Determination of Olefins in Gaseous Hydrocarbons B. R. STANERSON AND HARRY LEVIN, The Texas Company, Beacon, N. Y.
A method is described for determining olefin content of gaseous hydrocarbons by direct titration of the sample dissolved in cold chloroform. Bromine in glacial acetic acid is used for titration. The method has given satisfactory results on synthetic gaseous blends comprising the complete range of unsaturation for COand Cd hydrocarbons as well as C5 olefins in small amounts. The higher gaseous olefins (C3, C,, and C5) can be determined to the exclusion of ethylene if not more than 10 per cent of it be present. The method is suitable for routine plant control purposes because of its simplicity and rapidity. Hydrogen sulfide, mercaptans, and 1,3butadiene interfere and must be removed before analysis.
The method of Uhrig and Levin (8) for determining bromine addition number of liquid hydrocarbons prompted the investigation of a similar procedure for gaseous hydrocarbons. The principle (bromination by a solution of bromine in glacial acetic acid, under conditions of titration) remained the same b u t modifications were made to adapt the procedure to gases. This was easily done for gaseous hydrocarbon mixtures containing CS, Cd, and Cg olefins. The method was also extended to determine these olefins to the exclusion of ethylene if the latter did not exceed 10 per cent of the sample.
0 DRYING TUBE
T
HE importance of determining unsaturation in hydrocarbon gases is evident from the comparatively numerous publications on the subject in recent years. Some of these have dealt with improvement of existing procedures (5, 7) and others with new methods of attack ( 4 , 6 ) The present paper describes a method developed to satisfy the need for one that is rapid and suitable for routine plant control purposes. I
The methods commonly used for determining unsaturation in gases involve absorption in sulfuric acid solutions of various concentrations, absorption in bromine solutions of various concentrations, or catalytic hydrogenation. Methods such as bromination in the vapor state (6),fractional desorption (S), and desorption-thermal conductivity (2) are not suitable for routine use.
FIGURE1. APPARATUS An adaptation of the method of Uhrig and Levin (8) has recently been published by Benson ( I ) , who used it on a micro scale for determining olefins and obtained very good results, particularly when the end point of the direct bromine titration was illuminated with a blue-white lamp against a white background.
Matuszak (5) extensively reviews and presents the Principles and limitations governing the selection of conditions for determining gaseous olefins by absorption in sulfuric acid. By developing an apparatus which requires very small fortions (aPproxlmately 1 ml.) of reagent, he minimizes such actors as reversibility of absorption, solubility of gaseous paraffins, increase in solubility of hydrocarbons because of acid-soluble absorption products, solubility of hydrocarbons in precipitated polymerization products, and liberation of unabsorbable gas. Matuszak reports good results with olefins UP to and including the C4 hydrocarbons. The time required for analysis may be slightly longer than that required for other absorption methods or catalytic hydrogenation. Both McMillan et al. (4) and Savelli et ai. (7) show that conventional bromine water methods for determining olefin give high results. The latter had considerable success in using quarter-saturated bromine water, both with and without excess potassium bromide. This solution is weaker than usually employed and its superiority over the more concentrated solutions is attributed to its inactivity to butanes. Failure to obtain exact reproducibility was attributed to absorption of small quantities of saturated hydrocarbons in the liquid dibromides formed by reaction of bromine with unsaturated constituents. McMillan’s catalytic hydrogenation method (4) for determining unsaturation of gases has been used successfully by this laboratory and others; it is accurate but has certain disadvantages for routine use. Many plant gases contain impurities that poison the catalyst, necessitating frequent regeneration of it or delaying hydrogenation. Deviations from perfect gas laws and adsorption of sample on catalyst cause erroneous results unless proper corrections are applied.
Materials Used The butanes, propane, propene ethylene, and methane used in the experimental work were d l purchased as c. P. materials and tested 99.5 per cent pure or better. A commercial grade of propene, stated to be 95 per cent pure, was used in a few of the preliminary experiments on this compound. The amylenes and butenes were prepared in this laboratory by conventional methods and were 99+ per cent pure. The ethane used was not purchased or prepared in the purified state but added in the form of certain plant gases.
Reagents and Apparatus Bromine (1 per cent by volume) in c. P. glacial acetic acid, 0.1 N sodium thiosulfate, c. P. chloroform, 10 per cent potassium iodide solution, starch indicator solution, kerosene, and dry ice. Standardize the bromine solution by adding 5 ml. to 25 ml. of 10 per cent potassium iodide solution and 5 ml. of chloroform in an iodine flask. Titrate this mixture immediately with 0.1 N standardized sodium thiosulfate solution using starch indicator. It is advisable to standardize the bromine solution daily until its stability is established under the conditions of use. In addition to the apparatus shown diagrammatically in Figure 1 it is desirable to have a 10-mi. self-filling buret for the bromine solution.
782
RESULTSON C4 BLER’DSUSIR’GMETHODAa TABLE I. TYPICAL Sample
1
2
3
4
5
6
7
8
9
1
0
1
1
,
12
Composition, Volume Per Cent Isobutene 1-Butene 2-Butene Isobutsne n-Butane
12.8
24.2 9.0 1.6 12.9 4 . 0 0.8 2 . 7 13.2 15.9 1.5 5.9 6.7 3 4 16 5 1 6 . 1 1.8 7.4 6.8 4 2 : 6 55:O 2 4 : 5 30:O 3 7 . 4 1 3 . 1 1 3 . 4 1 5 . 9 44.6 42.7 45.2 31.3 29.0 70.7 69.3 69.8
..
2.3
..
6.1 1.8 0.7 2.7 0.8 3.4 59.1 60.3 33.3 31.8
0 . 3 12.45 2.5 8.25 2 . 5 11.45 69.0 37.80 25.7 30.05
Unsaturation, Volume Per Cent Theory 12.8 2 . 3 3 0 . 3 38.7 3 3 . 6 1 6 . 2 17.3 1 4 . 3 7 . 6 7 . 9 5 . 3 32.15 Found 13.1 2 . 6 2 9 . 7 3 9 . 5 3 3 . 8 1 6 . 8 17.5 1 4 . 2 7 . 8 8 . 0 5 . 2 32.10 Error, absolute 0.3 0.3 0.6 0.8 0.2 0.6 0.2 0.1 0 . 2 0 . 1 0.1 0.05 a Except that a 30-second end point was used.
TABLE11. TYPICAL RESULTSON BLENDSCOR’TAINIXG PROPENE Sample
(Method Aa Used on Blends 1 to 6 and Method B on Blends 7 to 10) 1 2 3 4 5 6 7 S 9 Composition, Volume Per Cent
n-Butane Ieobutane Propene
783
ANALYTICAL EDITION
October 15, 1942
9 0 . 3 83.45 9:7
16155
75.55 71.1 21.3 24:45 7.6
58.8 56.8 2 9 . 6 2 4 . 8 57:55 11.6 18.4 42.45
Unsaturation, Volume Per Cent Theory 9 . 7 16.55 24.45 7 . 6 11.6 18.4 Found 8 . 7 1 5 . 3 22.60 6 . 6 10.7 17.1 Error, absolute 1.0 1.25 1.85 1.0 0.9 1.3 Apparent purity of propene, % 9 0 . 0 9 2 . 5 9 2 . 3 8 8 . 0 9 2 . 5 9 3 . 0 a 30-second end point used.
Procedure
...
80.0 20.0
42.45 42.30 0.15
20.0 20.1 0.1
99.5
100.5
used in all cases except on samples containing ethylene. The excess bromine reacts with some ethylene, but not quantitatively. Under the conditions of Method A ethylene does not react appreciably. C A L C U L A T I O N SCalculate . unsaturation as follows: Per cent unsaturation AXBX100 (gas volume basis) = C X 7.14 where A = ml. of bromine solution reacting with olefins B = mg. of bromine per ml. bromine solution C = ml. of gas condensed One milliliter of gaseous olefin, measured or calculated to standard tem erature and pressure, is equivalent to 7.14 mg. ofbromine on the basis of one mole of bromine reacting with one mole of olefin.
10
Precision and Accuracy
The precision and accuracy of the proposed method are indicated b y the tables. In Table I are results obtained by Method A on blends containing all three Ca olefins. The average 11.6 56.3 11.6 5 5 . 5 deviation from the theoretical is ~ 0 . 3per 0.0 0.8 cent, absolute. 100 99.0 Table I1 shows the results obtained with propene as the olefin. The first six blends were analyzed by Method A and contained (95 per c e n t pure)- propene from one source. The remaining four samples were analyzed by Method B and contained propene from another source having a purity of 99.5 per cent. A further comparison of the methods is given in Table 111. Method B gave slightly higher results in those cases where propene and 1-butene are present. These two olefins are slower to react with bromine and by using Method A i t is possible that small amounts of sample escape during the period of slow additions of bromine. 8814 43:7 11.6 56.3
METHODA. Introduce approximately 5 ml. of chloroform into a 50-c~.Erlenmeyer flask and attach it to the apparatus below E (Figure 1). Place a dry ice-kerosene bath around this flask, so that it is a t least half submerged, and keep the bath at -55 F. (-48” C.) or lower. After a minute or two in the cold bath evacuate the flask for 1 or 2 minutes by opening it to the vacuum through stopcocks E , D, F , and J. After closing D to the flask, evacuate the Shepherd buret (1 mm. of mercury or less pressure) through stopcocks A , B, D, F , and J. Collect 100 ml. of gas sample in the buret by making the appropriate connections with a sample storage bulb or the original sample container. After recording the temperature of the water in the jacket as that of the sample and the pressure from manometer F, transfer the sample into the Erlenmeyer flask by mercury displacement to tube C. Agitate the flask to hasten solution and condensation of the gas in the cold chloroform. Turn stopcock A , so that air will be drawn into the flask through the hole in the end of the stopcock (Figure a), thus displacing the sample from tube C. After the sample has condensed and dissolved in the chloroform, remove the flask and promptly titrate its contents with a 1 per cent solution of bromine in glacial acetic acid. The solution is not cooled during titration. The end point is that at which a faint color of bromine persists for at least 60 seconds. The color of very dilute bromine water (made by diluting a saturated solution with 100 volumes of water) is used as standard. METHOD B. This alternative p r o c e d u r e is different from Method A only in the manner of titration. After the sample has been condensed, remove the flask from the apparatus and titrate rapidly witti the bromine solution until 0.5 to 1.0 ml. excess of solution has been added. Allow the flask to stand a short time (30 seconds are sufficient but longer periods up to 30 minutes are unobjectionable) in order to complete the bromination, add 10 ml. of 10 per cent potassium iodide solution, and titrate the liberated iodine with standard sodium thiosulfate solution. This method is more accurate than Method A and should be FIGURE2. STOPCOCK
TABLE 111. COMPARISON OF RESULTS BY METHODS Aa Sample
1
2
3
4
AND
B
5
6
19.8
27.6 26.8 45.6
Composition, Volume Per Cent 1-Butene 2-Butene Isobutane Plant gas Ab Plant gas Bb
17.1
..
11.2 34.2
82:s
5416
..
..
20.05
... ...
7i:95
10.0
.. ..
80:2
9o:o
..
.. ..
19.8 20.5 19.2
54.4 53.8 52.9
..
Unsaturation, Volume Per Cent Theory Found, Method4 Found, Method B, back-titration after: 30 seconds 2 minutes 10 minutes 30 minutes
t . .
59:O 59.0
32.8 32.8
..
..
38:6 38.8 38.9 38.8
60:l 60.8 60.8 61.0
33.3 33.5
3+:2 36.8
.. ..
....
.. .. . ,
...
...
33.4 33.7
24:O 23.9
..
19.9 19.8
..
.. .. ..
24:s 24.8
19:s 19.9
24:7 24.6
.. 54:6 54.5 54.7 54.6
....
0 30-second end point used. b Plant gases A and B were samples of polymerization charge stock and off gas containing Ca and C I unsaturates but neglible amounts of ethylene.
The effect of allowing the small excess of bromine (added according to the directions of Method B) to stand in contact with sample for varying lengths of time up to 30 minutes, is shown in Table 111. It is evident that bromination of paraffins is not significant. The proposed method has been used on very concentrated, as well as very dilute, blends of olefins in paraffins. The results of typical experiments are shown in Table IV. On
INDUSTRIAL AND ENGINEERING CHEMISTRY
784
Vol. 14, No. 10
melted maleic anhydride. The presence of hydrogen sulfide has an apparent inhibiting effect on the bromination of olefins conducted Sample I 2 3 4 5 6 7 8 9 1 0 1 1 in the analytical method proposed-for exComposition, Volume Per Cent ample, a sample containing 21.5 per cent isoIsobutane 13.1 24.3 37.0 45.4 . . . . 3.7 2.6 11.8 butene, 3 per cent hydrogen sulfide, and the n-Butane 85.6 7 3 . 3 59.4 53.8 .. .. ..... . 100' :: 8::; balance butanes gives a value of approxiIsobutene 1-Butene i:3 2:4 3:s 0 : 8 100' ... . . . . . .. .. .. .. .. mately 7 per cent olefins by bromination. 2-Butene . . . . . . . . . . 100. Propene . . . . . . . . . . . . . . 100' . . . . . . The presence of hydrogen sulfide causes an Method used B B B B Aa A" Aa B AS A4 A" early -appearance of an- apparent end point, which renders a sample containing it inappliUnsaturation, Volume Per Cent cable to Method A and makes it impracticable to 1.3 2.4 3.6 0 . 8 100 100 100 100 89.9 8 0 . 2 69.9 Theory Found 1.3 2.3 3.0 0 . 8 98.8 97.7 99.8 97.8 88.7 8 1 . 9 68.9 use MethodB. This limitation on themethod 1.3 2.3 3.5 0 . 8 100.4 101.4 9 9 . 9 9 8 . 0 9 0 . 8 81.6 69.5 . . . . . . . . 9 9 . 7 1 0 0 . 5 .. 9 8 . 3 8 8 . 7 .. 6 9 . 8 detracts practically nothing from its utiIity,since . . . . . . . . 100.7 . . . . . . . . . . . . hydrogen sulfide is easily eliminated by scrubAverage error, absolute 0.0 0.1 0.3 0.0 0.1 0.1 0.2 2.0 0.5 1.6 0.4 bing the samde with doctor solution. Methvl mercaptan h i s little effect, but results tend t o 0 30-second end point used. be higher than theoretical when it is present in high concentrations (5 to 10 per cent), and butadiene brominates very slowly, giving erratic blends containing not more than 5 per cent of olefin, an abresults by either Method A or B. solute error greater than 0.1 oer cent is not common; on The method can be used for the determination of unpure olefins thue results are not as good, saturation of C1 and C4 cuts obtained by low-temperature I n some of the preliminary work listed in Tables I to IV a 30-second end point was used for all direct titration. For blends containing TABLE V. RESULTS OBTAINED propene and 1-butene more accurate results [Bromine titration (Methods A and B), bromine water absorption (quarter-saturated were obtained with a 60-second end point as solution), and catalytic hydrogenation.] specified in Method A. This was used in 1 2 3 4 5 6 7 8 9 Sample the analysis of the complex samples shown Composition, Volume Per Cent in Table V. Here also is shown the utility Amylene8 0.6 2.0 2.4 7.0 1.5 1.4 0.4 of combining the method of bromine titration Isobutene 1.4 4.8 5.9 3.3 3.6 3.5 1.0 2:2 1-Butene 1 . 2 4 . 0 2 . 9 0.9 .. i:Q .. 4 . 9 2 . 2 3 . 0 with that of a method for total olefins (bro2-Butene 1.3 5.7 3.2 3.4 3.3 19.5 4.7 mine water absorption or catalytic hydrogenaPropene 1.2 3.1 3.1 1.0 8:6 7:3 4.3 5.2 2.7 Ethylene 3.4 9.0 6.8 14.7 27.1 tion). Isobutane 84:l 73:2 54:7 56:l 53.8 32.2 62.8 52.8 45.2
TABLEIV. ANALYSIS OF BLENDS CONTAINING VERYHIGHAND VERYLOW CONCENTRATIONS OF UNSATURATES
i;:;it::
~~
Discussion
n-Butane Propane Methane
..
..
..
l0:2
i:O
2i:2
~~
25:5
..
~
23.1
32.1
..
5:1
l2:5
7:s
Unsaturation, Volume Per Cent
~~
2i:7
..
18:5
..
The two variations of the method are given Theory total 5.7 19.8 24.1 18.4 18.0 23.2 29.6 25.5 36.3 because of their utility in the analysis of plant Theor less CzHd 14.6 14.2 22.8 10.8 9.2 Founb: Method A 5 : 8 19: 2 23:4 l7:5 23.0 1 3 . 9 13.7 11.4 10.3 gases of different composition. Method A 5.5 19.2 23.2 .. 17.5 13.9 13.8 23.0 Found, Method B may be used on all types of samples except .. .. .. .... ....* . 18.2 5.8 19.6 23.7 18.4 5.7 19.7 23.7 those containing ethylene in excess of 10 per Found, bromine *. .. .. .. 1 5 . 7 18:7 24:o 30:O .. Water absorption cent. The method finds all the higher olefins (1/4 saturated but not ethylene. This type of test is particusolution) .. .. 1 6 . 3 1 8 . 4 22 33 .. 72 22 99 .. 83 .. *. Found, catalytic larly valuable on plant gas charge stocks hydrogenation .. .. 2 3 . 4 2 9 . 2 .. .. *. that consistently contain ethylene below the specified limit and in which the higher olefins are the only unsaturated compounds reactive fractional distillation as well as for the numerous plant gases in the commercial operation. This is true in certain polythat contain less than 10 per cent of ethylene. I t has been merization processes. The unsaturation test can therefore be used in this laboratory with considerable success, new applied to the gas samples, thus avoiding the necessity of operators experiencing very little difficulty in acquiring the fractionation by distillation to eliminate the less reactive necessary technique. ethylene. Method B is more accurate and more precise than method A and should be used when ethylene is not present. There Literature Cited is very little difference in the time required for analysis by these methods. (1) Benson, S. W., IND. ENQ. CHEM.,ANAL. E D . , 14, 189-91 (1942). Since rapidity of analysis was desired every effort was made (2) Edse, R., and Harteck, P., Angew. Chem., 53, 210-13 (1940). to develop such a method, even a t the expense of extreme (3) Kiiohler, L., and Weller, 0. G., Mikrochemie, 26, 44 (1939). accuracy. It is believed that the accuracy of the method is (4) McMillan, W . A,, Cole, H. A . , and Ritohie, A. V., IND. E N O . better than certain routine tests (conventional sulfuric acid CHEM.,ANAL.E D . , 8, 105-7 (1936). (5) Matussak, M . P., Ibid., 10, 354-60 (1935). and bromine water absorption) and equal to that of catalytic (6) Rossmann, E . . Angew. Chem., 48, 223-6 (1935). hydrogenation within certain ranges of olefin concentration. (7) Savelli. J. J.. Seyfried, W. D., and Filbert, B. M . , IND. ENG. The proposed method is faster than the others, duplicate CHBM., A N A L .ED., 13, 868-79 (1941). analysis being completed in 15 to 20 minutes after the sample (8) Uhrig, K.,and Levin, H., Ibid., 13, 90-2 (1941). has been introduced into the apparatus. Hydrogen sulfide, mercaptans, and 1,3-butadiene interfere PREssNTmo before the Division of Petroleum Chemistry a t the 103rd MeetCHIDMICAL SOCIITY,Memphis, Tenn. ing of the AMEBIOAN and must be removed by scrubbing with doctor solution and
:
..
