-induced fluorescence. We obtain rate coefficients, k(T), in the range (3.6–5.4) × 10⁻¹⁰ cm³ s⁻¹ with no obvious temperature dependence between 15 and 295 K, confirming that the CN + C₆H₆ reaction remains rapid at temperatures relevant to the cold ISM

Do the effective heat conductivity and the heat transfer coefficient at the wall inside a packed bed depend on a chemical reaction? Weaknesses and applicability of current models

Many studies have been on the effective heat conductivity (λeff) and the transfer coefficient at the wall (αw) inside packed beds. It has been mentioned that the values of λef and αw are changed when a chemical reaction occurs in the packed bed. We give an explanation for such a phenomenon. The properties λeff and αw are lumped parameters which usually are determined by both the measured temperature profiles and the model used to calculate the temperature profiles from λeff and αw. If either the experimental data are wrong or the model is erroneous the error will manifest itself in the values of λeff and αw. At least a part of the change in the values of λeff and αw due to a chemical reaction is caused by the fact that a homogeneous model with catalyst and gas having the same temperature is chosen, whereas a heterogeneous model with catalyst and gas having different temperatures should be used. If no reaction occurs the catalyst and gas will have the same temperature and the homogeneous model yields a good description. Hence, when fitting temperature profiles with this model the correct values of λeff and αw are found. If reaction does occur the catalyst and the gas will have different temperatures because the heat of reaction must be transferred from the catalyst to the gas. If, despite this fact, a homogeneous model is used to calculate the temperature profiles, an error is introduced which is reflected in the values of λeff and αw. As a consequence we create an apparent dependence of λeff and αw on the reaction rate. We derive criteria to determine which model must be used. We discuss results presented in the literature on the dependence of λeff and αw on the chemical reaction. The explanation is both qualitative and quantitative

Methyl esters selectivity of transesterification reaction with homogenous alkaline catalyst to produce biodiesel in batch, plug flow, and continuous stirred tank reactors

Selectivity concept is essential in establishing the best operating conditions for attaining maximum production of the desired product. For complex reaction such as biodiesel fuel synthesis, kinetic studies of transesterification reaction have revealed the mechanism of the reaction and rate constants. The objectives of this research are to develop the kinetic parameters for determination of methyl esters and glycerol selectivity, evaluate the significance of the reverse reaction in transesterification reaction, and examine the influence of reaction characteristics (reaction temperature, methanol to oil molar ratio, and the amount of catalyst) on selectivity. For this study, published reaction rate constants of transesterification reaction were used to develop mathematical expressions for selectivities. In order to examine the base case and reversible transesterification, two calculation schemes (Case 1 and Case 2) were established. An enhanced selectivity was found in the base case of transesterification reaction. The selectivity was greatly improved at optimum reaction temperature (60 C), molar ratio (9 : 1), catalyst concentration (1.5 wt.%), and low free fatty acid feedstock. Further research might explore the application of selectivity for specifying reactor configurations

Methyl esters selectivity of transesterification reaction with homogenous alkaline catalyst to produce biodiesel in batch, plug flow, and continuous stirred tank reactors

Selectivity concept is essential in establishing the best operating conditions for attaining maximum production of the desired product. For complex reaction such as biodiesel fuel synthesis, kinetic studies of transesterification reaction have revealed the mechanism of the reaction and rate constants. The objectives of this research are to develop the kinetic parameters for determination of methyl esters and glycerol selectivity, evaluate the significance of the reverse reaction in transesterification reaction, and examine the influence of reaction characteristics (reaction temperature, methanol to oil molar ratio, and the amount of catalyst) on selectivity. For this study, published reaction rate constants of transesterification reaction were used to develop mathematical expressions for selectivities. In order to examine the base case and reversible transesterification, two calculation schemes (Case 1 and Case 2) were established. An enhanced selectivity was found in the base case of transesterification reaction. The selectivity was greatly improved at optimum reaction temperature (60 C), molar ratio (9 : 1), catalyst concentration (1.5 wt.%), and low free fatty acid feedstock. Further research might explore the application of selectivity for specifying reactor configurations

Percolation-dependent Reaction Rates in the Etching of Disordered Solids

A prototype statistical model for the etching of a random solid is investigated in order to assess the influence of disorder and temperature on the dissolution kinetics. At low temperature, the kinetics is dominated by percolation phenomena, and the percolation threshold determines the global reaction time. At high temperature, the fluctuations of the reaction rate are Gaussian, whereas at low temperature they exhibit a power law tail due to chemical avalanches. This is an example where microscopic disorder directly induces non-classical chemical kinetics.Comment: Revtex, 4 pages, 5 figure

The Ti/c-Si solid state reaction : III. The low-temperature reaction kinetics

Thin Ti layers (≈10nm) are grown on top of a clean Si(111) substrate. Heating these layers initiates a solid state reaction, yielding a monosilicide phase at ≈350°C and a C49 disilicide at ≈450°C. The present study concerns the growth kinetics of both phases by means of ellipsometry. A diffusion-limited growth kinetics is found for the monosilicide formation. However, two growth rates are observed, a fast initial one and a slow terminal growth rate. An enhanced Si diffusion in atomically disordered regions as compared to well ordered regions (grains or clusters) could be an explanation. From the measurements we have found a value of 2×10-15 cm2/s for the diffusion coefficient at ≈370°C and an activation energy of 0.62 ± 0.1 eV. Both values correspond to the fast process. Subsequently increasing the temperature to ≈450°C permits the growth of the homogeneous C49 TiSi2 phase. For this process, both planar layer growth and intermixing are observed, however, quantitative results could not be derived from the present study

MHD Flow of a Uniformly Stretched Vertical Permeable Membrane in the Presence of Zero Order Reaction and Quadratic Heat Generation

We present a magneto - hydrodynamic flow of a uniformly stretched vertical permeable surface undergoing Arrhenius heat reaction. The analytical solutions are obtained for concentration, temperature and velocity fields using an asymptotic approximation, similar to that of Ayeni et al 2004. It is shown that the temperature field and the velocity field depend heavily on the thermal grashof numbers, heat generation/absorption, magnetic induction, chemical reaction parameters and reaction order. It is also established that maximum velocity occurs in the body of the fluid close to the surface and not the surface

Gas phase bond dissociation enthalpies and enthalpies of isomerization/reaction for small hydrocarbon combustion related compounds between 300 and 1500 K: A comparison of Gaussian-4 (G4) theoretical values against experimental data

Gas phase calculations at 1 atmosphere pressure between 300 and 1500 K at 200 K intervals were conducted using the Gaussian-4 (G4) composite method level of theory on a representative set of reactions having broad relevance in hydrocarbon combustion chemistry. Reasonable agreement between the experimental and theoretical data was obtained across the temperature range under consideration for all bond dissociation enthalpies, isomerization enthalpies, and enthalpies of reaction. For some reaction schemes, chemical accuracy for the theoretical method was maintained over the complete temperature range, whereas other systems displayed up to several kcal mol^-1^ deviations from experimental data. The direction of signed errors generally increased as the temperature was raised, and no general error trends were related to molecular size or reaction class