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

摘要

A simple, economic diode laser based cavity ring-down system for trace-gas applications in the petrochemical industry is presented. As acetylene (C_2H_2) is sometimes present as an interfering contaminant in the gas flow of ethylene (ethene, C_2H_4) in a polyethylene production process, an on-line monitoring of such traces is essential. We investigated C_2H_2-C_2H_4 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 4.5 x 10~(-8) cm~(-1) Hz~(-1/2) was achieved, corresponding to a detection limit of 20 ppbV C_2H_2 in synthetic air at 100 mbar. In an actual C_2H_2-C_2H_4 gas-flow measurement the minimum detectable concentration of C_2H_2 added to the C_2H_4 gas stream (which may already contain an unknown C_2H_2 contamination) increased to 160 ppbV. Moreover, stepwise C_2H_2 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.