Spinning Band Distillation Column Still Heads R. E. Jentoft, W. R. Doty, and T . H . Gouw Chevron Research Co., Richmond, Calv. 94802
WITHthe advent of more efficient laboratory distillation columns, it becomes important to improve the condenser and the reflux dividing section of the system. Once a fractionation system is selected, it is often the still head that is the limiting factor in the attainable separation sharpness ( I ) . Poor design may result in large liquid holdups which limit the sharpness of fractionation because of partial remixing of the products separated in the column. A large volume head can result in erratic overhead temperature observations because of dilution effects. Poor reflux ratio control causes confusion in the interpretation of results. These deficiencies are especially significant if small amounts of a component have to be isolated in high purity. Relatively little has been published on this subject because it is difficult to detect still head deficiencies from efficiency data as they are usually measured. Column efficiency measurements are generally carried out by equilibrating a binary test mixture at total reflux for a considerable amount of time. Under these conditions, the measured column efficiency is independent of the still head holdup. In an actual distillation where finite reflux ratios are employed, the influence of the poor still head design would be directly obvious. However, determination of HETPs at partial reflux is much more complex than when total reflux is employed. Computations have to take into account the unequal quantities of liquid and vapor entering and leaving a plate during any given time interval. Another complicating factor is the gradual change in composition characteristics of a batch distillation at partial reflux. Hence, HETP data at partial reflux are seldom given. Only a few good still heads for distillation columns with stationary packing have been described in the literature ( 2 , 3). For spinning band columns, the choice is even more limited. Many still head designs suffer from either a large holdup, internal and external mixing, and/or a poor reflux control system. New Distillation Head. A drawing of the new distillation head is given in Figure 1. This particular model has been designed for a spinning band column, but with certain simplifications the design can also be employed for packed columns. Care is necessary in the construction of the internals of the column head. The glass piece, which is both the seat of the bushing and also the valve seat, is first constructed by vacuum forming the glass over a special mandrel (see Figure 1). Perfect alignment of the seat of the bushing, the valve guide, and the column proper is obtained by using another stainless steel mandrel mounted in the column proper and mounting the head together while the unit is still on a lathe. In operation, the rising vapors go through the vertical vapor inlet ports to the condensers. These ports should be at least equal in area to the cross-sectional area of the open column. The returning condensate flows over the edge of the valve guide onto and over the metal valve, past the valve cup, and (1) W. J. Podbielniak, IND. ENG.CHEM., ANAL.ED., 5, 126 (1933). (2) E. Krell, “Handbook of Laboratory Distillation,” Elsevier, New York, N. Y.,1963. (3) N. Adler, ANAL.CHEM., 35, 724 (1963).
back into the column. There is no holdup around the valve cup. [In some designs holdup around the valve cup may amount to 0.5-1.0 ml. Besides contributing to the external holdup-random mixing ( 3 t t h i s holdup can represent as much as 1 to 2 seconds of boilup. With a valve timed to operate at 10 seconds off and 1 second on, 2 to 3 seconds of boilup is taken each time the valve is lifted. The actual reflux ratio would, therefore, be closer to 9:2 or even 8:3 instead of the nominal 10:l.l When the valve is lifted, the condensate flows from the valve guide onto the valve and then drips off the tip of the metal valve into the outlet line. Reflux ratio control is carried out by a timing device which activates the solenoid around the reflux hat. This lifts up the valve and allows the condensate to enter the outlet line. This type of reflux ccntrol is very accurate and very reproducible and is independent of the boilup rate, the viscosity, and the surface tension of the liquid overhead product. T o ensure the absence of leakage when reflux is returning to the column, the glass seat and the bottom of the metal valve are ground and finally lapped together with a 400-mesh silicon carbide grinding powder (Carborundum Co., Niagara Falls, N. Y.). Because the bushing is subject to extreme conditions of temperature and wear, the choice of material is critical. Of the many compounds tested, the best results, from the point of machinability and performance, have been obtained with a 25z special fi11-75z Teflon TFE compound (Saunders Engineering Corp., Los Angeles, Calif.). These bushings have been operated to temperatures of more than 550 “F for several hours without any problems. To relieve any stress, the */4-inch rod is first heat-treated in a closely fitting glass tube at 600 O F overnight before machining. In time, and especially when operation at high temperatures has been carried out for extended periods, these bushings have to be replaced because leakage may then develop around the threads. The bushing is tightened into place with a very long screw driver until the flat surfaces are mated. It is kept seated during operation because the friction of the shaft of the spinning band tends to tighten the bushing. The rotating shaft enters the column through an O-ring seal which is leak-free even at pressures of 0.1 torr and rotating speeds of up to 3000 rpm. The bronze bearing is connected to the reflux hat by a standard Swagelok ‘/Anch to l/c-inch Zytel reducing union. The spinning band shaft goes through the hollow metal valve and through the bushing to the column proper. Alignment of the hollow valve shaft around the band shaft is accomplished by a rather close fit in the reflux hat at the top and by its position around the top of the bushing at the bottom. At some speeds the band shaft may vibrate sufficiently to come into contact with the metal reflux valve stem. Because a variable speed drive is used, the rotor speed is simply adjusted to avoid these critical regions. A good condenser design should emphasize efficient contact between cold surface and rising vapors with a low internal holdup. The latter should be kept as low as possible because it reduces the fractionation efficiency of the system. This is achieved by using a small cross section while maximizing the area of heat transfer, such as with a bulb-type condenser or VOL. 41, NO. 1 , JANUARY 1969
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P;
BOSTON GEAR NONPOROUS BRONZE BEARING 8-24-4
SPINNING BAND SHAFTSWAGELOK 8 10-6- 4 SOLENOID
7
I
a
BUNA N OR
3
SOFT IRON CORE LEVEL ADJUSTERS REFLUX HAT REFLUX HAT-
1
\-O-RING
SEAL
METAL REFLUX VALVE IIII A
VAPOR INLET PORTS
VACUUM JACKET
VALVE CUP
VALVE GUIDE ERMOCOUPLE INLET
BRONZE MANDREL
UTED, HOLLOW, FLUX VALVE VALVE SEAT OUTLET LINE
BUSHING ,
MATING SEAT TO GLASS
Figure 1. Spinning band distillation head
by a tight coil of tubing in a narrow confinement. The latter kind causes a somewhat greater liquid holdup because the flow of returning condensate is somewhat retarded between the individual coils. With the exception of the top condenser, the whole head is encased in a silvered vacuum jacket (evacuated to 10-7 torr at 800 "F) t o approach adiabatic conditions. A simplified design, obtained by vacuum-jacketing only until the bottom of the lower condenser, is adequate for most distillations. At high vapor temperatures, however, we have observed that the more elaborate design, and use of compressed air as coolant in the bottom condenser, results in more stable vapor temperatures because of smaller thermal losses from the head. The two sets of condensers are used for added versatility. This is particularly advantageous when the product tends t o solidify a few degrees below its boiling point. The coolant
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in the bottom condenser can then be maintained at a temperature above the melting point, while the top condenser can be used with a much colder coolant to ensure total condensation of all vapors. Temperature determinations are carried out with a l/lsinch sheathed metal thermocouple. It is inserted through a short 1/4-inch,drilled Teflon rod, which is then mounted on the column with regular Swagelok fittings. The output of this thermocouple is connected t o the central electronic control system of the distillation laboratory ( 4 ) .
RECEIVED for review August 19, 1968. Accepted October 7, 1968. (4) R. E. Jentoft, J. F. Johnson, and J. Y.Beach, Am. Chem. Sor., Diu. Perr. Chem. Preprinfs 5 (3), 109 (1960).
