Spotlight on the Gulf Coast Conference - Analytical Instrumentation
33
Figure 1. LiqMix Cascade schematics
Figure 2. Schematics of GC-Precon-PHID and standard gas generator
The liquid components were tared in advance using a laboratory balance, loaded in the reservoir and pressurised with helium 5.0. With its Coriolis mass flow controller LiqMix Cascade accurately delivered the liquid hydrocarbon mixture into a vapouriser where it evaporated into a flow of a carrier gas (hydrogen 5.0). In a gas phase, it was transferred to a mixing chamber where it was later mixed with a second stream of calibration gas C1-C4 (Praxair standard gas mixture) introduced into the mixing chamber by an accurate thermal mass flow controller.
To achieve very low concentrations a portion of homogenous gas/ vapour mixture is delivered to the second dilution stage, so-called Cascade, where it is further diluted by a stream of pure hydrogen and transferred to a sampling point by heated line.
Optimisation of cold trap using GC-Pre-concentration-PDHID
The cold trap was a key performance component in the study on fuel hydrogen impurities and was examined by using a separate GC-system with PHID detector.
Standard gas mixtures were prepared in the same way as described above by LiqMix Cascade. The samples were introduced through a heated (70 degrees Celsius) 20 mL sulfinert gas loop for 6,5 minutes. 30 seconds was allowed for the gas pressure to stabilise before the loop was emptied through sulfinert tubing, using purified helium 5.0, into a PTV-injector. The PTV temperature held -10°C by use of liquid nitrogen.
After 6 minutes of purging the loop the PTV temperature was increased 5 °C/min until reaching the end temperature of 110°C. The content of the cold trap was focused on a Varian Poraplot Q column (52m, 0.32mm ID, 10 micrometer) at -10°C with helium carrier at constant flow.
The oven was kept at -10°C for 15 minutes to separate air and methane and then ramped to 170°C at10°C/min rate. The compounds were detected by use of Photo Discharge Helium Ionisation Detector (VICI PDHID for Agilent 7890A). One of chromatograms is presented on the Figure 3.
Uncertainty and standard deviation associated with lowest achievable concentrations of analytes in this study are presented in the Table2. Initial concentrations of components were approximately 0.1% mol/mol.
Picture 2. Photo of installation
In the worst case of operation at maximum dilution ratio and lowest concentration, the LiqMix Cascade system did show the reasonable relative error at about 3% for gas standards from
Table 2 Uncertainty and standard deviation associated with lowest achievable concentrations by Liqmix Cascade.
Analyte Concentration, ppb-mol
1-Pentene 1-Butene Ethylene
Isopentane Neopentane Pentane Propene
Isobutane Butane Propane Ethanol
Methanol Ethane
Acetone
0,981 1,019 3,000 3,846 3,904 3,913 4,798 9,808 9,808
12,500 13,396 15,094 19,615 20,010
Liqmix
Uncertainty, ppb
0,057 0,057 0,127 0,164 0,166 0,166 0,205 0,415 0,300 0,456 0,445 0,491 0,721 0,630
RSD % Liqmix
5,810 5,594 4,233 4,264 4,252 4,242 4,273 4,231 3,059 3,648 3,322 3,253 3,676 3,150
Figure 3. Chromatogram GC-Preconcentration-PHID.
AUGUST / SEPTEMBER •
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