Distillation




Distillation

Distillation is the process of vaporizing a liquid and recovering it by condensing the vapors. The liquid formed by this condensation is called the distillate. Distillation is chiefly employed to separate a liquid from non-volatile matter dissolved or suspended in it; or to separate one liquid from a mixture of liquids of different boiling points; that one having the lowest boiling point being the first to begin to pass off as vapor.
But the separation of two liquids which are miscible with each other is never complete by this means, and is less perfect the nearer their boiling points are together. Liquids which are miscible in all proportions, may be separated quite completely, provided there are a few degrees difference in their boiling points, by employing the principle of fractional condensation of the vapors. This consists in passing the mixed vapors through a condenser which is kept at a constant temperature between the boiling points of the liquids. Thus the vapors of the high-boiling liquid being cooled below the boiling point of that liquid are condensed, while the vapors of the low-boiling liquid being still hotter than its boiling point, cannot condense, but pass on to another part of the apparatus, where they are condensed separately. 'When the high-boiling distillate condenses, it carries with it more or less of the low-boiling liquid, and hence should usually be returned to the boiler and redistilled. In such mixtures as this, there is a gradual rise in the boiling point during the entire distillation.
The chief parts of every distilling apparatus are the boiler or still and the condenser In practical work the appliance for fractional condensation is placed between the still and the condenser. It may be an apparatus, called a "dephlegmator," in which the vapors are forced to bubble through a layer 01' column of the condensed higher boiling liquid; or the mixed vapors may pass through a tower or

pipe, kept at a constant temperature, just above the boiling point of the low boiling liquid. The still is usually iron, copper, or other metal, heated directly by a furnace, a steam jacket, or a coil.
In Coupier's still (Fig. 5) a towel' (A) is placed on top of the boiler (B); between the tower and the condenser is a series of bulbs (C, C) surrounded by a water bath, which may be kept at any desired temperature. 'While the mixed vapors are passing through the bulbs, the high-boiling constituents are condensed, and only the vapor of the more volatile liquid passes through (E) to the condenser (F). From each bulb a pipe (D) leads back to the tower, into which the condensed heavy liquid is delivered, to be redistilled or dephlegmated.

The French column (Fig. 6) is very similar to the Coupier's apparatus, but instead of bulbs, a series of U-tubes (C) surrounded by a water bath is used. The column or dephlegmator (B) is divided into chambers by plates, each of which has a central opening covered by a dome; a small overflow pipe passes fro111each plate to the next.

The vapors from the boiler (A) pass up through the central openings and bubble out under the edges of the domes through the layer of liquid on each plate. The liquid thus condensed flows down through the overflow pipes, and returns to the boiler.

The Coffey still (Fig. 7) is much used for alcohol and gas liquor distillation. This consists of two towers, one, called the "analyzer" (E), receiving free steam fro111the boiler, and the other, called the "rectifier" (G), containing a long coil of pipe (C, C), through which

the liquid to be distilled flows on its way to the analyzer. The analyzer is divided into a series of chambers by horizontal, perforated plates (A); from each plate an overflow pipe (F) passes down and dips into a shallow cup (H) on the next plate below and holding liquid enough to form a hydraulic seal at the lower end of each overflow pipe. 1'hese pipes project about an inch, or an inch and a half above the plate in which they are set, thus determining the depth of the liquid layer on each plate. '.The rectifier is also divided into chambers by perforated plates, but it has overflow pipes
in its lower half only. In the chambers lie the coils of pipe (C) through which the liquid to be distilled passes on its way to the analyzer. This still works as follows: Steam from the boiler is blown through (K) into the analyzer, and passes from the top of the
analyzer through the pipe (L) to the rectifier. The liquid to be distilled is pumped through the pipe (B) and the coil (C) in the rectifier, and is delivered at the top of the analyzer through the pipe (D). The cold liquid is heated by the steam surrounding the coils, and is delivered hot at the top of the analyzer.
Since steam is being forced up through the perforations, the liquid cannot pass down through them, but is forced to spread out over the plate, and run down the overflow pipe (F) to the next plate, and so through the analyzer. The steam, bubbling up through the thin layers of liquid, heats it very hot, and causes the volatile substances
to distill off with the steam. This mixture of steam and volatile matter passes from the top of the analyzer, through (L), to the bottom of the rectifier. During its passage up the rectifier, the steam is condensed by coming into contact with the cold pipes (C, C), through which the liquid is flowing to the analyzer.Thus only the more volatile matters pass out at the top of the rectifier, and go to the condenser (0). The water condensed in the rectifier contains some volatile matter, so it is pumped to the top of the analyzer and mixes with the fresh liquor to be distilled. From the bottom of the
analyzer a waste pipe (J) carries off the spent liquor which has been deprived of its volatile matter.
Distillation in vacuum is sometimes employed, and will be described in connection with the industries in which it is used.


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