Improved Fractionating Column For Gases HAROLD SIMMONS BOOTH
AND
RALPH McNABNEY Ohio
Morley Chemical Laboratory, Western Reserve University, Cleveland,
The improved automatic fractionating column for low-boiling gases described i s of the constant-pressure type and is of greatly improved efficiency and capable of precise temperature control over a wide temperature range at least up to 10' C.
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SK oi the difficulties in the smooth operation 01 autoinnti-
cslly controlled fractionating columns for purification of gases with low boiling points, in which the head temperature is maintained by the injection of liquid air, is the lag in the injection period behind the actual need. This results in considerable tcmperature fluctuations in the head and overcooling a t the end of the injection cycle. Booth and Bozarth (1) partially overcame thib difficulty by clamping almost shut theliquid air exit tube from the head, so that only a small amount of liquid air went through the head a t each injection, but even then at very low temperatures small variations in head temperature were disclosed as a slightly wavy line on the recording. potentiometer chart. Such temperat ure variations interfere with the purification of gases, particularly when the boiling points of the gases to be separated are close.
The column shown in Figure 1 is of the constant-pressure type described by Booth and Bozarth ( I ) , in which the pressure is maintained by controlling the cooling of the still-head condenser by a contacting manometer connected to the still head. Its control of the head temperature is so delicate that liquid air has been used in it as the refrigerant successfully to distill fractionally a gas boiling at 48" C. without freezing up of the head and -4th the temperature holding perfectly constant. FRACTIONATING C O L U M N
REFLUXCONDENSER.A sectional view of the still head is shown in Figure 2. The condenser jacket consists of a brass tube 2.5 cm. in diameter and 22 cm. long, the ends of which are turned to a slight taper to accommodate the cork stoppers which close the ends. At the bottom of the condenser jacket, the space between the outer tube of the jacket and the glass tube of the column is filled with a solid brass cylinder 4 cm. long. This solid section of brass serves as a heat reservoir to smooth out fluctuations in the rate of cooling. The liquid air inlet tube opens into a groove in the top of this cylinder. In the condenser jacket, the liauid air is fowed tn follow a longer path by a spiral baffle,
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Figure 1. Lek, electrical control center.
Diagram of Fractionating Column Center, air-operated control appuatus.
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Rlsht, dintilling column
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made by winding No. 6 copper wire around a suitable mandrel and slipping the finished s iral into the jacket. The spiral is securely soldered to the jac%et and the center bored out in the lathe to fit snugly over the glass tube. Just above the air outlet, a washer is fastened t o the jacket to prevent the cork stopper from being pushed over the air outlet. The corks are compressed to give a tight seal by means of clamping okes pulled down by tie rods running between the two ends g o t shown in drawing). These tie rods are imbedded in the cork insulation. For the first few runs, the nuts on the tie rods must be tightened a t the beginning of each run to ensure a tight - seal a t the end of the conaenser-jacket. Since the liauid air comes in direct contact with the elass tube of the still heid, rapid response to the control action isobtained m d this condenser has a very high cooling capacity for its size. For gases boiling below -50" C., the still head may be cooled with liquid air, using a liquid air injector of the kind described by Booth and Bozarth (1). For cooling the still head in the distillation of higher boiling gases, a dry ice-acetone cooling system may be used, although liquid air has been used successfully up to +8"
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the contact point and causes fluctuations in the pressure. This, in turn, causes large changes in the rate of flow of reflux, which seriously diminish the efficiency of the column. To decrease these pressure surges and to check the variation in the flow of reflux, a system of control was devised which minimizes the effect of lag on the response to the control action. A mechanism is employed which gradually lifts the control contact while liquid air is being injected into the still-head jacket, result ing in the interruption of injection a t a pressure higher than that a t the beginning of injection, and thus shutting off the liquid air ahead of the normal time of cutoff. This "anticipator" device eliminates the excess cooling which otherwise would cause the pressure to drop below the normal cutoff position.
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RECTIFYINQ COLUMN. The column consists of a straight glass tube of 8-mm. inside diameter containing an inner glass tube of 5-mm. outside diameter and 90 cm. long, which is sealed off a t both ends. The inner tube is spaced from the outer by means of a wire spiral consisting of one No. 26 Nichrome wire wound around another one. This is made by coiling one piece of Nichrome wire around a mandrel about l mm. in diameter, such as a steel knitting needle. The coil is taken off the mandrel and a straight piece of wire is slipped through the center, the two are clamped together at one end, the straight wire is pulled taut And clamped a t both ends, and the coil is pulled out until it fits snugly over the straight wire. This is then wound around the inner glass tube in the form of a spiral with a pitch of about 9 mm. This spiral spaces the inner tube from the outer, makes the ascending vapors follow a spiral path up the column and consequently travel a greater distance, increases the surface of contact between vapor and liquid, and gives better distribution of the liquid around the column. Most of the descending liquid runs straight down the surfaces of the tubes, but at each turn the spiral picks up a certain amount of liquid and carries it around t o a different point in the column. A t the bottom of the column, just above the still pot, a dropper is located for observation of the rate of reflux and to return the liquid directly to the surface of the liquid in the still pot. THERMAL INSULATION. The still head and upper half of the column are insulated with a layer of cork about 2.5 cm. thick, which is coated with waterproof paint. The lower part of the column is insulated by a vacuum jacket made from a Pyrex condenser, in order to be able to observe the liquid in the column. It can be insulated entirely with cork, with a small peephole to observe refluxing if desired. STILLPOT. The body of the still pot is about 4 em. in diameter and 6 cm. high with a neck 2 cm. in diameter. Clamped against the outer edge of the bottom is an electric heater unit wound on a brass ring. A single layer of thin asbestos tape is put between the metal ring and the glass to prevent direct contact between metal and glass, which might cause breakage of the still pot. The outside of the heater unit is insulated with asbestos to prevent heat from reaching the upper part of the still pot and causing superheating of the vapors passing into the column. To prevent heat radiation from the outside the still pot is insulated by a silvered Dewar flask, the top of which is tightly closed by a cork stopper. By the use of a larger diameter still pot, more evaporating surface and a shallower body of liquid are obtained, both of which diminish the tendency of the liquid to superheat and produce pressur? surges. CONTROLS
SYSTEM OF CONTROL.This column, like the one described by Booth and Bosarth, is operated a t constant pressure, which is maintained by controlling the cooling of the still-head condenser by a contacting manometer connected to the still head. This system of control, shown in Figure 1, operates by injecting liquid air into the still-head condenser when the pressure rises enough to close the manometer contact. This cools the still head, lowen the pressure, and then opens the contact again. Since the head is cooled intermittently, the use of a column having a low liquid holdup produces a number of difficulties, which require some modification in the method of control. Because of the lag between the control action and the effect as transmitted to the control manometer, the mercury tends to overshoot
Figure 2.
Still Head
As shown in Figure 1, liquid air injection is controlled by the magnetic valve, A , made from a telegraph sounder as described b Booth and Bozarth (I), which closes the leak in the air line l e a x ing to the injector. Another magnetic valve, B, controls a leak of the air line leading to the contact drive cylinder shown in Figure 3, made from a small bicycle ump, the piston of which is connected by a lever to the glass tug, carrying the control contact. The coils of the two magnetic valves and of the holding circuit relay are connected in parallel, so that all close simultaneously when ener ized. (The holding circuit relay, C, is a 2500ohm, Type PC, fllied No. 77-070 relay frpm Allied Control Co., Inc., New York, N. Y.) A pair of electrical contacts, known as the starting contacts, are located on the frame of the contact drive mechanism and close when the piston is in the down position. These contacts are connected in parallel with the make contacts on the holding circuit relay. The 4volt direct current power supply to the control circuits passes first through the manometer contacts, then through the
ANALYTICAL EDITION
February, 1944
starting and holding contacts] and finally through the relay and magnetic valve coils. In operation, when the control manometer contact closes, the magnetic valves and relay close, injecting liquid air into the still head, and slowly lifting the control contact. Although the starting contact is opened as soon as the piston starts upward, the holding relay contact in parallel with it is still closed, so that current continues to flow through the valve coils until the manometer contact opens again and releases the valves and relay. After this, the electrical circuit cannot be vompleted again until the piston reaches the down position, even though the manometer contact might have closed, because the circuit cannot be completed until the starting contact closes. The drive piston is connected by R lever and linkage to the tube (wrying the movable contact in such a way that a large movement of the piston produces a small Figure 3. Contact Drive movement of the contact. A 200Cylinder gram lead weight is carried on the Diston rod to return the Diston t o the how, position, and a stih spring connected to the top of the contact tube keeps the system of linkages i n tension and thus eliminates lost motion. The stop on the contact drive piston (not shown) which restricts its upward motion is set t o allow the movable contact to be lifted about 3 mm. The control contact consists of a platinum tip soldered t o a copper lead wire and sealed with de Khotinsky cement into the end of a glass tube 2.0 mm. in diameter to give stiffness to the contact wire. A fixed platinum contact is sealed through the manometer tube below the surfare of the mercury to complete the circuit through the mercury. Regulation of the speed of rise and descent of the control contact is accomplished by adjusting the screw clamps in the line from the air supply to the magnetic valve and in the line from the valve to the contact drive cylinder. The method of delaying injection of liquid air until the moving manometer contact has reached the down position was adopted to improve the stability of the system and t o prevent chattering of the controls.
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I n all other respects, the operation of this column is almost the same as that described by Booth and Bosarth. This apparatus is intended primarily for purification of a single gas, so that an automatic stopcock is not used to control the take-off from the column because it is unnecessary, With a mixture of gases, the automatic stopcock described by Booth and Bozarth should be used. Exhaustion of a fraction from the column was indicated by a slight rise in the time-temperature curve of the still-head thermocouple, indicating the presence of a higher boiling fraction in the still head. With the automatic stopcock this would be indicated by increasingly infrequent operation of the stopcock. LITERATURE CITED
(1) Booth,
H. S., and Bozarth, 9.R.,IND. ENQ.CHEW.,29, 470
(1937).
W e i g h i n g Funnels MILTON S. SCHECHTER AND H. L. HALLER U. S. Department of Agriculture, Bureau of Entomology and Plant Quarantine, Beltsville, Md.
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types of easily constructed weighing funnels have been used in this laboratory t o facilitate the accurate weighing and quantitative transfer of solids or liquids t o volumet,ric flaska or other containers for analytical determinations. I n the accompanying sketch funnel A is suspended from the balance hook during the weighing by a wire attached to the Sshaped stem by twisting, the solid or liquid sample is placed in the shoulder, S, of the funnel, and the exact weight of sample desired is adjusted. The funnel is then placed as shown at the left, and the sample is nTashed quantitatively into the flask by a stream of solvent from a wash bottle, The intermittent siphoning action facilitates the washing of solids into the receptacle.
OPERATION
OPERATION OF THB COLUMN. First, the gas to be purified is
condensed in the still pot with liquid air and, after transfer, any noncondensable gases in the system are pumped out. The Dewar of liquid air is removed and the heating coil is put in place under the still pot after the coil has been immersed in liquid air for a moment to avoid sudden warming of the still pot. Then an empty silvered Dewar flask is placed mound the still ot and the current is turned on in the heating circuit, While t i e pressure in the column is rising, the injector Dewar flask is filled with liquid air nhich is injected into the still head manually before the operating pressure is reached. This cools the still head and column mole quickly. After the operating pressure has been reached, the contacting manometer holds it constant. When the column ha- been operating for a short time, the contact drive mechanism is adjusted to give the best compensation for the response lag of the system. The rate of ascent and de. scent of the drive piston and the rate of liquid air injection are adjusted so that the total movement of the contact point is between l and 2 mm. The pause between the return of the drive piston to the down position and the start of the next injection is about a half second in duration. If the injection period, which is normally about 2 or 3 seconds, becomes too long and the control contact rises t o o far, the rate of liquid air injection should be cut down. When properly adjusted, this control system can hold the pressure within 1 or 2 m a . of the control point, even during change of components in the still head, when the pressure is subject to wide fluctuations with the usual method of control. This column will handle a low-boiling gas like tetrafluoromethane easily, while previous types have been so unstable that operation with such a compound was very difficult.
Funnel B is made with a flat bottom for placement on the balance pan. After the sample is weighed, the funnel is tilted and the sample washed into a flask with an appropriate solvent. Either funnel may be fitted with ground-glass caps if desired. The diameter of A which the authors have used is 2.5 cm. and the over-all length is 7 cm. The diameter of B is 2 cm. and the over-all length is 6 cm. The shape and dimensions may be modified to meet the requirements of the analyses t o he performed.
