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Chemical Kinetics
Published in Armen S. Casparian, Gergely Sirokman, Ann O. Omollo, Rapid Review of Chemistry for the Life Sciences and Engineering, 2021
Armen S. Casparian, Gergely Sirokman, Ann O. Omollo
Chemical kinetics is the study of reaction rates and mechanisms of chemical reactions. The reaction rate is a kinetic property and depends on the mechanism of the particular reaction. In contrast, thermodynamic properties are independent of mechanism. Thermodynamics indicates whether or not a reaction is spontaneous, but kinetics indicates how fast the reaction occurs and if it happens fast enough to be of any interest or value.
Reaction Kinetics in Food Systems
Published in Dennis R. Heldman, Daryl B. Lund, Cristina M. Sabliov, Handbook of Food Engineering, 2018
Ricardo Villota, James G. Hawkes
Chemical kinetics encompasses the study of the rates at which chemical reactions proceed. The area of kinetics in food systems has received a great deal of attention in past years, primarily due to efforts to optimize or at least maximize the quality of food products during processing and storage. Moreover, a good understanding of reaction kinetics can provide a better idea of how to formulate or fortify food products in order to preserve the existing nutrients or components in a food system or, on the other hand, minimize the appearance of undesirable breakdown products. Unfortunately, limited kinetic information is available at present for food systems or ingredients that would facilitate the development of food products with improved stability or the optimization of processing conditions. A major consideration, however, is that indirectly some of the information available may be used to predict kinetic trends and thus establish major guidelines in formulation, storage, and process conditions. Thus, it is within the scope of this chapter to (a) present a general discussion on general kinetics, outlining some of the fundamental principles, (b) provide information on a variety of food systems, indicating their reactivity and reported kinetic behavior, and (c) provide an understanding of current changes resulting from external influences, including updated analytical methodologies and technological advances. It is considered that a better understanding of kinetics in food systems will facilitate the development of a more complete and sound database.
Introduction
Published in Nayef Ghasem, Modeling and Simulation of Chemical Process Systems, 2018
Chemical kinetics is the study of chemical reactions with respect to reaction rates, re-arrangement of atoms, formation of intermediates, and effect of various variables. At the macroscopic level, the interest is in the amounts reacted, formed, and the rates of their formation. The following items are essential in discussing the mechanism of chemical reaction at the molecular or microscopic level: During the chemical reactions, molecules or atoms of reactants must collide with each other.To initiate the reaction, the molecules must have adequate energy.The alignment of the molecules throughout the crash in certain cases must also be considered.
On a thermodynamic foundation of Eyring rate theory for plastic deformation of polymer solids
Published in Philosophical Magazine Letters, 2023
In the Eyring rate theory, the plastic flow process is treated as the movement of a cooperative mobile element, which is referred to as a flow unit, in the stretching direction over a free-energy barrier. The model is based on chemical kinetics and describes the changes in the reaction rate with temperature. In the absence of stress, the detailed balance requires that an equal number of flow units move forward and backward over the potential barrier at a frequency ω given by: where denotes the free-energy barrier, β indicates the inverse temperature (Boltzmann’s constant is assumed to be unity for convenience), and represents the universal frequency, ( is the Dirac’s constant).
Experimental study on transport behavior of cesium iodide in the reactor coolant system under LWR severe accident conditions
Published in Journal of Nuclear Science and Technology, 2020
Naoya Miyahara, Shuhei Miwa, Mélany Gouëllo, Junpei Imoto, Naoki Horiguchi, Isamu Sato, Masahiko Osaka
One was the chemical equilibrium calculation using Thermo-Calc software [37] with SSUB4 substance thermodynamic database [38]. The calculation was conducted for equilibrium gas compositions at 3 points in the TGT, namely at 1000 K point above the crucible and the following 1273 K and 1000 K points. In the other calculation method, chemical reaction kinetics was considered using the ECUME [23]. The chemical reaction calculation was conducted using the chemical kinetics simulation software ANSYS CHEMKIN 18.2 [39]. An ideal linear temperature distribution coupled with a simple flow pattern (plug flow) was assumed. As the inlet boundary conditions, chemical equilibrium composition above the crucible and the experimental flow rate, 2 NL/min, were given. The SSUB4 substance thermodynamic database [38] was used to determine the chemical equilibrium compositions.
Methane/Air Auto-Ignition Based on Global Quasi-Linearization (GQL) and Directed Relation Graph (DRG): Implementation and Comparison
Published in Combustion Science and Technology, 2020
Chunkan Yu, Felipe Minuzzi, Viatcheslav Bykov, Ulrich Maas
The history of model reduction of chemical kinetics goes back to three conventional reduction methods, namely rate-determining step (RDS) (Campbell 1994; Dumesic 1999), quasi-steady state assumption (QSSA) (Bodenstein, 1913) and partial equilibrium assumption (PEA) (Chapman and Underhill 1913). In order to implement the assumptions applied in these methods, one must decide which species can be assumed being at quasi-steady state or which elementary reactions can be assumed equilibrated. Verification of these assumptions before the numerical experiments are conducted is difficult. Thus, only post-processing can verify the validity and accuracy, requiring a significant amount of time validation for human resources. The task becomes even more challenging when the assumptions are valid only for limited ranges of initial conditions or in a certain stage of system evolution (e.g. during an induction period of the ignition). It was shown in (Boivin 2011), for example, that even for the hydrogen/air system, one needs two different QSSA strategies to describe the auto-ignition processes for low and high initial temperatures.