Noncatalytic reactions examples

G-S noncatalytic reactions examples, modeling, effect of sintering... 

Fluidized bed noncatalytic reactors. Fluidized heds are also suited to gas-solid noncatalytic reactions. All the advantages described earlier for gas-solid catalytic reactions apply. As an example. 

The earliest examples of analytical methods based on chemical kinetics, which date from the late nineteenth century, took advantage of the catalytic activity of enzymes. Typically, the enzyme was added to a solution containing a suitable substrate, and the reaction between the two was monitored for a fixed time. The enzyme s activity was determined by measuring the amount of substrate that had reacted. Enzymes also were used in procedures for the quantitative analysis of hydrogen peroxide and carbohydrates. The application of catalytic reactions continued in the first half of the twentieth century, and developments included the use of nonenzymatic catalysts, noncatalytic reactions, and differences in reaction rates when analyzing samples with several analytes. 

Noncatalytic Reactions Chemical kinetic methods are not as common for the quantitative analysis of analytes in noncatalytic reactions. Because they lack the enhancement of reaction rate obtained when using a catalyst, noncatalytic methods generally are not used for the determination of analytes at low concentrations. Noncatalytic methods for analyzing inorganic analytes are usually based on a com-plexation reaction. One example was outlined in Example 13.4, in which the concentration of aluminum in serum was determined by the initial rate of formation of its complex with 2-hydroxy-1-naphthaldehyde p-methoxybenzoyl-hydrazone. ° The greatest number of noncatalytic methods, however, are for the quantitative analysis of organic analytes. For example, the insecticide methyl parathion has been determined by measuring its rate of hydrolysis in alkaline solutions. 

Some specific examples include the noncatalytic reaction of acrylamide with primary amines to produce a mono or bis product (5). 

Fluidized-bed appHcations in the 1990s may be separated into catalytic reactions, noncatalytic reactions, and physical processes. Examples of fluidized-bed appHcations include the foUowing ... 

TABLE 11.6 Examples of Fluid-Solid Noncatalytic Reactions... 

When solid particles are subject to noncatalytic reactions, the effects of the reaction on individual particles are derived and then the results are averaged to determine overall properties. The general techniques for this averaging are called population balance methods. They are important in mass transfer operations such as crystallization, drop coagulation, and drop breakup. Chapter 15 uses these methods to analyze the distribution of residence times in flow systems. The following example shows how the methods can be applied to a collection of solid particles undergoing a consumptive surface reaction. 

CAUTION Comparing k values between different types of reactions is an all-too-common mistake. Although we use the same letter k for both first-order and second-order rate constants, their units differ A first-order rate constant has units of [time]-1, while a second-order one has units of [time]-1 [concentration]-1. Comparing these two numbers is like comparing apples and tomatoes - it is nonsense. This is also the reason why comparing the rate constants of a reaction with and without a catalyst is meaningless Consider, for example, the noncatalytic reaction A I B C that obeys the second-order rate law shown in Eq. (2.21). 

CSTR for most reactions. These conditions are best met for short residence times where velocity profiles in the tubes can be maintained in the turbulent flow regime. In an empty tube this requires high flow rates for packed columns the flow rates need not be as high. Noncatalytic reactions performed in PFRs include high-pressure polymerization of ethylene and naphtha conversion to ethylene. A gas-liquid noncatalytic PFR is used for adipinic nitrile production. A gas-solid PFR is a packed-bed reactor (Section IV). An example of a noncatalytic gas-solid PFR is the convertor for steel production. Catalytic PFRs are used for sulfur dioxide combustion and ammonia synthesis. 

With the balance for the number of available forms of active centers of a catalyst, the concentrations and, correspondingly, the thermodynamic mshes of the reaction complexes catalytic intermediates)—that is, the thermalized intermediate compounds of the reactant molecule (or molecular fragment) with the active center—appear interrelated through these balance relations in respect of each type of active center. The balance is of primary importance to the kinetics of the stepwise transformations and causes a number ofpecuHarities of the stationary kinetics of the stepwise processes. This makes the kinetic description of the catalytic transformations differ considerably from the descrip tion of the preceding schemes of noncatalytic reactions. For example, in the simplest catalytic stepwise transformation of substance R to substance P,... 

To arrive at rate equations of catalysis by a single species that is present almost exclusively as free catalyst, the formalism developed for noncatalytic reactions can be used with the catalyst appearing as both a reactant and a product. In fact, many of the examples used in earlier chapters to illustrate the deduction and application of rate equations were from catalytic reactions of this type. Thus, all the rules derived in Chapter 7 for network elucidation remain valid in such cases. 

Apart from these two complications, the formalism developed in Chapter 6 for noncatalytic reactions remains applicable as far as rate equations are concerned, but must be combined with modeling of the catalyst equilibria Where rate equations of transition-metal catalysis appeared in examples in earlier chapters, the quantity CMt is the concentration of the free catalyst and may have to be supplied by a catalyst equilibrium model. 

An example of cmcentration and temperature distributicm is given in Fig. 88. Reaction 1 is an ordinary reaction with the activaticm energy of 83.6 kJ/mole (20 kcal/mole) the rate of reaction 2 is chosen as one-tenth of that for reaction 1. The noncatalytic reactions 3-5 are rather violent with the activation energy of 167.2 kJ/mole and heats of reaction (A D) of 3 X 10 kJ/mole. This reacticm does not occur at the temperature of the dense phase, but is quite active at higher temperatures because of the... 

In conclusion, the possibility of highly exothermic noncatalytic reactions must be carefuUy eliminated, for all values of solid mixing. This is done by the control of composition of the feed mixture so as, for example, to suppress an excess of oxygen. Also, appropriate cooling is necessary in the dilute phase. 

Studies on the mechanisms of catalytic and non catalytic reac tions undertaken over the past 15-20 years have led to significant progress in the theory of reaction mechanisms. Most of the reactions involving homogeneous, metal-complex, and enzymatic catalyses were shown to be no less complex in terms of their mechanism compared with the mechanisms of radical chain processes. Infact, they appear to be much more complicated. Numerous examples of complicated mechanisms can be found in the literature. At present, multiroute mechanisms (with 2 to 4 reaction routes), involving as many as 8 intermediates and up to 12 elementary steps, are widely known to exist even in heterogeneous catalysis by metals and nonmetals where the simplest two-step schemes have hitherto been very popular. The existence of many routes and elementary steps is the most important general feature of the mechanisms of catalytic and also many noncatalytic reactions. 

Gas-solid heterogeneous reactions may be noncatalytic. An example is the hydrofluorination of uranium dioxide pellets referred to in Sec. 7-1. Since one reactant is in the solid phase and is consumed, the rate of reaction varies with time. Hence such processes are basically transient, in comparison with the steady-state operation of gas-solid catalytic reactors. The process for smelting ores such as zinc sulfide,... 

In many noncatalytic types a solid product builds up around the reacting core [for example, Na2S04(j) is deposited around the NaCl particles in the last illustration]. This introduces the additional physical processes of heat and mass transfer through a product layer around the solid reactant. A somewhat different form of noncatalytic gas-solid reaction is the regeneration of catalysts which have been deactivated by the deposition of a substance on the interior surface. The most common is the burning of carbon (with air) which has been gradually deposited on catalyst particles used in hydrocarbon reactions. Many of the physical and chemical steps involved here are. the same as those for gas-solid catalytic reactions. The chief difference is the transient nature of the noncatalytic reaction. This type of heterogeneous reaction will be considered in Chap. 14. 

We will continue with our excursion to the reaction systems with an interface but this time we will deal with noncatalytic gas-solid reactions. These types of reactions are quite common in industry, and even in everyday life, burning of coal being the most common example. The difference between the treatment of the gas-solid catalytic and gas-soM noncatalytic reactions are several fold. We will list the most important ones that will differentiate the analysis here ... 

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