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Show 3.3.3 Drager/Gastec Tubes These -reaction- tubes are another example of a wet chemical method that has been adapted for direct online use. Most refinery personnel are familiar with Drager-type batch (non-continuous) sampling tubes for HzS. A small hand sampling pump draws a measured amount of gas sample through a glass tube filled with chemically impregnated support material. The amount of chemical on the support material is cahorated to quantitatively identify a particular species in the gas stream. Apparently there are sampling tubes for methyl, ethyl, and propyl mercaptans and COS; in addition to the familiar HzS tubes. The labels must be read carefully, however, for there are many interferences possible in using some of these measuring tubes. For instance, the presence of HzS interferes with measurements in both the mercaptan and COS tubes. Some of the refiners have tried -stacking- these Sulfur tubes; i.e. drawing a sample through a series of tubes starting with an HzS tube. The accura(.j' of such a method has not been compared with more accurate laboratory methods. 3.3.4 Other Detectors Currently, there are a number of other detectors that are used for gas analysis; typically after conversion of all sulfur compounds to S02" Some typical ones include : Thermal Conductivity Detectors (TCD) Flame Photometric Detectors (FPD) Flame Ionization Detectors (FJD) Hall Electrolytic Detectors. Ultraviolet Detectors (UVD) The first three detectors listed are often used after a gas chromatographic separation step. The various detectors are used for measuring different types of compounds. Apparently the FP and UV Detectors are two that can accurately measure the very low levels (200 ppm or less) of the sulfur compounds as they elute from a gas chromatograph. The techniques that are used for sampling RFG with a GC followed by an FP detector generally require a laboratory type of precision that is difficult to maintain in daily, online refinery practice. 3.4 Onstream Analyzers There are no onstream analyzers that are available or in refinery use at this time that -speciate- all of the sulfur compounds in fuel gas. Such a measurement will be of interest to refiners in analyzing gaseous sulfur compounds both qualitatively and quantitatively. Such knowledge will allow them to choose new treatment schemes and to troubleshoot the existing ones. Analytical firms are working to develop such an instrument, and the development time that will be required is undefined at this time. The meter that will be required for environmental mOnitoring purposes may not need to speciate, but must at least be able to convert and totalize the sulfur content. Even after these types of instruments become available, there will be some time period required before such an instrument can be officially "certified" by the appropriate regulatory agencies. 3.4.1 Speciation It appears at this time that to identify, or "speciate", the individual sulfur species, a gas chromatographic step will most likely be necessary. A G.C. with an S02 conversion step will also allow quantifications of the sulfur species. As mentioned above, the conversion of all sulfur species can in general be accomplished either by hydrogenation or combustion. -6- |