The Octane number rating of a gasoline is an indication of how the gasoline will perform under various engine conditions. Two different ratings are included: Research Octane Number (RON) and Motor Octane Number (MON). Finished gasoline must meet certain Octane number specifications. Thus refineries control this parameter during production and must certify that gasoline meets specification before it is released. In addition, the Octane number of gasoline blending components (reformate, etc) is an important parameter in unit production. The traditional laboratory method for Octane number determination is the knock engine method in which gasoline is burned and its combustion characteristics compared to known standards. This method is time and labor intensive, and provides no ability for real-time control of production. The use of Guided Wave NIR spectrophotometric analyzers allows for easy measurement of Octane number in gasoline and refinery production units using fiber optic-based, Near-Infrared (NIR) spectroscopy. Guided Wave analyzers are designed for process use with built-in multiplexing capability. NIR can be applied in real-time directly in-process monitoring and proves to be a time and money-saving alternative to traditional methods. The use of NIR spectroscopy for Octane determination is well documented in the literature with initial studies being carried out in 1989. (Kelly)
Figure 1: Gasoline Spectra Collected with Guided Waves NIR-O Full Spectrum Process Analyzer
The NIR region of the electromagnetic spectrum contains information from the overtone and combination bands of the C-H, O-H, and NH fundamentals. This information is related to the chemical composition and can be used for both quantitative and qualitative analysis. (D. Burns) By measuring the NIR spectra of a series of fuel samples of known Octane number, a quantitative model can be developed allowing the measurement of future samples based only on their NIR spectrum. Guided Wave analyzer systems use fiber optics to allow the sample probe to be located in remote locations away from the spectrometer itself.
The Guided Wave system is a multichannel system, allowing one analyzer to measure multiple parameters on multiple streams (up to 12). The multiplexing capability reduces both cost and complexity of installation for each measurement point compared to stream switching.
Feasibility Study: Can NIR be used to Monitor Octane Number of Gasoline?
Figure 2 Distribution of dataset used for Octane Number of Gasoline by NIR Process Spectroscopy Feasibility Study
The NIR spectra of a group of different process gasoline samples with known Octane numbers were measured between 1000 and 1600 nm using a Guided Wave NIR Spectrometer. Figure 1 shows the absorbance spectra of some representative gasoline samples collected using an on-line process probe with a 1 cm pathlength. The distribution of Octane numbers for these samples is shown in Figure 2. The RON values range from 89.7 to 101.4 and the MON values range from 80.8 to 89.0. These are standard ranges for blended gasoline. A quantitative calibration model was created using the NIR spectra and laboratory octane data. The calibration is generated using PLS regression in the Unscrambler™ software. For a discussion of PLS and other multivariate calibration techniques please see Martens & Naes (H. Martens)and ASTM E1655 (E1655).