high speed stirring and flask design

7/29/2019 High Speed Stirring and Flask Design

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A n Apparatus for Hig:Effect of Flask Design and Some(

AVERY A. MORTON AND DONMassachusetts Instituteof Technolc

h-Speed Stirringhher Factors on Stirring

ALDM. KN OT T,gy, Cambridge, Mass.

TIRRING is one of the most common practices in the The y~up eiw ) I U ~W iuii i a GILL. I L - ~G U I ~ q u u l i iece oflaboratory and is indispensable for many reactions stainless sheet steel, No. 18 gage. A 0.3-em. (0.125-inch) hole is

first drilled in the center. The 0.6-cm. (0.25-inch) shaft isthenIts benefits increase ( 1, &, 8) as the Velocity increases: Some turned down to 0.3-em. (0.125-inch) diameter for 8. distanceofreactions indeed occur to an appreciable extent only when 0.3 em. (0.125 inch) at one end. The rod isnext fitted into thethe velocity becomes very great (4,6). I n spiteof the mani- hole and the tN 0 are riveted together by afew good blows witha

fold to stirring has long beenputand the obvious hammer while the rod is held upright inavise. This attachmentis firm enoughto permit mounting the Shaft ina lathe and cutting

one apparatus (4) designed for operationsat 10,000r. p. m., will just pass through the neck of the flask. The disk is thenof such constmetion asto be unsuitable foreveryday labor&- slotted radiallyasmany timesasdesired withahacksaw and thetory use, and little, if any, serious study has been devoted to

s e ~ ~ ~ ~pf\hepp~~tusfactors such as flask shape, stirrer design, et% requisite for mns and appreciable vibration. The baseofmaximum effectiveness. Possibly the complexity of a sub- the ring stand can be anchored to the desk with Screwsor C-ject in which an unusually large number of variables exists clamps, hut this precaution is usually unnecessary. The noisehas deterred many would-be investigators, but the likelihood is not unduly great, considering the high speeds involved.

The apparatus is operated with a variable transformer of 5Of greatly improving the fields for many ampere capacity in series with the motor. The revolution rateand of discovering new reactions warrants a real effort to is measured by astroboscope (Strobotrtc) whose light is fixed onacquire knowledge of ideal conditions inanoldart. amarked metal disk attachedto the shaft.

This paper gives directions for constructing a laboratory The small motor becomes very hot, particularly on long runs,but 8. blast of air directed into rtn opening on the undersidecools

stirring apparatus which can operate at speeds near 11,000 itr. p. m., yet issosimple that it can be mounted on an ordinary to remove grease.ring stand. Results of some studies on the typeof flask suit- At speeds of 10,000 r. p. m. some liquid may he forced outable with stirrers of the propeller typeatrelatively high speeds through the glass bearing unless it is fitted very well. In later

work the authorsweregreatly indebted to Mr. Davidson for hisare suggestion of avery effective hut simple ruhher sed. A sectionofciencies of flasks, stirrers, and other factors has been devel- rubber tubing, 0.6 cm. (0.25 inch) in outside diameter by0.94 cmoped. It isshownconclusively that achange in flask design (0.375 nch) long, ispushed over the lower endof the glass bear-has adirect bearing onefficiencyof stirring and that the best ing until the overhanging end of the rubber rests lightly on the

propeller shaft. In this way the escaping liquid presses againsttlask isonefromwhich swirling, obstmctions, and pockets me the md it more tightly. Another solution of thislargely eliminated. problemisto sink the neck far enoueh in the ton of the flask to

S

advantages of high speed and efficiency, there has been only the head to a diameter (slightly ess than 2.5 1 nch) which

A cotton filter on the air line is usual l~equired

A simp1e method for comparing

High-speed StirringApparatus

in thicpaper. a0.d4-homenowerseries-woundball-hearineamotor(G.E.)with rated8 eedat 10,000r. p. m was used. It i i boltedto a O.94cm. (037C!ineh) steel block into the center of whicha1.25-cm. (0.5-inch) steel rod is screwed,so that the motor is supoorted from an ordinerv clamn fastener. This motor is satisfac-tory for many ordinar? uses but lackspower to stir some mx-turesBit the highest speeds.

The equipmenthas since been supplementedby addition of a0.25-horsepower series-wound hall-hearing motor also with B

rated speed of 10,000r. p. m., and mounted similarly ona 1.25-em. (0.5-inch) steel block hut supported by two insteadof onerod and clamp device (Figure1). The total weight of the largemotor, block, and rods is about 3.7 kg. (10 pounds). The lastunit isrecommended for general use, since it is sufficiently power-ful to stir at high speeds nearly all mixtures met in laboratorypractice.

The propeller shaft is a 0.6-cm. (0.25-inch) stainless (18-8alloy) steel rod 37.5 em. (15 inches) long, supported by tN 0shortsections of pressure tubing (3) in & N O ballhearing assemblies(about 2.2 em., 0.875 inch, m outside diameter) which are placedabout 15 cm. (6 inches) apart. The end-tmnd contact betweenpropeller ahaft and motor shaft is closed by a short length ofpressure tubing which is slipped over each end and held by acouple of turns of wire. Perfect alignment of motor and pro-peller shaft snot required in this arrangement.

A holder for the hall-hewing assemblies is made from two17.5-cm. (7-inch) lengths of angle iron, the larger one 3.75cm.

Q.5 inches), and the smaller one 2.5 cm. (1inch), held togethertightly (Figure 1) by four machine bolts passed through bothpieces. A little mlling was necessary an the smaller angle ironin order to make it fit tightly against the ball-bearingassembl y.Two sections of 1.25-em. (0.5-inch) iron rad aretapped into thelarger angle iron to support the holder in two clamp fasteners.

649



650 I N D U ST R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol. 13, No. 9

the liquid surface is composed of drops of liquid which havefallen anor climbed up the shaft. Figure3shows the effect n thecreased flask. Uniform mxingissecured. The positions of thecreases are shown by the reflections as bright lines of the edges.The circular spot a little heow the neck is the roughened placeforpencil marking present onall commercial flasks.

I n addition to this visual evidence, it was found that the

oxidation of toluene by permanganate at 10,750 r. p. m. for30minutes inthe ordinary flaskwassoslight astuproduce notemperature rise over the 1.6" C. due to mechanical agita-tion. With a creased flask other factors being constant, atotal temperature rise of 12.3' was noted, of which 7" wasthe result of chemical heat.

The unusual successes which have attended use of thiscreased flask warranted studv of the effect of additional varia-

FIGURE. AI R AND WATER M IXTURESTIRREDIN ORDINARY FLASK

tions from this type. The four varieties considered in thisstudy are labeled as:

0S

An ordinary 3-necked flask with four vertical creasesSameas 0 but with powderedElass sintered to the bottom

in order to act asai abrasiveSameas0but with the bottom pushed inward about2.5 cm

(1 inch) in the form of an inverted funnel which is aboutI

4.4cm.'(1.75 inches) in dimeter atSameas0but with the bottom pushemventiond pear shape

P the bottom.d outward in the con-

n. . ..The0,I,andP flasks are pictured in rlgure 4. w were

made from commercial 500-ml. 3-necked Pyrex flasks. Fourvertical creases were made in each to a depth of about 2.5em (1 inch) at the deepest point. Save for-thefour distine-tions made above, constructionof each wasasnearly identicalas could be accomplished by a competent glass blower. Alarge Thiele tube wasalsousedasa flask inafew experiments.

Measurement of Efficiency

Efficiency was judged on thebasis of the temperature rise,starting from room temperature, with the vessel shielded by

makearing seal that directs the liquid downward. This methodisvery satisfactory, although the effectivenessof the flask as a

vehiclefor stirringa pears to be reduced considerably.Metal insteadof grass propellers are used because the latter areconsidered dangerous at high speeds. There have been severalinstances in this laboratory however, when glasshas heen usedat 5000 r. p. m. and even digher. The glass stirrer was usuallyconstructed from 0.6cm. (0.25-inch) or larger glass rod. Ingeneral, its useisnot recommended unless necessary to avoidcor-rosion and then only with adequate safeguards to avoid injury ifa section of the glass breaks and goes through the h k .

The apparatus as a whole is exceedingly convenient andeasily constructed. It is a decided simplification over theone previously described (4)which includedasubstantial ironpipe framework, numemu8 pulleys and belts, and aspecial lo-cation.

Flask Design and Oi

The performance of this apparatus, using a stirrer of thepropeller type, was observed in several seriesof experimentsin which the type of flask, number of propeller segments,position of propeller, and a few other variables were studied.Major interest centered in the design of the f l ask. A specialflask having deep vertical creases (6, 7) has been responsiblefor a considerable increase in yield in many reactions and,in one case (7), for eliminating altogether a certain productwhich was obtained in an ordinary flask.

The comparative efficiency of this special flask over the com-mon one in mxingairand water can he observed visually in Fig-ures 2 and3,taken with rnEdgerton high-speed hash lamp, withexposureof l / ~ ~ +msecond. In both pictures, the propeller wasmoving at 11,000r. p. m. All conditions were identical save for

the h e f flask used. In Figure 2 the characteristic swirlina

~ ~ ~ ~ ~

ihotogrrtphic amp. The little ring on the propeller shaft above



September 15, 1941

the propeller having six segments, and that inthe P flask with a propeller having eight sec-tions. Lines have been drawn between pointsrepresenting different flasks in which comparableconditions were present-for example, 0-4, S-4,

0 O R s

FIGURE. TY PESF FLASKSSED S TESTS

50 60 70 80

M E C H A N I C A L H E A T

FIGURE . EFFECT F VARYINGFLASKSND STIRRERST10,750R. P.M . AS TESTEDY EVOLUTIONF HEATN OXIDA-

TION OF TOLUEXE

an insulated can. The reaction was the permanganate oxi-dationof toluene (at 10,750 r. p. m.) or xylene (at6000r. p. m.)over a 30-minute period. Part of the total temperature riseis due to the mechanical effect of stirring,sothat an independ-ent experiment, identical save for the absence of permanga-nate, was necessary. The respective areas on a timetem-perature graph were then estimated. Numerical values forchemical and mechanical heats used in all graphs in thispaper are merely these areas which are proportional to thetrue heat units. The total temperature increase was as

much as 27" C. in the studies at 10,750 r. p. m.This method has the great advantage of rapidity, so thata large number of observations can be made of many factors.Interpretation of data, however, is subject to the criticismthat production of mechanical heat will automatically increasethe total chemical heat according to the general expectationthat achemical reaction is doubledfor every 10" temperatureincrement. Fortunately any such effect is negligible com-pared with the greater influence which the flask exerts on theefficiency of agitation.

Effectof Variables

Figure5pictures the ratio of chemical to mechanical heatplotted against mechanical heat for a study of the oxidationof 25 ml. of toluene by 2 grams of potassium permanganate

in 300 ml. of water at 10,750 r. p. m.

relatively large mechanical effort. - The re-sults show clearly that in every series the Zflask was superior. With one exception the

pear-shape flask, P,was the most ineffective of the lot.Lines ,could also have been drawn between points repre-

senting different propellers for otherwise comparable condi-tions-e. g., 1-4, I-6, and 1- 8. In such a comparison theeight-bladed propeller gave poor results, although in everyseries but one it produced the largest mechanical heat. Theperformances of the four- and six-bladed stirrers were in gen-eral very good. If a line be drawn diagonally towards thenortheast, so as to separate the six best from the six worstconditions, the I flask and the four-bladed propeller are rep-resented three times each among the better group. Thesere-sults are contrasted with the occurrence of theP flask (twice)and the &.bladed propeller (three times) in the poorer half.The single occurrence of the eight-bladed propeller among thebetter group can be attributed to its being used with the bestflask; likewise the single appearance of the four-bladed pro-peller with the poorer group can be explained on the groundthat it was used with the poorest (P ) lask. The results arein general striking, particularly in view of the later discovery(Figure6) that the position of the stirrer at 2.5 cm. (1 inch)from the bottom was far from ideal.

Figure 7 shows a parallel study at6000 r.p. m. of the oxida-tion of 25 ml. of xylene by 16grams of potassium permanga-nate with 4 grams of sodium hydroxide in 300ml. of water.The position of the propeller was again 2.5 cm. (1 nch) fromthe bottom. This figure exhibits the clear superiority of the

5I

2

zU2 1. 5

e2y 1.0

2

IWI

u

0. 5U

I I I I I I I I

20 25 30 35

M E C H A N I C A L H E A T

FIGURE. EFFECT F VARYINGPOSITIONF STIRRERS

S H O W N BY EVOLUTIONF HE.4T I N OXIDATI ON OF X Y L E N E



652 I N D U S T R I A L A N D E N G I N E E R I N G C H E M I S T R Y Vol . 13, No. 9

100-6

I

I

%.ppreciably inferior in the third. The position of the

stirrer had an immense effect on efficiency. When

a decided superiority resulted. I n the light of this .L

sensitiveness to position, the good showing of the I u

a I-....

...-...orrectly placed 1.25 cm. (0.5 inch) above the bottom,0--8.p..

flask, in Figures 5 and 7, is all the more surprising. f o 6 yThe general improvement noted in the flask having 2 1 1

0 2 1 I 0--4i

i 1 o o - 8 ~ + Ii -t"*:y...,the inverted bottom was confirmed by constructing - 0 4 I -2-8'"...two more of that general pattern but with slight

across the bottom spaced between the creases in the

5modification.

side. The other had a number of Vigreux indenta- I PRO PEL L ER SEG MENT S

One had two deep intersecting creases 6000 R P M

V A R I A B L E S .

SCREEN MESH -0.16.100

tions in the lower half, the purpose of which was to " I 4 ,b .B

promote more mixing by having the currents flow inand around the projections. Both innovations re- a 0 . I

I I

I

four-bladed propeller with every flask tried-(for example,compare1-4, I-6, and 1-8 or 8-4, 8-6, and S-8) and the pro-nounced difference between the I and P flasks with this best(No. 4) stirrer. The series with the six-bladed propellershows also a marked difference between these two flasks(1-6 and P-6) but the superiority of I over 0andS is not so

evident. It should be remembered, however, that the posi-tion of the stirrer in this set of experiments is not ideal for re-vealing any superior qualities present. Indeed, when the pro-peller is moved to a position 1.25 cm. (0.5 inch) from thebottom (Figure6)other conditions being the same, the resultswith the six-bladed propeller confirm in every respect the de-cided superiority of theI flask over the three other types.

2-

I W , T .

/ ~ l p o p i i l l ~RO PEL L ER - I"FR0M BOTTOM

V A R I A B L E S

T YPE OF F L A S K . 0 , S . I . P

PROPELLER SEGMENTS

I20 2 5 30 3 5 40

I1

MECHANICAL HEAT

FIGURE. EFFECT F VARYINGFLASKSND STIRRERST

6000R.P.M. AS SHOWNY EVOLUTIONF HEATN OXIDATIONOF XY LEI ~E

The erratic results (Figure 7) with the stirrer having eightsegments are attributed both to i ts, general ineffectivenessand unfavorable position. The mechanical heat generatedby this stirrer, however, is 50 to 100per cent larger than thatrealized by the four-bladed one.

Figure 6 depicts the effect of varying the position of thepropeller with six segments at 6000 r. p. m. The samemix-

ture was used as in the preceding test at this velocity. Theresults are decisive. The pear-shaped flask was, in all cases,the poorest of the lot. The position of the stirrer had littlebearing on its effectiveness. TheI flask, on the other

A further attempt to direct the flow of liquid inside theflask was made with the flask of the I type having its necksunken so that the liquid would be forced down toward thepropeller. For some reason not yet apparent, this innova-tion reduced the efficiency to apoint where i t was not muchbetter than the pear-shaped flask. The change, however,

did eliminate collection of liquid in the neck and any splash-ing through the stopper which sometimes occurred at thehighest speeds.

A Thiele tube flask (from 3.4-cm., 1.375-inch, tubing)was also tested with the three propellers at6000 r. p. m., usingthe same mixture and the same amounts employed for thecreased flasksat that velocity. The volume of the tube wassuch as to be just illled by the mixture. Figure8shows thatthe mechanical input was considerably larger than before,the lowest value of 36 units in this series being near the high-est value of 39 in Figure 7. So great was the power requiredto stir the mixture in this apparatus that i t was impossible toattain a velocity higher than 7620 r. p. m. with the smallmotor used in all these tests. This series was designed totest the effect nf introducing a screen, supported at a level

0.6-cm. (0.25 inch) below the propeller, which would breakthe current into small particles. A 16-mesh screen provedof little value, but one of 100-mesh showedadefinite improve-ment. The propeller with four blades was again generallysuperior.

Interpretation of Results

As far as can be judged from this beginning study of acomplicated problem, the following opinions may be ven-tured:

A . The roleof the container in stirring appearsto be that offurnishing a surfacefor rapid transfer of liquid away from andto a position above the propeller. In the ordinary round flask,the liquid swirls around the outside wall. The propeller throwsthe mixture outward and keeps it rotating but does not efTec-

tively mx the components. The creased flask directs the liquidflow upward, from whence it falls again upon the stirrer to behurled once more in fast-moving currents from the propeller.

The additional superiority of a creased flask with an indentedbottom is due to the further facilitationof movement of liquidwithin the flask. The effect is simlar to that of streamlining.The pocket immediately beow the propeller where liquid cancollect and escape mxing is eliminated.

The poor results with the pear-shaped flask come from inter-ference of the movementof the liquid. The propeller churns theliquid at the bottom, does a great dealof mechanical work in try-ingtoforce the liquid around an unnatural and difficult path, andis unable to perform its proper function of mxing because theflow of liquidisimpeded.



September 15, 1941 A NA L Y T I CA L E D I T I O N 653

B. The roleof the propeller w ~ l le to generate fast-movingA propeller witha argeurrentsof liquid through the mixture.

number of blndcj operatinpxt high speed Giglit conreivahly crUcthp liquid inru mm ~y art, and accomplisha degreeof nixing.k : x 11 srtmcnt. however. nwdr enouehnurlsrr t o servem B mod~ ~ ~ ~

driver. -The general iu6ffectivenessof the eight-bladed pro&ie;at 6000 and 10,750 r. p. m. was pronounced. The mechanicalheat generated by its use was in nearly every nstance the greatest(sometimes by 50 to 100per cent)of the three, yet i t appeared towhirl around in a generally ineffective manner. The propellerwith four blades givesasiltisfaotow uerformauce. Evenagood

tical"impo2ance n large-scale a&arat;s.C. Impediments, whether points on the inner surface of the

flpsk or nalli to ronsrrict the~flo\r,rsulr in a reduction in etl-rimcg. A very finewrePii which suhdividrstheliquid intosmallonniclmndiistj the rmrtion. but the entrxv rpauirrd to O V P ~I P

stirring apppratw of that'type is very mnch'greater &d theratio of chemical to mechanical heat is less than in a properlydesigned flask.

Acknowledgment

The authors are indebted to I. Amdur for very helpfulsuggestions inbanding the data.

L iterature Cited

(1) Brunner,E J., 2. pfiysik.Chem., 47, 56 (1904).(2) Friand, J . A. N.. and Dennett, J . A. , J . Cfiem. Sm, 121, 41

(3) Hershberg, E. B., IND.Em. Cwnr.. h t , . ED ,11, 170 (1939).(4) Huber,F. C.. andReid,E. E.. IND.Em.CHEM.,8,535 (1926).(5 ) MUigan,C. H.. and Red. E. E., I bid.. 15, 1048 (1923).( 6 ) Morton, A.A., IND.ENU.CHEM.,ANAL.ED. , 1, 170 (1939).

(7) Morton. A. A,. O son, A. R., and Blattenherger,J. W., J . Am.

(8 ) VanName. R.G., andEdgar,G..2. pfiy,sik.Chem..73,97 (1910).

Connuanrron romthe Research Laborstory of OrgsnioChemistry. Mass*ohusetta Institute of Teohnology.No. 248.

(1922).

Chem.Soc.. 63, 314 (1941).

PotentiometricTitration Stand AssemblyLOUIS LYKKEN AND F. B ROLFSON, Shell Development Company, Emeryville,Calif.

HE u e of electrometric titration methodshas slowly be-

tiometric titrations have long been used by academic workersfor special determinations, this tme of analysis has foundonly limited application in many industrial laboratories be-cause the apparatus has had to be assembled largely frommiscellaneous parts at hand. Generally, this resulted in acumbersome assembly which was inconvenient and unwieldyto use in routine determinations. Also, expeneucehasshownthat the use of loosely assen'' * '

motor; the Eastern Engineering Company variable-speed stirrer,a laboratory practice. Although poten- Model 1, has proved satisfactory. The motor and rheostat are

conveniently mountedon the same support, as shown inFigure3nes ,

Pencil- orstick-type electrodesare usedthroughout; wheneverasalt bridge isused, contact is made with the titration medium

~~~~~~~~~~~~~~~~m~~~~~~~~~~~~~~~~

nical Laboratories, have been found ideally suited for use in thetitrationassembly. For convenience, metallic electrodes-i. e,Silver, Platinum, and tungsten-have been mounted,a8shown in

T

. . . . wim.r.?,,nF+, "~""~-~--"":~..:-'---~--.- 11, ~ 1 ~ . ~ ~ 1

considerable part of the ope]nance.

In designing the assemblymade to producea titrationroutineas well asspecial detthat a satisfactory assemblyordinary apparatus requiredburet clamp, stirring motor, 1

port have been semipermanerporting rod. The table sup]been designed to be the only e

The buret may be readily reielectrodes may be convenient

During two yearsof contintitration stands have beenfoifor routine titrations. Theytain, are simple to operate, i-though unitized into compactmost electrometric titrations.

APPA photograph of one farm of

and some details of constructiorcustomary "universal" clampsnector-blocks containing holesUristo se1scrPwz. so ;tten,ptrnnnector-blockforall purpowrcplwes th r custmury huret huolidied IQkrlitc is used as D

described here, every effort wasstand which could be used for

erminations. Itwas recognizedmust he as easy to use as thefor indicator titrations. The

beaker cover, and electrode supttly attached to the vertical supiorting the titration beaker hasiasily movable partonthe stand.

noved from ita support and thely removed and replaced.ued useatEmeryville, five suchund satisfactory and convenientare durable, are easy to main-

Laveaneat appearance and, al-units, are sufficiently flexible for

aratus

the assembly isgiven in Figure II arcshorn in Figures2 to4. 'l'he~ S V Pepn renlnwd bv metal con-of apnronrike size&d headlesshas%ee<made to use the same. A modified thermometer clampolderor clamp. A thick pieceof1 comhination electrode support,port. The titration vessel, which,,-Axr- hnolm* i. a..nnnd-l nm a

FIGWE1. TITUTION SSEMBLY EEPAREDOR NEUTULI-rn

beaker cover, and buret-tip supisa250-m. tall-form electrolyt... _ _ _.I_., ."Y~rV.U'U y.lBakelite to africtionclamp. Theglass, propuer-type stirrer is supported and operatedby asmal l variablespeed

*A I I "N I l l&%X,"Nb

Beckman Model 0 deatronic voltmeter and Beckman glass and oalome~electrodes

high speed stirring and flask design

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