EngFnyrin g
Liquid-Liquid Extraction from Single Drops
p*cess development
FRANK B.
WEST, P. A. ROBINSON’, A. C. MORGENTHALER, Jk2,T. R. BECK,
AND
D.
K. McGREGOR3
UNIVERSITY OF WASHINGTON, SEATTLE, WASH.
with shorter tube lengths. D a t a are presented on the extraction of acetic acid from RVATION of liquidliquid extraction from benzene drops of known volume by water. Approximately Each was provided with a feed system and raffinate single d r o p s h a s t h e ad14 to 20Yo extraction was obtained during drop formation. removal system -,hi& difThe subsequent extraction during the time of rise could be fered somewhat from one vantage that the extraction can be carried out under correlated empirically by equations for unsteady-state difstudy to the next. A typical known conditions of interfusion i n stagnant spheres. These results differ as much setup (Iis )shown in Figure 1. facial area and time of conas fivefold from those previously obtained in similar equipEach feed system conment by similar procedures by Sherwood, Evans, and tained a nozzle, a standpipe, tact. The results should be directly applicable to spray Longcor. Sherwood’s data for the period of free rise are and a feed reservoir. The towers and perforated-plate interpreted by Higbie’s unsteady-state equation for insertedstopper verticallJT throughwas a rubber at “transient films” moving around the drops, k = 1.13 the base of the extraction extractors, and should help 1 /z explain the mechanism of ex* It was necessary to introduce a correction factor The were traction in other types of prepared with inside diamof 0.31 into this equation for benzene drops and another eters ranging from 0.024 to equipment. 0.162 cm. by draaing out of 0.68 for drops of methyl isobutyl ketone. The first reported study of glass tubing and fire-polishthis type is that of Sherwood, ing the ends. The nozzle was fed from a standpipe made Evans, and Longcor ( l a ) ,who of glass and Tggon tubing and containing a stopcoclr for regulatextracted acetic acid from drops of benzene and methyl isobutyl ketone with water. The iesults indicated considerable agitation ~ ~ ~ ; n l ~ g ~~ ~ t ~~ r ~~ in the interior of each drop and showed that a large portion of the the second study (1) the device shown in Figure 1 maintained a constant feed level in the standpipe. In spite of the constant extraction occurred durlng the formation of the drops. The delevels, the feed rate decreased erratically with time unless the for manyfold greater than that gree of extraction stopcock was either tapped or reset a t frequent intervals. This rigid stagnant spheres. difficulty was overcome in the third study (9) by reverting to A series of investigations to confirm and extend the above reSherwood’s method of maintaining a constant standpipe level by running in feed manually from a b i r e t used as sults has been initiated in this laboratory. I n the feed reservoir. the first of these, Donelson (2) obtained results At the top of the column the benzene rafvery similar to the above when extracting finate was collected in a cone-shaped collector which had been formed by hollowing out a cork acetic acid with water and when extracting stopper. This collector funneled the raffinate acetic and benzoic acids with 0.8 S sodium into an inverted glass U-tube which discharged hydroxide from drops of technical grade beninto a buret receiver. A small amount of zene. purge water was introduced either through the bottom stopper or through a side arm near The present article is devoted to certain the top of the column to force some water sharply contradictory results which lead to enout with the raffinate. The combined bentirely different conclusions. These results are zene and purge water raffinate was collected well confirmed, being based on three indeand measured in the buret. pendent studies (1, 9, 2 1 ) . They are confined to the aqueous extraction of acetic acid from EXTRACTION PROCEDURE thiophene-free AXERICANCHEUICALSOCIETY The procedure for the extraction runs was specification benzene, although fairly similar as follows, with the few exceptions noted in results were obtained with the technical benTable I. zene available at the time of these studies. The column was filled with distilled water EXTRACTION APPARATUS which had been preneutralized to the phenolphthalein end point and adjusted to 25’ * The apparatus used in studying extraction 1’ C. Feed consisting of acetic acid in thiofrom individual drops was very similar to that described by Sherwood, Evans, and Longcor phene-free, AMERICAN CHEMICAL SOCIETY speci(12). The extraction columns were constructed fication benzene mas charged to the feed resof glass tubing 44 mm. in inside diameter, apervoir and standpipe. Neither phase had proximately 152 cm. long, except for a few runs been saturated with the other. The feed solution was then admitted through the nozzle 1 Present address, Preservative Paint Co., Seattle, Wash. at as nearly constant a rate as possible. Pre* Present address, General Electric Co., Hanford, neutralized distilled water was run in simulWash. Figure 1. Apparatus Used taneously a t one fourth t o one half as large 3 Present address, Superior Portland Cement, Inc., in Series I1 for Extraction a rate to keep benzene from collecting a t the Concrete, Wash. from Individual Drops
(g)
$is
234
i~~)f~$
~
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1951
235
OF ACETICACID FROM INDIVIDUAL BENZENE DROPSBY WATER TABLE I. EXTRACTION
Run No.
Feed Rate, Ml./Min.
Time of Rise, Sea.
6.81 1.81 3.60 2.00 4.87 3.78 5.23 5.10 5.50 1.17 1.05 1.89 1.99 1.86
300.0 39.4 85.6 56.6 158.0 103.5 54.5 60.0 60.0
3.2 3.0 2.95 0.9 1.0 0.8 12.5 12.3 12.35 15.0 14.9 14.5 14.6 0.5
0.0615 0.0615 0.0615 0.0615 0.0615 0,0615 0.0615 0.0615 0.0615 0.0615 0.0615 0.0615 0.0615
148.0 148.0 148.0 148.0 148.0
6.83 7.13 4.28 2.88 1.32
51.2 50.0 141.0 80.6 103.0
143.0 143.0 143.0
3.81 14.80 5.20
40.0 214.0 60.0
60.0
63.0 120.0 114.0 120.0
% of Acid Feed Not Accounted for
Acid Concentrations, G. Moles/Liter Feed Raffinate
Drop Rate, Drops/hlin.
Extraction Height, Cm.
Extraction,
%
Drop Volume, Ml./Drop
0.0227 0.0460 0.0421 0.0353 0.0308 0.0365 0.0959 0.0850 0.0917 0.0195 0.0167 0.0157 0.0178 0.0155
-0.3 -0.4 2.9 -1.8 -4.1
0.061Z
0.0432 0.0473 0.0504 0.0511 0.0453 0,0488 0.0424 0.0415 0.0406 0.0322 0.0339 0.0328 0.0317 0.0473
-1.2 -4.2 3.5 -2.0 -2.5 2.4 4.0 0.6
29.8 23.1 18.1 16.9 26.4 20.6 31.0 32.5 34.0 47.6 44.9 46.7 48.5 23.1
12.8 12.7 14.3 14.2
16.6
0.0713 0.0691 0.0652 0.1800 0.0645
0.0550 0.0535 0.0429 0.1136 0.0372
0.3 0.3 -0.2 -0.7 -2.0
22.8 22.6 34.2 36.8 42.4
0.1333 0.1426 0,0304 0.0388 0.0128
12.8 12.4 12.7
0.0635 0.0650 0.0650
0.0444 0.0465 0.0465
-3.8 -0.3 -1.5
30.1 28.4 28.4
0.0952 0.0692 0.0867
-1.6
Series I1 ( 1 ) 335 344 350 49 50 Series I11 ( 9 ) 1
30 31 Q
x
Q
Water temperatures in runs 33, 34, and 35 were 27.0°, 23.4', and 23.5' C., respectively.
top of the column. The water displaced by this purge stream left with the benzene and carried out any abnormally high acid concentrations resulting from impact of the drops with the top of the column. At frequent intervals during each run measurements were made of the droD rate and of the time of rise of the drom from the tip of the nozzle to the tip of the cone in the hollowed-out stopper. The benzene feed was discontinued after collection of 30 t o 50 ml. of raffinate, and the purge water was shut off after all benzene had been displaced from the column. The column was then drained and rinsed, and the entire combined extract and rinsings were titrated to the phenolphthalein end point. The volume of benzene raffinate was measured, and in the third study (9) was found to run about 1%less than the feed volume used. The raffinate and displaced water were combined and titrated to the phenolphthalein end point. A sample of the feed was titrated similarly. A number of runs were also made using faulty technique to determine the importance of such errors (9). I n some, no flush water was used until the end of the run, and then only the bare minimum to remove the benzene from the tower and outlet tube. The apparent extraction efficiency increased markedly, but still fell far short of those reported by Sherwood, Evans, and Longcor. Tipping the column out of line so that the droplets struck the upper stopper considerably off-center had a negligible effect when the usual purging procedure was followed. Substitution of tap water had little effect save on the material balances ( 1 ) .
vestigation, or about as much as was extracted during the formation of Sherwood's drops. The difference is of the order of fivefold, if allowance is made for the mean concentration driving force in the two cases.
RESULTS
Figure 3 shows the unextracted acid remaining in 0.030-ml. drops as a function of the extractor height for the present investigation as compared with Sherwood's results. The curved solid line was obtained by interpolating to 0.030-ml. drop volume on the curves for 9, 35, and 150 cm. in Figure 2. It extrapolates to 14% extraction a t zero height. The experimental data are represented equally well by the straight dashed line with an intercept of 20% extraction at zero height. The straight line corresponds to interpolated points obtained by lowering the 35-cm. correlating line in Figure 2 by 2y0 and raising the 9-cm. line by lyo,both changes being well within the scattering of the data. I n either case, the extraction during drop formation is only a fraction of the 4oy0 found during the formation of Sherwood's drops. Figure 4 shows the observed velocities of rise of the benzene drops through preneutralized distilled water in the extraction
The principal data and results are listed in Table I. The acetic acid balances for the runs reported show a maximum discrepancy of about 4y0 with an average of less than 2yo. The raffinate concentration reported includes all acid found in the displaced water collected with the raffinate. The extraction efficiencies are based upon the feed and raffinate ooncentrations, which are more reliable than the extract concentrations. The per cent extraction obtained with different drop sizes and extractor heights is shown in Figure 2. With a height of 143 to 149 em. the extraction increases from about 30% for 0.100-ml. drops to 48% for 0.015-ml. drops. Where Sherwood, Evans, and Longcor obtained 90% extraction from drops averaging 0.031 ml. in volume, only 39y0 extraction was obtained in the present in-
60
I-
z
W
0
5 40
a
,
eu 4 0
w
20
c
0
U
I
a W
I 0.04
DROP
Figure 2.
I 0.08 VOLUME
I
I
-
CC.
0.12
0.16
Per Cent Extraction us. Drop Volume for Various Heights of Rise
INDUSTRIAL AND ENGINEERING CHEMISTRY
236
column. Points from the runs with deliberately inadequate Aush water are included here, although not listed in Table I. The scattering of points is somewhat greater than indicated by the probable precision of 0.2 second in the measured time of rise. The corresponding precision in the velocities of rise would be of the
Vol. 43, No. 1
benzene interface during the period of rise of the drop through the water. Various possibilities are indicated schematically in Figure 5. The upper horizontal line, ABC, represents the locus of certain water and benzene molecules relative t o the drop center a t one particular time, t l . Possible loci of these same molecules relative to the drop center some time later a t t z are represented by cases I, 11,and 111. I n case I there is so little resistance to internal circulation within the drop that point B on the interface has been carried along around the drop a t the same speed as point A far out in the continuous water phase. I n case I1 the internal resistance to flow is greater, so that point B lags behind point A as it is dragged along the surface by the water. I n case I11 the drop has so much internal resistance to flow compared to the water that it acts like a rigid body with point B on its interface and point C in its interior remaining stationary with respect to the drop center, Cases I and I1 give rise to “transient films’! which move along over the surface of the drop at approximately the same speed as the interface. As assumed by Higbie (41, these transient films are fornied along with new interface near the top of a drop as it pushes up through the water. They move down over the surface with the interface and dkappear near the bottom. The benzene film passes into the interior of the drop and the water film into the bulk of the water phase-hence the term “transient films.” Higbie showed that these transient films need not be very thick to be effectively infinite in thickness with respect to solute diffusing for times of the order of 0.01 to 1.00 second.
COLUMN
’
H E I G H T - CM.
PREDICTED F O R RIGID S P H E R E S
Figure 3. Extraction from 0.030-M1. Benzene Drops us. Height of Water Column
order of 8.2 cm. per second for the 143- to 149-em. runs, and of 0.8 cm. per second for the 35-em. runs. Accordingly, less weight was given the latter points in drawing in the solid correlating line in Figure 4. The broken line shows the velocities predicted from the usual settling equation and drag coefficients (IO)for rigid spheres of the same density. The flow of water around the drops must have been approaching fully developed turbulence, inasmuch as the predicted Reynolds numbers, based on drop diameters, varied from about 250 for the smallest drops to about 1100 for the largest. The observed velocities are slightly higher than predicted u p to drop volumes of a t least 0.03 ml. This may be due to the slow upward current of water in the center of the column, which rises under the influence of the stream of benzene drops. Thus the drops in runs 48 and 49 of series I were formed a t twice the rate and rose approximately 0.25 em. per second faster than the slightly larger drops from runs 46 and 47. The amount of flush water fed into the column during the time of passage of any drop through the column was so small as to affect the rate of rise by less than 0.01 em. per second. At the high drop volumes the observed velocities fall further and further below the predicted values and also show a greater amount of scattering. This may be due to progressive distortion of the drops with increased size as the interfacial tension becomes inadequate to maintain the spherical shape assumed in the predictions. The largest drops were very noticeably distorted and were observed to change shape &s they rose through the column. COMPARISON WITH DIFFUSION EQUATIONS
An attempt has been made to interpret the results of the present and previous investigations by considering the conditions that may exist within a drop of benzene and on either side of the water-
-
DROP VOLUME CC. Figure 4. Velocity of Rise of Benzene Drops through Water
The benzene and mater mass transfer coefficients for case I transient films may be estimated by Higbie’s equation k
=
1.13
This equation gives the effective mass transfer coefficient resulting from unsteady-state diffusion of a solute of diffusivity D through a surface into a stagnant film of infinite thickness for a time of diffusion, t,. I n case I the interface moves with the same velocity as the water, so that t, is the bime for the drop to rise a distance equal to its own diameter. Johnstone and Kleimchmidt (8) have derived an almost identical equation in a less rigorous fashion. The benzene and water mass transfer coefficients for c u e I1 transient films may be estimated by substituting a diffusion time
INDUSTRIAL AND ENGINEERING CHEMISTRY
January 1951
of te/ff into Equation 1. I n terms of Figure 5, f: is the ratio of vertical velocity of the interface to that of the bulk water, both taken with respect to the drop center. The resulting equation is
k = 1.13fc
(?)l‘’
The benzene mass transfer coefficients for the case I11 stagnant benzene drops may be predicted from the following equations taken from Geddes (3):
where
Dl?t x=-@U
PHASE
AT t2 FOR
CASE
I
Figure 5. Qualitative Picture of Water and Benzene Velocities On either side of interface relative to drop center
x
E, is the fractional extraction resulting from unsteady-state diffusion of a solute of diffusivity D through a rigid, stagnant sphere of radius r for a time equal to the time of rise, t. The solute concentration is assumed to be uniform throughout a t the start of the period of rise except at the interface, which is held constant a t a lower value throughout the extraction. Ei is expressed as the ratio of the solute extracted t o the total which could be extracted down to the interface composition. Equation 3 gives the corresponding effective mass transfer coefficient. Values of 1 Ei have been tabulated by Geddes for various values of X. For still lower values of X , Equation 3 may be approximated satisfactorily by substituting t for ts in Equation 1. The water mass-transfer coefficients for t,he case I11 water film around the benzene drops are more difficult t o estimate and should be much smaller than the Equation 1 transient film values used by Geddes in a similar situation. Having obtained the individual mass-transfer coefficients, it should be possible t o estimate the over-all mass-transfer coefficients based on benzene concentrations hy means of Equation 6:
-
1
where m is the slope of the appropriate portion ( 1 3 ) of the equilibrium curve for the distribution of acetic acid between water and benzene. I n this investigation m reduced to the water to benzene ratio of the acid concentrations a t the interface. The over-all coefficients may then be converted to the fraction extracted by Equation 7 which is analogous to Equation 3 :
KB=
-ltIn (1 - E)
(7)
where E is the ratio of solute extracted to the total which would be extracted down to equilibrium with the water phase, the equilibrium value being negligible in the present experiments. The above equations and E all apply only to the extraction subsequent to drop formation. Sherwood’s results during the time of rise can best be explained in terms of case I1 transient films.
As an example, consider one of Sherwood’s 0.030-ml. benzene drops rising through 150 cm. of water of negligible acid concentration. From Figure 3 the observed extraction is 90.9%, but this includes 40% extraction during drop formation. The extraction subsequent to drop formation is 84.870 of the 60% acid remaining after drop formation. Introducing E = 0.848, t = 14.16 seconds from the velocity of rise in Figure 4, and r = 0.193 cm. into Equation 7 yields the observed K B of 0.00862 for the period of rise. Next turning to Equation 2, ts = 0.0364 second. The diffusivity of acetic acid in water is taken as 1.25 X 10-6 s , cm. per second corrected t o 25’ C. from 18’ C. data (6). T%e diffusivity of acetic acid in benzene is taken as 2.31 X sq. cm. per second corrected to 25’ C. from 15’ C. data, (7). The corresponding values predicted for k , and k~ are 0.02O9fcand 0.0284f0,respectively. The predicted K B from Equation 6 becomes 0.0278 fo using a value of 72 for m from the distribution data (6) and the usual method for locating interfacial concentrations (13). The water phase accounts for only 2% of the predicted over-all resistance. Finally, if one empirically setsfc = 0.31, the predicted K B becomes identical with the observed value. The fact that the predicted K B is independent of tower height explains Sherwood’s success in correlating extraction with tower height by a semilog plot as in Figure 3.
and
CONTINUOUS
237
Similar application of case I1 transient films to Sherwood’s 0.013-ml. drops of methyl isobutyl ketone gave an empirical fc = 0.68. The diffusivity in the ketone was arbitrarily assumed to be the same as in the benzene, and the theoretical rate of rise for a rigid ketone sphere was used, 11.15 cm. per second. I n this case the predicted water side resistance is approximately 40% of the total. Higbie’s work on absorption from carbon dioxide bubbles into water ( 4 ) also showed approximately 60% as much absorption as predicted by Equation 1 for ts’s of the same order as in the investigations being considered here. However, Higbie attributed this discrepancy t o laok of equilibrium a t the interface due to the high rates of mass transfer encountered. This may be an additional complicating factor. The results of the present investigation cannot be correlated satisfactorily in terms of case I1 transient films. The predicted kt, for 0.030-ml. benzene drops would again be 0.0278 fc, but the observed K B corresponding t o the straight dashed line in Figure 3 is only 0.00123. This corresponds to fc = 0.0444 and would be equivalent to the drop interface moving a t only 0.2’% of the relative velocity between the water and the drop. The apparent time for a point on the interface t o move from top t o bottom of the drop, t&, would be 18.45 seconds compared t o the expected 14.16 seconds’ time of rise for a 150-cm. tower. The results of the present investigation during the time of rise can be correlated more satisfactorily in terms of case I11 stagnant spheres. Again consider 0.030-ml. benzene drops rising through 150 cm. of water of negligible acid concentration. X is 0.0866 for the benzene drops from Equation 5. The corresponding Ei is 0.292 by interpolation on a plot of Geddes’ solu-
238
INDUSTRIAL AND ENGINEERING CHEMISTRY
tions of Equation 4. No good method is available for estimation of the water film resistance; but even if Equation 2 is used arbitrarily with an f c = 0.05, the water side resistance is still essentially negligible, about 2 7 , of the total. Accordingly the water resistance has been neglected and E taken equal to E,, thus predicting 29.2% extraction of the acid remaining after drop formation. The ohserved extraction for these conditions was approximately 39%. If 14Tc extraction during drop formation is assumed as indicated by the curved line in Figure 3, the extraction subsequent to drop formation is then 29.1% of the remaining acid, in excellent agreement with the predicted value.
It R B S found that the results of the present investigation could be correlated empirically on the following assumptions: That drops of all sizes lost 14% of their initial acid concentrations during drop formation. That extraction after formation of the drops proceeded according to Equation 4 for a rigid, stagnant sphere with negligible water phase resistance. The correlating lines for per cent extraction cs. drop volume in Figure 2 were constructed on the above assumptions. The theoretical rates of rise were used for drops smaller than 0.030 ml. The data of Robinson and Morgenthaler fall slightly above the 150-em. line, while those of Beck and of MeGregor average about 4% below it. The 35-cm. curve is a little high on the average, and the 9-cm. curve is a little low compared to the observed points. Considering the assumptions made, the agreement 15 ith the observed data is excellent. The curved solid line representing this investigation in Figure 3 also necessarily agrees with the above assumptions, because it was made up from interpolated points on the solid lines in Figure 2. The above treatments neglect an) effect that simultaneous diffusion of water into the benzene and of benzene into the n-ater mag have had on the diffusion rate of the acetic acid. COKCLUSIONS
Acetic acid was extracted from benzene by water in the present investigation with entirely different results from those obtained by Sherwood, Evans, and Longcor in similar equipment by similar procedures. During drop formation the apparent eutraction amounted to only 14 to 20y0 as compared to Sherwood’s 40’%. For 0.030-nil. drops rising 150 em. the total extraction was found by interpolation to be only 39% as compared to Shern-ood’s 90.9%. The total extraction in the present investigation was about the same as that during the formation of Sherwood’s drops. If allowance IS made for the greater average driving force in the present investigation, the over-all discrepancy is of the order of fivefold. Sherwood’s extraction results during the period of rise can be correlated satisfactorily by Lquation 2 for case I1 transient films. For O.03O-ml. benzene drops the correction factor,f,, is 0.31. For 0.013-ml. drops of methyl isobutyl ketone f c is 0.68. The results of the present investigation can be correlated empirically by the unsteady-state diffusion equation for a rigid,
Vol. 43, No. 1
stagnant sphere, provided allowance is made for approximately 1401, extraction during drop formation. Actually the drops are nonspherical and can hardly be considered to be completely rigid. The discrepancies between the results of the two investigations cannot be accounted for by any reasonable errors in equipment or procedure. I t seems likely that they are due to differences in purity of the benzenes used with resultant differences in some important physical property. This possibility is being studied further. YOMEYC LATURE
D E
=
=
E, = k
= =
k~ k,
= =
m
=
n r
= =
t
=
t,
=
fc
KB =
X
=
T
=
diffusivity of solute in either phase, sq. cm. per second ratio of solute removed t o total which would be removed by reduction to equilibrium with other phase ratio of solute removed to total which would be removed by reduction to interface concentration correction factor in Equation 2 effective mass-transfer coefficient for either phase, gram moles/(sq. cm.)(sec.)(gram moles/ml.) k for benzene phase kforwaterphase over-all mass-transfer coefficient based on benzene phase, gram moles/(sq. cm.)(sec.)(gram moles/ml.) appropriate slope of equilibrium curve, gram moles of solute per ml. of water over gram moles of solute per ml. of benzene any integer from 1 to infinity radius of drop considered as a sphere, cm. time for a newly formed drop to rise through continuous phase, seconds time for a drop to rise a distance equal to its o ~ diameter, n seconds D.rrZt/ra 3.1416 LITERATURE CITED
(1) Beck, T . R., B.S. thesis in chemical engineering, University of Washington, 1949. (2) Donelson. R . N., B.S. thesis in chemical engineering, University of Washington, 1948. (3) Geddes, R. L., Trans. A m . I m t . Chem. Engrs., 42,79 (1946). (4) Higbie, R a l p h , Ibid., 31,365 (1935). (5) International Critical Tables, 5-01. 111, p. 429, S e w York, RIcGraw-Hill Book Co. (6) Ibid., 1-01. V,p. 69. (7) I b i d . , p. 74. (8) Johnstone. H. F., and Kleinschmidt. R . IT.. Trans. A m . 1 ~ ~ 3 t Chem. Engrs., 34, 181 (1938). (9) MeGregor, D. K., B.S.thesis in chemical engineering, University of Washington, 1949. (10) P e r r y , J. H . , “Chemical Engineers’ H a n d b o o k , ” 2nd ed., p. 1855, New York, MIcGraw-Hill Book Co., 1941. (11) Robinson, P. A., and hlorgenthaler, A. C., Jr.. B.S. t,hesis in chemical engineering, University of Washington, 1948. (12) Sherwood, T. K., E v a n s , J. E., a n d Longcor, J . V. A , , IND.ENG. CHEM.,31, 1144 (1939). (13) Walker, IT. H . , Lewis, W ,K., McAdams, W. H., a n d Gilliland, E. R., “Principles of Chemical Engineering,” 3rd ed.. pp. 4512, Kew Y o r k , McGran--Hill Book Co., 1937.
RECEIVED December 14, 1949. Presented before the Division of Industrial and Engineering Chemistry a t the 118th Meeting of the AXERICANCHEMICAL SOCIETY, Chicago, Ill.
.
