System competitive parallel

The two routes (one is Eqs. 2-37b and 2-37c the other is Eqs. 2-37a and 2-37d) together constitute a complex reaction system that consists simultaneously of competitive, consecutive and competitive, parallel reactions. 

Knight CS. Experimental investigation of the effects of a recycle loop/static mixer/ agitated vessel system on fast, competitive-parallel reactions. PhD dissertation, University of Arkansas, 1994. 

P 35] A reaction system with two competitive parallel reactions was used for mixing characterization [36], The Dushman reaction involves the mixing of iodate, iodide and sodium acetate in one solution and a strong acid such as sulfuric acid or hydrochloric acid in another solution. If mixing is fast, the neutralization of the acid and the base dominates as the faster reaction. The redox reaction of iodide and iodate then is a slow process nearly no iodine is formed as the redox product... 

C.S. Knight, Experimental Investigation of the Effects of a Recycle Loop/ Static Mixer/Agitated Vessel System on Fast, Competitive-Parallel Reactions, MS Thesis, University of Arkansas, 1995. 

Fig. 1 illustrates the mixing problem for the competitive-consecutive case. R is the desired product and S is the undesired overreaction product. During the time from when the reactants are first contacted to when they are completely mixed on the molecular scale, reaction of A with B to form the desired product R occurs along with the undesired reaction of R with B to form S. When A and B are well mixed at the molecular scale, mainly R is formed, but when there is a boundary between A and B, a significant amount of undesired S appears. Competitive-parallel reactions can be subject to similar mixing effects where the first reaction is the desired one and the second is a simultaneous decomposition of A to form the undesired U. While these two reaction systems have received the most attention, the course of any reaction that is... 

Villermaux [141, 566] used an inorganic system of competitive parallel reactions. The first reaction was an acid/base neutralization, the second an oxidation reaction (oxididation of hydrogen iodide HI to iodine I2 by iodic acid HIO3) and in the third reaction iodine was complexed with an iodide ion to a triiodide ion, which could be spectrophotometrically monitored at 353 nm (Table 1.3). 

The chemical compositions of many reacting systems can be expressed in terms of a single reaction progress variable. However, a chemical engineer must often consider systems that cannot be adequately described in terms of a single extent of reaction. In this chapter we are concerned with the development of the mathematical relationships that govern the behavior of such systems. We treat reversible reactions, competitive (parallel) reactions, and consecutive reactions, first in terms of the mathematical relations that govern the behavior of such systems and then in terms of the techniques that may be used to relate the kinetic parameters of the system to the phenomena observed in the laboratory. 

In general, an analysis of a system in which noncompetitive parallel reactions are taking place is considerably more difficult than analyses of the type discussed in Chapter 3. In the analysis of competitive parallel reactions, one must deal with the problems of determining reaction orders and rate constants for each of the individual reactions. The chemical engineer must be careful in both planning the experimental work and analyzing the data so as to obtain values of the rate constants that are sufficiently accurate for purposes of reactor design. 

The use of a single reaction requires the online measurement of the local species concentration along the flow. With such systems, one experiences the main drawback of physical methods with the local measurement and the influence of the probe size on the mixing quality estimation. For that reason, the so-called test reactions are very attractive. Two main systems, based on competitive chemical reactions, have been proposed for the investigation of mixing effects, that is, the competitive consecutive reaction system (Scheme 6.1) and the competitive parallel reaction system (Scheme 6.2). Let us consider the following simplest reactions schemes which do not exactly match the published real systems, but which facilitate the comparison ... 

Competitive-parallel reactions can also be subject to mixing effects, as shown by Baldyga and Bonme (1990) and Panl et al. (1992). Many variations are possible, but the basic reactions of these systems are as follows ... 

The system is equivalent to two competitive parallel readions with the particularity that one of them is dependent on pH. The best pH value to observe the colour contrast is thus the result of two opposite effects that follow immediately the irradiation by one hand higher proton concentration favours the appearance of the flavylium cation, by the other hand the proton concentration should not be excessive otherwise flavylium cation is the most stable specie (colour exists prior to the irradiation). Once the coloured species are formed, flavylium or quinoidal base, the system reverts completely back to the equilibrium according to eq.(8).[7] In other words, the bleaching of Ct due to the irradiation is recovered in two steps i) faster one from Cc in competition with the hydration that leads to the coloured species, ii) slower one from the coloured species via hydration followed by ring opening and isomerisation, eq.(8). 

In contrast to consecutive reactions, with parallel competitive reactions it is possible to measure not only the initial rate of isolated reactions, but also the initial rate of reactions in a coupled system. This makes it possible to obtain not only the form of the rate equations and the values of the adsorption coefficients, but also the values of the rate constants in two independent ways. For this reason, the study of mutual influencing of the reactions of this type is centered on the analysis of initial rate data of the single and coupled reactions, rather than on the confrontation of data on single reactions with intergal curves, as is usual with consecutive reactions. 

An interesting parallel was found while the microwave-enhanced Heck reaction was explored on the C-3 position of the pyrazinone system [29]. The additional problem here was caused by the capability of the alkene to undergo Diels-Alder reaction with the 2-azadiene system of the pyrazinone. An interesting competition between the Heck reaction and the Diels-Alder reaction has been noticed, while the outcome solely depended on the substrates and the catalyst system. Microwave irradiation of a mixture of pyrazinone (Re = H), ethyl acrylate (Y = COOEt) and Pd(dppf)Cl2 resulted in the formation of a mixture of the starting material together with the cycloaddition product in a 3 1 ratio (Scheme 15). On the contrary, when Pd(OAc)2 was used in combination with the bulky phosphine ligand 2-(di-t-butylphosphino)biphenyl [41-44], the Heck reaction product was obtained as the sole product. When a mixture of the pyrazinone (Re = Ar) with ethyl acrylate or styrene and Pd(dppf)Cl2 was irradiated at 150 °C for 15 min, both catalytic systems favored the Heck reaction product with no trace of Diels-Alder adduct. 

Van Vliet, E., Derksen, J. J., and Van den Akker, H. E. A., Modelling of Parallel Competitive Reactions in Isotropic Homogeneous Turbulence Using a Filtered Density Function Approach for Large Eddy Simulations . Proc. PVP01 3rd Int. Symp. on Comput. Techn. for Fluid/Thermal/Chemical Systems with Industrial Appl., Atlanta, GE, USA (2001). 

As briefly discussed in Section 1.2, chemical-reaction engineers recognized early on the need to predict the influence of reactant segregation on the yield of complex reactions. Indeed, the competitive-consecutive and parallel reaction systems analyzed in the previous section have been studied experimentally by numerous research groups (Baldyga and Bourne 1999). However, unlike the mechanical-engineering community, who mainly focused on the fluid-dynamics approach to combustion problems, chemical-reaction... 

Looking back over the steps required to derive (5.290), it is immediately apparent that the same method can be applied to treat any reaction scheme for which only one reaction rate function is finite. The method has thus been extended by Baldyga (1994) to treat competitive-consecutive (see (5.181)) and parallel (see (5.211)) reactions in the limiting case where k -> oo.118 For both reaction systems, the conditional moments are formulated in terms of 72(X> and can be written as... 

Carrier-mediated passage of a molecular entity across a membrane (or other barrier). Facilitated transport follows saturation kinetics ie, the rate of transport at elevated concentrations of the transportable substrate reaches a maximum that reflects the concentration of carriers/transporters. In this respect, the kinetics resemble the Michaelis-Menten behavior of enzyme-catalyzed reactions. Facilitated diffusion systems are often stereo-specific, and they are subject to competitive inhibition. Facilitated transport systems are also distinguished from active transport systems which work against a concentration barrier and require a source of free energy. Simple diffusion often occurs in parallel to facilitated diffusion, and one must correct facilitated transport for the basal rate. This is usually evident when a plot of transport rate versus substrate concentration reaches a limiting nonzero rate at saturating substrate While the term passive transport has been used synonymously with facilitated transport, others have suggested that this term may be confused with or mistaken for simple diffusion. See Membrane Transport Kinetics... 

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