1892
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
The obvious conclusion is that the distillation of large uiireactivrl molecules from solutions in the absence of residual gas cannot be expressed by known relations, though this can scarcely be accepted as proved until much further investigation has been made. It is with the same feeling of uncertainty t h a t we avoid cxpressing t h e equilibrium data in terms of activity coefficients. I:ntil an adequate technique has been cvolvcd for measuring the total pressure of mixed vapor a t each temperature, we prefer to post pone this part of the discussion.
Projective Distillation of NOP-EHS
In Ihe practical range the relative volatility of EHP-EIIH, 2.5 to 4.5, is too high for testing multistage fractionating at,ills. Consequently i t was thought desirable t o duplicate the measurements with a mixture providing much smaller values of relativc: volatility. Projective distillation for the system, NOP-EHS, was studied in the temperature range 100” t o 180’ C. for six liquid mixtures between 0 and 100%. The NOP was picpared by esterification and purified by four distillations under high vacuum, as described in the first,paper of this series. While absolute purity waa not and indeed never can be attaiiied, since a liquid is pure o d y until a device capable of fractionating it is developed ( 1 ), the NOP was considered sufficient,ly homogeneous for the purpose. It is possible t h a t the data would be altered slightly if a higher degree of purity were attained. Analysis of samples was accomplished by nioasuring the refractive index, and variation of refractive irides with composition n-as estahlifihed
Vol. 44, No, 8
using nine samples made up by weight. ‘L‘hcse data are listed in Table 1’. The falling-stream tensimeter was fitted with a discharge tube, 0.719 em. insidc diameter. and a single collectmg zone 7.34 em. high draining into a piprt with a volume 2.023 w, The distance between stream and condenser wa8 about 3.0 ern Numerical data for rate of distillation, composition of vapoi, and relative volatility are presented in Table VI, and from theqe data x-y curves have been plotted for loo”, 140”, and 180” C. in Figure 4. Unlike the system ERP-EHS, this pair of fluids exhibits a considerable change in the relative volatility at constant temperature as liquid composition is varied. At all temperatures in the range 100” to 180’ C., the relative volatility falls rapidly with increasing content, of NOP in the liquid, as shown in Figure 5. It is therefore not possible to assign for a n y temperature in this range a mean value of the relative volatility, independent of composition. For this reason, it is concluded t h a t the system is not suited to the testing of vacuum Btilfs. Literature Ciked (1) Hickniaii, K. C. D., Rea. Sci. Instruments, 22, 141 (1981). ( 2 ) Hickman, K. 0. D., and Trevoy, D. J., IND.ENG.CHIM.,44, 1882 (1952). (3) Ibid., p. 1903. (4)Perry, E. S,, and N’eber, M’. H., * J . -4m.CI~evu.Soc., 71, 3726 (1949). RECEIVED for reiiew September 13, 1951. ACCrnPPED May 2, 1952. Communication No. 1441 from the Kodak Research Labomtorim.
(Studies in High Vacuum Evaporation)
SURFACE BEHAVlO
K.
T STH.
C. D. HICKMAN
Eastman Kodak Co., Rochester 4, N. Y.
X T H E first two papere of this series we have examined the process of evaporation from clean new surfaces of certain liquids, using specialized means for so doing, and it is now oyr task to continue the study with “ordinaryJ’ surfaces, if a single word can describe the great variety of situations where liquids are ordinarily distilled. -4t the start it is apparent that our outlook is changing. JVe used t o be concerned with liquids as chemical entit,ies, such as H20,CHCI,, CIH90H,which presumably exposed surface mosaics of self-compositions, the chief requirements for continuous rapid distillation being t h a t thermal energy should diffuse t o the surface, and electrical forces should remain balanced. Kow, we find ourselves envisaging liquid conglomerates where the smallest trace of impurity can monopolize the surface and modify t,he rate and quality (from gross mixtures) of distillation. I n the first concept, an ordinary liquid is a pure substance; in the present, it is axiomatic that it is contaminated. The extreme care that is required t o polish a solid absolutely clean is well understood-yet we are dealing only with a single layer, or a t the most, a few layers of molecules. When a clean surface is melted, contaminants at once diffuse into it from below and they can be removed permanently only by scraping or its hydraulic equivalent,
“overflowing.’ ’ Liquids as a. class are, thed’ore, iufierently self-soiling We can thus make a broad &tinction between those stills or portions of distillaCion equipment where there is overflowing and those where there is not. All closed boilers and pot stills, all beakers, test tubes, and pail evaporators have what we shall call “ocean” status; t,liey are the final sinks from which nothing involatile escapes but into which everything ‘wmbles, where anything with an affinity for the surface must cont,inue to clutter that surface. All the molecular flotsam from the feed bat?ch or produced later by hydrolysis, and all the jetsam from column pipes and superstructure-to borrow nautical terms-must collect, perpetually repressing or complicating the act of evaporation. In the column above the still, the reflux on the wall may be said t o have “river” status. This is continuously renewed, and it should be substantially free from nonvolatile constituents. In a bubble tray or ite equivalent, the liquid has pond or “lake” status. It is resting partially stagnant but is being renewed and overflowed. We hope t h a t the falling-stream tensimeter, described in the first two papers of this series ( 3 , 9) presents good river conditions. Examples of liquid surfaces with ocean status are found in
August 1952
INDUSTRIAL AND ENGINEERING CHEMISTRY
many of the researches on accommodation coefficients that are scattered through the literature. Rideal (7) studied the evaporation of water in a slanting, inverted U-tube, and Baranaev ( 1 ) measured it from a vertical test tube. The evaporation of glycerol into a high vacuum was done by Wyllie (10) from a beaker.
1893
molecules will become reoriented by physicochemical forces, heads in and tails out, etc., until finally a “mature” surface will be established. In this, molecular species will congregate selectively in the top layer, presumably in reverse order of their contributions to surface tension, and there will be a dynamic equilibrium between the composition of the surface and the interior of the liquid, which will proceed to a limiting reproducible value. The orientation will have reduced the number of holes in the surface t o a value less than h/n, which will approach the minimum possible with the geometry of the molecules available. Contaminated Surfaces
In many mixtures, particularly when these are confined in “pot” containers, there will be one or more constituents, generally in minor amounts-for instance, tars produced during an organic synthesis-which will have an outstanding affinity for the surface, and these will gather there, presenting a front not the least representative of the bulk composition. Contamination with smaller molecules from either the liquid or vapor side of the interface will occur, and it is not difficult to envisage a mosaic, consisting of a few (a) larger, many medium-ie., representative-and fewer smaller molecules, which is substantially Figure 1. Still and PumpingjTrain for Examining Vacuum Distillation free of surface holes. This could be a static situation or a dynamic one where the mosaic had an All of these are pot stills, and it is not surprising, according to our unusual capacity for repair (a punctureproof inner tube of an arguments, t o find very low coefficients of evaporation reported. automobile tire comes to mind). The surface would have many In the pot or beaker, the ocean conditions can be smooth or properties of a solid, but it is not a unit layer of crystal lattice stormy; the surface can be left stagnant or mechanically stirred. because of its mixed population. It is, in fact, a unique and imWherever there is surface motion, some areas will be under comportant condition of matter. pression, some under tension, and in the latter, fissures of comThe surfaces just mentioned, in inverse order beginning with pletely new, clean fluid should appear for-fleeting instants. the contaminated, would develop best a t low temperatures. It is a useful preparation for the study As the rate of evaporation rose, the surface would contain an increasing proporahead t o visualize some stages that small tion of holes or, in other words, would areas of the surface of a liquid might pass through in changing from clean to soiled. become increasingly agitated by exit and The cleanest condition, which we shall entrance of vapor moleculee. At the label LLimmaculate,” is presumably what critical point, the holes and molecules would be revealed if a liquid could be would have equal status, orientation pulled apart without disturbing the would be random, and all surfaces would motions of any of the molecules along the become immaculate a t the moment they plane of fracture. A knife cutting inceased to exist. stantaneously would produce an immaculate surface if it could do so without imDepleted Surfaces pressing an over-all movement on the surface. If we accept the theory of “holes” Under rapid distillation there is yet ( 2 ) , the surface would display h holes for another category, “depleted” surfaces, its TL molecules, exactly the same fraction which have lost their more volatile conh / n as the H holes for N molecules in the stituents, including hotter and lighter bulk of the liquid. There would be no molecules. By definition an immaculate selective concentration of surface-active surface cannot be depleted and a prismolecules, no mechanical or physicotine surface is unlikely to be. The imchemical orientation; the surface would portant cases in the vacuum still or tennot yet have responded t o displacement simeter are the mature and contaminated of van der Waals forces. surfaces that are also depleted. There The nearest approach to this ideal is, thus, the paradox that the various that can be obtained practically is by postulated surfaces that could modify cutting with a knife or disturbing with a distillation could exist undistorted only paddle or the edge of an orifice, to prowhen there is no distillation. Their duce a “pristine” surface which would difplace is taken in common usage by defer from an immaculate only by the pleted surfaces that are not fixed entities mechanical orientation or spin of the surbut vary with every operating nuance. Figure 2. Molecular Pot Still under Total face molecules. Almost immediately the It is indeed from such a motley selecReflux
.
INDUSTRIAL A N D E N G I N E E R I N G CHEMISTRY
1894
Vol. 44, No. 8
no halfway measures; the surface is &her working or torpid. Generally, the two surfaces exist side by side in the same vessel, each holding absolute sway in its o w i area. This can happen when a well-degassed phlegmatic mixture containing a contauiinant (of chemical nat’ureas yet unexplored) is heated in a flask by il burner Ti.ith a sinall point flame. .1 convection currmt rises which ma)- establish a local, turbulent surface, ringed around by ;i torpid area n-hich, in contrast, is almost completely quiet. The torpid surface is at a higher griivitat’ional level, by a goodly fraction of a millimeter, and it is evident that a batt,le is being \raged a t the rim, nith the workiiig eur’tace always at a disadvantage. It maintains its place by virtur of its higher vapor pressure itnd the vigorous st’reaming of thc convection current, both of which push t h e barrier aivay. Ti the source of heat is removed, torpidity will supervene, and it may he necessary to raise the temperat.ure of t’heflask 10’ to 15” C. t o re-establish activitJ-. It is thus seen t h a t the militant, efficient surface that could evaporate with such vigor arid excellent sctpaiatory power is always tending t o disappear or turn t’orpid. It is t’hemost aggressive and yet the shyest of entities and, for thi:: reason, v-e offer the terms ”paranoid,” or better, “schizoid,” t o dcscriix?, as a class, all liyuitis
Figure 3.
Camera, Pot Still, and Band-Lighting
t,iori t,hat the accommodation coefficients t o which we have referred appear to have been measured. There emerge, however, two readily recognizable surfaces or habit,s thereof that h a w great practical significance, for, as will be appreciated from the experiments reported subsequentlj-, one or the other or both are alrrays present during lorn pressure dist’illation. The first n-ill be called rt ”torpid” surface, though passive, lethargic, or other synonrm could be applied. It appears to evolve from a mature surface that becomes contaminated and then depleted, until it acquires the power of spreading over all parts of the liquid that are not protected bg a stronger force. It is not known yet whether the torpid area has a surface tension greater or less than that of the “ n o r ~ n d ”liquid. The convex meniscus at the interface with thc rest of the (active) surface suggests a higher tension, but t,he ability to spread and the concave meniscus which it forms with the wall of the cont’ainer suggest a lower t,ension. X torpid surface is readil\- recognized hy its shin>-, varnished appearance and absence of convectivca 111ovement. The rate of evaporation is greatly depl,essetl, with correspoudirig ext,raordinary alterations in cwmposition of vapor, from a mixture. It is as though trace sulistiinces that cannot readily evaporate lock theniselves into the Furi’ac-eand Irpress t,he very convective movements that could ( w r y them :i\.r-ayagain-they “pour t,reacle on troubled oil.” Members of the other broad class a w thought t o h a w pristine origin, and because the?. are rich in hotter and more volatile molecules, t,hey exert a greater vapor pressure than t’he torpid surface, which they can thrust away laterally or even disrupt explosively. These working surfaces are in rapid motion and can thus be identified. Under special circumstances, the a-orking surface can persist indefinitely, but it is aln-ays in danger of depletion and relapsing suddenly into torpidity. There seem to be
Figure 4.
Pot Still after Degassing
present,ing working areas \\-hicaha r c in abrupt twnt,act with torpid surfaces. The organic chemist as noted earlier, has been familiar, consciously or otheruise, \\-ith many of the aspects of torpidity and schizoid behavior. Ihplosive degassing and writhing patterns on the surface of his phlegmatic mixtures in the distilling flask are examples. -4bubbler or a mechanical stirrer has been the practical ansxer. The general point at issue is whether t h e effects are just a commonplac~echain of events originating in and completely explainable by viscosity. diffusion, thermal gradient’s, and surface tension forces or xl-hether in addition to these there are specific, hitherto unexplored processes in the molecular layer on the liquid side of t,he interface. ~
INDUSTRIAL AND E N G I N E E R I N G C H E M I S T R Y
August 1952
Figure 5.
A Working Surface of EHP
Experimental Thp means which we have chosen to study these questions is t o make measurements and visual and photographic observations of organic liquids heated under vacuum in pot stills. The kind of procedure is suggested in Figure 1, where the well-jacketed flask, A , is attached to a mechanical vacuum pump, B , through a condensation pump, C, and two freezing traps, D, D’. Alternatively, the flask a is not jacketed but is cooled as vigorously from above as it is heated below [Figure 1 ( a ) and Figure 21. The lagged-flask experiments are reported in the fourth paper of this series ( 4 ) and those with the bare flask, used as a molecular pot still under total reflux, are described here. Patterns have often been observed on the surfaces of liquids during quiet non-ebulliative evaporation, and these patterns will now be examined fully but qualitatively. The observational device is a simple form of schlieren or venetian-blind lighting, whereby the reflection from the surface shows distortion with the slightest movement of the liquid. An element of surface that “sees” a bright band will see a dark one if the surface is tipped, a n d this will be noticed by eye or lens. The flask of Figure 2 is thus mounted between the striped windows and the camera shown in Figure 3. The lower vertical blind allows the hot striae t o be photographed as they rise through the liquid. A lens of wide aperture and long focal length is used, the large diameter improving the quality of thp photographs by keeping the reflection
Figure
7. A Divided Surface with Working and Torpid Areas
Figure 6.
1895
Working Surface Observed from Below
of the light and dark bands out of focus. The long focus enables the observer t o sit between the camera and the flask, moving away to record events of interest. Exposures of * / b o to l/l~o second at f/S (actuallyflll tof/16), on Kodak Super-XX Panchromatic film, illuminated by two Photoflood lamps, RFL2, provided the photographs. The lamps were connected through a variable transformer set a t 90 volts, which was raised t o 115 volts for the moment of exposure. The permanent illumination was eomfortable, and the life of the lamps was prolonged indefinitely. Typical Cycle of Events. The liquid first exainined was the plasticizer and pump fluid, 2-ethyl hexyl phthalate. One liter was placed in the 2-liter flask of Figure 2, and vacuum was applied, After the first evolution of air, degassing continued in spasmodic bursts, until the surface became permanently quiet, as evidenced in Figure 4. These early experiments were done with a two-necked flask and a primitive form of band lighting, but the photographs remain acceptable. Distorted reflections of the bands are seen on the surface, marred by a defect in t h e flask which appears as a slanting black mark A glass chimney (not visible), supported by three legs, has been placed in the flask t o guide convection currents. After the Bunsen burner has been lighted, striae of hot liquid float upwards, causing a “hump” under the surface. This hump, which is the natural consequence of the greater hydrostatic depth of the hot, lighter column of liquid, continues t o increase until it vanishes in Figure 5 . The
Figure 8.
Divided Surface Observed from Below
INDUSTRIAL AND ENGINEERING CHEMISTRY
1896
Figure
9. Crater Formed Drop of Distillate
by
Figure 10. Working Area Increases
temperature is now 125” C., the vapor pressure about l o p , (residual gas pressure
