Analysis of Impurities in Ethylene/Ethane and Propylene/Propane Streams Using a Pulsed Flame Photometric Detector (PFPD)

Some of the key processes in the petrochemical industry are conversions of high-grade ethylene (C2) and propane/propylene (C3) feedstocks into end products (polyethylene, polypropylene) and intermediates such as 1-butene. These are the building blocks for plastics and a wide range of products, and are a large industry, with 55 million metric tons of polypropylene produced in 2013.1 Unfortunately, even trace levels of sulfur species H2S and COS, which are often entrained in C2 and C3 feedstocks, corrode pipes and equipment, inhibit or damage catalyst beds, and lower product yield and purity. The need for a fast, reliable analysis method for H2S and COS in both C2 and C3 feedstocks is obvious, but sulfur in C2 and C3 is a difficult application, owing to the poor separation of the impurities from the matrix when coupled with the quenching of the PFPD detector signal by the matrix carbon. Reactivity of the sulfur species, especially H2S with all surfaces in the calibration and analytical system, adds additional complexity to this application. We present here a fast, reliable and robust method for the analysis of sulfur contaminants in C2 and C3 feedstocks that makes use of an automated gas loop injection system, separation by gas chromatography, and pulsed flame photometric detection (PFPD) that can detect sulfur at better than 0.1 ppmv.