Apparatus for Pressure Control in Vacuum Distillations L. M. ELLIS, J R , , ~University of Florida, Gainesville, Fla.
A
COMMON difficulty in carrying out accurate distillations under vacuum is that of maintaining the pressure within sufficiently close limits. Of the many devices which have been suggested for overcoming this difficulty, probably the most satisfactory has been that of Cox ( I ) , who used a manometer with an adjustable electrode so that, when the pressure had reached a desired value, contact was made between the electrode and the rising mercury surface, thus actuating a relay and shutting off the pump motor current. The apparatus to be described here is similar in principle to that of Cox, and requires, as did his, the use of a vacuum pump which will not leak or suck back oil when stopped while connected to an evacuated system. The manometer with the adjustable electrode is replaced, however, by another device which requires no machined parts or packing (although it is not stated, these appear necessary to insure satisfactory operation of the manometer which Cox has described), and provision is made for wiping out most of the pressure fluctuations. The apparatus is illustrated by Figure 1. The “pressure balance” at the center, which replaces the previously mentioned manometer, is about 6 inches (15.24 cm.) tall and 7 inches (17.78 cm.) wide. The bulb on the left-hand vertical arm is about 1.5 inches (3.81 cm.) in diameter, the tube just below it 1 inch (2.54 cm.), and the rest of the tubing 7 mm. inside diameter. The relative positions of the sealed-in
I BOTTLE
e
TOPUMPMTCR
//Ob! LINE
FIGURE 1. DIAGRAM OF APPARATUS electrodes and the height to which the balance is filled with mercury are correct as illustrated. These electrodes must be gas-tight, and stopcock B must be kept well lubricated with a good grade of stopcock grease. The lower end of the righthand electrode should be fairly accurately centered in the tube. The pressure balance is mounted on a board of suitable size, and the board itself supported by a screw or bolt through its center which allows it to be rotated slightly in its own plane, but which should be tight enough to prevent any accidental jarring from altering the position of the balance. Stopcock A should have a rather small bore; stopcock C should be of 3 mm. inside diameter. Glass tubing of 5 mm. inside diameter is used for making the connections between 1
Formerly Research Fellow, American Petroleum Institute.
the various parts of the apparatus. The relay used should be fairly rugged. For use with the prescribed voltages, one with about 100 ohms resistance will be satisfactory. To operate the apparatus, stopcock A is closed, B and C are opened, and the pressure balance adjusted so that the right-hand sealed-in electrode just fails to make contact with the mercury. The pump motor is then started and the system evacuated to the desired pressure. Stopcocks B and C are then closed. Any further decrease in the pressure causes the mercury to rise in the right-hand vertical tube of the balance, completes the relay actuating circuit, and stops the pump motor. Stopcock A is then opened sufficiently to admit a stream of air which will keep the pump in operation about one-third of the time. Final adjustments of pressure are made by tilting the pressure balance-clockwise to increase pressure, counter-clockwise to decrease it. At the end of a distillation, stopcock B should be opened before admitting air into the system, as the entering air will otherwise bubble past the mercury into the bulb of the balance and may blow mercury into the upper horizontal tube. The inertia of the moving pump and motor parts causes the pump to operate an additional stroke or so after the motor current is shut off, and these additional strokes of the pump are believed to be the main cause of the pressure fluctuations which occur in the apparatus. Presumably these could be overcome by using a sufficiently large volume in the system, but the arrangement illustrated gives exactly enough regulation with reservoirs of moderate volume. The introduction of the section of capillary tubing cuts down the rate of diffusion of the gas in the system so that a decrease in pressure in the left-hand bottle only slowly affects the pressure in the right-hand one, and when the apparatus is in operation such changes in pressure are compensated by the air admitted through stopcock A before any noticeable pressure change occurs in the right-hand bottle. The effect of the capillary is illustrated by the fact that only faint pressure changes can be detected when 250-cc. flasks are used instead of the size bottles specified. The higher the pressure a t which the system is operated, the greater the effect of the additional strokes of the pump, and hence the smaller the bore of the capillary used. For the usual range of distillation pressures, 10 to 150 mm., a 3-inch (7.62-cm.) section of 1.5-mm. bore capillary is satisfactory. For pressure in the neighborhood of 300 mm., a similar section of 0.3-mm. bore capillary has been used. At very low pressures, stopcock C can be left open. Although leakage across stopcock B prevents satisfactory operation of the apparatus, the pressure difference between the two sides is so slight that this never occurs if a wellground stopcock is used and kept well lubricated. This type of control apparatus has been employed quite successfully in a number of installations in this laboratory. When the apparatus is properly adjusted, the pressure deviations are too small to be detected on an ordinary manometer, or to affect the rate or temperature of the distillation. Presumably the only changes needed to adapt the device to the control of pressures above atmospheric, up to the limit of the strength of the glass parts, would be the substitution of a suitable pressure pump for the vacuum pump and the reversal of
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July 15, 1932
INDUSTRIAL AND ENGINEERING CHEMISTRY
the contactpoints on the relay so that the pump motorwould operate when the relay actuating circuit was completed. LITERATURE CITED (1) Cox, IXD.EKG.CHEM.,Anal. Ed., 1, 7 (1929).
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RECEIVEDMarch 3, 1932. This work was oarried out a t The Johns Hopkins University, and the apparatus described was developed in connection with Research Project No. 28 of the American Petroleum Institute The work was supported by a research fund of the Inatitute donated by John D. Rockefeller and administered b y the Institute with the cooperation of the Central Petroleum Committee of the National Research Council.
Determination of Olefins by Bromine Titration J. C. MORRELL AND I. M. LEVINE, Universal Oil Products Co., Chicago, Ill.
T
.
HE present procedure has been developed as a rapid method for the determination of olefins in a mixture of hydrocarbons, particularly in cracked hydrocarbon distillate. It is based upon the idea that a solution containing olefins will absorb bromine in proportion to the olefin concentration. This idea is, of course, not new; it is the factor which governs the determination of the bromine number. Instances of direct bromine titration also appear in the literature. A chloroform solution of ice-cold nerol (4) has been so titrated, bromine entering both double bonds quantitatively. The degree of unsaturation of ethyl stilbene (5) has been determined in like manner. Similarly, Kurt Meyer (3) titrated a keto-enol mixture (methyloxaloacetate) for the determination of the enol portion. Although the application to nerol and ethyl stilbene is satisfactory, the method has potential sources of error which prevent its general adoption, The errors involved, as a result of substitution, incomplete additions, etc., have vitiated the usefulness of the bromine or iodine number as applied to cracked petroleum distillate ( 2 ) . Some of the objections are overcome when the olefin concentration of an oil is calculated from the ratio of its bromine titer to that of a standard solution containing a known concentration of known olefins. This is the basis of the present work. PROCEDURE The titration of the experimental standard and unknown is carried out under the same conditions of light and heat in the following manner: The oil (2 cc. or a volume which requires 1 to 5 cc. of the bromine solution) is diluted with olefin-free naphtha to a volume of 10 cc., and a 4 per cent solution by volume of bromine in carbon tetrachloride is added from a buret, five drops (0.1 cc.) a t a time, stirring after each addition. The end point of the titration is that point where a definite orange color persists for 30 seconds. This end point is one which each operator has to fix in his own mind. The titration should be carried out in diffused light, since direct sunlight causes the production of excessive hydrogen bromide. All values of concentration in this paper are expressed in terms of volume per cent. For the case in which the standard and unknown solution contains the same single olefin, Equation 1 is used to calculate the olefin content of the solution.
where U
= = T1 =
S
Tz =
olefin content of solution olefin content of standard titer of solution titer of standard
Solutions containing various amounts of octylene in olefin-free cleaner's naphtha were titrated. Tables I and I1 indicate the results obtained. TABLEI. TITRATION OF SOLUTIONS CONTAININQ OCTYLENE VOL. 12% CsHle SOLN.
VOL.Brz SOLN,
10 5 4 3 2
5.1 2.6 2.2 1.5 0.98
cc.
cc.
VOL.Br? SOLN. VOL. CeHie SOLN. 0.51 0.52 0.55 0.50 0.49
A volume of 1 cc. gave results that were entirely too low. TABLEI1 CSHMIN SOLN.
VOL. TITRATED
cc.
VOL. Brz SOLUTION USED (TITER)
cc.
VOL. Brz SOLN. FOR
2 00. OF 1% CsHis
cc.
The amount of bromine that can be absorbed by 2 cc. of a 1 per cent octylene solution is 0.00664 cc. This corresponds to 0.166 cc. of a 4 per cent solution. The average quantity of bromine solution actually used is 0.166 cc. The standard may also consist of an oil which contains the same type of olefin mixture as that in the oil under investigation. Equation 1 will apply in this case also. I n a concurrent paper it is shown that the olefins in cracked gasolines are probably of the same type in approximately the same relative proportion. Thus a cracked gasoline in which the olefin content has been accurately determined may be used as a standard. Results are shown in Table 111. OF BROMINE AND SULFUR MONOTABLE111 COMPARISON CHLORIDE METHODS r
No. 1 2 3 4 5
VOL. Brz SOLN.
Brz method
cc
%
. 6.74
6.14 8.72 6.90 13.0
OLEFINS SzClz method (8)
29.6 standard 27.0 38.2 30.4 57.0
% 29.6 28.0 35.0 29.8 61.4
These analyses indicate that the distribution of various olefins (including those of various types, as well as of different molecular weights) is the same in cracked distillate from various charging stocks and operations. Equation 1 has also been found to give good results when small concentrations of olefins are to be determined. I n cases of this kind larger samples are used for analysis. The method may also be used where the solution contains known olefins different from that contained in the standard. I n this case the specific gravities and the molecular weights of the olefins must be known. If the solution contains a
