Automatic Receiver Changer for Vacuum Distillation. Donald A. Simpson and Maurice D. Sutherland, Chemistry Department, University of Queensland, Brisbane, Australia. t o make fullest use of the highly efficient laboratory Iandstills now available, prolonged initial periods at total reflux very high reflux ratios must be used. These requirements N ORDER
make continuous operation almost imperative and call for some device to permit continuous operation and product withdrawal. At the high reflux ratios which render the use of an automatic fraction changer so desirable, the operation of the device described below has been very satisfactory. The automatic vacuum take-off arrangement of Brown and Coles ( 1 ) did not meet the authors’ specification of fraction cutting on the basis of an identical volume for each fraction. They therefore decided to use an intermittent siphon t o deliver the chosen volume of distillate into each receiver, which, after being so weighted, would Pink in a liquid and permit the rotatable delivery funnel t o be moved to a position over the next empty tube. A simple device for changing receivers under a chromatographic column, using a n arrestment under which only loaded tubes can pass, has been described by Phillips ( 4 ) , who also mentions another receiver (hanger described by Randall and Martin ( 5 ) in Grudoofed Z6e
which the weight of the fraction operates an escapement, bringing the siphon opposite the next receiver. The authors have found it advantageous t o use the intermittent current supplied by the repeat cycle timer (which controls the distillate withdrawal needle of the still through a still-head solenoid) to energize a small motor. This during the ‘Lon”period moves the delivery funnel if necessary to a new position over the next empty receiver. The apparatus (Figure 1) is contained within a sheet brass cylinder which is 5.25 inches in diameter and 7.75 inches high. This cylinder is divided into u per and lower portions, which meet on machined surfaces. &e upper portion, which is 2.5 inches high, is fastened rigidly to the framework on which the still is mounted and is closed on top by a sheet of 0.25-inch clear Perspex, which is cemented in place. The distillate enters the receiver changer from the buret by a tube passing through the center of the Perspex window and is caught in a siphon tube which rests loosely in a bracket and is readily changed for another of different volume when desired. The standard ground cone below the Perspex window permits a conventional type of receiver to be used interchangeably with the automatic apparatus. When the siphon tube overflows, the accumulated distillate is directed into one of ten rimless 13-ml. test tubes by a bent funnel, the tip of which is correctly positioned when the upper arm is pressed against the side of the test tube; both arm and funnel are attached to a brass block which turns freely in a thrust race. The upper arm is pressed upon by the lower arm, nhich is attached to the shaft of a 0.01-hp. alternating current motor to which current is supplied through the timer during the “on” period only. During the r r ~ f f ” period the lower arm is returned against the thrust race support by a return spring, so that there is little friction between the upper arm and thc test tube. Each of the test tubes floats in glycerol contained in a m-ider glass tube (diameter 1.125 inches) indented with guides which support the inner tube in a vertical position but permit it to move freely up or down. The level of glycerol is adjusted so that each test tube floatx and in turn obstructs the upper arm. The additional weight of the tube and contents, after the siphon has discharged into it, lowers the test tube sufficiently to allow the upper arm to pass over it xhen the motor is next energized. rhuq, each test tube receives in turn one discharge of distillate from the siphon. In addition to the ten floating tubes there is an eleventh receiver, a fixed tube of larger (25-ml.) capacity, to guard against loss of material if the rate of distillation greatly escrxeds expectation. The leads from the timer to the motor pass into the apparatus through a wax seal and are interrupted by a plug and socket and a variable resistance. Both marhined surfaces are greased before being brought into contact and are easily made gas-tight. The surfaces can be parted again by a seal-breaking screK set near the outer edge of the machined surfaces, and a pair of clamps is used to support the lower section in case of failure of the vacuum. The pressure in the device is adjusted by means of an auxiliary pump to that of the still before connection is established by opening the buret tap. The advantages of glycerol as a fluid support for the test tubes are low vapor pressure, low solubility for hydrocarbon vapors, and easy removal from the outside of the tubes by washing with water. Ten 13-mI. tubes have been found ample for overnight operation under reduced pressure of a 13-mm. Podbielniak HyperCal column operating a t a high reflux ratio. The pressure-regulating system is a simple version of that described by Palkin ( 2 ) and Palkin and Nelson ( 3 ) . The only significant modification has
Figure 1. Automatic Receiver Changer
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ANALYTICAL CHEMISTRY
been to move the controlled valve into a eeparate small vessel which is connected by a short wide tube to the sulfuric acid manostat. An automatic switch shuts off all power to the still if the pot pressure deviates appreciably from a chosen value. There is no real danger of loss of distillate through simultaneous turning of the funnel and discharge of the siphon. As the movement of the funnel is very rapid compared with the discharge of the liquid, only a small portion of any fraction can be lost, the rest being distributed between two tubes. However, if the off period is made long enough, it will exceed the limited period after the on period, during which siphoning can occur. This critical period depends on the on period, the boil-up rate, and the flow properties of the distillate, but in the authors' experience an off period of 5 minutes has proved more than sufficient for complete drainage of Cla alcohols. If siphoning does not occur within about 3 minutes after an on period, it will not occur until after the next on period. Therefore, if the on period is made sufficiently long, no loas of distillate need be feared.
provide high solvency and freedom from fire hazard. Tetrachloroethylene is somewhat preferable for laboratory use in that accumulation of water and resultant loss of solvent are minimized by its high boiIing point and Iow solubility for water, and a greater amount of solvent is condensable by an article of given thermal capacity. Precautions and Operating Rules. The unit should be covered at all times when not in use and should be placed in a hood or well-ventilated area to avoid the accumulation of toxic concentrations of vapors. Chlorinated hydrocarbon solvents are cumulative liver poisons; great care should be taken to avoid-inhalation of vapors. Flammable solvenh should not be used.
VAPOR ZONE
TO WATER T U tOOClNG MILS
