development
Rotary C Fractionating F. C. BENNER, A. DINARDO, AND D. 1. TOBIN NATIONAL RESEARCH CORP., CAMBRIDGE, MASS.
DESCRIPTION O F THE: STILL
T h e objective of these studies was to develop further a rotary condenser fractionating still which can be operated effectively a t lower absolute pressures than the more conventional bubble-plate or packed distillation columns. Data are presented for operation of the still in the pressure range of 0.005 to 1.5 mm. of mercury. Using binary test mixtures in a 32-inch long column, enrichments corresponding to separations as high as 18-inch theoretical plates have been observed. Pressure drop values from pot to column head range from 0.2 to 0.5 mm. This distillation unit combines fractionating ability with low pressure operation-a somewhat unique combination. Low pressure drop through the column permits distillation with vapor pressures in the distilling pot of below 1mm. This achievement may permit processing by distillation of liquid mixtures which are too heat-sensitive to withstand distillation in more conventional-type columns.
A
The present model of the still is slightly larger than the original Bowman design and differs in various respects. The over-all assembly is comprised of the distillation column, the vacuum pumping system, the coolant circulating system, and the control panel. Figure 1 is a photograph showing the complete still assembly and control panel, which recently has been made commercially available a s a standard laboratory unit. Figure 2 is a sectional drawing of the general assembly and principal components excnept for the circulating system, n-hich controls the flow rate and t,emperature of the coolant. DISTILLATIOS COLTXS. The distillation column consists of a rotating stainless eteel condenser v i t h a stationary inner cooling tube, a 1)orosilicnte glass coluniri heated by Xichrome windings
UXIQUE type of fractionating still which utilizes the princi-
ple of thermal rectification has been described by Bowman ( 2 ) . This still is a multiple redistillation unit in which the evaporator and condenser are concentrically arranged tubes. The inner tube is the condenser rotating a t moderately high speeds; the outer tube is the evaporator. Vapor rises in the annular space between these tubes, and reflux runs down the wall of the evaporator as a thin film. Partial condensation of the vapor stream takes place on the inner tube, and condensate is continuously returned t o the reflu1 film by the centrifugal action of the rotating condenser. Partial vaporization of the reflux film takes place over the entire surface of the film. The theory of operation of this still has been thoroughly discussed by Bowman. The unit can be used over any range of pressures, but i t offers particular advantages for distillation at low pressure. The pressure drop is very low as compared with packed or bubble-plate columns. Enrichment is not diminished by lowering of pressure, but within a certain range of low pressures may be twice as great as at atmospheric pressure by taking advantage of fractional condensation as well as fractional evaporation. The authors were particularly interested in the possibilities of using this unit in the high-vacuum range of pressures. The application of high-vacuum distillation has been definitely limited by the low order of fractionation available in the falling-film type of stills as used in molecular or pseudomolecular distillation. A program has been carried on to develop a still for general laboratory research use and t o obtain sufficient data for design of larger scale units. The present paper may be viewed as a progress report, since the evaluation is far from complete, even though much information has been obtained
Figure 722
I. Still Assembly
March 1951
INDUSTRIAL AND ENGINEERING CHEMISTRY
with a glass flask for containing the liquid to be distilled, and a metal housing which contains the mechanical drive for the condenser. Basic dimensions of the column are shown in Table I.
TABLEI. BASICDIMENSIONS OF COLUMN Glass column Length, inches Outside diameter, mm. Heated length, inches Joint to pot
Rntnr
Length, inches Outside diameter, inches Serrated length, inches
44 75 32
723
A copper tube extends down the inside of the stationary cooling tube t o within a short distance from the bottom to serve as exit for the coolant which is introduced a t the top of the cooling tube. As it is necessary t o control both the rate of coolant flow and t h e temperature of the coolant, a recirculating system has been designed t o supply either oil or water t o the rotor. Temperature of the coolant is controlled by a Fenwal thermostat controlling a bank of heaters. The flow rate is regulated by suitable throttling and by-pass valves.
65/40 spherical 68 1.5
46
VACUUMPUMPING SYSTEJI. The vacuum pumping system consists of a mechanical forepump, an oil-type diffusion pump, and a cold trap which may be cooled by dry ice-acetone mixtures. The entire column may be tilted by means of a gear and handwheel about the vacuum pumping line which serves as a support. The vacuum seal which permits the tilting is a grease-filled space between two neoprene grease retainers. Seals of this type have been very successful in providing positive vacuum-tight seals under low speed operation. The still assembly is tilted to permit removal of the glass column from the long rotating condenser. GLASSCOLUMN.The column is glass tubing with a flange from standard chemically resistant glass pipe which can be assembled t o the metal housing and made vacuum tight by means of a rubber gasket. A side arm with stopcock for product removal is located about 6 inches below the top of the column. The side arm permits the removal of a small fraction of the condensate flowing down the wall and was simple and adequate for the early studies. Nichrome ribbon, 1/8 X 0.008 inch, is wound on the column from the bottom t o a point just below the product take-off side arm for providing a variable amount of heat through a transformer control. The Nichrome ribbon is wound on the column in two sections so t h a t the heat t o the bottom half and top half of the column can be independently controlled. This is of value in the starting-up procedure, and there are occasions also when it is advisable to supply less heat to the top of the column than t o the bottom in order to maintain suitable film conditions. The lower end of the column is fitted with a 65/40 ground glass spherical joint. Various sizes of round-bottomed flasks with heating mantles are used for heating t h e charge of liquid to be distilled. ROTATING CONDENSER.The rotating condenser is machined from thin-walled stainless steel tubing (1.5-inch outside diameter) sealed off a t the bottom with a stainless steel plug, which supports the stationary cooling tube on the inside. The condenser is supported at the upper end by two bearings as indicated in Figure 2. The surface of the condenser below the motor housing is machined with a spiral, round-bottomed groove. The purpose of this machined groove is to provide serrations so t h a t condensate may be thrown off more readily by the centrifugal action of the rotating condenser. The condenser rotates a t 1725 revolutions per minute. To prevent vibration and whip, it is necessary to maintain close tolerances on straightness in its manufacture. It is customary to cold straighten the tube before machining and restraighten again if necessary. If a n y dynamic unbalance should result from machining, the rotor may tend to vibrate while operating. Small counterweights added t o the bottom plug are used to bring the rotor back into balance. COOLANT CIRCULATION. Coolant is circulated through a stationary inner tube around which the rotor revolves. There is a radial clearance of approximately '/8 inch between the inner wall of the rotor and the outer wall of the cooling tube. This annular space is filled with suitable liquid t o serve as a heat transfer medium from the rotor t o the circulating coolant.
Figure 2.
Assembly D i a g r a m of Still
1. Still pot 2. Glass evaporator wlumn 3. Stainless steel rotating wndenser 4. Product take-off 5. Motor drive 6. Bearings 7. Stationary woling tube 8. Rotatable mountinn 9. Vacuum seal assemkiy 10. Cold trap 11. Diffusion pump 12. Mechanical vacuum pump
UNIQUEDRIVE MOTOR. A unique feature of this still is t h e method of introducing motion to the rotor. The armature of t h e drive motor is machined down smaller than its original diameter and press fitted onto the condenser. The armature, as can be seen from the sketch, and the condenser are entirely within t h e evacuated part of the system. A thin nonmagnetic shell is inserted between the armature and the windings, and welded a t both ends of the housing. This shell permits the magnetic flux t o cause rotation without using sliding vacuum seals. There is some loss in efficiency due to the fact t h a t the air gap has been enlarged slightly, but this motor arrangement has worked very satiec factorily. The windings are cooled by a small blower attached t o the motor housing. The actual vacuum seal is made between the top flange of the motor housing and the flange through which the cooling tube enters. The lower section of the metal housing is jacketed and water-cooled in order t o prevent the product from
724
INDUSTRIAL AND ENGINEERING CHEMISTRY
reaching the top of the still. During normal distillation, no vapor ascends above the end of the glass section. CONTROL PANEL.A control panel, separate from the still itself, contains electrical circuit breaker switches, variable aut,otransformers for control of heat input t o the column and pot, meters for indicating power input, and meters for the thermocouple vacuum gage system. PRESSURE MEAST:REMEKT.Pressure within the system is measured by either a thermocouple gage, which is useful in the pressure range from 5 to 2 0 0 of ~ mercury, or with a radium source ionization gage, n.hich is useful in the range from 1~ to 10 mm. When operating in the very low pressure range below 50p, no control of pressure is used-Le., the system i s run at full capacity of the evacuating pumps. With the diffusion pumps on, pressures of less than 5, are obtainable. With only the mechanical pump operating, pressures ordinarily range between 25 and 5 O p . When i t is desired t o operate a t higher pressures, control is achieved by use of a controlled air leak to a pipe tap a t the cold trap. A4 simple but easi1;i cont,rolled air leak consists of a short length of rubber pressure tubing with a fine IT-ire running through it. h hose clamp is used to regulate the amount of opening around the wire. Pressures indicated on the radium source ionization gage located in the eva uation line a t the exit, from t,he motor housing indicate that this technique gives constant pressure a t the still head. OPERATIO1 O F THE STILL
GEXERAL.Operation of the still t o obtain best results requires careful control by the operator. The principles governing proper operation may be deduced from Bowman's report (2). There are three variables which must be considered. These are V ,the rate of vapor generation from the pot; and Qv and Qc, the rate of vapor generation and condensation per unit length for evaporat,or and condenser, respectively. In the case where Qc equals Qv and where Qc and Quareconstant over the entire length of the column, 2,QZ/V takes on a particular significance. Statistically, this ratio is t,henumber of t,imest'he material is redistilled in passing through the column. This paramet'er has been termed the "heat ratio" and has been shown to be nearly numerically equal to the number of t,heoretical plates or transfer units required for the same separation in contact-type columns. The operator therefore attempts to adjust the heat controls to obtain maximum heat to t,he column and minimum heat to t,he pot without overstepping the limits in either direction. Simultaneously, the rat,e of condensation must be controlled so that condensation rat,e equals t'he vaporization rate from the column. Maintenance of constant Vaporization from the pot is not, a major problem. Hoivever, various problems are encountered in controlling vaporization and condensation in the column section. These are enumerated and briefly discussed below. MAISTENAXCE O F BALANCE BETWEEN CONDENSATIOX AKD EVAPORATION. I t is possible to operate the column with Q u and Qe unequal. However, it is usually desirable to operate with the rate of condensation as nearly as possible equal to the rate of vaporization. If the condensation rate, Qc, is greater than the evaporation rat,e, Q , t'hen the boil-up rate, V , and reflux rate, L, will decrease with increasing height in the column, and the reflux film mill become thinner a t the top of the column. Q,Z cannot exceed Q,Z by a value greater than 8, since a t this point no condensate can reach the level of the take-off point. If Q E equals Qv, then V and L will be constant throughout the length of the column, and the reflux film will be uniform. Conversely, uniformity of film thickness is an indication of proper balance beAppearance of the film has been used as a tween Qc and &.. criterion of control by both Bowman and the present authors. The authors do not feel that this technique is entirely adequate, since it places too much responsibility on the judgment and visual perception of the still operat,or. The thickness of a film in gravity
Vol. 43, No. 3
flow varies as the cube root of the flow rate (9). Large increases in reflux rate, L,therefore, will not be manifested in correspondingly large increase in film thickness. Other possible control methods present difficulties. A direct method of control would involve measurement,of reflux rate at, both top and bottom of the column, to ensure constancy of L and V . r\f.4XI&K&i HEAT ISPUTTO THE EVAPORATOR. The heat ratio Q,Z/V is obviously limited by the maximum value of heat flux which can be obtained without disruption of the film. I n transferring heat to a thin film, the rate of heat transfer can be increased up to the point at which the filni is blown off the wall. One would expect the maximum value of Qv t o vary for different liquids. Qualitat'ively this appears t'o be supported by the data. EFFECTOF PRESSURE DROP AND COMPOSITION GRADIEST. Both pressure drop and composition gradients in the column have the effect of establishing a vertical temperature gradient in the reflux film. This situation will be discussed in terms of pressure drop only. ' The magnitude of this temperature gradient increases as the operating pressure decreases. Data will be presented later to demonstrate this factor. d temperature gradient in the reflux film requires a matching gradient a t the condenser which involves control of the coolant temperature and its rate of flow. TEST MIXTURES
BIXARY TESTMIXTURES INVEGTIGSTED. For evaluation of the still a t atmospheric pressure, the binary system n-heptane-methyl cyclohexane has been used. For evaluation at. low pressures, three binary mixtures were used, but only two have yielded useful results. The binary system of p-t,ricresyl phosphate and m-tricresyl phosphate studied by Bishop ( 1 ) has been used for a series of test runs ab total reflux. This test mixture is not entirely satisfactory and st,udies with it are not being continued. The relat'ive volatility ranges from 1.50 to 1.30 over the temperat,ure range 180" to 245' C. This value of relative volatility ~ o u l dbe adequate if accurate analyses of feed and product could be made. Analyses are made by freezing point determination and for mixtures rich in the meta isomer, serious supercooling effechs make the determination of freezing point on small samples quite difficult. The freezing point' in this range was determined by observing the transition from solid to liquid. I t is doubtful that changes in composition can be observed in the range 90 to 100% metal isomer. This limits the mixture to evaluation of separations equivalent to no more than 15 platea. The binary system n-octj-1 phthalate-2-ethylhexyl sebacatc studied by Perry and Fuguitt (6) should have been excellent as a test mixture bj- virtue of its low relative volatility and the ease and accuracy of refract,ive index measurements used for analysis. However, in it,s use, an unexpected difficult,j-arose. These liquids possess the characteristic of nonwetting toward glass a t the temperatures of the reflux film. Condensate would not' spread out as a film on the evapora.tor but broke up as drops which ran down the evaporator wall as dist,inct drops or as rivulets. The authors have not investigated the nonwett,ing by this mixture sufficiently t,o be able to state whether it, is a property of the esters themselves or whether it may be induced by small concentrations of impurities. The binary syst,em, di-n-butyl phthalate-di-n-butyl aeelat'e, described by Williams ( I O ) was not tried after an initial test run on dibutyl phthalate exhibited nonn-etting effects. In a search for neiv binary systems which might' be useful, mistures of n-hexadecane-1-hexadecene and n-hexadecane-l-octadecene appeared to be good possibilit,ies. An equilibrium still very similar in design t,o that used by Perry and Fuguitt ( 6 ) was used to obtain relative volat,ility data for these mixtures. The system n-hexadecane-1-octadecene was abandoned after preliminary studies on a mixt,ure 50% by weight shoxed the relative volatilit,y to be 3.50. A series of determinations of relative volatility for n-hexadecane-1-hexadecenewere made and are listed in
INDUSTRIAL AND ENGINEERING CHEMISTRY
March 1951
TABLE11. DETERMINATION O F RELATIVEVOLATILITY BINARY MIXTUREn-HEXADECANE-1-HEXADECENE
PROPERTIES OF TEST LIQUIDS TABLE 111. PHYSICAL
FOR
(Pressure, 1 mm. I-Ig) Mole Fraction Hexadecane Liquid Vapor Relative Voiatility 0 730 0.741 1.06 0.585 0.514 0.398 0.305 0 298 0,091
0.601 0.528 0,416 0.315 0 314 0 099
1.07 1.06 1.08 1.05 1.08 1.10
Mean
m-Tricresyl phosphate Melting point (exptl.) Vapor pressure (8)
log P
p-Tricresyl Phosphate Melting point (exptl.) Vapor pressure (8)
log
1.07 =e 0 . 0 1
EVALUATION OF STILLAT ATMOSPHERIC PRESSURE I-Hexadecene Refractive index, WITH n-HEPTANE-METHYLCYCLOHEXANE A T TOTAL REFLUX
d
75.1 95.2 95.3
(Relative volatility, 1.083) qat Heat Input, Pot Boll-up &*oh Fraction TheoretiW a t t s Tzmz., Rate n-Heptane Cc./H’r. Pot Overhead Plates P o t Column 200 150 130
1300 1300 1300
104.6 104.6 104
400 458
0.324 0.324 0.331
0.529 0.529 0..554
11 11 11.5
nL5
nk5
\I 1
I/
PRODUCT TAKEOFF
STATIONARY
II
cnnt ING
TII~F
U
11 \
ROTOR
1 Figure 3.
,EVAPORATOR
(b) Cross-Section Diagrams of Rotor Drive Motor and Distillation Column (0)
P
= 15.223
-
592s T
FRACTIONATION. Studies of enrichment by the still were made on three systems; one a t atmospheric pressure, and two in the low pressure region. Table I V lists typical data for the test mixture n-heptanemethylcyclohexane. These runs were made a t total reflux. Separation equivalent to 11 to 11.5 theoretical plates as calculated by the Fenske equation ( 4 ) was obtained. It may be observed that increase in heat ratio based on heat input values achieved by reducing the heat to the pot while maintaining constant heat to the column did not appreciably affect enrichment. Table V is a compilation of data taken for the tricresyl phosphate system. The value of “theoretical plates” for the tricresyl phosphate
NICHROME WINDING
I
T
EXPERlMENTAL RESULTS
CONDENSER
I
~
1.4207
WINDINGS
I1
5373
1 38779 0.68382
Refractive index, ng6 Density (20’ C.), g./ml. Methyloyclohexane
NON-MAGNETIC SHELL
I
-
n-Heptane, batch 27 (National Bureau of Standards test values)
ARMATURE
W
13.982
1.4392 112.7 113.2
STATIONARY COOLING TUBE
AIR BLOWER
21.5’ C.
1,4326 0.3 0.0
Iodine No. (exptl.) (calcd.)
Refractive index,
Table 11. The average of these is 1.07 * 0.01. Analyses of samples were made by determination of iodine number. The m- and p-tricresyl phosPURITY OF TEST COMPOUNDS. phates were purchased from Eastman Kodak Co. and were used without further purification. The hexadecane and hexadecene were purchased from the Humphrey-Williams Co., New Haven, Conn., and were distilled in the rotary fractionating column t o obtain fractions of constant refractive index. n-Heptane was obtained from Westvaco Chlorine Products Co., batch No. 27 with National Bureau of Standards test values of physical properties. This was used without further purification. Methylcyclohexane was obtained from the Dow Chemical Co. and used without further purification. Physical properties of test li’quids are listed in Table 111.
-
7 2 . 5 D C.
n-Hexadecane Refractive index, n’$,, Iodine No. (exptl.) (calcd.)
TABLE IV.
Sample No.
725
726
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
Vol. 43, No. 3
eluded because data on boil-up rate and heat removed a t the condenser are not very reliable. From the heat input values it is certain, however, t h a t the heat ratio cannot exceed 10 and is probably nearer a value of 5. FILMTEXPERATURES AND PRESSURE DROP. I t has already been pointed out that the over-all pressure drop from pot to head of the column has the effect of setting up a temperature gradient in the reflux film. Measurements of film temperatures have been made with systems operating a t head pressures from 25 microns up to several millimeters of mercury. The temperatures were measured by iron-constantan thermocouples fastened to metal rings which were located a t three levels in the column. One was located a t the bottom of the heated section, one a t the midpoint, and the third just below the product take-off point. Table VI1 summarizes data collected for two systems-the tricresyl phosphate isomers which were distilled a t pressures in the range 25 to 4 0 and ~ the hexadecane-hexadecene system which for this particular run was distilled a t pressures of 5 t o 9 mm. For
Figure 4. Vapor Pressure-Temperature Curves for Test Liquids (7, 8 )
isomers wai calculated using the Fenslie equation. A value of 1.45 for relative volatility was taken from the data of Bishop; this value corresponds to a temperature of 190" C., which was taken as an average of the film temperature in the column. If the ATE-AEFLUX FlLM value 1.35 corresponding to the pot temperature of 220" C. were Figure 5. Control Limits for used, better separation would be indicated. Data will be preTemperature Gradients in Fracsented to show t h a t film temperatures are appreciably lower than tionation of Tricresyl Phosphate Isomers the pot temperature. I n all the experiments, separation of the tricresyl phosphate isomers was complete within the limits of the analytical procedure. It may be noted that an increase in OF STILL WITH m- AND p-TRICRESYL PHOSPHATE TEST TABLEV. EVALUATIOX head pressure from 5 to 6Op did not affect ~ I I X T UAT RE TOTAL REFLUX the pot temperature or enrichment. (Relative volatilitv = 1,461 Over-all pressure drop as measured beComposition, Mole Measured Fraction mPot tween head of the column and the pot Head Pressure Heat Input, T ~ ~ ~Tricresyl . , Phosphate Theoretical Watts Sample Pressure, Drop, Pot Overhead Plates using a U-tube manomet'er with triNO. Mm. Pot Column C. P cresyl phosphate for the manometer 94.1 16.5 >16.5
