Determination of Kerosine-Rangen-Paraffins by a Molecular Sieves, Gas-Liquid Chromatography Method G . C.Blytas and D. L. Peterson’ Shell Development Company, Emeryuille, Calif.
n-Paraffins in kerosine fractions can be determined by a gas chromatographic method based on the quantitative elution of n-paraffin& which have been selectively sorbed on molecular sieves. The kerosine sample is injected into the carrier stream at a point upstream from the molecular-sieves column. The vaporized sample passes through the molecular sieves (at 350’ C) where n-paraffins are retained by sorption, while the branched paraffins pass on through a corn= bustion tube and the resulting C02 pulse is recorded. The flow of carrier is then raversed, and the n-paraf= fins are eluted by raising the temperature of the sieves to 550’ C, and are subscrqu8ntly condensed at the inlet of a GLC column held at Oo C. The n-paraffins are separated according to carbon number by temperature programming the GLC column. This method is applicable to the determination of n-paraffins up to eicosane. THEDETERMINATION of n-paraffins in complex hydrocarbon mixtures is of interest both academically and industrially. Academically, n-paraffin determinations are important in studies of the mechanism of chemical evolution of life processes on earth ( I ) ; industrially, a knowledge of the content and distribution of n-paraffins is often necessary in evaluating processes concerned with changing the “n-paraffinslbranchedparaffins” ratio of various petroleum fractions. Isomerization and catalytic reforming are well known examples of processes in which the n-paraffin content of gasoline fractions (C5-Cl0) is decreased. The determination of n-paraffins in kerosine fractions (CI1-Cle) is relevant t o the various processes which have recently been developed for the recovery of nparaffins for use in the manufacture of biodegradable detergents. Many of the analytical techniques used for determining n-paraffins in petroleum fractions use molecular sieves. For example, subtractive methods (2, 3, 4), in which samples are chromatographed through a conventional G L C column in series with a molecular-sieves column, are often used. The n-paraffins are determined by the difference between the chromatograms obtained in this way and with the G L C column alone. However, these methods are not well suited for the determination of small amounts of n-paraffins. To avoid this difficulty, Eggertsen and Groennings (5) used a G L C column and a molecular-sieves column in series for the direct determination of n-paraflins. In their method, the n-paraffins are first selectively sorbed on the molecular-sieves column while the non-normal paraffins are developed, then desorbed by heating and elution in the opposite direction,
Present address, Chemistry Department, California State College, Hayward, Calif. (1) M. Calvin, Meeting of California Section of ACS, April 11, 1966, Berkeley, Calif. (2) N. Brenner and V. J. Coates, Nature, 181, 1401 (1958). (3) B. T. Whitham, Ibid., 182, 392 (1958). (4) B. T. Whitham, Ibid.,192, 966 (1961). ( 5 ) F. ‘T.Eggertsen and S. Groennings, ANAL.CHEM.,33, 1147
(1961). 1434
ANALYTICAL CHEMISTRY
tt Room Temp.
\
Reference
He Flow Regulator Possible Position of Hydrocarbon-Detecting T h e r m a l Conductivity C e l l
i
I
Old Sample Induction Port
’
-1
A I Block Containing C a r t r i d g e Heaters- Dewar
CuO Combustion T u b e s , 70OoC
Molecular Sieves Column, 350 t o 5 5 0 T
SF-96 GLC Column O’C (Ice Bath)
to 2 5 0 T
Figure 1. Schematic diagram of apparatus
and finally determined according to carbon number. This method, however, is limited to gasoline fractions, which contain n-paraffins with no more than 10 carbon atoms, because of interferences from non-normal paraffins. Such interferences become appreciable with higher-boiling fractions such as kerosine. In this paper, we show that it is possible to obtain an accurate determination of the n-paraffins in kerosine fractions by a modified Eggertsen-Groennings method. Further, if the resolution of the non-normal paraffins is not essential in the analysis, and a “total-normal-paraffin” figure is sufficient, a particularly simple procedure is possible. We have analyzed kerosine fractions and shown that this method can be used for the determination of n-paraffins up to eicosane. EXPERIMENTAL
Apparatus. A schematic diagram of the apparatus used in this work is shown in Figure 1. It provides a regulated flow of helium through a G L C column past a sample introduction port and through a small bed of molecular sieves. A flow-reversing valve permits a rapid change t o the opposite direction of flow-Le., through the molecular-sieve bed, then the G L C column. In either event, the final exit gas passes through a copper oxide combustion unit where all hydrocarbons are totally oxidized to CO, and water. The water is removed on Drierite, and the COz is detected by a thermal conductivity cell and recorded. A summary of items of equipment used is given in Table I.
-1
I
I
Heating and 20 min Stripping Period
and Heating or
11
I
and Heating - , of
Program
Sieves l o 550.C
1
0
B. Chromatogram of kerosene with m0l.CYl.r sieve C O l Y m n .f 380.C
1
5
Marker
8
5
0
I
I
5
IO
LO
n-ClL
CLC Temp.
0
From O ' C
6
IX'L)
Figure 3. Chromatogram of kerosine obtained by injecting sample after the GLC column
n-Paraifi"S
"-C,
H e a m p and 20 min
O
Program Begun
Marker
Time. minutes
Non-Xormai Pardfins
Flow R e v ~ r s s l
' II,
, " &
Strippmg Period
I
I
5
IO
i5
20
T m c , minutes
Figure 2. Effect of temperature of MSSA column on interference by cosorbed non-normals Procedure. An analysis consists of three distinct steps: 1. Subtraction of n-paraffins o n molecular sieves. Initially, the direction of flow is forward, as shown in Figure 1, and the temperature of the molecular sieves is 350" C , while the G L C column is surrounded by an ice bath. A liquid sample of 1- to 3-pl volume is injected into a stream of preheated helium a t a point downstream from the G L C column, it is vaporized, and is carried over the molecular sieves. Here the n-paraffins are selectively sorbed. The nonnormal paraffins pass on through a combustion tube and the resulting COS pulse is recorded. The duration of this step is 1 to 3 minutes. 2. Desorption of n-paraffins from the sieves. With the non-normal paraffins thus removed from the system and recorded, the flow is reversed and the temperature of the sieves raised to 550" C for 20 t o 30 minutes. The desorbed n-paraffins are carried t o the inlet of the G L C column, still a t 0" C, where they condense. 3. Elution of n-paraffins. For the separation of individual n-paraffins, the ice bath around the G L C column is replaced by an insulating Dewar and with the flow still in the same direction as in Step 2, the temperature of the column is raised regularly t o 250" C, the maximum recommended operating temperature of the stationary phase (SF-96). The developed n-paraffins pass through the combustion and drying tubes opposite from those used for the non-normal paraffins and are recorded. During development, which requires 20 to 30 minutes, the molecular sieves are cooled to 350" C. Immediately after the recording of the heaviest normal components, the insulating Dewar is replaced by the ice bath, the flow is returned to the forward direction, and another analysis is begun as soon as the pressure has stabilized. The flow of the eluent gas is 50 ml/minute throughout the analysis. Maintenance. The proper functioning of the sieves can be monitored by over-riding the breakthrough of a light n-paraffin in a reference sample under standard conditions. A decrease in sorption rate or capacity will be evidenced by a n earlier breakthrough and a slower rise. Incomplete desorption of n-paraffins under standard conditions can be detected either by using internal standards, in this case a light and a heavy n-paraffin which should encompass the molecular weights of the n-paraffins in the sample, or by a repeat desorption and development. Each of the copper-oxide converters used here held sufficient oxygen as CuO for 200-300 samples. I n order to avoid unnecessary consumption of CuO, the effluent from a new SF-96 column being heated for the first time, or from a column being heated to supply silicone vapor for the treatment of fresh sieves (see below) should bypass the converters. Ex-
$ration or near-expiration of the CuO is evidenced first by reduced sensitivity, then by broad echo peaks due apparently to detention of incompletely oxidized hydrocarbons by Drieri te. DISCUSSION OF METHOD This method differs from that used by Eggertsen and Groennings in two important respects: The first modification is an increase of the sieves temperature during the subtraction of the n-paraffins. I n the present method the sieves are a t 350" C instead of a t 200" C. This is necessitated by the higher molecular weight of the kerosine samples used here. As Figure 2 shows, higher temperatures result in much less interference by the non-normal paraffins on the n-paraffin determination. The second modification is in the location a t which the sample is injected. I n the earlier version of the method, the sample was introduced upstream from the G L C column; in the present work, the sample is introduced downstream from the GLC column (see Figure 1). This modification ensures that the non-normal paraffins will not be present in the system when the flow is reversed for the desorption of nparaffins. A chromatogram of kerosine obtained by injecting the sample downstream from the G L C column, and with a subtraction temperature of 350" C, is shown in Figure 3. This chromatogram is essentially free of interferences from non-normal paraffins. A comparison of the non-normal paraffin peaks in Figures 2 and 3 is instructive. In Figure 3, the non-normal paraffins are completely developed within 2 minutes, while in Figure 2, their peak is still tailing out after 7 minutes because of the longer retention by the combined G L C and molecular sieves columns. Lengthening the duration of the n-paraffin sub-
Table I. Description of Equipment Used 7-ft X 1i4-inch(OD), 10% w SF-96 GLC Column on 60/80 mesh Chromosorb W. GLC Block Cartridge Heaters 2, 75 watts each. 10-inch X '/(-inch (OD), 1 to 2 Molecular Sieves Column inches of packing. Copper Oxide Combustion 2, 6-inch x 1/4-inch (OD) each. Capacity, 200-300 samples. Tubes Reference ( 5 ) . Copper Oxide Catalyst 10-inch x '/(-inch (ID) PVC Indicating Drierite Tubes tubing. Republic Teflon plug valve No. Flow-Reversing Valve 310-6, '/* D. Ideal - Aerosmith, Inc., needle Flow Regulator valve, Model 52-2-11 . Moore Products Co., Model Flow Controller 63BU-L. Linde 5A. Type Molecular Sieves 0.5, 1, 2 grams. Molecular Sieves Weight Molecular Sieves Particle Size 30/50 mesh.
VOL. 39,
NO. 12, OCTOBER 1967
1435
Table 11. Effect of Subtraction Temperature, of Amount, and of Age of Sieves on Interference by Non-Normals Normals Content of Sample: 1.74 wt Subtraction Wt. InterEXsieves, ference,a Temp, Time, periC minutes grams Sieves ment 360 7 2 New 73 1 18 405 5 2 New 2 I 460 2.3 2 New 3 0.5 31 New 350 4.5 4 53 350 4 Old* 2 5
z
z
360 360 365
6 7 8
2 2 0.5
6 11 2
Oldb
Table 111. Precision of n-Paraffin Analysesa n-Paraffin concentration, wt ~Sample Clo CII C12 C13 C I ~ CI: Total
z
0.60 0.63 0.64 0.67
Averages : B
0.58 0.59 0.59 0.62
0.34 0.34 0.32 0.36
0.13 0.17 0.25 0.18
1.65 1.73 1.80 1.83
0.64 0.59 0.34 0.18
1.75
0 . 0 5 5 . 3 7 5.10 4.69 2 . 9 5 0 . 0 6 5.56 5.17 4.69 3.41 0 . 0 6 5 . 3 3 5.14 4.44 3.49
0.43 0.47 0.34
18.59 19.36 18.80
Averages: 0 . 0 6 5 . 4 2 5 . 1 4 4 . 6 1 3 . 2 8
0.41
18.92
3 . 4 5 6 . 1 6 5.96 3.71 3.52 6 . 7 9 6 . 0 3 4.02
0.62 0.75
19.19 21.11
C
Averages : 3 . 4 9 6.48 6 . 0 0 3.87 0 . 6 9 20.53 a 10-20 wt % ii-Cle was added as an internal standard.
traction step in Figure 2 to allow for complete development of the non-normal paraffins would result in a loss of some of the lighter n-paraffins. The optimum subtraction time is a function of the composition of the sample, and in general increases with decreasing normal paraffin content. In kerosine samples, the maximum subtraction time can be assessed by adding a small portion of n-octane and recording, during analysis, the non-normal paraffin tail a t highest sensitivity. The first indication of the arrival of n-octane corresponds to the maximum subtraction duration. Further improvement in separation, if necessary, can be achieved by decreasing sample size, increasing subtraction temperature, or decreasing carrier flow rate. Essentially complete elimination of interference is thus possible. In general, elimination of interference from cosorption is much easier with aged sieves than with fresh, very active sieves. "Aging" of sieves can be accelerated by exposing the fresh sieves to silicone vapor at a relatively high temperature. New sieves, exposed for 20 hours at 380" C to the effluent gas from the SF-96 column held a t 250" C, showed no more interference than sieves which had been used repeatedly over a period of several weeks. This treatment lowered the saturation capacity for n-pentane at 0 " C by only 15 %, of which at least part could be due to a loss of nonselective sorption sites. Treating with hexamethyldisilazane (6), which is commonly (6) R. H. Perrett and J. H. Purnell, J . Chromatog., 7,455 (1962).
1436
ANALYTICAL CHEMISTRY
surface sorption Zeolitic Sorption 01 ~ a n - ~ o r n a l s
-Linde SA S:o:ecular ----_ S ~ l i c o n eTreating
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Sieves
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