Flask Design and High-speed Stirring AVER1 4. 3IOHTO3, BRADFORD DARLIKG, AIYD JOHN D-kVIDSOA Jlassachusettc Institute of Technolop?, Cambridge, Mass.
T
A comparison of this result with that of Huber and Reid (1) is complicated by the fact that the stirrer which they employed was different (a centrifugal instead of a propeller type), the diameter of their stirrer was somewhat larger, the volume of their reaction mixture was smaller, and the container was an ordinary beaker n i t h baffles rather than a flask. The first represents merely one of the differences, besides the container, 17-hich is a factor in stirring. The second difference can be adjusted approximately by comparing values at equal peripheral velocities, on the reasonable expectation that the liquid 11hich leaves the agitator will then have approximately an equal velocity in each instance. The third difference, ~ ~ l i i involves ch capacity, might also be corrected b y making the reasonable assumption that a larger volume of liquid will mean a proportionally fewer number of contacts of each particle with the agitator. I t is convenient, however, to make the compaiison a t equal peripheral velocity and thereby shon increased capacity due to the shape of the container as well a. better yield. The data shon- (Table I) that the special flaqk handled 71 per cent more liquid and gave a slightly bettei (2 per cent) yield than did the apparatus of Huber and Reid. Since the yield in each case was less than 100 per cent and the matimum conversion within one hour had not yet been reached, i t can safely be considered that each apparatus was accomplishing all that it could at that peripheral velocity, and that the values shou- the superiority of the present apparatus.
HE relative efficiency of flasks was judged in a previous paper (S) on the basis of comparative heats of reaction, and a creased flask n i t h an inverted bottom was found best. The present m-ork consists of a survey of the direction and force of the currents in a cross section of the flask, a coniparison of the resultb from oxidation of p-nitrotoluene obtained in the special flask with those in a n ordinary flask and with those observed by Huber and Reid ( I ) , and finally a description of a considerably imp1oved high-speed stirring apparatw for laboratory use. The direction and force of the currents generated by a propeller which forces the liquid down\\-ard a t the center oi the special flask already mentioned have been tested. The results in a cross section through the center shorn a thin section, not more than 5 mm. wide, adjacent to the n all in vihic11 a current of very high velocity moves upward; a region adjacent to this section in xhich the upward force is only about a tenth that in the high-velocity belt; a ! d e r section in mrhich no pressure is evident; and a n area nearer the propeller where n moderate wction is evident. Existence of this fast-moving current next to the wall explains a number of facts which hitherto have not been clearfor example, the reports ( 3 ) that sintered glass fused on and Vigreux indentations pressed in the wall, and that a neck sharply recessed in the flask reduced the efficiency, are now understandable. These obstructions ITere directly in the current of high velocity where their effect was greatly magnified. It is therefore more than ever apparent that the u-all serves chiefly, if not solely, as a surface for the rapid transit of a current of very high velocity to a position above the propeller; and this surface must be as smooth as possible and free from any obstructions, even when these are as distant from the propeller as the base of the neck. The effectiveness of a flask of proper design ab compared with a n ordinary flask has been measured by the percentage of p-nitrobenzoic acid obtained by oxidation of p-nitrotoluene with permanganate. This reaction is good for a comparative study, for i t progreases without stirring and I' therefore not dependent on a special type of agitation. More particularly is it of value because it was studied also by Huber and Reid (1) in their interesting work on high-speed stirring. K h e n thi5 reaction was carried out in two different special creased flasks, a yield of 51 per cent was obtained after one hour's stirring a t 9000 r. p. m. The yield in an ordinary three-necked flask under the wme conditions was 30 per cent.
T . m u I. CRE..WED
COMP.\HISOX
FLASK, .IND
OF .IS ORDISAHY FLASK, .I sPhLI.IL THE APPARATUS OF HUBER AND REID
Ordinary 3-Kecked Flask Type of stirrer Diameter of stirrer, cm. Total volume of reaction mixture, ml. Yield a t 9000 r. p . m . , 70 Yield a t equal peripheral velocity 1498 cm. per second, %
Propeller 3.18
Special Creased Flask Propeller 3.18
Huber and Reid's Apparatus Centrifugal 3.7
785 30
785 51
459 64
30
51
49
Such differences can be of practical importance. With larger quantities they represent a power factor which might become so large as to offset the advantages which would accrue from high-speed stirring. In reactions where high-speed stirring is absolutely necessary, such a difference would greatly influence the cost of production. The higher the speed the more important does the design of the container and stirrer become. The unit for laboratory operation has been considerably improved. This apparatus now runs smoothly and quietly, will operate over long periods of time without requiring adjustment, and is proving a laboratory asset. It has been used in work already reported ( 2 ) .
Experiments
FIQURE
1. SPECIAL FLASKUSEDI N EXPERIMENTS ON OXIDATIOS OF
NITROT TOLUENE
134
HIGHVELOCITYCURRENTS(J. D. and B. D,). A capillary tube of 2-mm. bore was inserted through the side neck of a threenecked flask which had four creases, the flask was filled with water, and the stirrer was rotated at 9000 r. p. m. Observations were made across the section of maximum flask diameter. When the capillary was touching the mall the water rose to a height of 25 cm. above the flask. When moved about 5 mm. from the wall, it fell t o less than one tenth of that value. This method of surveying was originally intended as another means of judging the efficiency of stirrers of the propeller type.
September 15, 1942
ANALYTICAL EDITION
OXIDATI~S OF ~-KITHOTOLUENE (B. D:), The flask (Figure 1) was made from an ordinary 1-liter round-bottomed flask by putting in four creases, pushing the bottom inward, slightly recessing the neck, making an inlet tube which extended to a oint above the stirrer where suction occurred, and inserting a tgermometer t'ube which extended beyond the width of the current of high velocity. The last two changes eliminated some pockets present in the ordinary three-necked flask and took advantage of the previous survey of currents. The special inlet tube was not, of course, needed for the experiments described in this paper but is of value in other reactions. The slight recession of the neck is a compromise between the flat base which mould be ideal from the efficiency standpoint and the sharply recessed position which eliminates filling the neck with liquid. Even with this modification: a rubber seal l i d to be placed on the bearing in order to eliminate loss of liquid. The propeller was constructed as described in the pi:evious paper. The diameter \vas 3.18 cm. (1.25 inches). The SIX segments were set at an angle of 45". It was located 12.5 mm. (0.5 inch) above the center of the inverted bottom of the flask. This propeller could not ntt:iin a speed of 10,000 r. p. m.,but the agitation was greatrr than that attained by a four-bladed propeller which was 2% cm. (1.25 inches) in diameter whic,h rotated ly a t that velocity. If the angle at which the blades were set less than 45", the agitation was visibly lessened. The motor (0.25 h. p.) and assembly were the same as described in the previous article (9). The reaction mixture consisted of 43.5 grams of potassium permanganate, 16.7 grams of p-nitrotoluene, 7.5 grams of sodium hydroxide, and 750 grams of water. Two grams of manganese dioxide from a former experiment were added in order to ensure constant conditions with respect to all surfaces. The mixture except for the permanganate \vas heated to 70°, the oxidizing
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L.
735
agent, added, and the stirring increased to 9000 r. p. in. The reaction was exothermic and the flask had to be cooled constantly by an external bath. Stirring was continued for an hour at 9000 r. p. m. with the temperature maintained at about 75" C., after which the mixture was cooled to 10" C. and filtered through asbestos in order to remove p-nitrotoluene and manganese dioxide. The filtrate mas then acidified with sulfurous acid, and the precipitate was filtered, redissolved in hot dilute sodium hydroxide, and filtered again. The acid was recovered by acidification and filtration. and was dried in an oven at 70" C. to constant n-eight. The manganese dioxide residue was extracted twice with ;io nil. of hot alcohol, from which the unused p-nitrotoluene was rwovered by precipitation wit,h n-ater. In t,wo experiments the yield of acid obtained was 10.1 grams each time, of which 0.4 gram was material that had been lost in the washings. The recovered p-nit'rotoluene m s 4.4 and 4.3 grams: respectively. The loss in each rase amounted to about 24 per cent. These results w r e substantially duplicated in a 3-necked flask diich had four vertical creases, an inverted bottom, and a flat (not recessed) surface at the junction of neck and flask. A compnrison run made in an ordinary three-necked flask ivithout crease. yielded 6.1 grams of acid and 10.1 grams of unc-h:itiged 7)-nitrotoluene. The low \vas 10 per wilt.
The percentage yields for the dame peripheral velocity calculat'ed from the dat'a of Huber and Reid are listed in Table I, along with the arerage from the experiments in the creased and the result with the ordinary flask. The proportions of reagents employed in this work are the same as those used by Huber and Reid, but the comparison is not exact because the temperature a t which they operated TWS that of the bath rather than of the reaction mixture. The temperature rise must have been consider10.000 R P M. MOTOR ATTACHED able from both the heat of reaction and the BY RUBBER T U B I N G mechanical work. The showing of the special flask is therefore probably better than ,PRESS - FIT WASHER the figures indicate.
Improved -Apparatus (J. D.)
M I! BALL' BEARINGS
IN-RING STAND
1.
/5
O I L HOLE
/ I
.
. I
ADDITION, TUBE
.
5 0 0 - C C. PYREX FLASK
II
-KWI I
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The design of a n apparatus now being used regularly in this laboratory is shown in Figure 2. This apparatus consist6 of tIvo ball-bearing assemblies supported in a 15 X 2.2 cm. (6 X 0.875 inch) brass pipe and coupling arrangement. The upper T is tapped in order to make a continuous screw. The upper ball-bearing rests on a shoulder made by boring the inside of the pipe, so that a press-fit results. A washer, likewise turned to a press-fit, holds the ball-bearing in place. The lower ball-bearing is similarly fitted and held in place. The washer at the bottom must be carefully made since there is no ot,her support for the bearing. The shaft (6.25 mm., 0.25 inch) is not attached to but fits closely in the bearings. The shaft with stirrer can therefore be removed without having to dismant,le the support. This apparatus is mounted on a laboratory ring stand, which in turn sits on four rubber stoppers in order t o dampen transmission of vibration t o the desk top. This last precaution is not always necessary for quiet operation. Dry nitrogen can he admitted through the lower pipe when desired. For reactions with organosodium compounds the nitrogen was not bubbled through the mixture but maintained at a very slight pressure by means of a mercury or oil trap. The flask now in use with this apparatus has two instead of four creases. The change is not believed to detract seriously from the value of the flask and makes const'ruction somewhat easier for the smaller sizes. For some purposes and particularly :tt moderate speeds a simple yet adequate bearing can be made from ordinary glass tubing in which a glass or other stirring rod rotates. The seal is effected by a short piece of rubber tubing which projects beyond the lower end of the glass bearing just far enough to touch
INDUSTRIAL AND ENGINEERING CHEMISTRY
736
and press gently on the rod. The upper end of the bearing is flared a trifle in order to eliminate undue friction. A drop of glycerol lubricates and improves the seal. This device is practically the reverse of the well-knonm bearing with the rubber seal at the top, used often for stirring at reduced pressures. The seal at the top is not tight enough t o permit use of the apparatus in preparation of organosodium compounds. That at the bottom has given entire satisfaction. The device is particularly good in place of a mercury seal because it offers less drag in stirring and spills no tiny particles of mercury. The seal should be a little distance above the flask, a change easily made with an adaptor, in order to reduce the softening action of organic solvents on rubber. Neoprene has proved better in this respect than ordinary rubber. A single rubber piece has been used
Vol. 14, No. 9
in as many as ten or twelve runs in the preparation of organosodium compounds before it was necessary to replace it.
Literature Cited (1) Huber, F. C., and Reid, E. E., IXD.ENG.CHEM.,18, 535 (1926). (2) Morton, 4 . A., Davidson, J. B., and Kewey, H. N., J. Am. Chem. Soc., 64, to be published (1942); Morton, -4.A., Davidson, J. B., and Hakon, B., Ibid., 64, to be published (1942). (3) Morton, A. A,, and Knott, D. M.,IND.ENG.CHEM.,ANAL.ED., 13 --, A49 " - , (1441) --, \--
CONTRIBUTION from the Research Laboratory of Organic Chemistry, l\.lassachusetts Institute of Technology, N O . 270.
Apparatus for Crystallization and Filtration at Low Temperatures F. W.QUACKENBUSH
AND
H. STEENBOCK, University of Wisconsin, Madison, Wis.
B
ECAUSE of the technical difficulties involved, fractional
crystallization has seldom been employed for the purification of lowmelting substances. Brown and his co-workers ( I , @ surmounted these difficulties b y carrying out the necessary operations in a special room a t a constant temperature of -20°, and were able to demonstrate the effectiveness of low-temperature fractionation as a means of separating unsaturated fatty acids. Wheeler and Riemenschneider (4) employed the method successfully in the preparation of pure oleic acid. I n this laboratory, the method has been adapted to a relatively small unit which permits both crystallization and filtration at temperatures as low as -75" C. The unit consists of a suitable assembly within a well-insulated chamber. The chamber is provided with means for external control, thus
I,
p FIGURE 1 . LARGECRYSTALLIZATION CHAMBER
eliminating discomfort to the operator, and the apparatus is sufficiently simple to be practicable in any laboratory. Two units were constructed which differed chiefly with respect t o size and means of effecting filtration. I n the larger unit t h e solvent with suspended crystalline material was forced onto the funnel b y means of air pressure. In the smaller unit transference of the funnel was effected b y manually rotating the crystallization flask and its supporting shaft.
Construction of the Larger Apparatus The apparatus consisted of an insulated chamber (Figure 1) the interior of which was divided into three compartments: A , for crystallization and filtration; B, for collection and removal of the filtrate; and C, a reservoir. The chamber (outside dimensions 62.5 X 40 X 75 cm., 25 X 16 X 30 inches high) was constructed of galvanized sheet iron and wood and insulated with a 7.5-cm. (3-inch) thickness of packing-box paper and rock wool. An observation window (17.5 X 22.5 cm., 7 X 9 inches, triply glazed) permitted visibility of operations. Compartment A contained the cooling liquid in which the crystallization flask and Buchner funnel were partially submerged. I t was provided with a mechanical stirrer and a chute, F (2.5 X 7.5 em., 1 X 3 inches), through which dry ice was introduced to produce the cooling. Circular openings, 1.25cm. (0.5inch) in diameter, at the lower end of the chute were covered with a sliding door to aid in controlling the rate of cooling. The crystallization flask (5-liter, round-bottomed) was set on the floor in a rigid steel collar (12.5 cm., 5 inches, in diameter X 2.5 cm., 1 inch, high), and was held in position by a removable wooden collar which was fitted over the neck of the flask and was anchored to the walls of the compartment. Two glass tubes extended into the flask from the exterior of the chamber. The smaller tube, D (14 mm.), which terminated in the neck of the flask, served for the introduction of the solution to be fractionated, for the insertion of the thermometer, and as an inlet for air during filtration. The T-tube, E (16 mm.), which extended to within 1.25 cm. (0.5 inch) of the bottom of the flask, served both as a bearing for a mechanically-driven glass stirrer and as a conduit for transferring the contents of the flask to the approximate center of a 20-cm. (&inch) Buchner funnel. The stem of the funnel was held by a rubber stopper within a conical collar which extended from the floor of A into B. Compartment B , which was accessible through a door a t the end of the chamber, contained a &liter flask into which the stem of the Buchner funnel extended. A small glass tube, I, connected the flask to an aspirator. Compartment C (25 x 25 x 20 cm.,.lO X 10 X 8 inches, high) was gas-tight and served as a reservoir for the cooling medium from A during the removal of a solid fraction from the funnel. A tube (1.25-cm., 0.5-inch steel pipe) originating at the deep end of A extended to within 1.25 em. (0.5 inch) of the floor of compartment C. A glass tube, H , connected the top of compartment C with a compressed-air line. I t served for the admission or removal of air to transfer the cooling medium from one compartment to the other. A drain pipe extended from the bottom of C to the exterior.
