734
I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y
Vol. 15, No. 7
Aut omat i c Fraction a ti n g Co1u m n s1 By Wm. A. Peters, Jr. E. I. nu PONTDE NEMOURS & Co., WILMINGTON, DEL.
HE first fractional distillations were carried out in batch stills, and the operation required careful watching a t all times and repeated distillations to obtain a high degree of separation. Later, the continuous rectifier was developed for certain fractionations and, since the rectifier might be regarded as a series of small stills operating simultaneously, the labor required for a given output was considerably reduced. If the composition of the feed and all other conditions were maintained absolutely constant, it might be possible to run a continuous rectifier indefinitely without any attention and obtain a satisfactory product. The important details of such a rectifier for separating alcohol and water are shown in Fig. 1. The divisions shown in the column represent theoretical plates.2 A theoretical plate is defined as one on which the liquid covering i t is of the same composition throughout, while all the vapor rising from i t is in equilibrium with this liquid. It therefore represents a certain height of column rather than one or more actual plates of a plate column, and this same conception can be applied equally well t o a packed column. I n fact, all our small automatic columns are packed with glass rings, a type of construction t h a t is much cheaper than the plate column for the smaller units. It will be noted that the steam supply is controlled by a float operating from the pressure a t the base of the column so t h a t this pressure is maintained constant.
T
1 Presented before the 15th Semiannual Meeting of the American Institute of Chemical Engineers, Wilmington, D e l , June 20 to 23, 1923. 2 THIS JOURNAL, 16 (1923),402.
It would be easy t o maintain both the feed and t h e drawoff constant, but the composition of the feed might vary. If, with constant feed, the amount of alcohol in the feed is increased, the liquid and vapor on the plates of the column will become richer in alcohol, and since a richer liquid in the column gives more back pressure for a given steam supply t o the base, the steam will be cut down and the rectifier will operate as shown in Pig. 2 until the feed is checked. Not only will the distillate be poorer alcohol, but all the excess alcohol in the feed will be lost in the slop. The operation of the column can be made fully automatic in several ways, so t h a t such changes in feed composition will be taken care of. One method is shown in Fig. 3. Here a double wier is placed in the distillate line so that the ratio of distillate withdrawn to distillate returned t o the column as reflux remains constant (0.408t o 1 in this case), and a thermostatic control is p u t on the steam supply to maintain a given temperature a t some point in thecolumn. The feed is controlled by a float operated from the base of the column. This method of control insures that as long as sufficient feed is available it will be supplied at the maximum allowable rate. Of course, if less is available it will simply run t o the column as supplied. After the column has come t o equilibrium under the conditions given in Fig. 3 (same feed and steam supply as shown in Fig. l ) ,let the amount of alcohol in the feed be increased t o 15 per cent. The first effect will be an enrichment of alcohol on the plates near the feed plate. This will cause the temperature a t the thermostat to drop and more steam will be
FEEOTANK
F E E D 1000*PER
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I FIG. STANDARD CONTINUOUS RECTIFIER
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CONTINUOUS RECTIPIER WITH EXCESS ALCOHOL I N FEED
I N D U S T R I A L A-VD ENGINEERING CHEAUSTRY
July, 1923
735
OOURLE W ~ E R
WITHURWN PER WR
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L FIG.3 - A U T O M A T I C
RECTIFIER
admitted to the column. But it will also cause the pressure a t the base of the column to rise, which will raise the float regulator and reduce the feed. Although the feed is richer, the column will not handle a greater amount of alcohol for the same back pressure a t the base, since it should already be working very nearly to capacity a t the top plates where the liquid and vapor are rich in alcohol. A very slight increase in steam will therefore start the column t o load near the top and run the pressure up at the base. Hence, when a balance is finally reached, the conditions in the upper part of the column will be almost exactly the same as before the feed was changed. The conditions in the lower part of the column will be shown as in Fig. 4. If the alcohol in the feed is reduced to 5 per cent, the column will no longer deliver 109.2 lbs. of distillate, since a much larger amount of steam is required to heat the feed. This will cause art increase of back pressure in the lower section of the column. When equilibrium is finally reached, we may be getting about 100 lbs. of distillate from 1850lbs. of feed. The slop will run 0.21 per cent alcohol. I n case i t is necessary to take care of such wide variations of feed, a small addition can be made to the bottom of the column. The equivalent of five extra theoretical plates below the feed plate will bring the slop down to 0.01 per cent in this case. It is seen that by maintaining the ratio of reflux to distillate constant, the composition of the distillate will be absolutely independent of the composition of the feed. The automatic rectifier will work just as well on many other mixtures, such as alcohol and acetone, acetone and water, ammonia, and water, and it will work nearly as well on any mixture of two liquids, It has also been arranged to make separations of mixtures containing more than two components. Variations of this arrangement have been used. For example, where it was required t o keep a given amount of distillate on hand, the feed was controlled by the level of distillate in a storage reservoir. One of these units has operated for over two months without any attention whatever. Patent applications covering this rectifier ha\-e been made.
FIG.&-AUTOMATIC
RECTIFIER
The Collapse of the Autoclave' By W. P. Mason RENSSELAER POLYTECHNIC INSTITUTE, TROY,N.Y .
This photograph explains itself and shows what can happen to a brass, cylindrical autoclave (about 15 x 25 in.), when a vacuum is formed after steam condensation takes place. This particular apparatus has been in use for several years. It collapsed only recently. After having been heated to 15 lbs. for 15 min., the gas flame was turned out and the autoclave was allowed to cool without opening the r e 1i e f valve. It is true that if the valve had been slightly opened this collapse would not have occurred, but it is not safe to depend upon the student slowly opening the valve, and if it is opened suddenly the cotton plugs will be blown out of the vessels within the autoclave. The design is a t fault. There should be a sufficicntly strong band around the middle of the cylinder to permit its withstanding a reversed pressure when the steam condenses. 1
Received M a y 11, 1923.
