Aug., 1921
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T H E JOURNAL.OF INDUSTRIAL A N D ENGINEERING CHEMISTRY
701
LABORATORY AND PLANT Distillation Apparatus‘ By Paul Gross and Alexander H. Wright CHEMICAL LABORATORIESO F
TRINITY COLLSGE, DURHAM, N.
The different types of apparatus herein described have been devised from time to time to meet experimental situations arising in the course of the authors’ work. For some of these no originality other than that of form is claimed. Their usefulness and simplicity, together with the general lack of knowledge concerning them, is deemed sufficient reason for recording them. FRACTIONATINQ COLUMN
Anyone who has had to perform the frequent fractional distillations incident to the laboratory preparation of pure organic chemicals has felt the need for a fractionating column which was of good efficiency, and a t the same time strong, simple of construction, easy to make or buy, and which could be quickly and thoroughly cleaned and set up. The still described below answers these requirements. It consists of a straight tube A (Fig. 1) with a series of small bulbs sucked in on opposite sides, each pair of bulbs lying between the pair next above and below. The bulbs, being thin, permit of rapid heat exchange with the air outside. Their large area, together with the baffling of the ascending vapors by the alternately projecting bulbs, brings about a thorough contact between the liquid as a thin film and the rising vapor. These provide the necessary conditions? for efficient separation. If made of Pyrex glass and sealed to a flask as shown in Fig. 1, this makes a durable, efficient still. The thin bulbs are well protected. There is a free passageway down through the tube, which makes cleaning and draining easy, and there are no movable parts to lose or break. The cylindrical profile permits of fastening as with an ordinary straight glass tube, with clamps a t appropriate points.3 The symmetrical bulbs will stand the lowest pressure without breaking, thus making the 0 still available for fractionations in vacuo.4 The efficiency of one of these columns (Table I) was determined by distilling a mixture of 50 g. each of purified benzene (b.p.80.20’ * 0.05’, f. p. 5.50’) and toluene (b. p. 110.60” * 0.05’) as recommended by Young.S The weights of the fractions between definite boiling temperatures were determined. The usual correctionE for pressure and thermometers were made. The data are for a tube of Pyrex glass 62 cm. long and 2 cm. in diameter, having 42 pairs of bulbs which approximated to three-fourths spheres of 1 cm. in diameter. This was sealed 2.3 cm. down the neck of a 250-cc. Pyrex distilling flask, and was provided with a delivery tube 4 em. from the
c.. AND QUEENSUNIVERSITY, KINGSTON, ONTARIO TABLE I Roiling Points of Fractions
Weight of Distillate Grams 80.2-83.2 4.587 83.2-86.2 22.590 10.883 86.2-89.2 7.322 89.2-92.3 92,3-96.4 6.086 4.877 95.4-98.5 3.267 98.5-101.6 101.6-104.6 1.931 5.356 104.6-107.6 107.6-110.0 5.425 5.656 110.0-110.6 17.192 110.6 110.61 3.938 TOTAL WEIGHT99.100 1 Residue remaining in column.
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Per cent. of Total Distillate 4.6 27.3 38.2 45.6 51.8 56.8 60.1 62.1 67.4 72.8 78.6 96.1 100.00
top. This represents an average column of this type which, though not the most efficient, is yet the handiest for allround use. COMPARISON WITH OTHERTYPESOF COLUMNS To obtain a more quantitative comparison between the various stills, each of the figures for the per cent of total distillate in Column 3 of Table I were plotted against the final temperature a t which the fraction was collected. Curve B in Fig. 2 represents these results, while the other curves represent the results obtained by Young for two of his types of stillhead: Curve C, the rod and disk, and Curve A, the pear. The figures given are for stills of the same length as that used here.
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Received November 23, 1920. Young, “Fractional Distillation,” 190 8 Compare Merriman, J . Chcm. Soc., 99 (1911), 994, who states, with refereuce t o ordinary types of column, *’a long fractionating column is a heavy piece of apparatus, and experience has shown that it is undesirable that any part of its weight should be,borne by a clamp.” 4 Noyes and Skinpor, J . A m . Chcm. Soc., 89 (19171, 2718, describe a fractionating column for distillation ‘ciple of which is the same as that of t S o ~ p other more quantitative method of 6 Young, LOC.cat., .p. 158. comparisoq such as that of Rob well have been used here, but f common forms of column with 1
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FIG. 2
The broken line DEHFG in Fig. 2 would represent a still which was 100 per cent efficient. That is, the curve would be identical with DEHFG, and the area between them zero. The benzene would all distil a t 80.2’. The boiling point would then rise abruptly to 110.6’ and the toluene would distil ob. I n a given case the area included between the first half of a curve and D E H plus that between HFG and the second half is a quantitative comparative measure of the still’s efficiency. The smaller the areas the more efficient the still. The values of these areas from Fig. 2 are given (in arbitrary units) in Table 11. TABLEI1 Rod and Disk Gross and Wright Pear
Area under First Half 2.6998 2.4421 2.3016
Area above Second Half 1.2027 1.1110 0.8120
TOTAL 3.9025 3.5531 3.1136
As the figures indicate, the efficiency of our column lies
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THE JOURNAL OF INDUSTRIAL A N D ENGINEERIATG CHEMISTRY
between that of the rod and disk and the pear of equal length. It must be remembered that the above column represents one for average use and is not the most effiB cient that could be made of this type. Also it is constructed of different glass. This method of making a quantitative comparison by means of the areas under the curves is, as far as we know, given here for the first time, although i t follows logically from Young's discussi0n.l
Vol. 13, No. 8
second type (Bruhl, Bredt, Gautier) the receivers are rotated' (by means of a lubricated joint or connection) beneath t h e
PROTECTION OF CORKS
It is very often necessary to fractionate liquids, the vapors of which either attack or exert a solvent action on cork or rubber stoppers. The usual methods of wrapping the stopper with tin or silver foil are only occasionally successful, and sometimes (as with high-boiling corrosive liquids like chloroacetic acid) fail utterly, necessitating recourse to expensive all-glass stills. The device shown in Fig. 3 overcomes this FIG.3 difficulty. It consists of a small ball condenser which condenses the hot vapor and keeps it from reaching the stopper. If the bulb is blown from Pyrex tubing or other good grade of glass no fear need be entertained of its cracking on account of the temperature extremes to which it is subjected. It works admirably, for example, in protecting the lower stopper in the flask in such an apparatus as shown in Fig. 1. . MICRO-DISTILLING FLASK
Occasion often arises to distil and determine the boiling point of as little as from 0.5 to 2 cc. of liquid. If the liquid is pure the value can readily be determined by the submerged bulblet method.2 In most cases, however, the liquid is a mixture, and it is necessary to separate the constituents and to learn the boiling points of the fractions to aid in their identification. The apparatus shown in Fig. 4 consists of a small bulb, A, about 1 cm. in diameter, with a side tube, B, sealed to it, which is the inner tube of the condenser C. I n the top of A is a narrow well, about 0.5 cm. deep, made by softening the top of the bulb and pushing in with a charred wood sliver. I n this well is put a drop of mercury about 1 mm. deep, into which dips the tip of a copper-constantin thermo~ o u p l e . ~The couple is best calibrated in the still by means FIG.4 of liquids of known boiling points.
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RECEIVER FOR VACUUM FRACTIONATIONS
Many attempts have been made to solve the problem of changing receivers in a vacuum distillation without altering the pressure of the system. These are of two types: the first employing a by-pass controlled by stopcocks.4 I n the Lac. cit., 15s. Smith and Menzies, J . A m . Chem. Sac., 32 (1910), 897. 8 Constructed as recommended by White, J . A m . Chem. Sac.. 36 (1Q14), 1868, 2011, 2292, and Adams, I b i d . , 36 (1914), 72. Instead of using an elaborate potentiometer, if great accuracy is not desired, there may be used a millivoltmeter, or better, one of the numerous compact, portable instruments now available which utilize the potentiometer principle. 4 Since writing this our attention ha8 been called b y Professor A. B. Lamb to a similar piece of apparatus described by Palomaa, Chem.-Ztg., 1902, 337. This method seems, however, to be generally unknown, although the only one which will accomplish the desired purpose simply and efficiently. 1
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ra O p G Z ?
,-I
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1
1 1
HI
1 'I
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hand) over the usual forms of the latter. It is quick operating, direct read.. ing, and simple in construction. T h e 10-cc. pipet A (closed a t C) is calibrated so that volume v between C and H is one-eleventh of the volume V between C and K. At D aleveling tube F a t least 90 cm. long is connected. The side tube B is 30 to 40 cm.
T H E J O U R N A L OF IhTDUXTRIAL A N D ENGINEERING CHEMISTRY
Aug., 1921
$v = P V and p =PV/v Also HI = f i - P . * HI = P(V/v 1) By the calibration V/v .H I = P ( l I - l ) and P = HI/lO
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= 11 1
Therefore, the height HI, in centimeters, is equal to the
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pressure in millimeters. It is obvious that by changing the ratio Vlv the sensitivity of the gage can be changed. Also in a given instrument two or more marks on CH with the accompanying knowledge of the volume ratios will increase the range of the gage. The one described will, however, be found the most convenient for general use, reading to 0.1 mm. or better, up to 30 mm., thus covering the range for most vacuum distillation.
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The Consistency of Starch and Dextrin Pastes”’ By Winslow H. Herschel and Carl Bergquist U. S. BUREAUOF STANDARDS, WASHINGTON, D.
This paper describes a preliminary investigation for the best method for determining the consistency of starch and dextrin pastes. While the results of tests are given, it is realized that they are approximate, and serve rather to show the value of the method employed than to indicate the exact qualities of the materials. There are a great many substances which are not fluid enough to run through a viscometer under hydrostatic head, as in the well-known instruments of Saybolt and Engler, and which a t the same time are not hard enough to be subjected to the Brinell test which is used for metals. Thus there are a variety of substances for which no standard method of measuring consistency is available, although many tests have been proposed. The term consistency is used to designate the degree of firmness or cohesion, and is applicable both to the viscosity of a liquid or to the plasticity of a plastic solid. A number of kinds of penetrometer are in use for testing a ~ p h a l t and , ~ other instruments which might be mentioned are the Legler con~istometer~ and the consistency tester of Hubbard and Pritchard.5 It is doubtful, however, whether any of the above-mentioned instruments could be used to distinguish between a plastic and a viscous substance, a distinction which must be made if a clear idea of consistency is to be obtained.
THE DISTINCTION BETWEEN A PLASTIC AXD A VISCOUS SUBSTANCE The word plasticity is not always used with the same significance. According to Bingham16plasticity is a measure of the consistency a t a given time, while, as pointed out by Emley,’ the plasticity of plasters and mortars depends upon the length of time during which they can be worked, and “consistency has little influence on plasticity.” I n this paper the usage of Bingham will be followed, and plasticity considered as independent of the change of,consistency with time. Thus with plasticity, as with viscosity, a constant consistency during the time of test is assumed, although it is admitted that wlth some substances this assumption may be in error. It is known that lubricating greases may be forced through a capillary tube with greater ease the second time than the first, thus showing a softening of the grease as a result of a breaking down of the structure, and undoubtedly this same action may take place with other substances. The difference between a viscous and a plastic substance may be seen in its behavior when subjected to successive pres1 Received
March 10, 1921 Published by permission of the Director of the Bureau of Standards. a See, for example, Herbert Abraham, “Asphalts and Allied Substances,” 1918, 480. The Dow penetrometer, supplied with a disk insteudof a needle, is described in Proc A m . SOC. Test. Moteriols, 6 (1906), 501. 4 J Lewkowitsch, “Chemical Technology and Analysis of Oils, Fats and Waxes.” 5th Ed., I , 362. 6 P. Hubbard and F. P. Pritchard, Proc. A m . SOC.Test. Materials, 17 (19171, 11, 605. 6Bureau of Standards, Scicntilic Paficr 278 (1916). 7 I b i d , Technologrc Paper 169 (1920), 21. 2
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sures of increasing intensity. For a viscous liquid and a given capillary, the flow is proportional to the pressure. With a plastic substance (which may be considered a soft solid) the flow is propartional to the pressure in excess of a limiting amount p , obtained by graphical extrapolation. If it is assumed that
p = -41d d
where f is the “yield shear value,” and I and d are the length and diameter, respectively, of the capillary; and similarly
where F is the shear due to the total pressure P; then, in general, with a viscous liquid flow is proportional to F, and with a plastic substance flow is proportional to F - f. The practical significance of the yield shear value for material such as paint, which is applied to a vertical surface, is shown by Table I, slightly modified from table given by Bingham and Green.’ TABJ.EI-INFLTJENCEOF CONSISTENCY UPON WORKINGQUALITIGS OF PAINTS,ACCORDING TO BINCHAM AND GREEN RIGIDITY YIELDSHEAR VALUE R f WORKING QUALITY O F PAINT Low Low Thin “runny” paint High
Low
Thick runny paint. Will probably drag under the brush (“stringy” paste)
Low
High
Will not run easily, nor will it offer much resistance to the brush
High
High
Will have good b o i y and covering power, and easily stay put”
The yield shear value is thus a measure of the ability of a substance to resist moderate forces, such as the force of gravity, and adhere to a vertical surface where a liquid, even when very viscous, would gradually trickle down. Rigidity is the term proposed by Bingham for the property of a plastic solid which corresponds to the viscosity of a liquid; the reciprocal of rigidity is known as the mobility. When there is a t least a possibility that the substances to be investigated may be plastic, it is necessary to use a plastometer in which the rate of shear may be varied. This excludes from consideration most forms of viscometer, and leaves the choice of an instrument to be made between the variable pressure capillary instrument of Bingham, and some form of rotating instrument, such as those of MacMichael,2 Doolittle,3 Hayes and Lewis14and S t ~ r m e rin , ~which the speed may be changed. Since only in the case of the Bingham instrument has the attempt been made to express R and f in c. g. s. units, this Proc. A m . SOC.Test. Materials, 19 (I919), 11, 640. G. S t . J . Perrott and R. Thiesen, THISJOURNAL, 12 (19201, 324; W. H . Herschel, I b i d . , 12 (1920), 282. 8 Seaton, Probeck and Sawyer, THISJOURNAL, 9 (1917), 35. 4 A m . SOC.Mech. E m . , 38 (1916), 626. SE. F. Higgins and E. C. Pitman, THIS JOURNAL, 12 (1920), 587. 1 2
