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
782
TABLE11. EFFICIEXCY TESTSou .4x OPES-TUBE CoLmm 13 11V. I S DI.I\IETER A N D 180 CV. (6 FEET)L O K G Head R a c k Prescure. ny Pressure, l l m Propylene Theoretical M m Hg
Glycola
Pot
Head
Plates
1.00h
1.? 32
1.4437 1,4437 1,4437 1.4148 1,4148
1.4610 1,4533 1.4554 1.4118 1.4122
4 ,5 5.9 3.0
1.00h 1 00b 720;
40
2 6
ix
-
2 9
3.8
1 mm. propylene glycol = 0.08 m m . mercury. b Di-n-butyl phthalate-di-n-butyl azelate test misture. a C
MethylcSi~loheuane-n-heptane test mixture.
TABLE 111. EFFI~IESCY TESTS AT 1.00 1111. OF ~ I E R C T 'HEAD RY PRESSURE os AS - k , c x I s i - i r SFIR.\L-PACRED C ~ L C I I40 X 1I)r. I N DIA~IETER ASD 120 CM. (4 FEET) LOSG Back Pres-ure,
M m Propylene
5'
Pot
Head
Theoretical Plates
11
1.44i3 1.4473 1.4473 1,4473
1 4830 1.4806 1,4787 1.4790
11.7 10.7 10.8
20 40 50 5
n
GIycolU
1Z.B
Vol. 39, No. 6
Rectification-efficiency tests twre made a t several back prrssures (Table 111). This column has 11 t o 12.5 plates in its useful operating range, and no difficulties due t o false head-temperature rradings Jwre encountered. The results of these tests were confirmed tiy act,ual dist,illations, in which the column performed w r y ,qati?factoi,ily in appliuat,ions n-here its limit,ed efficiency w:is sufficicnt to accomplish separation. In the cvcnt that low-pressure columns are encountered n-hich are t,oo efficient, to he evaluated with the n-butyl Psters of phthalic and azolaic acid, a trst mixture based on the corresponding ctt,hyl esters can probably tie developed which xvill he applicablr. .\rcording to the litrrature (S),ethyl esters of azelaic and phthalic. acids h:Lm a difference in boiling points of 5' C. at atmosp!irric pressure, and the phthalate ester is the higher boiling. If t hcw ralucs arc cwrrect, the differcnw in hoiling points ~ o u l t I probatily !ir somcivhat lem than 5' at, 1 mm. prcswre. An S-Y diagram of these esters would therefore permit. c~fEc~irxic.ic~s in exress of thirty plates to be determined. ACKNOWLEDGMENT
1 m m . propylene glycol = 0.08 mm. mercury.
temperatures in the experiments a t 1.00 mni. of mercury ~ ( ' r r higher by a t least 1.5' C. than was to he expected for the opcrating pressure and indicated that a core of hot vapor, superheat,edso far as the head pressure was concerned, \vas passing up t'hrougli the tube without, contacting the refluxing liquid sufficiently t o c:tu~t: it to lose its superheat. The second column, specifically designed for Iov pressure (iix'ration, consisted of a 40-nim. diameter Trubore t,ube fitted rvith a 120-cm. (4 feet) long spiral packing made by turning a squ:tre thread in a solid aluminum rod. T h e spiral had a root diarnc,tctr of 12 mm., a pit,ch of two turns per 25 mm. (1 inch), and a thread thickness of 2 mm. Heat losses w r e minimized by a heatcd jacket. Boil-up rate n-as maintained manually and indicated hy a back-pressure manometer. I n other respects the spiral-packcd column was similar to the open-tube column.
Thc. author n.iGhes to thank Jack G. Hershelnxtn of this labirratory for valuable assistance in designing and constructing the low pressure columns used in this xork, and for the plate tletc,rminations made a t atmospheric pressure on the open-tuhr collimn.
'
LITERATURE CITED
:I., P11.D. thesis. Univ. of Pittsburgh, 1952. ( 2 ) Fenske, l f . R.. ISD. ESG. CHmr., 24, 482 (1932). (3) Handbook of Chemistry and Physirs, 20th ed., Clevc,latitl, Ohio. Clieinical Rubber Pub. Co., 1945. (1)Hirkman, K., J . Am. Chem. Soc.. 5 2 , 4714 (1930). ( 5 ) AIcCahe, W. L., and Thiele, E. W., 1x11.E s c . CHEZI.,17, 605 (1925). ( ( 5 ) Othmer, D. F., ISD. E s o . CHEx.. . k t r , . ED., 4, 232' fIO32'j. (7) Stiiits. -is, Z. phusik. C'hem., 39, 385 (1902). ( l j Bishop, C .
T E D as part of the Thirteenth Aiinual Chemical 1,:nwiieeriw Synrpoeinm of the Diviqion o i Indiisrrial a n d Engineering Chemir. Tlle refrac>tive index-conil,o.itioi~ I t,i;,tioi.s uf these cnn he e x p r r ~ ~ eby d second-orc!ri. rquatioiis. For tlic E I 11'-Ells : n',"
diere
S
=
=
1.1187
+ 0.03105 + 0.0048X2
(1)
mole fraction of EI-IP
783
site the groove in lantern ring P is soldered :I small tube wliich permits oil to flax in and around lantern ring I' from reservoir cup S. Cup 1' attached to tlie s h f t collects :iny sealiiig oil which may pass through the gland and thus avoids contnmination of the test mixture. Disk I; serves to deflect tlie v a p o ~tu ' the cold walls of the condenser and thereby prevei:t;: tlie eacape of condensable vapors to the v:icuuni system. The internal heater, T7, is :I ~/,-incli-dian~eter coil cbf barc KO.27 Sichrome resistance wire having a total capacity of 300 Ivatts. 'The ends of the coil are welded to tungsten rod>which are, in turn, fused into the glass by means of graded gl wall of the boilel, is 11-nundwith ieveral turnsj T, of h r e Sichrome wire KO. 31 having :I total wattage of 260. The n.ire is f:istened t o t h r glass by small dali> of S s r i e ~ ~ e i cenieiit ~en S o . 7.
For the determination of liquid-vapor equilibrium diagrams in high vacuum an equilibrium still was constructed with features designed to give consistent data at pressures in the neighborhood of 0.1 mm. The liquid-vapor equilibrium diagrams for two binary systems, di-2-ethylhexyl phthalate-di-2-ethylhexyl sebacate and di-n-octyl phthalate-di-2-ethylhexyl sebacate, were determined at 0.1 mm. Relative volatilities were calculated, and the relation of refractive index to the number of theoretical plates at total reflux was estimated for each system b y means of the equation of Beatty and Calingaert (7).
The o h e r v e d data fit this equation wit11 :i maximum deviation ( J f 0,0002 and an average deviation o f 0.00009. For the s p t e m
SOP-EHS: rif:
wlic)r(x S
=
=
1.4487
+ 0.0280S
f 0.0052X2
(2)
mole fraction of S O P
The observcd data fit this equation with a niasiinuni deviatiori of 0.0002 and :in average deviation of 0.00006. T1iei.e data are given in T:il)les I :urd 11. Colunin 4 of these t:ililcs indicate:: the extent to n.liich the sniootlied data calculated from the equations dpvi:itrs from :I straight line relation..
ln W
- I
u
z
W' J
a
- 0
ln
I T P - I
I I/ I
EQUILIBRIUM STILL
Figure 1 shows the equilibrium still. The still proper is constructed entirely of Pyres, whereas the stirrer shaft and top closure are fabricated of metal. The still consists of boiler .-and I condensing region B , which is surrounded by water jacket C; A is separated from B by an alembic-that is, a collecting gutter. The alembic, 11, serves t o collect the distillate and conduct it t o collecting cup E. The condensate enters cup E a t the bottom and overflows tlirougii calibrated drop counter F into the boiler. Cup E has a volume of only 3 cc., so that the contents are continuously and rapidly renewed. I t is fitted x i t h a 14/35 standnrd tapered glass stopper to permit access t o the distillate for s:impling and nnalysis. The still is incorporated into a vacuum system a t G and G'. Stirrer sliaft H is a piece of 3 / 1 6 - i ~ drill ~ ~ h rod, the lover end of which is fitted with a spring-loaded, I , hemispherical bearing, J . Ue:iring J rents in depression K and thus prevents the lo~verend of the shaft from whipping. Spring I maintains the bearing in tension against the glass. The impeller, L, is a steel rod, X inch. attached t o the hub of the shaft. The upper end of the shaft passes through closure plate M and vncuum gland AT. Plate LWcontains a l/c-iiich recess to accomniod:ite a rubber ring gasket nliich forms the vacuum seal between the glass and the plate. The gland is silver-soldered to the plate. The packing consists of a layer of graphited asbestos, 0, a lanterii ring, P , having n circumferential groove and four holes, a second layer of graphited asbebtos, O', a collar, Q, and a nut, R. Directly oppo-
-G'
V
l
K
'
Figure 1.
'J
Equilibrium Still
INDUSTRIAL AND ENGINEERIhG CHEMISTRY
784
Vol. 39, No. 6
justed t o the correct value and maintained during the measuremerit. Mole Distil1:ttion is continued under these conditions until a steady Deviation Fraction rag Calcd. by from Straight state has been reached. To measure the effects of the component EHP .g Obsvd. Equation 1 Line 0.000 1,4487 1.4487 separations, the heaters are turned off and the pressure is imme0.149 1,4534 1,4534 - 0 : 0006 diately increased to a few millimeters to st,op distillation. \\-hen 0.247 1,4568 1.4567 0.0008 0 363 1.4606 1,4607 -0.0010 the fluid has cooled, the vacuum is released, first by opening stop0.486 1,4647 1,4648 -0.0012 cocks 10 and 14 and then carefully opening 13. The stirrer0.559 1.4675 1,4676 -0.0011 0.680 1,4722 1,4720 - 0.0010 gland msembly, 9, is removed, and a drop sample of the pot liquid 0.777 1,4756 1,4756 - 0.0008 is v-itlidrnwn by a long capillary. -4 sample is also taken from 0.848 1.4784 1,4785 -0,0005 1,000 1,4845 1,4845 .... the collecting cup, 15, by a similar procedure. Refractive iadices are determined as the samples are taken. Several experimental determinations using 49.5 mole TABLE11. REFRACTIVE IXDICES OF NOP-EHS ~ I I X T U R E S in E H S proved that a 30-minute period is required for equ Mole Deriation after distillation attains a constant rate. Continued distillation Fraction ."D" Calcd. by f r o m Straight NOP n y Obsvd. Equation 2 Line for periods as long as 2 hours results in no additional change in the 0.000 1,4487 1 4487 composition of the distillate. 0.137 1.4526 1,4526 -0:0007 0.224 1.4552 1.4552 - 0,0009 0,341 1,4587 1.4688 -0,0012 EVALUATION T.ABLE I. REFRACTIVE IXDICES O F EHP-EHS MIXTLRI,:.;
0.452 0.552 0,705 0,805 0.887 1,000
1,4624 1,4657 1.4710 1,4745 1.4776 1,4819
1.4623 1,4657 1.4709 1,4747 1 4776 1.4819
-0.0014 -0.0013 0.0012 - 0.0007 - 0.0005
-
....
OPERATION
In operation the equilibrium still, 1,is incorporated in a vacuum system as shown schematically in Figure 2. The butyl phthalate manometer, 2, and Pirani gage, 3, are both included for versatility in pressure measurements The cold trap, 4,is added to protect the fluid of the mechanical pump, 5, from volatiles anti possible products of decomposit'ion which may he generated by the boiler of the equilibrium still. The mechanical pump, 5 , ha, a speed of about '/2 liter per second. The 4-liter glass hottle, 6, serves as a ballast to even out pressure fluctuations. A11 tuhulations connecting the component parts of the system :ire m:& o f 15-mm. Pyrex tubing. The t'ubes are joined hy heavy-willed rubber tubing which has been anointed with castor oil. Jl'herc a valving arrangement is necessary, large Pyres stopcocks :ire used. The electrical energy supplied t o the internal heater. 7 , ol' the boiler and to the coliiniii henter. 8, is controlled by individii:il autotransformers. The still system shown in the dingrani should be I d i - t c x - t t . d i)efore the test niisture is adniitted for experimerit:il \vurk. tight s y k e m will 't)e e v x u a t e d t o 50-25 niicrouz iri 10 15 niiirrites. T o charge the test mirture into the still, the .tii,rer-gl:inti :I>sernl)ly, 9, is removed. .%bout 7 5 c c . of the misture ; I ! ' C S i)oii! cvl through a long-stemmed funnel itito the hoilpi,. The stirrer-&ind i.; replaced, making certaiii gasket is properly w i t e d , Stopcocks 10, 12, nnd ed to the ope11 pwitioir while stopcocks 11 and 13 m'e closed. The mechanical puml) is started, and, when the desired pressure is reached, >topcocks 10 and 14 are closed and 1 1 is opened. The still and ballast n,ystem are now under a static pressure, tlie value of which is indicated by t h r slim of the two gages. The electrical liraters of the still boiler and column are energized, and, as t,he mixture heats atrd di.>tillation finally sets in, the pressure is maintaiiied a t its prbper value by manipulation of *topcocks 10 and 13. When the desired distillation rat? is reached, the presstire is finally ad-
The evaluation of the performance of this equilibrium still elitailed a determination of the individual effects of the two accessories, the column heater and the pot stirrer. These effects were determined by comparing the performance of the still alone and with three combinations of these accessories. The relation of component separation to distillation rate wvau employed to compare these separate methods of operation a t 0.1 mm.pressure using the EHP-EHS mixture containing 49.5 mole r; E H P . The data are plotted in Figure 3. The scale of vapor conipositioris is exaggerated in the figure t o depict more clearly the variations produced by the different methods. The operation of the equilibrium still in conjunction Yith the colunin heater gave constant values a t distillation rates of less th:tn 180 cc. per hour. .It higher rates the difference between the liquid : ~ n dvapor concentrations decreased appreciably. Kithout the use of the column heater the vapor composition decreased coiistmtly with increasing rate. Extrapolation to zero rate of t1ic-e tiut:i gave a value of vapor composition almost equivalent ~ I the I constant values obtained when the column heater wvas em{11(1yed, TT-ithout the column heater much greater pot heat inpits \\ rre nreded to maintain a given distillation rate. The lower c~c~tii~~uiient separation obtained under these conditions may be due 1i:vti:illy to a superheating of the pot liquid. The u,e of the pot stirrer should minimize the effects of superIii,ating nrid might nlio contribute to a better state of equilibrium 11y m:iitit:iining a more uniform liquid concentration. At the s:liii(' time tlie rotating shaft might cause some fractionation of the tiistilkite. which is an undesirable factor. K h e n the pot stirrer \\-:is i i v d n-ithout a columii heater, all u i these factors apparently
Figure 2.
Equilibrium Still Assembly
INDUSTRIAL AND ENGINEER1
June 1947
G CHEMISTRY
785
various distillation rates and extrapolating the data to determine the equilibrium vapor compoPRESSURE : 0.10 mm. sition at zero distillation rate. Emplo-ing both the pot stirrer and the COLUMN H E A T E R . STIRRER +t ineffective, for the curve rapidly approached thnt producrd hy APPLICATION the hasic still alone. K h e n the equilibrium still was used in conju~ictionwith the IIIobtaining the liquid-vapor equilibrium data for the two syscolumn heater and the pot stirrer, constant results were obtained tenis under investigation, e:ich ohservation \vas made at a conat distillation rates of 30 to 150 cc. per hour. n-hich were slightly stnnt distillation rate betn.ern 30 :rnd 1-10 cc. per hour after dishigher than those observed n-ithout the pot stirrer. However, tillation had continued for a minimum of 30 minutes. The presthis difference is of small signiflcance relative to the large comw r c war d r a y s within the rnngr 0.10 = 0.01 mm. Nosignificant ponent separation effected behveen the liquid and vapor phases. variation in the vapor or liquid equilibrium compositions would be The stirrer continues to exert a fractionating effect at di~ti1l:rtion Prpected within this presure rnnge, since the relative volatility rates below 30 cc. per hour. Reproducible value$ may he obtained without the use of the pot stirrer. If the column heater is proper13- employed in distilling a t a rate of less than 180 cc. per hour, a single reading will suffice to T.ZBI.E11-. V . W O R - I J Q ~ TEQI-ILIBRIA I~ AT 0.10 h I x DI-~-OCTYL 1'HTH.\I,ATE AS11 DI-2-ETHTLHEXYL SEB.4CATE indicate t,he liquid-vapor equilibrium composition of a given Relative binary mixture. Kithout the uce of the column heater q i i i i i h o-r Liquid -_ .2 6 \ n p~. Volatility resiilt9 m:iy he o1it:IitierI b3- making :I series of o1werv:itioiis :it a D 1.ih-g;; I1 ;5 X(Y0P) XEnp
I N L l O U l D : 0.488
-
T.4BLE 111. 'I:.4POR-LIQL-lD EQI-ILIBRIA AT 0.10 1111. FOR DI-2ETHYLHEXYL PHTH4L.1TE . i S D DI-2-ETHYLHEXl-L SEBACATE Vapor
n
:'
1 4527 1 4562 1 1 1 1 1
4566 4585 4599 4636 4643 1 4664 1 4691 1 4708 1 4709 1 4728 1 4734 1 4740 1 4753 1 4758 1 4763 1 4777 1 4800 1 4811 1.4817 1,4824 1.4836 1.4837 1 4837
Relative \-olatilit>
Liquid n g5
~(EHP)
0 127 0 232 0 243 0 300 0 340 0 448 0 468 0 526 0 603 0 648 0 650 0 701 0 717 0 733 0 767 0 780 0 792 0 829 0 888 0 915 0.930 0,947 0.975 0.978 0 978
-=(E E P)
0 045 4501 0 097 4518 0 097 4518 4529 0 132 1 4533 0 145 1 4558 0 220 1 4561 0 229 1 4576 0 274 1 4597 0 335 0 387 0 4615 1 4618 0 396 1 4639 0 455 1 4642 0 465 1 4650 0 488 1 4665 0 532 1 4675 0 557 1 4683 0 580 0 642 1 4706 1 4745 0 747 0 785 1 4760 0 819 1,4773 1.4794 0.872 1,4823 0.945 1.4824 0,947 1.4620 0.937 -4verage Arerage deviation average deviation 1 1 1 1
U
3 10 L 81 2 99 2 82 3 04 2 68 2 96 2 97 7 02 2 92 2 63 2 83 2 91 2 88 2 90
5- 76 63
2 io 2 67 2 95 2.94 2.62 2.27 2.49 2.99 2.85 0 . 13 4 5
1,4502 1.4516 1.4529 1 4537 1 4550 1 4562 1 4569 1 4578 1 4569
1 4600 1 4600 1 4603 1 4620 1 4625 1 4635 1 4642 1 4662 1 4665 1 4666 1 4667 1 4662 1 4683 1 4702 1 4717 1 4724 1 4728 1 4741 1.4742 1.4752 1 4758 1.4769 1.4787 1 ,4803 1.4807 1.4809 1.4812
0.056 0.106 0.151 0.177 0.221 0 260 0 262 0 312 0 347
0 360 0 380 0 390 0 443 0 457 0 488 0 510 0 572 0 579 0 582 0 585 0 628 0 631 0 685 0 728 0.748 0.756 0.794 0 797 0 823 0 839 0 867 0 915 0 957 0.968 0.973 0 980
1 4498 1.4510 1,4521 1 4527 1.4540 1 4549 1 4557 1 4565 1 4577 1 4584 1 4585 1 4588 1 4605 1 4609
.
1 4620 1 4627 1 4647 1 4650 1 4650 1 4652 1 4666 1 4667 1 4687 1 4705 1.4711 1.4717 1.4730 1.4731 1.4742 1.4748 1,4759 1,4781 1.4799 1,4804 1.4807 1.4610
0.042 0.085 0.124 0.145 0.188 0 218 0 243 0 270 0 307 0 330 0 333 0 344 0 396 0 408 0 443 0 464 0 526 0 535 0 535 0 542 0 562 0 585 0 643 0 695 0.711 0.728 0.763 0,766 0.797 0.812 0.842 0.900 0.947 0.960 0.968 0.976
.4verage Average deviation 7' average deviation
1 35 1 28 1.26 1.27 1.23 1 26 1 23 1 23 1 20 1 24 1 23 1 22 1 22 1 23 1 20 1 20 1 21 1 20 1 21 1 19 1 22 1 21 1 21 1 18
1.21 1.17 1.20 1.20 1.18 1.21 1.22 1.20 1.24 1.26 1.19 1 21 1.21 0.023 1.9
I bo--
36
Vol. 39, No. 6
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
786
I
1
7
I
,og n
:
Such a graph may he developed by tlie u>e of Equation 4 from Beatty and Cnlingaert (I),
I
I
, __-____-
~ - _
Xn(l 'XOI
Xo(1 -Xnl
I
i n nliich the number of theoretical phtes'nt total reflux, n, necessary to maintain equjlihrium between the two liquid compositions (espressed :IS mole fractions) of a binary mixture Xo and X, m:iy be calculated, provided the relative vola--__ ___tility, a , is kno\vn for this coilcentration 'range. S,, m u d be greater than X3. T h e accuracy of n adepends on the conrtnncy of CY. The calculations are sumniarized i n T:ilih V 4 nnd VI. By anchoring a t the specific coricentra1.4460 I4540 1.4620 1.4700 1.4780 1.4860 tion Xn,a series of points are calculated relating REFRACTIVE I N D E X the concentrations of mixtures X, (column 1) to the number of plates a t total reflus, 71 (column 21, Figure 4. Refractive Index Plotted against Number of Theoretical Plates required to separate ' these niisturcs from the originally specified composition Xa. I n column 3 of a binary system does not vary appreciably Ivithin sniall prestefriirtive indices a t 25' C. are listed for the corresponding sure changes. However, care vvas taken that the pressure did liquid concentrations X,. The resultant curves for both not vary during an individual measurement, since this would sudsystcms are plotted in Figure 4. denly alter the rate of distillation and thereby disturb the eqiiilibrium in the system. The apparatus s-as completely cleaned after every three o r four TABLEI-. REFRACTIVEINDEX us. KCJIBERO F PLATES FOR SYSTEM 2-ETHYLHEXYL PHTH.ALATE-2-ETHYLHEXYL SEBACATE readings to reduce the possibility of erroneous results due to conAT Xo = 0.010 tamination or possible decomposition of the materials. The Mole JIole observed d a h are recorded in Tables 111 and IV. The relative Fraction S o . of Fraction Su.of EHP i n plater, 71%' for EHP in plates, it2 for volatilities given in the last column of these tables zre calculnted Vapor, S,. n x n Vapor, X n n X, by Equation 3:
(3) where x, y = equilibrium liquid and vapor mole fraction,
