Automatic Distillation Apparatus For Gasoline Analysis LESTER STEFFENS AND D. P. HEATH Socony-Vacuum
Oil Company, Inc., G o n d Laboratories, 419 Greenpoint Ave., Brooklyn 22, N. Y ,
A n apparatus i s described which maker possible tho automatic operation ef an efficient laboratory distillation unit. While tho device was designed lor gasoline analysis, i t can be adapted to the ~ o n h o 01 l other distillationr. I t automatically collects the fractions of distillate and plots the distillation curve. It has been operated 24 houn per day for well over a year with an operator present only 8 houn per day.
THE' .
high level of the quality requirements speciEed for aviation gasoline makes it necessary to determine tbe composition of gasoline stocks in terms of individual hydrocarbons. in order to be able to produce the maximum amount of the desired components, and to eliminate or minimize productiou of materials of low quality. The first step in the analytical procedure is separation of the sample by distillation into fractions consisting of substantially pure hydrocarbons where possible, or, as a compromise, a mixture of a very few components. Analytical distillation of gasoline requires s fractionating eolumn with a large number of theoretical plates, and a t the same time, small holdup per theoretical plate. I n addition, as Rose (Z,S, 4) and his axsoeiates have shown, analytical distillations frequently require operation at reflux ratios of over 100 to 1, and use of a charge that is large compared to the holdup of the column in order to be able to isolate components present in sm 11 amounts. Under these conditions the distillation of B single sample may require several weeks of continuous operation. For this r e w n a n automatic distillate-collecting device was constructed which makes possible the operation of a laboratory distillation unit with only occasional attention from the operator. This device uses some of the principles of the unit described by Bruun and Falconer (1) but is more nearly fully automatic, It was developed to be used with Podbielniak Super-Gal Model B distillation unit with a column containing 90 cm. (3 feet) LI 22mm. Heligrid packing, but it could, of course, be used with any type of Labomtory distillation units.
Fiquro P. Automatic Distillation Apparatus DESCRIPTION
A Bketch of the distillate-collecting device is given in Figure 1, and a photograph of the entire unit in Figure 2. The photograph was taken during the development of the apparatus and does not show several minor improvements.
FLOAT CONT4
Figure 1. Sketch of Distillate-Collecting Device
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INDUSTRIAL A N D ENGINEERING CHEMISTRY
The pins extend through the turntable and serve as cable guides on the undewide. An ice bath surrounds the bottles in the turntable. Ice is placed in a screened-off portion in the center of the bath, so that a n ice jam cannot prevent the table from turning. The cover late and turntable have holes through which the ice bath can be
Elled.
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GLASS TUBE ON BOTTOM OF FLOAT CHAMBER. PACKING NUT
BRACKET N U T
L E 4 0 GASKET
COIL 6 - 8 v DC. RESISTANCE
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Vol. 16, No. 8
ture reaches some redetermined point, or (3) when the temperature goes off eitter end of the recorder scale. As an alternative the unit can be made to go on total reflux. There are two alternating current circuits, one supplying the heater load and the other supplying the control load. The heart of the shutdown circuit is a sensitive direct current relay having one side of the alternating current control line across its contacts. This relay is energized from the battery booster when the reset switch is closed. There are four contacts, any of which when closed will short out this relay and thus break the alternating current control circuit. One contact is attached to the turntable, so that contact is made after the last bottle is filled. A contact is attached to each end of the recorder scale to prevent the temperature from going off the scale. The fourth contact is also in the recorder and can be set to make contact a t any desired temperature, thus stopping the operation. Since t,he contacts on the sewitive direct current relay are not large enough to carry the entire alternating current load, another relay is used to break the alternating current heater circuit. This relay is energized by the alternating current power from the control circuit. Thus, both safety relays are opened when any of the four safety contacts are closed. A switch is provided, so that the heater circuit may be kept on even if the alternating current control circuit is off. By the w e of this switch the unit can be made either to go on total reflux or to shut down entirely when one of the four safety contacts is closed.
NO. 2 6 WIRE
STEEL NEEDLE
ELECTRICAL LEADS
Figure 3.
Cross Section of Solenoid-Operated N e e d l e Valve
The only temperature recorder available was a potentiometertype instrument covering the range of 0" to 400" F. with an iron constantan thermocouple. In order to improve the accuracy of the temperature measurement, a two-junction thermopile was used, thuq doubling the sensitiv.ty of the instrument for a given temperature change. The internal cold-junction compensator was balanced out by the use of a manganin resistor. For the low range of the instrument the external cold junction was placed in an ice bath, and for the high range in a steam bath. In this manner temperature measurements precise to j=0.6' F. were obtained. The measurements must, of course, be corPected to a standard barometric pressure. ELECTRICAL CIRCUIT
The clectrical circuit as shown in Figure 5 has three different functions: (1) to control the collection of fractions, (2) to supply power to the recorder and cause the time a t which a fraction is taken t o be recorded, and (3) to stop the distillation when necessary. When the float rises far enough to strike the adjustable float contact, a sensitive 6-volt direct current relay is energized. The closing of this relay causes a latrh-in relay to close, and starts a vacuum tube timer. While the latch-in relay is cloQed,power is supplied to the float chamber needle valve, holding it open. After about 1 minute the vacuum tube timer releaces the Iatchin relay. This allows the needle valve to close and energize4 the turntable solenoid just long enough to allow an empty bottle to move into position. A push-button is provided, so that this cycle can be started manually. During the period that the latch-in relay is closed, a small 110-volt alternating current relay is also closed. The thermocouple leads are connectgd across this relay, so that when it is closed the thermocouple is shortened. This a w e s a break in the temperature record that shows when a fraction was bottled. A circt.it, is provided to shut down the unit (1) when all 18 receiving bottles have been filled, (2) when the vapor tempera-
a=&sTo; ROLLER
TURNfABLE
ABLE GUIDE
Figure 4. Turntable Solenoid Arrangement
For safety, &volt direct current power is used on all contacts. This prevents sparking when these contacts are closed and minimizes the danger of fire. The alternating current control circuit includes the timer used to control the reflux ratio, which in this unit is an electronic timer having independently adjustable open and closed periods. OPERATION
The distillation is run much the same as any other efficient laboratory distillation. A debutanized sample is put in the di9tilling flask and the column brought to equilibrium a t total reflux. The control circuit is turned on a t the start of heating; however the diqtillate valve solenoid is disconnected. The ice bath is hled and the float contact adjusted for the size of fraction desired. When equilibrium is reachcd, the collection of di3tillate is started by plugging in the distillate valve solenoid. This starting time is recorded by holding the thermocouple shorting relay closed for a few seconds. Any pentane in the sample is diitilled over whire the operator is present, so that he may remove the fractions from the ice bath immediately. The pentane can be taken over rapidly, usuallj. in 5 to 6 hours. After the pentane has beeu distilled over, the heat input is adjusted to the desired boilup rate and the timer set for the desired reflux ratio. From this point until the end of the i l l + tillation the operator gives little attention to the unit. Eyery morning and evening he removes fractions from the turntable,
Auguat,
1944
ANALYTICAL EDITION
1
120V.A.C.
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1
1 1
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BiNDING POSTS FOi T H ER MOCOUPLE
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I VACUUM TUBE
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I
I I
I I
FETY CONTACT ON l I m I 30TTLE NO. 18
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SAFETY CONTACTS
I MICROMAX I RECORDER L----------l LATCH-IN RELAY
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L I I
I 7 I
PODBIELNIAK CONTROL BOARD
DISTILLATE VALVE SOLENOID SOCKET
II
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IL
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I
L ,
- - - - -- - TURNTABLE SOLENOID Figure 5.
'-' NEON LAMP
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-T
11,
NEEDLE VALVE SOLENOID
5
FLOAT CONTACT
1111
Wiring Diagram of Distillate-Collecting Device
winds up the weight, and checks the heat input to the column and the flask. Since less than 10% of the charge is normally taken overhead per day the heat inputs need only occasional adjustment. A variation of 5% in a reflux ratio of 100 t o 1 h a s no appreciable effect on the efficiency of separation obtained. If, however, the unit were operated more rapidly or wide boiling range samples distilled, automatic control of the boilup rate would be necessary. When the temperature approaches the end of the lower range of the recorder, the operator shifts the cold junction from the ice bath t o the steam bath. Since there is a 25" F. overlap between the upper and lower scales, this change can usually be made sometime during the day. If the operator is not present, the unit shuts down when the end of the temperature scale is reached. The size of fraction taken in this apparatus can be varied from 0.6 to 370 of the charge. Thus, small enough fractions can be taken to avoid overlapping of components and it is not necessary to be able t o take cuts a t given temperatures. If a number of cuts are obtained on a plateau, they can be blended to simplify the subsequent inspection work. At the end of a run the recorder chart is removed and the distillation curve plotted. A section of a recorder chart showing the interruption in the distillation curve obtained when a fract'ion is bottled is shown in Figure 6. Since the chart speed of t,he recorder is fixed a t 5 cm. (2 inches) per hour, the rate a t which each fraction was distilled can be calculated from the size of cut and the length of chart covered by one fraction.
Figure 6.
Scdion of Recorder Chart
ACKNOWLEDGMENT
The authors wish to acknowledge the assistance of A. E. Traver, who designed the electrical circuit, and to express their appreciation to J. B. Rather for permission to publish this paper. LITERATURE CITED
The fraction-collecting device described has been in use for over a year, during which it has been found extremely reliable. The presence of an operator is required for a n &hour period a t the start of a run, but thereafter for only an hour each morning and evening. Duplicate distillations check within the accuracy of the temperature recorder.
(1) Bruun and Falconer, IND. ENG.CHEM., ANAL.ED.,9,193 (1937). (2) Rose, IND.ENQ.CHEM.,32,675 (1940). (3)Ibid., 33,590 (1941). (4) Rose, Welshans, and Long, Ibid., 32,673 (1940). PRESENTIED before the Division of Petroleum Chemistry at the 106th Meeting of the A M ~ R I C A CHEMICAL N SOCIETY. Pittsburgh, Pa.
