Multiple reactions series

Multiple reactions Series or parallel reactions that take place simultaneously in a reactor. For example, A -(- B C and A + D - E are parallel reactions, and A + B C + D E + F arc series reactions. 

In this chapter we discuss reactor selection and general mole balances for multiple reactions. There are three basic types of multiple reactions series, parallel, and independent. In parallel reactions (also called competing reactions) the reactant is consumed by two different reaction pathways to form different products ... 

There are four basic types of multiple reactions series, parallel, complex, and independent. These types of multiple reactions can occur by themselves, in pairs, or all together. V en there is a combination of parallel and series reactions. they are often referred to as complex reactions. 

Closure. After completing this chapter the reader should be able to describe the different types of multiple reactions (series, parallel, complex. and independent) and to select a reaction system that maximizes the selectivity. The reader should be able to write down and use the algorithm for solving CRE problems with multiple reactions. The reader should also be able to point out the major differences in the CRE algorithm for the multiple reactions from that for the single reactions, and then discuss why care must be taken when writing the rate law and stoichiometric Steps to account for the rate laws for each reaction, the relative rates, and the net rates of reaction. 

Reactors for multiple reactions (series or parallel reactions) are designed to achieve maximum yield or selectivity of desired products (Section 2.1.8). Consider a series reaction... 

Multiple reactions in series producing byproducts. Rather than... 

Multiple reactions in series producing byproducts. Consider the system of series reactions from Eq. (2.7) ... 

Multiple reactions in series producing byproducts. For the series reaction system in Eq. (2.18), the series reaction is inhibited by low concentrations of PRODUCT. It has been noted already that this can be achieved by operating with a low conversion. 

Because the characteristic of tubular reactors approximates plug-flow, they are used if careful control of residence time is important, as in the case where there are multiple reactions in series. High surface area to volume ratios are possible, which is an advantage if high rates of heat transfer are required. It is sometimes possible to approach isothermal conditions or a predetermined temperature profile by careful design of the heat transfer arrangements. 

For multiple reactions in which the byproduct is formed in series, the selectivity decreases as conversion increases. In this case, lower conversion than that for single reactions is expected to be appropriate. Again, the best guess at this stage is to set the conversion to 50 percent for irreversible reactions or to 50 percent of the equilibrium conversion for reversible reactions. 

For a single equation, Eqs. (7-36) and (7-37) relate the amounts of the several participants. For multiple reactions, the procedure for finding the concentrations of all participants starts by assuming that the reactions proceed consecutively. Key components are identified. Intermediate concentrations are identified by subscripts. The resulting concentration from a particular reaction is the starting concentration for the next reaction in the series. The final value carries no subscript. After the intermediate concentrations are ehminated algebraically, the compositions of the excess components will be expressible in terms of the key components. 

If electronic effects are not constant in the reaction series, a multiple LFER is used ... 

This reaction cannot be elementary. We can hardly expect three nitric acid molecules to react at all three toluene sites (these are the ortho and para sites meta substitution is not favored) in a glorious, four-body collision. Thus, the fourth-order rate expression 01 = kab is implausible. Instead, the mechanism of the TNT reaction involves at least seven steps (two reactions leading to ortho- or /mra-nitrotoluene, three reactions leading to 2,4- or 2,6-dinitrotoluene, and two reactions leading to 2,4,6-trinitrotoluene). Each step would require only a two-body collision, could be elementary, and could be governed by a second-order rate equation. Chapter 2 shows how the component balance equations can be solved for multiple reactions so that an assumed mechanism can be tested experimentally. For the toluene nitration, even the set of seven series and parallel reactions may not constitute an adequate mechanism since an experimental study found the reaction to be 1.3 order in toluene and 1.2 order in nitric acid for an overall order of 2.5 rather than the expected value of 2. 

Example 14.6 derives a rather remarkable result. Here is a way of gradually shutting down a CSTR while keeping a constant outlet composition. The derivation applies to an arbitrary SI a and can be extended to include multiple reactions and adiabatic reactions. It is been experimentally verified for a polymerization. It can be generalized to shut down a train of CSTRs in series. The reason it works is that the material in the tank always experiences the same mean residence time and residence time distribution as existed during the original steady state. Hence, it is called constant RTD control. It will cease to work in a real vessel when the liquid level drops below the agitator. 

A batch or plug-flow reactor should be used for multiple reactions in series. 

Multiple reactions in series producing byproduct. Consider the system of series reactions from Equation 5.68. Selectivity for series reactions of the types given in Equation 5.7 to 5.9 is increased by low concentrations of reactants involved in the secondary reactions. In the preceding example, this means reactor operation with a low concentration of PRODUCT, in other words, with low conversion. For series reactions, a significant reduction in selectivity is likely as the conversion increases. 

When this potential is utilized in a series of A-dynamics, or CMC/MD simulations, the WHAM equations for multiple reaction coordinates and at constant temperature can be readily applied to obtain the best estimate of free energy using all of the data from n previous simulations... 

Multiple regression on j and og-type parameters employs the dual substituent-parameter equation, which may be written as in equation 891. (The combining of the k and k° terms implies that there is no intercept term allowed, and k° is now the actual value for the parent system, cf below.) For any given reaction series the equation is applied to meta- and para-substituents separately, and so values of pi and pr characteristic both of reaction and of substituent position are obtained. The various op-type scales are linearly related to each other only approximately. In any given application the scale which gives the best correlation must be found92. 

For multiple reactions a change in the observed activation energy with temperature indicates a shift in the controlling mechanism of reaction. Thus, for an increase in temperature Eq s rises for reactions or steps in parallel, Eobs falls for reactions or steps in series. Conversely, for a decrease in temperature E s falls for reactions in parallel, E s rises for reactions in series. These findings are illustrated in Fig. 2.3. 

Since multiple reactions are so varied in type and seem to have so little in common, we may despair of finding general guiding principles for design. Fortunately, this is not so because many multiple reactions can be considered to be combinations of two primary types parallel reactions and series reactions. 

For multiple reactions the effect of this flow is much more serious still. Thus for reactions in series the lowering in amount of intermediate formed can be and usually is quite drastic. 

Most multiple-reaction systems are more comphcated series-parallel sequences with multiple reactants, some species being both reactant and product in different reactions. These simple rules obviously will not work in those situations, and one must usually solve the mass-balance equations to determine the best reactor configuration. 

At the same time, as a chemist I was disappointed at the lack of serious chemistry and kinetics in reaction engineering texts. AU beat A B o death without much mention that irreversible isomerization reactions are very uncommon and never very interesting. Levenspiel and its progeny do not handle the series reactions A B C or parallel reactions A B, A —y C sufficiently to show students that these are really the prototypes of aU multiple reaction systems. It is typical to introduce rates and kinetics in a reaction engineering course with a section on analysis of data in which log-log and Anlienius plots are emphasized with the only purpose being the determination of rate expressions for single reactions from batch reactor data. It is typically assumed that ary chemistry and most kinetics come from previous physical chemistry courses. 

Because the intron itself is chemically altered during the splicing reaction—its ends are cleaved—it may appear to lack one key enzymatic property the ability to catalyze multiple reactions. Closer inspection has shown that after excision, the 414 nucleotide intron from Tetrahymena rRNA can, in vitro, act as a true enzyme (but in vivo it is quickly degraded). A series of... 

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