Conventional heating

Even if the reactor temperature is controlled within acceptable limits, the reactor effluent may need to be cooled rapidly, or quenched, to stop the reaction quickly to prevent excessive byproduct formation. This quench can be accomplished by indirect heat transfer using conventional heat transfer equipment or by direct heat transfer by mixing with another fluid. A commonly encountered situation is... 

One disadvantage of fluidized heds is that attrition of the catalyst can cause the generation of catalyst flnes, which are then carried over from the hed and lost from the system. This carryover of catalyst flnes sometimes necessitates cooling the reactor effluent through direct-contact heat transfer hy mixing with a cold fluid, since the fines tend to foul conventional heat exchangers. 

The reactor effluent might require cooling by direct heat transfer because the reaction needs to be stopped quickly, or a conventional exchanger would foul, or the reactor products are too hot or corrosive to pass to a conventional heat exchanger. The reactor product is mixed with a liquid that can be recycled, cooled product, or an inert material such as water. The liquid vaporizes partially or totally and cools the reactor effluent. Here, the reactor Teed is a cold stream, and the vapor and any liquid from the quench are hot streams. 

Heat Exchangers Since most cryogens, with the exception of helium 11 behave as classical fluids, weU-estabhshed principles of mechanics and thermodynamics at ambient temperature also apply for ctyogens. Thus, similar conventional heat transfer correlations have been formulated for simple low-temperature heat exchangers. These correlations are described in terms of well-known dimensionless quantities such as the Nusselt, Reynolds, Prandtl, and Grashof numbers. 

Radiation is not generally considered in conventional heat transfer equipment except for direct gas/oil-fired heaters and cracking units. These later types are not a part of this chapter, because they are specialty items of their own as far as design considerations are concerned. 

There are different heat tests, some being specific to a product environment. There are those for temperature and also humidity. With certain materials, humidity combined with elevated temperatures has a significant effect on the material s behavior. This effect would not be evident in the conventional heat distortion test (HDT). 

Keywords microwave and conventional heating in Diels-Alder reactions... 

Scheme 10 Various types of functionalizations of 2-pyridones using conventional heating...

The use of microwave irradiation for this reaction, compared to conventional thermal heating, was investigated. Chloroform used as solvent under the conventional heating did only allow a temperature of 60 °C and a direct comparison between the two methods is therefore somewhat unfair imder these circumstances. Nevertheless, the microwave-assisted method is attractive and proved useful for both primary and secondary amines resulting in highly substituted pyrazolo ring-fused pyridones 40 in 68-86% yields within only 10 min. 

Fig. 8 A comparison of the results from microwave and conventional heating for amino-dehalogenation on 2-pyridones...

Fig. 10 2-Pyridone carboxylic acids 64 can be selectively decarboxylated to the saturated derivatives 65 via a reagent-free microwave-assisted method. Decarboxylation was also conducted under conventional heating but then copper cyanide was required resulting in mixtures of saturated and unsaturated 2-pyridones...

It was shown in the same article that the decarboxylation could also be performed by conventional heating but then copper cyanide was required and a mixture of saturated and imsaturated 2-pyridones 65 and 66 was obtained in a ratio of 1 10 (Fig. 10). A tentative mechanism was suggested for the reagent-free MAOS method where the carbonyl in the 2-pyridone ring is supposed to assist in the decarboxylation yielding an yUde 67 (Fig. 11). The decarboxylated bicyclic 2-pyridone 68 is thereafter obtained after protonation by the solvent. In agreement with the mechanistic suggestion, it was shown that a selective deuteration occurred when deuterated dimethyl sulfoxide (DMSO-de) was used as solvent. 

Manganese(III)-promoted radical cyclization of arylthioformanilides and a-benzoylthio-formanilides is a recently described microwave-assisted example for the synthesis of 2-arylbenzothiazoles and 2-benzoylbenzothiazoles. In this study, manganese triacetate is introduced as a new reagent to replace potassium ferricyanide or bromide. The 2-substituted benzothiazoles are generated in 6 min at 110°C imder microwave irradiation (300 W) in a domestic oven with no real control of the temperature (reflux of acetic acid) (Scheme 15). Conventional heating (oil bath) of the reaction at 110 °C for 6 h gave similar yields [16]. 

In all cases, besides resulting in good to excellent yields, the microwave-assisted multistep syntheses resulted in much faster reactions compared to earlier published procedures at atmospheric pressure under conventional heating conditions. It is also noteworthy that in some cases the strong thermal effect due to graphite/microwave interaction, can efficiently be used for the synthesis of heterocyclic skeletons, especially benzothiazoles but, in fact, there is no general rule and some reactions performed in the presence of solvent may sometimes be more convenient than the same dry-media conditions. 

Potent antimicrobial l,2,4-triazolo[3,4-fc]-l,3,4-thiadiazepines derivatives were prepared from readily accessible substituted 2-mercapto-l-aminotria-zoles and substituted chalcones on basic alumina in a solvent-free microwave-assisted synthesis (Scheme 28). Exposure of the reaction mixtures to microwaves led to an important decrease of the reaction time, which has been brought down from hours to seconds, accompanied by improved yields as compared with conventional heating [36]. This facile, rapid, and economic... 

Fig. 13 Synthesis of oxazoles on JandaJel. Reagents and conditions a toluene, alkyl acetoacetate (R0(C0)CH2C0R R =t-Bu), reflux, 6h or alkyl acetoacetate (R = Me, Et), toluene, LiC104, reflux, 6h fc dodecylbenzenesulfonyl azide, EtsN, toluene, rt, 16 h c benzamide, Rh2(oct)4, toluene, 65 °C, Ih rf Burgess reagent, pyridine, chlorobenzene, MW 100 °C, 15 min (or 80 °C, 4 h with conventional heating) e AICI3, piperidine, CH2CI2, rt, 16 h...

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