Pear still-head

The tube with thirteen bulbs was somewhat more efficient that the slightly shorter tube with rod and discs, but the quantity of liquid and vapour in the still-head was half as large again. 

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). 

The pear still-head is more efficient than the rod and disc tube of the same length and possesses the same advantages exeept that it is somewhat more difficult to construct. It may be especially recommended for liquids of high boiling point. 

The Evaporator Still-Heads.—Greater efficiency, for a given vertical height, and less condensation, for a given efficiency, is attained by the evaporator still-heads. 

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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. 

With a larger quantity, 250 grams, a fairly satisfactory result was obtained even with a pear still-head. In this case the distillate was divided into three fractions, and the following results were obtained. 

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. 

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

With the rod and disc, the pear and the evaporator (3 and 6 sections) still-heads, nearly as good results were obtained with 25 as with 50 or more grams of the mixture, and in all these cases the residual toluene was quite pure. 

Much time will be saved if an improved still-head is employed for liquids of high boiling point, the pear, and for volatile liquids the evaporator apparatus may be especially recommended. 

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