chlorofluoride. The aluniiiium electrode might also be used as an indicator in those titrations in which no mineral acid is present and the chloride ion is absent. Although this research is incomplete, it is reported now because this laboratory has no further interest in its derelopment. The data presented nlay be of sufficient interest to stimulate further work with these electrodes.
sheet, all of which were used in this study.
ACKNOWLEDGMENT
The author is indebted to the Aluminum Co. of America for its donation of various alloys, the Brush cO. for its donation Of liuni foil, the Carborunduni Metals C O . for its donation of zirconium n-ire, and the Fansteel 1\Ietallurgical Corp. for its donations of niobium and tantalum
LITERATURE CITED
(1) Baker, B. B., Morrison, J . D., h . ~ . CHEM, 27, 1306 (1955). ( 2 ) Willard, H. H., \\-inter, 0. B., Isu; E S G . CHEM., .kS.kL. E D . 5 , 1 (1933). R~~~~~~~ for review september 12 1956. Accepted February 19, 1987.
Testing of a Rotary Concentric-Tube Distilling Column BEVERIDGE J. MAIR, NE0 C. KROUSKOP, and FREDERICK D. ROSSlNl Petroleum Research laboratory, Carnegie lnstifute o f Technology, Pittsburgh
b A large laboratory concentric-tube distilling column, with rotor 4.871 inches in diameter and 60 inches in length, and with an annular space of 0.0465 inch, has been tested for throughput and separating power a t speeds up to 4000 r.p.m. For speeds up to 2400 r.p.m. the results confirm data previously obtained on a smaller column. Above 2 4 0 0 r.p.m. the separating power is much lower than was expected. This is attributed to the generation of heat by friction in the vapor phase.
I 3, Pa.
T
AMERICANPetroleum Institute Research Project 6 published ( 3 ) in 1 9 4 i a description of the assembly and testing of a small model of a rotary concentric-tube distilling column. The rectifying section was the empty annular spsce formed by the outside surface of a rotating, closed, inner cylinder and the inside surface of a stationary outer cylinder. The annular fractionating section was 0.043 inch in width, 23.0 inches (58.4 em.) in length, and 2.93 inches (i.44 cm.) in smaller diameter. For high values of throughHE
put, 2000 to 4000 ml. per hour, the column when operated a t 4000 r.p.m. had a n efficiency factor (the throughput divided by the holdup per theoretical plate. or the number of equivalent theoretical plates through which the material being fractionated passes in unit time) about 10 times t h a t of the best values previously reported for other rectifying columns. The efficiency factor increased markedly with the speed of rotation of the inner cylinder. Extrapolation of the results indicated that a column with a rotor 5
Figure 1 . Assembly of rotary concentric-tube distilling column
Motor B. Transmission Baffles D. Shaft of rotor Guide bearings, graphital Wall of rectifying section, 4.964-inch inside diameter G . Cylindrical rotor, dynamically balanced, 4.871-inch outside diameter H . A4nnularspace, 0.0465 inch in width, 5 feet in length Transite shell, 2 inches thick I. J . Heating jacket for rectifying section K . Corrugated sheet asbestos covered Tvith aluminiim foil (Alfol) L. Guide bearings, graphital M. Pot, stainless steel, 3-gallon capacity *Y. Tube for introducing charge and withdran.iiig samples 0. Thermocouple, copper-eonstantan P . Insulation, magnesia asbestos, 2.5 inches thick (2. Heaters for pot R. Tube connecting to manometer, detcrniines pressure difference between pot and head S . Thermocouple, copper-constantan, determines teniperature of vapor-liquid equilibrium in pot 1'' to 114. Thermocouples. single and differential, to determine temperatures of vall of stationary cylinder and heating jacket, and difference between these temperatures a t four positions along rectifying section C . Condenser on distillate line I-. Connection from head of column to condenser, insulated with magnesia asbestos tf-. Thermocouple, copper-constantan, 10-junction, to determine temperature of vapor-liquid equilihrium a t head X. Ball and socket joint, borosilicate glass Y. Reflux regulator Z. Condenser A. C. E. F.
i
VOL. 29, NO. 7, JULY 1957
1065
inches in diameter and 5 feet in length, with an annular space of 0.043 inch and spinning a t 8000 r.p.m. would have a separating power in the neighborhood of 300 to 400 theoretical plates with a throughput of 5000 ml. per hour. A larger column was ordered from the Specialized Instruments Corp., Belmont, Calif. Testing of it was completed in March 1954. The present report summarizes the results of these tests. Hawkins and Burris (1) constructed a similar column with a 4 by 53 inch rotor and placed it on test in December 1954.
___
.
-.
A
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Figure 2. Relation between power input to pot and throughput
r
-
I
1
-
APPARATUS
The apparatus is shown in assembly form in Figure 1. The rectifying section is the empty annular space which is 0.0465 inch (1.18 mm.) in width, 60 inches (152.4 cm.) in length, and 4.871 inches (12.37 cm.) in inner diameter. The available speeds of rotation are 1000, 2000, 3000, 4000, 6000, and 8000 r.p.m. Both the inner cylinder (rotor) and the outer cylinder are made of stainless steel. The rotor is balanced dynamically. As shown in Figure 1, the stationary outer cylinder is surrounded with the Transite shell, I, the heating jacket, J , and corrugated sheet asbwtos covered with aluminum foil, K . The heating jacket consists of a stainless steel tube (0.083-inch waI1) having four electrical heaters on the outside, uniformly wound. The heaters are insulated from the steel tube with two layers of flexible mica plate, and are held in position with a layer of Sauereisen cement 5/16 inch in thickness. Four thermocouple tubes (3/16 inch in outside diameter) are spaced uniformly along the steel tube and are embedded in the cement. Four single-junction copper-constantan thermocouples, 21' to T,,give the temperature of the wall of the stationary cylinder and four give the temperature of the heating jacket a t the corresponding positions. Each pair may be used to read directly the difference between these temperatures. Another single junction thermocouple, 0, gives the pot temperature; a ten-junction thermoelement, W , gives the temperature of the vapor liquid equilibrium a t the head of the column. The 3-gallon stainless steel pot, M , is electrically heated and is thermally insulated with a 2.5-inch layer of magnesia asbestos. A manometer (attached a t R ) gives the pressure drop in the rectifying section.
Figure 3. Relation between heat generated in rectifying section and speed of rotation at constant power input to pot (350 watts)
- ...-Figure 4. Number of theoretical plates as function of power input to pot for several speeds of rotation I6O1
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.
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3000--,
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A '00" 640r
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RCI;
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TESTING THE COLUMN
Tests of separzting efficiency and throughput were performed a t 0, 1000, 2000, 3000, and 4000 r.p.m., with power inputs to the pot of 200, 250, 300, 350, and 400 watts. For the throughput tests, 2,2,4trimethylpen1066
ANALYTICAL CHEMISTRY
01 150
I
POWER
I
200
250
300
INPUT TO THE POT, WATTS
350
400
Figure 5.
Number of theoretical plates
4 as function of throughput at several speeds of rotation
CT) W
tane was used, and, for the testa of separating efficiency, mixtures of 2,2,4trimethylpentane and methylcyclohexane were used ( 2 ) .
I-
4 P 1
a
0
+ W I I :
s
I + LL 0 I I : W
m
5z
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I
I
IO00
2000
3000
THROUGHPUT, ML(LIQUI0)
I
5000
4000
PER
6900
HOUR
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Y I
.
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Figure 6. Pressure drop as function of throughput for several speeds of rotation
q
The procedure for starting the column and placing it on test was as follows: About 4 liters of the test liquid was placed in the pot, the power input to the pot was set a t the value selected for the experiment, and the column heaters were adjusted to bring the mall of the stationary cylinder and the heating jacket to the same temperature. Observations mere made a t 20-minute intervals of the temperature of the head and pot, and of each of the four portions of the rectifying section. The pressure drop and the voltages applied to the jacket heaters were noted. Slight adjustments of pori-er input to the heating jacket were made as required. I n determining the separating efficiency, small pot and head samples (about 2 ml. each) were removed 1 hour after the rotor was started and a t 6-hour intervals thereafter for a total of 31 hours. A steady state mas usually established within 13 hours after starting the rotor; the separating efficiency remained substantially constant to the end of the experiment. For the throughput experiments the column was started and adjusted in a similar manner, the reflux rc.gulator was opened, and the time required for the collection of 12 to 20 ml. of distillate was observed. Three observations a t one value of power input to the pot w r e made a t 15-minute intervals before proceeding to the next desired value of power input. In addition, experiments a t 11000 r.p.ni. were performed with the poivvcr input to the heating jacket less than that required to keep it a t the temperature of the wall of the stationary cylinder.
RESULTS OF TESTING
The relation between the power input to the pot and the throughput arc shown in Figure 2. For rotations of 0, 1000, and 2000 r.p.m. the results are represented by a single straight line which is extrapolated to the value 170 watts for zero throughput. This is the power required to take care of all un-
Figure 7. Number of theoretical plates per meter length of rectifying section, 4 for 3- and 5-inch columns, as function of peripheral speed of rotor a t several values of vapor velocity VOL. 29, NO. 7, JULY 1957
1067
compensated heat losses from the column and still pot a t this operating temperature. It includes the power required to maintain the pot and its charge at the boiling point of 2,2,4trimethylpentaiie plus that required to maintain the head of the column (above the heating jacket but beIoLv the condenser and reflux regulator) a t the same temperature. The calculated line for the throughput, based on the hest of vaporization of 2,2,4-trimethylpentane, assuming a net loss of 170 watts from the still pot and column head, is also shonn in Figure 2. The higher values of throughput obtained a t 3000 r.p.m., aiid particularly a t 4000 r.p.ni., may be attributed to the generation of heat due to friction in the vapor phase. Some part may be due to friction in the rotor bearings. The h e i t caused the evaporation of a portion of the reflux, giving apparent higher values of throughput. The heat generated in the vapor is proportional to the cube of the speed of rotation according to the equation, Q =
12
Figure 9. Number of theoretical plates a t rotor speed of 4000 r.p.m., as function of throughput a i several values of power input to heating jacket
(r.p.m.)3
The difference between the throughput a t 0 and 4000 r.p.m. gives a measure of the heat generated a t the latter speed. From the heat of vaporization of 2,2,4-trimethylpentane it n-as computed that &, the heat generated st 4000 r.p.ni. with a 350-watt input to the pot, was 102 watts. From this value the constant IC n as evaluated and n-as used to compute the heat generated a t rotational speeds from 1000 to 8000 r.p.m. This calculation appears to be valid only for equal rites of throughput. These results are shown in Figure 3. Attempts to operate the column a t 6000 r.p.m. nere not successful. After darting the rotor a t this speed with a 350-n-att input to the pot and no heat applied to the jacket, the temperature of the stationary cylinder increased to a vxlue above the boiling point of 2,2,4-trimethvlpentane, the pressure drop rose rapidly, and in about 45 minutes the column flooded. Evidently the heat generated by friction evceeded the loss of heat from the rectifying section to the surroundings. Figures 4 aiid 5 give the nuinber of theoretical plxtes plotted as a function of pon-er input to the pot and as a function of throughput. Figure 6 gives the pressure drop as a function of throughput for the several speeds of rotation. The results for the present 5-inch and the previous 3-inch columns are given together on a comparable basis in Figure 7 . The number of theoretical plates per nieter length of rectifying section are plotted with respect to the peripheral speed of the rotor for a number of different vapor velocities for both columns. 1068
Figure 8. Throughput as function of power input to heating jacket for several values of power input to pot
ANALYTICAL CHEMISTRY
For experiments i t 4000 r.p.ni., the effects on throughput and separating efficiency of changing the poner input to the heating jacket are shown in Figures 8 and 9. The power required to maintain the heating jacket a t the temperature of the stitionary cylinder was 250 +IO watts. The results for power inputs of 167, 84, and 42 watts on the jacket refer to experiments in which heat was allowed to leak through the nall of the rectifying section.
DISCUSSION
The results for the tn-o columns are in fair agreement throughout the range covered by the data for the 3-inch column, which covers speeds up to 4000 r.p.m. for the 3-inch column and up to about 2400 r.p.ni. for the &inch column. The separating efficiency of the 5-inch column a t speeds of rotation above 2400 r.p.m. is much lower than nould be expected from a linear extrapolation of the data for the 3-inch column to higher speeds of rotation. Failure of the 5-inch column to give the anticipated results a t higher speeds is associated with the generation of heat in the gas phase. Even with the continual removal of heat through the \?-all of the rectifying section, the vapor in the annular space may be expected to remain superheated above the equilibrium temperature. As a result, the interchange of molecules betn-een the vapor and liquid in the direction of the equilibrium composition n ill be ad\ ersely affected.
For this type of apparatus with the conditions indicated, it appears that a peripheral velocity greater than about 1600 cm. per second produces more heat through turbulence in the gas phase than can be dissipated without adversely affecting the separating efficiency. These results are in general agreement with those reported by Hail kins and Burris (7).
ACKNOWLEDGMENT
The authors gratefully acknowledge advice received from Charles B. Willingham in the design of the apparatus.
LITERATURE CITED
(1) Halvkins, J. E., Burris, IT. A , AXAL. CHEM. 2 8 , 1715 (1956). (2) Willingham, C. B., Rossini, F. D., J . Research S a t l . Bur. Standards 37, 15 (1946). ( 3 ) Willingham, C. B , Sedlak, V. A., Kesthaver, J. IT , Rossini, F. D., Ind Eng. Chem. 39, 706 (1947).
RECEIVED for review April 9, 1956. Accepted February 23, 1957. Investigation performed as part of the work of American Petroleum Institute Research Project 6.
