Hot spot formation

Hot spot formation witliin tlie reactor can result in catalyst breakdown or physical deterioration of tlie reactor vessel." If tlie endothermic cyanide reaction has ceased (e.g., because of poor catalyst performance), the reactor is likely to overheat. Iron is a decomposition catalyst for hydrogen cyanide and ammonia under the conditions present in the cyanide reactor, and e. posed iron surfaces in the reactor or reactor feed system can result in uncontrolled decomposition, which could in turn lead to an accidaital release by overheating and overpressure. 

Impact and Friction Initiation of PETN have been extensively studied. This general subject is summarized in Bowden Yoffe (Refs 13 19), Afanas ev Bobolev (Ref 85a), and Vol 7, 135—55, which contains a rather complete description of impact initiation with many refs to the impact and friction initiation of PETN. There is general agreement that impact and friction initiation proceed via a hot spot mechanism (see Vol 7, HI 70—75). There is, however, considerable doubt and controversy about the mechanisms of hot spot formation... 

Heavens Field suggest that both gas cavities, sealed off during fusion of the sample, and high flow rate shearing near the impacting striker edge contribute to hot spot formation during PETN impact initiation... 

The maintenance of uniform flow distribution in fixed bed reactors is frequently a problem. Maldistribution leads to an excessive spread in the distribution of residence times with adverse effects on the reactor performance, particularly when consecutive reactions are involved. It may aggravate problems of hot-spot formation and lead to regions of the reactor where undesired reactions predominate. Disintegration or attrition of the catalyst may lead to or may aggravate flow distribution problems. 

Signal intensities in MALDI MS not only depend on the amount of the analyte but also on its chemical composition, for example, the sequence of amino acids in the case of peptides, and may underlie suppression by other components present in the sample [51,52]. Further, the hot-spot formation described above leads frequently to poor shot-to-shot and spot-to-shot reproducibilities. Both factors hamper the use of MALDI MS for quantitative... 

On the other hand, Dudek et al. found that the impact sensitivity (h50% ) of e CL-20 28 cm is comparable to that of [5-HMX (h50% = 30 cm) [124]. Agrawal et al. have not only given impact sensitivity of CL-20 but also the mechanism of initiation on impact leading to its detonation. This is in line with the mechanism of initiation by hot spot formation on impact followed by detonation similar to other secondary explosives [125]. 

Marken, F., Tsai, Y.-C., Coles, B.A., Matthews, S.L. and Compton, R.G., Microwave activation of electrochemical processes convection, thermal gradients and hot spot formation at the electrode-solution interface, New. Chem., 2000, 24, 653. 

Fixed-bed catalytic reactors and reactive distillation columns are widely used in many industrial processes. Recently, structured packing (e.g., monoliths, katapak, mella-pak etc.) has been suggested for various chemical processes [1-4,14].One of the major challenges in the design and operation of reactors with structured packing is the prevention of liquid flow maldistribution, which could cause portions of the bed to be incompletely wetted. Such maldistribution, when it occurs, causes severe under-performance of reactors or catalytic distillation columns. It also can lead to hot spot formation, reactor runaway in exothermic reactions, decreased selectivity to desired products, in addition to the general underutilization of the catalyst bed. 

Figure 3 Schematic of temperature profiles and hot spot formation in a multitubular reactor.

The benefits of applying micro structured reactors for autothermal reforming are manifold. Besides the well-known narrowing of the residence time distribution and the low pressure drops, hot-spot formation may well be reduced owing to the axial heat transfer of the wall material. 

The reaction takes place at temperatures around 1 000 °C, a pressure of 25 bar and residence times in the order of few to several hundred milliseconds. Conventionally the process is performed with metal-loaded ceramic supports. From calculations, hot-spot formation up to 2320 °C might occur, which was expected to be minimized by the metallic catalyst. 

Early work by Tonkovich et al. [46] dealt with a heat exchanger/reactor containing catalyst powder for the partial oxidation of methane for distributed hydrogen production. The intention was to run the reaction safely in a micro structured reactor owing to the short residence times applied and the improved heat removal avoiding hot-spot formation. 

The phenomenon of increasing combustion catalyst activity was frequently observed in literature before [64, 65]. At both levels of Pt loading, the coolant temperature at the exit was higher than the temperature of the products which was explained by hot-spot formation. 

When considering the use of microreaction technology, from the perspective of high-throughput organic synthesis, the main benefit that this technique offers is increased reaction control, which in itself affords many practical advantages to the user. As a result of the small reactor dimensions, rapid mixing of reactants, and an even temperature distribution are observed, which not only increase the uniformity of reaction conditions, but also afford increased reaction safety, selectivity, reproducibility, and efficiency when compared to conventional batch reactors where hot spot formation can lead to the formation of by-products and the risk of thermal runaway. 

Cyclohexene hydrogenation is a well-studied process that serves as model reaction to evaluate performance of gas-liquid reactors because it is a fast process causing mass transfer limitations for many reactors [277,278]. Processing at room temperature and atmospheric pressure reduces the technical expenditure for experiments so that the cyclohexene hydrogenation is accepted as a simple and general method for mass transfer evaluation. Flow-pattern maps and kinetics were determined for conventional fixed-bed reactors as well as overall mass transfer coefficients and energy dissipation. In this way, mass transfer can be analyzed quantitatively for new reactor concepts and processing conditions. Besides mass transfer, heat transfer is an issue, as the reaction is exothermic. Hot spot formation should be suppressed as these would decrease selectivity and catalytic activity [277]. 

Local non-uniformities present the danger of hot spot formation where the current forms filaments. Two adverse scenarios can develop (1) local thermal run away that engenders regions of high defect density which gradually grow in size, and (2) spectral hole burning in which certain spectral modes are locally depleted. For all of these reasons, uniform material properties have always been a mandatory virtue. 

Figure 1. Calculated hot-spot formation in modeling hydrocracker operation. Reprinted with permission from S. B. Jaife, Ind. Eng. Chem. Process Dev. 15,410 (1976). Copyright 1976 American Chemical Society.

Dlott DD, Fayer MD. Shocked molecular solids vibrational up pumping, defect hot spot formation, and the onset of chemistry. J Chem Phys 1990 92 3798-3812. 

Operators will often watch the readings from an array of thermocouples in a reactor, as an indication of imminent hot spot formation. The historical danger point for the reactors of Table I has been a band of 35°F. It is interesting to note that the average positive temperature fluctuation within a disturbance, from the integral of Equation (21), is about 30°F. 

Description The Uhde (Edeleanu) technology features a two-stage reactor system of which the first reactor is operated in the recycle mode. With this method, a slight expansion of the catalyst bed is achieved that ensures very uniform concentration profiles in the reactor and, most important, avoids hot spot formation. Undesired side reactions, such as the formation of di-ethyl ether (DEE), are minimized. 

Thus we will not directly address, for example, the mechanisms of hot spot formation in the crystal [11-13], or the role of particle size [10,12,14],... 

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