Methanol gasoline detection using Raman spectrometer

Application background

Methanol gasoline is a "blended oil" composed of ordinary gasoline with a certain proportion of methanol and other additives added. There are many ways to classify methanol gasoline. One of them is to divide it into three categories according to the methanol content: low alcohol gasoline (M3-M5), medium alcohol gasoline (M15-M30) and high alcohol gasoline (M85-M100), the number after M represents the volume percentage of methanol in methanol gasoline. The advantage of adding methanol is that it can improve the quality of ordinary gasoline and increase the octane number of gasoline. Compared with ordinary gasoline and diesel, methanol gasoline burns more fully under the same conditions, and the emissions of conventional CO and other pollutants and PM2.5 particulates in the exhaust are significantly reduced.

Different methanol addition ratios have different combustion performance and engine requirements. Low-proportion gasoline does not require modification of the engine and device and can be used directly. Methanol gasoline with a medium proportion has certain requirements on the engine and often requires the addition of corresponding co-solvents. Methanol gasoline with a high proportion cannot be used with ordinary gasoline and requires modification of the engine structure. Using the wrong proportion of methanol gasoline in an ordinary engine will cause the calorific value of the gasoline to decrease, easily cause vapor lock and affect the fuel supply, and easily cause corrosion and wear to the engine sealing system and other adverse effects. Therefore, many parts of the world regard methanol content as a core indicator of methanol gasoline and have strict regulations on its content.

Accurate and efficient detection of methanol content is of great help to the quality control of methanol gasoline production and the performance evaluation of market products. The classic detection method for methanol gasoline is gas chromatography. However, this method has low testing efficiency and is difficult to achieve rapid detection due to cumbersome pre-processing, complex calibration, and long test cycle. Near-infrared spectroscopy can quickly determine methanol content, but because the near-infrared spectra of other alcohols in gasoline are not much different from methanol, the test results are easily interfered by fluctuations in the content of other alcohols. Professor Dai Liankui of Zhejiang University discovered that a portable Raman spectrometer can quickly, non-destructively and efficiently detect the content of methanol gasoline.

Measurement process and methods

Professor Dai Liankui used a Raman spectrometer to measure the Raman spectrum data of a series of methanol gasoline samples.

In the experiment, the spectrum measurement range was 0-2100cm^-1, the integration time was 25s, the central wavelength of the laser was 785nm, and a 10mm quartz cuvette was used at the end of the sampling pool. After the sample is placed in the cuvette, the laser reaches the fiber probe through the excitation fiber. After irradiating the sample, a Raman signal is generated, which is then collected by the fiber probe. After passing through the collection fiber, it enters the spectrometer. The original Raman signal of the sample is obtained after subtracting the dark background from the signal.

The gasoline sample uses multiple portions of 90# and 93# gasoline provided by the refinery as base oil samples, and then adds anhydrous methanol with an analytical purity greater than 99.5% to form a series of methanol gasoline samples with different methanol contents in volume fractions from 10% to 90%. Due to the extremely low water content, the methanol gasoline sample to be tested did not significantly stratify at room temperature, so there was no need to add additional co-solvent during the experiment.

In order to accurately extract effective spectral information, the original Raman spectrum needs to be preprocessed. The main steps include spectral band selection, smoothing and noise removal, fluorescence background elimination and standard normalization. The standard normalization selects the saturated hydrocarbon characteristic peak (1460cm^{- 1}) intensity as the base light intensity. The figure below shows a comparison of the Raman spectra of the sample before and after pretreatment.

Figure 1 Raman spectrum of sample before pretreatment.

Figure 2 Raman spectrum of sample after pretreatment.

The characteristic peak spectrum of methanol is distributed between 1021-1091cm^-1. The scientific research team found that the intensity of the methanol characteristic peak in the sample has a high correlation with the methanol concentration. There is a significant linear correlation between the intensity value and the methanol concentration at 1050cm^-1, which is suitable as the methanol concentration characteristic observation peak.

Results and discussion

The spectrometer is easy to carry, powerful, easy to operate and easy to integrate, providing strong support for the team's research. In addition to being used in laboratory environments, this spectrometer is also suitable for various fields that require portable testing equipment such as methanol gasoline production quality control, performance evaluation, and field testing.