Studies on the Urea-Dewaxing of Lubricating Oils*

Transcription

1 Studies on the Urea-Dewaxing of Lubricating Oils* Naoki Yata** Summery: A behavior of activators and inhibitors of the urea adduct formation was investigated, and these two factors seemed to be of primary importance of all which control the adduct formation, especially when dewaxing lubricating oil fractions with urea. The inhibitors, which were predominant in the resin fraction of mineral oils, were separated and fractionated by the silica gel liquid chromatography using ethanol as an eluent. According to the tests by infrared sepctroscopy, extractive fractionation by acids and interfacial tension of the resin derived from spindle oil fraction (from Yabase crude) in connection with the adductibility, the strongest inhibitors appear to be concentrated in the acidic constituents of oil and they are remarkabley surface active. Activators play an important role to introduce urea and n-paraffin molecules into the homogeneous phase, and their characteristics depend upon the solubility to urea and oil. Addition of a small amount of water to the poor activators such as higher alcohols (e. g. n-butanol) is very effective for the improvement of their activation power. The urea dewaxing of lubricating oil fractions was observed to be more efficient for the lower boiling point fraction than the higher for the purpose of lowering the pour point. The action of pour point depressant such as Santopour or Lanolin is especially notable for urea-dewaxed oils. treating of a spindle oil fraction. Action of Inhibitors on Urea Adduct Formation. Inhibitors Originally Present in the Petroleum Fraction It has been well known that the inhibitors for urea adduction may be removed by an adsorbent such as silica gel. For instance, when the lubricating oil fraction is separated chromatographycally by means of silica gel, a fraction which contains no inhibitor is separated as a "saturated hydrocarbon portion". Spindle oil fractions from the locally produc- were fractionated by means of a liquid chromato- graphic technique, using petroleum ether, benzene, and ethanol as elution solvents alter- natively; 0.4g of each separated fraction was dissolved in 10cc of a mixture of toluene and Volume 4-March 1962

2 Yata: Studies on the Urea-Dewaxing of Lubricating Oils n-cetane (TC solution), containing 10 vol. % by the action of a small amount of water of the latter. Three grams of urea and 0.01 naturally present in the air, but if this oil is cc. of water were added to this TC solution. forced to oxidize under the oxygen atomosphere by means of an oxidization test appara- After one hour the adductibility (the ability to form an adduct) was determined from the tus such as specified in JIS-C , the change in refractive index, and these adductibilities are compared with the properties of tion proceeds.3) These facts lead to a conclu- adductibility is gradually reduced as the oxida- each chromatographical fraction in Table I. sion that the oxidation products are one of the main constituents of inhibitors. Table 1 The effect of the chromatographycal fraction As an example, the Yabase spindle oil (by silica gel) on the adductibility distillate was treated by two methods; one with 98% sulfuric acid (45min. stirring followed by neutralization with ca. 5% aqueous sodium hydroxide, and then sludge separation) another with furfural using a batchwise countercurrent extration system. The "A" oils in Table 2 are the samples taken immediately after the treatment, and "B" oils are those taken two weeks after the treatment (kept in beakers), and these oils were treated with urea. The time required to reach an equilibrium of adduct formation was compared with the properties of these oils in Table 2. It is clearly seen from the results shown in Table 2 that refractive indexes of furfural treated oils are less than those of sulfric acid treated oils; and the aromatic content in the former is lower than that of the latter and yet the furfural refined oils require more time than acid treated oils to reach the equilibrium. In both cases, the resin which acts as a strong inhibitor is removed by the treatment, and the induction time (the time required for completion of the equilibrium) is shortened as com- pared with that of untreated oils. For instance, even after a period of about 1 month, untreat- oils will not adduct with urea at all (in ed this case, the induction time is shown as infinite), while "1-A" oil in Table 2, which is treated with furfural, is able to adduct in 500 minutes to 24 hours. Generally, oils just after treatment ("A" oils in Table 2) are more adductible than oxidized oils of higher acid values which stood for 2 days in the open air ("B" oils in Table 2). The resin content of these oxidized oils was measured by the liquid chromatographic technique, and the results obtained were 0.23% and 0.10% for oils treated with acid in dosages of 20% and 15% by volume, respectively. On the other hand, resin contents of 0.52 and 0.75% were obtained for furfural treated No. 5- B and No. 4-B oils respectively. It is evident, therefore, that the resin content of furfural treated oil are greater than that of sulfuric acid treated oil. Resin content of the original oil Bulletin of The Japan Petroleum Institute

3 Yata: Studies on the Urea-Dewaxing of Lubricating Oils Table 2 The properties of sulfuric acid and furfural treated oils and the induction time for adductions Fig. 1 Infrared spectra of resinous matter derived from oxidized oil by means of chromatography (Infrared Spectrometer-Type IRS, Nippon Bunko) chromatographic fraction of oxidized oils by Dr. Luther et al.4) A strong absorption band at about 1700cm-1 is attributed to the presence of carbonyl group, and the theory that acid value correlates with the inhibition action and the action is strengthened with the increasing amount of acidic, oxidized materials appears correct. The resinous materials separated from the Yabase oil (as shown in Table 1) were diluted about 50 times by volume of petroleum ether, Volume 4-March 1962

4 Yata: Studies on the Urea-Dewaxing of Lubrieating Oils inhibition action and this supports an assumption of the presence of suraface active inhibitors. The infrared absorption spectra of several components separated were shown in Figure 2, Fig. 2 Infrared Spectra of Resins (Apparatus is similar to that used in Fig. 1.) Table 3. Inhibition and I. F. T. by the separated fraction from "resins" and as observed in Figure 1, A1 and A2 show a strong absorption band at 1700cm-1, indicating the presence of acidic oxidized materials, and the absorption which is supposed to be originating from petroleum acids (such as naphthenic acids) appeared at a 1740cm-1 to 1760cm-1 range. Absorption at about 1600cm-1 is believed to be due to aromatic hydrocarbons, and this absorption is more predominant in B and N components rather than A1 and A2. The intensity of absorption at 1380cm-1 is supposed to be attributed to the presence of naphthenic hydrocarbons. In component B, the absorption due to oxidized materials is not clear, but weak absorption at 1660cm-1 and 1240cm-1 suggests the presence of these materials and a small amount of amines is also traced. Esters are assigned to 1165cm-1 and 1182cm-1 bands in the N component, but as determined by Dr. Luther et al.,4) the absorption which is Bulletin of The Japan Petroleum Institute

5 Yata: Studies on the Urea-Dewaxing of Lubrieating Oils 39 as ionic or nonionic series, may also act as strong inhibitors.7) These results lead to a theory that the inhibition action at the adduct formation is a kind of surface phenomenon and the surface active materials which were adsorbed on the surfaces of n-paraffins or urea mole- cules form a strong surface film. This surface film will interrupt the association of two molecules to produce the adduct. Activators for urea adduct formation It seems to be correct as far as the phenomenon is conerned, that the adduct formation occurs during a spiral growth of urea molecules around the n-paraffin molecules which act as the crystal nucleus.8) This theory is confirmed by the fact that the activaters of adduct formation are not only the solvent of urea, but also the solvent for n-paraffins. Consequently, the activators have a mutual solubility and form a homogenous solution between urea and n- paraffin molecules. In this section, the action of activators which are composed of organic polar solvents were investigated, and also the effect of coexisting water with these activators Table 4. The effect of caustic treatment. (Sample oil were determined. is similar to that of sulfuric acid 5 vol. %- B oil in Table 2) Alcohols Pure n-paraffins or their solution with benzene or toluene may be adducted easilly with the aid of solid urea and small amount of water, but lubricating oil fractions which contain inhibitor materials may not be able to adduct without the aid of the activators (for example, methanol) other than water. Figure 3 (a) shows the relationship between the amount of alcohols as the activator and adductibility, which was determined as follows: 20cc of the unrefined oil, the same one as shown in Table 2, is treated with 10g of powdered urea and 0.2 to 0.6cc of methanol, ethanol, propanol, and butanol alternatively shaked mechanically for 2 hours, filtered and after the petroleum ether (100cc) washing, weight of adduct precipitate was determined. For this figure it is seen that the higher the molecular weight of alcohols, the lower the activation, and methanol is actually the best activator. It is also known that the maximum yield was obtained when the molal ratio of activators to urea is about It appears that an active complex was formed between urea and alcohols. The results for the ketone activators were shown in Figure 3 (b), which was obtained by the similar method with alcoholic activators. In the case of acetone and methyl ethyl ketone Volume 4-March 1962

6 Yata: Studies on the Urea-Dewaxing of Lubrieating Oils Table 5. Difference of the adductibility between the different addition methods of activators.* (a) Alcohols From the results shown in Table 5 (A, B), it was concluded that in the case of methanol, the induction time does not differ greatly whether activator is added into an oil-urea phase (method 4) or oil is added into a urea-activator phase (method B). In the case of ketones (acetone and MEK), however, method B clearly gives shorter induction periods than method A. These (b) ketone facts mean that the activation by acetone or MEK, is carried out first by the urea-ketone adduct formation and then the replacement between the ketones in adduct and n-paraffins in the oil occurs. This mechanism was proposed by the author as " replacement adduction" in order to explain the phenomenon that when the adduct of lower molecular weight n-paraffin such as n-heptane adduct is mixed with the higher n-paraffin such as n-hexadecane, a selective replacement occured and n-hex decane adduct was newly formed, ousting the n-heptane back to the mother liquid15). Miscellaneous Activators Selective aromatics such as aniline, furfural, and phenol, and several organic acids, nitrills also act as the activators.10) These materials exhibit a superor activation ability than water. Liquid ammonia9) is also found to be a slightly better activator than water. Table 6 summarlizes the activation effects which were determined by the method similar to that shown in Figure 3. Bulletin of The Japan Petroleum Institute

7 Table 6. Comparison of the effect of miscellaneons activators. Yata: Studies on the Urea-Dewaxing of Lubricating Oils Table 7. The effect of water (Ten cc of sample oil and 5cc of benzene with 8g of urea were used 41 contrary, the activation ability is improved by the addition of water. A similar tendency was found for higher ketones such as MEK or MIBK, and in general, these activators which have a poor solvency power to urea are effective in the adduct formation at higher tempera- tures or in the aqueous urea solution. Discussions About the Action o f Activators From the above mentioned results, it has been found that activators should have an affinity to both urea and n-paraffins. That is to say, an effective activation is obtained only when a balance is established between the two opposing characteristics, and from this theory, methanol is believed to be one of the best and the most realistic activators. Table 8 compares the critical solution temperatures (C. S. T.) which is considered as a measure of the affinity of n-paraffins to solvents, and the solubility to urea which is con- sidered as a measure of the affinity of these solvents to urea. From Table 8, it may be said that the adductibility by the aid of anhydrous solvents is correlated with both of C. S. T. and solubility to urea, and the higher the C. S. T. values and Table 8. C. S. T. for n-paraffins and the solubility for urea of activators Volume 4-March 1962

8 42 Yata: Studies on the Urea-Dewaxing of Lubricating Oils the urea solubility, the better the activation. In general, the higher the oleophillic charactor of solvents, the lower the solubility to urea, and a satisfactory activation resulted when these two factors were well balanced; namely, in Table 8, a 75% aqueous solution of n-nropanol The solubility to urea is extremely increased by the addition of water and accordingly the adductibility is lowered in the case of methanol or ethanol, while opposite results were obtained for n-propanol and MEK. Urea Treatment of Lubricating Oil Fractions. Combination of Urea Method with Solvent Dewaxing A comparatively favorable dewaxing effect was obtained when the urea dewaxing was combined with that of solvent method. In this combination method, which process comes first, urea or solvent dewaxing, will depend on the type of feed oils, particularly the n-paraffin content. Mr. Gopalan found13) that the effect of urea dewaxing of lower boiling fractions is superior to that of higher boiling fractions, and similar results were obtained by the author and others14) using Yabase oils. It was also found by the author that for the lower boiling fractions the solvent-urea gives better results, while for the high boiling fractions the reverse or the urea-solvent combination is better. Table 9 summarizes the characteristic change Table 9. Comparison between the dewaxing techniques. conclusion that non-n-paraffin constituents of high melting point are present in B oils in comparatively large quantities. The choice of combination between urea-solvent or solvent-urea does not too much affect the pour point of dewaxed oils as seen from a comparison of No. 4 with No. 6 results. However, an analysis of refractive indexes of these dewaxed oils reveals that the urea-solvent process gives better results than the solvent-urea process. Dewaxing of Spindle Oil Fraction The oil A in Table 9 was treated by furfural extraction, followed by solvent dewaxing. The crude spindle oil thus obtained has a re- * One hundred grams of urea are excessive amount for both A oil and B oil. Bulletin of The Japan Petroleum Institute

9 Yata: Studies on the Urea-Dewaxing of Lubricating Oils Fig. 4 Effect of Pour Point Depressant effect of pour point depressant, especially that of esters, is not noticeable. In general, the by means of the con- ventional solvent dewaxing alone, and the urea method may be con- sidered as one of the best techniques for further lowering the pour point. Table 10. Comparison of the classification for Jet Lube and properties of dewaxed oil. Conclusions The inhibitors for urea adduction are adsorbed on each surface of urea and n-paraffin molecules simultaneously, and the resultant surface film probably checks the adduct formation. Water itself is poor in its affinity to oils and cannot break this surface film, but a suitable solvent such as methanol exhibits an affinity both to oil and urea (mutual solubility) and easily disorders the orientation due to the surface active substances which form the surface film, thus destroying the action of inhibitors. Furfural or phenol is poorer in its affinity to urea than MEK or methanol, and does not dissolve urea too well but if the solvent is used in the presence of small amount of water, it will act as an excellent activator. This fact introduces the following conclnsions; (1) if the the refined oil by furfural or phenol extraction is treated with urea without removing the solvent and in the presence of small amount of water, an almost complete adduction results, and similary, (2) after the solvent dewaxing, using a MEK-toluene mixture, the adduct for- mation is easilly obtained using urea and small amount of water without removing the solvent. These conclusions are important for the technical application of urea adduction to commercial refinery operations, especially when the urea method is combined with solvent refining and Volume 4-March 1962

10 Yata: Studies on the Urea-Dewaxing of Lubricating Oils dewaxing processes. The superiority of the urea method to the solvent method has been confirmed in the manufacture of jet lube oils of lower pour point. References Bulletin of The Japan Petroleum Institute