More than a decade after Phillips' first published work this article reviews recent developments in comprehensive two-dimensional gas chromatography (GC x GC). Special attention is devoted to the further development and diversity of modulation devices. These include heated sweepers, cryofocused modulators, and a variety of diaphragm valve-switching strategies. It is demonstrated that all modulation approaches can be very well suited to GC x GC, depending on the particular application. Diaphragm-valve modulation is very powerful for volatile organic compounds. Slotted heater and cryofocused modulation are preferred for samples that contain non-volatile components. Applications ranging from petroleum to environmental and biological samples are illustrated. Extension of the technique to GC x GC-mass spectrometry (MS) is also discussed and trends for future research activity are pointed out.
Most modern ethylene cracking furnaces use closecoupled extractive or in situ oxygen analyzers based on a zirconium oxide sensor in the stack area to provide fast and reliable measurement of the oxygen content of the flue gas exiting the firebox. This allows for precise control of the excess combustion air in the firebox, resulting in higher energy efficiency for the furnace than is typical using conventional extractive sample-based analyzer systems where the analyzer might be several hundred feet away from the sample point. These analyzers use a zirconium oxide sensing element that operates at 7008C, which is well above the auto-ignition temperature of many flammable hydrocarbon mixtures. Although flame arrestors can be installed on these analyzers to prevent flame propagation from the probe element into the firebox there are still situations that can exist that the analyzer can pose as an ignition source. This paper will discuss the two typical zirconium oxide oxygen analyzer designs and how the analyzer can be a source of ignition, and review an incident in a Dow cracking furnace, which resulted from an in situ oxygen analyzer igniting a flammable mixture in a furnace. Corrective actions are included in this paper.
AtmosphericPressureFluctuations and OxygenEnrichmentin Waste Tanks Atmospheric Pressure Fluctuations and Oxygen Enrichment in Waste TanksExecutive Summary .Oxygen enrichment in waste tanks can reach and SRTC's Climatology Site were analyzed to hazardouslevels if normalpurging is interruptedby determinethe mean andmaximumpressureincreases power failures. In such cases it is necessary to ina5-yearperiod 1986-1990. estimate the time it would take for oxygen buildup to reachacriticalconcentration.The mean pressure increase in an 8-hr period was found to be 1.7 mb and 5.0 mb for a 24-hr period. Mixing of outside air into closed tanksthrough smallThese pressure increases would induce an air leaks will be slow without externalforcing. One such enrichment of ,...0.2%and -0.5% for 8 and 24-hour external forcing mechanism is natural atmospheric periods,respectively. Since oxygen is 21%of air by pressure fluctuations, which can force air into tanks volumethe in-tankoxygen enrichmentwouldbe -1/5 throughsmall cracks or vent holes. The efficiency of of the percentageslistedabove. this process is a functionof the atmospheric pressure increases over time periodscomparable to the in-tankThe above values over-estimate the enrichmentrate mixing rate.becausethey do not include the reductionin efficiency as the in-tank oxygen level increases and becausethey Two in-tank mixing rates were assumed (8 and 24 assume complete mixing of oxygen entering the tank. hours) and pressure measurements from Bush Field
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