Superheating

This is an endothermic reaction accompanied by an increase in the number of moles. High conversion is favored by high temperature and low pressure. The reduction in pressure is achieved in practice by the use of superheated steam as a diluent and by operating the reactor below atmospheric pressure. The steam in this case fulfills a dual purpose by also providing heat for the reaction. 

The output from the turbine might be superheated or partially condensed, as is the case in Fig. 6.32. If the exhaust steam is to be used for process heating, ideally it should be close to saturated conditions. If the exhaust steam is significantly superheated, it can be desuperheated by direct injection of boiler feedwater, which vaporizes and cools the steam. However, if saturated steam is fed to a steam main, with significant potential for heat losses from the main, then it is desirable to retain some superheat rather than desuperheat the steam to saturated conditions. If saturated steam is fed to the main, then heat losses will cause excessive condensation in the main, which is not desirable. On the other hand, if the exhaust steam from the turbine is partially condensed, the condensate is separated and the steam used for heating. 

So far the emphasis has been on substituting hazardous materials or using less, i.e., intensification. Let us now consider use of hazardous materials under less hazardous conditions, i.e. at less extreme temperatures or pressures or as a vapor rather than superheated liquid or diluted, in other words, attenuation. ... 

Tsup = temperature of the superheated liquid BT = normal boiling point If the mass of liquid vaporized is rriy, then... 

In fact, the true fire load will be greater than the energy release calculated in Example 9.1. In practice, such a release of superheated liquid generates large amounts of fine spray in addition to the vapor. This can double the energy release based purely on vaporization. 

There are two esdsting steam mains. These are high-pressure steam at 41 bar superheated to 270°C and medium-pressure steam at 10 bar saturated at 180°C. Boiler feedwater is available at 80°C and cooling water at 25°C to be returned at 30°C. 

From steam tables, the outlet temperature is 251°C, which is superheated by 67°C. Although steam for process heating is preferred at saturated conditions, it is not desirable in this case to desuperheat by boiler feedwater injection to bring to saturated conditions. If saturated steam is fed to the main, then the heat losses from the main will cause a large amount of condensation in the main, which is undesirable. Hence it is better to feed steam to the main with some superheat to avoid condensation in the main. 

From steam tables, the outlet temperature is 160°C, which is superheated by 16°C. Again, it is desirable to have some superheat for the steam fed to the main to avoid condensation in the main. 

Fats are hydrolysed to glycerol and fatty acids by boiling with acids and alkalis, by superheated steam and by the action of lipases. If alkalis are used for hydrolysis, the fatty acids combine with the alkalis to form soaps. Alkaline hydrolysis is therefore sometimes called saponification. 

Frasch process A process for obtaining sulphur by passing superheated water down a shaft to liquefy sulphur which is blown to the surface with compressed air. 

The paper focuses on the presentation and discussion of the results of the application of long term, continuous, AE structural monitoring to 2 large superheated (SH) steam outlet headers, belonging to 2 different full-size (600 MW, supercritical multifuel) ENEL power units. Continuous AE surveillance of the 2 SH headers started in October 1996 and is still ongoing. 

Two large Superheated Steam outlet headers with potential stmctural integrity problems, belonging to 2 600 MW multifuel ENEL power Units, have been submitted to continuous AE... 

Classic nucleation theory must be modified for nucleation near a critical point. Observed supercooling and superheating far exceeds that predicted by conventional theory and McGraw and Reiss [36] pointed out that if a usually neglected excluded volume term is retained the free energy of the critical nucleus increases considerably. As noted by Derjaguin [37], a similar problem occurs in the theory of cavitation. In binary systems the composition of the nuclei will differ from that of the bulk... 

Figure A2.5.4. Themiodynamic fimctions (i, n, and C as a fimction of temperature T at eonstant pressure and eomposition x = 1/2) for the two-eomponent system shown in figure A2.5.3. Note the diflferenee between these and those shown for the one-eomponent system shown in figure A2.5.2. The fiinetions shown are dimensionless as in figure A2.5.2. The dashed lines represent metastable extensions (superheating or supereooling) of the one-phase systems.

Blackman M, Peppiat S J and Sambles J R 1972 Superheating of bismuth Nature Phys. Sc/. 239 61... 

Wen X, Tolbert W A and DIott D D 1992 Multiphonon up pumping and moleoular hot spots in superheated polymers studied by ultrafast optioal oalorimetry Chem. Phys. Lett. 192 315-20... 

Appreciable quantities are also obtained as a by-product in the manufacture of hydrogen from naphtha-gaseous hydrocarbons. In this process the gaseous hydrocarbon and superheated steam under a pressure of about 10 atmospheres and at a temperature of 1000 K are passed over a nickel-chromium catalyst. Carbon monoxide and hydrogen are produced ... 

In America, the sulphur deposits (mostly in Louisiana and Texas) are dome-shaped layers about 30 cm thick, between limestone above and anhydrite below. From these, the sulphur is extracted by the Frasch process. A metal tube, about 15 cm diameter and containing two concentric inner tubes (Figure 10.1) is sunk into the top of the deposit. Water, superheated to 450 K, is forced... 

It has already been pointed out that a liquid even when subjected to simple atmospheric distillation may become superheated and then bump violently in consequence this danger is greatly increased during distillation under reduced pressure and therefore a specially designed flask, known as a Claisen flask, is used to decrease the risk of superheating. In Fig. i2(a) a Claisen flask D is shown, fitted up as part of one of the simplest types of vacuum-distillation apparatus. ... 

The action of this and other anti-bumping devices e.g., minute carborundum chips) is dependent upon the fact that the transformation of a superheated liquid into the vapour will take place immediately if a vapour phase e.g., any inert gas) is introduced. The effect may be compared with that produced by the introduction of a small quantity of a solid phaM into a supercooled liquid, e.g., of ice into supercooled water. 

The effect of superheated steam may be illustrated by reference to baizaldehyde, which boils at 178° at 760 mm. It distils with steam at 97-9° (Pj = 703-5 mm. and pg = 56-5 mm.) and the distillate contains 32-1 per cent, of benzaldehyde by weight. If one employs steam superheated to 133°, the vapour pressure of benzaldehyde (extrapolated from the boUing point - pressure curve) is 220 mm. hence pj = 540 (water), Pg = 220 (benzaldehyde), and... 

In practice superheated steam is generally employed for substances with a low vapour pressure (< 5-1 mm.) at 100°. Thus in the recovery of the products of nitration or aromatic compounds, the ortho derivative e.g., o-nitrophenol) can be removed by ordinary steam distillation the... 

The size of the flask to be employed depends upon the quantity and nature of the liquid to be distilled it should be one half or at most two-thirds full. If too large a flask is used, superheating and, sometimes, decomposition may occur. 

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