Kinetics positive order

The piston flow reactor has an advantage over a stirred tank reactor when the kinetics is of positive order, but the reverse is true when the... 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

Thus in Table 4.3 we add to Table 4.2 the last, but quite important, available piece of information, i.e. the observed kinetic order (positive order, negative order or zero order) of the catalytic reaction with respect to the electron donor (D) and the electron acceptor (A) reactant. We then invite the reader to share with us the joy of discovering the rules of electrochemical promotion (and as we will see in Chapter 6 the rules of promotion in general), i.e. the rules which enable one to predict the global r vs O dependence (purely electrophobic, purely electrophilic, volcano, inverted volcano) or the basis of the r vs pA and r vs pD dependencies. 

Inspection of Table 6.1 shows the following rule for electrophobic reactions Rule Gl A reaction exhibits purely electrophobic behaviour ((dr/dO)PA 0) when the kinetics are positive order in the electron donor (D) reactant and negative or zero order in the electron acceptor (A) reactant. 

Inspection of Table 6.1 shows that reactions exhibiting volcano-type (maximum type) behaviour with respect to positive order in A or D and at the same time negative (not zero) order in D or A respectively. 

The kinetics depicted in Figures 9.4 in conjunction with Figure 9.5 and 4.30 provide an excellent example of promotional rules L2, and G2 (electrophilic behaviour), as well as rule G3 (volcano type behaviour). As long as the rate is negative order in C2H4 and positive order in po2 (Fig. 9.4)... 

Figure 26.1 represents the heat profile of the benzophenone hydrazone and hexylamine reactions. At the same conditions, at 90°C, the reaction involving hexylamine is considerably faster. The heat profile of the hexylamine reaction at 70°C shows how the reaction has positive order kinetics, while the benzophenone reaction shows overall zero order kinetics. 

The heat profile shows that the reaction has zero order kinetics at first, and then switches to positive order kinetics. The benzophenone hydrazone reacts first only when it is completely consumed, the reaction involving hexylamine begins. Samples were taken and analyzed by and NMR. One sample was taken when the aryl halide conversion was low, at about 5%, and the profile was overall zero order the second when the profile had switched to positive order and the conversion of the halide was greater than 50%. 

We studied the competitive amination of two amines (benzophenone hydrazone and -hexylamine) and one aryl halide (3-bromobenzotrifluoride), catalyzed by Pd(BlNAP). We showed that, when reacting alone at the same conditions, n-hexylamine is considerably more reactive and shows positive order kinetics benzophenone hydrazone shows zero order kinetics and forms a very stable intermediate, the BlNAP(Pd)Ar(amine) we also observed by NMR. During the competitive reaction of the two amines, the benzophenone hydrazone reacts first and only when it is completely consumed, the hexylamine starts to react. In this case it is the stability of the major intermediate, and not the relative reactivity, which dictates the selectivity. 

Note that 1 jr goes to infinity as Ca 0 for any kinetics because reaction rates must go to zero when reactants have been consumed. This is equivalent to saying that the kinetics of all reactions must become positive order in the limit of any reactant disappearing. 

For a reversible reaction the rate goes to zero before the reaction reaches completion, and 1 /r therefore goes to infinity. A plot of 1/r versus Cao — Ca, with positive-order kinetics will look as shown in Figure 3-14. The residence time for a given conversion obviously approaches infinity as the conversion approaches equilibrium in either a PFTR or CSTR. Just as for irreversible reactions, the CSTR requires a longer T for a given conversion. 

The PFTR will always give a higher maximum yield of an intermediate if aU reactions obey positive-order kinetics. 

For zeroth-order kinetics the maximum selectivities are identical, and for negative-order kinetics the CSTR wiU give a higher maximum selectivity. [What type of reactor will be better if one reaction is positive order and the other negative order ]... 

From this example we see that the CSTR requires a longer residence time for the required 90% conversion (it must since the kinetics are positive order), but the CSTR gives a higher selectivity to B. One can always design for a larger reactor, but the A that was converted to C is a continual loss. Thus the CSTR is the clear choice of reactor in selectivity for this example, where we wanted to favor a lower-order reaction rather than a high-order reaction. 

For a single reaction in an isothermal reactor the design principles involved primarily the reactor configuration for niinimum residence time. This generally favors the PFTR over the CSTR for positive-order kinetics. However, the CSTR frequently is less costly and easier to maintain, and one or more CSTRs can frequently be preferred over a PFTR. 

For series reactions with an intermediate desired, there is always an optimum T for maximum yield, and the PFTR gives a higher maximum yield if both reactions have positive order, while the CSTR gives a higher maximum yield if the reactions are negative order (a rather rare occurrence). For series reactions with the final product desired, the PFTR requires the shorter time and gives less intermediate for positive-order kinetics. 

Thus we see that for nonisothermal reactors this 1/r versus Cao Ca curve is not always an increasing function of conversion as it was for isothermal reactors even with positive-order kinetics. Since the 1/r curve can have a rninimum for the nonisothermal reactor, we confirm the possibility that the CSTR requires a smaller volume than the PFTR for positive-order kinetics. This is hue even before the multiple steady-state possibilities are accounted for, which we will discuss in the next chapter. This is evident from our 1 /r plot for the PFTR and CSTR and will occur whenever r has a sufficiently large maximum that the area under the rectangle is less than the area under the curve of 1/r versus Cao Ca-... 

Therefore, we can generalize the previous discussion to say that aU qualitative features of multiple steady states in the CSTR remain unchanged for the nth-order irreversible reaction as long as is obeys positive-order kinetics. We will consider zeroth-order and negative-order kinetics in problems. 

Recycle will always give a lower conversion in a PFTR for positive-order kinetics because it produces backmixing, in which product mixes with reactant at the entrance to the plug-flow section. 

Kinetics There have been few comprehensive studies of the kinetics of selective oxidation reactions (31,32). Kinetic expressions are usually of the power-rate law type and are applicable within limited experimental ranges. Often at high temperature the rate expression is nearly first order in the hydrocarbon reactant, close to zero order in oxygen, and of low positive order in water vapor. Many times a Mars-van Krevelen redox type of mechanism is assumed to operate. 

Electrophilic replacement constants crXr have been obtained for all the positions of benzo[6]thiophene from the solvolysis of isomeric l-(benzo[ >]thienyl)ethyl chlorides in 80% ethanol-water. These constants signify replacement of the entire benzene ring by another aromatic system (74JOC2828). The positional order of reactivity was determined to be 3>2>6>5>4>7, all positions being more reactive than benzene. The same order was also derived from the kinetic data for pyrolysis of the isomeric l-(benzo[6]thienyl)ethyl acetates (78JCS(P2)1053). A modified extended selectivity treatment has been developed to correlate electrophilic substitution data in benzo[Z> ]thiophene, which assumes a dual activation mechanism (79JOC724). 

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