FEATURES FLOATING DROPS AND LIQUID BOULES—A FURTHER

liauid A Further Look The conditions under which a liquid

drop canjoat and grow on a pool o f the same liquid before spontaneous coalescence occurs are established in a series of fascinating expe rim en ts . T h i s inuestigation raises the intriguing question as to what is meant by a ccclean”

he name boule, borrowed from the synthetic gem

Tmaker, has been applied to those masses of a liquid,

larger than individual drops, that can be caused to float for periods of seconds, minutes, and occasionally hours, on the surface of the same or a similar slightly superheated liquid, without mixing. Boules were encountered during a 1962-64 study (74) of sessile drops, and a broad qualitative account was previously published in this journal (73) with references stemming back to Plateau (77), Rayleigh (ZO), Boys (4,Mahajan (75), and especially Prokhorov (79). I t was Prokhorov who postulated that there will always be an expanding or renewing layer of vapor between drop and drop, or drop and support liquid, if the two are to resist coalescing longer than a few seconds. A superheated support liquid can provide a controlled supply of such vapor. It was hoped that the previous paper ( 7 3 ) would serve as the background for individual studies of boule systems. With three exceptions (5, 7, ZZ), this has not proved to be the case. Each unit of research as it progresses has needed to draw on observations and data coming from other often unfinished investigations. There has thus accumulated a mass of interrelated data, incomplete in many sectors, which nevertheless, and taken as a whole, carries knowledge of the boule system a useful step forward; this article is such an across-the-board account and it is again offered as foundation for narrower studies in depth. Some sections, new methods, and key observations are credited to individual authors-see author box. While the introductory material is arranged in logical order, the experimental work is reported sequentially, from simpler to later, more difficult techniques. The unraveling of boule behavior is displayed step by step as the experiments proceed. Early Findings and Present Program

liquid surface

18

I N D U S T R I A L A N D E N G I N E E R I N G CHEMISTRY

It was shown (73) that sessile drops will stay on the physically clean surface of any liquid heated 0.5’ to 5.0’ C. above its boiling point, preferably without ebullition. For large drops or boules to accumulate and persist it was known that (1) there should be exclusion of air or foreign gas so that the surface should be covered by

Kenneth Hlckman Jer RU M a Andrew Oavidhazy Olivia Mady

I

-BOULE

\SHROUD

substantially saturated vapor; (2) boule and support liquid should be kept electrically neutral, conveniently by short-circuiting with a wire immersed in both; and (3) natural vibration of the boule shall be avoided by selecting a low, critically narrow range of superheat and, in addition, by immersing a mechanical wave suppressor in the boule. (4) A fourth requirement has since been recognized: that the boule-making device shall be held in a thermostat and shielded from electrical, magnetic, and perhaps other external influences. I t was early appreciated that the boule rests in a bath ofvapor formed in a cradle of the support; the vapor provides a shroud leas than 0.1 mm. thi& which engulfs the boule and then issues from a circular rim around the surface. There was no information concerning the relative quantities of vapor distilled from the support liquid and the boule, but it was suspected from the revolving whirlpool generally observable near the top center of the boule surface that the vapor rising through the shroud was driving the contents of the boule in rapid doughnut involution. I t was not known why some boules spontaneously merged or "burst." So that many characteristics of boules were not attributed to the peculiarities of water, experiments had been repeated routinely with a wide selection or organic liquids, including fluorocarbons, which demonstrated that boule systems are due to liquid-vapor interaction and are largely independent of chemistry. However, water, because of its chemistry and consequent high surface tension, readily acquired film that altered boule behavior, introducing the suspicion that rearrangement of the uncontaminated water molecules at the liquidvapor interface might determine some of the properties of water boules. There thus arose a natural division of attention between large stable boules on organic liquids (common solvents in the boilingpoint range 30" to llO°C.) and the smaller, sensitive boules ordinarily obtainable with water. Some properties of large water boules could only be inferred from measurements on organic boules. In compensation, the water boule emerged as a potential sensitive test for contamination of the water surface and now

V O L 5 9 NO. 1 0 O C T O B E R 1 9 6 7

19

T A B L E I.

A T,

2O

’

c.

1-2O

of Boule

Liquid

Small, vibrating

htrong

Organics

1 Larger, vibrating

/Strong

\Yater

1

Assured

1

c.

’

Type of Boule

’

Water

I

1

c.

l

I

Water Water

~

Duratzon 10-90 sec.

1 ’

tamznation

Very large

1 Good

10-30 min.

IPoor Fair

5-10 min. 20-60 min.

I

1

I N D U S T R I A L A N D ENGINEERING C H E M I S T R Y

Research

Variation of properties with ambient pressures. Study of vibration and waves

’

Considerable1 Effect of artifacts

/Fair

Uncfrtain 1 Large Uncertain Large, mechanically stabilized

for

1

Slight

None

1-3 min.

Larger, vibrating

shows promise of becoming a quantitative tool. The state of the art at the commencement of this research is summarized in Table I . The present program pursues the characterization of the boule with emphasis on water boules and leads to the study of the resting water surface through studies of boule formation. Steps covered are: (1) Improving the boule-making apparatus: (a) for rectilinear photography of boules, analysis of boule shapes, measurements of the thickness and contour of the vapor shrouds, and recording the sequence of events during merging or “bursting” (b) for producing boules at pressures below atmospheric (c) to provide better stabilization of the boules, and instruments for their continuous monitoring (2) Providing high purity source water and means for continuous in-process repurification (3) Constructing apparatus for collecting and measuring simultaneously but separately the vapor evolved froiii known areas of surface of the support liquid, the cradle, by way of the rim, and the top of the boule (4) Studying the influence of confined and expandable surfaces on boule formation (5) Examining the effect of selected additives to the water boule system ( 6 ) Unraveling the mechanics of boule flotation ( 7 ) Offering a tentative appraisal of the state of the surface of “clean” water resting at atmospheric boiling point in well leached borosilicate glass containers (8) Projecting some future trends in boule research The account now reverts to the sometimes different order in which the work was performed. 20

1

Assured

Organics

0.2-1”

PREFERRED AREAS FOR BOULE RESEARCH At atmospheric boiling point

Slight

1

High Variable

1

Measurements of heat and mass transfer Study of contaminated water surfaces; susceptibility of boules to extraneous in-

I

Size and Shape of the Boule

The boule maker for this photographic study followed the established double flask arrangement in Figure 3 of reference 73, and in Figures 6 and 13 of this article, where the lower vessel receives overflow from the boule flask and supplies vapor for blanketing the working surface and distillate for growing the boules. Borosilicate glass flats were sealed into opposite sides of the working flask to provide two parallel windows and a tubular extension was sealed onto the bottom to accommodate two electric heating pads which were applied to the sides of the cylinder normal to the windows. This avoided striae of hot liquid arising in line with the light source, boule, and camera. The arrangement is shown in Figure 1. The upper drawing shows the borosilicate glass flask with plane parallel windows, collimating lenses on either side, camera at light source with filament nearly end-on. The lower drawings show the heater arrangement, convection currents and plunge lines of evaporatively cooled liquid. The glassware and heaters were housed in a doublewalled tempered glass box that was maintained at a temperature near the boiling point of the liquid studied. The photographic equipment and methods have been detailed elsewhere (5). Except that the shielding was insufficient for growing large water boules, the arrangement has served excellently for still and motion pictures. The photographs were rectilinear and, with a small angular correction, revealed the true cross section of the boule, the dimensions being determined from the known diameter of the wire gauze stabilizer. The small water boules, not examined here, had given nearly hemispherical silhouettes. Organic boules pul-

Figure 7.

Superheat flask for boule photography, heater arrangement and convection currents, and plunge lines of evaporatively cooled liquid

sated and changed shape so that only an average contour could be assigned to a particular liquid. The contours were found to approximate catenaries with the values of the constant a of the catenary formula changing progressively during growth of the boule. The catenary formula is y = a [cosh

(:)

CARBON TETRACHLORIDE

ACFTONF

- I]

where x and y are coordinates of points along the contours and a is a constant to be found by trial. The formula was checked experimentally for carbon tetrachloride, benzene, 2-propanol, acetone, and n-heptane, from small to the largest stable sizes. Approximately 20 pictures were taken of each liquid and the x , y coordinates were measured from an enlarged image. Then these x , y values were analyzed to determine the a value of the catenary equation. From the determined value of a, the x’s corresponding to a given AUTHORS: Kenneth Hickman, Professor and Head of the Distillation Research Laboratory, is the author of a previous IMEC paper on boules (June 7964); he directed this work, and designed and constructed much of the equipment. Jer R u M a a , Research Fellow, performed the work relating boule sire and pressure. O h i a Mady, Research Assistant, made the measurements of boule shape and growth and the manipulative work in the 3-ring experiment. Andrew Davidhazy took the photographs and devised and constructed the boule monitoring equipment. The authors are associated with the Rochester Institute of Technology, Rochester, N.Y. This study was aided by a grant from the Ofice of Saline Water, U.S. Department of the Interior.

Figure 2. Photographs of growing boules VOL. 5 9

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21

3.8

-

I

3.6 3.4

3.2 3.0

I

2.8 26

a 2.4

I

J

2.2

2.0

I

I .8 1.6

1.4

I 1.2

I

I .o 1

0.9

.

1.1

1.3

1

1

1

1.5 VNUi ff a

1 1 1 1.7 1.9

1

1 2.1

I

!

I

l

23

0.8

1.0

1.2

1.4

1.8 2.0 VMUi OF a

1.6

2r

A

26 2.8

Figure 3. Curves illushating calmly equation for boules of cmbon lehachloride and acetone

Figure 4. Comparimn of catenary curves far 2-prapanoI, hcplane, benunc, acetone, and cmbon tetrachloride

y were recalculated to judge the closeness of fit to the pure catenary equation. Thus, a value of a for each progressing boule size was determined and plotted to give graphic illustration of the individual shape changes as each liquid boule grew. Boule size was gaged byy,,, the depth of the boule. As the boule grows, it may remain flat, or become sagging and pointed. This is reflected in the a value; if the boule becomes flat and rounded on the bottom as it -, .grows, a will increase more rapidly in value as ymm increases. On the other hand, if the boule becomes ''pointed and sagging with growth, the (1 value for the deepest boule will be less than the corresponding value for a flat boule. This can be seen pictorially from Figure 2 where five prints each of carbon tetrachloride and acetone show the stages of growth. The former becomes pointed as it grows while the latter remains flat and rounded. This is also seen in Figure 3 where (1 in the case of carbon tetrachloride appears to approach a limiting value as size increases, but with acetone increases with boule size. A graph of the ymu values (the depth of the boule) versus the a value in the catenary equation for all five liquids is given in Figure 4. Despite some scattering of the data from which the curves are constructed, because of the persistent fluctuations of the boule, each liquid has a distinctive curve which reflects its most probable shape. The three liquids whose graphs curve up-

ward furnished boules which were more pointed and sagging in shape than the benzene or acetone boules.

22

INDUSTRIAL AND ENGINEERING C H E M I S T R Y

Shroud Thickness

The thickness of the layer of vapor that separates the boule from the cradle would be expected to vary from a minimum at the lowest point where the supply is least and the overburden greatest, to a maximum near the exit rim, with a mean thickness girdling the boule somewhere in between. Early (reference 73) calculations of mean thickness were based on loss of weight of the resting, unfed boule. The loss multiplied by the latent heat of evaporation should provide the quantity of heat (Q) transmitted by the shroud and from this, assuming that the vapor streams in laminar flow through the exceedingly thin channel and assuming a thermal conductivity approximating known values of k for the stationary vapor, the thickness e (cm.) would be

e=-=-kAAT Q

kAATt who

(1)

for w grams lost in t sec. from a cradle area of A sq. cm. Weight loss was calculated from photographs of the rim taken repeatedly during the l i e of a boule as previously described (74). Whereas Hickman had estimated a weight loss of 1.7 grams from a 35-gram boule in 30 min. and from this derived an equivalent shroud thickness of 10 p, repeat measurements failed to find appreciable

weight change in water boules lasting 0.5 to 1.0 hr. However, benzene, acetone, carbon tetrachloride, and nheptane showed measurable evaporative losses as listed in Table I1 but Equation 1 led to thicknesses of shrouds which would be too large to retain vapor to float the boule ;the assumptions made concerning laminar flow and thermal conductivity of vapor did not apply. Concordant thickness readings have since been obtained for an isopropyl alcohol boule using double reflections from the outside of the boule. A miniature medical lamp was immersed in the support liquid opposite a plane glass window of the boule-making assembly, as suggested in Figure 5a. A camera was placed on the axis normal to the window so that the light from the filament took the path shown in Figure 5b. Two images were found on the negative, one reflected from the wall of the cradle, the other from the wall of the boule. The apparent separation of the surfaces-e.g., as seen in the two images of the filament (Figure 5c)-depends on the real separation, the angle of observation, and the image magnification or reduction. The boule is immersed in solvent of refractive index greater than air, and thus appears nearer to the lens than it really is. The virtual distance S, is all that matters and this is readily derived from the known focal lengthf of the lens and the distance S, from lens to film : 1

1

1

s,+s,=j

(2)

TABLE II. W E I G H T LOSSES I N BOULES DUE TO EVAPORATION

Volume loss/hr.-unit surface evaporating area, ml./hr. sq. cm. Heat loss from boule (cal./sec. ) Volume loss/hr., yo A T , C. Calculateda shroud thickness, p

Benzene

Carbon nTetraAcetone Heptane chloride

0 0516

0,058

0.095

0,0776

0.0208

0.0274

3.12

3.61

0.65

0.48

0.0260 0.0262 5.55 4.87 0 34 0.55

449.2

208.6

204.6

147.0

aration, d, of the filament images on the film was 0.25 mm., so that shroud thickness, uncorrected for the angle of reflection, was d = 0.25 mm./5.3 = 0.047 mm. The ratio of actual thickness to photographed thickness e is 0 = d / ( 2 tan B cos A )

where A is the angle of incidence and B the angle of refraction at the first surface as pictured in Figure 56. In the present case the refractive index was n = 1.35, the angle of incidence an estimated 33' 30', and the angle of refraction 48' 15', which leads to a ratio

With a camera extension of 63 cm. and focal length 10 cm., the virtual distance from boule to lens was 119 cm. and the image magnification 5.3. The measured sep-

e

- =

d

I 2(1.11)(0.834)

Figure 5a. Arrangement of camera I and immersed light source for

I

determining shroud thickness

I

U

I Figure 5b. Optical path diagram for dettrmining shroud thickness

Figure 5c. Double image of filament VOL. 5 9

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OCTOBER 1 9 6 7

23

from which e = d/1.85 and the real thickness of the shroud becomes 0.047 mm./1.85 = 0.025 mm. (25 p) in the region photographed. Vapor Pnrrun and Boule Size

In the absence of indication that atmospheric pressure is most conducive to boule flotation, it is desirable to test other pressures-conveniently below atmospheric. As pressure decreases, the specific volume of vapor released beneath the boule increases and the support pressure Ap, corresponding to a given degree of superheat A T . or of energy input, cal. s ~ . - ~ will , be less. The larger vapor volume should entail higher vapor velocities, faster involution of the boule, and possibly earlier and more violent vibration, all tending to diminish the size and l i e of the boule; this has proved to be the case. It remains to be seen whether pressures above atmospheric will encourage much larger boules.

Figure 6. Apparatus for making boules under reduced prassura

The apparatus for growing boules under varying presdura is shown in Figure 6 which follows the design of the third figure in reference 73,with substitution of a watercooled condenser for the air-cooled one and attachment of a gas ballast bottle, and vacuum pump and gage. The 1-liter boule flask (operating contents 0.5 liter) and heater rest in a large glass Dewar vessel. The upper portion of the flask and attached purge boiler are thermally insulated with a transparent plastic cover. The boule flask heater with variable transformer and wattmeter controls the degree of superheat, while the rate of supply of distillate passing the drop counter is determined by the adiustable heater under the purge flask. The true boiling point is measured by a thermometer terminating in the drop counter; and the degree of superheat is recorded by the differential thermocouples which bridge the distillate applicator and support liquid. The same thermocouples serve to short-circuit the boule with the 24

INDUSTRIAL AND ENGINEERING C H E M I S T R Y

support. Because the readings were made on a shortterm basis with much repetition, and upon organic liquids, the need for better shielding of the apparatus was not encountered. Many liquids were examined superficially but f o u v water, 2-propanol, n-heptane, and carbon tetrachloridewere well studied. Because size is critically dependent on energy input, each maximum size was determined over a range of inputs and thus superheats, beginning with the largest A T , were chosen. A drop of distillate growing quietly on the liquid at the end of the applicator would begin to shake, and when it became a little larger it would appear to burst by merging with the support liquid. As the energy input and superheat A T , were decreased, the onset of vibration was delayed until at a critical AT., vibration did not occur and the largest boules were obtained. With still less heat input, drops would refuse to float, or if they did, gave smaller boules. Each ballast pressure thus produced a family of boules with a greatest size occurring at a particular energy input. When the families were plotted either against input or the resulting degree of superheat, the maximum boule sizes were staggered both as to height and position on the input and superheat axes. Water. The size of the water boules ranged from a fraction of a milliliter at low pressures to about 17 ml. at 700 mm. Hg, but at whatever the equilibrium vapor pressure, the excess pressure Ap, represented by the superheat A T , was the same for maximum size, namely 7.5 to 9.5 nun. Hg, corresponding to a 10- to 13-cm. depth of water. When, therefore, boule size is plotted against head-of-water support pressure, the maxima fall in line as shown in Figure 7. Indication of a greatest boule sue was encountered at 700 nun. Hg, b.p. 98" C.; we hope this maximum will not be confirmed because we hope to grow large boules at higher pressures. Boule sue drops sharply at half an atmosphere. Organic liquids. Because the apparatus, primarily designed for water, would not accommodate the largest organic boules, measurements have so far been restricted to below 450 mm. Hg working pressure where the boules are smaller. It is intended to continue measurement to 2 atm. in new apparatus, at which time fuller working details can be given. Trends, however, are already discernible; the common solvents examined give larger boules than water at any pressure, n-heptane, Figure 8, yielding 1.2-ml. boules at 100 mm. at which pressure water will give only occasionally a 0.5-ml. boule. Again, the coincidence for water of boule maxima at a single support pressure Ap8 is not repeated for the organic liquids. The ascending limbs of the carbon tetrachloride curves (Figure 9) fall nearly on the same sloping straight line. If instead of pressure Ap*, the volume of vapor generated beneath the boule is chosen for abscissa as in Figure 10, it is the descending limbs that coincide on a line of reverse slope. That this relationship is fortuitous is suggested by the curves for 2-propanol (Figure 11) which are positioned over a wide range of vapor support pressures. Curves have also been plotted for maximum boule size us. AT,, and for AE, the energy or wattage

I4rAp HUD OF UPUID, cm

Figure 9. Relation between bode siu and available infano1 pressure of suppmt lip'd-carbon tetrachloride Wmking prcssurcs, m. Hg: ( 7 ) 745; ( 2 ) 209; (3)3a?; ( 4 ) 406

Ap IYllllwIL *MIIIIWIS WD OF W A I E t cm

Figure 7. Relation between maximum boule siu and mailable intnml pressure of support lip'd-water

2

4

6

8 IO 12 M A I N E YAM1 ME

14

16

18

Figure 70. Boule siu as function of vapor rate and wo!king pmssure for carbon tclrachloride Based on the s m i

doto a Figure 9

Figure 8. Relation between maximum boule siu and m ' l a b l c interm1 prc"ac of support Iip'd-/wptam Working pruwtr, o m . Hg: ( 7 ) 703; ( 2 ) 777; ( 3 ) 254; ( 4 ) 324; ( 5 )432

Ap HUD OF LIPUIO, m

Figure 71. R h t i o n between boule size and available inrSnrnl pressure of support liqui&2-fiopanol Waking prasnaar, nm. Hg: ( 7 ) 88; ( 2 ) 725; (3) 755;

(4)

786; ( 5 ) 566; ( 6 ) 420 V O L 59

NO. 1 0 O C T O B E R 1 9 6 7

25

,

0

1

Figure 12.

(a) Ap, and (b)

IM

AT. a i m m ‘ m bode si= arfuncfionr of 0Par.lingpreswe

supplied to the boule flask, but are omitted as uninformative. Instead, Ap. and AT,at the maximum boule sizes for two liquids, n-heptane and carbon tetrachloride, are plotted in Figure 12, (1 and b, with operating pressure an abscissas. Any boule, whether the largest possible size under the conditions, or smaller if left unfed in the boule maker, will eventuallyappear to burst and merge with the support liquid, an unpredictable event. The disappearance

26

0

Ofa(llW6 !MSSUIf, mm II(

OPEU~N ntruur, M II(

INDUSTRIAL AND ENGINEERING CHEMISTRY

of the separating skins is complete in about 0.01 sec. (5). There is clear distinction between the spontaneous demise of a boule that is not being fed and one that is being grown until its weight cannot be supported. If boules do not grow to size under optimum conditions for largest size, it is perhaps because of spontaneous “pricking.” Boule growth not interrupted by pricking will be limited by the vapor pressure, Ap,, available in the support liquid, lessened by a coefficient of evaporation and

further lessened by the pressure drop at the cooler surface of the boule where condensation takes place with a different effective coefficient; both causes of demise are examined in another section. Quantitating the Boule

Difficulties in securing stable water boules with repeatable properties have been traced during two years’ observation to external conditions and to surface impurities. So sensitive is the boule to surface structure that the water boule system may today be one of the most sensitive tests for surface anomalies. The external conditions for growing large stable boules of distilled water were listed earlier. I t was evident that a reliable boule maker should be insulated thermally, electrically, and possibly magnetically in a suitable cabinet. We were also advised that radiation from outside the laboratory, perhaps extraterrestrial ( 6 ) ,might be pricking the boules. (See also correspondence on page 87 of this issue.) A further call then was for a monitoring system to record continuously the birth, life, and demise of boules under varying experimental conditions. Apparatus partially meeting these requirements is shown in Figure 13. The borosilicate glass boule maker is mounted in a plywood box 70 X 64 X 45 cm., the inside lined with glass wool batting and stout aluminum foil, The front door with Plexiglas window is hinged and there is a side window for a telescope. The foil sheets are electrically grounded and connected with wire screen which may be used to shield the windows. Temperature near the boule flask is maintained constant to * l o C. by a thermostat and 500-watt space heater thermally shielded from the boule flask. The boule maker, following the previous pattern, comprises a half-liter flask attached at midpoint to a steam generator of the same size, each warmed by out-of-contact convective heaters, of 60-watt and 250-watt maximum capacity, the former for the boule flask controlled by a wattmeter and variable transformer. This system is less than ideal because it carries the house current inside the box. In sensitive cases the heaters are energized by direct current. Each flask has a flexible siphon (the plastic tube is outside the hot box) for emptying and filling, which also allows the steam generator (the purge flask) to be fed from an inverted reservoir to make up for small longterm losses from the condenser. Various attachments to the boule flask, including the slanting (to reject air) condenser, project through the roof of the box and are thermally lagged where necessary. T o record the behavior and life of the boules, a small telescope and photocell are trained on the underside of the liquid surface to receive light reflected from a miniature lamp. When a drop of liquid condensate disturbs the surface, light reflected to the photocell is interrupted and the pen of the attached potentiometer recorder moves on the chart. Sharp transverse lines away from the longitudinal traces record a burst, and a return line the establishment of a new boule. A series of jagged peaks record a succession of drops that do not float. Representative boule histories are shown in Figures 14 and 15,

where the chart travels one large division in 15 min. Row a of Figure 14 shows the chaotic situation prevailing before use of sufficient shielding. Row b shows a condition sometimes attending the production of the largest sizes where each stable boule is preceded by a succession of merging drops. Rows c, d, and e show the certainty of formation that is restored by higher energy input at the expense of size and longevity. Figure 15 shows rhythmic fluctuations in size sometimes encountered but as yet unexplained. The angle and focus of the telescope-whether it concentrates on the first drop that floats or embraces the largest boule-influences the shape of the pen trace, as does the brightness of the lamp. For a fixed setting of the telescope the lamp can be energized to give a simple yes-no trace or record growth and swings by distortions of the vertical portion of the trace. This will account in part for the lack of uniformity in the pen traces throughout the paper. All, however, have one quality in common, a horizontal peak marks a burst and restart. Numerical Evaluation of Pen Traces

The pen traces can carry a useful visual message, as in Figures 14 and 15; other trace records require analysis to extract information, which can comprise (a) largest size encountered, (b) most probable size, (c) size frequency distribution, (d) number of false starts-Le., number of merges before sustained flotation, and

Figure 74. Pen traces from monitored boule maker

VOL. 5 9

NO. 1 0 O C T O B E R 1 9 6 7

27

(e) average size, or inversely (f) the number of starts in a selected period, including large and small boules and individual drops. Since the very large boules are often interspersed by 2 to 20 false starts, accounting method (d) used indiscriminately can be misleading. Surprisingly, (d) evaluation is a sensitive detector for agents affecting boule behavior and has been used in part for Figures 36 to 39. Purity of Source Water

There are constant reminders that the water in the boule flask is less than pure with respect to the phenomena examined. Three sources of impurity are recognized : from distillation by re-solution of volatile artifacts and solid carryover; from the apparatus by solutes derived from borosilicate glass, glass joints, electrodes, and plastic tubing ; and from spontaneous decomposition, if any, of the solute at boiling point induced by radiation, or of high energy particles, and unknown agents. The source water was city supply, Lake Ontario, average 170 p p.m. total dissolved solids plus iron from 70-year-old plumbing. The water was distilled in a radiant-heated glass still (10) with partial steam discard to remove some volatile artifacts; the distillate was stored in a closed 5-gallon bottle until siphoned into the feed flask of Figure 13. The boule flask was filled by redistillation from the purge flask, which provided steam and minute traces of carryover from nucleate boiling. Thereafter the working fluid was continuously redistilled, supplying distillate with a volume equal to the contents of the flask every 2.4 hours. The period of half dilution, disregarding the shortening effect of the boules, would approximate (In 2) X 2.4 = 1.66 hours. The concentration of nonvolatile solutes present at any instant should fall to one-half that level 1.66 hours later and to about 2.3 X 10-5 that level in 24 hours The concentration next day would be negligible. After some months a purge flask was found to be ringed with silica deposits while the boule flask had been leached of readily soluble material. The purge flask was thus drained and refilled from time to time. With respect, then, to distillation the working fluid was extremely pure. The glass joint connecting the condenser was sealed on with the inside portion facing upward, and by keeping the box 10' C. below boiling point there was always a distillate seal in the joint to exclude air. This distillate, if it leaked at all, did so out of the apparatus. Equivalent precautionary measures were taken at the junction with the boule applicator rod, the thermocouple exits, and the ball valve rods. The only procedure which remained questionable was the use of polyethylene or gum rubber tubing between the service funnels and the boule maker. However, the areas exposed to the incoming water for 1 cin. between the glass tubes were outside the hot box, and a natural thermocline pre28

INDUSTRIAL AND ENGINEERING C H E M I S T R Y

Figure 15. Periodicit? of boule sizes at steady superheat

vented the tube water from mixing with the flask water. Xfter fabrication the boule maker was washed and flamed to annealing temperature, and following assembly was completely filled with 1% NaOH for 16 hours, drajned, and again washed. I t was then charged at operating level with dilute ammonia and run for 24 hours, lagging the condenser so that steam and ammonia escaped. I t was then drained, steamed with pure water, and allowed to fill by distillation from the purge flask. Large boules hvere secured at once at the appropriate superheat and it is important to note that the boules declined to a smaller but steady maximum as further self-purification of the apparatus continued during the next week or two. Jt'ithin the means employed, the apparatus and operatiiig water were extremely pure. H o w nearly complete purity for the purpose was reached is problematical; what follows is an evaluation. The Stabilized Water Boule

The boules now behaved differently from the way described in our preceding publications (73, 14) There was

less pulsation, and the boule took up a position directly below the applicator without requiring an immersed stabilizer and grew to a volume of 15 to 30 ml. Now, the major difference between the double flask arrangement of Figure 13 and the previousdouble flask constructions is that the surface overAow OCCU’PS at the steam entrance through a tube, 2 cm. in diameter; the steam is not brought in through a separate tube, as formerly. It seemed that the large, relatively flat weir over which the support water escaped could provide a low resistance path for the liquid surface to push contaminants awayif indeed contaminants do accumulate on “clean” multiply distilled water in c l o d glasa systems. In early attempts to operate the 3-ring apparatus, described below, with water, Mady noted that drops from the applicator would not float at superheats low enough to grow large boules unless the surface was overflowing. When the effect was checked further, the rate of overflow was diminished to less than one drop in 5 to 10 min. The essential point was to keep the weir wet during this period. If the casing temperature were raised to dry the weir, flotation ceased. It was as though a “boulephobic” (paradoxically, hydrophilic) film was present which, under compression, exerted a veto which was lifted if the film could be pushed away. The concept of a compressed or tensioned film was now tested in another automatic boule maker. The usual double flask arrangement waa constructed but with t h e e alternate exits for liquid, p c d o d at points equilateraUy distant, each at the half-full level. A photograph of the apparatus i s shown in Figure 16. (Figure 17 show the modified apparatue, d e d b e d below.) Exit A was the broad-weir, steam entrance above described. Exit B was a slot which would permit surface water to escape by a narrow up-and-over path, the rise being caused by capillary action. Exit C was below the surface, a secondary overflow weir being provided outside the flask. This apparatus was mounted in the hot box with puppet strings attached to the three water exits, for manipuIation above the box. If the assumptions were c o m t , boule formation should suffer when overflow wan from underneath the surface, but be improved if overflow was through the narrow gate, and reach full dimensions when exit was over the broad weir. For objective appraisal, the photoelectric monitor was installed. Comparative readings are shown in Figure 18 where we note immediate boule interruption on changing from over- to underflowing even though t h i s occurs only through a limited range of superheat. Equally noteworthy is the re-establishmedt of flotation on changing back to overflowing. The anticipated difference between a wide and a narrow overflow path waa not always observed. T o test whether the supposed film was related to the area or volume of liquid on which the boule was to be

I Figure 17. Mod$ed fmn of aun-undnpowpask

VOL 5 9

NO. 1 0 O C T O B E R 1 9 6 7

29

floated, boules were grown in successively smaller flasks, each, however, attached to a full-size purge flask. The photograph in Figure 19 shows boule growth in the smallest working flask (100 cc.) ; the smaller surface area permitted boules to grow larger, until they displaced almost entirely the original contents of the flask. One reason to anticipate larger size boules is that the energy of circulation of the support liquid is conserved in the smaller flask; see later section on maximum boule size. The Resistant Film; Native or Foreign?

A currently debated question (8, 9 ) is whether a polar liquid of high surface tension can develop a surface so oriented that planar polymerization can alter both chemical aiid physical properties of the surface, for instance, modifying the rate of mass transport (16) across the vapor-liquid interface. A dependent question is whether solvent molecules amenable to orientation at the surface can serve as preferential host molecules for certain impurities that might be present or are being slowly, generally accidentally, admitted to the system. These questions would seem to be open to examination by the over-underflow technique. The apparatus was modified (as shown in Figure 17) by the addition of two ball valves to obviate swinging the flask. Wires attached to the balls were brought out of the apparatus through glass capillary tubes which become sealed by long slugs of condensate. Raising one ball and lowering the other switched from underflowing to overflowing without altering the distance of the liquid surface to the drop applicator.

Figure 18. Pen records of over-underjow changes

Underflowing and Superheat

T o plan tests in this area it is desirable to adopt some elementary theory of boule flotation. The surface of a liquid that is undergoing mass and heat transfer is not a fixed entity but is a mosaic of hotter aiid cooler areas ( 2 ) with upwellings under the former and downstreamers (21) under the latter. Extraneous material whether adsorbed or floating fortuitously is driven from the hotter upwellings toward the cooler, less emissive patches, where it may be discharged into the vapor or may be piled up in ridges on the liquid surface, from which it may be carried back into the liquid by the downstreams; indeed both may occur. If a drop presented for flotation lands on an emissive element of surface it should float, while on an encumbered area it might merge. However, as the drop approaches very near the supporting surface, the escape path of the vapor is narrowed and the surface layer tends to be blown away laterally and to exert pressure on the surrounding area. If the available support pressure A), is high, as at high superheat, there will be enough support above the least 30

INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY

Figure 79. Largest boule in smallest flask

emissive areas to ensure flotation which will be prevented neither by underflowing nor by mild contamination. In any event, once the drop has become supported, vapor-induced circulation maintains a clean surface beneath the boule and an even supply of heated liquid and the boule can now grow; the sequence is sketched in Figure 20. In borderline conditions of superheat and/or contamination a drop which can push away an unsuitable landing patch because the surrounding surface has an easy escape weir is likely to float, while another drop, facing conditions which differ only in that lateral escape is impeded, will not. Referring again to the question of surface orientation us. contamination, when the carefully cleaned boule maker-operated, for instance, with water and 30-watt energy input-has been left running for many hours or days without ingress of air or addition of test chemicals, the boules settle down to a rate of one each 5.5 min. (11 to 12 ml.) in place of the larger 7- to 8-min. boules encountered at the start. The smaller growth appean to be characteristic of the cleaner water surface. If change IS made to underflow, boule flotation is no longer at once interrupted as described earlier, but the succession and iize remain unaltered perhaps for half an hour. The iize of established boules increases but two or more drops may merge before a boule starts. Presently the number Jf merging drops increases until no boules will form at dl. On return to overflow, flotation is restored, often at the next drop, and the boules will be larger than the steady minimum size, for the next few hours. The pentrace sequence is shown in Figure 21 which may be compared with the traces in Figure 18. An ad hoc explanation, seemingly inescapable, is that the quiet surface of our continuously redistilled and overflowed water is, at the moment of switching to underflowing, almost free of surface contaminant. During the next hour and with the escape weir closed, the surface acquires contaminants that it cannot discharge and loses ability to float a boule. This single experiment weights the argument heavily toward the conclusion that the changes in support properties of the water surface are due to acquired impurities and not primarily to rearrangement of the surface water molecules. Another tentative conclusion is that to form large water boules a special type of contaminant is required. The surest indication of the presence of contaminant is the alternation of merging drops and boules; generally contaminants give smaller boules, occasionally larger. Discussion of the behavior pattern is continued after the next section. Vapor Exchange in Boule Sy&m

So far there has been no information concerning the relative rates of vapor emission from the free surface of

Figwe 20. Conditions lo support a sessile drop; left, acending middle, descdng shim; right, surface maintaincd clean by draft bcncath bade striae;

Figwe 21. Pen traces showing delayed onset of baric fornorion after rwitching to undarpow. Traces on left of each sfrip are boule sepenlcs; trams on ri& arc superhot fonpnorures VOL 59

NO. 1 0 O C T O B E R 1 9 6 7

31

F i p t 22a.

Thec-ring collecting yJtm

,

*=

N

i

I c

-

Figure 226. Sing18 frame from motion picture of 3-ring c o w above n-hcptana boule

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Figura 23. Latbfabricated p u t s for 34% apparatus 32

INDUSTRIAL AND ENGINEERING C H E M I S T R Y

I

the support liquid, the surface where it is covered by the boule-Le., the cradle-and from the top of the boule itself. A direct way to measure these quantities (to be altered slightly by the means proposed) involved isolating three areas of surface and collecting the vapor emitted from each. The areas are the top of the boule, the rim surrounding the boule from which vapor from the shroud escapes, and an area of the freely evaporating support liquid. Measurement can be accomplished by lowering three concentric tubes, each attached to a condenser, into the boule and surrounding liquid as suggested in Figures 22a and 226. The essential parts of what is conveniently called the “3-ring apparatus” are shown in Figure 23. The concentric tubes (on the left in photograph) are placed over the evaporating liquid A boule is grown under the central tube and, when sufficiently large, the liquid level is raised to attach the boule to the tube. The level is then raised further until the two surrounding tubes have met the support liquid, after which the stabilizer can be withdrawn from the top of the boule and the liquid level lowered until the tubes have been withdrawn as far as possible without breaking contact with the surface. The first 3-ring apparatus was made with a detachable head so that thermocouples could be put in place and other adjustments made. The detachable head was found satisfactory for many organic liquids, none of which presented serious difficulties. But closure could not be made sufficiently tight nor, without the use of gasket material, could it be made useful for water. Water boules simply could not be grown to sufficient size in the demountable apparatus. We therefore completed two constructions and give quantitative data for the organic liquids and one set of readings for the only water boule yet captured and held for sufficient time. To the upper part of the apparatus, which required lathe construction (from Blaessig Glass Specialties, Inc., Rochester, N. Y.), small water-cooled condensers were added, each having a liquid drop counter and means for returning the liquids to the system. A working drawing of the operating assembly is shown in Figure 24. Here, 1 is the b o u l e - d i n g vessel of 100-mm. diameter having a flanged opening at the top on which rests the cover, 2. Vapor is supplied by tube 3, which rises from a I-liter generator flask, 4, having a trapped overflow tube, 5. Excess vapor from the assembly is condensed and returned by pipe 6. The inner concentric tube ?, 48 mm. in diameter, will retain the anchored boule. The vapor from the boule is condensed in the slanting aerial condenser, 8, and condensate passes through the drop counter, 9, to a common header, 10. Surrounding 7 is the annular tube, 11, which collects vapor from the rim of the boule and discharges condensate from the watercooled condenser, 12, with its dependent drop counter, also into manifold 10. Finally, concentric tube 13 collects vapor from the support liquid by means of condenser 14. At the bottom of vessel 1 is a flexible tube coming to a leveling bulb, 15, of diameter similar to vessel 1. This bulb is suspended from a laboratory

~~

Figrrra 24. Demountable 3-ring aMmahrr fw organic liquids

clamp so that the height of the bul screw 16. It is necessary also to provide maintain the level in the working , place. This is accomplished thoughnhe wide side tube, 17, by means of a hinged overtlowj,member, 18, also controlled by screw clamp 19. &uid to form the bodes is distilled from flask 20 into4ondenser 21 whence it flows from the drop counter and calibrating bulb, 22, onto the applicator-stabilizer, 23. When the boule has grown to sufficient sue, the lamp Under flask 20 is deenergized. When the apparatus is rhnning undisturbed, the test filling is distilled into bt from flasks 4 and 20, causing the content in vessel lrjo rise. When the level of contents in this and fla&t5’are both equal to the overflow point in the regulaWr 18, liquid flows back into the neck, 3, of flask 4 and.,thus back into the system, eventually to flask 20. Except for trivial losses through the condensers, the status quo can be maintained indefinitely. To raise the level for mating the boule and support liquids onto the concentric tubes, the regulating ScTPIwB, 16 and 19, are twisted simultaneously. It is fd‘this reason that the diameter of the leveling fl&.&d the operating vessel should be the same so that equal number of screw turns serves for raising the Aask and the overflow. Heat energy is applied to the operating flask by an electric resistance winding, 24, which has the greatest concentration of coils down at the base, tapering off to nothing just below the overtlow pipe, 17. It is convenient to operate this between 0 and 60 watts input and ,

an

higher for warming up. The running input is ordinarily between 5 and 20 watts direct current to avoid accidental electrical ‘‘priakiig” of the boule. The appropriate parts of the apparatus are kept warm by creating a double-walled transparent plastic house, excluding the parts that need to be kept cool, as suggested by the dotted lines, 28. Cellulose acetate sheeting obtainable from art stores has withstood temperatures up to 100’ C. Temperature can be maintained sufficiently uniform by a blast of hot air from an electric blower projecting tangentially into the upper part of the casing. The standard blowers, as purchased, are altered so that the motor can be kept running at full speed while electrical input to the heater is varied. It is necessary only that the temperature of the upper parts of the apparatus, up to put not including the drop counters, should be rrtafi&hed 2*.or 50 C. above condensing temperature while the lower part falls naturally to below evaporating temperature. Mercury thermometers placed in the casing allow the operator to secure a suitable temperature distribution. One further precaution: to ensure good calorimetric conditions within the operating vessel, asbestos twine is tied around the body of 1 at points 25, and a tubular plastic cover, 26, is lapped over these. A final wrapping of glass wool batting, 27, completes the insulation and leaves enough of the upper part of vessel 1 visible to watch the growing boule. O n at least one of the drop counters-e.g., 22-we placed a small calibrated receiver with a magnetic bail plug. The ball can be moved into place and the number VOL 5 9

NO. 1 0 O C T O B E R 1 9 6 7

33

of drops counted to fill the calibrator, after which the ball is removed and the rate of boule growth is estimated by counting drops. Construction of %Ring Apparatus for Watw Boules

The failure to grow water boules with the previous apparatus suggested the following requirements for water service: The apparatus of all glass should be hermetically sealed, leveling arrangements should entail no exposure of liquid to the atmosphere, there should be optional means for in-cycle filtration of the contents, and the water collected by the condensers should be purged of dissolved air before returning to the main cycle. The ramifications of equipment imposed by these considerations are omitted here. It is sufficient to say they have been constructed, have provided the anticipated improvements, and have been detailed elsewhere (2.2). Graphs of %Ring Data

In contrast with the difficulties anticipated-and actually encountered with water boules-an isopropyl alcohol boule was anchored, and the two outer rings were submerged without bursting the boule, within a few hours of putting the apparatus into commission. The second meniscus afforded by the intermediate concentric tube just outside the rim further stabilized the boule which would hang, unmoving in a pure catenary for as long as needed, often for some hours, while the wattage input to the support liquid could be varied in leisurely steps over the working range. It was now unnecessary to time the drops from the three condensers simultaneously. They could be counted serially after each change of input and the readings adopted when each had reached steady repeat values. A rough estimate of isopropyl alcohol emission was thus made during the first afternoon and plotted to furnish Figure 25. The abscissas here are energy input, and the ordinates are the relative masses of distillate per unit areas of the boule top, the surface of the cradle in which the boule rested, and the surface of the support liquid. Now the vapor collected by the middle annulus has come partly from the cradle but partly also from those portions of the support and boule that are exposed between the rim and the two concentric tubes. The assumption, only approximate, is made that these two annular areas are giving off vapor at the same unit rates as the main areas of support and boule top, respectively. Since both these rates are measured, they can be subtracted pro rata from the total yield from the rim collection to furnish the emission from the cradle. The findings dispose at once of the earlier assumption (73,page 29) that there is abnormally rapid emission of vapor from the boule top; the reverse is true-the top of the boule exhibits symptoms akin to torpidity (7.2). Measurements were now carefully made of the six liquids that had been used in the boule-shape experiments. Among these, the repeat experiment with 2-propanol agreed with the first run in kind but not in degree, Figure 26. Also, acetone in these earlier experiments registered zero 34

INDUSTRIAL AND ENGINEERING CHEMISTRY

.,

I

I”

wrm irrur Figure 25. “First a f t m o n ” emission chart for 2-propad

41

1 1.42 1

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0.71:

0.35:

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14

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16

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Figure 26. Repeat of emission CUNCS for 2-propanol oftm oddirion of water-cooled codemms

b

emission from the boule. Both anomalies were traced t&&j&.&ia por from the air-cooled condensers first employed, which were now replaced by the water-cooled condensers described above. With substantially complete condensation higher readings were obtained, particularly with the slower evaporating surfaces. The three liquids chosen for intensive study were 2-propanol, carbon tetrachloride, and benzene. Benzene was selected because of its molecular symmetry, low dipole moment, and low specific gravity. I t is also one of the few liquids reported ( I ) in a form&% experimental era to have an evaporation coefficientneai unity (3). In the 3-ring device, benzene exhibited high ' emission from the cradle (Figure 27) in spite ofthe v a w t having to escape against the overburden of the boule. . Under the experimental conditions, zero heat input ,). occurs with a meter reading of 2 watts, and th cradle emission increases linearly with each additidn watt. Relative emission of the support surface is less &ddoes not increase linearly with input at higher val~ar;.? . boule emission is small throughout the range of energy inputs. Carbon tetrachloride resembles benzene in the . . molecular characteristics cited but has twice the specific gravity. Emission from the support surface has now overtaken the cradle, presumably because of the later's heavier overburden; the boule, lagging far behind in emission, actually evaporates less from the top surface as energy input increases (Figure 28). 2-Propinol typifies a simple polar compound with oat hydroel group, low specific gravity, and higher viscosity. Except that apparent zero input was 10 warn-. because the jacket temperature was a little too low, t k ' , . three curves are hardly distinguishable from those of the very dissimilar liquid, carbon tetrachloride (see graphs : in Figure 26). The findings for the three liquids are at first sight paradoxical. The largest quantity of vapor per uait area comes from the top of the support liquid but is nearly equaled by vapor from the cradle generasd ' . against a hydraulic head and thus with lesser Ap. T h k cradle thus exhibits the largest evaporation coeflicient Emission from both surfaces increases regularly withitr creased heat input. However, the boule which floats increasingly heated vapor becomes cooler, approachiq ,. ever more nearly the equilibrium boiling point, a i d . , emits less and less vapor from its top surface. To rationalize the paradox, it is necessary to construct a 1 plausible mechanism for the system. It is regrettable . that operational difficulties have hitherto discouraged temperature measurements so that evaporation c-; : cients, category E, have not been calculated for selected areas; nevertheless, .even without AT d i n & much can be inferred from Figures 26-28. '

3.04 u d l TIRICHLORIDE

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Figurs 28. Caban t4trachim'de emission arms in &"ng

Mechanics of Floating Boule

apporahls

It has been conjectured and amply confirmed by rl injection and motion pictures that a doughnut vortex occupies most of the upper half of the boule. i : tation causes a downward spiral in the cente- .. :ii. .-eVOL 5 9

NO.

10 OCTOBI'>

: c " . j

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.

.

tutns fthe boule' ind' &am 'outward to reioin the surface. The d e d u d vapor transactions bet&cradle a n 6 boule &e pictured in Figure 29. The'M haltsupposes a CI%& sectional contour of the 'shroud with. the thickness exaggerated. I n the right half of the figure the contouis &e rimoOthcd but the thicknesses are greatly exaggerated to accommodate probable transfer vectors, Primary vapor necessarily originates from the cradle wall. .Denshies of (net) emission aie suggested by the varyiag. crowding of the row of shbtt, center-facing a&oys, while (net) cond&ation.'on the boule is shown by the inner arrows. T h e excess of uncondensed vapor, depicted by long arrows, will stream outward and. up ward through the shroud, gathering volume and velocity on the way and driving along the .liquid surfaces of the cradle and boule. The cradle wall will grow progressively colder through loss of latent hait, and the, boule will will become hotter by gain of latent heat. In the drawing, the. hotter portions of liquid are shown unif d y shaded, regardless of a c t i d temperature, while the cooler portions are lefi white. The convention can-, not. be interpreted skctly because the shaded lining of the boule is cooler, at lea& ii the lower regions, than the white liming of the cradle. The thicknesses of the two liniings suggest the temperature differences and depths of gradient penetration. Evolution of vapor from the cradle is probably greatest at the base where the heat barrier (white) is thinnestthh, in'spite of the overburden of the boule. The excess press& in the shroud will be equal to the depth of immersion'at any point, modified slightly by the outwad streiming of the vapor. At a ceitain distance from the bottom center, the vapor pressure of the heated boule surface will necessarily begin to exceed the diminishing pressure in the shroud, say at the annul& region or isobaric ri,ng indicated by' lines a, a', above which the boule will contribute vapor to the shroud and the temperature of the streaming layer will drop. M&t of the vapor contributed by the cradle has been returned to the shroud just bdore issuance from the rim, and it is near here that the vblume'and streaming velocity are greatest and the pressure is least. I t is here, tod, that vibration is observed to be induced. It'often looks as if there' is a bulge in the cradle in the vibrating region, as suggested on the left-hand side of Fig& 29. Reedgenerated vibrations will be in opposition on the boule and cradle surfaces. Pmibly where the channel is temporarily most n&w and the vapor velocity,is high&, the lateral pressure is less than in the vapor above the boule, inducting evapdration from boule and'cradle to create' a surface temperature below the equilibrium boiling point of the system.

-

,

Maximum b u l ) Sir.: Supwrtins the b u l i

Forces and Mbchanlrmr

A boule cannot float on a vapor layer unless the exceaP vapor pressure Ap,, expressed as hydraulic head, is greater than the boule depth. For water, by experiment, the 36

INDUSTRIAL AND ENGINEERING C H E M I S T R Y

minimum potent&lly.available Ap, required three to four times the hydraulic head. The primpy limitation to size is the rev&= vapor pre&ure exerted by the boule surface at any point, that is to say, Ap,,ofthe boule surface co&ponding to AT,,must always exceed the hydraulic head at any given point. Now ATn, and thus Apn, depends largely on the evaporation and condensation coefficients, c a t e o i y I? and (77) obtaining at the two surfaces, the &dents, in turn,depending on the rates of brnover or replacement,: n ti& per second, of the thermally insulating boundary layers..,. This rate is determined near the base of the boule by the outward sweep.of u n b n d e n d vapor and near the top by the accumulated vapors and tho& &evapotated.fiom the,boule above the isobdtic region. The rate'of compdsite streaniing-of vapoa is finally determined by the degree'of superheat. The upward q h n i c a l streaming enof the cradle. wall is dissipated by the large bulk of..support,liquid. and is o p p d by the heated liquid's rising up the walls of the v&el and flowkg inward toward the boule. The conten% of the boule sui& no extraneous damping so that inelution and n increase cumulatively in propoytion to superheat: The condensation coefficient of the boule &on outdistanc& the exaporation .cc.?licient E (the coefficients corresponding Wiih the b u k temperatures of the two liqui4) of the support. The boule absorbs a greater porportion of the @esented vapor; and the press w e ~ b e t w e athe two surfaces falls below'the hydraulic head-at the b,& of the boule.. Merging follows. next. region where merging can 'be kpected is near the rim. The ascending walls ,ofboth cradle and boule have become cooled to a point where they have ceased to bk emissive and cannot repel one another. Although there is little hydraulic head to bring them together, the Bernoulli effect of the rapidly issuing vapor causes constriction and the vibration that is observed. Once contact has been made between the smallest element of o p p i n g waves, the boule is doomed.

c

me

Figure 30. Instantaneous photograph of a 2-propanol boule '/loath sec. after an electrically triggered burst

Figure 31. Photograph of a 2-propanol boule as in Figure 30, but later

Photographic Confirmation

While the boule can be pricked anywhere by mechanical means, merging induced by electric charge or allowed to occur spontaneously has, so far, originated only near the base or at the rim, as shown in the photographs of Figures 30 to 33. The various techniques which employed flat or Schlieren lighting, stroboscope, and drum camera have been described elsewhere ( 5 ) . Figure 30 shows a 2-propanol boule electrically burst and photographed in flat lighting. Merging has started at the right rim, apparently near the electrode which however is actually at the far side of the flask. Walls of the boule and shroud have joined and are crumpling back, preceded by surface waves. In Figure 31 the last remnants of wall are seen, enclosing what remains of the vapor contained in the shroud. Motion pictures show that these remnants rapidly become spherical bubbles which either vanish or rise to the surface according to size, ascending at 50 to 70 cm./sec The elapsed time of the burst of a 2-propanol boule of 4-cm. diameter is 0.01 to 0.1 sec. A similar boule bursting spontaneously is shown in the Schlieren photograph of Figure 32 where merging is seen to have spread simultaneously from points at both rim and base. (The camera was triggered by electrical changes in boule during burst.) The pseudomorph of what was once a boule is shown 0.02 sec. after burst in Figure 33. The sharply defined contents are less than 1' C. cooler than the surrounding liquid.

Figure 32. Spontaneous burst starting at base of 2-propanol boule

Environmental and Chemical Factors Influencing Boule Flotation and Size

Physical factors. Devices which decrease the rate of involution of the boule decrease the condensation coefficient, raise support pressure, and increase the maximum boule size. Arrangements which increase the velocity and turnover Ofthe wall to bring new, hot support liquid into the neighborhood of the boule also

Figure 33. .As in Figure 32, but later. cooler boule contents remains VOL. 5 9

NO. 1 0

Only an isomorph of the

OCTOBER

1967

37

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0.150 c

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1 0 2 0 3 0 4 0 5 0 6 0 7 0 mL h 101 iimv ou SEAMAI lWc

Figwe 35. As in Fe’i

favor increased sue. Surprisimgly, altering the position of the heater to provide a central upwelling of superheated liquid (water) to aid, instead of oppose, cradle flow was found to result in a less stable and thus smaller maximum boules; the reasons are as yet unexplained. Increasing the size and effectiveness of the stabilizer increases boule size. By lowering a bundle of wire gauze that almost filled the boule, thus destroying the upper doughnut vortex, water boules were grown large enough to anchor to the center tube in the 3-ring apparatus. If the stabilizer was withdrawn before both boule and rim were anchored, the boule generally burst. The simplest mechanical device so far encountered for increasing boule size is to generate the boule in a flask just a little larger than the largest anticipated boule diameter, as in Figure 19, SO that.the velocity ofcirculation of the cradle may be conserved and thus amplified. Physicochemical Factors

A broad survey of chemical additions that affect water boule flotation is beyond our present scope. Instead, we select three examples as pointers: (1) foreign gases, which interfere with boule formation; (2) an amphophyllic contaminant typified by octylamine; and (3) an effect accidentally encountered, produced by small concentrations of gelatin. Foreign gas. Gases which have been injected into the steam above water boules include air, Na H,,0%and CO,. Variations of boule behavior with chemistry, 38

INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY

34, 50 waUs ked input

concentration, and superheat represent a vast area for data collection, again beyond our present scope. Exploratory additions were made of cylinder hydrogen at 30 watts superheat input (Figure 34) and 50 watts input (Figure 35) and with cylinder oxygen at 20 watts input (Figure 36). The pen traces resulting from adding air from the laboratory supply lines were intermediate with those of Figures 34 and 36 but were not reproducible. A reference plot of boule characteristics us. energy input is furnished in Figure 37. The gases were injected through a fine glass capillary, d i n g exit below the liquid surface in the purge flask. With distillate returning through the counter at 38 drops/min. weighing 0.07 gram/drop, the volume of the steam generated at normal pressure was 4.5 liters/min. The gas entered this volume of steam at the mume and received further mixing on the way to the boule flask. From there the mixture passed to the reflux condenser which rejected most of the permanent gas from the upper end. The condensate was water at the boiling point saturated with gas at the partial pressure supplied. After a sufficient interval (more than 12 hours), the boule flask would contain this saturated mlution. Any change of gas rate would cause an immediate change in steam composition and in the gas content of the water forming the boules, followed by a gradual change in the quality of the support water, specifically to one half saturated in about 105 min., three quarters saturated in 210 min., and so on.

0.10

0.05

,

IO

20

30

40

Figure 36. Effect on boules of oxygen added to the f m a t i u e W watts superheat input

SLCMI;

Dircusrion

At energy inputs conducive to large boules and with small concentrations.-e.g. 1 volume of gas per 1000 volumes of steam-02, N2,and HZincrease the number of single drops that merge, decrease the average size established boules will attain, and decrease the sues of the largest boules encountered. It is thus desirable to record the total number of starts per hour, the number of persisting boules-i.e., inverse of average boule size-and the greatest boule sue also expressed as the inverse or virtual number per hour. This has been done in Figures 34 to 37. At larger energy inputs, boule formation is d a tively insensitive to hydrogen additions of less than 5 parts per 1000 of steam. Hydrogen can be used as a carrier for admitting selected volatile contaminants. Discontinuing the addition of gas after some hours of treatment at a concentration of two to five volumes per 1000 volumes of steam, unusually large boules are noticed, separated from one another by two to three merging drops. The effect is pronounced with oxygen, as seen in Figure 36. Three possible explanations are: the permanent gas has blown a volatile contaminant out of the condenser; the gas remaining in the solution, -lo-' mole/liter during the first how after halting addition, hinders condensation on the boule and increases support pressure; or that in the case of oxygen, an artifact has been oxidized (78). Twenty-four hours later boule sue distribution returns to normal for the prevailing conditions and another gas test can be made.

Figure 37. Relationship between heat input and boule formalion with "pe" watn and s t e m

Aliphatic amine. I n exploratory tests small quantities of volatile organic acids and amines were added to the steam by saturating the hydrogen (2 volumes/ 1000 volumes of steam) fed to the boule maker. Acetic and propionic acids and monoethanolamine, severally, caused no notable changes in boule sequences but noctylamine, which exerts a vapor pressure of less than 2.5 mm. at room temperature and was thus added as 1 volume to about 3 X 106 volumes of steam, pmduced the cumulative effect shown in Figure 38. Trace a shows average of 9 boules per hour without hydrogen; b, the effect of adding hydrogen (the number of HI bubbles per minute is marked); c shows the reduction of hydrogen to 50 bubbles per min. and the addition of octylamine vapor. Trace d shows the cessation of bubble formation, c the discontinuation of hydrogen and octylamine and the beginning of restoration of boule formation. Trace f is the same as e, but at a time 24 hours later. Boules were quenched completely after 85 min. and were interrupted severely for days following cessation of the addition. The quantity transferred to the flask during the 85 min. was equal to or less than 11 mg. However, since the hydrogen leaving the condenser smelled strongly of octylamine only a fraction could have remained in the flask or collected at the liquid surface. The experiments show that more air leakage can be tolerated by the boule system than had been supposed and indicate that air does not leak harmfully into the VOL 5 9

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50

c,

H,

50

0 70

72 40

a

b

C

e

Figure 38. Pen traces showing effect of addition of hydrogen hydrogen plus octylamine

present boule makers. They further show that foreign gas and volatile contaminant in trace quantities can alter boule size and influence the receptivity of the support water for single drops. Gelatin. Gelatin additions were first made to the boule in the hope of raising viscosity, reducing turnover rate, and thus increasing the size of boule. Powdered Knox gelatin (70 mg.), in the form of melted 5% jelly, was thoroughly degassed by introduction through the condenser in the customary manner. The addition burst the boule and then quenched boule formation completely. Sucrose (100 mg.) added in a similar way, affected neither the boule receiving the sugar nor the support liquid in which it became diffused; boule sequence continued with frequency unchanged. In contrast, the gelatin, diluted to about 1:5000 by the 350 ml. contents of the boule flask, formed a solution which accepted every drop without flotation. The boule maker was continued in operation, and some hours later intermittent flotation was observed interspersed with occasional boules 1.4 to 1.6 times larger than before. This sequence persisted overnight, by which time the concentration of gelatin had fallen through exponential dilution to less than 1 p.p.m. The effect continued until the second morning when the gelatin in the working flask had been diluted to Iess than one part in a billion. Could the support-enhancing material be coming from the purge flask and thus be volatile? Changing the purge flask water twice in succession gave an affirmative answer; boule size and sequence pattern were returned to normal-Le., one boule every 5 to 7 min., by removal of the gelatin residues. 40

INDUSTRIAL A N D ENGINEERING CHEMISTRY

The sequence of events illustrated in Figure 39 can be explained by supposing that gelatin in the support liquid lowered surface turnover, reducing vapor emission beneath the applied drop which consequently would not float. After some hours of boiling, the gelatin was hydrolyzed, restoring surface emission and furnishing a volatile hydrolyzate to the shroud that hindered condensation onto the boule and thus permitted larger boules. In row a of Figure 39, boules have been forming regularly at 10 per hour. Gelatin solution (30 mg.) is added to form 0.01% solution in the support liquid, Boule formation is immediately stopped. After 4 hours small boules appear which later develop to large size with intervals of no boule formation. I n row b more gelatin is added, and the phenomena are seen again. I n row c, 13 hours after the first gelatin addition, purge water is changed and boule size returns to normal. Finally, gelatin is added to the purge flask only and within an hour boule size increases, suggesting that the interfering substance is a volatile hydrolysis product of gelatin. Repeat experiments, with as little as 10 mg. of gelatin, confirmed the sequence of events. Raising the heat input from 36.5 watts to 60 watts and thus increasing the degree of superheat, just after adding gelatin to the support liquid, restored drop flotation at the expense of maximum boule size, as shown in row a of Figure 39. With pure water in the boule maker and 100 mg. of gelatin added to the purge flask, enhancement of boule size was noted after 3 hours (upper row c, Figure 39). The effect was checked in many different ways, including addition of meat extract and gravy from a can of dog food. The main hydrolysis products of gelatin-lysine and alanine and the lesser moieties, phenylalanine and methionine, whether added directly to the support water or to the purge flask-were without effect then or later. Indole, less than 1 mg., added to the purge flask water, duplicated the performance of hydrolyzed gelatin. Here for the moment the matter will be left. The examples given-modification of boule sequence by inert gas and by nonvolatile and volatile solutesdemonstrate the effects of small quantities of impurities and permit completion of the preliminary picture of boule mechanics in water. Broken Boule Sequence

During its residence on a host liquid, a large boule has been sweeping upward any artifacts liberated in the shroud to join others floating on the surface and is likely to have formed a ring of molecular debris around the rim. LYhen .merging occurs, the boule pseudomorph does not spread out on the surface but is seen to fall through the support liquid at 1 to 3 cm./sec., visibly drawing surface water to the center. The centripetal flow will crowd the artifacts and carry many of them back into the liquid but leave a dense patch, just where the next drop will fall. This drop, robbed of vapor support, falls into the liquid faster than the burst boule sank and drags a core of contaminated surface water with it, at the same time pulling more contamination to the

Points of crucial interest are that contamination sufficient to modify boule behavior can be demonstrated as present on very carefully purified water; that the contamination is real and not a modification of the water substance. The contamination effect can be duplicated by foreign gas, for instance Hz and 0 2 , by volatile surfactants, and by substances such as gelatin hydrolyzate not ordinarily considered surfactants. Summary and Forecast

Figure 39. Pen traces showing injuence of gelatin addition on boule formation

center. After a succession of drops the surface is sufficiently cleaned to let the next drop float. Contaminants taken into solution or suspension are now available for reliberation at the cradle wall whence they can pass across the shroud and diminish the condensation coefficient of the boule and encourage large size; they are also liberated at the top surface of the support liquid. Some will find their way to the overflow weir or out of the condenser, but much of the material will collect in another reject ring around the rim and interrupt boule formation after the next burst; the familiar cycle of larger boules and false starts repeats until the contaminants have escaped. A test then for moderate (in the present context) contamination of the water surface is a succession of larger boules interrupted by a repeating sequence of individual drops, a “Morse Code” of -. . A test for the highest purity obtainable in the apparatus is to change to underflow and find that the sequence of “normal” sized boules persists. .-a

.-e

a-

Dimensional and heat transfer parameters have been outlined for the floating liquid boule. The floating behavior of the boule has been brought under control by standardizing apparatus and procedure. Quantitative pen traces of boule life and stability have been secured by a photoelectric monitoring technique. Boules floating on water monitored in this way reveal the presence of foreign material on the purest water that has so far been produced by continuous redistillation in the boule-making apparatus. A pictorial theory of boule flotation (and demise) has been sketched in relation to the surface properties of the supporting liquid and the manner of superheating. Future plans call for boule makers with extensive instrumentation, improved shielding, and inbuilt sources of purer water. Since any glass device will contribute contaminants from glass, the effects of these on the boule process can be discovered only by duplicating the apparatus in other materials. Many of the experiments that we have described need repeating in depth and other leads invite inquiry, particularly the possible influences of radiation from outside the laboratory. The relation, if any, between torpidity effects noted under high vacuum, and boule acceptancerejection phenomena on quiet liquid surfaces is open for exploration. The quantitative boule maker is still in an early stage of development but it is already in a position to attack the chemistry, as opposed to mechanics and physics, of water systems as hosts for the floating boule. REFERENCES Baranaev, M., J. Phys. (U.S.S.R.) 13,1635 (1939). Berg, J. C., Boudart, M., Acrivos, A., J.Fluid Mech. 24, 721 (1966). Boudart, M., Chcm. Eng. Sci. 17, 1 (1962). Bo s V. C., “Soap Bubbles and the Forces Which Mould Them,” Reissue, Douglkday, Garden City, N. Y.,1959. (5) Davidhazy, Andrew, Phot. Sci. Eng. 10, 160 (1966). (6) Fischer, W. H., private communication, 1966. (7) Fischer, W. H., Lodge, J. P., Jr., unpublished work. ( 8 ) Fletcher, N. H., Fletcher, N. Y., Phil. Mug. 7 (a), 255 (1962). (9) Frank, H. S “Summation of Forms of Water in Biologic Systems,” Ann. N . Y. Acad. Sci. 125,”730 (1965). (10) Hickman, K., Anal. Chem. 36, 1404 (1964). (11) Hickman, K., “Evaporation Coefficients of Liquids,” Proc. Is1 Intl. Symp. on Water Desalination, 1, 180-(1965). (12) Hickman, K., IND. ENC.CHEM.44,1832 (1752). (13) Hickman, K., Ibid., 5 6 , 18 (1964). (14) Hickman, K., Nature 201, 985 (1964). (15) Mahajan, L. D., Kolloid-2. 6 5 , 20 (1933). (16) Mortensen, E. M.,Eyring, H., J . Phys. Chem. 64,846 (1960). (17) Plateau, “Statique Expirimentale e t ThCoretique des Liquides Soumis aux Scules Forces Moliculaires,” Paris, 1873. (18) Powers, R. W., Electrochem. Tech. 2, 163 (1964). 18, 4 1 (1954). (19) Prokhorov, P. S., Bircussions Faraday SOC. Roy. SOC. (London) 28, 406 (1897). (20) Rayleigh, Lord, PYOC. (21) Spangenberg, W. R., Rowland, W. R., Phys. Fluids 4, 743 (1961). (22) U.S. Dept. Interior, Res. & Dev. Progr. Rept. 200, “The Mechanism of Boule Flotation on Water and Other Liquids,” September 1966.

(1) (2) (3) (4)

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xcept for the phenomenon of scale formation, the

E desalting of seawater by evaporation would be an easy way to solve many of the growing shortages of

Carbon dioxide is nature's way of controlling many chemical changes in the oceans; therefore, it may be the best

way to combat scale in future d

desalination plants

42

INDUSTRIAL A N D ENGINEERING CHEMISTRY

fresh water throughout the world. Even so, production is now about 100 million gal. a day, and by 1975 daily production may exceed 400 million gal. This amazing prospect for growth shows the incentive to develop the lowest cost processes for saline water conversion. By far the most important process is multistage flash, and by 1975 more than 90% of the world's production of fresh water from the sea may come from large multistage flash plants, similar to the one illustrated in Figure 1. For many years the submerged tube evaporator provided fresh water for merchant and naval ships. During World War 11, however, the character of marine warfare and the rigid requirements for evaporators on submarines and small naval vessels stimulated the development of more efficient and more compact machines. As a result, the vapor compression and the multieffect submerged-tube evaporators were developed and widely used. Yet, the most serious problem was the formation of scale which impaired the efficiency of the evaporator and reduced production. Concurrently with the development of better evaporators, chemical and mechanical methods for controlling scale were devised, but they were successful only within certain limits and the cost of these methods was of secondary importance. Prior to the 1950's the economics of water production did not seriously concern the engineer, except for the initial cost of the evaporator as a competitive piece of equipment. Not until large plants began to supply water for overseas domestic and industrial uses was it necessary to consider the total economics and design more efficient plants, which in turn required effective scale prevention methods. The conventional submerged tube and the newer vapor compression evaporators met the needs of shipboard uses, but because of serious scale problems, were not adaptable to large land-based installations. The flash evaporator being less susceptible to scaling was regarded as the better process, and an intensive program of research and development began. The major stimulus for this activity was the Office of Saline Water (OSW) established by Congress in 1952 to develop economical