Superheating and bumping

Superheating and bumping. If a hquid is heated in a flask by means of a Bunsen burner and wire gauze placed below it, the formation of bubbles of vapour at the lower surface of the liquid in contact with the heated glass is facilitated by the presence of air dissolved in the liquid or adhering as a film to the glass and by roughness on the surface of the 

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) wUl deliver vapour 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 pressure 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 

A useful device to prevent bumping of hquids during distillation consists of a glass tube, 2-3 mm. in diameter, bent in a U-form with one arm somewhat shorter than the other it should be long enough to extend from the bottom of the flask for a short distance into the neck in order that it should remain in an upright position (Fig. 1,3, a). If for 

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Calculation of the boiling point at selected pressures Superheating and bumping. ... 

It sometimes happens that a liquid bumps violently even with the addition of a few porcelain clips and the passage of a fine gas current through the flask. The best plan in such cases is to immerse the flask so far into the heating bath (see p. 38) that the level of the latter is above the level of the liquid in the flask. In this way the vapour is superheated, and bumping does not occur. 

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 boiling point of mercury at atmospheric pressure is 356.9°C, and at 1 torr it is about 200°C. It follows that the mercury diffusion pump boiler should not be heated until the pressure is reduced to 1 torr. Should the pressure be allowed to rise considerably, for any reason, the heating should be discontinued. Otherwise, when the pressure falls the mercury will be superheated and may bump violently. Mercury is a dense liquid, 13.55 g/cm a at 0°C, and large fast moving quantities of it may break the delicate jets or the boiler. 

However, before any volatility tests are carried out it, all water must be removed because the presence of more then 0.5% water in test samples of crude can cause several problems during distillation procedures. Water has a high heat of vaporization, necessitating the application of additional thermal energy to the distillation flask. Water is relatively easily superheated, and therefore excessive bumping can occur, leading to erroneous... 

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 distilling flask. To minimise superheating of the liquid (due to the absence of minute air bubbles or other suitable nuclei for forming bubbles of vapour), and to prevent bumping, one or more of the following precautions should be taken ... 

The cellulose specimen under examination is refluxed in the acid-oxidant mixture and the gases formed are swept continuously into an absorption train by a carrier stream of air free of carbon dioxide. Conrad and Scroggie20 have added a number of important improvements which apparently increase the reproducibility of results. One of their modifications is a stirrer in the reaction chamber which reduces the danger of bumping caused by superheating. The latter is undesirable since the reaction is apparently quite sensitive to the temperature. 

Distilling flasks should be chosen of such a size that the bulb is half or two-thirds full of liquid. In order to avoid bumping and superheating a few pieces of porous plate (pot) half as large as peas are dropped into the flask before each distillation. If boiling is again delayed fresh pieces of pot must be added, not to the superheated liquid, however, but after brief cooling. 

A Boiling Liquid Expanding Vapour Explosion, or BLEVE, is an industrial event related to the laboratory bump occasioned when the inadequately mixed bottom of a vessel of liquid becomes superheated, then explosively boils. In the industrial version, rupture of a pressurised container is usually involved. Although strictly speaking a non-reactive physical hazard, chemical fires and explosions, with fatalities, often follow. Means of estimating risk and prevention, with a fist of incidents are given[l], A more ferocious version, the Boiling Liquid Compressed Bubble... 

When a reaction mixture is heated, there is a tendency for it to boil violently as large bubbles of superheated vapour suddenly erupt from the mixture. This is prevented by the addition of the anti-bumping granules. A condenser is placed in the mouth of the flask and connected to a cold-water tap (water goes in at the bottom and out at the top). The flask is then heated and the vapours produced are condensed back into liquids when they reach the cool condenser. 

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