November 19%
INDUSTRIAL AND ENGINEERING CHEMISTRY
With reference to distillation of heabscnsitive materials, however, the advantages of inverted batch operation have apparently not been recognized. No comparison is made in this paper with respect to time required to reach equilibrium or to relative ease of operation of the two processes on a laboratory and pilot plant scale. CONCLUSIONS
The theoretical considerations substantiate the conclusions presented by Lloyd (6)to the extent that the advantage leans to the batch distillation aa operation approaches a rectification and swings toward the continuous still as the procedure becomes that of stripping. However, the calculated advantage of continuous distillation disappears altogether if inverted batch distillation is considered the appropriate batch operation for stripping. The theoretical calculations indicate that the batch type of distillation is superior over a much wider range than is indicated by Lloyd if the distillations are performed in identical columns. The batch process is indicated as having its greatest advantage for charges With proportions Of the two components and this is enhanced by the presence of appreciable holdup and total reflux
2611
start-up. Batch operation has some advantages because of more complete use of column capacity throughout ita entire length and also because it is not always necessary to distill the complete charge of a binary mixture in batch operation. UTERATLJREW E D
(1) Cannon, M. R., IND. ENQ.CHEM.,41, 1953-5 (1949). (2) Houston, R., M.S.thesis, The Pennsylvania State College, 1947. (3) Kahn, H. A., Ibict., 1949. ENQ.CEEM.,ANAL.ED.,17,590-2 (1946). (4) Langdon, W. M., IND. (5) Lloyd, L. E., Petroleum Refiner, 29, 135 (February 1950). (6) Prevost, C. F.. M.S. thesis, The Pennsylvania State College, 1948. (7) Rose, Arthur, Johnson, R. C., and Williams, T.J., IND. ENO. CEEM.,42, 2145-9 (1950). (8) I b X , in press. (9) Rose,Arthur, and Williams, T. J., Ibid., 42, 2494-7 (1950). (10) Rose, Arthur, Williams, T. J., and Prevost, C. F., Ibid., 42,18769 (1950). (11) Williams, T. J., M.S. thesis, The Pennsyivania State College, 1950. RBCBIVBD August 80, 1960. Presented before the Division of Induetrjal and Engineering Chemistry at the 118th Meeting of the AMERICAN C~EMI~AI, SOCIETY,
Chiaapo. 111.
Performance of Packed Columns during Batch I
J. ERSKINE H A W K I N S AND 1. A L L E N BRENT, JR. UNIVERSITY OF FLORIDA, GAINESVILLE, FLA.
The studies were made because no single test mixture was known for testing columns operating at 20 to 760 mm, of mercury, no data were available on the effect of reduced pressure on column Performance, and no satisfactory method of comparing column performances at different pressures and finite reflux ratios was available. These studies have produced the test mixture, ethylbenzene-chlorobenzene, which can be used to determine the number of theoretical plates over the pressure range indicated above, and have shown that at total reflux the
columns tested have the same number of theoretical plates regardless of the pressure, and have resulted in a simplified method of comparing column performance under different conditions when the reflux ratio is finite. I t should now be possible to evaluate columns with 60 to 70 theoretical plates at various pressures and over a wide range of operating conditions. Columns having 100 to 200 theoretical plates may be evaluated by using the mixture rc-heptane-methylcyclohexane at a pressure which will give an appropriate value of the relative volatility.
MIXTURES FOR TESTING DISTILLATION COLUMNS AT ATMOSPHERIC AND REDUCED PRESSURES
AS
PART of the program of determining the effect of reduced pressure on packed column performance, it was desired that a test mixture be available for testing columns at both atmospheric and rcduced pressures. Since it has been shown that different mixtures sometimes give different results (39),it was desired to have a mixture that could be used a t both atmospheric and reduced pressures. To investigate the effect of change of relative volatility with pressure on column performance it was desired to have two mixtures available, one of which would show an increase and the other a decrease in relative volatility as the pressure was lowered. A number of mixtures are available for testing columns at at-
mospheric pressure. Some of the most frequently used are ethylene dichloride and benzene (19),benzene and carbon tetrachloride (39),benzene and toluene (fO),n-heptane and toluene (39),nheptane and methylcyclohexane (8Q and rnethylcyclohexane and toluene (27). However, for determining the number of plates a t reduced preasures only a few mixtures have been described, and these do not have all the properties desired. The data for ethylene dichloride and benzene have been determined a t pressures from 760 to 100 mm. of mercury by Bragg and Richards (7). These data show R deviation from ideality, with the relative volatility changing with concentration. Furthermore, because ethylene diahloride is
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
2612
subject to hydrolysis, its corrosive action makes it undesirable to use in columns containing metal screen packing. Recently two mixtures, ndodecane-cyclohexylcyclopentane and tridecanedicyclohexyl have been recommended (14) for evaluation of vacuum rectification columns. Unfortunately these materials are not readily obtainable. In addition, the properties of these mixtures deviate from ideality and because the relative volatility increases as the pressure decreases, columns with more than 20 plates at 20 mm. cannot be tested with these substances. The mixture n-ocbe-toluene ( 6 )has been used at atmospheric and reduced pressures. However, the wide variation of the relative volatility with composition is an undesirable property. The mixture n-decane-trans-Decalin (18, 37) has also been suggested for use at reduced pressures, but at pressures of 50 mm. of mercury or higher the relative volatility is too large to permit testing columns with a large number of theoretical plates. Also the magnitude of the change in relative volatility with small pressure changes is not desirable. Following are the desirable characteristics which a test mixture should have if it is to be used a t pressures from 760 to about 20 mm.
Their refractive indexes were measured a t 25' C. using a Bausch and Lomb Abbe refractometer. These data are shown in Table I. The equation, x = a 610.252n - 190.682 nsrepresents therelation between the mole fraction of chlorobenzene, x , and the index of refraction, n, at 25' C., where a = -486.079 for values of x b e tween 0.20and 0.80 and a = -486.084 a t other values of x . The maximum deviation from the experimentally observed values ia about 1%. Also, ng = 1.4933 0.02501 0.003182z*.
+
+
The mixture ethylbenzene-chlorobenzene was found to be satisfactory over the pressure range 760 to 20 mm. The relative volatility a t 760 111111. of mercury was found to be 1.10and a t 20 nim. it increased to 1.12. The mixture n-heptane-methylcyclohexane previously used at 760 mm. was found to be satisfactory for use at reduced pressures as low as 300 mm. The relative volatility at 760 mm. was taken to be 1.07 (1,6) and it was found to decrease to 1.05 a t 300 mm. Due t o the increasing difficulty of separation with lower values of relative volatility, a value of (Y is reached below which errors in determining the number of theoretical plates are magnified. ETHYLBENZENE-CHLOROBENZENE
SYSTEM
After checking the properties of many liquids, the mixture of ethylbenzene and chlorobenzene appeared to offer possibilities for use. The boiling points a t atmospheric pressure seemed to be about the correct distance apart and high enough so that the boiling point at 20 mm. would be above room temperature. Samples of these liquids were obtained by courtesy Dow Chemical Co., Midland, Mich. They were further purified by dietillation in an efficient column until they had the following properties:. Ethylbeneene Chlorobenzene
B.p. at 760 mm., 136.15" C., n y 1.4933 B.p. at 760 mm.,131 60° C., 1.5215
?&v
The values for ethylbenzene check closely the values given by the National Bureau of Standards (1)-a boiling point a t 760 mm. of 136.187' C., and ng of 1.49324. The values for chlorobenzene check exactly with values listed by Dow (23) and several of the many values listed by Huntress (211.
REFRACTIVE INDEX. The differencein refractive indexes of the two liquids permits their rapid analysis. Allowing an error of &0.0001 in reading the refractometer, the difference of 0.0282 unit between the refractive indexes of the pure liquids at 25" C. indicates an error of less than 0.5% in the analysis of the mixtures by this method. Known mixtures of ethylbenzene and chlorobenzene were made by weighing out quantities in a glass-stoppered weighing bottle.
+
OF ETHYLBENZENE AND TABLE I. REFRACTIVEINDEXES CHLoROBENZnNE Weight Ethylbenaene, Gram
....
1.8144 1.9876 1.8350 1.2948 1.4115 1,2300 0.8369 0,6781 0.4250 0.2178 f
Its components should be obtainable in a high state of purity or be easily purified. Its constituents should form a nearly ideal solution, with no maximum or minimum boiling mixtures. It should be stable and noncorrosive in the column. It should be capable of easy and accurate analysis. Ita boiling point a t the lowest reduced pressure should be above room temper2tture. Its relative volatility should be in the range 1.05 to 1.15 to permit its use with columns of high performance.
Vol. 43, No. 11
.
.
.
Weight Chlorobenzene Grams .
.
.
I
0.3803 0.4472 0.5180 0.8580 1.1651 1.3809 1.4251 1.7797 2.3006 2.4128
*...
Mo1.e Fraotion Chlorobenzene 0.0000 0.1806 0.1751 0.2103 0.3843 0.4378 0.5145 0.6173 0.7123 0.8362 0.9127 1.0000
nV 1.4933 1.4974 1.4978 1.4986 1.6034 1.6048 1.5070 1.5100 1.5128 1.5165 1.5187 1.5215
VAPOR-LIQUIDEQUILIBRIUM.The vapor-liquid equilibrium for the system ethylbenzene-chlorobenzene was determined at 760, 300,and 20 111111. of mercury using the Kjeldahl-flask type of apparatus described by Othmer (96). This apparatus was modified by having a condenser built around the tube leading to the vacuum line to prevent losses when operating at reduced ressures. An internal-type heater w&~lused-and the peck o f t h e flask and side tube were wound with reslstance w r e down to about 0.5 inch above the liquid level in the flask. This resistance wire was found necessary after runs were made with the ap ara tus lagged first with glass wool and then with the wire. i o & condensation occurred when the runs were made a t atmospheric pressure using the glass wool lagging. This was evidenced by the greater separation obtained than during a corresponding run with the neck and side tube heated with resistance wire. The current to the resistance wire was maintained high enough so that no condensation occurred. Slight superheating of the vapors would introduce no error. At the temperatures involved when the pressure was maintained at 20 mm., either lagging was found to be satisfactory. When operating under reduced pressure a manostat devised by Hershberg and Huntress (80) was used to control the pressure. To minimize pressure fluctuations a 5-gallon bottle was included in the system. Pressures were determined by use of a Zimmerlitype gage which could be read to 0.1mm. No variation in prepsure was detected during an experiment. The boiling points of the mixtures were approximately 35"C.at 20 mm. and 100' C. at 300 mm. Water a t about 18' C. was circulated throu h the condensers. OPERATING%ROCEDURE. About 200 ml. of sample were laced in the apparatus and the manostat was set for the desireipressure. The liquid was allowed to reflux gently for approximately 2 hours to ensure equilibrium. Samples of distillate and li uid were removed and analyzed by use of the refractometer. %he composition of the samples varied from pure ethylbenzene to pure chlorobenzeneat 760 and 20 mm. The data a t these pressures are shown in Table 11. The shape of the curves obtained by plotting these data indicates the mixture is behaving almost ideally. Using the equation y/(1 -y) = ( Y X / ( ~-x), the relative volatility, 01, was calculated for values o f t from the vapor-liquid equilibrium data. These data are also shown in Table I1 and are plotted in Figure 1. A straight line through CY = 1.10 a t 760 mm. and (Y = 1.12 at 20 mm. represents the data between the limits of 1 0 and 90 mole % chlorobenzene. In addition to the complete vapor-liquid data obtained at 760 and 20 mm., three points were determined a t a pressure of 300 mm. These data are included in Table 11. Since the relative volatility for this PLATECALCULATIONS. mixture is constant the Fenske equation (15) may be used for calculating the theoretical plates at total reflux. Using the equation in the form
November 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
2613
n-HEPTANE-METHY LCYCLOHEXANE SYSTEM
where xo * the mole fraction of the more volatile component in the distillate and x1 = the mole fraction of the more volatile component in the kettle, n, the number of plates in the column, waa calculated as a function of the composition.
71.08 A
I-
Ob
760 WW.
' 1
' 0:2 014 ' 016 ' 0:8 ' MOLE FRACTION CHLOROBENZENE I N L I Q U I D
1?O
Figure 1. Relative Volatility at Various Concentrations for System Ethylbenzene-Chlorobenzene at Pressures of 20 and 760 Mm. of Mercury 0.02, which remains conBy arbitrarily starting with 5. stant, and changing xo from 0.02 to 0.98 by intervals of 0.04, the equation wae solved for pressures of 760 and 20 mm., where a is 1.10 and 1.12,respectively. These data are shown in Table 111. In order to simplify the use of these data for oalculating the number of plates the compositions were expressed in terms of refractive indexes. These data are also shown in Table 111. A plot of refractive index us. theoretical plates for use with the Wt mixture ethylbensene-chlorobensene at pressures of 760 and 20 mm. is shown in Figure 2. It indicates that columns of 60 to 70 theoretical plates may be tested with this mixture.
One of the best mixtures available for determining the number of theoretical plates of columns at atmospheric pressure is nheptane-methylcyclohexane, using the data of Beatty and Calingaert (6)and Bromily ltnd Quiggle (9). Its use, however, has not until now been extended below atmospheric pressure. The vapor pressures of these pure liquids have been determined at the National Bureau of Standards (I). The vapor pressuretemperature data were plotted on a large graph and the relative volatilities calculated as the ratio of the vapor pressures at 760, 600,600, 400, and 300 mm. These data are given in Table IV, and show that the relative volatility decreases with decreasing pressure from 1.07 a t 760 mm. to 1.05 at 300 mm. It reaches a value of 1.00a t about 80 mm. It is suggested that this mixture may be used to test columns of a hundred or more plates. Such testa should be conducted at a pressure at which the relative volatility of the test mixture has an appropriate value. Samples of n-heptane and methylcyclohexane were obtained and these were purified further by distillation until they had the following properties : B.p. at 760 mm.. 98.40' C., ns8 1.3852 B.p. at 780 mm., 100.90° C., n v 1.4206
n-Heptane Methyloyclohexane
1.5200 U W
4
1.5100
W w
5 1.5000 0s
TABLE11. LIQUID-VAPOR EQUILIBRIUM OF ETHYLBENZENECALOROBBNZENB AT 760, 300, AND 20 Man. OF MERCURY Chlorobensene in Liquid np Mole fraction
Chlorobenzene in Vapor n58 Mole fraction
1.4933 1.4948 1.4980 1.4978 1.4990 1.5004 1.6018 1.6032 1 .SO47 1.5055 1.5070 1.5080 1.5093 1.5112 1.6123 1.5135 1.6147 1.5160 1.5173 1.6182 1.5190 1.6216
Total Preasure of System, 760 Mm. 0.000 O.Oo0 0.060 0.067 0.115 0.106 0.191 0.176 0.240 0.221 0.291 0.273 0.348 0.326 0.399 0.378 0.457 0.432 0.486 0.461 0.539 0.515 0.574 0.550 0.818 0.594 0.678 0.658 0.716 0.696 0.756 0.736 0.793 0.777 0.833 0.820 0.874 0.864 0.904 0.895 0.927 0.921 1.000 1.OOO
1 . Eo20 1.5065 1.5161
Total Pressure of System, 300 Mm. 1.5027 0.359 0.333 1.6073 0.625 0.497 0.824 1.5186 0.840
1.4933 1.4960 1.4971 1.4987 1.5007 1.5027 1.5049 1.5070 1.5079 1.5090 1.5112 1.5132 1.6159 1.5177 1.6190 1.6216
T o t d Pressure of System. 20 Mm. 1.4933 0.000 1.4963 0.107 0.118 0.150 0.165 1,4978 0.226 0.210 1.4991 0.285 0.311 1.5014 1.5035 0.359 0.388 0.439 0.468 1.5057 0.614 0.543 1.5078 0.546 0.574 1.5087 0.584 1.5098 0.611 0.659 0.685 1.6120 0.726 1.5139 0.749 0.817 1.5164 0.833 1.5180 0.888 0.877 0.930 0.921 1.6193 1.000 1.5215 1.W 0.000
Relatiye Volatility
...
.126 .OW .lo6 .116 .094 .lo4
1.4900
PRESSURE PRESSURE
20 MU.
760 MU.
60 80 100 PLATES Refractive Index at 25' C. vs. Theoretical 20
40 THEORETICAL
TABLE111. DATA BOR DETERMININQ THEORETICAL PLATES USING ETHYLBENZENE AND CHLOROBENZENE AT 760 AND 20 MM.,
.lo8 .lo4 .lo2 .lo1 .lo8
00
0.02 0.06 0.10 0.14 0.18 0.22 0.26 0.30 0.34 0.88 0.42 0.46 0.50 0.54 0.58 0.62 0.66 0.70 0.74 0.78 0.82 0.86 0.90
.096
.loo .111
.099
.OW .093 ,105
.OS8 ... -12 .ll .I2
...
...
--
Figure 2. Plates for System Ethylbenzene-Chlorobenzene
.094
.117 ,121 .loo ,132 ,130 .124 ,118 ,118 ,119 ,126 ,126 ,118 102 135
A B
P
xn =e 0.02 Theoretioal Plates 760 mm. 20 mm. 00.0 00.0 12.0 10.1 17.8 14.95 21.8 18.3 24.9 21.0 27.6 23.2 29.9 25.1 32.0 26.9 33.9 28.5 35.7 30.0 37.5 31.5 39.2 32.9 40.8 34.4 42.6 35.8 44.3 37.2 46.0 33.7 47.8 40.2 49.8 41.8 52.0 43.7 54.3 45.6 56.8 47.3 60.0 50.3 64.0 53.8
n l"s 1.4933 1.4948 1.4958 1.4969 1 I4979 1 .4990 1.5001 1.5011 1.5022 1.6033 1.6044 1.5055 1.5066 1.5077 1.5089 1.6101 1.5113 1.5125 1.6186 1.5148 1.5160 1.5172 1.61%
TABLE IV. RELATIVEVOLATILITY OF I~-HEPTANE-METHHYLCYCLOHEXANE TESTMIXTURES AT V ~ I O UP~ESSURES S I
Pressure Mm. Merohry 760 600 500 400 300
Relatipe
Volatlllty 1.070 1.088 1.062 1.055 1.050
2614
INDUSTRIAL A N D ENGINEERING CHEMISTRY
TABLE v. REFRACTTVE INDEXES O F n-HEPTANE-METHYLCYCW HEXANE MIXTURLS
Mole Fraction n-Heptane 1 0000
.
1.3852 1.3880 1.3902 1.3925 1.3948 1.3978 1 ,3996 1.4017 1.4048 1 ,4080 1.4817 1.4150 1.4182 1.4206
0.9118 0.8527 0.7712 0.6997 0.6125 0.5538 0.6000 0.4130 0.3275 0 2268 0.1392 0.0622 0.0000 I
-
-
+
&HEPTANE AND METHYLCYCLOHEXANE AT PRESSURES OF 760 AND 300 MM., xn = 0.02.
20
These values are in close agreement with those given by the National Bureau of Standards (18). Since this laboratory is equipped to make routine analysis by use of the refractometer a t 25"C.,it was believed more desirable to determine the refractive index us. composition data for mixtures of n-heptane and methylcyclohexane at this temperature, than to apply a temperature correction to the data which previously had been obtained a t 20°C. These data are listed in 287.230n 92.0252 n2repreTable V. The equation, x = a sents the relation between the mole fraction of methylcyclohex&ne,x,and the refractive index, n,a t 25' C., where a = -221.307 for values of z between 0.1 and 1.0 and a = 221.337 for values of z below 0.1. The values of x calculated from this equation vary on the average about 1.5% from the experimentally determined values. Also n%' = 1.3852 0.030873 0.004531 x2. In order to verify the value for the relative volatility obtained
+
TABLEVI. DATA FOR DETERMININQ THEORETICAL PLATES USINa
nV
+
Vol. 43, No. 11
Theoretical Plates 760 mm. 300 mm. 00.0 00.0 16.9 23.4 25.1 34.8 30.7 42.0 35.1 48.8 38.8 53.9 42.4 58.3 45.1 62.4 47.7 66.2 50.3 69.7 52.8 73.2 55.2 76.4 57.5 79.8 59.9 83.1 62.4 86.4 64.8 89.9 67.3 93.3 70.1 97.2 73.0 101.2 76.2 105.6 80.0 110.9 84.4 116.8 90.0 124.9
n 66 1.4198 1.4182 1.4166 1.4160 1.4134 1.4119 1.4104 1 ,4090 1.4075 1.4061 1.4047 1.4032 1.4018 1.4004 1.3990 1.3876 1.3962 1.3948 1.3936 1.3922 1.3908 1.3892 1.3884
a t 300 mm. by taking the ratio of the vapor pressures, several experiments were run using the O t h e r apparatus. The value thus obtained agreed with the calculated value of 1.05, which shows that the two methods of evaluation give comparable results for this mixture. Using the Fenske equation described above, the data for the number of plates as a function of composition, were calculated for a = 1.07 and a = 1.05 for pressures of 760 and 300 mm., respectively. These data are shown in Table VI.
EFFECT OF REDUCED PRESSURE ON PACKED COLUMNS OPERATING AT TOTAL REFLUX
A
T T H E time that this work wss started there were no ref-
APPARATUS
erences available on the effect of reduced pressure on Two columns, one representing a fixed-type prtcking and the other a random type, were used in these tests. column efficiency. Before the completion of the investigation reI was a Viral originally ported herein, an article by Feldman, Myles, Wender, and Orchin Lecky and Ewe11 ($3) and improved in this laboratory by S t a l l appeared (14). They reported that their cohmn Was n-~ore cup, Fuguitt, arid Hawkins (36). Column 11 W'RS packed cient a t 150 mm. of mercury pressure at certain throughput rates, with 4mm. glass Raschig rin s. The column specifications and pro erties of the packing are stown in Table I. and less efficient a t other reduced pressures, than a t atmospheric &e kettles for both columns were 500-ml. round-bottomed pressure. They used different mixtures a t atmospheric pressure flasks with 24/40 standard taper for connection to tj,e column for testing their column than were used at reduced pressures. and a sample thief arrangement for withdrawing a sample from Since the completion of the present work, articles by Struck the kettlcs for analysis. The sample of condensed vapor was obtained by means of an especially designed thiol which has been and Kinney (37) and by Berg and Popovac (6)have been pubdescribed previously (8). lished. They conclude that the efficiency of the columns they The kettle heaters were electrical resistance heaters of the type tested was practically independent of pressure. made in this laboratory. At no time did bumping oorur. The test mixtures, previously used in testing cohmns a t a t m m The distilling heads the total condensation type a pheric and subatmospheric pressures either behaved far from low holdup and provided with a calibrated drip point for throughideally or showed a lnrge change in relative volatility as the total put rate determination. pressure was changed from 760 to below 100 mm. of mercury. Thus it appears that additional information is desirable. TAB- I. DATAON DIs'MLLINo COLVMNS The present work uses the Nondrainahlea Operatin Holdu b MI. at 44 MI. Presaure Drop' same mixtures for testing at Column Holdup, per%our TIroughput Column Specifications MI. 760 mm. a00 mm. 20 mm.0 800 mm. 20 mm. Packing both atmospheric and reduced pressures. The mixtures used 50 X 50 mesh Inside diameter 17 mm. stainless steel Inner glass tibe outside also illustrate the two possispiral acreen diameter 6 mni 4 8.6 8.8 10.0 0.6 1.7 Paokin height 35 6 cm. bilities of change of relative Inside 8ismete;. 18 mm. 5 6.0 7.4 1.5 8.a volatilitv with Dressure-one Packing height, 50.8 cm. 5 Determined by pourink 100 ml. of n-he tane through column and collecting drainage in a graduated aylinder, mixture showing an increase 100 ml. - drainage in ml. nondrainable !oldup. and the other a decrease in b Determined by method of Tongberg Quiggle, and Fenske (SO), uaing steario acid dissolved in ethylbenzene and a throughput rate of 44 ml. per bout. relative with deAll preaaurea and preasure drops are in mm. of mercury. creased pressure.
-
..
Novambe~1951
tive volatility remains constant for varying com osition, the equation of Chilton and Cokurn (11) may be used for calculating the number of transfer units.
TABLE 11. SPIRAL SCREEN COLUMN PERFORMANCE AT 760 MM.AND 20 MM. h b R CURY FOR THROUQHPUT RATESUSINQETHYLBENZENE AND CBLOROBENZENE Throughput, MI. per Hour 27.3
43.7
65.6 87.S
Total Pressure of System, 20 Mrn. H.E.T.P., H.T.U., n 1 inch n 1 inch
Total Pressure of system, 760 Mm--
+
n 1 24.6 24.7 23.7 24.2 24.6 25.2 21.6 22.1 21.3 22.0
..
110.0 136.0 273.0
18.0 17.0
-
H.E.T.P., inch 0.58 0.59 0.62 0.60 0.59 0.58 0.68 0.65 0.69 0.67
n
H.T.U.,
+1
Z4:S
0:is
..
..
26:O
0:59
..
.. ..
0.78 0.82
19.2 16.9
0:66 0.68
..
+
25:7 24.8 25.8 26.0
0:67 0.69 0.56 0.56
24.3
0.60
.. .. ..
..
22:l 21.15
..
+
inoh
1
.
..
21.4
0.77 0.82
..
..
.. .. ..
0.69
..
All work was carried out in a constant temperature room and all voltages used to control the heating of both the column and kettle were taken from a constant voltage transformer. TEST MIXTURES
The test mixtures used were ethylbenzene-chlorobenzene and n-heptane-methylcyclohexane. The properties of these mixtures are described above. For operating a t reduced pressures two recautions were taken to prevent loss of material. A second conjenser (spiral tube) was connected to the head with a standard-taper joint. This second condenser was connected to the vacuum h e by means of a short
EXPERIMENTAL RESULTS
SPIRALSCREENPACKING. The first 2i:o 0:51 series of runs was made with the spiral screen column using the mixture ethyl26:s 0:&3 benzene-chlorobenaene at a pressure of .. .. .. .. 760 mm. mercury. Throughput rates were varied from 27 to 273 ml. per hour. 25.5 0.57 .. These data are shown in Table 11. The column performance, expressed in terms .. ... . of theoretical plates and height of 21.7 0 68 theoretical unit, are shown plotted against throuvhput rate in Figure 1. .. .. Next a series of runs was made with .. .. the same column and mixture exce t a t a aressure of 20 mm. Throu&ut races were varied from 27 to 11'0 m1. per hour. These data are shown in Table TI and are'plotted in Figure 1. In both these runs the volume of test mixture in the kettle waa maintained a t about 250 ml. A com arison of the data in Table I1 and Figure 1 for these runs shows tgat the column performance is at least as good a t 20-mm. pressure as at atmospheric ressure up to the through ut rate a t whivh flooding occurs. Idreover, it is seen that in goth of these runs the performance varied inversely with the throughput rate. One run was made a t 300 mm. with the mixture ethylbenzene and chlorobenzene using the s iral screen column and a throughput rate of 44 ml. per hour. $he following results were obtained: . I
.
I
nY Using a value of a by
n=-
I
I
I
I
I
I
I
I
I
a4
-
1 log 1.11
1.11 the total plates may be calculated 0.888 X 0.623 log 0.112 X 0.377
PROCEDURE
The t a t mixture waa placed in the kettle and the manostat set for the desired pressure. Heat was a plied to the kettle and the li uid vaporized into the column. $he packin was wetted b dziberately flooding the column. Then the voqtage to the c o l umn heater was ad usted so that the column thermometer read about 2O C. less &an the head thermometer. In general, 24 hours were required for the columns to reach e uilibrium. After equilibrium had been attained s m d samples were removed Rimultaneously from the head and the kettle, and analyzed by means of the refractometer. The performance of the columns was exprewed as total plat-, bei ht of equivalent theoretical late, number of transfer unite, a n d height of a transfer unit. &e calculation of the number of lplates using theae mixtures has been described. Since the rela-
24.6
(1)
This value checks within experimental error with the value of 24.6 and 25.8 obtained a t 760 and 20 mm. with the same throughput rate. Next a series of runs was made using the spiral screen column and the test mixture n-heptane-methylcyclohexane a t a pressure of 760 mm. with throughput rate varying from 27 to 273 ml. per hour. These results are shown in Table I11 and plotted in Figure 2. 26
length of heavy-walled rubber tubing. An liquid formed in this condenser was returned through the caibrated drip point. A cold trap showed that no material escaped throu h these condensers. Also the Cenco Hy-Vac pump waa throttfed, by means of a pinch clamp on the rubber tubing which connected the pump to the pressure regulator, thereby reducing the frequency of tho fluctuation of the valve on the manostat. No premure fluctuations were observed in the Z i e r l i gage which could be read to 0.1 mm.
Mole Fraction Chlorobenzene Kettle Head 0.377 0.888
Head 1.5180
Kettle 1.5032
14
2615
INDUSTRIAL AND ENGINEERING CHEMISTRY
COLUMN-
SPIRAL SCREEN
VI
5j
0
24
Y I
--
j 20 22
0 . 8 ~
c
0.6{ c
c
r
16
(1.
I
I
60
I
I . 100
I
I I I 150 200 THROUGHPUT RATE MVHR.
I
I
250
I
1
Figure 2. Theoretical Plates and H.T.U. us. Throughput Rate Using Test Mixture n-HeptnneMethylcyclohexane The same column was tested using the same test mixture at a pressure of 300 mm. with throughput rates from 27 to 136 ml. per hour. These results are shown in Table I11 and are plotted in Figure 2.
2616
INDUSTRIAL AND ENGINEERING CHEMISTRY
Vol. 43, No. 11
110 ml. per hour. These data are COLUMN PERBORMANC~ AT 760 MM, A N D 300 IMM.MNRTABLE 111. SPIRALSCREEN shown in Table I V and are plotted in CURY FOR THROUGHPUT RATESUSINQ n-HEPTANE A N D METHYLCYCLOHEXANE Figure 3. Total Pressure of system, 300 Mm. Total Pressure of System, 760 ML Thoughput, M1. per H.E.T.P.. H.T.U., H.E.T.P., H.T.U., This column flooded slightly a t about Hour n +1 inch n +1 inch n +1 inch n +1 inch 100 ml. per hour a t a pressure of 20 mm. 26.8 26.0 25.6 0.57 0.55 0.54 0.56 26.4 27.3 but up to this point it performed a well 0.69 0.58 0.57 0.68 25.3 24.8 26.7 25.1 43.7 under reduced pressure as a t atmos24.9 0.56 0.69 26.1 pheric pressure within the limits of ex0.63 0:63 0.61 Z3:6 23: 2 24.0 0:iz 23.2 66 .'5 0.67 0.67 21.9 21.9 87.3 perimental error. 2i:2 0:69 2 i : g 0:67 110.0 The Raschig ring-packed column wae .. 19:a 19:8 0174 0:i4 .. 142.0 next tested a t atmospheric pressure with .. 17.5 0.84 0.84 .. 275.0 17.6 the test mixture n-heptane-methylcyclohexane at throughput rates varying from 38 to 275 ml. er hour. The data for TABLE Iv. RASCHIQRING COLUMN PERFORMANCE AT 760 MM. AND 20 M M . MERthese runs are sffown in Table V and are CURY FOR THROUGHPUT RATESUSINGETHYLBENZENE A N D CHLOROBENZENE plotted in Figure 4. Here again almost Total Pressure of System, 20 Mm. Total Pressure of System, 760 Mm. constant performance was ahown as the Throughput Rate MI H.E.T.P.. H T.U H,E.T.P., H T.U throughput rate varied greatly. per kou; n +1 inches n +1 inches' n + 1 inches n + 1 iicha;' The same column was thcn tested with 44 8.6 2.6 8.5 2.7 8.5 2.7 8.5 2.7 the same mixture under a pressure of 300 8.5 2.7 8.5 2.7 8.0 2.9 66 mm. with throughput rates varying from 8.4 2.7 8.i 2,'s 44 to 130 ml. per hour. Slight flooding 8'.5 2.7 f3.4 2.7 6.6 3.6 6.5 3.6 lib 8.0 2.9 7.9 2.9 .. .. .. .. 190 occurred at the higher rate. Thesedata 275 7.4 3.1 7.2 3.2 .. .. .. .. are shown in Table V and are plotted in Figure 4.
TABLEv. RASCHIGRING COLUMN PERFORMANCE USINQ ?&-HEPTANEAND METHYLCYCLOHEXANE AT 760 MM. AND 300 MM. MERCURY FOR VARIOUS RATES THROUGHPUT
Examination of the data in the Tables 11, 111, and I V for the column performances at various throughput rates reTotal Pressure of System, 300 Mm. Total Pressure of System, 760 Mrn. Throughput veals that there are several runs at idenRate MI H.E.T.P., H3XiH,E.T.P., H.T.U tical throughput rates with different per kou; n +1 inches n + 1 inches" n +1 inches n +1 inohes' 8.6 2.6 9.0 2.5 kettle compositions. S c h u l t r a n d 38 0.4 2.i 8.4 2 . i 44 Stage (84) investigated this factor using 8.6 2.6 d.4 2.6 8.0 2.9 8.0 2.9 66 .. .. .. .. 7.8 2.9 7.8 2.9 88 the mixture benzene-toluene, and stated 7.3 3.2 7.3 3.2 130 8.6 2.0 8.8 2.6 .. .. .. .. 131 that with decreasing benzene concentra.. .. .. 8.1 2.8 8.4 2.7 152 tion the number of plates increased by aa 276 7.6 3.0 8.0 2.9 .. .. .. .. much as 23%. Richter (2.99)claim Schultz and Stage did not take into account an anomalous behavior of the test mixture. The work reported herein did not show a significant Once again the column performance appears to be as good under difference in the column performance a t either atmospheric or reduced pressure as at atmospheric. At both pressures the reduced pressure as the kettle composition was varied when the column performance varies inversely with the throughput rate. total volume was kept a t about 250 ml. at all times. This veriRASCHIQRING PACKING.The column packed with 4-mm. fies results obtained at atmospheric pressure by Tongberg, Raschig rings was next tested at 760 mm. using ethylbenzene and Quiggle, and Fenske using benzene and carbon tetrachloride (39). chlorobenzene with throughput rates varying from 44 to 275 At reduced as well as a t atmospheric pressure the column perml. per hour. These data are shown in Table IV and are plotted in formances were found to vary inversely with the throughput rate. Figure 3. This agrees with work done at atmospheric pressure by John and The performance of this column is not so good as that of the Rehberg (W), Podbielniak (M), Fenske, Tongberg, and Quiggle spiral screen column and does not change so sharply with varying ( l 7 ) , Tongberg, Lawroski, and Fenske (B), Lecky and Ewe11 throughput rates. This essentially constant performance, re(M), and McMahon (24). Stedman (56), using a column of hie gardless of throughput rate, has been noted for single turn helices design, reported an optimum throughput rate for maximum per( 17 , 3 8 )which are also a loose-type packing. formance at atmospheric pressure. Stallcup, Fuguitt, and The same column was then tested with the same test mixture Hawkins reported an optimum throughput rate at atmospheric at a pressure of 20 mm. with throughput rates varying from 44 to
..
I 1 1 1 1 COLUMN-
RASCHIG RlHG
I ln
Y
4 s w
24
t; 0=
L
I
50
I
I
"
"
"
J
100 150 200 THROUGHPUT RATE IL/HR.
250
Figure 3. Theoretical Plates and H.T.U. US. Throughput Rate Using Test Mixture Ethylbenzene-Chlorobenzene
50
100 150 200 THROUGHPUT RATE MUHR,
250
Figure 4. Theoretical Plates and H.T.U. US. Throughput Rate Using Test Mixture n-HeptaneMethyloyclohexane
November 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
pressure using a spiral screen column (36). Redd and Hawkina (88),using a twisted stainless steel-wire analytical column, found the performance to vary inversely with the throughput rate; when using a spiral screen column they reported an optimum throughput rate, all work being at atmospheric pressure. EFFECT OF RATIO OF HOLDUP TO CHARGE
Another factor which was investigated under conditions of total reflux was the effect of ratio of holdup to charge. Rose and Houston (81) investigated this for f d t e reflux ratios but they repoxted nothing for total reflux. A material balance requires that the smaller the charge in the kettle, the lower must be the composition of the distillate because of the fact that the column has a dgnificant holdup, However, consistent column performance depends on whether or not the kettle composition changes proportionately with the distillate composition. To investigate this factor WE were made using a constant charge composition of 37 mole % chlorobenzenewith test mixture volumes of 250,100, and 60 ml. at pressures of 760 and 20 mm. using the spiral screen column. These data are shown in Table VI.
261'1
TABLE VI. EFFECTO F RATIOOF IfOLDUP TO CHARGE AT TOTAL REFLUX Charge Volume
Holdup Volume, %
Prassure Mm. MeroLry
250 100 60 250 100 60
3.2 8.0 16.0 4.0 10.0 20.0
780 760 780 20 20 20
n
+1
24.6 25.0 24.6 26.0 25.6 26.0
In all cases the kettle composition decreases with decreasing charge volume. Accordingly the head compositions, with the smaller charge, should be lower if the column is to show the same performance. This is true in all cases with the exception of the runs at 760 mm. with 250- and 100-ml. charges. These showed the same head composition due to the fact that the run with 100ml. charge showed 0.4 of a plate better separation, which is withm the range of experimental error.
PACKED COLUMN PERFORMANCE WHEN REFLUX RATIO IS FINITE
T
HE usual method of expressing the results of a batch distillation is by means of sharpness of separation curves (33). In this method the distillate composition or some function of the distillate composition such as head temperature, refractive index, etc,, is plotted against volume distilled or per cent distilled. This is satisfactory for comparing columns using the same mixture and a t the same pressure. However, when testing is to be done at different pressures the effect of the pressure change on the ease of separation of the mixture must be taken into account. Only the sharpness of separation curves for a given column at different pressures would not be sufficient to tell whether the column itself was operating as effectively at one pressure as a t another. For example, if a mixture shows an increase in relative volatility as the pressure is decreased, it becomes easier to separate a t the lower pressure. If the sharpness of separation curves were identical it would be assumed that the column itself was not operating as effectively a t the reduced pressure, since under this condition the mixtute is easier to separate. EQUIVALENT PLATEB.Equivalent plates are defined as the 8 m i n i u mnumbcr of plates that a column operating under total .reflux would need to exert in order to produce a separation equivalent to that obtained at finite reflux ratio, all conditions being the same except the reflux ratio. Thus, the equilibrium diagram or the Fenske equation (16), if the relative volatility ib constant, for example, at 20 mm., could be used for calculating the number of equivalent plates when runs are made a t this pressure. Ap,propriate equilibrium diagrams or relative volatilities would be used a t other pressures, thereby taking into account the effect of ithe change of pressure on the ease of separation. From these data the equivalent plates may then be plotted against per cent distilled, thus indicating the performance as the distillation proceeds. Baker, Barkenbus, and Roswell ( 4 ) utilized a similar method of expressing performance at atmospheric pressure which they called plate equivalence. However, they used a large volume .of test mixture in the pot and took small samples with the idea of keeping the pot composition constant. They therefore were getting results similar in part to continuous distillation and not to batch distillation with its corresponding change in pot composition. A simple method is proposed which takes into account the effect of change of pressure on the test mixture. This method is
to remove small portions-3 or 4 drops-periodically from the kettle and the head for analysis. The kettle composition as the distillation proceeds is then known and may be plotted on the same graph as the head or distillate composition. With this information the kettle composition and the head composition a t any period of the distillation are known. The difference in these compositions may be interpreted in several ways. The simplest method for comparison of any one column under different conditions, such as different pressures or d8erent test mixtures, is in terms of equivalent plates. OPERATINGEFFICIENCY. Another method of expressing column behavior when product is withdrawn will be called the operating efficiency. This is defined as the number of equivalent plates exerted, when the takeoff is finite, divided by the number of theoretical plates the column shows under total reflux for the same throughput rate. The operating efficiency therefore will have a value of 1.0 for its upper limit. Baker, Barkenbus, and Roswell used a similar method, but the development of their terminology is not the same.
(e)
GENERAL OPERATING PROCEDURE
The charge was placed in the kettle and the head was connected to the ga e, pump, manostats, ete., for tests under reduced pressure, as tescribed in the preceding section. For tests at atmospheric pressure the connection from the head was left open. The celumn temperature was adjusted to within about 2' C. below the head temperature and the kettle heater adjusted to give the desired through ut rate after f i s t flooding the column by applying a larger vo%age to the kettle beater. The throughput rate of 44 ml. per hour was chosen because at this rate the column was operating a t high performance and a convenient amount of material assed through the column. The column used in all of the fofiowing experiments was a spiral screen ty e with a maximum number of theoretical lates of about 26. A s characteristics have been described in t i e preceding section. After 20 to 25 hours a few drops of distillate were removed and analyzed, This was repeated a few hours later. If the reading were almost the same the column was adjudged to be a t equilibrium and a few drops of liquid were removed from the kettle. The differencein the two readings gave the number of theoretical plates at total r e flux. The combination head sample thief and receiver (8) designed in this laboratory was then attached and the stopcock in the head opened. The distillation heads used were the total con, densation type with a low holdup. The reflux ratio was controlled with a swinging clapper activated by an external electro-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2618
Vol. 43, No. 11
TABLEI. OPERATINGEFFICIENCIES O R SOMESPIRALSCREENCOLUMNS Throughput rate 44 ml. per hour, varying oharges and reflux ratios 250 MI. of A 250 MI.of Bb 250 MI. of A' 250 MI.of CC 100 MI.of A 24:l 24:l 8.1:1 24: 1 24: 1 7% 760 mm.d 20 mm. 760 mm. 20 mm. 760 mm. 20 mm. 760 mm. 20 mm. 760 mm. 20 mm. 0 0.90 1.0 1 .o 1.0 1 .o 1.0 1.0 1 .o 1.0 1.0 0.74 0.94 0.91 0.71 0.93 0.95 0.94 0.90 5 0.97 0.96 0.74 0.96 0.91 0.69 0.99 0.99 0.98 0.92 0.99 10 0.98 0.90 0.72 0.94 0.94 0.78 0.96 0.88 0.95 0.98 15 0.97 0.87 0.79 0.84 0.73 0.92 0.90 0.83 20 0.90 0.91 0.95 0.76 0.84 0.81 0.87 0.88 0.71 0.68 0.74 30 0.82 0.88 0.67 0.64 0.80 0.78 0.86 0.86 0.66 0.79 40 0.66 0.84 0.67 0.86 0.87 0.72 0.64 0.57 0.78 0.64 0.61 0.83 50 A. 37 mole chlorobenzene in ethylbenaene. B. 36 mole n-heptane in mathylcyclohexane C. 25 mole % chloTobensene in ethylbenzene. Pressures given are m mm. of meraury.
-
Volume Distilled,
a
b C
d
50 MI. of A 24:l 760 mm. 20 mm. 1.0 1 .o 0.96 0.97 0.99 0.99 0.98 0.98 0.95 0.94 0.89 0.81 0.77 0.83 0.67 0.83
@
magnet. A Cramer ercentage timer, which may be purchased from R. W. Cramer Eo., Inc., Centerbrook, Conn., controlled the electromagnet; it was set to the desired reflux ratio, which was determined by revious calibration, and the takeoff commenced. Distillate s a m p k were taken a t frequent intervals and kettle Sampies less frequently since its composition did not change rapidly. EXPERIMENTAL W O R K
TESTS AT CONSTANT CHARQE VOLUME.The first run was made a t atmospheric pressure with a charge of 250 ml. of a mixture of ethylbenzene and chlorobenzene which contained 37 mole % of the latter. The timer was set for 25% which gives a reflux ratio of 3. The curve for these data showed that this reflux ratio was too small to separate this mixture appreciably, so no more runs at this ratio were made. 100 I
I
I
I
I
I
REFLUX RATIO 8.1-1 THROUGHPUT RATE 44 Y V H R
40 80 70
0 760 M. x 20 MM.
20 MY.
60 50 40 30 20 IO
pressure. However, when the separations are calculateJ as equivalent plates and plotted in Figure 2, it is seen that the operating efficiencies are almost identical. The sharp initial drop in the product composition, when product is removed, haa been noted for runs a t atmospheric pressure by Rose and Houston (81) and others, and it is now seen to occur at reduced pressure also. Because of this sharp initial drop, the equivalent plates also drop rapidly; but as the distillation curve flattens out, the equivalent plates build up to a maximum and then fall off more gradually. This effect will be noted in all runs to follow. Accurate values of equivalent plates after 50% has been distilled cannot be determined because the kettle composition becomes too low. In order to determine the reproducibility of these distillation curves, a check run was made at 20 mm. and at a reflux ratio of 8.1. These data are shown in Table I and (another filed with the American Documentation Institute) and are plotted in Figure 1. The excellent reproducibility of these data indicates that experimental errors are small. Next, runs were made at 760 mm. with a charge of 250 ml. of 37 mole % chlorobenzene with the timer set at 4%, giving a reflux ratio of 24. The data are shown in Table I (and others filed with American Documentation Institute) and are plotted in Figures 2 and 3. A comparison of Figures 2 and 3 shows the effect of increasing the reflux ratio in terms of distillation curves and
A=REFLUX E-REFLUX C-REFLUX D-REFLUX
0
Figure 1. Composition of Distillate a n d Kettle us. Per Cent Distilled Using Test Mixture Ethylbenzene-Chlorobenzene Charge.
250 ml. of 37 mole
26
E 24
24, 8, 24, 8.
2 0 MM. 20 HM. 760 MM. 760'MM.
2 22
% ehlorobeneene
The next run waa made a t atmospheric pressure with the same charge hut with the timer set for 11% giving a reflux ratio of S:1. These data are shown in a table filed with the American Documentation Institute. and are plotted as composition bf d i 5 tillate and kettle us. volume per cent distilled in Figure 1. By use of Figure 1 and data reported in the first paper of this series, the head and kettle compositionsa t various values of per cent distilled were determined. The equivalent plates at various values of per cent distilled were then calculated, using a Iarge-scale plot of Figure 2 of the first article in this series. The operating efficicncy waa calculated also. These data are included in Table I. A plot of the equivalent plates against the per cent distilled is ehown in Figure 2. Next, a run waa made with the same charge and reflux ratio but a t a preaaure of 20 mm. These data are shown in Table I (and another filed with American Documentation Institute) and are plotted in Figures 1 and 2. It is noticed that the distillation curve in Figure 1 shows a better separation a t 20 mm. than a t 760 mm. This is because the mixture is easier to separate at the lower
RATIO RATIO RATIO RATIO
y
20 18
f
16
e
1
I
I
I
1 1 - 1
IO 20 30 40 VOLUME PERCENT DISTILLED
0
1
50
Figure 2. Equivalent Plates us. Per C e n t Distilled Using Test Mixture Ethylbenzene-Chlorobenzene Charge. 250 ml. of 37 mole % ohlorobenzene
equivalent plates. I t is noticed that with the greater reflux ratio the initial drop is not nearly as great as with the lower reflux ratio, and the equivalent plates almost regain the value shown at total reflux. This verifies the statement of Rose (SO) that the optimum reflux ratio is equal to the number of plates and to exceed this number is, in general, impractical. CALCULATION OF THEORETICAL PLATES AND TRANSFER UNITB. The number of theoretical plates and number of transfer unita were calculated for the runs a t 760 and 20 mm. of mercury and a
INDUSTRIAL AND ENGINEERING CHEMISTRY
November 1951
reflux ratio of 24, using the values of head and kettle composition at 10% distilled. The method of McCabe and Thiele given by Badger and McCabe (3) WlW used t0 C a h h t e the number Of theoretical plates. The appropriate operating lines were placed on the equilibrium diagrsms for ethylbenzene-ohlorobenzene a t 760 and 20 and 44 the number Of plates stepped Offa The Obtained and 42, respectively, for 760 and 20 mm. A column cannot be 100 .--
i
I
REFLUX RATIO 2 4 - 1 THROUGHPUT RATE 44 ML/HR.
y 90
El70
+
l
= Y'r
-AY
(1)
and the McCabe-Thiele diagram. This done for the points as above, plotting values of l/(y* -y) as the ordinate against values of y aa the abscissa gives a curve; the area under the curve shows the 100 90 80 70 60 50 40 30 20 10
60 50 l3 40
::
P so
3
N
2619
20 10 0
'0 VOLUME PERCENT DiSllLLED
Figure 3. Composition of Distillate and Kettle V I . Per Cent Distilled Using Test Mixture Ethylbenzene-Chlorobenzene Charge. 250 ml. of S7 mole
I
I
I I
80
I
REFLUX RATIO 2 4 - 1 THROUGHPUT RATE 44 H m R .
20
SO 40 50 60 VOLUME PERCENT D I S T l LLED
70
80
Figure 4. Composition of Distillate and Kettle us. Per Cent Distilled Using Test Mixture n-Heptane-Methylcyclohexane Charge. 250 ml. of 36 mole % n-heptane
ahlorobenaene
more efficient a t any finite reflux ratio than it is at total reflux; therefore these values are meaningless since the column shows only 26 plates at total reflux. The inability of the McCabeThiele method to express correct column performance for batch distillation at atmospheric pressure has been reported by Rose and Houston (8), Colburn and Stearns (If?), and Rose and Pfeiffer (st?),and now it is seen that it is not applicable when operating under reduced pressure. 100 80
10
number of transfer units to be 39. Similar calculations for 20 mm. shows 42 transfer units. This indicates that the transfer unit values calculated by this method for batch fractionations are also meaningless since only 26 transfer units were found when operating a t total reflux. The above calculations, which not only are long and tedious to perform but also are without meaning, emphasize the desirability of using equivalent plates and operating efficiency when describing the performance of a column a t finite reflux ratios.
I
10 0 VOLUME PERCENT DISTILLED
Figure 5.
Equivalent Plates us. Per Cent Distilled
Charge. A B
dd
E: F.*
210 ml. of 37 mole % ahlorobenuroe
250 ml. of 36 mole % n-heptane
zso ml. of 25 mole w ahlorobeugena
VOLUME PERCENT DISTILLEI
Figure 6. Composition of Distillate and Kettle us. Per Cent Distilled Using Test Mixture Ethylbenzene-Chlorobenzene Charge. 260 ml. of 25 mole % ahlorobexmane
Berg and Popovac (6) attempted to use this method for evaluation of column performance a t atmospheric and reduced preseures for reflux ratim of 20 to 1 and 30 to 1. They also, with but one exception, get values for the number of theoretical plates at reflux ratios of 20 to 1 and 30 to 1which are aa high a ~ and , in some case8 15% higher than, those obtained a t total reflux. The fact that the text mixture they used waa very nonideal makes it di5cult to obtain significant results. This probably accounta for their findings. The number of transfer units may be calculated when the reflux ratio is finite, using the equation ( I 1 )
Next, runs were made at 760 and 300 mm., using the mixture nheptane-methylcyclohexane with a charge of 250 ml. of 36 mole % n-heptane and a reflux ratio of 24. These results are shown in Table I (and otKers filed with American Documentation Institute) and are plotted in Figuree 3 and 4. These curves are similar to those in Figures 2 and 3 for ethylbenrene-chlorobenzene mixtures. In Figures 3 and 4 the distillation curves are much leks sharp than those with ethylbenzene-chlorobenzene. This is because the relative volatility of n-heptane-methylcyclohexane ie lower and therefore more difficult to separate. Also, the separa-
INDUSTRIAL AND ENGINEERING CHEMISTRY
2620
tion was poorer at the reduced pressure since this mixture shows a decrease in relative volatility with decreasing pressure. However, the equivalent plates a t the two pressures are very close, showing that the column performs as well a t 300 mm. as at 760 mm. if the effect of pressure on the relative volatility of the mixture is taken into consideration. Furthermore, the performance with the different mixtures, ethylbenzene-chlorobenzeneand 100,
I
I
I
I
I
REFLUX RATIO 24-1 THROUGHPUT RATE 44 HL/HR.
w90 =
80 70 8 60 50 40 P 30 20 w 0
d
$0
PO
'0
30
20
40
50
60
70
80
VOLUHE PERCENT D I S T I LLED
Figure 7. Composition of Distillate a n d Kettle us. Per Cent Distilled Using Test Mixture EthylbenzeneChlorobenzene Charge. 100 mi. of 37 mole Yo chlorobenzene
n-heptane-rnethylcyclohexane, are almost the same until distillation has proceeded for some time. Both mixtures started with the s m e volume and with almost the same charge of more volatile component. At any value of per cent distilled the kettle volumes would be the same; and yet, as the distillations proceed, the equivalent plates and operating e5ciency in the n-heptanernethylcyclohexane run remain high, while they fall off more rapidly with ethylbenzenechlorobenzene mixture. From the distillation curvea, Figures 3 and 4, for the two mixtures, it is seen
2 2
3 * E
E
REFLUX RATIO 24-1 THROUGHPUT RATE 44 MVHR, A,E,C 20 MM. B D F 760 MM.
26 24 22 20 18 16
I i
0
,A
i 20i i 30i i 40i i 501 VOLUME PERCENT DISTILLED
Figure 8. Equivalent Plates ws. P e r C e n t Distilled Charge. A B. 50 ml. of 37 mole % chlorobenzene
Vol. 43, No. 11
24 and a charge of 250 ml. of 25 mole % chlorobenzene at pressures of 760 and 20 111111. These data are shown in Table I (and others filed with American Documentation Institute) and are plotted in Figures 5 and 6. These runs, with their lower charge of more volatile comporient, showed a lower kettle composition for corresponding values of per cent distilled. Figure 5 shows that the column performance falls off more rapidly than for the runs with higher charge composition. This indicates that the column performance varies directly with the kettle composition. This explains why the curve of the equivalent plates shown in Figure 2 for ethylbenzene and chlorobenzene at 20 mm. croases the curve a t 760 mm. a t about 45% distilled. The greater ease of separation of the mixture at 20 111111. results in the kettle composition being lowered more rapidly, thus causing the column performance to fall off at a higher rate. EFFECT OF CHARGE VOLUME. Next, runs were made with the same charge composition but different charge volumes. This waa investigated by Rose and Houston (31) at atmospheric pressure and they found that the runs with the smallest charge showed the sharpest separation. This was checked and, in addition, runs were made at reduced pressure. The first runs were made with 100 ml. of 37 mole % chlorobenzene a t 760 and 20 mm. with a reflux ratio of 24. These data are shown in Table I (and others filed with American Documentation Institute) and are plotted in Figures 7 and 8. Next, runs were made under conditions identical to the above except 60 ml. of charge were used. These data are shown in Table I (and others filed with American Documentation Institute) and are plotted in Figures 8 and 9. The runs previously described, with a 250-ml. charge, in Table I (and others on file a t American Documentation Institute) round out this series. These data are replotted in Figure 8. A study of Figures 3, 7, and 9 verifies statements by Rose and Houston (SI)that the initial drop in product composition is less for the smaller charges at atmospheric pressure. It also shows the same relation holds at reduced pressures. Figures 8 shows that the equivalent plates for the smaller charges do not drop as rapidly at the beginning, but after about 20% distilled the performance is almost independent of the starting volume. The characteristic rapid drop in performance near the end of the runs at 20 mm. of mercury pressure is again observed for the lower charge volume. This is to be expected since the kettle composition for the runs is almost the same at equal values of per cent distilled regardless of actual volume of material in the kettles. SEPARATION OF ALPHA-AND BETA-PINENE. All of the experimental work indicates that these distilling columns operate quite as effectively a t low pressures as at high pressures. Therefore the decision as to what pressure will give the greatest separation will be governed by the particular mixture which is to be separated. 100 I
I
I
I
REFLUX RATIO 24-1 THROUGHPUT RATE 44 HVHR.
C* D 100 ml. of 97 mole % chlorobenzene E: F.* 250 ml. of 37 mole % chlorobenzene
that the kettle composition in the case of n-heptane-methylcyalohexaneremains much higher in the more volatile component than when using ethylbenzene-chlorobenzene. This is due to the lower relative volatility for the first mixture and the resulting lower distillate composition. Thus it appears that the kettle composition ia one of the controlling factors in batch fractionation. This would account for the performance falling off in all cases. As would be expected, the more rapidly the kettle composition becomes poorer in the more volatile components, the more rapidly the performance decreases. TOverify this statement, runs were made with a reflux ratio of
Figure 9. Composition of Distillate a n d Kettle V I . Per Cent Distilled Using Test Mixture EthylbenzeneChlorobenzene Charge. 50 ml. of 37 mole % ahhlorobenzene
I N D U S T R I A L AND E N G I N E E R I N G CHEMISTRY
November 1951
Mixtures of LY- and &pinene have been separated at 20 mm. in this laboratory primarily to avoid reactions which result a t higher temperaturee. Data of Armstrong (a) show that at 500 mm. the relative volatility of a- and B-pinene is 1.30 and a t 20 mm. it is 1.42. These were calculated as vapor pressure ratios. Stallcup, Fuguitt, and Hawkins (236)determined the vapor-liquid equilibrium of a-and @-pinenea t 20 mm., and from these data the relative volatility at 0.5 mole fraction is 1.43 which is in good agreement with the value 1.42.
I
I
=
w W
2
90 80 70
REFLUX RATIO 24-1 THROUGHPUT RATE 44 NL/HR.
4 60 a.
-J
50
g
40
4
2 so
20
2 10 = o
VOLUME PERCENT D l S T l LLED
Figure 10. Composition of Distillate us. Per Cent Distilled Using Mixture LY- and &Pinene Charge. 190 ml. of 30 mole %I u-pinene
Fractionations were conducted a t 600 and 20 mm. with charges of 100 ml. of 30 mole yo a-pinene. These data are shown in a table filed with American Documentation Institute and are plotted in Figure 10. They show that the separation at 20 mm. is much better than at 500 mm., which is in conformity with the earlier statements concerning the relation between the change in relative volatility with pressure and the effectiveness of the distillation process. LITERATURE CITED
(1) Am. Petroleum Inst., “Selected Values of Properties of Hydro-
carbons,” Research Project 44,Natl. Bur. Standards, Tables 2 K and 7K. (2) Amstrong, G. T.,theais, University of Florida, 1943. (3) Badger, W. L., and McCabe, W. L., “Elements of Chemical Engineering.” p. 351, New York. McGraw-Hill Book Co., 1936. (4) Baker, R. H.. Barkenbus, C., and Roswell, C. A., IND.ENQ. CHEM.,ANAL.ED., 12,468 (1940). (5) Beatty, H. A., and Calingaert, G., IND.ENQ.CHEM.,26, 504 (1934).
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(6) Berg, L,and Popovac, D. O., Chem. Eng. Progress, 45, 883 (1950). ENO.CHEM.,34, 1088 (7) Bragg, L. B.,and Richards, A. R., IND. (1942). (8) Brent, J. A., Jr., and Hawkins, J. E., Anal. Chem., 22,,374 (1950). (9)Bromily, E., and Quiggle, D., IND.ENQ.CHEM.,25,1136 (1933). (IO) Bruun, J. H., IND. ENQ.CHEM.,ANAL.ED., 2,187 (1930). (11) Chilton, H. T., and Colburn, A. P., IND.ENO. CHEW.,27,255 (1935). (12)Colburn, A. P.,and Steams, R. F., Trans. Am. Inst. Chem. E n ~ r s .37, , 291 (1941). (13)Dow Chemical Co., “Dow Organic Solvents,” 1938. (14) Feldman, J., Myles, M., Wender, I., and Orchin, M., IND.ENQ. CHEM.,41, 1032 (1949). (15) Fenske, M. R., Ibid,, 24,483 (1932). (16) Fenske, M. R., Myers, H. S., and Quiggle, D., Ibid., 42, 649 (1950). (17) Fenske, M. R., Tongberg, C. 0.. and Quiggle, D., Zbid.. 29, 957 (1937). (18) Foraiati, A. F., Glasgow, A. R., Jr., Willingham, C. B., and Rossini, F. D., J. Research Natl. Bur. Standards, 36, 129 (1946),Research Paper 1695. (19) Glasgow, A. P.,Jr., and Sohicktana, S. T., J . Research Nat. Bur. Standards, 19,693 (1937),Research Paper 1049. (20)Hershborg, E. B., and Huntress, E. H., IND.ENQ.CHEM.,ANAL. ED., 5, 344 (1933). (21)Huntress, E. H., “Organic Chlorine Compounds,” p. 1077,New York, John Wiley & Sons, 1948. (22) John, H. J., and Rehberg, C. E., IND.ENQ.CHEM.,41, 1056 (1949). (23) Lecky, H.S.,and Ewell, R. S., IND.ENQ.CHEM.,ANAL.ED., 12, 544 (1940). (24) McMahon, H.O.,IND. ENQ.CHEM.,39,712 (1947). (25)Othmer, D.F., Zbid., 35, 614 (1942). (26) Podbielniak, W. J., IND.ENQ.CHEM.,ANAL.ED., 13,639 (1941). (27) Quiggle, D., and Fenske, M. R., J . Am. Chem. SOC.,59, 1829 (1937). (28) Redd, J. B.,and Hawkins, J. E., J . Electrochem. SOC.,97, 178 (1950). (29)Richter, H., Oel u. Kohle, 40,67 (1944). (30) Rose, A., IND. ENQ.CHEM.,33, 594 (1941). (31) Rose, A,, and Houston, R., unpublished data. (32) Rose, A,, and Pfeiffer, C., unpublished data. (33)Rose, A., and Welshana, L. M., IND. ENO.CHEM.,32,668(1940). (34)Schulta, 0.H., and Stage, H., Oel U. Kohle, 40,68 (1944). (35) Stallcup, W. D.,Fuguitt, R. E., and Hawkins, J. E., IND.ENQ. CHEW.,ANAL.ED., 14,503 (1942). (36) Stedman, D.F., Natl. Petroleum News, 29, 125 (1937). (37)Struck, R. T.,and Kinney, C. R., IND.ENQ. CHEM., 42, 77 (1950). (38)Tongberg, C. O., Lawroski, S., and Fenske, M. R., Zbid., 29. 957 (1937). (39) Tongberg, C. O.,Quiggle, D., and Fenske, M. R., Ibid,, 26, 1213 (1934). Rncmrve~June 16, 1950. Copies of the tables not printed here may be obtained by ordering Dooument 3231 from Amerioan Dooumentation Inatitute, 1719 N St., N.W., Washington 6, D. C., remitting $1.00 for microfilm (images 1 inch high on standard 36-mm. motion pioture film) or $2.10 for photooopies (6 X 8 inches) readable without optical aid.
