Near-infrared laser based cavity ringdown spectroscopy for applications in petrochemical industry

A simple, economic diode laser based cavity ringdown system for trace-gas applications in the petrochemical industry is presented. As acetylene (C2H2) is sometimes present as an interfering contaminant in the gas flow of ethylene (ethene, C2H4) in a polyethylene production process, an on-line monitoring of such traces is essential. We investigated C2H2-C2H4 mixtures in a gas-flow configuration in real time. The experimental setup consists of a near-infrared external cavity diode laser with an output power of a few mW, standard telecommunication fibers and a home-made gas cell providing a user-friendly cavity alignment. A noise-equivalent detection sensitivity of 4.5x10(-8) cm(-1) Hz(-1/2) was achieved, corresponding to a detection limit of 20 ppbV C2H2 in synthetic air at 100 mbar. In an actual C2H2-C2H4 gas-flow measurement the minimum detectable concentration of C2H2 added to the C2H4 gas stream (which may already contain an unknown C2H2 contamination) increased to 160 ppbV. Moreover, stepwise C2H2 concentration increments of 500 ppbV were resolved with a 1-min time resolution and an excellent linear relationship between the absorption coefficient and the concentration was found.