J. W. Panon
and I. D. Johnson Marathon Oil Co. Littleton, Colorado
I1
Magnetic Stirring For Glass Pressure Reactors
A
modified Carius combustion tuhelJ has been described. Stirring has been achieved3 by a "magma-dash" operation using Teflon-covered Alnico magnets. The Carius combustion tube is satisfactorv for hinh pressures but suffers from the limitation of size. We have used Fischer & Porter aerosol compatibility tubes for pressure reactions because they offerlarger volumes, although they have lower working pressures than the Carius tubes. The aerosol compatibility tubes are sealed with O-rings with a special metal coupling. Three standard sizes are available, 3, 6, and 12 oz, with working pressures of 500, 200, and 150 psig respectively. The latter two sizes may be conveniently stirred by Teflon-covered magnetic stirring bars with a magnetic stir plate, but the 3-02 size is test-tube shaped and, like the modified Carius tube, is stirred best by a "magnadash" operation. We used the 3-02 size aerosol compatibility tube even when working a t relatively low pressures because of the added safety of its higher worlting pressure. This necessitated the design stir- of a better "mama-dash" ring device. We used a double-wound solenoid to agitate a Tefloncovered magnetic dasher. The double winding permitted a ~ u s h - ~ uarrauwment: ll . hv" this method, more rapid agitation is possible than when gravity alone is used on the down stroke. However, this necessitated the addition of a ring-shaped permanent magnet4 below the solenoid to repel the dasher and prevent its hitting the bottom of the aerosol tube on the down stroke.
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'ANTHONY, P. Z., J. CHEM.EDUC.,36. 489 (1959). zObtained from Fischer & Porter Co., County Line R o d , Warminster, Pennsylvania. 18974. a SHORR, L. M., ROGOZINSKI, M., AND HASHMAN, U.,Chem. Ind. ( L a b ) , 52 (1964). 'Obtained from Bunting Magnetics Co., 9245 Cherry St., Franklin Park, Illinois.
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The arrangement is shown in Figure 1. Tefloncovered magnet wire5 was used in the solenoid so that it could be immersed in a hot oil bath (up to 240°C) without adverse effects.
Figure 1.
Diagram of "magno-dal" stirring devise.
The circuit used to drive the solenoid coil is shown in Figure 2. The coils are driven alternately by the power transistors Q I and Q6. These power transistors are turned on and off by the flip-flop circuit containing transitors Qz and Qa, which are in turn driven by the unijunction transistor Q,. Q, is in a relaxation oscillator timer circuit in which R,, Rz, and CI determine the SObtained from Consolidated Wire, 1635 S. Clinton St., Chicago, Illinois.
110
SOLENOID
POWER SUPPLY
Figure 2.
TIMER
FLIP-FLOP
SOLENOID DRIVER
Circuit used t o drive solenoid coii.
frequency of operation. The values used cover a frequency range of 1-20 cycles per second. Details of the coil construction are shown in Figure 3. The coil was wound on a spool made from thin wall brass tubing, 1'/2 in. in diameter, with soft. iron rings in. thick for end pieces. Since the system was designed for high temperatures, the rings were silver soldered to the brass tube. Two wires were wound onto the coil form a t once in order to have two coils on the same form with the same resistance and similar geometry. The coils had 2200 turns each of No. 28 Teflon-insulated magnet wire. The coils were connected as shown in Figure 2, so as to reverse the magnetic field when one and then the other is energized.
(dissimilar metals to prevent galling) were prepared The die was of two-piece construction, the bottom removable, so that the molded Teflon article could be easily removed. A cylinder of F E P Teflon was cut to fit the die as illustrated in Figure 4. The cylinder
,11/2" 0i.m.t.r Th1"W.U Bralr Tubiw 2112.~1f16"Soft Imn Ring
400 Turns Center Toppad ifilar Wound Coii
Figure 3.
Detoilr of coil conrtruction.
The magnetic dasher was constructed in two ways. For temperatures up to 175-20O0C, a snap-fit construction was satisfactory. Two small grooves were cut near the ends of a cylindrical T F E Teflon-covered magnetic stirring bar. Two Teflon discs, with a number of small holes drilled near the periphery and an inside hole slightly smaller than the outside diameter of the cylindrical stirring bar, were forced over the ends of the bar until they snapped into position where the shallow grooves were cut. Unfortunately, a t temperatures near 200°C or over, the Teflon was prone to coldflow and distort so that the end discs would fall off during agitation. This difficulty was surmounted by constructing a one-piece FEP Teflon-covered dasher. FEP Teflon has a melting point and can be molded more easily than TFE Teflon. An aluminum die and steel punch
Figure 4.
Construction of mognetic dasher.
was partially bored out to accept a small cylindrical Alnico magnet (such as is found in TFE Teflon-covered magnetic stirring bars). A small plug of FEP Teflon, with a small hole drilled through its center (to allow air to escape during the molding process), was cut to fill in the space above the magnet. This was placed in the assembled die and heated in a furnace to 350°C. It is best if the punch is heated separately at this time. The furnace is opened periodically to inspect the Teflon and when the top of the F E P Teflon turns glassy, the die is removed from the furnace, the punch inserted, and the assembly placed in a hand press; slight pressure should be applied to seal the openings. If the small hole in the plug seals while the assembly is in the furnace, it may be necessary to prick it open with a needle to allow residual air to escape and prevent bubble formation. After allowing the die to cool, the bottom is removed and the FEP Teflon cylinder pushed out. The resulting one-piece cylinder of FEP Teflon, containing on Alnico magnet in the center, can now be turned down on a lathe to a configuration similar to the three-piece TFE Teflon "snap-fit" dasher shown. This onopiece F E P Teflon dasher was usable up to 240°C. We gratefully acknowledge the advice and assistance of James E. Wilson in the construction of the FEP Teflon dasher. Volume 43, Number 7, July 1966
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