| OCR Text |
Show on the details concerning the trace analysis ($ SO ppmv) of the individual sulfur compounds that are present in refinery fuel gases and sulfur plant gases. The measurement of individual sulfur compounds in RFG is generally done in four distinct steps, as follows: SEPARATION ~~ CONVERSION ~~ DETECTION ~~ QUANTIFICATION 3.1 Separation First the individual sulfur compounds need to be separated from each other so that they can be identified and quantified independently. At the present time, the most widespread method of separating and quantifying compounds for analysis in a gas stream is gas chromatography (GC). This well known, established technique allows the individual compounds to release, or -elute-, from chromatographic adsorptive media at different times and in a particular order. The GC prints out a -peak- as the compound is released and detected, and the size of the peak relates' directly to the quantity of the eluted product. Problems can sometimes be encountered when the elution peaks for various compounds are very close to each other or when the peak for one compound is much larger than an adjacent one. 3.2 Conversion Each gaseous compound must be detected as it elutes in order to be quantified. However, since RFG sulfur compounds can vary in their chemical nature and reactivity, a number of different detection methods might be necessary. To avoid this complication, it is most convenient to -convert- all of these compounds to a common, reactive form; so that a common detection method can be employed. The conversion forms which have been found to be most convenient are HzS or SOz- Conversion to HzS is accomplished by hydrogenation, and SOz is formed by combustion. 3.3 Detection & Quantification The detection of the various converted sulfur species is presently performed by a number of different methods which will be briefly discussed. 3.3.1 Metal Sulfide Tape Meters The most common HzS meter in use at this time is the metal sulfide -tape- machine. This instrument photometrically measures the rate of formation of a metal sulfide (e.g., Zns, CdS, etc.) onto a porous tape, which is moving at a known rate. The black sulfide forms when a metal salt solution (such as an acetate) undergoes an irreversible chemical reaction with the HzS-laden gas stream. This instrument can perform a -total- sulfur measurement when a hydrogenation bed is installed prior to the sulfide-forming step. 3.3.2 Wet Chemistry - UOP Test Methods Most of the sulfur compounds in RFG can be scrubbed out of the gas with caustic, and then measured quantitatively with classical wet chemical test procedures. UOP has test methods (# 212 and others) which utilize a potentiometric titration to measure HzS, COS and mercaptan concentrations in caustic solutions. These tests are employed in their licensed Merox (regenerative caustic wash) units. Other wet chemistry methods are also available, but are generally not suited for routine online analyses. The metal sulfide tape method described above is an example of a wet chemical test which has been adapted to an online, continuous format. -5- |
