High-Vacuum Fractionation with Falling-Stream Still D. J. TREVOY and W.A. TORPEY Research La6orator;es, Eastman Kodak Co., Rochester 4,
N. Y. However, by using an attached pot still as a primary reservoir for distilland, it was possible to increase the separatory power without appreciably adding to the complexity of apparatus and this is the scheme that was employed. The arrangement is shown diagrammatically in Figure 1. Thus, the condensate from the pot still (stage 1) flowed by gravity into the falling-stream tensimeter (stage 2 ) a t a point just ahead of the impeller. Overflow from stage 2 was returned to stage 1 by a tube connected behind the impeller. The falling-stream tensimeter was like that described earlier, except that three small streams, rather than one large one, were used. This made possible the use of smaller tubing in the liquid circuit, with a corresponding decrease in the volume of fluid in the stage, and a much simpler liquid distributor above the jet tubes was required to obtain smooth streams. The effective area of the three streams was not determined since the rate measurements were relative. The jet tubes were 8.5 em. long with an internal diameter of 0.28 cm. The condensing zone was 19.5 cm. long and the condensing surface was cooled with jets of air. By a simple arrangement of ball valves, shown in Figure 1, the condensate from stage 2 could be returned to that stage while the receiver was being changed without breaking vacuum.
The fractionation of a phlegmatic fluid by high-vacuum distillation is ordinarily evaluated by applying analytical tests to the distillate which is collected i n a series of fractions. Neither the pressure of vapor in the still nor the equilibrium vapor pressure is usually measured, owing to experimental difficulties. If, however, the falling-stream tensimeter is used as a still, the rate of evaporation a t constant temperature can be measured as a function of the amount distilled by a simple timing operation. Because the rate is almost directly proportional to the equilibrium vapor pressure and is readily measurable to 0.5% or better, the method provides a sensitive evaluation of the distillate while the distillation proceeds without interruption. In the apparatus described, a modified falling-stream tensimeter is fed by distillate from a simple pot still. This procedure provides a reservoir for the initial charge and increases the over-all separatory power, without unduly complicating the operation of the still. Data are reported for the purification of di-2-ethyl hexyl phthalate and di-2-ethyl hexyl sebacate, and for the fractionation of a 50% solution of the two esters. Rate data may reveal the presence of isomers or members of a homologoits series differing by one or two CH2 groups in concentrations which are not detectable by refractive index measurement with a precision refractometer. The twostage assembly is operated with a holdup of 250 ml. in the pot and 150 ml. in the flow tensimeter. At the end of the distillation, a total of 160 ml. of liquid remains in the apparatus. The still requires an initial charge of a t least 2 liters and provides rate data which are highly reproducible. Such data are considered a useful supplement to refractive index measurement and other tests in following the course of a fractionation.
MECHANICAL VACUUM PUMP
I
? FRACTIONATIOS i of liquids having high molecular weights, the fluids are often distilled under high vacuum and the condensate is collected in a series of fractions. Following the distillation, various properties of the fractions, such as refractive index and acid value, are measured, and cuts of relatively homogeneous material are selected on the basis of these data. Unfortunately, one of the most sensitive tests of homogeneity, the constancy of vapor pressure from fraction to frartion, cannot ordinarily be applied, owing to the difficulty of accurately measuring either the pressure of vapor in the still or the equilibrium vapor pressure in the 1- to 100-micron range. This difficulty may now be overcome by employing the falling-stream tensimeter (1) as a still and measuring the rate of evaporation, a property which is almost directly proportional to the vapor pressure. Xeasurements of the rate may be made by n simple timing operation M ithout interrupting the distillation in any way. I n a single operation one obtains not only an efficient stage of fractionation, but simultaneously the accurate measurement of a property which is V P I v sensitive to small differences in chain length of the molecules.
Figure 1. Falling-Stream Still with .Attached Pot Still
APPARATUS
The pot still was similar to that described earlier ( 2 ) but was built from a 5-liter flask, the liquid being agitated by a cone stirrer. I n order for the two stages to operate satisfactorily together, it was necessary to supply feed liquid to stage 1 a t a substantially constant rate. This was accomplished by suspending a piece of drill rod in a slightly larger glass capillary about 7 em.
The simplest way to apply the principle would be t o enlarge the liquid reservoirs of the falling-stream tensimeter already described ( I ) , a t the same time decreasing the volume of the connecting tubes and circulating pump, in order to reduce holdup or the quantity of liquid left in the apparatus a t the end of a run. 492
V O L U M E 2 6 , NO. 3, M A R C H 1 9 5 4 long, as shown in Figure 1. Feed liquid flowed down the annular space b e t w e n rod and capillary, being forced into the still by atmospheric pressure. By raising or lowering the rod or by changing the size of the rod, any desired flow rate could be obtained. No clogging was experienced and the rate remained constant indefinitely, a result which could not be obtained by using partially open stopcocks. With a stopcock for on-off control, a given setting could be maintained from day to day. Considerable degassing of the feed liquid occurred in a small bulb below the capillary, the bulb being connected to an auxiliary vacuum pump. The rate of evaporation in stage 2 was determined by observing the time required to fill an 11-ml. pipet in the condensate line.
493 Table I.
Refractive Index and Rate of Evaporation of Fractions of Purified Fluids Di-%ethyl Hexyl Phthalate Timefor 11 ml. of condensateo.
Fraction
Di-2-ethyl Hexyl Sebacate Time for 1 1 ml. of condensateb, n sec.
'z
sec.
NO.
82.0 82.2
1,44882
i8:3
1 ,44862
78.3 78.0 78.7 78.8
82.5 82.7 82.8
1 ,44852
1.44862 1.44862
83.0
1.44885
..
.....
78.3
Liquid a t 150' C. b Liquid at 165' C.
a
MODE O F OPERATION
.
Two general methods of operation were employed. When only one distillation was required, the entire charge was placed in the pot of stage 1 and, after degassing, distillate was caused to enter stage 2 at a rate at least twice that a t which final distillate was withdrawn from the apparatus. The residue from stage 2 overflowed continuously back into the pot still and became a t once incorporated with the original distilland. Distillate from the second stage was collected in 200-ml. fractions. When more than one distillation was required, the second and subsequent distillations were accomplished as follows: The still assembly was cleaned with acetone, drained, dried, and brought under vacuum. Fractions 1 and 2 (total 400 ml.) of the previous distillation were admitted to the pot and heated until distillation hegan and distillate filled and overflowed the second stage. Then, as the latter n-as brought into operation, fraction 3 was admitted to qtage 1 at a rate approximately equal to the rate of withdra\vJ of the second-seiies S o . 1 distillate. As KO. 1 distillate was completed and the receiver changed, the No. 4 distillate of the first series was poured into the now nearly emptv supply funnel of stage 1. In this way there was always approximately 250 ml. of charge in the pot, 150 ml. of first residue in the flow tensimeter and 200 ml. in the combined volumes of serial feeds and distillates. At the finish of each run, the pot still was taken nezrlv to dryness so that the ultimate holdup was about 160 ml It is not claimed that this procedure \vas the best possiblefrom a sepnratory point of view, since the composition of distillates from run n - 1, serving as feed for run n, did not always have the composition of the contents of the pot at the time of addition. However, the compositions were not too far apart and the procedure n-as simple and precise. PURIFICATION O F FLUIDS
The test fluids used previously (1), di-2-ethyl hexyl phthalate
(EHP) and di-2-ethyl hexyl sebacate (EHS) were purified individually by repeated passes through the two-st,age still. After three passes, the di-2-ethyl hex>-1 phthalate showed constant refractive index and constant rate through fractions 5 to 10 but the tii-2-ethyl hexyl sebacate required seven passes before rates mere constant within about, 1%. After the fourth pass, di-2-ethyl hexyl sebacate showed a constant refractive index in fractions 4 through 12 but the rate decre:rsed 7.670 over this range. Refractive index and rate data for :,he center cuts of the purified fluids are given in Table I . FRACTIONATIOY O F A BIYARY MIXTURE
To demonstrate the use of rate measurements in follon-ing the course of a fractionation, a binary mixture of the purified phthalate and sebacate esters was adjusted to a concentration of 50 volume % and 2300 ml. were charged to stage 1. Thrce successive passe. through the two stages were carried out, as described previouslv. The voltage supplied t o stage 2 vias adjusted to give an approximately constant rate, with 120 seconds to fill the 11-ml. pipet, and the IiJuid temperature was measured as a function of the volume distilled. This mode of operation corresponded approximately to that of constant heat supply to stage 2, and for this reason it was easy to mzintain a constsnt rate even in the transi-
1
'0
1
I
I
40
60
80
I
20
I
Kx)
Volume % distilled
Figure 2. Effective Relative Volatility for Each Stage and Over-all Separation Coefficient for Fractionation of Di-2-ethyl Hexyl PhthalateDi-%-ethylHexyl Sebacate Mixture h
Stage1
0 Stage 2
Over-all
tion zone near the middle of the pass. S o provision was made t o take samples between the stages but the distillate from stage 2 was analyzed by measuring the refractive index and converting to concentration, using data reported pieviously for the mixture (3). Results from the first pass lend themselves to mathematical analysis since the composition of condensate from the falling stream depends only on the temperature and composition of the stream, as was established previously for this mixture ( 1 ) . If the composition of distillate from stage 2 is known, one can calculate the composition of liquid in stage 2, and by a material balance the composition of liquid in stage 1 is obtained. Values of the effective relative volatility may then be assigned to each stage. I n this treatment the over-all separation coefficient is defined as y2(1 - rl)/zi(1 - y2) and the effective relative volatility for each - y"), where n is the number of the stage is y n ( l - r,&)/z,,(l stage and z and y are the compositions of liquid and vapor, respectively, expressed as mole yo di-2-ethyl hexvl phthalate. Some of the observed data for the first pass and the calculated effective relative volatilities are listed in Table 11, average values for each fraction being reported. I n this table, RJ is the residue in stage 2 at the end of the pass (about 150 ml.) and R, is the residue in stage 1 (about 20 ml,). ht the beginning of the first pass, thr initial distillate contained 92.1 mole yodi-2-ethyl hexyl phthalate, from which the initial over-all separation coefficient is 10.6 and the relative volatility for the first stage is 3.32. The effective relative volatility data and over-all separation coefficients are shown graphically in Figure 2 and it is observed that the efficiencv of stage 1 is about 65% of that of stage 2 for the operating procedure described.
ANALYTICAL CHEMISTRY
494
but the refractive indices differ by 0.0376. Contamination of the phthalate by 0.1% of suberate can readily be detected by Effective Over-all Relative measuring the refractive index, while Separation Volatility, rate measurements to 0.5% can deterCoefficient Stage 1 7.69 2.40 mine a minimum of 4.2% of suberate. I t 6.29 1.98 may also be shown that isomers having 6.20 1.97 6.68 2.13 the same number of carbon atoms are 6.31 2.02 6.98 2.26 often detectable as a result of large dif6.69 2.20 ferences in rate of evaporation, while the 6.13 2.07 5.28 1.83 refractive indices are almost identical. 5.43 1.92 .. .. From these simple examples it is clear .. .. that measurements of the rate of evapo.. .. ration and the refractive index form a sensitive cross-check in assessing the homogeneity of high-molecular-weight fluids being purified by vacuum distillation and in determining the degree of fractionation of complex mixtures. Using a falling stream as the source of vapor, a high order of accuracy and reproducibility in the rate measurements is readily and conveniently obtained. There is sufficient latitude in operating the two-stage
Table 11. Observed Data for First Pass and Effective Relative Volatility in Each Stage Fraction No. 1
Val. Distilled 4.4
2 3
21 13 .. 91
5 6
39.4 48.1 56.0 65.6 74.4 83.2 90.1
4
30.6
7 8
9 10 11 (110
Rn
ml.)
RI
.. ..
Temp., c. Stage 1 Stage 2 165 147.0
88.9
Effective Relative Volatility, Stage 2 3.20
~1
ZI
70.9 61.0
167 168 168 168 169 172 173 174 175 178
11 44 78 .. 95 148.8 149.6 150.4 152.0 155.0 157.9 l59,6 159.8
85.0 82.5 80.3 74.9 69.7 57.8 39.7 18.8 5.2 0 2
3.17 3.15 3.14 3.12 3.09 3.04 2.96 2.88 2.83 2.82
64.2 60.0 56.5 48.9 42.7 31.1 18.2 7.4 1.9 0 7
..
..
0 0 0.0
..
0 0 0.0
..
,.
1
20
..
1
I
I 40
1
I
60
80
47.4 43.2 37.9 32.1 24.8 17.0 9.7 4.2 1 0
.. .. ..
Volume % distilled
Figure 3.
Variationof Liquid Temperaturewith F-olume Per Cent Distilled at Constant Rate 0 First pass A
Secondpass
0 Third pass
1 V
The two esters were separated further in the second and third passes through the stills, with the results shown in Figures 3 and 4. These data are not easily expressed in terms of relative volatility but they demonstrate the degree of fractionation obtained with each pass. After three passes, about one third of the original di-2ethyl hexyl sebacate had been separated in a relatively pure state (better than 99.5%) and one third of the original di-2-ethyl hexyl phthalate had a purity of better than 98%. DISCUSS103
I n a homologous series of phlegmatic liquids, the addition of one or two CH1 groups alters the refractive index only slightly but produces a large change in the rate of evaporation. For example, di-2-ethyl hexyl suberate has a rate 170% greater than that of di2-ethyl hexyl sebacate a t 150” C., but the refractive index is only 0.0014 greater. By measuring the rate to 0.5%, which is readily done by the falling-stream technique, a 0.3% contamination of suberate in a sebacate sample can be detected. With the precision refractometer, measuring refractive index to about 0.00003, a minimum of 2.1% suberate can be detected. On the other hand, two compounds from different homologous series may have nearly the same vapor pressure and rate of evaporation, but the refractive indices are often very different. As an example, the rate of evaporation of di-2-ethyl hexyl suberate is only 12% less than that of di-2-ethyl hexyl phthalate a t 150’ C.
20
I
’
40 60 Volume % distilled
-80
0
Figure 4. Composition of Condensate from Stage 2 as a Function of Volume Per Cent Distilled 0 First pass A
Second pass Third pass
still that most of the adjwtmentv in heat supply and feed rate can be made at the time the condensate receivers are changed. The holdup of liquid in each stage is large but where the initial charge is 2000 ml. or more, the apparatus should have wide applicability. ACKNOW LEDG\I EXT
The authors gratefully acknowledge the assistance of K. C. D. Hickman, both in the experimental work and in the preparation of the manuscript. LITERATURE CITED
(1) Hickman, K. C. D., and Trevoy, D. J., Ind. E ~ L Q Chem., . 44, 1882 11952).
(2) Ibid.,p’1903. (3) Trevoy, D. J.. Ibid., 44, 1888 (1952) RECEIVEDfor review October 8, 1953. Accepted November 19, 1953. Communication KO.1619 from the Kodak Research Laboratories.
