INDUSTRIAL AND ENGINEERING CHEMISTRY
November, 1934
Fresh sections of collenchyma were treated with Schweitzer’s reagent (8) in the customary manner. B y this treatment the cellulose con’stituents were removed, leaving the pectic compounds intact. After this treatment the sections showed the absence of cellulose by the same four tests used above (8). Figure 16 A is the bright-field image of the collenchyma treated for the removal of cellulose and B is the Spierer image. The Spierer image of this section, containing no cellulose, cannot be distinguished from the one in which cellulose is known to be present as distinct lamellas (Figure 15 B). ACKNOWLEDQMENT The authors are indebted t o A. G. Lang for assistance with the botanical material here reported.
1213
LITERATURECITBD Anderson, D. B., Sitrbe-r. Akad. Wiss. Wien, Math.-naturw. Khsse, Abt. I, 134,429 (1927). Mark and Meyer, Ber., 61, 593 (1928). Markand Meyer, 2.physik. Chem., 2B, 115 (1929). Sanders, J. P., and Cameron, F. K., IND.ENQ.CHEM.,25, 1371 (1933).
Seifrim, W., Colloid Symposium Monograph, VIII, 174-202 (1931); J . Phys. Chem., 35, 118-29 (1931). Spierer, C., Arch. sci. phys. nut., 8 , 2 1 (1926). Thiessen, R.,IND. ENQ.CHEM.,24, 1032 (1932). Tunman, O., “Pflanmenmikrokemie,” 2nd ed. rev. by L. Roeenthaler, Gebriider Borntraeger, Berlin, 1931. Zeiss, Carl, Inc.. directions for uee of Spierer dark-ground illuminating system, Forn., F. 533a/Mi. RECEIVZD August 3, 1934. Presented before the Diviaion of Cellulose Chemistry at the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
Efficient Small-Scale Fractionating Equipment C. 0. TONGBERG, D. QUIGGLE,AND M. R. FENSKE Pennsylvania State College, State College, Pa.
I
N THE l i g h t e r
TUBE RIR LIQUID
FIGURE X. GLASSSIPHON COLW
petroleum p r o d ucts, such as gasoline, more efficient fractionation is d e s i r a b l e , particularly for making specialty products. Small-scale fractionating columns a r e n o t only useful a s a n a l y t i c a l apparatus but are also helpful in the design and operat i o n of l a r g e - s c a l e equipment. While small columns can be made much more efficient than large ones, they usually lack the means of m e a s u r i n g and controlling the important operating varia b les-name1 y , rate of distillation and reflux ratio. Two efficient f r a c tionating columns are described. One is an all-glass apparatus capable of d i s t i l l i n g a charge of 50 to 100 cc. The other is a n i c k e l column with nickel p a c k i n g to which 11 liters can be charged. In both these columns
it is possible to know the rate of distillation and the reflux ratio at any time. These columns have been tested with binary mixtures of carbon tetrachloride and benzene, methylcyclohexane and toluene, n-heptane and methylcyclohexane, n-heptane and toluene, and benzene and toluene. Thus mixtures of the various types of hydrocarbons present in gasoline-namely, paraffins, naphthenes, and aromaticshave been used. ANALYTICAL DATA
A more complete description of the analytical data and methods used in this work is given elsewhere (4). All the materials used were of high purity. I n order to facilitate the presentation of the data and conserve space, the following terms are defined : RATE OF BOILING. This is the amount of liquid boiled up from the still as vapor in a given time and represents the forward flow in the column. If the latter is operating under total reflux, then this vapor is entirely condensed and returned to the still through the column countercurrent to the equivalent amount of rising vapor. The higher the rate of boiling under total reflux, without flooding, the greater is the capacity or throughput of the column. REFLUXRATIO. This is the ratio of the volume of condensate returned t o the top of the column to that withdrawn from the top as product. H. E. T. P. This is the height in centimeters (or inches) equivalent to a theoretical plate when determined under total reflux. It is obtained from analysis of the distillate and liquid in the still, and from vapor-liquid equilibrium data. VAPOR-LIQUID EQUILIBRIUM DATA. This is the composition of a vapor in equilibrium with a liquid a t a constant pressure of about one atmosphere. Such data are usually determined experimentally. The following notations pertain to the vaporliquid data used in this work. CARBONTETRACHLORIDE AND BENZENE.The vapor-liquid data were correlated by Varteressian (7) from the data of Rosanoff and Easley (6). Analysis was by means of refractive index (7). BENZENEAND TOLUENE.The va or liquid dia am was calculated from the data of Rosanoff, %hion, and &%dse (5).
1214
INDUSTRIAL AND E N G I N E E R I N G CHEMISTRY WSITION ON
Vol. 26, No. 11
result that the heat loss from the column is compensated for by a slightly higher rate of MEDIUM xx, W'WIND'NG 500 w. ONLY boiling in t h e s t i l l . This MONE method has the advantage 5 0 0 5 3 0 3 W . WINOo-'Loo w. that the proper temperature RESISTANCE INGS IN SERIES. level for the column is automatically maintained. This 4coW. HIGH l$s'l~o~~$~",~ 700 W.ONLV manner of reducing heat loss %OW. MEDIUM 403ALONE W*WIND'NG 403 R ONLY from the c o l u m n has been found suitable for materials mw I ~ $ ~ s ~ ~ ~ & 180 D W.- ONLY boiling up t,o 140' C. For higher boiling liquids a small COMBINATIONS OF THE A W E GIVE F W M 0-IMO W. electrical-resistance winding FIXITIONS CONNECTED ON 3-HElT SWlTWN around the air jacket surHIGH 1 AND 3 , 2AND 4. MEDIUM L A N D 2 L A N D 3 rounding the column may be LOW lAND'?. ! used to afford ample addiHEAT IN^ ClRCUlT ON STILL tional control of heat loss. The v o l u m e of refluxing liquid is measured by a small siphon cup having a capacity of ap roximately 1 cc. and is attacKed to the lower end of .& S. Asbestos mweredMrmel the column as shown in Fig10 nun on cenfers. FIGURE 2. ELEVENLITER ure 1. Simply counting the CAPACITY FRACTIONATING COLUMN number of times per minute this cup trips and noting the rate of product withdrawals from the columns enables the reflux ratio to be known. The heating of the still has been designed so that 90 or more per cent of a charge may be distilled off without interfering with or slowing down the rate of boiling. A Variac transformer' has been found suitable for controlling the heat input. 3 HEAT SWITCHK
APPR3X.W..
'lRCUiT
5W6MOW.WINDINGS IN PARALLEL
%Ow'
5oo-800yL
TABLE I. H. E. T. P. TESTSAT TOTAL REFLUXIN GLASSCOLUMN (FIGURE1) This mixture follows Raoult's law, and on this basis boiling point-composition curves were calculated for pressures a t and near atmospheric and were used in analyzing the mixtures. *HEPTANE AND TOLUENE.The vapor-li uid diagram was determined by Bromiley and Quiggle (3). Anajysis was by means of refractive index. *HEPTANE-METWLCYCLOHEXANE. The value 1.07 for the relative volatility of this mixture as determined by Beatty and Calingaert (1) was used to represent the vapor-liquid equilibrium data. Analysis was by means of refractive index (3). *HEPTANE-TOLUENE.The vapor-liquid equilibria as well as the data for analysis were determined by Birnstiel (I). Analysis was by means of refractive index.
RUNNo.
1 2 3 4 5 6
7 8 9 10
MOLEPERCENT
MORBI VOLATILE TOTAL COMPONENT THEORETICAL Distillate Still PLATES H. E. T. P. CC./hT. Cm. Inches NORMA.L HEPTANB AND TOLrlENE 110 88.5 19.0 12.5 3.6 1.4 12.5 130 87.2 15.5 1.4 3.6 240 11.5 3.9 1.55 87.0 22.2 360 12.5 3.6 1.4 85.0 9.3 11.5 410 85.6 19.7 3.9 1.55 410 86.0 18.8 11.5 3.9 1.55 11.5 3.9 1.55 410 85.0 15.2 420 88.2 26.4 11.5 3.9 1.55 12.0 420 87.0 17.0 1.46 3.7 89.4 32.0 11.5 3.9 1.55 420
OF RATBI LIQUID BOILINO
CARBON TETRACHLORIDE AND BENZINB
1 2 3 4 5
ALLGLASSCOLUMN The all-glass column having the still and condenser directly attached to it is shown in Figure 1. Since it is all glass, the contents, column, still, and condenser are completely visible. The apparatus measures about 100 cm. (39 inches) in over-all height. The column is 10 mm. inside dlameter and is packed for a length of 41 cm. (16 inches) with one-turn glass helices (8). When tested with the binary mixture of wheptane and toluene, the column was found to have the equivalent of 11 to 12 perfect plates, as the data of Table I show. As also indicated in this table, the mixture of carbon tetrachloride and benzene does not behave like the hydrocarbon mixture. This has been observed before (4) and is probably due to the wider differences in hysical and chemical pro erties of these two components. Wit{ hydrocarbons a rate of foiling of 400 to 500 cc. of liquid per hour under total reflux is possible without flooding. A large variety of pnckin materials for fractionating columns has been studied and t&s simple type of packing has been found best, both from the standpoint of efficiency and throughput (4). To reduce heat loss from the column, it'is jacketed by vapors rising from the still. At the start of the fractionation the stopcock a t the left of the column is opened. Because the pressure drop through this open annular space is less than through the packing, the vapors quickly rise and heat the column. These rising vapors expel the air in the annular jacket, and, after they have reached the condenser, the sto cock is closed. The vapors are now forced up the column, an,! as heat loss occurs, some condensation takes place in the annular space, the correct temperature level being thus maintained. Their condensation then draws more vapor into the annular space with the net
360 370 390 450 490 I i \ .
50.0 29.2 46.3 43.0 46.3
27.5 9.3 22.0 21.5 21.0
8.0 8.0 9.0
8.0 9.0
5.9 5.9 5.1 5.9 5.1
2.3 2.3 2.0 2.3 2.0
METALCOLUMN
This larger apparatus has a steel still capable of taking a charge of 11 liters. The column, which is a nickel tube 33 mm. (l6/'6 inches) outside diameter X No. 20 Stubs gage wall thickness, and 2.74 meters (9 feet) tall, is connected t o the still by a flared compression fitting. The condenser is a section of 25-mm. (1-inch) Pyrex glass piping with one end flanged. The companion flange is attached to the nickel tube. This gasketed joint of Pyrex glass to nickel has been found entirely satisfactory. The product is withdrawn as vapor from the column just below the condenser. The product line is a, 6-mm. (l/Anch) 0. d. tube brazed onto the side of the nickel column but does not project into the column. With this arrangement of connections t o the still, condenser, and product receiver, the column can be quickly dismantled and repacked or inspected for corrosion. A sketch of the apparatus is shown in Figure 2. Following is a more detailed description of the parts: The still is made of a 445-mm. length (17.5 inches) of 205mm.
i. d. (&inch standard) steel pipe, with circular pieces of boiler plate 1
Purchased from General Radio Company, Cambridge, M w .
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
1215
TABLE II. H. E. T. P. TESTSU s I N G U-TYPE PACKING
RUN No.
VELOCITYOF LIQCID AT T O P
L./hr
(In nickel column, 32 mm. i. d. and 2.74 meters packed section) MOLEPERCENTMOREI TOTAL THEORETICAL VOLATILB COMPONEINT PRESSURE DROP Distillate Still PLATES
Mm. of Hg
H. E. T. P. Cm.
Inches
NORMAL HEPTANB AND MBTHYLCYCLOHEXANE'
1.8 2.0 2.1 2.2 2.3 4.0 4.8 5.0
7.0 7.0 6.5 5.0 8.0 37.0 44.0 36.0
81.0 82.0 76.0 75.0 80.0
25.5 24.0 24.0 22.8 26.6 20.5 25.6 28.0
CARBON TETRACHLORIDE AND
BENZENE^
80.5
84.0 90.0
38.0 42.5 50.5 40.5 35.0 38.0 33.0 35.5
7.4 6.6 5.6 7.1 8.1 7.4 8.7 7.9
2.9 2.0 2.2 2.8 3.2 2.9 3.4 3.1
35.5 7.9 3.1 5.5 8.1 35.0 3.2 8.0 9.0 31.5 3.5 9.5 34.5 8.1 3.2 38.0 2.5 2 5 . 0 1 1 . 5 4.5 33.0 8.2 9.0 32.0 3.5 4.0 38.0 9 . 0 3 2 . 0 3 .5 40.0 6.0 10.0 4.0 41.0 27.5 10.0 a Maximum rate of liquid flow from top of column without flooding a t start of teat was 4.7 litera per hour; later i t was 4 . 2 liters per hour. b Maximum rate of liauid flow from too of column'without floodine - &hen packing was new was about 4 . 7 litera per hour; a probable average value is 3.8 liters. 1.6 1.6 2.7 3.6 3.7 3.8 4.0 4.8
10.0 10.0
8.0
76.5 78.4 76.0 72.2 66.8 71.5 74.6 71,5
brazed on each end. Around the length of the still is wrapped a 5-mm. thickness of asbestos paper which serves to insulate the heating wire from the steel still. The heating element consists of four separate windings wound side by side over the entire length of the still. Two of these windings are of 1.5 mm. ( l / ~ inch) X No. 36 B and S gage, and the other two are of 3 mm. ( I / * inch) X No. 35 B and S gage Chrome1 A ribbon, the former giving wattages of 300 each and the latter of 500 and 400. As shown in the heating circuit for the still in Figure 2, by means of three-heat switches and a variable resistance connected in series with only one of these 300-watt windings, it is possible to obtain combinations from 0 to 1500 watts. This circuit arrangement makes possible the minimum of external heat dissipation and power loss, while covering the whole range of wattage input to the still. By having four separate windings wound side by side over the entire length of the still, no matter what combination is used, the distribution of heat is always even. Standard 85 er cent magnesia pipe lagging is used to insulate the still from t e a t loss. The nickel column is covered with a 5-mm. thickness of asbestos paper. Over this, No. 16 asbestos-covered monel resistance wire2 is wound in order that the proper amount of heat may be supplied to give adiabatic conditions. A variable resistance in series with this winding enables up to 75 watts per linear foot (30.5 cm.) to be supplied. Over this winding is placed standard 85 per cent magnesia pipe lagging. Asbestos-covered chromel-alumel thermocouples are brazed onto the nickel tube, and others are brazed to copper strips about 250 mm. long, 25 mm. wide, and 2 mm. thick, placed between the asbestos paper insulation and the heating wire. These latter couples attached to the copper plates enable average temperatures to be obtained. By adjusting the heat input to the column so that the temperature difference between the column and the copper strips is either zero or small, substantially adiabatic conditions are maintained. For accurate control and a knowledge of the performance of the column, other couples are attached to the apparatus as shown in Figure 2. The product line is a 6-mm. (1/4-inch) 0. d. copper tube fitted a t the lower end with a small needle valve. It is not necessary to have any cup or collecting device on the end of the tubing leading into the column. Once the product line, n-hich is always inclined downward, has liquid in it, the withdrawal of liquid, as product, sucks more vapor into the line. For low-boiling materials it is best to have this product line water-jacketed. This is easily done by fitting the copper line inside of EL slightly larger copper tubing and flowing water in the annular space. A manometer, consisting of an 8-mm. (6/le-inch) 0. d. copper tube leading from the still to a mercury U-tube, allows the column to be run a t maximum capacity without flooding or priming. This copper tube should lead upward, a t least for a reasonable distance from the still so that any vapors condensed in it can flow back into the still and not interfere with the mercury manometer reading. The observed pressure dropr are given in Tables I1 and 111. A condenser of the form shown in Figure 2 has been found suitable. By having it as close to the top of the column as possible and the cooling water in the annular space as shown, the heat losses to the room are reduced to negligible proportions. In this way it is a simple matter to determine the amount 2
Purchased from H. E. Trent Co., Philadelphia, Pa.
of material being condensed per hour and so returned to the column as reflux. Thermocouples in the outlet and inlet condenser water are connected so that they read the temperature rise in the cooling water. The exit water from the condenser passes into a 500-cc. graduated buret with a stopcock on the bottom. Noting the time for a given amount of water to flow and the rise in its temperature, the heat picked up per minute or per hour is readily determined. For most materials the heat of vaporization is known or may be calculated with sufficient accuracy so that by observing the rate of product withdrawal from the column, the reflux ratio is knorvn. The entire apparatus is mounted on a light iron framework so that it may be moved about readily. The necessary control devices are mounted on a panel attached t o t h i s framework. This convenience facilitates the keeping of accurate records of the fractionation as well as the efficient operation of the column. Such centralized control is a significant factor in the successful performance of any c o l u m n . The panel is shown in Figure 3. The two rheostats, one controlling the heat to the column, the other the heat to the still, are shown, tog e t h e r w i t h small ammeters which read their re spect ive currents. D o u b l e - p o l e , doublethrow switches have been f o u n d very suitable for thermocouple s w i t c h e s , each accommodating two couples. The product-line valve is shown leading into the glass graduate. The FIGURE 3. EFFICIENTSNALL-SCALE mercury U-tube manomeFRACTIONATING EQUIPMENT ter reading the pressure in the still is also shown. The packing material is the most important part. It controls the capacity of the column as well as the enrichment produced in it (4). Two wire form packings have been tested in this column. One has the shape of a staple or carding tooth made from wire about 0.5 mm. in diameter, the staple measuring 4 X 6 mm. The legs of the sta les are about 4 mm. long, the distance between the ends of t f e legs being about 6 mm. These staples are packed into the column a t random. They have about 72 per cent free space. Results of testing this packing are given in Table 11. Here again the mixture of carbon tetrachloride and benzene behaves differently from hydrocarbon mixtures. As seen from Table 11,
INDUSTRIAL AND ENGINEERING CHEMISTRY
1216
Vol. 26, No. 11
The other packing material consists of one-turn nickel wire helices or rings. This is made by winding No. 26 nickel wire on a Pmm. (6/rinch) rod, and cutting each turn of the helix formed to give single rings. This pack. ing has about 86 er cent free space. A 2.74meter length of these hefcces is equivalent to about twentytwo perfect plates when tested with n-heptane and methylcyclohexane at total reflux. A rate of boiling of about 6 liters of liquid per hour under total reflux is possible with hydrocarbons without flooding. The results of tests on this packing are given in Table 111. In tests of this sort the time allowed for the column to come to equilibrium conditions varied from 1.6 to 8 hours.
ul
rb
1.4
i
LIQUIDHOLDUPIN COLUMNS VOLUME PERCENT DISTILLED
Figure 4
VOLUME PERCENT DISTILLED
Figure 6
ul
& r4 =-'
'$
60
auz 0 w
i VOLUME PERCENT DISTILLED
Figure 6
VOLUME PERCENT DISTILLED
Figure 8
Figure 7
VOLUME PERCENT DISTILLED
Figure 9
The liquid holdup in any column being used for batch fractionations is an important factor in detennining the sharpness or cioseness of separation, It is for this reason that packed columns are particularly suitable for batch fractionations since their liquid holdup is a small fraction of that of bubblecap columns. While it is a relatively simple matter to determine the liquid holdup of a packing in a static way-i. e., simply the amount of liquid that will adhere to a packing for a given drainage t i m e i t is not as apparent how the holdup might be determined when a column is in actual operation, and the holdup is dependent not only on the liquid that will adhere to the packing, but also on the rate of boiling and reflux used. However, a simple method has been found suitable. The method consists in adding a definite amount of a nonvolatile substance to a definite amount of liquid charged to the dry still and column. When the desired operating conditions are reached, when operating under total reflux, a known volume of the liquid in the still is withdrawn. The liquid is then evaporated leaving the nonvolatile material which is weighed accurately on an analytical balance. Knowing the amount of liquid in the still associated with the total amount of nonvolatile substance a t the start and a t the point of test, the liquid retained in the column is readily determined. Stearic acid has been found suitable. Tests of this sort on the all-glass column in Figure 1 show that, when the rate of boiling under total reflux is about 550 cc. of benzene per hour, the holdup is about 9 cc. of liquid; when the rate of boiling is about 250 cc. per hour, the holdup is about 8.0 to 8.5 cc. of liquid.
PERFORMANCE OF
THE
COLUMNS
In order to conserve space, the results of testing the columns as well as their actual performance in fractionating UI known mixtures are given in Figures 4 to 10, and Tables 2 W I to 111. For the most part the data are self-explanatory, gs All the test liquids used were pure materials so that the FIGURES~TO RESULTSOF ~~. 5l u :v , COLUMN analyses are reliable. The data as set forth in the tables TESTING are the principal factors determining the performance of the $0 :I column a t that time. Ample time was allowed to insure J equilibrium conditions to be established in the H. E. T. P. 2 tests. The data from which Figure 7 has been made are given in tabular form in Table IV for the reason that the analyses l 0 ~ 3 0 4 0 5 0 6 0 1 0 of the distillate and bottoms cannot be presented accurately VOLUME PERCENT DISTILLED in Figure 7. Data of this sort, where the still and distillate Figure 10 compositions are obtained a t the same time, are very useful a 2.74meter (%foot) length of this packing is equivalent to in analvsine the ooeration of batch fractionations. about thirty-five perfect -lates when bsted -%th a-mixture of fable" further d a b of interest to the problem n-heptane and rnethylcyc~hexrtneat total reflux. With hydro- of whether or not the H. E.-T..P. is-dependent . on the concen. . carbons a rate of boiling under total of about 4 liters of tration of the components being fractionated. 'l'hese tests liquid per hour is possibl; without flooding. u
v are
_.-.
November, 1934
INDUSTRIAL AND ENGINEERING CHEMISTRY
1217
TABLE III. H. E. T. P. TEEsTS USING ONE-TURN HELICES RUN No.
(In nickel column, 32 mm. i. d. and 2.74 meters packed section) MOLEP E ~ CBNT R MORB V ~ L O C I or T YLIQUID VoLnnLm COMPONENT TOTALTEEOR~TICAL AT TOP PRSBBURE DROP Diatillate Still PLATES H. E. T.P. L./hr. M m . Hg Cm. Inchrr N O R M A L EEPTANm A N D METHYLCYCLOHEXANEQ
51.0 47.0 48.0 45.8 52.2 63.8 62.0
5.2 5.5 5.6 5.8 5.9 6.0 6.3
20.7 19.3 18.0 17.5 23.4 20.7 19.3
21.5 20.5 22.0 21.5 18.0 29.0 29.5
13.5 14.0 13.0 13.5 16.0 9.9 9.7
5.3 5.5 5.1 5.3 6.3 3.9 3.8
17.0 17.5 20.0 19.0 18.0
17.0 16.5 14.5 15.0 16.0
6.7
19.5 18.5 20.0 21.0
15.0 15.5 14.5 13.5
5.9 6.2 5.7 5.4
CARBON TKTRACHLORIDE A N D BENPENEb
4.2 4.7 5.4 5.6 5.7
4 5
56.5 56.2 64.5 69.0 62.2
19 24 34 36 33
13.0 11.6 14.5 24.5 16.5
6.6
5.7 6.0 6.3
METHTLCYCLOHEXANE A N D TOLUENE
14 4.3 14 4.4 6.4 16 4 5.6 20 Maximum liquid rate without flooding was 6.2liters per hour. b Maximum liquid rate without flooding was 5.7 litera per hour.
90.4
TABLEIV. FRACTIONATION OF 100 cc. OF *HEPTANE AND TOLUENE MIXTUREIN GLAas COLUMN (FIGURE 1)O ?+HEPTANE DISTILLATE Distillate Still Vol. % Mole % Mole % 2 89.4 32.0 5 85.0 10.5 86.4 88.2 14.5 26.4 84.3 17.8 24.5 83.0 29.5 17.0 87.0 81.5 33.5 85.0 39.0 9.3 74.5 43.0 77.8 48.5 31.5 50.7 54.5
