Separation and Analysis of Mixtures of Hydrocarbon and Inorganic Gases by Gas Chromatography G. J. CVEJANOVICH Lago Oil & Transport Co., ltd., Aruba, Netherlands Antilles
b A rapid and accurate gas chromatographic method has been developed which permits the component analysis of C1 to Cr paraffins and olefins as well as the quantitative determination of Hz, Oz Nz, CO, and COz. Those hydrocarbons that have boiling points above that of 2-methyl-2butane (38.5" C.) are determined collectively by back washing and reported as C6+. The significance of this method is that the composition of the sample is obtained in a single analysis eliminating the need for calibration of the inorganic constituents that may be present. The technique involves the use of multiple columns that can be operated singly or in combination and arranged in such a manner that certain components can b e isolated. A single analysis can be made and the results calculated in less than 3 hours.
T
HE OBJECTIVE in this investigation was to develop a method that would provide a qualitative as well as quantitative analysis of samples containing the common fixed gases, the hydrocarbons from C1 to C6, including the mono- and certain diolefins, and finally to determine the amount of any heavier material that may be present. Many procedures have been proposed for this type of analysis, but none has combined them into a single analysis suitable for routine use in a laboratory. Greene and Pust (2) use two analyses to achieye their results. The heaviest components detected are the butanes, all of which are not resolved, nor are oxygen and nitrogen separated. Kyryacos and Board ( 5 ) resolved all the fixed gases, but not the hydrocarbons. Knight (4) obtained excellent resolution of the pentane-pentene isomers, but this too is only a portion of the required analysis. The method of Rombaut and Fodderie (9) requires two analyses to achieve a separation; a Molecular Sieve column for the fixed gases and a tetraisobutylene column for the hydrocarbons are needed. Furthermore, this substrate will not resolve all of the butane-butene isomers and has the objectionable feature of rapid elution even at ambient temperatures. Taylor and Poli (11) used a specially designed dual de-
654 *
ANALYTICAL CHEMISTRY
tecting thermal conductivity cell with a detector a t the exhaust of each column, requiring calibration curves or internal standards for quantitative results. Leggoe, Brewer, and Hoffman (6) have developed a method that separates the fixed gases and hydrocarbon vapors up to and including the butane-butenes, but they did not determine the heavier components. The procedure of Madison (7) is similar to the method described in this paper. However, in the former, no attempt is made to determine components heavier than the pentanes. I n addition, calibration curves are required for quantitative results. The necessity of recalibration a t regular intervals as well as the need for rigid control of the system does not lend itself readily to routine use. A fast and accurate method has been developed that circumvents these difficulties and permits the analysis of these complex mixtures in a single determination. By merely measuring the areas of the resulting peaks, applying the necessary thermal conductivity cell response factors, and normalizing the results, a complete analysis is obtained. The method is applicable to a wide range of sample types and has been used successfully for the analysis of flue gases and other inorganic mixtures as well as those containing hydrocarbons. The method has the added advantage that it can be programmed easily for high speed computers, eliminating tedious calculations. APPARATUS
,4 Perkin-Elmer, Model l54B Vapor Fractometer, equipped with a thermistor-type thermal conductivity cell and a standard Perkin-Elmer gas snmpling value, was used. Helium is used as the carrier gas throughout. The column support for the gas-liquid partition columns is Chromosorb (JohnsManville Corp.), 30- to 60-mesh as received. A third column contains 5A Molecular Sieves screened to 30- to 60mesh (Fisher Scientific Co.). All columns were constructed of copper tubing, inch in outside diameter. Two four-way valves (Type 316-6-'/~, Republic Manufacturing Co., Cleveland, Ohio) equipped with the necessary fittings were used to hold the various columns and to permit rapid switching from one column to another. To obtain
the peak areas, a Perkin-Elmer Model 194 printing integrator (0-6000 C P M ) , coupled to a Leeds & Northrup Speedomax G recorder (0-5 mv.), was used to record the chromatograms. COLUMN PREPARATION AND ASSEMBLY
Three columns are needed to perform these analyses. For the hydrocarbon analysis, 30 inches of squalane on Chromosorb followed by 30 feet of adiponitrile on Chromosorb are used in tandem. A 3-fOOt column of 5A Molecular Sieve, 30- to 60-mesh, is used to determine the inorganic gases. Partition Columns. Approximately 200 grams of Chromosorb is acid treated with 4 N nitric acid for 10 minutes, water washed until neutral to litmus paper, and dried in a n oven a t 180" C. T o 150 grams of t h e acidtreated and dried support are added 52.5 grams (0.35 gram per gram) of adiponitrile (Eastman-Kodak Co.) dissolved in sufficient acetone to form a loose slurry with the Chromosorb; the solvent is slowly evaporated on a hot plate with constant stirring. The substrate and support are then packed into a 30-foot length of copper tubing, which is coiled to conserve space. To 10 grams of acid-treated Chromosorb are added 3 grams (0.3 gram per gram) of squalane (Eastman-Kodak Co.) dissolved in acetone and finished in the manner d,epcribed above. The substrate and support are then packed into a piece of copper tubing 30 inches long. Absorption Column. Approximately 20 grams of 5-4 hlolecular Sieves, 30- to 60-mesh, are placed in a small porcelain evaporating dish and heated in a n oven a t 500' C. for 4 hours. il %foot section of 1/4-inch copper tubing is purged with d r y nitrogen and packed with t h e solid substrate. T h e column is coiled, again placed in the oven a t 500' C. and simultaneously purged with dry nitrogen for a n additional 2 hours. Column Assembly and Operation. T h e assembly used is shown schematically in Figure 1. T h e two fourway valves are connected with a very short pipe nipple and the Molecular Sieve column is installed across adjacent ports of one of the valves (V-1). One end of the adiponitrile column is placed in the remaining port of valve V-1, while the other end is placed in either of the auxiliary ports of the thermal conductivity cell block. The squalane column is connected be-
GAS SAMPLING VALVE
P-
I
LlQUID INJECTION BLOCK
O ' , ~ ~ ~ ~ U R E
0
ROTOMETER
V E N1 -
Figure 1.
Schematic of system
tween the other auxiliary port of the thermal conductivity cell and the port opposite the pipe nipple in the second four-way valve (V-2). Either of the remaining ports in the second four-way valve is connected to the sample inlet of the thermal conductivity cell while the other is connected to the detector inlet. I n arranging the columns, it is important that the sample pass through the squalane column first, then the adiponitrile column, and finally the Molecular Sieve column, if the desired results are to be obtained. To permit rotating the valves with the cabinet door closed, I/ginch holes were drilled into the side of the cabinet, and extension handles were attached to the valves. I n this way, temperature equilibrium is maintained during analysis. PROCEDURE
Analytical Method. T h e columns are installed, as described above, a n d purged with helium until free of air and other contaminants. It is advisable to isolate t h e Molecular Sieve column b y means of t h e four-way valve until t h e squalane a n d adiponitrile columns have been thoroughly purged t o prevent contaminating i t with high boiling materials. Prior t o sample introduction, the two four-way valves are preset to the positions shown in Figure 1. One milliliter of sample is introduced either by the PerkinElmer gas sampling valve, or a hypodermic syringe, and the course of the chromotograrn is carefully followed on the recorder. As soon as the hydrogen peak has been recorded, the four-way valve (V-1) to which the Molecular Sieve column is attached, is rotated, by passing this column, and isolating any oxygen, nitrogen, methane, and carbon monoxide that may be present
Table I.
in the sample. The next peak to emerge is ethane followed b y the remaining hydrocarbons in the order shown in Table I. Following the elution of 2-methyl-2-butene, the second four-way valve (V-2) is rotated, the heavier components back washed through t h e detector and recorded a s a single peak. The back washing valve (V-2) is then returned to its original position, the instrument is stahilized, and the valve used to isolate inorganic gases (V-1) is then returned to its original position permitting the elution of the trapped components. There is a temporary disturbance in the recorder base line when the position of the valves is changed. This has no adverse effect upon the analysis when trapping the inorganic constituents because recovery is very rapid or when back washing because sufficient time is available for the instrument to stabilize. Hoa ever, when these valves are returned to their original positions, the disturbance to the base line is pronounced. Therefore, sufficient time must be allowed for the base line to stabilize after the back washing valve is returned
Operating Parameters, Retention Data, and Thermal Response Factors
OPERATING PARAMETERS Temperature 28" C. Column pressure 20 p.s.i.g. Detector voltage 8 . 0 volts Helium flow rate Dual column 41 ml. per minute Triple column 36 ml. per minute RETENTIOS AND THERMAL RESPONSE DATA Compound
$bbreviation
Relative Retention Time (n-Butane = 1.00)
Thermal Response Factorsc 69 2d 1.96 2 09 1 43 5 2gd 2 09 1 43 1 19
0 437 Hydrogen H2 0 474 Ethane CZ 0 494 Ethylene CF= 0 604 Propane CS 0 604 Hydrogen sulfide HzS 0 648 Carbon dioxide co, Propylene C3= 0 748 Isobutane i-C4 0 795 Acetylene C2S 0 939 n-Butane n-C4 1 0000 1 18 1-Butene n-C4=l 1 284 1 24 Isobutylene i-C4= 1 22 1 355 trans-2-Butene tc4=2 1 545 1 18 Isopentane i-c5 0 98 1 647 cis-2-Butene Cc4=2 1 15 1 758 n-Pentane n-C5 0 95 1 892 3-Methyl-1-butene 3~hfc4=1 1 02 2 028 1,3-Butadiene c4=1,3 2 109 1 25 1-Pentene ca=l 1 02 2 549 2-Methyl-1-butene 2MC4=1 2 941 1 01 trans-2-Pentene tcs=2 0 96 2 955 cis-2-Pentene Cc6=2 1 02 3 306 1,4-Pentadiene Cs=1,4 1 04* 3 535 2-Methyl-2-butene 2MC4=2 1 04 3.715 0 . 038b 2 50 Oxygen 0 2 0.554b 2 38 Nitrogen Nz 2 i8 1 ,0006 Methane C1 Carbon monoxide co 3.347b 2 38 0 83 Hexanes and heavier cS+ Retention time of n-Butane is 16.8 minutes past injection. b Measured from the instant the value is turned. Retention time of methane is 5.1 minutes. 0 Reciprocals of values reported by Messner et al. (8). Determined independently.
VOL. 34, NO. 6, MAY 1962
655
to ita original position. When the valve, t o which the Molecular Sieve column is attached is returned to its original position, the disturbance that occurs markedly affects the oxygen peak, and the shift in the base line must be taken into consideration when measuring the area of thie peak. The determination of nitrogen does not offer this problem because there is sufficient time for the base line to stabilize before this component emerges. Preparation of Standards. A series of gas blends were carefully prepared using pure gases of 99.5+ mole % purity. Known quantities of pure hydrogen, oxygen, nitrogen, carbon
Table II.
w
* I Y
fflz
%
2; a
e
MINUTES
Figure 2.
Analysis of Synthetic Blends
(Each Analysis Is Average of Five Determinations) Blend 1 Blend 2 Blended. Found. Std. Blended. Found. Component mole %’ mole % dev. mole %’ mole 7’ Hydrogen 19 0 16.0 18.6 20.0 2.8 2 2 2.1 Oxygen 0.5 3.0 3.3 Nitrogen 7.9 7 8 0.6 11.9 11.7 Carbon monoxide 5 0 5.0 0.3 1.6 1.5 25 5 Methane 1.1 25.0 24.7 25.0 47 Ethane 4.6 1.3 1.6 0.4 Propane 8 0 6.1 7.6 6.8 0.8 Propylene 8 2 8 0 7.3 7.7 0.5 5 1 Isobutane 5.0 8.3 8.1 0.3 n-Butane 1 9 1.8 3.2 0.3 3.4 3 4 Isobutylene 3.4 5.5 0.6 5.9 2 0 1-Butene 2.0 3.2 0.8 3.9 1 7 trans-2-Butene 3.6 ( 2 . 0 (0.5 cis-2-Butene 0 3 0.7 0 4 Isopentane 0.4 0.3 0.4 0.1 0 1 I-Pentene 0.1 0.2 ... 0.2 2-Pentene 0.1 0.1 0.1 0.2 0.1 46 n-Hexane 5.0 - -100 0 Total 100.0 100.0 100,o
b,i
Table 111.
Std. dev. 1.2 0.1
0.3
