Fuel degradation and deposit formation are key challenges for future fuels and biofuels development. This protocol allows one to study fuels oxidation under controlled conditions, and can be useful to develop new formulations and preventive strategies. This method can help answer key questions in the field of fuel quality, such as how the fuel composition and operating conditions impact the oxidation mechanisms and product.
The main advantage of this technique is the use of commercially available reactors, coupled to powerful analytical methods to perform controlled oxidation and monitor the evolution of the oxidation product. This method can provide insight into the degradation mechanism of diesel and biodiesel fuels and it has been successfully applied to study the oxidation ability of that fuels and lubricants. Individuals new to this method will struggle because interpretation of the GCMS results may be challenging.
The oxidation forms multiple products, and the chromatogram may contain several peaks. To begin this procedure, remove dust or contaminants from the reaction vessels using compressed air. Fill the reaction vessels of Block A with seven milliliters of a fresh fuel sample of methyl oleate using a pipette.
Then fix the air tube onto the reaction vessel cover, and place it on the reaction vessel. To configure the test, set the heating set point temperature of Block A to 110 degrees celsius. Then set the air flow rate to be bubbled through the sample to 10 liters per hour.
Set the shut-off criteria to either a conductivity threshold of 400 microsiemens per centimeter, or an induction period determined by the software. Next enter the name and reference of each of the Block A samples in the software. Proceed to place the reaction vessels in heating Block A.Launch the test by pressing the start button in the user interface of Reactor 1, and check that the reactor is working by visualizing air bubbles.
Once the test is finished, extract the reaction vessels from the heating block. Using a pipette, carefully aspirate the contents of each reaction vessel, and transfer each extracted sample to a 10 milliliter glass flask. Then place the flasks vertically in the refrigerator.
Determine the average induction period from the four samples by the method of tangents intersection. Repeat the previous steps using Block B, modifying the shut-off criterium by indicating an ending time equal to four times the average induction period. After the test is complete, manually extract the first reaction vessel from the heating block, at 0.5 times the induction period.
The second reaction vessel at the induction period. The third reaction vessel at two times the induction period. And the fourth reaction vessel at four times the induction.
Then transfer the contents of each reaction vessel to a 10 milliliter glass flask. After labeling each sample, place them vertically in the refrigerator. To determine the TAN of methyl oleate, sampled at increasing oxidation stages, first rinse all vessels and parts in contact with the sample with ultra-pure water, followed by isopropanol.
Next place two grams of the sample in the measurement vessel. Dilute the sample with 10 grams of isopropanol. Now insert a glass electrode and gradually add a solution of 0.1 mols per liter of potassium hydroxide diluted in isopropanol until the equivalence point is reached.
Report the TAN of the sample in milligrams of potassium hydroxide per gram of fuel. To analyze methyl oleate sampled at increasing oxidation stages, inject one microliter of the sample into a GCMS instrument equipped with an FFAP column using a split ratio of 1 to 75. To perform the first oxidation cycle in Reactor 2, set the heating temperature to 150 degrees celsius in the software.
Set the pressure in the test cell to seven bar, and then set the shut-off criterium. Prepare the test cell by removing dust or contaminants using laboratory paper soaked with acetone. After replacing the sealing ring, fill the test cell with five milliliters of the fuel sample using a pipette.
Close the test cell with a screw cap followed by a protective locked cover. Next launch the test by pressing the run button in the user interface of Reactor 2, and observe the curve of the induction period in the software. After the cell automatically depressurizes, open the protective locked cover and the screw cap.
To perform x successive oxidation cycles, repeat the previous steps the desired number of times. At the end of x tests, transfer all of the oxidized fuel remaining in the cell to a five milliliter glass flask, then place the glass flask vertically in the refrigerator. Clean the cell using laboratory paper soaked with acetone.
To analyze the generated samples by FTIR-ATR, first clean the ATR diamond cell using laboratory paper soaked with ethanol. Following this, set 100 scans to build the FTIR spectrum, fix the resolution at four inverse centimeters and set the spectral range from 600 to 4, 000 inverse centimeters. After removing the fuel sample from the refrigerator, agitate it thoroughly in order to homogenize the fuel composition.
Using a pipette, sample 10 microliters of fuel from the flask, and place the droplet on a horizontal ATR diamond cell at ambient temperature. Finally start the analysis. The TAN analysis results suggest that acid species formation is slow during the initial oxidation process and becomes significant at intermediate and advanced oxidation.
There is an increase in intensity for several peaks in the methyl oleate chromatograms from zero IP to four IP, and some species not present initially were formed at mid to high oxidation levels with increasing intensities. Methyl 6-heptenoate is produced during the oxidation process. Additionally, several short chain molecules are formed through molecular cleavage.
Aldehydes and methyl esters are formed through reaction of molecular oxygen with alcohol-radicals or hydroperoxides. The fuel color changes during the oxidation from transparent to yellowish, then brownish, which is due to the formation of polar compounds. The molecular weight of these products increases along with the oxidation level.
Oxidized B0 samples show the formation of a dark viscous phase composed of high molecular weight polar products. The carbonyl peak in the FTIR spectra of the B0 samples increases with the oxidation level and is attributed to aldehydes, ketones, and acids. The peak in region R3 shows the formation of hydroxyl groups indicating the presence of alcohols and acids.
Once mastered this technique can be done in few hours, according to fuel nature, reactivity, and test conditions, which is lower than standard storage tests performed in several months, and the lowest one to gain information on the oxidation mechanism. When attempting this procedure, it's important to remember to store the oxidized sample and those are appropriate condition, and to make sure the volume analyzed with infrared spectroscopy, GC, or TAN is homogenous and representative of that oxidized sample. Other methods can be applied to understand the deposit properties, like thermal graphic analysis, which shows several molecular weight products.
Also scanning electron microscopy, which provides information about morphology and chemical composition of deposits. Such information allows one to validate the measured trends obtained with FTIR and GCMS. After watching this video, you should have a good understanding of how to perform accelerated fuel oxidation and how to analyze the samples with infrared spectroscopy, GCMS, and TAN, to identify the oxidation products.
Don't forget that working with fuels in solvent can be extremely hazardous, and precautions such as protective equipment, including gloves, lab coats, and goggles, should always be taken when performing this procedure.
