ANALYTICAL EDITION
January 15, 1935
Methane in methane residues was quantitatively oxidized when twice passed over 3.5 cc. of cobalt oxide catalyst a t a rate of 20 to 25 cc. per minute and 550" C., followed by flushing a t this temperature, provided the ratio of oxygen to methane was at least 3 to 1. Comparisons made between the slow-combustion and catalytic oxidation methods show complete agreement a t all concentrations of methane. Two procedures are given for the use of this catalyst: the utilization of a single furnace with a compound combustion tube, and the use of two furnaces employing separate
11
combustion tubes with a modified manifold. The authors recommend the latter. LITERATURE CITED (1) Braun-Knecht-Heimann Co., Catalog, p. 382 (1934). (2) Burrell and Oberfell, J. IND. ENQ.CHEM.,8,228 (1916). (3) Dennis, L. M.,"Gas Analysis," 2nd ed., p. 198, New York, Macmillan Co., 1913. (4)Kobe, IND. ENQ. CHEM.,Anal.Ed., 3, 159 (1931). (5) Steacie, J. Am. Chem. Soc., 52,2811 (1930). (6) Yant and Hawk, Ibid., 49,1457 (1927). RECEIVED September 6, 1934.
,Molecular Weight of Cracked Distillates OGDENFITZSIMONS AND E. W. THIELE, Standard Oil Co. (Ind.), Whiting, Ind.
M
OLECULAR weights are an important property of petroleum products, finding extensive use in the design of refinery equipment. However, the routine determination of molecular weights is not practicable, because of the care and time required in this type of work. Some means of estimating molecular weights from other more easily determined properties is therefore desirable. For straight-run stocks this need was met by a previous study (S),and in this paper the results of an extension of this work to cracked stocks are given. Some modifications have also been made in the method of making the determination.
The results of a series of determinations, by the cryoscopic method, of the molecular weights of various cracked stocks, all f r o m Midcontinent gas oil, and of a few Midcontinent pressure-still charging stocks are given. Some novelties in the cryoscopic determination are presented. The molecular weight results are correlated with boiling point, viscosity, and density to simplifv the estimation of the molecular weight of any given cracked stock.
PREPARATION OF SAMPLES The cracked stocks used in this work were all prepared in pilot-plant cracking equipment. One set of samples was prepared in this plant from a Midcontinent gas oil by simulating conditions of a commercial liquid-phase process in which the product boiling above naphtha end point and below a heavy tar is recycled. A product of 450' F. (232" C.)
BECKMANN
end point on the A. 8. T. M. distillation was produced. The distillate and cycle stocks were cut into 5 per cent cuts in a Hempel-type column and the samples thus obtained were studied. (These samples are numbered from 200 up in the tables and data which follow.) Another set of samples was obtained by the successive once-through vapor-phase cracking of a Midcontinent gas oil. In this process no stock was recycled with the fresh feed, but the product from the first once-through cracking was fractionated to eliminate gasoline and heavy tar, and the residue was fed through the still in a separate operation. This was repeated five times. Selected cuts of the various cycle stocks were chosen for study. Certain Midcontinent straight-run cuts were also examined. APPARATUS AND PROCEDURE
ATINUM STIRRER LINE
, TEST
TUBE FLASK
[VACUUM RELEASED]
DOUBLE WALLED BRASS CAN
ICE 6 WATER MIXTURE
PERFORATED BRASS
FIGURE 1
MOLECULAR WEIGHTS. The apparatus used is illustrated in Figure 1, which will require little explanation. Attention is called to the stirring device which lifts the stirrer slowly and after a pause drops it suddenly, thereby producing the maximum of agitation with a minimum and uniform heat development in the solution. It also prevents the mercury thread from sticking and reduces supercooling of the solution. The Dewar flask with vacuum released was found to give the right amount of cooling of the solution for convenient reading when the temperature difference between the bath and solution was 4" to 5" C. (7.2' to 9" F.). The same results have been obtained by other investigators by maintaining a smaller temperature differential between the bath and solution, but for accurate determinations this requires means for varying the bath temperature in order to keep the temperature difference constant. In making a determination, the solvent was measured out with a pipet, the temperature being noted. Two grams
INDUSTRIAL
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AND ENGINEERING CHEMISTRY
VoI. 7, No. 1
52
50
c)
2
48
0 46 $44
0
4
34
iT
t; 3 2
g 30 I i-
28 26 24
TIME IN MINUTES
FIGURE2
of pulverized barium perchlorate were added, this drying agent having been found more satisfactory than sodium sulfate, The thermometer and stirrer were placed in the solution and the whole immersed in an ice bath and stirred until crystals appeared, then removed and warmed in the hand until they melted. This was repeated once or twice more. This precaution, which is due to Gullick (Q), improves the constancy of the freezing points. The whole apparatus was then put together, the stirrer started, and readings were made of the thermometer a t regular intervals. When a steady state had been reached after crystals had formed, a cooling curve was plotted and the freezing point determined as described by Steed (6). The freezing point of the solvent having been obtained, the weighed solute was added in a sealed thin-walled glass bulb, and broken by the stirrer and the freezing point determination was repeated. In a number of cases the freezing points corresponding to three or more different concentrations of solute were determined, and the apparent molecular weight determined by the formula (F
M =
- d)W
FIGURE3
exact value is found by plotting the concentration against apparent molecular weight and extrapolating to zero concentration. In an attempt to avoid determinations a t more than one concentration, a study was made of the cryoscopic constant F. The values used for 0 and A H e in Equation 3 to obtain the cryoscopic constants given apply to the solvent, and hence Equation 1 is exact only a t infinite dilution. No satisfactory method of obtaining AHe a t other than infinite dilution was found. However, it was found that the equation M =
(F - 2d)W
(4)
as
gave extremely constant values for the molecular weight, regardless of the concentration, and these values were identical with those obtained by the extrapolation method of Wilson 700
(1)
as
e 00
obtained by rearranging the usual equation W
XT d=FS+W B
LL
g (2)
500
a m 4w
2w
where M = apparent molecular weight F = cryoscopic constant of the soolvent d = freezing oint depressionin C. W = weight o f solute added S = moles of solvent
$ 300 W a 0
In 200
Since widely varying values of the cryoscopic constant were found in the literature, F was calculated by the equation
2
t-
o)
d
100
Res F - --
(3)
AHe from recent data of Parks and Huffman (5)
0 0
where R = gas constant 8 = freezin point of solvent, degrees Kelvin AH^ = heat off solidification of the solvent at the freezing point The values of F used for the three solvents were Benzene Cyclohexane Nitrobenzene
65.60 248.9 55.78
When computed in this way, the apparent molecular weight tends to increase with increasing concentration, and the
100
MOLECULAR
200
WEIGHT
300
FIGURE4
and wylde (7). A study of the thermodynamics has diSclosed no theoretical basis for this equation, and since in the case of nitrobenzene Equation 4 produces only about half the required correction over Equation 1, this method is advanced as empirical only and to be used only on benzene and cyclohexane a t present. Since there might be some question about the results obtained, the samples for which only a single determination was made are indicated in Table 11. The nature of the results obtained with the second formula
ANALYTICAL EDITION
January 15. 1935
13
is illustrated in Table I by some determinations of the molecular weight of pure cyclohexane and sample 225. TABLE I. RESULTS WITH EQUATIOX 2 WEIGHT SAMPLE Cyclohexane in 0.5582 moles of benzene
OF
SAMPLE Grams 0.5073 0.7930 1.1283 1.446
DEPREBBION
c.
0.694 1.072 1.505 1.907 Av. Actual
Sample 225 in 0.5632 moles of beneene
0.5571 0.9081 1.4121 2.2448 2.8428 3.5376
MOLECULAR WEIGHT EQUATION 4
BY
0.416 0.679 1.040 1.628 2.036 2.487 Av.
NO.
212 213 215 21G 218 22 1 223 225 228 231 232 235 238 242 246 249 250 251 300 305 306 307 do8 308 309 310 315 330 350 355 '0 Several
7
0
PENNSYLVANIA REF 2 NORMAL PARAFFINS IC
84.09 84.10 84.07 83.93 84.05 84.10 154.0 152.6 153.1 152.6 152.5 153.1 153.0
In selecting a solvent, benzene was used in the majority of the determinations, but nitrobenzene was used for light cuts which might possibly contain benzene or cyclohexane. Cyclohexane was used chiefly on samples which contained wax, which has a low solubility in benzene. The two determinations on paraffin distillate, sample 308 in Table 11, illustrate the error that may be introduced if wax crystallizes out of the solution. Because of its high cryoscopic constant, cyclohexane has found a good deal of favor for this type of work; observations made during the course of this work, however, indicate that the low latent heat of fusion which produces this high constant gives a steep and somewhat curved time-temperature curve after freezing has started, so that it is hard to determine the exact freezing point with precision. This is shown in Figure 2 where the moles of solvent in the case of benzene and cyclohexane are about the same. Benzene was found on the whole to be more satisfactory. OTHER PHYSICAL PROPERTIES. Viscosities were determined on a set of three modified Ostwald viscometers, constructed in accordance with the specifications of the British Engineering Standards Association (1) and calibrated a t 100" and 210' F. (37.78" and 98.89' C.) with a series of eight oils standardized by the Bureau of Standards. Temperatures of 100" and 210' F. were maintained within limits of 0.1" F. by immersion under reduced pressure in vapor baths of carbon disulfide and water, respectively. Check determinations were possible to 1 part in 1000, but
SAMPLE
2mb
02
,I
,;,
0.1
MOLECULAR
WEIGHT
FIGURE5 the uncertainty in the absolute value of the viscosity of even standard substances is several times this. Densities were obtained with precision float hydrometers which were checked against a dilatometer. The accuracy is probably of the order of 5 parts in 10,000. In the A. S. T. M. distillation corrections were made for barometer and loss.
RESULTS The results of the actual determinations are summarized in Table 11. The relationships between molecular weight and other properties are given in Figures 3 to 5.
.
50%
A., S. T. M. 129 162 212 230 264 312 348 383 442 206 303 434 470 489 528 571 626 676
0:522 0.570 0.658 0.829 1.012 1.237 17.00
330 270 444 722
0:803 1.690 2.025 2.43 3.08 4.78 9.08 101.7 5.65 0.922 0.692 1.85 23.7
574 564 529 546 53 1 501
5.59 4.78 3.49 4.14 3.74 2.77
556
...
cases.
KINEMATIC VISCO~ITY IN CENTIBTOKES 10O'F. 210° F. (37.78' C . ) (98.89' C. 0.359
...
... ... ...
0: 402 0.481 0.459 0.647 0.803 0:467 0.779 0.897 1.054 1.199 1.581 2.352 8.58 1.728
...
0:856 4.26
...
:
1 523
1:311
1.276
...
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Fym
3 shows a lot of A. P. I. gravity versus molecular weig t for the normafparaffins and for Midcontinent strai htrun and cracked stocks. The line for straight-run stocks digers slightly from that previously presented. This difference is due to a slight difference in the Midcontinent pipe-line crude, and not to a change in procedure, as was shown by checking some of the old samples. Figure 4 is a similar plot of molecular weight against 50 per cent A. S. T. M. boiling point (760 mm.). Since there is no stem correction in the A. S. T. M. method, the 50 per cent point shown by pure substances of various boiling points was actually determined, and a correction plot prepared so that the 50 per cent boiling point which would be shown by each pure hydrocarbon could be computed. These computed values, not the true boiling points, are used in Figure 4. Figure 5 is a plot of the kinematic viscosity versus molecular weight. In this plot the results of various determinations in the literature, as well as some other unpublished work, have been incorporated, in the hope of correlating viscosity, vis-
Vol. 7, No. I
cosity index, and molecular weight for the heavier fractions. The data are somewhat conflicting, and the lines as drawn are, at the upper end, to be regarded as only a first attempt to obtain the desired result. LITERATURE CITED (1) British Engineering Standards Association, No. 188 (1929). ENQ.CHHM.,24, 1369 (1932). (2) Epperson and Dunlap, IND. (3) FitzSimons and Bahlke, Am. Petroleum Inst., Proc. 10th Ann. Meeting, 11, No. 1, Sect. 111, 70-2 (1930); Oil Gas J., 28, 164 (December 5, 1929). (4) Gullick, J . Inst. Petroleum Tech., 17, 541 (1931). (6) Parks and Huffman, IND.ENQ.CHEM.,23, 1138 (1931). (6) Steed, J.Inst. Petroleum Tcch., 16,783 (1930). (7) Wilson and Wylde, IND. ENQ.CHEM.,15, 801 (1923). R ~ C E I V EAugust D 27, 1934. Presented before the Division of Petroleum Chemistry a t the 88th Meeting of the American Chemical Society, Cleveland, Ohio, September 10 to 14, 1934.
t
Colorimetric Determination of Small Quantities of Chlorides in Waters H. B. RIFFENBURG, Virginia Polytechnic Institut,e, Blacksburg, Va.
T
HE method generally adopted for the estimation
of the chlorides in water has consisted of titrating a known quantity of water with a standard solution of silver nitrate, using potassium chromate as the indicator, and calculating the chloride content in parts per million or a similar denomination (1). The chlorides in most rain waters and many ground waters are present in too small a quantity to be determined accurately by this method. A study of over two hundred articles on the composition of rain water in different parts of the world made by Riffenburg (6) in 1923 showed the chloride content to vary from a trace to over 50 parts per million, with the average about 3.0 parts per million. Some authors used a 0.1 N or weaker solution of silver nitrate to titrate the chlorides, and many did not discuss,the method used. Such a variable chloride content indicates that either the method used or the analysis is faulty, and this is confirmed by determination of chloride content in lakes, rivers, and many ground waters. Jackson’s chloride maps (4) show that some natural waters j n the northeastern part of the United States contain as little as 0.2 p. p. m, Lake Superior water showed 1.1 p. p. m. and sixteen rivers in the United States showed less than 2.0 p. p. m. of chlorides (3). Twenty-three samples of rain water collected in Washington, D. C., in 1923-1924 and analyzed by Riffenburg (6) contained a maximum of 3.0 p. p. m. and an average of about 1.3 p. p. m. Samples collected in central and upper Michigan and Blacksburg, Va., averaged about the same. Williams’ (2) analyses of rain waters collected in ten different places in the United States showed averages from 1.4 to 0.25 p. p. m. Williams evaporated a larger quantity down to 25 cc. and titrated it with a silver nitrate solution, 1 cc. of which is equivalent to 0.5 mg. of chloride. These analyses show the importance of developing a method that will more accurately estimate very small quantities of chlorides in waters. An attempt was made to increase the accuracy of the titration by using dilute solutions and greater or smaller volumes of water, but this failed until a modified colorimetric method was used. The colorimetric modification consists of filling one, or preferably two, Nessler tubes to the mark with the sample to be tested, adding 1 ml. of potassium chromate prepared as in ( I ) , and mixing
well. One tube is used as the control and to the other is added the silver nitrate solution until the usual red color can be detected in the sample when compared to the control tube. The silver nitrate should be added drop by drop, shaking the tube well, and comparing it with the control after each drop. The silver nitrate used contained an equivalent of 0.05 mg. of chloride per cubic centimeter; it should not be stronger than this and should not be too weak. Test tubes may be used, but Nessler tubes have been found more satisfactory, since they have olished bottoms and present a longer column of liquid through wkch to view the color change. Sharper color changes can be produced by allowing the sun to shine on a piece of white paper or white porcelain under the tubes and protecting the tube from the light. Distilled water may be used as the control if the quantity of sample is not sufficient to fll two tubes, and the same control may be used for a number of samples if none shows a different shade of the chromate color. If a different shade of color appears, the sample itself will have to be used as the control. The burets and pipets used in these analyses were certified by the U. S. Bureau of Standards and the other volumetric apparatus used was calibrated. Fairly close checks were obtained by evaporating 500 ml. to 50 ml. and titrating by the usual method, using a yellow light to increase the sharpness in the end point. Analyses of twenty samplek of rain water collected at Blacksburg, Va., during the past year and a half show a maximum of 0.4 p. p. m., a minimum of 0.05 p. p. m., and an average of 0.23 p. p. m. LITERATURE CITED (1) Am. Public Health Assoo., “Standard Methods for the Examina-
tion of Water and Sewage,” 7th ed., 1933. (2) Collins, W. D., and Williams, K. T., IND.ENQ.CHEM.,25, 244 (1933). (3) Ddle, R: B.,U. 8. Geol. Survey, Water Supply Paper 236 (1909). (4) Jackson, D. D., Ibid., 144 (1905). (5) Riffenburg, H. B., Ibid., 560,31-53 (1925). RECEIVED October 4, 1934. Preaented before the Division of Water, S e a age, and Sanitation Chemistry a t the 88th Meeting of the American Chemios1 Society, Cleveltlnd, Ohio, September 10 t o 14, 1934.
