Apparatus and Methods for Precise Fractional-Distillation Analysis 11. Laboratory Columns for Precise and Rapid Fractionation of Gaseous and Liquid Samples
WALTERJ. PODBIELNIAK, P. 0.Box 567, Tulsa, Okla. General design and construction of fractionating This paper reports the findings of intensiae theoretical and experimental study of precise low- and columns and apparatus. Fractionating columns are described having a n high-temperature laboratory fractionating columns under the headings of the following factors which are entirely separate sleeve-like metal reflector type shown to be of fundamental importance as well as vacuum jacket for thermal insulation at all temhighly convenient for direct experimental determina- peratures ranging from -190” C. u p to 300” C., within which jacket any of a large variety qf distion or control: Composition and distilling characteristics of tilling tubes of various diameters may be inserted for the rapid and precise fractionation of gaseous or sample. liquid samples ranging from a fraction of a liquid Total time of distillation. Regulation of distillation rate, reflux ratio, dis- cubic centimeter to as much as 20,000 cc. These tillation pressure, and all other variables except total distilling tubes are packed with highly effectiue, spiral, continuous, uniform, wire-coil packings time of distillation. Ratio of 10 times the hold-up of material in the shown to be si perior to the usual chance-arrangement fractionating section (excluding rejlux or condenser) noncontinuous column packings. Complete distillations with the apparatus assembly described take to maximum column capacity. Ratio of 10 times the hold-up of material in reflux from one hour to several hours’ time, depending on the requirements; and the separation accomplished in a condenser section to maximum column capacity. Effectiveness of column packing in securing re- single distillation is materially superior to that pospeated and intimate contact of rejluxed liquid and sible with previously described laboratory fractionating columns including even extremely tall experiascending vapor. mental columns operated at exceedingly slow rates. Eficiency of thermal insulation of column.
T
HIS article presents the results and conclusions of exhaustive theoretical and experimental investigations and seven years’ experience in the development and intensive use of laboratory columns of high-fractionating effectiveness suitable for distilling components with boiling points ranging from - 190O C. to 500 O C. This extreme range comprises practically all components which would be considered distillable, except a few gases like helium and carbon monoxide, noncondensable in liquid air, and the very highboiling petroleum and coal-tar pitches, asphalts, etc. I n the course of this work, all available results of previous and contemporary investigators of laboratory fractionating columns and apparatus were carefully reviewed and in many cases experimentally investigated. The literature on laboratory fractionating columns is exceedingly voluminous and has already been reviewed by Young (26), Leslie (17))Hill (16), and others.
columns to be used for precise analytical work naturally presents itself. The problem is highly complicated, including such variables as amount of sample taken, total time of distillation, method of varying rate of distillation, efficiency of column insulation, column and accessory apparatus design, etc. Previous investigators (7, 16, 18, 26) are not in agreement on the correct method of comparing and rating columns, as is evident from the discrepancies in their conclusions as to the merits of the same column designs. It is assumed in this paper that the user of a precise fractionating column is primarily interested in the most accurate possible analytical determination of a more or less complex sample in least total time for the distillation. The size of the sample distilled will be an important but usually a secondary consideration. In some cases, it will be desired to separate largest possible quantities of pure or substantially pure compounds or very close-cut fractions in reasonable time. However, the second requirement is not necessarily opposed FUNDAMENTALS OF DESIGNAND RATIXGS OF COLUMNS to the first, and confusion will be avoided by considering I n many ways the fractionating columns described in this only the first requirement. The analytical fractionating column is of necessity a batch and a previous paper (19) differ radically from other laboratory fractionating columns previously described. In making distillation column as opposed to continuous distillation apcomparisons of efficiency, the question of the fundamentals paratus. The logical way of expressing the results of an of design and quantitative comparison of laboratory batch analytical batch distillation is in the form of a plot of the overhead temperatures or some other physical property such 1 Book rights reserved except by permission of author. as density, refractive index, etc., and the corresponding total The original features of the apparatus described in this article are covered collected distillate expressed in per cent of the original by foreign patents and United States and foreign pending patent applicasample, or the so-called “fractional distillation curve.” If we tions and are manufactured and distributed exclusively by the inventor. 119
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ment and methods is the object. The author has therefore found it expedient to express arbitrarily all the important variables which affect the size of the intermediate fraction per cent as eight factors, each subject to exact measurement and to greater or lesser control. These factors are not necessarily independent of each other and the form of the connecting functions may be obscure. The only justification for setting them up is to facilitate the exact experimental study of changes in column design and operation. The size of intermediate fraction per cent is a function of and is completely determined by the following factors: 1. Composition and distilling characteristics of sample and of the vapor entering the column. 2. Total time of distillation. 3. Regulation of (a) distillation rate, (b) reflux ratio, ( c ) distillation pressure, and ( d ) all other operating variables except total time of distillation. 4. Ratio of 10 times the hold-up of material in the fractionat-
ing section (excluding reflux or condenser) to maximum column capacity. 5. Ratio of 10 times the hold-up of material in reflux or condenser section t o maximum column capacity. 6. Scrubbing effectiveness of column packing. 7. Efficiency of thermal insulation of column. 8. General design and construction of fractionating column and apparatus. All these factors have been investigated experimentally, usually by actual intensive use of a large variety of apparatus modifications supplemented by fundamental research on the more important factors.
1. COMPOSITION AND DISTILLING CHARACTERFIGURE1. DISTILLING TUBESAND PACKINGS USEDIN EXPERIMENTS FACTOR Single-wire coil, 7.9 turns per inch (3.12 turns per cm.), No. 26 (0.037 om. diameter) monel wire B . Single-wire coil, 6.2 turns per inch (2.05 turns per om.), No. 26 monel wire C. Single-wire coil, 7 turns per inch (2.76 turns per om.), No. 20 (0.0812 om. diameter) monel wire Double-wire cqil: Outer coil 7 turns per inch 2.76 turns per om.), No. 20 monel wire. Inner coil. 4 turns per in06 (1.67 turns per cm.), No. 20 monel wire Single-wire coil 7 turns per inch (2.76 t u p s per cm.), No. 20 monel wire Double-wire co!1: Outer coil, 7 turns per inch, No. 20 monel wire. Inner coil, 4 turns per inch, No. 20 monel wire Single-wire coil 7 turns per inch, No. 20 monel wire Double-wire odil: Outer coil, 7 turns per inch, No. 20 monel wire. Inner coil, 4 turns per inch, No. 20 monel wire 38 turns, a p roximately 43 mm: outside diameter of 5.0 mm. inside diameter &rex standard tubing (unpacked) : total vertical length of coiled portion 50 8 om (20 inchea). uncoiled 400 om. Single-wire coil, 5.5 t&s pkr inch (2.16'turns per 6m.), No. 18 (0.1024 om. diameter) monel wire Double-wire coil: Outer coil, 5.5 turns per inch (2.16 turns per cm.), No. 18 monel wire. Inner coil, 4.5 turns per inch (1.77 turns per om.), No, 18 monel wire Flat-strip coil, 6 turns per inch (2.36 turns per om.) of copper strip about 0.21 om. X 0.09 cm. in cross sectlon €3. Single-wire coil, 4.5 turns per inch (2.16 turns per om.), No. 18 monel wire Double-wire coil: Outer coil, 4.5 turns per inch, No. I8 monel wire. Inner coil, 3.5 turns per inch (1.38 turns per om.), No. 18 monel wife Flat-strip coil, 6 turns per inch (2.36 t y s per om.) of copper strip about 0.21 om. X 0.1 cm. in croas section Jack chains 8 stranda No. 22 brass single jack chains I Jack chains: 23 strand's, No. 22 brass single jack chains Glass cylinders, 5 mm. qutside diameter X 5 mm. long glass cylinders, walls about 1.2 mm. thick A.
assume that the separation obtained is sharp enough to result in perfectly horizontal plateaus for the pure components (or constant-boiling mixtures) in the sample, separated by more or less sloping breaks, then the amount of the intermediate mixed fraction indicated by the curve is a very definite and very useful criterion of the performance of the column. The smaller the intermediate fraction (approaching the value of zero for the ideal staircase break), the more accurate the analysis. For difficult separations or for inefficient columns, the break will be less sharply defined; however, the concept of the intermediate fraction as a criterion of analytical accuracy is of general usefulness. The investigation of the mechanism of fractional distilIation by theoretical reasoning alone, while obviously most desirable, is insufficient without recourse to extensive experimentation, especially when development of actual equip-
ISTICS OF
SAMPLEAND
OF
VAPORENTERING COLUMN
The following general observations are made on the basis of a great number of low- and high-temperature distillations of a variety of commercially important as well as synthetic mixtures. The observations are in substantial agreement with previous literature on this subject. For ideal solutions it can be shown that the amount of the intermediate fraction is approximately inversely proportional to the difference in.boiling points of the two components separated. However, highly complex mixtures are more difficult to separate than less complex mixtures, even though the components to be separated are present in the same percentage and the differences in boiling points of these components from the other components adjacent on the fractional distillation curve are the same for both cases. This fact should be kept in mind in testing columns with synthetic mixtures of only a few compounds as against distillation of commercial solvents, petroleum products, or other exceed" ingly complex products. The lower the average molecular weight of a mixture, the more amenable i t is likely to be to separation by fractional distillation. For instance, natural gas *and in general all gaseous mixtures consist of comparatively few components with large differences in boiling point. Practically any normally gaseous mixture can be completely analyzed into its individual components by a combination of precise lowtemperature fractionation and supplementary tests as previously discussed (20, $3, 27). With increasing molecular weight, the difference in boiling point between successive normal hydrocarbons decreases, and the number of isomers present increases considerably. This is shown clearly in the graphical presentation of boiling points of typical hydrocarbons and other compounds and in the illustrative fractional distillation curves in Part 111of this series. The composition of petroleum has long been a fascinating mystery, except in the case of the lower-boiling hydrocarbons up to about decane. With the aid of the apparatus described
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INDUSTRIAL AND ENGINEERING CHEMISTRY
in this and succeeding papers, it is possible to distil petroleum products fractionally and obtain very well defined and consistently recurring plateaus. These plateaus may be pure hydrocarbons, mixtures of hydrocarbons of almost identical boiling point, or constant-boiling mixtures. Many of the breaks between plateaus are poorly defined and seem t o show poor separation. In every case, when such apparently poor breaks were investigated by extremely careful refractionation, intermediate plateaus of components (or constantboiling fractions) occurring in small percentages were discovered.
FACTOR 2. TOTALTIME OF DISTILLATION Given a particular apparatus, there will be a certain optimum total time of distillation, below which the accuracy of separation suffers unduly and above which the increased sharpness of separation does not justify the extra time. The more effective the fractionating column, the shorter will be the total time of distillation to secure the desired results. The amount of hold-up of material in the reflux or condensing section of the column is frequently a controlling factor as to time of distillation and the time of distillation may be increased several times, by the use of large-diameter condensing tubes with bulky seals or with reflux partitioning devices, over the time which would be required with a column designed for least practical hold-up in the reflux. With the apparatus described in this series of papers, the time of distillation varies from as little as 1.5 hours to 5 or more hours (averaging 3 to 4 hours for most routine determinations), depending entirely on the nature of the sample and on the requirements of the analysis, but including a range of sample size from one to several thousand cubic centimeters and for either low- or high-temperature distillations. Typical results showing extremely sharp fractionation are given in Figures 4 to 13 of Part I11 of this series, which show far sharper separation than the results of similar distillations in columns several stories high (6,14) and taking as many as five days. It will be shown in this paper that even infinite time cannot compensate for poor rating in factors 4,5, 6, and 7.
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in steps, in order t o be able to control the distillation pro per1y , and to f a c i l i t a t e the recording and correction for pressure of the temperature readings. The second d i s t i l l a t i o n pressure may conveniently be about 40 mm., and the third pressure 5 mm. or lower. For distillation pressures lower than about 1 mm., packed fractional distillation columns are not suitable because of excessive pressure drop and for other reasons (24). (d) For
precise f r a c t i o n a t i o n it is essential t o h a v e precise and practically lagless control of heat supply to still, reflux cooling, distillation pressure, etc. Otherwise it will neither be possible t o adjust the reflux r a t i o in t h e manner described above nor to maintain the column precisely a t the desired condition of wetness of packing. All t h e s e o p e r a tional variables a p p e a r complicated and difficult to standardize and record. H o w e v e r , in p r a c t i c e , it has been found p o s s i b 1e 6 0 to stadardize on Operation
FIGURE 2. EXPERIMENTALLY DETERMINED TOTALHOLD-UP OF LIQUID IN VARIOUS DISTILLING TUBEAND PACKING COMBINATIONS S P Standard Precision distilling tubes, dbeled A , l3, 9, H , and I in Figure, 1 R ,Figure Regular 1 distllllng tube, labeled C In
e,
54,Semi-Regular distillingtube,labeled D
FACTOR3. REGULATION OF DISTILLATION RATE, REFLUX that different operain Figure 1 S, spiral distilling tube, labeled F in RATIO,DISTILLATION PRESSURE, AND OTHEROPERATING tors will Secure very Figure 1 silnilar r e s u l t s in Subspripts: u , unpacked. s, single-wire VARIABLES coil. d, ,double-wire coil. f, flat-strip coil. j , jack chains. 8 , glass cylinders The following variables comprise the procedure of frac- practically the same tional distillation analysis. The same column and apparatus total distillation time. will give widely different results according to the procedure It has also been found possible to develop an automatic recordused. The following generally valid observations are made, ing and control mechanism, adapted for low-temperature frackeeping in mind the fundamental criterion of least size inter- tionation analysis, in which practically all the important operational variables as included under factor 3 are fixed by a few mediate fractions per time of complete distillation. simple adjustments of the apparatus. (a) The amount of vapor entering the bottom of the columnis of course proportional t o the heat supplied to the still. Contrary to most accepted practice, it has been found in the present inFACTORS 4 AND 5 . RATIOOF TEN TIMESHOLD-UPOF vestigation that best fractionation can be obtained by regulating MATERIAL IN FRACTIONATINQ AND CONDENSING SECTIONS the heat to still and reflux cooling so that the packing of the TO MAXIMUM COLUMN CAPACITY column is barely wet (84). The reasons for this will be evident from the discussion under factors 4 and 5. Factors 4 and 5 are evaluated on the basis of the same set (b) Reflux ratio should be varied according t o difficulty of fractionation rather than maintained constant. More specifi- of experimental data. Otherwise, they are entirely distinct cally, the reflux ratio should be inversely proportional to the and independent, and two columns may have the same rating tangent of the fractional distillation curve; as in the case of the of the one factor and differ several hundred per cent on the automatic recording and control mechanism for low-temperature fractionation analysis described in Part IVZ of this series. This rating of the other. Both factors, especially factor 5 , have procedure is far more economical of time and yields better frac- been found to be of great importance in their effect on the tionation than maintaining the reflux ratio constant or following amount of the intermediate fraction per cent. Accordingly, any other procedure. a complete experimental study was made of a variety of dis( c ) Distillation pressure is preferably maintained constant at atmospheric. When the upper limit of the reflux temperature is tilling tube and packing combinations to determine their reached or when the still temperature approaches the temperature ratings for factors 4 and 5 . of incipient thermal decomposition of sample, it becomes necesDESCRIPTION OF DISTILLIKG TUBES. Figure 1 illustrates sary to lower distillation pressure. This should be done abruptly and gives all dimensions and details of nine distilling tubes of varying diameter in combination with several different types 2 Part I V is t o be printed in the Analytical Edition of May 15, 1933
ANALYTICAL EDITION
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AND COMPUTED DATAON HOLD-UP,MAXIMUM DISTILLING CAPACITY, AND FACTORS 4 AND 5 RATINGS TABLEI. EXPERIMENTAL ON VARIOUS DISTILLING TUBESAND PACKINGS (SEE FIGURE 1)
HELICAL GLASS CHARACTERISTIC SINGL~-WIREDOUBLE-WIRE FLAT-STRIP JACK CHAIN CYLINDER No PACKIXG COIL PACKINGCOIL PACKING PACKING PACKINQ PACKINQ PROPERTIES~ a2 0.133 0.21 ... .. ... b 0.028 0.028 .. 0.002 e 0.0000 0.236 d 0.161 ... e 0.020 0.020 ... 0.0045 0.0045 ... f 0.0245 0,0245 , . . .. n 0.2115 h 0.1365 ... .. ... .. i 2.75 Zero ... .. m 0.77 ... j .. m k 0.089 ... a B . Standard Precision micro0.11 0.14 ... .. ... .. b 0.038 0.038 distilling tube 3.0 mm. .. ... .. 0.002 inside diameter 0.00 .. . . . .. $ 0.148 0.176 . . . . . e 0.016 0.016 .. ... 0.006 0.006 ... .. f 0.022 0.022 ... ... 0.154 0.126 . . .. ... .. 4.0 3.5 i ... .. 0.385 0.359 .. j .. k 0.055 0.0629 .. C. Regular distilling tube 3.8 a 0.29 0.81 0.91 .. .. b 0.081 mm. inside diameter 0.081 0.081 ... .. e 0.000 0.006 0 0091 .. .. d 0.885 0.371 0 9ft2 .. 0.120 0.120 0.120 0.030 0.030 0.030 0.176 0.176 0.176 ... 0.710 0.196 0.806 ... 4.36 7.01 3.14 .. ... .. 1.63 2.57 0.279 .. .. 0.403 0.250 0.659 * .. D. Semi-Regular distilliflg tube 0.22 0.74 0.81 .. ... 3.8 mm. inside diameter 0.079 0.079 0,079 .. ... .. C 0.006 0.009 0.00 .. d 0.813 0.299 0.880 .. e 0.075 0.075 0.075 0.028 0.028 0.028 .. f 0.103 0.103 0.103 .. 0.710 0.196 0.777 .. ... 4.36 7.01 3.14 .. .. 1.63 0.279 2.47 .. 0.237 0.147 0.328 .. E . Standard Precision distil0.17 0.63 0.72 .. ... ling tube 3.8 mm. inside 0.06 0.06 0.06 .. ... C 0.009 0.006 0.00 diameter .. *. 0.771 0.684 d 0.23 .. .. 0.025 0.025 0.025 0.009 0.009 0.0344 0.034 .. ... n 0.196 0.649 h 0.736 ... i 7.01 4.36 3.14 .. ... 0.279 1.49 0.0234 .. j ... *. k 0.015 0.109 0.0788 .. 1 a and k experimentally determined, others derived or computed. . . . . . 2 a Total hold-up of liquid in distilling tube q, Total liquid and vapor hold-up in condensing section, obtainea Dy b: Total hpld-up of vapor in distilling tube adding e to f C. Correction for packing volume, to be subtracted from b h, Hpld-up of liquid and vapor in fractionating section only,obtained by d? Total liquid ana vapor hold-up in distilling tttbe, obtained by subsubtra@:ng,g from.$ tracttng c from the sum Of a and b 2, Maximum atsti‘llinn caoacitv of distillinn tube j Factor 4 rating , obxainkd by dividing h b y i e, Hold-up of liquid in condensing section k’, Factor 5 rating , obtained by dividing g by i f, Hold-up of vapor in condensing section DESCRIPTION OF DISTILLINGTUBE A . Standard Precision microdistillins tube 2.6 mm. inside diameter
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f
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of packings as shown. These distilling tube elements may of course be combined with any desired type of lagging, still heating and reflux cooling parts, etc., to make up a complete column assembly. They may be used as parts of complete vacuum-jacketed fractionating assemblies, sych as those shown in Figures 16 and 17. It will be shown that a highly efficient vacuum jacket is necessary to develop the full usefulness of any of the distilling tube packing combinations. The tubes labeled (‘Standard Precision” are so termed because of their specific fractionating and refluxing lengths, and certain other details which have been standardized for use in a vacuum jacket described below; they will also be shown to be most effective for precise fractionation use. The “Regular” tube is taken from the Regular low-temperature vacuumjacketed fractionating column previously described (19). The “Semi-Regular” tube is introduced only for the purpose of determining the liquid hbld-up of the triple seal of the Regular tube. The “spiral” tube is part of a spiral vacuumjacketed column constructed to the dimensions and specifications of the modification described by Davis (9). Drippers were constructed on the bottom ends of all the tubes except the 12.5-mm. inside diameter tube, as otherwise drops of liquid would tend to collect a t the ends and obstruct the vapor passage.
8::
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DESCRIPTION OF PACKINQS. The wire coil and flat-strip coil packings were made of dimensions and pitch as seemed best from previous experience. Changes in size of wire or pitch, or in the spacing of the inner and outer coil, or both, would of course affect the characteristics of the packing, depending on the extent of the changes. To a considerable extent the effect of such changes can be estimated from the experimental data. Other packings or distortions of the glass’wall of the distilling tubes were not tried. It was not intended to survey all known packings, but merely to evaluate those which promised best fractionation. EXPERIMENTAL DETERMINATION OF HOLD-UPOF LIQUID. The hold-up of liquid on the walls and packings was determined experimentally by carefully pouring exactly 10 cc. of c. P. toluene from a buret into the top of the exactly vertically aligned distilling tube, and measuring (in a 10-cc. finely graduated buret) all the liquid drained from the bottom of the tube after one minute of time elapsed from the pouring of the 10 cc. From three to eight or more such tests were made on each tube and packing combination, and the average of all readings was taken, except those obviously in error. The draining period of one minute was chosen as it most nearly represented the condition of wetness of column packing found
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AND COMPUTED DATAON HOLD-UP,MAXIMUM DISTILLINGCAPACITY, AND FACTORS 4 AND 5 RATINQ~ TABLEI. EXPERIMENTAL (Continued) ON VARIOUS DISTILLINGTUBES AND PACKINGS
F.
DEECRIPTION OF DISTILLINQ TUBE Spiral distilling tube 5 mm. inside diameter, 400-cm. distilling tnbe length
HELICAL CHARACTERISTIC S I N G L E - W I R ~DOUBLE-WIRE FLAT-STRIP JACKCHAIX PACKING P R O P E R T I E S ~No PACKING COILPACKINGCOILPACKING PACKING ... ... aa 2.68 ... .. b 0.389 ... ... C 0.00 . . , . . ... .. d 3.069 ... ... .. 0.170 ... 0.042 ... .. ... ... 0.212 ... ... .. h 2.857 ... ... ... i 7.02 ... ... ,. ... 4 . 0 7 . . . . . j k 0.303 ... .. 1.30 1.67 1.22 0.44 0.171 0.171 0.171 0.17 0.012 0,018 0.019 0.00 1.372 1.823 0.61 1.459 0.066 0.066 0.066 0.066 0.026 0.026 0.026 0.026 0.091 0.091 0.091 0.091 1.281 1.732 1.368 0.520 10.8 6.9 14.75 24.0 1.6 1.86 0.217 0.927 0.132 0.0845 0.0618 0.0380 3.40 2.23 2.13 1.77 a 0.61 0.254 0.254 0.254 0.254 b 0.254 0.040 0.021 0.029 0.014 C 0.00 3.614 2.463 2.354 2.010 0.864 @ d 0.092 0.092 0.092 0.092 e 0.092 0.035 0.035 0.035 0.035 0.035 f 0.128 0.128 0.128 0.128 0.128 0 3.486 2.227 2.335 1.883 h 0.736 14.5 8.4 26.5 21.20 i 36.3 1 . 5 3 4.15 0 . 7 1 1 . 1 0.203 j 0.152 0,088 0.0601 0.0481 k 0.0351 ... 4.92 a 1.17 ... ... .. 0.651 0.651 b ... ... 0.114 0.00 C ... ... .. 5.457 1.821 d 0.174 ... ... e 0.174 . . . , . . . . 0.098 0.098 f 0.272 0.272 0 5.186 h 1.550 ... ... .. 32.50 i 106.0 . . . . . 1.60 . . . 0.146 j 0.0835 ... ... .. k 0.0256
7.
. I
GLASS CYLIND~R PACKINQ
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0. Standard Precision distilling tube 6.4 mm. inside diameter
H . Standard Precision distilling tube 7.8 mm. inside diameter
I.
Standard Precision distilling tube 12.5 mm. inside diameter
by actual experience to be most conducive to good fractionation. The resulting data are plotted on log paper (Figure 2) and seem to correlate very well, considering that the different wire-coil and flat-strip packings were not strictly similar with respect to column diameter, The unpacked Regular (C) and Semi-Regular (D)tubes show much higher hold-ups than the corresponding 3.8-mm. inside diameter Standard Precision (E) tube; and the Regular tube has higher hold-up than the Semi-Regular. These differences can be due only to the higher hold-up of liquid in the larger condensing tubes of the Regular and Semi-Regular tubes, and in the triple-wall seal of the Regular as against the Semi-Regular tube. The spiral tube ( F ) shows a very high liquid hold-up, due in part to the extra length (400 cm.) of the fractionating length proper as against the length (101.6 cm.) of the other tubes; but due mostly to the increased surface of the large condensing tube (7.8 mm. inside diameter) and capillary action of the triple-wall glass seal. Contrary to the statements of Davis (9) and Young (18),an inclined spiral tube holds up much more liquid (rather than less) than a vertical tube of same uncoiled length and bore. The single-wire coil and helical flat strip show least hold-up of all the packings, with the double-wire coil next in order. The jack chains show a much higher proportionate hold-up, while the glass cylinders hold up an abnormal amount of liquid compared with all other packings. For the unpacked tubes or for tubes packed with practically similar packings, the liquid hold-up seems to vary directly as the 1.6th power (approximately) of the distilling tube diameter. Tubes A and B show erratic behavior due to the use of wire coils dissimilar to those used in the larger tubes and to the increasing effect of capillarity in these small diameters. Unpacked tube A tends to fill with liquid, instead of draining smoothly. The apparent discrepancy between the 2.6-mm. and the 3-mm. inside diameter single-wire coil packed tubes is due primarily to difference in the number of turns of wire per unit of length, and can be greatly lessened by using a packing with fewer turns.
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7.15 0.651 0.134 7.667 0.174 0.098 0.272 7.396 21.60 3.42 0.126
vapor to liquid, as explained below. It should be observed that the hold-up of material in vapor form is by no means inappreciable, whether in the fractionating or in the reflux section. Young (98) shows similar results of experiments on the liquid and vapor hold-ups of various distilling tubes; however, his values for unpacked distilling tubes of various lengths and bores are erratic when plotted as in Figure 2, and are in some instances more than 100 per cent greater or smaller than the present results. Inasmuch as the method used in the present investigation is a direct and accurate one, involving only measurement of liquid in finely graduated burets, and the results obtained are highly concordant when plotted, it must be concluded that Young’s data are considerably in error, or at least not run under standardized conditions. COMPUTATION OF HOLD-UPIN REFLUX.The hold-up of liquid in the condensing section of each distilling tube is computed either by proration of the total liquid hold-up in the case of the Standard Precision tubes, A, B, E, G, H , and I , or by suitable comparison of tubes C, D , E, and F. The hold-up of vapor is computed from the dimensions, using the factor of 1/225 to convert volume to cubic centimeters of liquid. Upon inspection of figures under g in Table I, it will be noted immediately that tubes C, D , and F have a hold-up far greater than corresponds to their distilling tube diameter. This fact is of great importance as will be apparent from the discussion under factor 5.
EXPERIMENTAL DETERMINATION OF MAXIMUM DISTILLINQ
CAPACITY. Direct test of maximum distilling capacity by distillation of a test compound or mixture is very uncertain and unreliable unless the variables of sample composition, column temperature, column and still insulation, etc., are COMPUTATION O F HOLD-UP O F M.4TERIAL I N VAPOR FORM.exactly fixed and measured. Essentially, maximum disInasmuch as the column holds u p material in vapor as well as tilling capacity is fixed by the amount of liquid which can be in liquid form, the hold-up of material in vapor form was coursed down the column without priming against an upcomputed from the dimensions of the distilling tubes as coming vapor of quantity bearing a fixed volumetric relation shown in Table I. A factor of 1/225 was used to convert to the quantity of reflux liquid. It seemed, therefore, far
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IMIDE DIAMETER CF DISTILLING TUBE
FIGURE3. EXPERIMENTALLY DETERMINED MAXIMUM DISTILLING CAPACITY OF VARIOUSDISTILLING TUBE AND PACKING COMBINATIONS Sp, Standard Precision distilling tubes. R Regular distilling tube. SR, Bemi-Reguh distilling tube. S, spiral distillin tube. s, single-w:e coil. d , double-wire coil. f, flat-strip C O I L i, lack chains. 0, glass cylinfers.
preferable to use and to standardize permanently on a much simpler and what is believed to be a more positive and reliable determination of maximum distilling capacity, as follows: Toluene is poured into the top of the column at a measured constant rate (using a mercury flowmeter for measurement), and simultaneously air is blown into the bottom of the distilling tube at a gradually increasing, exactly measured rate, until the column just begins t o prime or flood. The rate of pouring the liquid toluene into the distilling tube is then changed, and the air flow again increased gradually, until incipient priming occurs. The quantity of toluene in cubic centimeters per minute is then plotted against the ratio of quantity of toluene in cubic centimeters per minute over quantity of air in cubic centimeters per minute required for incipient priming; and the resulting curve interpolated at a ratio of 1 cc. of toluene to 225 cc. of air, or to obtain the maximum vapor capacity of the distilling tube. A filter flask attached by means of a cork to the bottom end of the distilling tube is used to direct air flow and to collect the toluene draining from the column.
e
While the priming point is a very unstable phenomenon, easily affected by slight variations in the packing, by dirt on the packing, etc., the apparatus and procedure themselves are very simple. . The actual determination of the priming point is difficult and requires some experience and skill. It was found that the best criterion is the first sign of periodic slugging or momentary complete entrainment of liquid en bloc in sections of the tube, followed by a burble of air dispersing the liquid. Such slugging would of course ruin the effectiveness of the column as a fractionating instrument. Maximum vapor capacity is taken as practically equal to the slugging point capacity thus determined. Above this point the column is unstable and priming is sure to occur a t the slightest disturbance.
Subscripts: u, unpacked.
The factor of 1/225 is chosen t o convert air volume to equivalent cubic centimeters of liquid because it can be shown that this ratio generally holds true for most liquids and their vapors a t atmospheric boiling point temperatures, including the range of aliphatic hydrocarbons, ethane to octane and beyond. The objection may be raised that toluene at room temperature is not the same thing as a particular liquid sample to be distilled; nor is air the strict equivalent of a saturated vapor in contact with its particular equilibrium liquid. However, the usefulness of such a standard simple and easily reproducible test is apparent. Actually, the discrepancies introduced by the assumptions of this test are not great, and its findings have been well substantiated by actual distillations made with the distilling tubes and packings tested. The various distilling tubes and packings, already described, were thus tested for maximum vapor capacity and the results incorporated under i in Table I and plotted on log paper in Figure 3. Tubes C, D, and E have about the same capacity within the limit of experimental error, as would be expected, although their liquid hold-ups are different. The capacities of the single-wire coil packed tubes are nearest to those of the unpacked tubes with double-wire coil, helical flat-strip coil, jack chains and glass cylinders following in order. The order of capacities is the reverse of the hold-ups in Figure 2. The spiral tube shows an abnormally low capacity, just as it had previously shown an abnormally high liquid and vapor hold-up. This is, of course, due to the fact that the liquid draining down the spiral column does not have full gravity head and is therefore more easily entrained and slugged upward. On the whole, the data fall on practically straight smooth curves, showing the substantial accuracy of the experimental determinations. Jack chains show both materially less
March 15, 1933
INDUSTRIAL AND ENGINEERING CHEMISTRY
125
hold-up and materially higher capacity than glass cylinders, of the sharpness of separation that may be secured at the top even though they present a great many more liquid-vapor of the column packing for the same sample and conditions of operation. Figure 6 serves graphically to illustrate this aontacts and changes of path per unit column length. Tubes A and B are again erratic, for the reasons discussed statement, which follows from theoretical considerations as above. The capacity of the unpacked tube A is zero, signify- reviewed in Young's book (d6), and from the nature of the ing flooding a t no capacity; whereas the introduction of a criterion of column effectiveness as defined in this paper, fine wire spiral into the tube bestows upon it a fairly respect- namely, least intermediate fraction per cent in least total time of distillation (without regard to sample size). For the two able capacity, due to the draining effect of the wire. From the experimentally determined maximum capacities columns, A and B, as shown, the composition and temperature i t is possible to approximate the minimum time of distillation gradient are exactly the same. Both columns are assumed t o required for a sample of a given size distilled in one of the have the same sample capacity, and hence the size of sample tube and packing combinations tested; or conversely, to is the same for both. However, the size of the intermediate determine the maximum size sample which can be distilled fraction as shown on the corresponding fractional distillation in a given total time of distillation. An average reflux ratio curves above the columns is widely different-column A of 6 to 1 may usually be assumed for routine determinations, is three times as effective as column B. It is not necessary for yielding a sample size of 43.6 cc., for each hour of total time the general truth of this relation that the separation between of distillation, in the case of the 3.8-mm. Standard Precision components be as sharp as indicated in the illustration. single-wire coil packed tube; and 212 cc., for each hour of Factor 4 is not distinguished by a determination of the total time of distillation in the case of the 7.8-mm. Standard height of equivalent theoretical plate (H. E. T. P.) as dePrecision double-wire coil packed tube. (All cc. figures given scribed by Peters (18)and Carswell (7) ; nor by any procedure in this paper are in liquid equivalent unless otherwise stated.) for determining the effectiveness of a column which is based Much higher average reflux ratios may, of course, be used for only on the composition and temperature gradient throughout precise work with corresponding diminution in size of sample the column. It is interesting to compare the relative importance of factor per hour total time of distillation. The maximum capacity figures are thus an index to the dis- 4 as defined and of factor 6, which expresses the scrubbing tilling capacity of the column. However, as regards the size effectiveness of the column. It is well known from general of the intermediate fraction per cent, the criterion of sharpness of separation used in this FRACTIONAL DISTILLATION CURVE paper, neither the maximum capacity figure i , nor the total hold-up figures, h and g, are of 5a 2 any significance. It is the relation between the 8 0 $ hold-up and maximum capacity that is both z a0, significant and important. DERIVATION OF FACTORS 4 AND 5. From the D IO 20 30 40 59 SO m 80 90 100 C C - DI 3x1 C C DiSTLLLED data thus far presented and incorporated in Table I, it is possible to compute factors 4 and 5 as listed under j and k , respectively, in the table. I n order t o give these factors a more A S S U M E INSIGNIFIC immediate physical significance, the maximum LD UP ABOVE COLUMN capacity was multiplied by 10 to r e p r e s e n t , roughly, the size sample which would be taken for a total time of distillation of 2 or 3 hours; TO CONDENSER and the quotient of the hold-up figure over ten times maximum capacity, in turn, multiplied by 100 to represent the percentage of the hold-up in terms of the sample size as assumed. In the case of factor 4,the percentage figure OPERATING ON MIXED thus obtained must be considered together with O F EXACTLY SAME NUMBER O F the rating of factor 6 before any final conclusions CH E.TP'5. IN BOTH CASES can be made. In the case of factor 5, the percentage figure indicates the minimum obtainable xx *xr size of intermediate fraction regardless of all v *\* other considerations. The final percentage fig*"' LEGEND ures for factors 4 and 5 are plotted on logarithmic = LOWER BOiLlNG COMPONENT >:: cross-section paper in Figures 4 and 5, respec,\ == HIGH BOILING COMPONENT :a* tively. \\\ I : : : : : j = MIXTURE COMPONENT%
