ANALYTICAL CHEMISTRY
190 Table VII.
Concentrates Prepared from Gas-Oil Fraction Gas Oil, Wt. %
Nitrogen concentrate Aromatic concentrate Olefin concentrate Saturate concentrate
43 21 16
20
in Table VII. As the aromatic concentrate contains some sulfur and oxygen compounds, it can be seen that nonhydrocarbon compounds make up nearly half of the gas oil. Determinations on the nitrogen concentrate yielded the following data: nitrogen, 3.7995; oxygen, 2.55%; and molecular weight, 290. Calculation from these data indicates that many of the molecules in this concentrate must contain more than one atom other than carbon and hydrogen. The three classes of hydrocarbons are present in about equal amounts. From examination of the aromatic concentrate it appears that this material contains about equal quantities of one-, two-, and three-ring aromatic compounds. The saturate and olefin concentrates are predominantly straightchain materials. SUMMARY
A series of separation techniques, outlined in Figure 1, has been applied to a gas-oil distillate from shale oil. The techniques used were adsorption employing several different absorbents, vacuum distillation, thermal diffusion, and complex formation. The gas oil was a complex mixture of saturated, olefinic, and aromatic hydrocarbons, as well as nitrogen-, oxygen-, and sulfur-containing compounds. Hydrocarbons made up approximately half of the gas oil, This material was separated into a number of fractions, each of which was less complex than the original gas oil. Concentrates obtained were those composed of nitrogen compounds
and aromatic, saturated, and olefinic hydrocarbons The roncentrates containing the saturated and olefinic materials were further subdivided into straight- and branched-chain compounds, and the aromatic concentrate was subdivided into fractions containing different numbers of condensed aromatic rings. For the nitrogen concentrate, srparation was made into fractions containing different relative percpntages of basic and nonhasic nitrogen. 4CKVOW LEDGMENT
This project was part of the SJ-nthetic Liquid Fuels Program of the Bureau of .\lines and was performed a t the Petroleum and Oil-Shale Experiment Station under the direction of H. P. Ruc, H. I f . Thorn(,, and J. S. Ball. The authors wish to thank 1finer.r.a Landers and Eli7abeth A. Pratt for analytical determinations in connection with this project. The work was done under a cooperative agreement between the University of Wyoming and the P.S. Department of the Interior, Bureau of hlines. REFERENCES (1) Charlet, E. AI., Lanneau, K. P., and Johnson, F. B., A N . i L . CHEM., 26, 861 (1954). (2) Jones, A. L., and AIilberger, E. C., I n d . Eng. Chem., 45, 2689 (1953).
(3) Lipkin, 11. R., Hoffecker, W. A., Martin, C. C., and Ledley, R.E., h h L . C H E X . , 20, 1 3 0 (1948). (4) Aloore, R. T., AIcCutchan, P., and Young, D. -4., Ibid.,2 3 , 1639 (1951).
(5) O’Donnell, G., Ibid.,2 3 , 8 9 4 (1951). ( 6 ) Orchin, AI., and Woolfolk, E. 0.. J . A m . Chem. SOC.,68, 1727 (1946). (7) (8)
Schiessler, R. W., and Flitter, D., Ibid.,74, 1720 ( 1 9 5 2 ) . Smith, J. R., Smith, C. R., Jr., and Dinneen, G. U., ANAL.CHEX, 2 2 , 8 6 7 (1950).
RECEIVED for review July 26, 195%. Accepted December 20, 1954.
A Fifty-Stage Apparatus for Distillation at Very l o w Pressures BEVERIDGE J. M A I R , A R T H U R J. P I G N O C C O , snd FREDERICK D. R O S S l N l Carnegie
institute o f
Technology, Pittsburgh, Pa.
This report describes the design, assembly, operation, and testing of a 50-stage apparatus for distillation at very low pressures, in the range 0.01 to 0.1 mm. of mercury. Results are presented for two distillations, one performed on a commercial mixture of normal paraffins C18 to C22, and the other on a concentrate of normal ~ obtained from petroleum. Both disparaffins C I toC26 tillations were carried out at a vapor pressure near 0.03 mm. of mercury, a throughput of 80 ml. (liquid) per hour, and a reflux ratio of 55 to 1, w-ith a total distilling time near 1900 hours. Continuous distillation for about 6 hours was required to obtain a steady state in this 50-stage apparatus.
I n the intervening years, multistage apparatus for distillation a t very low pressures have been described by Fawcett and MeCowen ( S ) , Wollner, Matchett, and Levine ( I S ) , Brewer and Madorsky ( 2 ) , Madorsky, Bradt, and Strauss (6),Madorsky ( 4 ) , Aldershoff, Booy, Langedijk, Philippi, and Waterman (I), and Melpolder, Washall, and iilexander (9). The API Research Project 6 began in 1951 active work on the development of a multistage apparatus for distillation a t very ~ Q Wpressures. This apparatus has been referred to in a preliminary way (11). The present report gives a detailed description of the design, assembly, operation, and testing of a 50-stage ap. paratus for distillation a t very low pressures. DESCRIPTION OF APPARATUS
I
S I T S work on fractionating hydrocarbons of higher molecular weight occurring in the heavy gas-oil and lubricant fractions of petroleum, the American Petroleum Institute Research Project 6 reported in 1929 and 1935 some simple apparatus for the distillation of such material a t low pressures ( 7 , I d ) . Similar apparatus was used in the distillation a t low pressures of the “water-white oil” and the “extract oil” portions of the lubricant fraction of petroleum in an investigation that was concluded in 1938 (6, 8). It was realized, however, that really effective separation by distillation a t very low pressures would require an apparatus involving many stages of separation.
Figure 1 shows the main part of the apparatus for distillation a t very low pressures, consisting of the pot, A , the two halves of the rectifying section, B, and the reflux regulator, C. The rectifying section consists of gently sloping glass tubes (at an angle of about 3.5” to the horizontal) with internal stainless steel fittings arranged in such a manner as to constitute a succession of plates. The two halves, each containing 25 plates, are connected in series to give a total rectifying section of 50 plates. Details of the pot and the lower portion of the rectifying section are shown in Figure 2.
V O L U M E 2 7 , NO. 2, F E B R U A R Y 1 9 5 5
191 Figure 4 gives additional details, with section N taken near the middle of a plate and section M a t the end of a plate.
‘4! Figure 1. Schematic diagram of 50-stage apparatus for distillation at ter? low pressures A B C
Connection between the two halves of the rectifying section is effected by the coiled glass tubes, B1o, &I, and BIZ(Figures 2 and 3). Tube B12 provides for the removal of air during the evacuation of the apparatus. Tube B,I provides for the passage of distillate from the top of the lower half of the rectifying section to the bottom of the upper half, while tube Bla provides for the return of reflux from the upper to the lower half of the rectifying section. The reflux regulator, consisting of the tapered glass stopper, Ca, the tungsten rod, C,, and the control mechanism, C1 (Figure 3), has been described (10, 11).
Pot Rectlfging section Reflux regillator
I
Details of the upper portion and of the reflux regulator are shown in Figure 3.
i
/CI
The cylindrical pot, A (Figure 2), made of borosilicate glass, contains an annular ring for collecting the distillate that condenses on the upper portion of the walls. The pot is connected to the rectifying section a t two points. The upper connection, Aa, is for the passage of distillate from the collecting annulus to the rectifying section, B, and the lower connection, A,, is for the passage of reflux from the rectifying section t o the bottom of the pot. The outer containing tube of the rectifying section, BL! is made of borosilicate glass with the lower side flattened. Inside this tube is a stainless steel sheet, Bz, curved to fit the walls and bent to form troughs, B5. Successive plates are separated from each other by liquid-tight seals, B4,in the troughs, B;, and t)y vapor barriers, BB,as well as by ridges, B;, in the lower flattened side of the glass containing tube. For each plate the troughs have small stainlws steel tubes, BeJ sealed as shown. 0 1 6
6421
Figure 3.
Upper portion of 50-stage apparatus for distillation at very low pressures
B I t o B;, Bm, and B N have significance indicated in Figure 2 B13. Opening for charging and cleaning rectifying section Bir. Thermocouple inlet with thermocouple t o liquid in plate 27 P r e to Pw. Plates 26 to 50 C. Reflux regulator CI. Metal housing and control for reflux regulator C2. Tungsten rod, 12 AU-G C3. Ground-glass stopper Cd. Drop counter Ca. Connection to receiver D I CB. Connection t o source of low pressure, 0 2 1
Figure 2.
Lower portion of 50-stage apparatus for distillation at very low pressures
TOP SECTION
Si
A. P o t assembly A1. Opening for thermocouple -42. Opening for introduction of charge, with standard ground joint, 24/40
-42. S n n u l a r ring for collecting condensate Tube for carrying condensate t o rectifying section Thermocouple well A s . Glass pot .47. Tube t o return reflux to still pot B . Glass rectifying section, with internal stainless steel fittings B I . Wall of rectifying section, 85-mm. O.D. glass tube €31. Condensing surface, stainless steel, 0.02 inch thick B I . Vapor barrier, stainless steel, 0.02 inch thick, spot-u-elded a t one point t o top,of condensing surface Bc. Liquid barrier a n d spring s t r u t , hard-soldered in gutter, stainless steel, 0.03 inch thick B I . Gutter, t,o collect condensate Be. Tube for carrying condensate t o next higher plate B;. D a m 5/32 inch in height for retaining some liquid a t each plate Be. Overflow. for returning lihuid to still pot Bo. D a m , 1/2 inch in height, t o provide liquid head required for condensate to pass t o upper section Bio. Coiled tube, 6-mm. O.D.. for return of reflux t o bottom section B I , . Coiled tube, G-mm. O.D., for passage of condensate to upper section B12. Coiled tube, 10-mm. O.D.. for evacuation of apparatus 1’1 to PX. Plates 1 t o 25 Al. A b .
111111 11111 5 IO
0
CENTlM E TE R S Figure 4. Cross section and top section of rectifying portion of apparatus N a n d M are taken a t middle and end, respectively. of one plate. B I t o Be have significance indicated in Figure 2
192
ANALYTICAL CHEMISTRY
In practice, the bottom side of the rectifying section is heated to vaporize a portion of the liquid. The vapor condenses on the cooler upper walls. The condensate flows to the troughs and is delivered through stainless steel tubes to the next higher plate. I n this way distillate passes upward through the rectifying seetion to the head, where most of it is returned as reflux. The reflux returns to the bottom of the rectifying section by passing over the ridges in the lower flattened side of the glass tube. The ridges serve to maintain a small amount of liquid in each plate. rlt the junction between the halves, the high ridge, B9 (Figure 2), maintains a level of liquid sufficient t o cause the flon of condensate through tube BII to the upper half. -4s the level of liquid in plate 26 is above that in plate 25, reflux returns through tube Blo to the lower half. Some warping may occur during the hard-soldering and spotwelding operations involved in the construction of the internal stainless steel fittings. The effect of such warping may he minimized by constructing the fittings in relatively short lengths. Thus, for each half of fractionating section two lengths of fitting are used. The junction of the two lengths of fitting occurs a t the division between two plates. Only one vapor barrier is required a t this point, but both lengths of abutting fitting require liquidtight seals in the troughs. Figure 5 shows the receiving assembly and system for producing the low pressure. Figure 6 gives a cross section of the rectifying section and its supports. Each half of the rectifying section is supported independently by means of a segment of an aluminum tube, E,, which is itself eupported on two Cnistrut brackets, E2,one near each end. Each length of aluminum tube is fastened a t the lower end only
CENTIMETERS
Figure 6. Cross section of support for rectifying section Frame. Unistrut Bracket. Unistrut Collar., Trensite ~~_~~..._ E,. S e 6 i e n t of aluminuin tube Ea. .4sbestos paper Ea. .4sbestos fiber E;. Fractionating section E8 . Bolt fastening aluminum tube to Traneite collar E O . Transite blocks EM. Bolts fastening Transite collar to bracket
El. E,.
Et.
and is free at the upper end to slide through the Transite collar, Ea, as it expands or contracts. The glass rectifying section comes in direct contact with a layer of asbestos fiber, Es, which provides a cushion sufficiently resilient to permit expansion or contractioii of the aluminum tube without excessively straining the glass. The lower ends of both halves of the rectifying section are held firmly in position, while the upper ends are free to move. The coils between the two halves of the rectifying section, Blo, BIZ, and B12(Figures 2 and 3), and between. the upper half of the rectifying section and receiver D Iand source of low pressure, D21 (Figure 5 ) , rovide for expansion or contraction of the g l w . The pot is helawith a clamp on its neck and supported with 3 ring at the bottom. Figure 7 shows the diagram of the electrical heating and thermocouple circuits.
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I
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?a
CEV-IKETERS
The temperature is measured with single-junction copperconstantan thermocouples in the pot well, F1, and in the liquid in plate 27, Fs,as well aa on the outside between the glass and the asbestos fiber cushion at positions FZ, Fa, Fd, Fs, F I , and F,. Two special electrical Glascol heating mantles, H1 and H,,are used on the pot. Five heaters are used on the rectifying sectionH,,H,,H,, HB,and H I . The small heaters, Hs, H I , and H;, are
Figure 5. Receiving assenibly arid evacuating s) stein CI.
Metal housing and control for reflux regulator DroD counter Coilkd tube, 10-mm. O.D., connecting fractionating section with receiver Dz, Di, De. Monel t o glass standard-taper joints, soldered Dt, D;. Diaphragm valves, Alone1 metal DS. Receiver Stopcocks, 4 nun. oblique bore D E ,0 s . Traps, refrigerated with liquid nitrogen Dlo, D n . Mercury vapor diffusion Dumu DIZ. Di8. Connection t o gas reservoir Dl,. Connection t o mechanical oil pump Dl5. Connection t o XlcLeod gage D I G DI;. , Openings t o traps, standard-taper joint, 12/30 Die. Ionization gage tube Stopcock, 15-mm. oblique bore DIP. Dio, Opening for charging and cleaning rectifying section Coiled t u b e , , IO-mm. O.D., connecting fractionating section D71. with evacuating system
c4.
D1.
Figure 7. Diagram of electrical heating and thermoelement circuits E?.
Brackets supporting rectifying section (see Figure 6)
F1. Thermocouple to pot F2,Fa, F L Fs, F ; , Fg. Thermocouples leading t o glass on lowe,r side of fa. F9.
Fio.
H I , H2. Hd, Ha. Ha, Ha, HI. Ha. GI to Ga.
rectifying section (between E6 and Ea in Figure 6) Thermocouple t o liquid in plate 27 of rectifying section (see Blr in Figure 3) Selector switch Potentiometer Electric heating mantles on pot Electric heating mantles on rectifying section .4uxiliary electric heaters on rectifying section Electric hcnter for mercury vapor pump Variable transforinera. 5 amperes, 110 volts, a.c.
193
V O L U M E 27, NO, 2, F E B R U A R Y 1 9 5 5 made of asbestoeinsulated resistance u-ire wound directly on the aluminum tube and thermally insulated with magnesia asbestos. A Trmsite box (not shown) filled u-ith glass wool surrounds glass coils,B,! and Bll (Figures 2 and 3) connecting the two halves of the rectlfying section. Figure 8 shows the design of the special heating mantles, H4 and Ha. Figure 9 shows the entire apparatus. OPERATING CH.4RACTERISTIC.5
The effective volume, or charging capacity, of ihe pot is 2.i liters. The haldup'of the entire rectifying section was found to be 0.44 liter, as determined by recovering the oil from the rectifying section after the pat had been taken to dryness. This rather high v d u e of holdup can be reduced (to shout 0.3 liter), by in-
creasing the slope of the rectifying section so that the liquid at, the upper end of each plate, which with the present slope appears to be about 2 mm. in depth, is reduced to a depth of 0.5 mm. or less. The head of the apparatus (above plate 5 0 ) was constructed to be slightly different from the design shown in Figure 3 and had B "holdup" of 17 ml. This mms determined with the apparatus in operat,ion by opening the reflux regulator just long enough to permit msterisl in the head to pass to the receiver, during which time (20 seconds) distillation into the head amounted to 0.5 ml. After the head had been emptied in this fashion, the throughput was determined by observing the time required for the head t o fill again to the point where distillate first began to pass back over the ridge into plate 50. With n-eicosane in the rectifying section a t n temperature of 102O C., oorresponding to a vapor pressure of about 0.03 mm., the throughput mas found to be 80 ml. per hour. The time required to establish a steady state was determined by charging the apparatus (pot and rectifying sect,ion) with a test mixture consisting of a concentrate of normal paraffins from petroleum, starting the distillation process, and operating under total reflux until no change in composition occurred a t the head of the apparatus. T h e composition in the head nas fallowed from the refractive indices of a succession of I-ml. samples removed st 0.5-hour intervals, starting with the onset of active distillation, .4 period of about 6 hours wm required to estahlish a steady state. PROCEDURE FOR DISTILLATION
T h e normal procedure for performing a distillation is 8 s follows:
I ( I I I I ( I I I I
0
5
Figure 9.
IO
The material to be charged is introduced to the pot through opening .42 (Figure 2). If desired, the rectifying section may also be filled through openings Blr (Figure 3) and DZO(Figure 5). Evacuation of the system is begun. When the permanent gas pressure in the apparatus ha6 been reduced to about 1 mm., heat is applied to the pot m d rectifying section and the temperatures are lowly raised. Evacuation is continued and the permanent gas pressure reduced to less than 2 x mm. The temperature and temperature gradient are adjusted so that the desired rate of distillation is obtained and the rate of distillation is uniform throughout the length of the rectifying
Assembly of SO-stage apparatus for distillation at very low pressures
ANALYTICAL CHEMISTRY
194 section. These adjustments depend on observations of the number of drops leaving tubes B, in a given time a t several positions along the rectifying section. .4fter 24 hours of normal operation in this way with total reflux, the material in the head of the apparatus (above plate 50) is allowed to flow to the receiver and is withdrawn to constitute a fraction. I n this operation] g i t h valve D3 (Figure 5 ) in the open position, stopcocks Dg and D19 are closed and the reflux regulator, CI, is opened to permit the fraction to flow to the receiver, then closed. Valve D?is closed and stopcock 0 8 is opened to the atmosphere. Valve D1 is opened to permit withdrawal of the fraction, then closed. Stopcock DS is closed, stopcock D S opened, and the receiver evacuated. Finally, the fractionating section is reconnected to the low pressure source by opening stopcock Dig. The apparatus may be used for the continuous flow of distillate to the receiver, and this is preferable, but it has been found convenient to operate with total reflux, removing material from the head to constitute fractions a t given intervals, as described above. When waxes or other materials which are solid at room temperature are being distilled, the receiver and connections D1 t o D7 (Figure 5 ) are heated electrically. I n this case it is also necessary to heat tube A7 (Figure 2). PERCENTAGE BY VOLUME 60
20
0
40
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VOLUME
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40
80
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VOLUME IN LITERS
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Results of distillation of commercial mixture of normal paraffin hydrocarbons, CISto Cx
Figure 10.
The ordinate scales give freezing point in C. and refractive index, nD at 80” C. The scale of abscissas gives the volume of distillate in liters. The crosses give the freezing points and refractive indices of the indicated pure compounds.
The apparatus requires little attention. L-svally, slight adjustments of distillation rate are made once in 24 hours, fractions are withdrawn every 12 hours, and the traps are refrigerated with liquid nitrogen once every 8 hours. The apparatus has been operated both continuously 24 hours per day, 7 days per week, and intermittently in periods of 120 hours each week. With the latter operation, because liquid remains in the plates, only several hours are required after the shutdown to reach normal operation. RESULTS OBTAINED
Figure 10 shows the results of the distillation in this apparatus of a charge of 2.7 liters of a commercial mixture (supplied through the courtesy of the Atlantic Refining Co.) of normal paraffins near (220. Figure 11 shows the results of the distillation of a charge of 2.8 liters of a concentrate of normal paraffins, C17 to C2jl separated from petroleum by molecular compound formation with urea. Both distillations were performed at a throughput of 80 ml. per hour with a reflux ratio of 55 to 1. The vapor pressure of the liquid under distillation was about 0.03 mm. of mercury. It is apparent that the distillation referred to in Figure 10 served to separate the mixture easily into its component normal paraffin hydrocarbons, Cis to (222. The normal paraffin hydrocarbons obtained from the distillation referred t o in Figure 11
Figure 11. Results of distillation of concentrate of normal paraffins, CISto Czj, from petroleum The ordinate scales give freezing point in C, and refractive index, I L D at 80° C. The scale of abscissas gives the volume of distillate in liters. The crosses give the freezing points and refractive indices of the indicated uure compounds.
were not of very high purity, primarily because of the presence of considerable amounts of hydrocarbons other than normal paraffins in the original charge. Experience on the assembly and operation of this apparatus to date indicates that it is practical to make, if needed, a 100stage apparatus for distillation at very low pressures. LITERATURE CITED (1) .4ldershoff, W. G., Booy, H., Langedijk, S. L., Philippi, G. Th., and.Waterman, H. I., J . I n s t . Petroleum, 39, 688 (1953). (2) Brewer, A. K., and Madorsky, S. L., J. Research Natl. BUT. Standards, 3 8 , 1 2 9 (1947). (3) Fawcett, E. W., and RlcCowen, J. L., U.S. Patent 2,073,202 (March 9, 1937). (4) Madorsky, S.L., J . ResearchNatl. Bur. Standards, 4 4 , 1 3 5 (1950). (5) Madorsky, 9. L., Bradt, P., and Strauss, S., I b i d . , 4 1 , 2 0 5 (1948). ( 6 ) RIair, B. J., and Schicktanz, S. T., I n d . Eng. Chem., 28, 1446 (1936); J . Research Natl. Bur. Standards, 17, 909 (1936). (7) Mair, B. J., Schicktanz, S. T., and Rose, F. W., J r . , Ibid., 15, 557 (1935). (8) Mair, B. J., and Willingham, C. B., I b i d . , 21, 535 (1938). (9) Melpolder, F. W., Washall, T. A., and Alexander, J. A., Division of Petroleum Chemistry, 126th meeting, - ~ M E R I C A NC H E M I C A L Kansas City, Mo., 1954. SOCIETY, (10) Rossini, F. D., and Glasgow, A. R., Jr., J . Research S a t l . B w . Standards, 2 3 , 5 0 9 (1939). (11) Rossini, F. D., Mair, B. J., and Streiff, -4. J., “Hydrocarbons from Petroleum,” Reinhold, New York, 1953. (12) Washburn, E . W., Bruun, J. H., and Hicks, M.>I.,J . Research Natl. Bur. Standards, 2 , 467 (1929). (13) Wollner, H. J., Matchett, J. B., and Levine, J., ISD. ENG.CHEM., ANAL.E D . , 16, 529 (1944).
RECEIVED for review September 13, 1954. Accepted December 7, 1954. Part of the work of the American Petroleum Institute Research Project 6 in the Petroleum Research Laboratory, Carnegie Institute of Technology. Material taken from a dissertation submitted t o the Carnegie Institute of Technology in partial fulfillment of the requirements for the degree of doctor of philosophy.
[End of Symposium]
