Kinetic limitations, chemical

When soil thickness is at the stable value (F), erosion is transport limited. Chemical weathering is also transport limited. This is, however, not because of reaction kinetics instead this limitation is primarily controlled by physical factors, most probably, restricted access of water to the primary minerals. 

Many semibatch reactions involve more than one phase and are thus classified as heterogeneous. Examples are aerobic fermentations, where oxygen is supplied continuously to a liquid substrate, and chemical vapor deposition reactors, where gaseous reactants are supplied continuously to a solid substrate. Typically, the overall reaction rate wiU be limited by the rate of interphase mass transfer. Such systems are treated using the methods of Chapters 10 and 11. Occasionally, the reaction will be kinetically limited so that the transferred component saturates the reaction phase. The system can then be treated as a batch reaction, with the concentration of the transferred component being dictated by its solubility. The early stages of a batch fermentation will behave in this fashion, but will shift to a mass transfer limitation as the cell mass and thus the oxygen demand increase. 

Each of the intermediate electrochemical or chemical steps is a reaction of its own (i.e., it has its own kinetic pecnliarities and rules. Despite the fact that all steps occur with the same rate in the steady state, it is true that some steps occur readily, without kinetic limitations, and others, to the contrary, occur with limitations. Kinetic limitations that are present in electrochemical steps show up in the form of appreciable electrode polarization. It is a very important task of electrochemical kinetics to establish the nature and kinetic parameters of the intermediate steps as well as the way in which the kinetic parameters of the individual steps correlate with those of the overall reaction. 

When anodic polarization is appreciable AE 0), the CD will tend toward the value and then remain unchanged when polarization increases further. Therefore, parameter i, as defined by Eq. (13.44), is a limiting CD arising from the limited rate of a homogeneous chemical reaction when Cj drops to a value of zero it is the kinetic limiting current density. 

The treatment of chemical reaction equilibria outlined above can be generalized to cover the situation where multiple reactions occur simultaneously. In theory one can take all conceivable reactions into account in computing the composition of a gas mixture at equilibrium. However, because of kinetic limitations on the rate of approach to equilibrium of certain reactions, one can treat many systems as if equilibrium is achieved in some reactions, but not in others. In many cases reactions that are thermodynamically possible do not, in fact, occur at appreciable rates. 

A Kinetic Limitation on the Conversion of Light to Stored Chemical Energy... 

Biochemical transformations of organic compounds are especially important because many reactions, although thermodynamically feasible, occur extremely slowly due to kinetic limitations. For example, we might be interested in the question of whether benzene can be biodegraded under naturally occurring methanogenic conditions (see Illustrative Example 17.1). Such natural attenuation of this toxic aromatic substance may be thermodynamically allowed under the perceived conditions. But these conditions may not be accurate (e.g., the benzene and methane chemical activities in the system). Also other environmental factors may cause the rate to be unobservably slow. One possibility is that the relevant microorganisms are simply not active in the environment of interest. 

Nevertheless, the kinetic modelling of spurs is by far the most complex problem to which diffusion-limited chemical reaction theory has been applied. The radiation chemistry of water is of especial importance to both radiotherapy and nuclear engineering. 

If the sulfur-containing species were in chemical equilibrium, the dominant species at high temperatures would be SO2, which would largely be converted to SO3 as the temperature decreased, and finally below 500 K, hydrogen sulfate (H2SO4) would be predominant. Observations from combustion systems show that the conversion of SO2 to SO3 and H2SO4 is kinetically limited, and that most of the sulfur is emitted as SO2, in contradiction to the equilibrium predictions. 

The more selective kind of ion exchange, the chelating ion exchange [22], suffers often from kinetic limitations, which limit the application range to cationic compounds with fast ligand exchange kinetic for the inner coordination sphere. Ion exchange is well suitable for preconcentration as well as for separation of chemically similar compounds. 

A number of Ya.B. s papers were devoted to the properties of states close to the fluid-vapor critical point, to periodic crystallization, and to the limiting chemical kinetic laws of bimolecular and chain reactions. We will not, however, attempt to take the place of the bibliography at the end of the book which provides a complete list of Ya.B. s papers on the subjects of this volume. 

Because of the relative ease of determining the system state by equilibrium calculations relative to experiments or detailed kinetics models, chemical equilibrium analysis has been the traditional approach to CVD process modeling. An extensive literature exists for the Si-Cl-H CVD system (7) and the As-Ga-Cl-H VPE process (I). The analysis of MOCVD systems has been limited by the lack of thermodynamic data. A recent equilibrium analysis of the MOCVD of GaAs (83, 84) is a good source of data for the GaAs system. 

Basically, using these technologies one would like to move forward to the theoretical optimum of a chemical process, which is that there are no other limitations than chemical kinetics. Normally a chemical process is influenced by more than just kinetics hydrodynamics (mixing), heat transfer, and mass transfer determine the quality of the process. Process intensification focuses on removing these three limitations to reaching the goal of kinetically limited processes. This is schematically depicted in Figure 2. 

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