Superheating localized

Diluted ODA may be fed directly to saturated and superheated steam without adverse effects however, ODA acetate solution should not be fed directly to superheated steam because dehydration of the salt occurs, resulting in the production of various amine acetamides. These are not corrosion inhibitors and may cause gunking and deposits to occur. Similarly, ODA acetate should not be fed directly to saturated steam ahead of pressure reducing (PR) valves or other types of appurtenances that may cause localized superheat conditions. 

In a subsequent paper [32], however, Berlan himself cast doubt on the existence of nonthermal effects, attributing the observed rate increases to localized hot-spots in the reaction mixture or to superheating of the solvent above its boiling point. He also mentioned the difficulty of measuring the temperature accurately in MW cavities. Furthermore, kinetic studies by Raner et al. [33], showed that the Diels-Alder reaction of 3 with 23 (Scheme 4.12) occurred at virtually the same rate under MW and conventional heating at the same temperature. 

The small increase in racemization rate observed when an aqueous solution of L-pro-line was heated under reflux on a MW oven at atmospheric pressure could be attributed to localized superheating or a generalized superheating of the solvent. It is known that water superheats by 4—10 °C when boiled in a MW oven [39, 40]. 

We have found it convenient to compare MW and conventional reactions using reflux conditions, since the temperatures are constant at the boiling point of the solvent. To eliminate the problem of the time required to reach the reflux temperature, reaction mixtures without one of the reactants or catalyst are heated to reflux and then the other reactant or catalyst quickly added. The reflux times required to give similar yields for a reaction, taken only partially to completion by MW and classical heating, are then compared. Small rate enhancements might still be expected merely because of superheating by up to 40 °C by the MW [39, 40, 46], and localized heating... 

Moreover, some MW absorbing impurities on the graphite surface, such as the Fe304 crystallites, could induce other local superheating. 

Because the reaction is driven by protonation of the carbonyl functionality, reacting species were expected to be localized on the bed of the acid catalyst subjected to microwave irradiation. Hexane was used as a nonpolar solvent to minimize solvent absorption and superheating. Elimination of catalyst superheating in a continuous-flow reactor was most probably the reason why no significant differences were observed between the reaction rates under the action of microwave and conventional heating. 

Superheating can occur as superheating of a liquid reaction mixture in liquid/solid reactions or as localized superheating in gas/solid reactions. 

Reactions accelerated by microwaves 363 Reactions not accelerated by microwaves 363 Superheating of liquid reaction mixture 3 64 Localized superheating in the solid phase 365 Selective heating 365 Hot spots 366... 

The superheated liquid concept, in any case, may only supply a local trigger to initiate the RPT. Other ways to trigger such events are possible, e.g., detonation of small explosive charges can sometimes be employed. To produce large-scale, coherent RPTs, the trigger may cause collapse of vapor films in adjacent masses of volatile liquid and lead to the escalation of the small triggering event. 

In this article, we suggest that a modified superheated-liquid model could explain many facts, but the basic premise of the model has never been established in clearly delineated experiments. The simple superheated-liquid model, developed for LNG and water explosions (see Section III), assumes the cold liquid is prevented from boiling on the hot liquid surface and may heat to its limit-of-superheat temperature. At this temperature, homogeneous nucleation results with significant local vaporization in a few microseconds. Such a mechanism has been rejected for molten metal-water interactions since the temperatures of most molten metals studied are above the critical point of water. In such cases, it would be expected that a steam film would encapsulate the water to... 

A free flame should be avoided, because local superheating causes charring and decomposition. The fumes from the decomposition cause the product (which normally is colorless) to darken. 

The thermal efficiency of a steam power cycle is increased when the press and hence the vaporization temperature in the boiler is raised. It is also increas by increased superheating in the boiler. Thus, high boiler pressures and te peratures favor high efficiencies. However, these same conditions increase capital investment in the plant, because they require heavier construction more expensive materials of construction. Moreover, these costs increase ev more rapidly as more severe conditions are imposed. Thus, in practice po plants seldom operate at pressures much above 10,000 kPa and tempera much above 600°C. The thermal efficiency of a power plant increases as pressure and hence the temperature in the condenser is reduced. However, condensation temperature must be higher than the temperature of the coo medium, usually water, and this is controlled by local conditions of climate geography. Power plants universally operate with the condenser pressure as 1 as practicable. 

H. Richterova and M. Hajek. Localized superheating effects in heterogeneous reactions. Proceedings from the International Conference on Microwave Chemistry, Prague, Czech Republic 1998. 

On the other hand, microwaves heat all of the sample fluid simultaneously without heating the vessel. Therefore, with microwave heating, the solution reaches its boiling point very rapidly. Because the rate of heating is so much more rapid, substantial localized superheating can occur [14]. Table 5.1 shows the effect of microwave heating on several solvents [20]. 

We have seen that the temperature spike involves a relatively large number of atoms, which, after having been superheated, undergoes a subsequent rapid cooling. A certain number of atoms are thus quenched in metastable positions, increasing the local density of defects. 

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