Mixed parallel-consecutive reactions Mixing

The kinetics of the ammoxidation of xylenes over a vanadium catalyst and mixed vanadium catalysts were studied. The reaction rate data obtained were correlated with the parallel consecutive reaction scheme by the rate equations based upon the Langmuir-Hinshelwood mechanism where the adsorption of xylenes was strong. The reaction rates of each path are remarkably affected by the kind of xylene and catalyst. The results of the physical measurement of catalysts indicated that the activity and the selectivity of reaction were affected by the nature and the distribution of metal ions and oxygen ion on catalyst surface. 

Parallel-consecutive reactions belong to the mixed type which have the characteristics of both parallel and consecutive reactions. The following example comprises two parallel chains, each composed of three simple reactions ... 

Mixed Parallel-Consecutive Reactions Reducing the Size of Kinetic Models... 

Imposing oscillations in the feed concentrations for a continuous back-mixed reactor can also result in beneficial changes of reaction selectivity [58]. Such changes are likely to be more significant with intermediates in consecutive reactions than with products from simultaneous reactions in parallel [59]. 

Equation (19-22) indicates that, for a nominal 90 percent conversion, an ideal CSTR will need nearly 4 times the residence time (or volume) of a PFR. This result is also worth bearing in mind when batch reactor experiments are converted to a battery of ideal CSTRs in series in the field. The performance of a completely mixed batch reactor and a steady-state PFR having the same residence time is the same [Eqs. (19-5) and (19-19)]. At a given residence time, if a batch reactor provides a nominal 90 percent conversion for a first-order reaction, a single ideal CSTR will only provide a conversion of 70 percent. The above discussion addresses conversion. Product selectivity in complex reaction networks may be profoundly affected by dispersion. This aspect has been addressed from the standpoint of parallel and consecutive reaction networks in Sec. 7. 

Currently only competing parallel and consecutive reactions are used for determining micro-mixing. 

If the reaction rate is comparable with the rate of the local micro-mixing, similar information can be obtained from competitive parallel reactions as from competitive consecutive reactions. However, parallel reactions offer the experimenter greater degrees of freedom with respect to different feed sequences or different stoichiometric ratios. 

In many catalytic systems multiple reactions occur, so that selectivity becomes important. In Sec. 2-10 point and overall selectivities were evaluated for homogeneous well-mixed systems of parallel and consecutive reactions. In Sec. 10-5 we saw that external diffusion and heat-transfer resistances affect the selectivity. Here we shall examiineHEieHnfiuence of intrapellet res ahces on selectivity. Systems with first-order kinetics at isothermal conditions are analyzed analytically in Sec. 11-12 for parallel and consecutive reactions. Results for other kinetics, or for nonisothermal conditions, can be developed in a similar way but require numerical solution. ... 

Imperfect mixing can also affect the selectivity for consecutive reactions of the following type, which are sometime called consecutive-parallel reactions ... 

The previous examples dealt with the effect of mixing delays on fast parallel or consecutive reactions. Mixing effects are also found in bioreactors, even though the cell growth and product formation reactions are relatively slow [5]. Because of the low solubility of oxygen in water, the dissolved... 

Now for kjCjio kj, or o, > 02, it seems reasonable to expect that the parallel reaction is more critical than the consecutive step in decreasing the yield of Q, and based on the above paragraphs the optimum choice would be a perfectly mixed reactor rather than a plug flow reactor—this will be verified by calculations. Also, for kjCjio < k2< or o, <02, the consecutive reaction should dominate, and the plug flow reactor should be best However, for a, 02, it is not so clear which is (he optimum reactor type. 

In both cases the first reaction (Equations 4.33a and 4.34a) is instantaneous (e.g., a neutrahzation) while the second reaction has characteristic reaction time comparable to the characteristic mixing time. The reactant A2 is added in an overall stoichiometric defect to the stream of Aj in consecutive reactions or to the stream of solution containing A and A in parallel reactions. If the aggregates of A2 are rapidly mixed forming homogenous solution with a rate much faster than the rate of the second reaction t. ), species A2 will be almost totally consumed by... 

The use of a single reaction requires the online measurement of the local species concentration along the flow. With such systems, one experiences the main drawback of physical methods with the local measurement and the influence of the probe size on the mixing quality estimation. For that reason, the so-called test reactions are very attractive. Two main systems, based on competitive chemical reactions, have been proposed for the investigation of mixing effects, that is, the competitive consecutive reaction system (Scheme 6.1) and the competitive parallel reaction system (Scheme 6.2). Let us consider the following simplest reactions schemes which do not exactly match the published real systems, but which facilitate the comparison ... 

Characteristic Reaction Time. As shown in Example 13-1, mixing can affect the selectivity of a reaction, not just the rate. Reactions that show selectivity are usually two-step reactions which are either consecutive or parallel. One reaction is usually so fast that it is mixing controlled. The second reaction has a characteristic time constant of the order of the local mixing time. The reaction time is usually given by... 

Note This protocol is focused on mixing effects for the classic competitive-consecutive reaction system. Reaction systems may also include parallel reactions in which A, B, or R are reacting to form unwanted products that are not represented by the consecutive-competitive system as used to derive eq. (13-5). To keep these reactions from making more unwanted products on scale-up, the overall reaction (addition) time may have to be held constant. In this case, the mesomixing issue for the primary reactions, A - - B R and R - - B S, would predict that more S would be formed. These issues may require selection of an alternative reactor, such as an in-line mixer, for successful scale-up. 

Mixing-Kinetic Problem. The reaction scheme that has received the most attention in both theoretical and experimental investigations of the effects of mixing on selectivity is the competitive-consecutive reaction. In addition, the parallel reaction system is receiving attention for its importance in reactions and pH adjustments. These systems are discussed in Chapter 13 and highlighted here because of their fundamental importance in the fine chemicals and pharmaceutical industries. The reaction scheme is as follows ... 

Mixing is a basic unit operation for a chemical reaction and has a decisive impact on the product yield and selectivity and the performance of chemical reactors. If the mixing time is of the same order as the characteristic reaction time, the product distribution is strongly influenced in the case of complex reaction networks with parallel and/or consecutive reactions [6,7]. To avoid the negative influence on product distribution, the mixing time should be at least 10 times shorter than the characteristic reaction time. 

This reaction system is a kind of complex van de vusse reaction, a typical reaction process involving consecutive and parallel reactions. It is, therefore, sufficiently complex to illustrate the algorithm. Due to the limit to the space of the article, we only select temperature, concentration and back-mixing to be the object of research to illustrate the use of the algorithm. 

Mixing may occur on several scales on the reactor scale (macro), on the scale of dispersion from a feed nozzle or pipe (meso), and on a molecular level (micro). Examples of reactions where mixing is important include fast consecutive-parallel reactions where reactant concentrations at the boundaries between zones rich in one or the other reactant being mixed can determine selectivity. 

Additional aggravating circumstances arise from the fact that chemical steps which are transfer limited will proceed differently in the industrial plant than on laboratory-scale. The selectivity of multiple reactions such as competing consecutive and parallel reactions depend very much on the extent of micro-mixing in the system. These facts are well known from Chemical Reaction Engineering textbooks. Conversely, these reactions are carried out to obtain details about the extent of micro- and macro-mixing in stirring. 

The choice of the appropriate reactor applied for kinetic measurements is determined by the type of reaction (simple, parallel, or consecutive), the reaction heat and the phase state of the reaction mixture. In general, reactors with simple, almost ideal mixing behavior are preferred in order to obtain simple material balances. 

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