Evaporator still-heads

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. 

Original Form of Evaporator Still-Head.—The general form of the apparatus, as originally designed, is shown in Fig. 37. Each section consists of three separate parts—... 

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... 

Length for length, the modified evaporator still-head would be more efficient, and would contain only about half the amount of condensed liquid. 

Benzene and Toluene.—That much time is saved by the use of an improved still-head is seen by comparing the results of a fractional distillation of 200 grams of a mixture of equal weights of benzene and toluene with a modified evaporator still-head of 6 sections with those obtained with a plain vertical tube 30 cm. in height. 

When the evaporator still-head was used it was not necessary to divide the distillate into more than five fractions. In the first distillation (Table 52), more than half the total quantity of toluene was recovered in a pure state, and it will be noticed that the values of Aw/A for the middle fractions are relatively very low. 

Methyl, Ethyl and Propyl Acetate.—The separation of a mixture of 100 grams of methyl acetate, 120 grams of ethyl acetate and 100 grams of propyl acetate was carried out by distillation with a modified evaporator still-head of 8 sections. Details of the fractionations are given in Table 54 (p. 194). 

The great superiority of the evaporator still-head is clearly shown by the results given in Table 55. 

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. 

Estimation of Loss by Evaporation. —The following may be taken as an example of the estimation of loss by evaporation. Mixtures of benzene and methyl alcohol, one with benzene, the other with methyl alcohol in excess (these liquids form a mixture of minimum boiling point), were distilled through an evaporator still-head of five sections of the original form, the distillation being stopped in each case when the middle point was reached. The following results were obtained ... 

A mixture containing 90 grams of benzene and 10 grams of toluene was distilled through an evaporator still-head of three sections. 

Advantages of Efficient Still-Head.—With an evaporator still-head of eight sections the following results were obtained (p. 193). 

When, however, the boiling points are not so far apart, as, for example, in the separation of ethyl acetate from methyl and propyl acetate, the process is tedious if an ordinary apparatus is used (p. 128), and even with a 5-column evaporator " still-head several fractionations arc required (p. 193), for the middle substance is always more diilieult to separate than the others. 

If the original mixture contained 270 grams of ben ne and 30 grams of toluene, and it was desired to recover the toluene, it would again be best to employ a very efficient still-head, and as, in this case, a most essential point would be that the liquid should return very completely from the still-head to the still at the end of the distillation, an evaporator still-head of many sections would be the best for the purpose. 

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