Bulb still-heads

Bulb Still-Heads.—A vertical tube with a series of bulbs blown on it was recommended by Wurtz (3). For a given diameter of tube and bulbs, the greater the number of bulbs the higher is the efficiency, but the greater also is the quantity of liquid and vapour in the still-head (Table 48). ... 

The inlet tube for steam should reach to the bottom of the distillation flask. A i6-mm. bulb on the end of this tube with four 0.8-mm. openings helps to insure thorough mixmg of the heavy residue. If this is not well stirred the aldehyde distils very slowly. It is well to connect the flask to the condenser through a large Hopkins still head in order to prevent the entrainment of foam during the distillation. 

Method A. Huang-Minlon modification of the Wolff-Kishner reduction. Place 36.0 g (0.3 mol) of redistilled acetophenone, b.p. 201 °C, 300 ml of diethylene glycol, 30ml of 90 per cent hydrazine hydrate (CAUTION) and 40g of potassium hydroxide pellets in a 500-ml two-necked round-bottomed flask fitted with a reflux condenser insert a thermometer supported in a screw-capped adapter in the side-neck so that the bulb dips into the reaction mixture. Warm the mixture on a boiling water bath until most of the potassium hydroxide has dissolved and then heat under reflux for 1 hour either by means of a free flame or by using a heating mantle. Remove the reflux condenser and fit a still-head and condenser for downward distillation. Distil until the temperature of the liquid rises to 175 °C (1). Separate the upper hydrocarbon layer from the distillate and extract the aqueous layer twice with 20 ml portions of ether. Dry the combined upper layer and ethereal extracts with magnesium sulphate, remove the ether on a water bath and distil the residue. Collect ethylbenzene at 135-136 °C the yield is 20 g (62.5%). 

In simple distillation still heads form the connecting link between the still pot and the condenser. As a rule they have provision for a thermometer. Fig. 238—1 and IS show common types. Those having a ground joint for the thermometer are to be preferred, as they ensure that the thermometer bulb will always be in the same position in the vapour tube, a fact of importance in comparative distillations. 

The still-head, as supplied by dealers, is often too short. It should, if possible, be long enough for the thermometer to be placed in such a position that not only the mercury in the bulb but also that in the stem is heated by the vapour of the boiling liquid otherwise a troublesome and somewhat uncertain correction must be applied (p. 12), and, if the distillation is not proceeding quite steadily, a little air may be carried back from time to time as far as the thermometer bulb and the temperature registered by the thermometer will then fluctuate and will, on the whole, bo too low (p. 28). 

If a distillation bulb with exceedingly short still-head bo employed, this will practically be the percentage of b in the first portion of distillate. 

The Pear Still-Head.—The Wurtz still-he vdmay be improved, to some extent in efficiency, but chiefly as regards the quantity of condensed liquid, by blowing pear-shaped instead of spherical bulbs on the tube (Fig. 36). 

Modified Evaporator Still-Head.—A modification of the evaporator still-head is shown in Fig. 39. In this, the rather fragile funnel on the three legs is done away with, and the top of the middle tube c is blown into a flattened bulb o, on which the drops of liquid from the depression above fall and collect into a shallow pool which soon overflows, and the liquid then spreads itself as before over the... 

The Glinsky dephlegmator (2) has only one reflux tube, which carries the excess of liquid from the large bulb to the tube below the lowest obstruction, practically back to the still. In its original form (Fig. 41), the dephlegmator otherwise resembled the Linnemann still-head very closely, but, as now constructed, there are bulbs on the vertical tube and spherical glass beads, instead of platinum cups, rest on the constrictions between the bulbs. 

Choice of Still-Head.—A plain wide tube or one with spherical bulbs is the least satisfactory, but the pear still-head, owing to the diminished capacity of the bulbs and the increased efficiency, gives much better results. Of all forms, the evaporator is the best, because the capacity is very small relatively to the efficiency, and the amount of condensed liquid in it is smaller than in any other equally efficient still-head moreover, almost the whole of the condensed liquid returns to the still at the end of the distillation. With a liquid of low viscosity, like one of the lower paraffins, the quantity left in the still head is almost inappreciable, and in other cases it may bo reduced to a very small amount by disconnecting the apparatus, while hot, from the condenser, tilting the tubo from side to side to facilitate the flow of liquid back to the still and, if the original form of evaporator still-head is used, shaking out any liquid remaining in the funnels. 

When, for example, benzene is nitrated by treatment with nitric and sulphuric acids, some of the benzene is usuaUy unacted on, while a certain amount of dinitrobenzene is formed. Here the boiling point of the chief component, nitrobenzene, is very much higher than that of benzene and far below that of dinitrobenzene, and the separation is therefore an easy one. If pure benzene is used for the preparation and the mixture is distilled through a pear still-head with, say, 12 bulbs, the collection of pure nitrobenzene may be commenced after the second or third fractionation. 

The apparatus consists of a 250-ml distillation flask connected to a condenser through a still-head, ground-glass joints being used throughout. Sealed into the bulb of the still-head is the stem of a separator, so that liquid can be introduced into the distillation flask without disconnecting the apparatus. 

The light in her head wasn t quite coming on. Maybe the bulb needed replacing. Trix set it aside, knowing it would come to her. Why not, she said. I d love to hear it too. She was warming to the family atmosphere, and warming to her role. And still managing to keep one eye on Aphrodite. And another on Victor Padorin. 

It should be possible to measure the column load at any moment. The measurements should still be reliable at loads above 500 nil/li, when the liquid fs a rule no longer flows in drops but as a continuous stream. A particularly important point is the accuracy of the temperature readings to ensure this, the head should be so constructed that liquid from the condenser cannot reach the thermometer bulb and that the pressure at the point of temperature measurement is the same as that at which the pressure is read. A flowmeter may be used to check the rate of the cooling water (section 8.6), as excessive sub-cooling leads to a false reflux. 

Nature of still-bead. 3 bulbs. 7 bulbs. 13 bulbs. Pear" stiU-head, 13 bidbs. 

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