Anal. Chem. 1904, 56, 1995-1997
1995
Gas Sample Transfer by Gaseous Displacement Ewan R. Colson Gas & Fuel Corporation of Victoria, Scientific Services Department, P.O. Box 83, Highett, Victoria 3190, Australia Gas samples for analysis by gas chromatography or mass spectrometry are frequently trapped, stored, and discharged to the analyzer at pressures close to ambient. Typically, the storage container may be (1) a gas sample tube of metal or glass with a valve at each end, (2) a collapsible plastic bag with a single valve, or (3) a gastight syringe or miniature gas holder. It is feasible to automate the collection of gas samples into banks of such storages, but there are difficulties in the automation of the transfer of such collected samples to the analyzer and the subsequent purging of the containers for reuse. For gas sample tubes, liquid is normally used as the displacing medium, and care must be taken to prevent the liquid from contaminating the analyzer intake system. This paper reports the automated displacing of sample tube contents with carbon dioxide gas.
EXPERIMENTAL SECTION Apparatus, A home-made sequencer organized valve operations in the systems illustrated in Figures 1and 2 and initiated chromatographic runs at appropriate times. The purge and displacement system of Figure 1was controlled to produce nitrogen flows from point U and carbon dioxide or nitrogen flows from point L by the operation of valves B, D, and F. Two flow rates of nitrogen from U were determined by the state of valve A. A flow regulator FR set a basic carbon dioxide flow rate which could be adjusted to two different values by the operation of valves C and E. Outlets U and L of Figure 1 were bridged to points U and L of a sample tube module shown in Figure 2. Coordinated operation of the valve pairs, 1L-lU, 2L-2U, 3L-3U, etc., with the valves in the Figure 1 system, enabled the displacement of gas in a selected sample tube in volumetric stages suited to the dead volume of the sample transfer line to the chromatograph. The sample tubes are of brass, about 290 mm X 17 mm i.d., with a capacity of about 65 mL. The end caps are machined conically to ease the gas velocity transition at the connections. The tubes are housed six to a box and connected to the upper and lower manifolds by miniature solenoid valves (type EVO-3, Clippard Instrument Laboratory Inc., Cincinnati, Oh). The nitrogen flow system of Figure 1was controlled to purge the box upper manifold between tube selections and to purge each tube of carbon dioxide and sample residues in preparation for another field sampling operation. Typically, the between-tube, upper manifold, purge volume was 32 mL, and each tube was finally flushed with 490 mL of nitrogen, The point labeled 1/0in Figure 2 serves as a sample inlet for the reverse procedure of sequentially filling the tubes by purging downward and out of point L. A separate battery-powered controller has been developed for the particular application of sampling natural gas at field locations at preset times, for subsequent analysis of a sulfur hexafluoride flow tracer. Multiple sample boxes can be handled by connection of the 1/0points of several boxes to the inlet lines of an accessory stream selector valve attached to the chromatograph. The dead volume of the sample system was about 10 mL from point U of Figure 2 to the gas sampling valve. This sample path included the upper manifold of a six-tube box, a 12-stream selector valve, a porous metal filter, and necessary interconnecting tubing. A Hewlett-Packard 5880A chromatograph, incorporating a thermal conductivity detector was used with helium carrier gas for three experimental procedures. One of the sample tube banks was filled with pure nitrogen, and the carbon dioxide displacement medium transferred the tube contents to the gas sampling valve of the chromatograph, in volumetric stages, separated in time by 1 min. This experiment was repeated with pure methane as the initial tube fill, and again with a four-component mixture of methane,
carbon dioxide, propane, and isobutane. Because the analytic data were required at 1-min intervals, the separating conditions were adjusted to achieve this. The column used for these separations was 1200 mm X 2.1 mm i.d., packed with 60/80 mesh Carbosieve B (Supelco, Inc., Bellefonte, PA). A Quantitative Study. A sulfur hexafluoride (0.24 ppm, molar) in natural gas standard was prepared and stored at 1400 kPa in a gas cylinder. A sample tube module (box) containing six nitrogen-purged tubes was prepared by separately passing 300 mL of the standard through each tube at a rate of 150 mL/min. The standard gas was then fed directly to the analyzing chromatographic system, which logged the heights of 30 sulfur hexafluoride peaks at 1-min intervals. About an hour after the sample tubes were charged the box was coupled to the system and automatic, staged, sample transfer and analysis commenced. Six samples were displaced and analyzed from each of the six tubes. Finally, 30 more peak heights were recorded from the directly coupled standard gas. The equipment used for these sulfur hexafluoride analyses consisted of a Shimadzu Mini I1 chromatograph, with an electron capture detector and nitrogen as carrier gas. Its output was processed via a Hewlett-Packard 3388A integrator. This chromatograph was fitted with a Valco gas sampling valve (Valco Instruments Co. Inc., Houston, TX) and two series columns were arranged in a Deans backflush configuration ( I ) . The sample volume was 150 pL and there was an outlet splitter ( l : l O ) , and a detector makeup and a column flow, each of 50 mL/min. The oven and detector temperatures were 65 "C and 100 OC, respectively. The stripper column was 400 mm X 2.1 mm id., packed with 60/70 mesh Tee-Six (Analabs, Inc., North Haven, CT), coated with 15% w/w Carbowax 400 (Alltech Associates, Deerfield, IL). The analytic column was 850 mm X 2.1 mm i.d., filled with 80/100 mesh, washed molecular sieve 5A (Alltech Associates, Deerfield, IL). The stripper column was backflushed at 3.6 s after the sample inject command, to prevent most of the carbon dioxide and ethane and all heavier materials from entering the second column. The molecular sieve column eluted sulfur hexafluoride before oxygen, as reported by Simmonds et al. (2). The sulfur hexafluoride peaks had a width at half-height of 1.2 s, and a retention time of 7.8 s after the sample inject command.
RESULTS AND DISCUSSION Although this method was specifically developed to accommodate up to 60 sulfur hexafluoride analyses each day, it seemed worthwhile to also present some data to give a wider appreciation of possibilities and limitations. Figure 3 shows the carbon dioxide content of the effluent of a tube as it was purged from either pure nitrogen or pure methane. Points from seven analyses are interpolated in each case over a 6-min period during a total carbon dioxide displacement volume of 70 mL. It is apparent that, during the first 1.3 min, or 29 mL of carbon dioxide displacement, the composition of the sample gas at the chromatograph was stable. This was because the displacement aliquots (7.2 mL), after the first aliquot (20.5 mL), were less than the dead volume of the transfer system (10 mL), and, on this time scale, negligible gaseous diffusion occurred within the transfer lines because of small cross sections, relative to that of the sample tube. Figure 4 shows the interpolated plot of seven normalized analyses of the four-component mixture, sampled downstream
0003-2700/84~0356-1995$01.50/0 0 1984 American Chemical Society
1996
ANALYTICAL CHEMISTRY, VOL. 56, NO. 11, SEPTEMBER 1984
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Minutes elapsed after tube valves opened 1 2 3 4 5 6
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