Chemical stability of hydrotreated fuels, and fuel stabilization by means of antioxidants

Abstract

more susceptible to oxidation than are untreated fuels [1-3], since the treating removes not only the undesirable components, but also natural oxidation inhibitors. Comparative studies are therefore needed on the oxidizability of such fuels, with a view toward finding means to improve oxidation stability. Here we are reporting on a study of oxidizability under artificial aging conditions, with samples of new types of jet fuels produced by means of hydrogenation andhydrotreating ; we also investigated the effectiveness of commercial antioxidants in chemical stabilization of these fuels. The hydrotreated fuels were commercial materials meeting the appropriate specifications. These studies were performed by means of repetitive oxidation of the same fuel sample at 120~ [4]. The tests were run in apparatus used to determine fuel corrosivity (GOST 20449-75). The fuel sample (110 ml) was placed in the beaker of this apparatus and heated to 120~ with the beaker connected to a reflux condenser through a ground glass joint. Four beakers were placed in the apparatus at the same time, with identical fuel samples. The fuel was held at this temperature for four heating periods of 6 h each, the apparatus being shut down between heating periods. After each heating period, the degree of fuel oxidation was rated by determining the peroxide number, optical density, acidity, and existent gtun content. The peroxide number was determined by potentiometric titration (with 0.1 N Na2S203 solution) of a fuel sample in a mixture of isopropyl alcohol and glacial acetic acid, plus a saturated solution of NaI in absolute isopropyl alcohol. The optical density was determined in a photoelectric colorimeter/neph elometer (Model FEK-56) in a cuvette with a light path of 30.085 ram, with a No. 4 filter (maximum transmission of filter at 434 nm) in comparison with an isooctane standard. The acidity was determined by a modernized ASTM D-974 method. In these determinations, the sample was dissolved in a mixture of toluene and isopropyl alcohol containing a small amount of water, and the resulting single-phase solution was titrated at room temperature with 0.1 N KOH solution in the presence of phenol red indicator up to the color change from yellow to rose-red. The existent gum contentwas determined in accordance with GOST 1567-56. The oxidation of the fuel samples was further characterized by IR spectroscopy, through which the kinetics of end-product accumulation could be observed on the basis of the absorption bands for the C-----O, C-O, and O-H bonds [5-7]. The conditions for recording the spectra were described in [7]. Results from these studies are shown in Table 1 and Fig. 1. The severely hydrogenated fuel (T-6 type) and the hydrotreated fuels (T-8 and RT types) are oxidized significantly under these particular conditions. In the first period of oxidation, active oxygen is found in the T-6 and T-8 fuels (respective peroxide numbers 119.6 and 135.4 mg O2/kg); in the subsequent heating periods, the peroxide compounds are transformed into other products, resulting in increases in optical density and in the quantities of organic acids in the fuel. This transformation is indicated both by the results of the chemical analysis (see Table 1) and by the IR spectra of the oxidation products. Under these particular conditions, no insoluble products (sludge) are formed in the fuels. After 24 h of oxidation at 120~ the fuel is oxidized quite severely. The increases in optical density are 1.08 for the T-6, 0.49 for the T-8, and 0.24 for the RT; the respective acidities are 14.8, 6.0, and 8.8 rag KOH/100 ml. The kinetic curves shown in Fig. 1 for the accumulation of the oxidation end-products in the bulk fuel RT or T-6 were obtained by measuring the optical density (log I0/I) of the bands at 1720 cm -1 ( v C ~O ), 1100 cm-1 (VC-O, SO-H *), and 3570 cm -1 (vO_ H) in the IR spectra of samples that had been oxidized at 120~ These data indicate that the course of the fuel oxidation is not the same in the two fuels. In the RT fuel, during the first 6 h of oxidation, hydroperoxides accumulate; by the end of this period, alcohols have appeared, and *As in Russian original - Translator.