Homogeneous reactions, definition

For homogeneous reactions we obtain the conventional definition of the reaction rate u. as rate of conversion per volume... 

The rate of a chemical reaction can be described in any of several different ways. The most commonly used definition involves the time rate of change in tlie amount of one of the components participating in tlie reaction tliis rate is usually based on some arbitrary factor related to tlie reacting system size or geometry, such as volume, mass, or interfacial area. Tlie definition shown in Eq. (4.6.7), wliich applies to homogeneous reactions, is a convenient one from an engineering point of view. 

In order to exemplify the potential of micro-channel reactors for thermal control, consider the oxidation of citraconic anhydride, which, for a specific catalyst material, has a pseudo-homogeneous reaction rate of 1.62 s at a temperature of 300 °C, corresponding to a reaction time-scale of 0.61 s. In a micro channel of 300 pm diameter filled with a mixture composed of N2/02/anhydride (79.9 20 0.1), the characteristic time-scale for heat exchange is 1.4 lO" s. In spite of an adiabatic temperature rise of 60 K related to such a reaction, the temperature increases by less than 0.5 K in the micro channel. Examples such as this show that micro reactors allow one to define temperature conditions very precisely due to fast removal and, in the case of endothermic reactions, addition of heat. On the one hand, this results in an increase in process safety, as discussed above. On the other hand, it allows a better definition of reaction conditions than with macroscopic equipment, thus allowing for a higher selectivity in chemical processes. 

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. 

Where the rate of reaction depends upon the presence of an accidental catalyst, measurements are characterized by great lack of reproducibility. Many homogeneous reactions, on the other hand, have quite definite and reproducible rates. For example, the measurements of Bodenstein and of Kistiakowsky on the rate of decomposition of hydrogen iodide agree excellently, as do those of numerous investigators of the rate of decomposition of nitrogen pentoxide. 

We adopt this kinetic definition of reaction order without reference to the actual number of molecules involved in each act of chemical transformation. In homogeneous reactions the kinetically determined order is equal to the number of molecules participating in the actual change of which the rate is being measured. In heterogeneous reactions this equality is not necessarily preserved. It will be convenient to call the order inferred from the effect of pressure on the time of half-change the apparent order, and to refer to the number of molecules involved as the true order of the reaction. We have now to consider the relation of the true and the apparent order in various cases. ... 

Thus far, the discussion of reaction rate has been confined to homogeneous reactions taking place in a closed system of uniform composition, temperature, and pressure. However, many reactions are heterogeneous they occur at the interface between phases, for example, the interface between two fluid phases (gas-liquid, liquid-liquid), the interface between a fluid and solid phase, and the interface between two solid phases. In order to obtain a convenient, specific rate of reaction it is necessary to normalize the reaction rate by the interfacial surface area available for the reaction. The interfacial area must be of uniform composition, temperature, and pressure. Frequently, the interfacial area is not known and alternative definitions of the specific rate are useful. Some examples of these types of rates are ... 

Although recent literature gives evidence for a homogeneous reaction on the catalyst surface (cf. Section 3.3.5.3.2) the commercial processes show typical features of heterogeneous catalysis and do not comply with the definition of homogeneous catalysis given in the preface. Therefore a description of the manufacturing process is not included here but it is dealt with in [59]. 

Other important definitions include those of homogeneous, heterogeneous, and irreversible reactions. A homogeneous reaction occurs within a single phase, as in a stream that is, + HCOj = H2CO3. Heterogeneous reactions occur between phases, as gas-water, water-mineral, or, rarely, gas-mineral, as illustrated by the following reactions... 

Entries 1 and 6 of Table 1 are the two cases of homophase homogeneous catalysis, either in aqueous or in organic solution. The evaluation (in view of catalyst separation) is negative, because in accordance with the definition of homogeneous reactions the catalyst is dissolved in the reaction medium and cannot be separated directly from the products. 

It would be very desirable to use the a constants, which have been carefully determined for homogeneous reactions in solution, for the reactions of solid catalysts also. This would overcome the difficult task of finding suitable catalytic transformation as a standard for the definition of special catalytic a values such a process must exhibit high reproducibility which is difficult to attain with heterogeneous catalytic reactions. However, the application of the liquid-phase a values requires the same blend of interaction mechanisms to operate in both homogeneous and heterogeneous reactions. This a strict condition which might not be always met. 

While photocatalytic reactions are frequently considered to be pseudo-homogeneous reactions with a rate based on either the unit volume of irradiated catalyst or the total reactor volume (Chapter I), definitions of the PTEF can be given as follows ... 

Thus, when proceeding with energy efficiency calculations, PTEF definitions using pseudo homogenous reaction rates r or r" + are preferred. For this reason data analysis is developed on the basis of equation (9-4). 

When analyzing kinetic data or designing a chemical reactor, it is important to state clearly the definitions of reaction rate, conversion, yield, and selectivity. For a homogeneous reaction, the reaction rate is defined either as the amount of product formed or the amount of reactant consumed per unit volume of the gas or liquid phase per unit time. We generally use moles (g mol, kg mol, or lb mol) rather than mass to define the rate, since this simplifies the material balance calculations. 

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