Precise Gas Chromatographic Determination of Hydrogen and C1 to C5 Hydrocarbons by the Simultaneous Use of Columns in Parallel Malcolm Shykles Universal-Matthey Products Limited, Underbridge Way, Brimsdo wn, Enfield, Middlesex, EN3 7PN, England
Quantitative analysis of H2 and C1 to C j hydrocarbons, which are the principal compounds found in refinery gas streams, is required for economic evaluation and operational control. Accurate analysis of these gases is difficult because there is no single detector which has a sensitive linear response to all compounds present, and also because their simultaneous separation on a single chromatograph in reasonable time is impossible without using temperature programming or column switching valves ( I , 2). The usual recourse is to analyze the hydrocarbons and hydrogen separately. This can result in lack of precision because of the difficulty of the injection of accurate sample volumes into each analyzer. The proposed method counters these difficulties by splitting the carrier gas flow through parallel columns in one chromatograph to flame ionization and katharometer detectors. The columns to the katharometer serve to separate hydrogen and to retain hydrocarbons until backflushed. T h e columns to the FID serve to separate the C1 to Cs hydrocarbons and backflush compounds eluted after npentane. The lengths of the columns are such t h a t hydrogen is eluted from column 2 (see Figure 1) ahead of methane from column 4. Thus only one data handling system need be used. The backflushing system is a variation of t h a t proposed by Deans ( 3 ) . No switching valves are used in the sample path. Dead volume and sorption effects are eliminated.
Rosman ( 4 ) . As a further precaution, the sample valve was returned t o its filling position exactly four seconds after t h e injection of the sample. T h e temperature of the gas sample valve was accurately controlled by utilizing the heater, platinum resistance thermometer, and heating control circuit contained in the PerkinElmer hot wire electronics unit, which are normally used to control t h e temperature of the katharometer detector. O p e r a t i o n . T h e flow system is illustrated in Figure 1. With toggle valves T 2 , T 3 , and T4 closed, the pressure on gauge G 1 is set to give t h e optimum flow through columns 3 and 4. T h e resultant pressure on gauges G2 and G3 is noted. Toggle valves, T 2 , T3, and T 4 are opened. T1 is closed, and t h e corresponding pressure regulators to gauges G2 and G3 adjusted so t h a t their pressures are slightly higher than before. This is t o prevent any diffusion of the sample into what would otherwise he dead space. T h e apparatus is now in the hackflush mode. T h a t is, columns 1 a n d 3 are hackflushed while the fldw is maintained through columns 2 a n d 4. For forward flow, it is necessary only t o open t a p T1 and close t a p T4. E x p e r i m e n t a l Conditions. Analyses were performed a t room temperature using nitrogen as t h e carrier gas. To give the optimum flow of 14 ml/min through columns 3 and 4, a pressure of 12 psig was required on gauge G I , resulting in a flow of 12 ml/min through columns 1 and 2. T h e resultant pressure on gauges G2 and G 3 was 6 psig. A sample loop of 0.2 ml was used. T h e integrator was set t o a range of 0 t o 100 m V throughout analyses. P r o c e d u r e . With t h e flow rates set! the sample is introduced using t h e gas sample valve. After the elution of hydrogen from column 2, t h e digital integrator is switched from the katharometer electronics unit t o the FID amplifier (set a t lower base-line potential to prevent premature integration). Following t h e elution of isobutane, columns 1 and 3 are hackflushed, leaving n-butane to npentane compounds t o continue through column 4.
EXPERIMENTAL
CALIBRATION
A p p a r a t u s . T h e analyses were carried out o n a chromatographic unit incorporating Perkin-Elmer flame ionization and katharometer detectors, amplifiers, and the standard Perkin-Elmer rotary gas sampling valve. A Kent chromalog-2 digital integrator with automatic base-line correction and a maximum integration rate of 40,000 counts per sec, was used t o measure t h e peak areas. Figure 1 is a flow diagram of t h e apparatus. Column 1 was 1st 2.5 ft, 20% dihutylmaleate on 60-80 mesh chromosorh P; 2nd 1.0 ft., 60-80 mesh 5A molecular sieves. Columns 2 and 5 were 2.5 f t , 60-80 mesh 5A molecular sieves. Column 3 was 3.5 f t , 2006 dihutylmaleate on 60-80 mesh chromosorh P. Column 4 was 12.0 ft. 20% dihutylmaleate on 60-80 mesh chromosorh P. Column 5 was the katharometer reference column a n d improved t h e stability of t h e pas flow obtained by using a pressure reducing valve alone. Column 1 was partly filled with the dihutylmaleate on chromosorb P packing material to prevent upsetting t h e split ratio hetween the columns (which could he caused by differences in sorption of t h e sample vapor by t h e stationary phases a t t h e column inlet) before all t h e sample had passed through t h e splitter. T h e columns were made of Ya-in. 0.d. copper tubing. Connecting unions were also filled with t h e appropriate packing material t o eliminate all void spaces. T h e line between t h e gas sample valve and t h e junction of t h e two columns was filled with 70 mesh glass heads a n d kept as short as possible (approximately 1 in.) t o reduce dead volume. T h e rotary gas sample valve was heated t o 180 "C t o help prevent sorption effects, which have been investigated by Myers a n d
Quantitative calibration of this apparatus is relatively easy because all compounds give linear responses and, therefore, calibration curves are not required. The hydrogen response factor to relate peak area to mol % was found from the response given by the injection of the pure gas. C1 to Cq hydrocarbon response factors were found by injection of a calibration gas mixture containing approximately equal volumes of these gases. Pentane factors were derived by plotting the relative area responses of the C1 to Cq hydrocarbons vs. their carbon number and extrapolating the graph.
RESULTS AND DISCUSSION Methane elutes with hydrogen and air from column 4. Experiments were made to determine if this affected the response of the FID to methane. A mixture of C1 to Cq hydrocarbons containing approximately 20% of each compound was analyzed, diluted to a 50% concentration in air, and reanalyzed. The ratio of C1 to Cq hydrocarbons in the diluted sample were checked against the original. The same procedure was repeated for hydrogen. The results obtained showed that the methane response was unaffected (within the.repeatability of the method) by the presence of hydrogen or air in the sample. Because of the insensitivity of the thermoconductivity detector to air when using nitrogen carrier gas, it was unANALYTICAL CHEMISTRY, VOL. 47,
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Table I. Precision Data Deburantzer overheads Mean analysis, Component
Hydrogen Methane Ethane Propane Propylene Is0 butane 11- Butane 1-Butene Isobutylene t m r z s -2 -Butene cis -2 -Butene Isopentane 11-Pentane C,+ compounds Total
mol
Meat1 Std dev
Repeat-
analjsis,
htean Std dev
Re?cat-
( 8 analbses)
abilit)
mol
Std de\
Repeat-
( E analjses)
abilit)
4.31 1.74 10.52
0.063 0.035 0.032
0.20 0.12 0.11
0.05
30.78
0.072
0.24
0.03 0.03
21.15 29.16
0.055 0.154
0.18 0.51
1.09
0.015
0.05
0.26 0.12 0.20 0.07 0.60 100 .o
0.011 0.011 0.005 0.014
0.04 0.04 0.02 0.05
(6 analyses)
ah>hI>
88.02 3.98 3.38
0.061 0.028 0.014
0.22 0.10 0.05
66.04 10.85 13.03
0.070 0.025 0.030
0.23 0.08 0.10
2.48
0.015
0.05
4.05
0.014
0.67 0.70
0.011 0.013
0.04 0.05
2.96 3.07
0.008 0,010
a%
moi
analbsis,
... ... ... 0.27 0.12 0.40 100 .o
0.012 0.012
0.04 0.04 100 .o
4
3
SAMPLE VALE
F I. D.
Figure 1. Flow diagram T1, T2, T3. T4, T5 = toggle valves. G1, G2, G3, G4, G5 = Pressure regulators with gauges
necessary to completely separate hydrogen and oxygen peaks. Injection of a 100% air sample produced a response equivalent t o that of 0.27% hydrogen. With samples containing less than 10% hydrogen, the oxygen peak was completely separated. Response of the apparatus to hydrogen was linear to within 0.5% which was the confidence limit of the calibration gas mixing apparatus. T h e object of this investigation was to provide an accurate quantitative method for the analyses of excess reformer gas for which the relative concentrations of hydrogen and the C1 to C j hydrocarbons are of primary interest. This gas also contains small quantities of ethylene and Cs+ compounds, which are not determined by this method. Therefore, for a total gas analysis, it is necessary to measure these compounds by other means and normalize the data obtained to 100%. Figure 2 shows a typical chromatograph for debutanizer overhead gas. Attenuation changes resulted in peaks between those labeled 7, 8, and 9. Standard deviation and repeatability data are summarized in Table I. Repeatability is that difference between two such single results as would be exceeded in the long run 950
ANALYTICAL CHEMISTRY, VOL. 47, NO. 6, M A Y 1975
Figure 2. Typical chromatogram (1) Hydrogen, (2) Methane, (3) Ethane, (4) Propane, (5) Propylene, (6) Isobutane, (7) nautane, (8) 1-Butene, Isobutylene, (9) frans-2-Butene. (10) cis-2Butene, ( 1 1) Isopentane, and (12) +Pentane
in only 1 case in 20 in the normal and correct operation of the test method (this is known as the 95% confidence level) ( 5 ) . Owing to the effect of normalization the maximum standard deviation is found a t the 50% level. T h e precision of the method compares favorably with data found in the literature (2, 6).
LITERATURE CITED (1) (2) (3) (4) (5) (6)
E. Malan and B. Brink, Chromatographia, 4, 178 (1971). C. N. Jones, Anal. Chem.. 39, 1858 (1967). D.R. Deans, J. Chromatogr., 18, 477 (1965). H. S . Myers and A. Rosman, J. Chromatogr. Sci., 7, 751 (1969). Appendix 1: 0-2-1968. 1969 Book of ASTM Standards, Part 18. N. G. McTaggart C. A. Miller, and B. Pearce, J. Inst. Petrol.. 54, 265 (1968).
RECEIVEDfor review August 6, 1974. Accepted December 16, 1974.
