Fast on-line analysis of process alkane gas mixtures by NIR spectroscopy

Proper operation of a molecular sieve process for the separation of iso- an d cyclo-alkanes front normal alkanes requires the fast online detection of normal alkanes breaking through the column. The feasibility of using near-i nfrared (NIR) spectroscopy for this application was investigated. Alkane mi xtures were prepared according to an experimental design. These mixtures, c ontaining small amounts of normal alkanes (C5-C7) and varying amounts of is o- and cyclo-alkanes (seven compounds), were analyzed simultaneously at 160 degrees C with the use of NIR spectroscopy and gas chromatography as a ref erence method. With an analysis time of 9 s, a critical level of 0.4% [mole /mole] of normal alkanes could be achieved. If a faster response is require d, the analysis time can he reduced to 4 s, at the cost, however, of an inc rease in the critical level to about 1% [mole/mole].