High performance direct absorption spectroscopy of pure and binary mixture hydrocarbon gases in the 6 - 11 μm range

The availability of accurate and fast hydrocarbon analyzers, capable of real-time operation while enabling feedback-loops, would lead to a paradigm change in the petro-chemical industry. Primarily gas chromatographs measure the composition of hydrocarbon process streams. Due to sophisticated gas sampling, these analyzers are limited in response time. As hydrocarbons absorb in the mid-infrared spectral range, the employment of fast spectroscopic systems is highly attractive due to significantly reduced maintenance costs and the capability to setup real-time process control. New developments in mid-infrared laser systems pave the way for the development of high-performance analyzers provided that accurate spectral models are available for multi-species detection. In order to overcome current deficiencies in the availability of spectroscopic data, we developed a laser-based setup covering the 6–11 μm wavelength range. The presented system is designated as laboratory reference system. Its spectral accuracy is at least 6.6×10⁻³ cm⁻¹ with a precision of 3×10⁻³ cm⁻¹. With a “per point” minimum detectable absorption of 1.3×10⁻³ cm⁻¹ Hz^−1/2 it allows us to perform systematic measurements of hydrocarbon spectra of the first 7 alkanes under conditions which are not tabulated in spectroscopic database. We exemplify the system performance with measured direct absorption spectra of methane, propane, iso-butane, and a mixture of methane and propane.