Real Reaction Systems and Modeling

We have considered several simple examples of multiple reaction systems. While the simplest of these systems have analytical solutions, we rapidly come to situations where only numerical solutions are possible. 

However, the ideas in the examples given are still very useful in dealing with more complex problems because simple examples with analytical solutions allow us to estimate reactor performance quickly without resorting to complex numerical solutions. Further, the kinetics of the complete reactions are frequently unknown, or, if rate expressions are available, there may be large errors in these rate parameters that will make even the complete solution approximate. This is especially true for surface-catalyzed reactions and biological reactions, where the rate expressions are complex with fractional orders and activation 

Therefore, complex processes are frequently simplified to assume (1) a single reaction in which the major reactant is converted into the major product or for a more accurate estimate (2) simple series or parallel processes in which there is a major desired and a single major undesired product. The fust approximation sets the approximate size of the reactor, while the second begins to examine different reactor types, operating conditions, feed composition, conversion, separation systems required, etc. 

Thus there are enough uncertainties in the kinetics in most chemical reaction processes that we almost always need to resort to a simplified model from which we can estimate performance. Then, from more refined data and pilot plant experiments, we begin to refine the design of the process to specify the details of the equipment needed. 

At some point in most processes, a detailed model of performance is needed to evaluate the effects of changing feedstocks, added capacity needs, changing costs of materials and operations, etc. For this, we need to solve the complete equations with detailed chemistry and reactor flow patterns. This is a problem of solving the R simultaneous equations for S chemical species, as we have discussed. However, the real process is seldom isothermal, and the flow pattern involves partial mixing. Therefore, in formulating a complete simulation, we need to add many additional complexities to the ideas developed thus far. We will consider each of these complexities in successive chapters temperature variations in Chapters 5 and 6, catalytic processes in Chapter 7, and nonideal flow patterns in Chapter 8. In Chapter 8 we will return to the issue of detailed modeling of chemical reactors, which include all these effects. 

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