24th International Colloquium Tribology - January 2024 59 Production of High VI Base Oils from Full Conversion Hydrocracker Residue with Solvent Refining Dimitrios Karonis 1* , Panorea Kaframani 1 1 National Technical University of Athens, School of Chemical Engineering, Laboratory of Fuels Technology and Lubricants, Athens, Greece * Corresponding author: dkaronis@central.ntua.gr 1. Introduction The production of low sulfur base oils with enhanced properties has gained attention compared to the classic base oils produced from classic solvent refining process of vacuum distillates. In a previous work, low sulfur refinery stock, the residue of a hydrocracker unit was evaluated as feedstock for the production of base oils with classic solvent treatment process. Some of the results were very promising. For this reason, the solvent refining process has been used for the upgrade of an ultra low sulphur content unconverted oil (UCO) from a full conversion fuel producing hydrocracker unit. The treatment used was focused on the removal of aromatics with polar solvent and the improve of pour point by solvent dewaxing. The results showed that the treated products have properties that can be classified as Group II base oils. The properties of the base oils were further improved by the addition of commercial additives (VI improver, pour point depressant). 2. Experimental Section 2.1 Feedstocks The feedstock used in this series of experiments was an ultra low sulfur residue derived from a full conversion severe hydrocracking process, used for transport fuels production; this residue is characterized as the unconverted oil (UCO) of the process. Table 1: Properties of the Feedstock Property UCO Method Density, 15 °C kg/ m 3 863.6 ASTM D4052 Viscosity mm 2 / s ASTM D445 40 °C 38.78 100 °C 6.184 Viscosity Index 104 ASTM D2270 Sulfur mg/ kg 22.1 ASTM D5453 Distillation °C ASTM D1160 IBP 329 10% 403 50% 449 90% 529 FBP 568 Pour Point °C 34 ASTM D97 The main properties of the feedstock are given in Table 1. UCO has a high VI as it is, a very low sulfur content, but high pour point. 2.2 Fractionation UCO was evaluated as received, but also after fractionation. The fractionating distillation employed was the ASTM D2892 under reduced pressure, using a fractionating column with no internals. UCO was separated into 4 fractions. The aim of the distillation was to remove the more volatile part of the UCO, and more precisely the fraction that boils below 400-°C. The cut point temperatures were 450 and 500-°C, in an attempt to produce cuts with different viscosity grades. The heavier part (+500-°C) wase the residue of the distillation. The properties of the distillation fractions from the UCO are given in Table 2. Table 2. Properties of UCO cuts Cut Cut Temp. °C Yield % m/ m v 40 mm 2 / s v 100 mm 2 / s VI I 400 14.3 13.30 3.182 102 II 450 41.7 25.24 4.714 104 III 500 29.8 54.71 8.038 109 IV +500 12.4 166.6 16.34 102 Loss 0.2 Cut I was not evaluated as is very volatile for a base oil (FBP at 400-°C), as well as Cut IV, due to its very high pour point (higher than 45-°C). Cuts II and III are satisfactory, with different viscosity grades and they were treated with solvents in order to improve their properties. 2.3 Solvent Extraction As shown in Tables 1 and 2, the fractionation produced two cuts for evaluation, one with VI similar and one with VI higher than that of the feedstock, but with different viscosity grades. The next step in the processing was to apply the classic solvent extraction process in order to remove aromatic hydrocarbons from the cuts and improve thus further the VI of the cuts. The extraction solvent used was NMP. Solvent/ Feed ratios used were 1.5/ 1, 2/ 0/ 1, and 2.5/ 1 m/ m. The contact time was 30 min, and the extraction process took place at 50-°C. The results of the VI improvement for Cut II and Cut III are shown in Figure 1. There was a significant improvement of VI for Cut II with increasing solvent/ feed ratio, while for Cut- III no significant improvement was noticed. The extraction yields were considerably high. 60 24th International Colloquium Tribology - January 2024 Production of High VI Base Oils from Full Conversion Hydrocracker Residue with Solvent Refining Figure 1: Impact of NMP extraction on VI and yields of the distillation cuts 2.4 Solvent Dewaxing The most common process in a solvent treating base oils production facility for the improvement of the pour point is solvent dewaxing. This process uses a mixture of methyl ethyl ketone (MEK) with toluene, in order to remove the wax that is formed during the cooling of the process feedstock, as the solvent mixture facilitates the separation of paraffins wax from the liquid phase. The improvement of the VI of the two distillation cuts had as result a slight increase of their pour points, which is normal since the extraction yields were very high, so the amount of aromatic compounds that were removed was small. The solvent extraction process while improves the VI of the feedstock, has a disadvantage on the cold flow characteristics of the product. The removal of the aromatic compounds results to the increase of the pour point of the extraction products. For Cut II, the extraction process with NMP increased the pour point from 24 °C to 26 - 27-°C, and for Cut III from 38-°C to 41 - 42-°C. The impact of solvent to feed ratio on the pour point was negligible (within the accuracy of the test method). The most common process to improve the pour point is the solvent dewaxing process, where the feed is mixed with the proper solvent, the mix is cooled, and the paraffins are separated by filtration. Based on results from previous similar experiments, the dewaxing solvent was a 1÷1 (on mass basis) mixture of methyl ethyl ketone (MEK) and toluene). The solvent to feed ratio was 3÷1 (on mass basis). The cooling temperature was -20-°C. The results of the dewaxing process are depicted in Figure 2. As shown in this figure, the pour point of the dewaxed products was significantly reduced, compared to the relevant values of the treated with NMP oil. Regarding the two distillation cuts, the pour point of Cut-II was reduced to values below zero, which is considered as a positive result, but a more intense reduction is required in order to achieve products that can be used as lubricating oil blending components. For Cut III the pour points were higher than 10-°C, at not acceptable level for lubricants. Figure 2: Pour point of extracts and dewaxed cuts 2.5 Pour Point Depressant In order to improve the pour point of the dewaxed oils, a commercial pour point depressant was added (polyalkyl-methacrylate (PAMA) type), at the level of 0.3% (m/ m). The results are shown in Figure 3. It is clear that pour point is reduced at all cases, but the reduction was not at the desired level. The decrease in pour point was higher for Cut II. Figure 3: Impact of PPD addition on pour point 3. Conclusions The upgrade of a low sulfur full range hydrocracker UCO with solvent refining was evaluated. The results showed that the production of high VI base oils, close to the requirements of Group II base oils can be achieved. The main problem of the base oils is their high pour points, a characteristic that must be further improved in order to produce acceptable quality products. References [1] T. R. Lynch: “Process Chemistry of Lubricant Base Stocks”, CRC Press, 2008, ISBN 978-0-8493-3849-6. [2] R. M. Mortier, M. F. Fox, S. T. Orszulik (Editors): “Chemistry and Technology of Lubricants”, 3 rd Edition, Springer, 2010, ISBN 978-1-4020-8661-8.