Homogeneous reaction systems

Vocabulary of Terms Used in Reactor Design. There are several terms that will be used extensively throughout the remainder of this text that deserve definition or comment. The concepts involved include steady-state and transient operation, heterogeneous and homogeneous reaction systems, adiabatic and isothermal operation, mean residence time, contacting and holding time, and space time and space velocity. Each of these concepts will be discussed in turn. 

Both homogeneous and heterogeneous reaction systems are frequently encountered in commercial practice. The term homogeneous reaction system is restricted in this text to fluid systems in... 

The solution procedure to this equation is the same as described for the temporal isothermal species equations described above. In addition, the associated temperature sensitivity equation can be simply obtained by taking the derivative of Eq. (2.87) with respect to each of the input parameters to the model. The governing equations for similar types of homogeneous reaction systems can be developed for constant volume systems, and stirred and plug flow reactors as described in Chapters 3 and 4 and elsewhere [31-37], The solution to homogeneous systems described by Eq. (2.81) and Eq. (2.87) are often used to study reaction mechanisms in the absence of mass diffusion. These equations (or very similar ones) can approximate the chemical kinetics in flow reactor and shock tube experiments, which are frequently used for developing hydrocarbon combustion reaction mechanisms. 

Lactam polymerizations (nonassisted as well as assisted) are usually complicated by heterogeneity, usually when polymerization is carried out below the melting point of the polymer [Fries et al., 1987 Karger-Kocsis and Kiss, 1979 Malkin et al., 1982 Roda et al., 1979]. (This is probably the main reason why there are so few reliable kinetic studies of lactam polymerizations.) An initially homogeneous reaction system quickly becomes heterogeneous at low conversion, for example, 10-20% conversion (attained at a reaction time of no more than 1 min) for 2-pyrrolidinone polymerization initiated by potassium t-butoxide and A-benzoyl-2-pyrrolidinone. The (partially) crystalline polymer starts precipitating from solution (which may be molten monomer), and subsequent polymerization occurs at a lower rate as a result of decreased mobility of /V-acyl lactam propagating species. 

Most industrial processes use catalysts. Homogeneous single reaction systems are fairly rare and unimportant. The most important homogeneous reaction systems in fact involve free radical chains, which are very complex and highly nonlinear. 

A homogeneous reaction system was used in which benzyltrimethyl-ammonium hydroxide was the base and pyridine the solvent. An oxidation mechanism is proposed that is consistent with observations on the reaction variables and possible oxidation intermediates of dihydroanthracene. 

This survey has been concerned with the enumeration of all possible mechanisms for a complex chemical reaction system based on the assumption of given elementary reaction steps and species. The procedure presented for such identification has been directly applied to a number of examples in the field of heterogeneous catalysis. Application to other areas is clearly indicated. These would include complex homogeneous reaction systems, many of which are characterized by the presence of intermediates acting as catalysts or free radicals. Enzyme catalysis should also be amenable to this approach. 

To avoid purification of the reaction product from the (colored) sensitizer or its oxidation and photolysis products, the use of insoluble sensitizers has been proposed, in particular for sensitized oxidations [14]. Whereas sensitizers adsorbed on solid supports, such as ion exchange resins, silica, or alumina [15], show considerable leading rates [16] and must be discarded, potential application might be found for sensitizers that are chemically bound to inert surfaces [14-17]. However, a loss of efficiency of at least 50% has to be taken into account when comparing overall quantum yields with those determined in homogeneous reaction systems [17]. 

Heinze et al. found that DMSO in combination with tetrabutylammonium fluoride trihydrate dissolved cellulose (degree of polymerization < 650) within 15 min at room temperature [38]. They also demonstrated that homogeneous esterification of cellulose is possible in this solvent system. The applicability of this new solvent system to cellulose grafting has recently been proved by adoption of cyclic compounds such as lactones and N-carboxy a-amino acid anhydrides (NCAs) [39]. e-Caprolactone was facilely graft-polymerized on cellulose at a graft rate of 65% (per trunk weight of 100), and NCAs at over 100%, in the respective homogeneous reaction systems at < 60 °C. 

The Role of Zeolites Where zeolites seem to play a more specific role appears probably when activities and selectivities are examined. Though few comparative studies were made possible upon examination of the litterature, it would appear that both rhodium and iridium exhibited lower activities than in the homogeneous reaction systems but far larger activities than exhibited when supported over traditional carriers (A1 0, Al 0 -Si0 , carbon, etc...). This is probably due to the better molecular dispersion of the rhodium and iridium catalytic precursors in zeolite media than over any of these carriers and still more important diffusion limitations than in solution. 

Equation 86 is commonly used for homogeneous reaction systems, but it is not exact in emulsion polymerization. The value of [Pp] is different for each polymer particle, and the value obtained for [Pp] when all of the particles are combined cannot be used either. Strictly, one needs to determine a discrete distribution function of polymer molecules in each polymer particle. 

Initiators are not used efficiently in free-radical polymerizations. A significant proportion of the primary radicals that are generated are not captured by monomers, and the initiator efficiency / in Eq. (6-10) is normally in the range 0.2-0.7 for most initiators in homogeneous reaction systems. It will be lower yet in polymerizations in which the initiator may not be very well dispersed. 

The occurrence of a homogeneous reaction system is also implicit i n the derivation of the copolymer composition equation. Some polymers, like poly(vinylidene chloride), are insoluble in their own monomer and are not highly swollen by monomer. In emulsion copolymerizations of such reactants the relative concentrations of the comonomers in the polymerizing particles will be influenced by the amounts that can be adsorbed on the surface or absorbed into the interior of these polymerization loci. 

Early work indicated that the nature of the reaction medium had no effect on the course of free radical copolymerizations in homogeneous reaction systems. More recent studies have not always supported this conclusion and it has been suggested that a bootstrap effect may be operating whereby there is a partitioning of the comonomers between the bulk of the reaction medium and the polymerization locus (i.e., the macroradical end) [29]. 

The discussion of free-radical polymerizations in Chapters 6 and 7 focused primarily on homogeneous reaction systems, in which monomer, polymer, and any solvent were all miscible. This conventional presentation makes it much easier to grasp the fundamentals of free-radical polymerizations. In fact, however, many large-scale processes are carried out in heterogeneous systems, because these offer advantages over alternative procedures. Their overall importance is such as to justify this chapter describing the effects of process conditions on polymer properties. 

For homogeneous reaction systems at constant temperature, agitation is not normally necessary. However, most reactions involve heterogeneous reaction mixtures and so require some form of agitation to ensure efficient mixing of the reactants. The most commonly used methods of agitation are outlined in this section. 

Thus, results obtained with some vanadium based homogeneous catalytic systems have also been explained in this manner. Christman has examined polyethylene obtained in a completely homogeneous reaction system, since the catalytic system was soluble and the polymerization took place under such temperature conditions as to keep the polymer dissolved in the reaction medium. 

The rate of change of concentration in a homogeneous reaction system can be described by the following system of ordinary differential equations (odes) ... 

For a spatially homogeneous reaction system, such as a constant-volume batch reactor, we write the following differential equation for each component... 

In the discussion that follows we will consider a homogeneous reaction system where three types of fundamental processes are the most interesting ... 

They also investigated the effect of water on the rate cind order of the homogeneous reaction system. For reactive aromatic substrates, the reaction in the absence of water is zero order in... 

Mechanistic Studies. - The application of radioactive tracer techniques to mechanistic studies in heterogeneous catalysis is, in principle, the same as their application to homogeneous reaction systems. A labelled compound, for example, a possible intermediate, is added to the reaction mixture. Gas-chromatographic separation of the reaction products and immediate radioactive assay of the separate components yields information about the chemical identity of the radioactive products and gives a quantitative measure of the degree of incorporation of the radioactive label. 

Both homogeneous and heterogeneous reaction systems are frequently encountered in commercial practice. In this book the term homogeneous reaction system is restricted to fluid systems in which the system properties vary continuously from point to point within the reactor. The team embraces both catalytic and noncatalytic reactions, but it requires that any catalysts be dispersed uniformly throughout the fluid phase. The term heterogeneous reaction system refers to a system in which there are... 

Quantitative changes in the concentration of chemical species with time in a spatially homogeneous reaction system under isothermal conditions may be described by a system of ordinary differential equations... 

For a homogeneous reaction system without exchange of matter with the surrounding medium, the rate of a stoichiometrically single reaction, either elementary or complex, can be expressed as... 

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