Superheated atmospheric-pressure

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. 

Eor printing on polyester, the fixation conditions are more rigorous than on other disperse dyeable fibers, owing to the slower diffusion of disperse dyes in polyester. Eor continuous fixation the prints are exposed at atmospheric pressure to superheated steam of 170—180°C for 6—8 min. A carrier may be added to the print paste for accelerated and fliU fixation. Dry-heat fixation conditions of 170—215°C for 1—8 min are less popular for printed fabrics, but are sometimes employed because of lack of other equipment. 

A two-step methanolysis-hydrolysis process37 has been developed which involves reaction of PET with superheated methanol vapors at 240-260°C and atmospheric pressure to produce dimethyl terephthalate, monomethyl terephthalate, ethylene glycol, and oligomeric products in the first step. The methanolysis products are fractionally distilled and the remaining residue (oligomers) is subjected to hydrolysis after being fed into the hydrolysis reactor operating at a temperature of ca. 270°C. The TPA precipitates from the aqueous phase while impurities are left behind in the mother liquor. Methanolysis-hydrolysis leads to decreases in the time required for the depolymerization process compared to neutral hydrolysis for example, a neutral hydrolysis process that requires 45 min to produce the monomers is reduced... 

If a minute bubble of air is formed (this will be at atmospheric pressure), it will serve as a nucleus for a larger bubble of vapour. At the boiling point the liquid (at 760 mm. vapour pressure itself) will deliver vapom in relatively large quantity to the air bubble. With the heat supply at hand, the total pressure inside the bubble soon rises above that of the atmosphere and is sufficient to overcome the pressiu-e due to the column of liquid a vapour bubble is then expelled. Hence, if a source of minute air bubbles or other nuclei is available in the liquid, boiling will proceed quietly. If, however, the liquid is largely free from air and if the walls of the flask are clean and very smooth, bubbles are formed with greater difficulty and the temperature of the liquid may rise appreciably above the boiling point it is then said to be superheated. When a... 

Closely related to the superheating effect under atmospheric pressure are wall effects, more specifically the elimination of wall effects caused by inverted temperature gradients (Fig. 2.6). With microwave heating, the surface of the wall is generally not heated since the energy is dissipated inside the bulk liquid. Therefore, the temperature at the inner surface of the reactor wall is lower than that of the bulk liquid. It can be assumed that while in a conventional oil-bath experiment (hot vessel surface, Fig. 2.6) temperature-sensitive species, for example catalysts, may decompose at the hot reactor surface (wall effects), the elimination of such a hot surface will increase the lifetime of the catalyst and therefore will lead to better conversions in a microwave-heated as compared to a conventionally heated process. 

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]. 

An evaporator operating on the thermo-recompression principle employs a steam ejector to maintain atmospheric pressure over the boiling liquid. The ejector uses 0.14 kg/s of steam at 650 kN/m2, and is superheated by 100 K and the pressure in the steam chest is 205 kN/m2. A condenser removes surplus vapour from the atmospheric pressure line. What is the capacity and economy of the system and how could the economy be improved ... 

FIG. 8 Schematic illustration of the steps in the phase diagram and the energy required for ice starting at — 20 °C to become superheated gas (steam) at 120°C at atmospheric pressure (1 atm). The type and amount of heat (sensible or latent) required to change the temperature or phase are given, where Cp is the specific heat and AH is the change in enthalpy. 

Impurities can be present in the boiling liquid itself. The evidence is strong that traces of dissolved material (excluding surface-active agents of course) have but a small effect on boiling, whereas suspended material is much more important. Wismer s important experiments on superheated liquids show a distinct difference between the maximum superheat attainable with water at atmospheric pressure in the presence of dissolved material and the values resulting for suspended matter. Some of the results are given in Table VI. 

If superheating of the surface is such that the equilibrium vapor pressure is a factor 1 + 0 greater than the external pressure, the evaporation rate, expressed as the linear velocity of vapor outflow from the surface, comprises 0cx/2. A combustion rate of 1 mm/sec of the liquid EM corresponds at atmospheric pressure to a vapor outflow rate of order 50 cm/sec, to which corresponds 0 from 0.001 to 0.002 and superheating of 0.02-0.04. For an accommodation coefficient a / 1 and otherwise equal conditions, the superheating increases proportionally to 1 /a. Finally, the superheating is proportional to the combustion rate. 

Superheating also can account for acceleration of reactions under microwave conditions70,71. Mingos has estimated that this can lead to 10-50-fold reductions in reaction times in comparison with conventional reflux conditions, but that rate enhancements of 100-1000-fold at atmospheric pressure would be required before specific microwave effects could be invoked72. 

If bubbles of air 10 7 m in diameter and no other nuclei are present in water just below the boiling point, by approximately how much could water be superheated at normal atmospheric pressure before boiling starts The surface tension of water at 100°C is 59 mN mT1 and its enthalpy of vaporisation is 2.25 kJ g 1. 

When does a liquid boil Clearly, boiling at constant pressure—say, atmospheric pressure—begins when we increase the temperature of a liquid or solution and the vapor pressure reaches a pressure of one atmosphere. Alternatively, the pressure over a liquid or solution at constant temperature must be reduced until it reaches the vapor pressure at that temperature (e.g., vacuum distillation). Yet it is well known that liquids can be superheated (and vapors supersaturated) without the occurrence of phase transfer. In fact, liquids must always be superheated to some degree for nucleation to begin and for boiling to start. That is, the temperature must be raised above the value at which the equilibrium vapor pressure equals the surrounding pressure over the liquid, or the pressure must be reduced below the vapor pressure value. As defined earlier, these differences are called the degree of superheat. When the liquid is superheated, it is metastable and will reach equilibrium only when it breaks up into two phases. 

Finite duration release of a subcooled, saturated, superheated or two-phase fluid at a temperature above its bubble point at atmospheric pressure ... 

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