Rapid flow reactors

Eigure 2 shows that even materials which are rather resistant to oxidation ( 2/ 1 0.1) are consumed to a noticeable degree at high conversions. Also the use of plug-flow or batch reactors can offer a measurable improvement in efficiencies in comparison with back-mixed reactors. Intermediates that cooxidize about as readily as the feed hydrocarbon (eg, ketones with similar stmcture) can be produced in perhaps reasonable efficiencies but, except at very low conversions, are subject to considerable loss through oxidation. They may be suitable coproducts if they are also precursors to more oxidation-resistant desirable materials. Intermediates which oxidize relatively rapidly (/ 2 / i — 3-50 eg, alcohols and aldehydes) are difficult to produce in appreciable amounts, even in batch or plug-flow reactors. Indeed, for = 50, to isolate 90% or more of the intermediate made, the conversion must... 

Batch reactors often are used to develop continuous processes because of their suitabiUty and convenient use in laboratory experimentation. Industrial practice generally favors processing continuously rather than in single batches, because overall investment and operating costs usually are less. Data obtained in batch reactors, except for very rapid reactions, can be well defined and used to predict performance of larger scale, continuous-flow reactors. Almost all batch reactors are well stirred thus, ideally, compositions are uniform throughout and residence times of all contained reactants are constant. 

The transformation of CF3CH2CI was studied at 320 C in a pulse flow reactor. Indeed, in a dynamic reactor, the agnificant alkene formation leads to a rapid deactivation of the catalyst. The reaction is carried out in absence of HF in order to favour the dehydrofluorination reaction. Products distribution is shown in Fig. 1. 

The first few experiments in the continuous flow reactor yielded inconsistent octene conversions (Figure 28.3). The experiment ran for 218 hours. Initially the conversion was consistent at 3-4% for several hours, then improved significantly to 16% and then rapidly dropped off to less than 2% (Figure 28.3). The selectivity was also very good for this ran, with an average normal to branch isomer ratio of 7 1. 

These initial experiments show that results can be obtained from this system that are comparable to those from the continuous flow reactor. The analytical system satisfies the requirements for accurate and rapid repetitive analysis. Scanning of 12 masses is possible at rates of approximately 100 ms/scan with good results. Further data manipulations are expected to yield additional results from this type of experiments. 

Figure 3.1 shows a typical laboratory flow reactor for the study of catalytic kinetics. A gas chromatograph (GC, lower shelf) and a flow meter allow the complete analysis of samples of product gas (analysis time is typically several minutes), and the determination of the molar flow rate of various species out of the reactor (R) contained in a furnace. A mass spectrometer (MS, upper shelf) allows real-time analysis of the product gas sampled just below the catalyst charge and can follow rapid changes in rate. Automated versions of such reactor assemblies are commercially available. 

The research group of Van Leeuwen reported the use of carbosilane de-ndrimers appended with peripherial diphenylphosphino end groups (i.e. 25, Scheme 26) [37]. After in situ complexation with allylpalladium chloride, the resultant metallodendrimer 25 was used as catalyst in the allylic alkylation of sodium diethyl malonate with allyl trifluoroacetate in a continuous flow reactor. Unlike in the batch reaction, in which a very high activity of the dendrimer catalyst and quantitative conversion of the substrate was observed, a rapid decrease in space time yield of the product was noted inside the membrane reactor. The authors concluded that this can most probably be ascribed to catalyst decomposition. The product flow (i.e. outside the membrane reactor)... 

There are many different aspects to the field of turbulent reacting flows. Consider, for example, the effect of turbulence on the rate of an exothermic reaction typical of those occurring in a turbulent flow reactor. Here, the fluctuating temperatures and concentrations could affect the chemical reaction and heat release rates. Then, there is the situation in which combustion products are rapidly mixed with reactants in a time much shorter than the chemical reaction time. (This latter example is the so-called stirred reactor, which will be discussed in more detail in the next section.) In both of these examples, no flame structure is considered to exist. 

Notice that the temperature difference between the cooling jacket and the reactor must be increased as the size of the reactor increases. The flow rate of cooling water also increases rapidly as reactor size increases. 

By placing the impeller within a draft tube within the reactor, the fluids are forced to pass through the impeller, where the bubbles are redispersed by impacting on the impeller surfaces. The draft tube is placed in the center of the reactor so the fluids recirculate repeatedly (a recycle reactor) to allow bubbles to be repeatedly redispersed in the draft tube. The overall reactor becomes well mixed and is therefore described by the CSTR equations. The rapid flow of this reactor enhances the mass transfer rate and thus increases the overall reaction rate if it is limited by mass transfer of a reactant from the liquid phase into the bubbles. 

In the US, two programs were reported High Temperature Solar Splitting of Methane to Hydrogen and Carbon, and Rapid Solar-thermal Dissociation of Natural Gas in an Aerosol Flow Reactor. 

A mixed-flow reactor requires uniform composition of the fluid phase throughout the volume while the fluid is constantly flowing through it. This requires a special design in order to be achieved in the case of gas-solid systems. These reactors are basically experimental devices, which closely approach the ideal flow conditions and have been devised by Carbeny (Levenspiel, 1972). This device is called a basket-type mixed reactor (Figure 3.6). The catalyst is contained in four rapidly spinning wire baskets. 

The quality of Mo03 combinations with iron, cobalt and manganese oxides has been investigated by Mazzocchia et al. [213], Table 9 presents some results, obtained with a flow reactor at 300—430° C using a C3H6/02 ratio of 0.22—0.28. The best catalyst is MnMo04. The addition of small amounts of water (up to 2.5%) further increases the selectivity, but larger amounts cause rapid deactivation. 

A commercial iron-promoted catalyst (Sn/Sb/Fe = 1/4/0.25) was studied by Germain et al. [92,93,135,137]. Iron is reported to improve the ammoxidation qualities of the catalyst although it has no effect on the oxidation [93], The kinetics, determined in a flow reactor at 445°C and with a feed ratio C3H6/NH3/air = 1/1.2/10, are essentially similar for this catalyst and bismuth molybdate. The initial selectivity is 80% and the maximum yield is 65% (at 445°C). The initial selectivity markedly depends on the temperature (e.g. 91% at 415°C and 72% at 507°C). The effect of water is hardly significant for this catalyst the acrylonitrile formation is slightly inhibited, while some more acrolein is formed. Presumably, water and ammonia compete in the interaction with the catalyst, which is much less reactive with respect to ammonia than bismuth molybdate. The acrolein ammoxidation is very rapid (about six times the propene ammoxidation rate) and selective (86%). A comparison of the Sn—Sb—Fe—O catalyst with bismuth molybdate is presented in Table 14. 

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