Experimental design chemical reaction processes

As mentioned in Sec. 1-1, the first step in a logical design procedure is to obtain a suitable expression for the rate of the chemical reaction process, and this requires experimental data. The data can be obtained in several ways ... 

The story of the ozone hole illustrates how important it is to learn the molecular details of chemical reactions. Some chemists use information about how reactions occur to design and synthesize useful new compounds. Others explore how to modify reaction conditions to minimize the cost of producing industrial chemicals. This chapter explores how chemical reactions occur at the molecular level. We show how to describe a reaction from the molecular perspective, introduce the basic principles that govern these processes, and describe some experimental methods used to study chemical reactions. 

Unfortunately, many of the chemical processes which are important industrially are quite complex. A complete description of the kinetics of a process, including byproduct formation as well as the main chemical reaction, may involve several individual reactions, some occurring simultaneously, some proceeding in a consecutive manner. Often the results of laboratory experiments in such cases are ambiguous and, even if complete elucidation of such a complex reaction pattern is possible, it may take several man-years of experimental effort. Whereas ideally the design engineer would like to have a complete set of rate equations for all the reactions involved in a process, in practice the data available to him often fall far short of this. 

Reaction characterisation by calorimetry generally involves construction of a model complete with kinetic and thermodynamic parameters (e.g. rate constants and reaction enthalpies) for the steps which together comprise the overall process. Experimental calorimetric measurements are then compared with those simulated on the basis of the reaction model and particular values for the various parameters. The measurements could be of heat evolution measured as a function of time for the reaction carried out isothermally under specified conditions. Congruence between the experimental measurements and simulated values is taken as the support for the model and the reliability of the parameters, which may then be used for the design of a manufacturing process, for example. A reaction modelin this sense should not be confused with a mechanism in the sense used by most organic chemists-they are different but equally valid descriptions of the reaction. The model is empirical and comprises a set of chemical equations and associated kinetic and thermodynamic parameters. The mechanism comprises a description of how at the molecular level reactants become products. Whilst there is no necessary connection between a useful model and the mechanism (known or otherwise), the application of sound mechanistic principles is likely to provide the most effective route to a good model. 

The field of chemical kinetics and reaction engineering has grown over the years. New experimental techniques have been developed to follow the progress of chemical reactions and these have aided study of the fundamentals and mechanisms of chemical reactions. The availability of personal computers has enhanced the simulation of complex chemical reactions and reactor stability analysis. These activities have resulted in improved designs of industrial reactors. An increased number of industrial patents now relate to new catalysts and catalytic processes, synthetic polymers, and novel reactor designs. Lin [1] has given a comprehensive review of chemical reactions involving kinetics and mechanisms. 

The secondary chemical reactions which follow the primary photo-process are highly specific and just as complicated as pure thermal reactions. It is the task of the kineticist to discover and record quantitatively and mathematically the various steps which follow the primary process to give the over-all, observed reaction rate. The most important aid in this work is the experimental determination of the quantum yield, i.e., the total number of molecules reacting for each photon absorbed or the number of moles per einstein. It is frequently designated by the symbol < . 

The derivation of a mechanism for a chemical reaction is by its very nature an uncertain process, being dependent critically on the nature and extent of the experimental evidence. Mechanisms that have at their heart a surface process or processes are even more uncertain and when the constraints imposed by the manipulation of HF are also taken into account, it is not surprising that there have been relatively few mechanistic studies made of heterogeneous catalytic fluorination. However a catalytic process cannot be said to be understood fully without a mechanism based on the experimental evidence available and such studies are helpful in the design of the next generation of catalysts. In most cases the work described below involves chromia or y-alumina based catalysts that have been pretreated according to the methods described above. Studies involving C2 and Q compounds are described in turn. 

---