Application of SpectraScan tunable diode laser instruments to fugitive emissions

Abstract

The advantages of lasers over broad‐band light sources for spectroscopic measurements have long been recognized. Laser techniques offer the ability to measure the concentrations of trace species in a gas stream in “real time” with sensitivity and molecular selectivity not possible with broad band spectroscopic techniques. Until recently, the use of laser‐based instruments has been largely limited to laboratory or one‐of‐a‐kind devices. However, the increased availability of solid‐state tunable diode lasers, recent advancements in fiber optic coupling and advanced signal processing have made it possible to develop instruments that offer customers enabling technology at an affordable price. Spectrum Diagnostix has developed a family of tunable diode laser‐based instruments used in a variety of applications including: fugitive release detection of HF and H2S in the refining and petrochemical industries, stack monitoring, and using extractive sample probe, in situ process control. SpectraScan instruments obtain their high sensitivity and chemical selectivity utilizing a technique known as wavelength modulated spectroscopy. Described simply, a near infra‐red diode laser’s wavelength is scanned rapidly and repeatedly through a molecular absorption line. A photodetector senses the instantaneous fraction of emitted laser power that is transmitted through the chemical bearing gas. Measurement of the relative amplitudes of offline to online transmission yields a precise value of the quantity of chemical along the laser beam’s path. The amplitude modulated signals are then detected using established radio receiver and signal processing techniques. The SpectraScan monitors are designed for permanent installation in harsh industrial and petrochemical environments and are approved for Class I, Division 2 hazardous area use. Describes the SpectraScan instruments, their field applications, and reviews operating data compiled from open path measurements of HF in refineries.