The study of reaction engineering combines the study of chemical kinetics (the study chemical reaction rate and mechanism) with the reactors in which the reaction occurs. Chemical reaction and the reactor are the heart of chemical processing industries to synthesis a chemical compound. The selection of reacting system is very important for the production of desired product in safest mode.
The knowledge of reaction rate (-rA) or rate law, principle of mass and energy balance, and phase-equilibria are the key steps to formulate and design a reactor. The rate of reactions indicates the speed with which a particular species disappear or produced. Basically, there are two types of reaction, homogeneous and heterogeneous reaction. In homogeneous reaction each species are in same phase, therefore rate of reaction is expressed in terms of volume of system. On the other hand, heterogeneous reactions involve more than one phase, so rate of reaction is expressed in terms of reaction surface area or weight of the catalyst. The rate law or equation is strong function of reacting materials and the condition of the reaction, such as temperature, pressure, species concentration, or the types of catalyst. However, the rate equation does not depend on the types of reactor, for example batch or continuous flow. In general, the rate law is written as: -rA = kCAn, where k is rate constant has strong dependency on the temperature of the reaction, CA the species concentration, and n order of the reaction. For elementary homogeneous reaction, the order of reaction is the stoichiometry coefficient, otherwise it has to be determined from the knowledge of the reaction mechanism (the slowest step of the mechanism determines the reaction rate) or by performing an experiment in a batch mode.
To write mole or mass balance equation, the system boundaries must be defined. The volume enclosed by theses boundaries is termed as system volume. For any species j, the mole balance is written as: Rate of flow of species j into the system volume –Rate of flow species j out of the system volume + rate of generation of species j within the system = Rate of accumulation of species j within the system. For a batch system, there is no input to and output from the system. Also, for steady state, the accumulation term is neglected.
As stated earlier, the reacting system is either batch or continuous. A batch reactor is used to synthesize product in small-scale, for testing new process that has not been developed, for the manufacture of expensive product, and a process that cannot be transformed to continuous mode. In batch reactor, the reacting species is kept inside the reactor for a long period of time, thus it has advantage of high conversion of reacting species. However, it involves high labor cost and prohibits large scale productions. The governing equations of batch reactor includes the accumulation and generation terms only, there are no input and output term in the rate equation.
Continuous flow reactor involves input, output and generation terms. In general, it is operated at steady state. There are three types of continuous reactor, namely continuous stirred tank reactor (CSTR), plug flow reactor (PFR), and packed bed reactor (PBR).
The CSTR reactor is usually used for liquid phase reaction, and assumed to be perfectly mixed and operated at steady state. It implies that every variable, for example concentration or temperature is independent of time and position inside the reactor. The design equation of CSTR: volume of the CSTR = (rate of flow of species j into the system – rate of flow of species j out of the system)/rate of consumption of species j. Thus, CSTR design equation gives the volume of reactor necessary to carry out the reaction.
In addition to batch and CSTR, there is another type of reactor called as PFR. To write model equation of PFR, it is assumed that there is no radial variations in the reaction rates and the operated at steady state to avoid complexity in solving the design the equations. The governing equation for PFR is: dF/dV = rA.
When chemical reacting species react, multiple reactions occur, such as series and parallel. In series reaction, the reactant forms an intermediate product, which reacts further to form another product. On the other hand, in parallel reaction the reactant is consumed in two different reaction pathways to form two different products.
Selectivity and yield are the two important aspects for the formation of product. Selectivity tells how one product is favoured over another product when we have multiple reactions. There are two types of selectivity, one instantaneous and other one is overall. The instantaneous selectivity of desired compound over undesired compound is the ratio of rate of formation of desired product to the rate of formation of undesired product. The overall selectivity is the ratio of exit molar flow rate of desired product to the exit molar flow rate of undesired product. Reaction yield are of two types, such as instantaneous and overall. The instantaneous reaction yield is the ratio of reaction rate of given product to the reaction rate of the key reactant. Similarly, the overall yield is the ratio of the moles of product formed at the end of the reaction to the moles of key reactant that has been consumed.
Most of the reactions are not carried out isothermally (at constant temperature), therefore it is necessary to understand the effect of heat on the progress of the reaction. Now, for non-isothermal energy balance equation is required in addition to the rate law, design equation and stoichiometry. The energy balance tells how the temperature effects the reaction and accordingly the reactor has to be designed.
Another important topic is the understanding of the reaction on the surface of the catalyst. The function of catalyst is to speed up the reaction without getting consumed. However, it gets degrades with the time. Therefore, we need to know or develop techniques that help in avoiding the loss of catalytic property. Also, fundamental knowledge of mass transfer is necessary to understand the molar flux of reacting species on the surface of catalyst.
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