able, the Soltrols are useful mixtures for analytical work. These samples were analyzed in duplicate by the method employed for the mixtures containing n-heptane. The results, calculated using Equation 2 n-ith a= 1.3951 and a = 1.4097 for samples containing octane and decane, respectively, are in Table TI. The results indicate that this technique is applicable to the determination of the n-paraffins from heptane to decane. The proposed procedure has been in routine use here for several months.. The n-paraffin content can he deducted from the total alkane value determined mass spectrometrically to give the isoparaffin content. Then ratios of iso- to n-paraffin can be computed. The samples analyzed here shon- a wide variation in this iso-tonoma1 ratio. A number of typical
results obtained for 84" to 112" C. and 162" to 185" C. distillates are listed in Table T'II. DISCUSSION
The proposed method should only be applied to distillates of known boiling point range which are free of olefins and water. The absolute accuracy depends upon the validity of the assumptions concerning the presence of only one n-paraffin in each of the distillates and the behavior of the distillates as ideal mixtures. The results shorn an average precision of better than 1% of the n-paraffin content, and an average error of less than 27, of the n-paraffin content. ACKNOWLEDGMENT
The authors wish to thank the Shell Development Co. for permiision to puhliqh this paper.
LITERATURE CITED
(1) Dimbat, M., Porter, P. E., Stros;. F. H., ANAL.CHEM.28, 290 (1956,. (2) Hibbard, R. R., Cleaves, 4. P.. Ibid., 21, 486 (1949). (3) Kurtz, S. S., Jr., in "The Cheniistr!of Petroleum Hydrocarbons," B. T. Brooks, others, eds., Vol. 1, Chap. 11, p. 315, Reinhold, New Torl;. 1954. (4) Linde Air Products Co., New Yorl; 17, N. Y., "Molecular Sieves for Selective Adsorption," 1955. ( 5 ) Rossini, F. D., Mair, B. J., Streiff. A. J., "Hydrocarbons from Petroleum,', p. 20, Reinhold, Ke-x l-ork. 1953. (6) Ibid., p. 184. (7) Ibid., pp. 226-36. (8) Sobcov, H., Ax.4~. CHEK 24, 1908 (1952). RECEIVED for review December 13, 1953, Accepted March 4, 1957.
Determination of Normal Paraffins and Normal Olefins in Petroleum Distillates KURT H . NELSON, M. D. GRIMES, and 6. J. HEINRICH Phillips Petroleum Co., Barflesville, Okla.
)A method for the determination of normal paraffins and normal olefins in 60" to 400" F. petroleum distillates is based on the selective sorption of straight-chain compounds by a synthetic zeolite, Linde Molecular Sieve, Type 5A. A weighed quantity of zeolite is brought in contact with the petroleum distillate and reweighed. After the unsorbed materials are removed by vacuum evaporation a t 100" C., the zeolite is again weighed and the weight per cent of n-paraffins plus n-olefins is calculated. The nparaffins are determined by analyzing an acid treat raffinate of the distillate, and the n-olefins are obtained as the difference between the two analyses. A single determination, requiring about 15 minutes of operator time, can be completed in approximately 1 hour. The accuracy and precision are of the order of +OS%.
A
for determining the straight-chain hydrocarbons in petroleum distillates during refinery operations would be highly desirable from the standpoint of product quality. As one example, the method mould be useful in the control of platformer columns to minimize n-paraffin contamination of heavy platformate. Another RAPID PROCEDURE
1026
ANALYTICAL CHEMISTRY
application would be to check the efficiency of the isomerization reactors in a inethylcyclopentane-n-hexaneisomerization unit. Several methods have been used to analyze petroleum distillates for nparaffin content. The formation of crystalline adducts containing urea and straight-chain hydrocarbons is the best known. This technique has been extensively investigated as a means of separation (4, 5, 11) and as an analptical method (f3,14). As a result the scope and structural limitations of complex formation are well known (S. 9, fZ). The ease of formation and stability of adducts increase as the chain length of the hydrocarbon increases, but a minimum chain length of si.; carbon atoms is necessary for adduction at room temperature. As a method of analysis, the best accuracy is obtained for compounds with chain lengths of more than 14 carbon atoms. Adduct formation by thiourea, the sulfur analog of urea, has also been studied, but thiourea is not specific for straight-chain hydrocarbons (8). Schindler and Kinsel (10)and Leithe (6) have determined indirectly the nparaffins present in gasoline, oils, and waxes. I n their methods the naphthenes and branched-chain paraffins are reacted with antimony pentachloride.
The unreacted n-paraffins are then extracted into carbon tetrachloride and determined by measuring the density of the resulting solution. The sorption of n-paraffins by natural zeolites has' been reported by Barrer and coworkers (1,Z), who investigated the size of molecules sorbed and the conditions for sorption. The best eeolite was chabazite. which quantitatively sorbed n-paraffins up to seven carbon atoms in length. Barrer used chahazite to separate n-paraffins quantitatively from prepared binary and ternary mixtures containing brancheclchain paraffins. naphthenes, or aromatics. Recently, Linde Co. has begun to market synthetic zeolites under the trade name RIolecular Sieves. Of these zeolites, Type 5A is a dehydrated calcium alumino silicate whose c r p tal lattice has pores 5 A. in diameter. This zeolite has the interesting property of sorbing straight-chain Compounds tin the exclusion of the other compounds thus providing a means for specificall>determining n-paraffins and n-olefin< ill the presence of other hydrocarbon:. APPARATUS A N D REAGENTS
The sorption tubes are constructed :Ishown in Figure 1. The portion of the
a,ppnratus containing zeolite is bent from a tube 1.2 om. in diameter by 18.0 em. long, and fits vertically in the furnace. The side arm of the three-way stopcock is sealed close to the stopcock barrel. The funnel has a capacity of about 3 ml. The 14/35 standard-taper joint permits filling and cleaning of the sorption tube. -4 furnace (Figure 2) capable of maintaining a temperature of 100' C. and permitting vertical insertion of the sorption tube is required. This furnace is essentially an electrically heated aluminum block housed in a small cabinet. The manifold above the furnace permits the sorption tubes to he attached to a vacuum pump and manometer.
n
ard-taper joint with the silicone lubricant and secure with small springs. Place the sorption tube in the furnace maintained a t 100" C. and connect to the manifold. Open the two-way stopcock and evacuate the sorption tube by means of the vacunm pump. After 10 minutes close the two-way stopcock and remove the sorption tube from the furnace. Cool before weighing. Determination of +Paraffins Plus n-Olefins. By means of the syringe, place 1 ml. of t h e sample in t h e funnel of the prepared sorption tube. Use 2 ml. of sample if the n-paraffin plus n-olefin content is less than lo'%. Then carefully open the three-way stopcock to drain t h e sample into t h e sorption tube. Close t h e stopcock \\-hen two or three drops of the sample remain in the funnel. Then position the stopcock plug so the liquid in the bore can drain into the sorption tube. With a stream of air, evaporate the few drops of sample remaining in the funnel. After weighing the sorption tube, heat it a t 100' C. in the furnace while evacuating with the vacuum pump. After 10 minutes remove the sorption tube from the furnace, allow it to cool, then weigh. Reheat the sorption tube for an additional 10 minutes, cool, and reweigh.
fins and n-olefins in the sample using t h e following equation. n-pamffis plus n-olefins, weight
00
=
(C - -41 100 (E - A )
where
A = weight of sorption tube, grams B = weirht of sorption tube and C
=
sample, grams veight of sorption tube and n-paraffins plus +olefins, grams
n-Paraffins. Calculate the concentration of n-paraffins in the original sample with t h e following equation. n-paraffins, weight %
=
(F - D )G (E- D )
where
D E
=
F
=
G
=
=
weight of sorption tube, grains weight of sorption tube m i l raffinate sample, grams weight of sorption tuhe niirl n-paraffins, grams weight per cent parafins plus naphthenes in origina,l sample
n-Olefins. Calculate the concentration of n-olefins in the sample by subtracting t h e weight per cent of nparaffins from the weight per cent of n-paraffins plus n-olefins. DISCUSSION
Figure I .
Sorption tube
Syringe. A 5-ml. hypodermic syringe equipped with a 5-inch, 20-gage needle is satisfactory. Vacuum pump. Manometer. Stopcock clips. Stopcock tension clips sold by Todd Scientific Co., Springfield, Pa., are satisfactory. Stopcock grease. A hydrocarhoninsoluble stopcock grease such as Phynal (Podhielniak, Inc., Chicago, Ill.) is required, as well as a silicone lubricant (Dom Corning). Synthetic zeolite. Calcium aluminosilicate zeolite, 14 to 30 mesh, Linde Co.; tradename, LindeMolecular Sieve, Type 5A. PROCEDURE
Preparation of Sorption Tube. Prior to each analysis remove any used zeolite from the sorption tube. Then lubricate the three-way stopcock with the hydrocarbon-insoluble stopcock grease and t h e two-way stopcock with the silicone lubricant. Secure t h e stopcocks with stopcock clips. (Except when introducing a sample into the sorption tube, t h e plug of the three-way stopcock is always positioned so its bore can be evacuated.) Yext, fill the sorption tube with the synthetic zeolite. Then lubricate the stand-
Figure 2. Furnace for determinotion of n-olefins and n-paraffins Determination of n-Paraffins. First, remove t h e olefins and aromatics from a sample using ASTM Method D1019-55T and reserve t h e raffinate for a n-paraffin analysis. Calculate the per cent paraffins plus naphthenes present in the sample. Then determine t h e n-paraffins in t h e raffinate using the same procedure as for t h e n-paraffins plus n-olefins in the original sample. CALCULATIONS
n-Par&s Plus -Olefins. Calculate the concentration of n-paraf-
Of the various structural types of hydrocarbons, the +paraffins and nolefins have the smallest critical crosssectional molecular diameter, about 4.9 A. The addition of a side-chain methyl group to the linear molecule, as in the isoparaffins, increases the critical molecular diameter to 5.6 A. When a paraffin, such as 2,2,4-trimethylpentane, has two side-chain methyl groups on the same carbon atom, the molecular diameter is increased to 6 A. All naphthenic and aromatic hydromrbons have critical molecular diameters greater than 6 A. Linde Molecular Sieve, Type 5A, which has pores 5 A. in diameter, can therefore selectively sorh the straight-chain. hydrocarbons to the exclusion of all other hydrocarbons. The unusual sorption property of the synthetic zeolite is shown in Figure 3. These curves, calculated from litcrature data (7), are plots of the equilibrium capacity of the zeolite for various hydrocarbons against the ratio, P/Po. P is the equilibrium vapor pressure of a hydrocarbon over the zeolite a t the equilibrium temperature. PO is the vapor pressure which the hydrocarhon would have a t the equilibrium temperature in the absence of the zeolite. The upper curve, A , represents the straightchain hydrocarbons while t h e lower curve, B, represents the other hydrocarbons. The sharp rise of curve A to the plateau beginning a t P/Po equal to about 0.01 indicates that the capacity for straight-chain compounds is cssentially independent of the straight-chain VOL. 29, NO. 7, JULY 1957
1027
hydrocarbon partial pressure. This rise also indicates that the straight-chain hydrocarbons are tightly held by the zeolite and are desorbed with difficulty. T o initiate desorption, the temperature must be raised and/or the pressure lowered until P/Po decreases below about 0.01. Therefore, a sample with a boiling range of 60" to 400" F. should be distilled into two fractions before analysis. The pentane-containing fraction can then be analyzed at a lower temperature to prevent loss of n-pentane. The other fraction is analyzed in the usual manner. Some preliminary experiments showed that varying the time interval between sample introduction and heating did not have a noticeable effect on the results. A period of 5 to 10 minutes is adequate for sorption of straightchain hydrocarbons. Heating for 10 minutes a t 100' C. is sufficient to remove the unsorbable components with only a minute loss of sorbed materials. Prolonged heating causes a slow loss of sorbed compounds from the zeolite. The quantitative aspects of the sorption of various straight-chain hydrocarbons at several temperatures were first investigated. For this, 1-ml. samples of high purity hydrocarbons were analyzed by a procedure similar to that outlined above. The data obtained are tabulated in Table I. The sorption of n-pentane, the most volatile of the n-paraffins listed, was determined at temperatures of O', 25", and 100" C. As may be seen, %pentane is retained by the zeolite at temperatures up to 100" C. The n-paraffins of longer chain length, therefore, should remain sorbed a t this temperature. This is confirmed by the data in Table I. Information on the sorption of Gtetradecane and nhexadecane at 200' C. was obtained because narrow, high boiling distillation fractions would be more readily analyzed a t temperatures above 100' C. Because of the olefinic bond, the nolefins are somewhat more strongly sorbed than are the n-paraffins. For this reason, the sorption of only one n-olefin was determined. At 100' C. no appreciable loss of 1-hexene occurred. K i t h their large molecular diameters, hydrocarbons other than straight-chain compounds cannot be sorbed by the zeolite. However, some surface sorption on the zeolite particles could occur. To determine this, several nonsorbing hydrocarbons were analyzed in the same manner as the straight-chain hydrocarbons. The data for some high purity Phillips Petroleum Co. Research Grade hydrocarbons are listed in Table 11. -4t 25' C. almost 2% of the methylcyclohexane and toluene remained sorbed on the synthetic zeolite. This amount of sorption was reduced to 0.570 or less when the furnace temperature was 100' C. Therefore. it may be 3028
ANALYTICAL CHEMISTRY
Table I. Amounts of Various Straight-Chain Hydrocarbons Sorbed by Synthetic Zeolite
Hydrocarbon n-Pentane
Source Phillips
Purity Research Grade
%-Hexane
Phillips
Research Grade
%-Octane
Phillips
Research Grade
n-Decane n-Dodecane n-Tetradecane
Eastman Eastman Eastman
Practical Grade Practical Grade Practical Grade
n-Hexadecane
Eastman
Practical Grade
1-Hexene
Phillips
Heaearch Grade
OS2
Furnace Temperature, Amount Sorbed, c. Weight yc 0 99.87,99. i o 25 99.93,99.91 100 99,7499.82 25 99.93,99.95 100 99.30, 100.00 25 99.99,99.72 100 99.84,99.94 100 99.99,99.82 100 99.61,99.89 100 99.59 200 99 20 100 99.25 200 99.67 100 99.57,99.78
If-
-
A
B I
0.0
Figure zeolite
0.2 0.4 0.6 RELATIVE PRESSURE, P/Po
3.
I
1
0.8
Sorption of hydrocarbons on synthetic A. 6.
Straight-chain hydrocarbons Other hydrocarbons
assumed that other nonsorbing hydrocarbons are not appreciably sorbed at 100" C. Because the straight-chain compounds are retained in the zeolite at i00' C., this furnace temperature was selected for the final analytical procedure. To evaluate the procedure, six synthetic samples representing various plant samples mere prepared and analyzed. Phillips Petroleum Co. Research Grade hydrocarbons were used to blend the samples shown in Table 111. The data given are for duplicate analyses. The accuracy in analyzing these synthetic samples was better than &0.5% by inspection. An analysis for the sum of n-paraffins and n-olefins will yield sufficient information in many cases, particularly for samples having a low total olefin content. However, data for the individual amounts of these two straightchain hydrocarbons present in a sample may be highly desirable. Such information can be readily obtained from two analyses, First, one portion of a
Table II. Amounts of Ring and Branched-Chain Hydrocarbons Sorbed by. Synthetic Zeolite .
Amount Furnace Sorbed, Tempera- Weight Hydrocarbon ture, ' C. 70 0.02,0.04 2-Rlethylbutane 25 0.50,o.55 2-Methylpentane 25 1,12,1.79 Methylcyclohexane 25 0.53,0.45 100 0.99,1.87 Toluene 25 0.13,0.52 100 0.10,0.39 Cyclohexene 100
sample is analyzed for the sum of n-paraffins and n-olefins. Then a second portion of the sample is acid treated according to ASTM Method D1019-55T to remove the olefins and aromatics. The resulting raffinate is analyzed for n-paraffins and the data are calculated to the whole sample basis. The nolefins are then obtained as the difference being the two analyses. Table
IV shows the results obtained with two synthetic samples blended from Phillips Petroleum Co. Research Grade hydrocarbons. The accuracy of the analysis is about iO.5%. Several plant samples were analyzed for either n-paraffins or the sum of
n-paraffins and n-olefins. The analytical results for some of these samples, given in Table V, again show a precision of about *0.5~o. The n-paraffin plus n-olefin results for the platformate fraction and the n-decane concentrate could be considered approximate n~~
Table 111.
Analysis of Synthetic Samples for n-Paraffins
Components Blended: Toluene Benzene AIethylcyclohexane 2,2,i-Trimethylpentane 2-Mcthylpentane n-Hesane n-Heptane n-Octane Found: n-Paraffin
Platformate
Weight % Light Gasoplatline formate
9.80
44,79
35.06
...
0.67
35.53 li:28
19.77 30 27
5.10 29.98 10.25
95.32 1.55 0.10 ...
Charge 3.07 90:i9 ...
32 i t 21 83
2.02
3.53
14 I10
. .
1.01
2.81
15 17
21 81
1 28 1 28
2.81
10:01
15 OT 14 98 dv. 15 02
Table IV.
...
Heavy platformate
10 08 10 28 10 18
14 89 15 03
21 86
21 84
1 28
...
2.97 2 89
Analysis of Synthetic Samples for n-Paraffins Plus n-Olefins, n-Paraffins, and n-Olefins ___ Sample 1, Wt cc
H y d ocarbon ~
Blended
Toluene \lrthvlcycloIieuane (‘1-clohexane 2.2,4-Trimethidpentane n - Heptane 1-Hexene Total n-heptane I-hexene
34 60 15 27
+
13 84 18 35 17 94
36 29
Table V.
Sample Platformer charge Platformer charge Platformate fraction n-Dccxne roncentrate
Found
18 78, 18 77
18 02 36 67,36 04
Sample 2, Wt 7 Blended Found 11 46 41 25 10 S i 22 91 8 97 8 99, 9 22 4 54
13 51
4 44
13 61,13 46
Analysis o f Plant Samples
Boiling Range, ’ F. 251 to 394 2j2 t o 381 115 t o 180 331 to 345
+
n-Paraffin n-Olefin, Kt. 5
ACKNOWLEDGMENT
The authors wish to e x p r w their appreciation t o H. J. Hepp and coworkers for the helpful discussions. and t o Phillips Petroleum Co. for making publication of this paper 1)ossihle.
~~~
Platformer charge
parafin values. This appro\inintion valid because the total olefin contents of these samples were 0.5 and 2.3 volume % respectively.
n-Paraffin, n-t. % 22 6,22 2 21 7,21 8
23 9 3 , 2 4 06 7 5 33,76 37, i 5 68
LITERATURE CITED
(1) Barrer, R. AI., Belchetz, I., J . SOC. Chenz. Znd. 64, 131-3 (1945). (2) Barrer, R. AI,, Ibbitson, D. A , , Trans. Faradau SOC.40, 195-206
(1944). (3) Domask, IT. G., Kobe, I
