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Regeneration of Used Lubricant Oil by Polar Solvent Extraction Jesusa Rinco´ n,*,† Pablo Can ˜ izares,‡ and Marı´a Teresa Garcı´a§ Departamento de Ingenierı´a Quı´mica, Facultad de Ciencias del Medio Ambiente, Universidad de Castilla-La Mancha, Avenida Carlos III, s/n. 45071 Toledo, Spain, Facultad de Ciencias Quı´micas, Universidad de Castilla-La Mancha, Avenida Camilo Jose´ Cela, 10. 13004 C. Real, Spain, and Escuela Universitaria Polite´ cnica de Almaden, Plaza Manuel Meca, 1. 13400, Almade´ n (C. Real), Spain
Solvent extraction is one of the cheapest and more efficient processes for waste oil recycling. In this work the action of some solvents (2-propanol, 2-butanol, 2-pentanol, methyl ethyl ketone, and methyl n-propyl ketone) on both yield and quality of the recovered oil has been investigated. The quality has been assessed through the measurement of metallic, polymeric, and oxidation compound concentrations in the extracted oil. Experimental results have shown that extraction yields increase with increasing solvent/oil ratios up to a point at which they stabilize. When comparing alcohols and ketones it has been found that yields obtained with solvents of equal numbers of carbon atoms are similar and increase with increasing solvent molecular weight for both families. On the other hand, metallic and oxidation compound removal was similar for alcohols and ketones of equal numbers of carbon atoms, but alcohols were more efficient than ketones when polymeric additive elimination was considered. All these results may be attributed to the combined effect of factors such as the system viscosity, the detergent-dispersant additive concentration, and the difference between the solubility parameters of the system components. 1. Introduction Large and increasing volumes of used lubricating oil are produced each year that, after use, are considered hazardous wastes. This is so because waste oils typically consist of a mixture of undegraded base oil and additives with high concentrations of metals, varnish, gums, and other asphaltic compounds coming from overlay on bearing surfaces and degradation of the fresh lubricant components. However, this waste oil pollution source may be eliminated by oil recycling. Over the years several re-refining technologies have been proposed for waste oil recycling. These technologies seem to converge on a three-step procedure:1-3 separation of water and light hydrocarbons by distillation at atmospheric pressure, separation of base oil from contaminant agents by high vacuum distillation (10-30 mmHg), and finishing of the base oil separated in the preceding stage by hydrogenation. The second step presents several problems related to fouling of heating and distillation equipment and cracking reactions, which usually originate both unpleasant smells (mercaptans) and low-quality base oils (poor stability, color, and smell). However, these problems could be overcome by introducing a pretreatment step before the high vacuum distillation such as, for example, extraction with solvents. The basics of this pretreatment is the use of a solvent to selectively extract all base oil components from waste oil in a process that would be quite similar to that commonly used in crude oil refining to separate out asphaltenes for producing heavy neutral base oil (bright stock). Furthermore, since waste oil constituents are * To whom correspondence should be addressed. Tel.: 34925-26 88 00. Fax: 34-925-26 88 40. E-mail: Jesusa.Rinco´n@ uclm.es. † Facultad de Ciencias del Medio Ambiente. ‡ Facultad de Ciencias Quı´micas. § Escuela Universitaria Polite´cnica de Almaden.
usually kept in stable dispersion by dispersant additives and electrostatic interactions between heteroatoms and asphaltic compounds, just as in the crude oil,2 a solvent to recover the base oil fraction must be miscible with the base oil contained in the waste oil being processed; also, when mixed with the waste oil, it must reject from the solution the additives and dispersed particles, allowing their aggregation to particle sizes big enough to separate from the liquid by sedimentation. It is well-known that some pure organic liquids (such as hydrocarbons, ketones, and alcohols) and their multicomponent solutions exhibit these properties, and a number of them have been used for base oil recycling.4-11 However, the earlier works using these solvents mainly focused on assessing the weight percentage of sludge removal and no attempt was made at determining the quality of the base oil recovered. This paper intends to complete the results drawn from these preliminary studies. Thus, the experiments have been designed to analyze the action of some solvents found useful in earlier works (three alcohols and two ketones). More specifically, in this work the influence of the solvent/waste oil ratio used for the extraction on both the extraction yield and quality of the oil obtained has been analyzed. The quality has been evaluated through the determination of undegraded polymeric additives, metallic compounds, and oxidation products. All these compounds are important used oil constituents that should be separated to obtain a base oil suitable for the formulation of new lubricants. Polymeric materials such as polyolefins are usually introduced into oil as viscosity index improvers, oxidation products mainly result from the oxidation of base oil components, and metallic compounds may have been introduced from different sources: lead from overlay of bearing surfaces, as well as grease, antiwear, and detergent additives, calcium from dispersant-detergent additives, zinc from antiwear additives and oxidation and corrosion inhibitors, and iron from gray iron cylinder liners, malleable iron pistons, hardened steel camshafts, gears, etc.
10.1021/ie040254j CCC: $30.25 © 2005 American Chemical Society Published on Web 05/11/2005
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Table 1. Used Oil Properties characteristic
used oil
viscosity at 40 °C (ASTMD-445), cSt viscosity at 100 °C (ASTMD-445), cSt sulfur (ASTMD-4294), % phosphorus (FRX), ppm acid number, mg of KOH/g metallic content, ppm zinc lead iron calcium
93.98 13.11 0.66 835 3.36 947 850 35 2000
2. Experimental Section 2.1. Materials. Organic solvents (2-propanol, 2-butanol, 2-pentanol, methyl ethyl ketone, and methyl n-propyl ketone) were supplied by Panreac. Used lubricant oil was supplied by Emgrisa, S. A. Prior to the runs the oil was treated in a rotary evaporator at 60 °C under vacuum (600 mmHg) to eliminate water and light hydrocarbons. Both types of compounds are undesirable for the formulation of new lubricants and may modify the solubility parameter of base oil components in the solvent. Used oil properties after this treatment are shown in Table 1. At the industrial scale this distillation to remove water and light hydrocarbons is usually performed at atmospheric pressure.3 We made it under vacuum to accelerate the separation process. 2.2. Extraction Procedure. Mixtures of about 10 g of used oil and solvent in weight proportions ranging from 1/1 to 15/1 solvent/oil were agitated to ensure adequate mixing (see section 3.2). Then, the mixtures were poured into glass centrifugal tubes that were later introduced in the support of a centrifuge (Selecta/ MIXTASEL). After centrifugation at 400 rpm for 10 min, a sludge phase (additive, impurities, and carbonaceous particles) was segregated from the mixture of solvent and oil. The solvent was separated from the solvent/oil mixture by distillation in a rotary evaporator and the recovered oil was weighed. The extraction yield was calculated as the mass of oil, expressed in grams, separated from 100 g of waste oil. The sludge phase was also weighed in all experiments. It has been observed that, on a solvent-free basis, the difference between the mass of raw used oil extracted and the masses of the two phases separated after extraction (base oil and sludge) was always less than 1%. 2.3. Analysis of the Metallic Content. The extracted oil samples were heated at 200 °C for 4 h and calcined at 650 °C overnight prior to the analyses. The noncombustible residue obtained (ashes) was further treated with hydrochloric acid, filtered, and diluted with deionized water to determine the metallic content. These analyses were performed by atomic absorption spectroscopy using a Varian Spectra 220 FS spectrometer. The concentration of metals in the sludge was determined only in some preliminary experiments, just to know that the amount of metals in the raw used oil was the sum of the amounts of metals in the two phases separated by extraction: base oil and sludge. 2.4. Other Analyses. Phosphorus content was measured by energy-dispersive X-ray fluorescence spectroscopy using an Oxford ED200 spectrometer. Acid number, sulfur content, and viscosities at 40 and 100 °C were determined according to ASTMD-664, ASTMD-4294, and ASTMD-445 procedures, respectively.
Table 2. Solubility Parameter of Base Oil and Viscosities and Solubility Parameters of Polar Solvents solvent oila
base 2-propanolb 2-butanolb 2-pentanolb methyl ethyl ketoneb methyl n-propyl ketoneb a
δ (J/m3)1/2 × 10-3 viscosity (25 °C), cP 3.0 5.6 5.3 5.2 4.5 3.9
2.0 3.7 5.1 0.41 0.51
References 11 and 22. b Reference 18.
3. Results and Discussion The accuracy of the experimentally determined extraction yields has been determined by comparing the results from four independent runs carried out under identical conditions: solvent, 2-propanol; solvent/oil ratio, 15/1 g/g; temperature, 25 °C; and mixing time, 30 min. In these experiments the extraction yields were similar (79.1, 79.0, 79.0, and 78.9%), indicating that reproducibility of the data was good. Nevertheless, to minimize experimental errors, each run was replicated twice. 3.1. Preselection of Solvents. As noted before, a good solvent to recover the base oil contained in the waste oil should be miscible with it and reject from the solution the additives and other impurities. It is wellknown6-9,12 that there are several solvents having these properties and, therefore, can be used for this operation but, to select the most suitable one, their action on the waste oil should be well understood. In this work, to select such a solvent we have followed a three-stage strategy considering, first, Burrel’s classification of the solvents,13 second, the existing background on the regeneration of waste oil by solvent extraction,4-11 and, third, the selectivity or capacity of the solvent to selectively extract base oil from waste oil. Burrel’s classification of solvents is based on their capacity to form hydrogen bonds. Thus, there are solvents with high capacity (alcohols, amines, acids, aldehydes, ...), moderate capacity (ketones, esters, ethers, ...) and low capacity (hydrocarbons, nitrogenated and chlorinated hydrocarbons). For this investigation we have chosen compounds belonging to two families (alcohols and ketones) of the first two groups that had earlier been used in the regeneration of waste oil.4-11 Results obtained with solvents from the third group have been discussed elsewhere.14,15 Once the alcohol and ketone families were selected, the individual components to be tested of each group were chosen on the basis of their selectivity. Therefore, solvents such as 2-propanol, 2-butanol, 2-pentanol, methyl ethyl ketone, and methyl n-propyl ketone, with solubility parameters (see Table 2) close to that of the base oil fraction were selected. Finally, it should be noticed that the solvents selected have between three and five carbon atoms in their molecules because alcohols and ketones with lower molecular weight cannot dissolve base oil and those with longer chain length may prevent aggregation of waste oil impurities.8,9 3.2. Preliminary Experiments. Two types of experiments were performed before analysis of the effect of the organic solvents used at different weight solventto-oil ratios on both yield and quality of the base oil obtained. The first group of experiments was made to determine the mixing time, t, required for the systems to attain equilibrium. Mixing or contact time between the solvent and the waste oil plays an important role
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Figure 2. Effect of time on metallic concentration of the extracted oil. Solvent, 2-pentanol. T ) 25 °C. Figure 1. Evolution with time of extraction yields.
in the separation of undesirable impurities from the base oil extracted for two reasons. On one hand, it should be long enough to allow the solvent to dissolve the base oil contained in the waste oil being processed, and on the other hand, it should also allow rejection from the solution of the additives and impurities by permitting their aggregation to particle sizes big enough to separate from the liquid phase by sedimentation. This group of experiments was performed at room temperature (25 °C) at weight solvent/oil ratios of 2 and 10 g/g. Results obtained at both ratios were similar, and Figure 1 shows those obtained at the higher ratio. It can be observed that the evolution of the extraction yield with mixing time depends on the solvent considered. For 2-propanol and methyl ethyl ketone it increases with time until it becomes constant at about 20 min. In other words, the mixing time required for these systems to reach equilibrium is 20 min. In the cases of 2-butanol and methyl n-propyl ketone the equilibrium is attained at shorter mixing times, less than 10 min, probably due to their larger chain length, and therefore to their larger capacity to dissolve base oil components. Finally, with 2-pentanol, the alcohol with highest viscosity (Table 2) and molecular weight, the extraction yield decreases with time until approximately 25 min and then remains constant. According to Alves dos Reis and Silva8 it may probably be imputed to that the higher viscosity of 2-pentanol increases the time necessary for the mixing and dissolution of base lubricant in this solvent and, obviously, it increases the difficulty of flocculation and sedimentation of the impurities to be removed. In other words, extraction yields are higher at shorter times because part of the impurities have not had time enough to aggregate and, as a consequence, they are coextracted together with the base oil. To test this last hypothesis the evolution with time of the metallic content of the extract obtained with this solvent was determined. As expected, it can be observed in Figure 2 that for the shortest mixing time the metallic content of the extracted oil was very similar to that of the untreated waste oil (Table 1) and that the concentrations of these metallic impurities decreased when mixing times increased from 5 to 25 min, being approximately constant from this moment. As regards the different behavior exhibited by 2-butanol and methyl ethyl ketone or 2-pentanol and methyl n-propyl ketone, solvents with comparable molecular weights, it may probably be imputed to that their
Figure 3. Effect of temperature on extraction yields.
viscosity at 25 °C (3.7 and 5.1 cP for 2-butanol and 2-pentanol versus 0.4 and 0.5 cP for methyl ethyl ketone and methyl n-propyl ketone) affects the time required for the impurities to aggregate to sizes big enough to separate from the liquid by sedimentation or centrifugation. According to all these results, and to ensure that equilibrium is reached with all solvents, a mixing time of 30 min was chosen for performing subsequent tests. The second group of experiments was carried out to determine the temperature, T, at which extractions should be performed, since it is well-known that this variable affects the solubility in classical organic solvents of both base oil and waste oil impurities.9,11,16 These experiments were performed with methyl ethyl ketone at a weight solvent/oil ratio of 10 g/g. The temperature was varied from 25 to 60 °C. Higher temperatures were not tested to prevent solvent vaporization, and consequently, that the solvent/oil ratio could change during the experiments. On the other hand, since temperatures lower than atmospheric would complicate the process, the lower end of the experimental range of this variable was set at 25 °C. Results obtained are shown in Figure 3. It can be observed that extraction yields slightly increase with increasing temperatures probably due to the combination of the following factors: on one side, the solubility of the base oil components in the organic solvent increases with temperature9 and, on the other side, the amount of base oil retained by impurities separated from the waste oil decreases
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Figure 4. Effect of temperature on metallic concentration of the extracted oil.
with increasing values of the variable.16 Furthermore, the increasing yield observed could also be due to an increase in the solubility of waste oil impurities with increasing temperature. To test this last hypothesis, the concentration of a type of impurities (metallic compounds) in the extracted samples was determined. Results from these analyses are shown in Figure 4. Confirming the suggested hypothesis, they indicate that the metallic concentration in the extracted oil increases with temperature. Considering these results, it may be stated that temperatures higher than atmospheric are not interesting for the waste oil regeneration process because, although extraction yields are slightly higher at higher temperature, they do not compensate for the lower extract quality (higher metal concentration) and higher energy costs associated with increasing temperatures. Thus, according to these and other authors’ results,8,17 an extraction temperature of 25 °C was selected for subsequent experiments. 3.3. Base Oil Yield. Waste oil impurities are usually kept in stable dispersion by dispersant-detergent additives and electrostatic interactions between heteroatoms and asphaltic compounds just as in the crude oil.8 Thus, the first thing to do to separate the undesirable compounds from the base oil fraction of the waste oil is to break such stable suspensions. To determine the capacity of the different solvents selected in section 3.1 (2-propanol, 2-butanol, 2-pentanol, methyl ethyl ketone, and methyl n-propyl ketone) to destabilize the dispersion and later separate impurities, they were mixed with waste oil at weight solvent/ oil ratios of 2, 5, 7, 10, and 15 g/g. Then the extractions were performed at conditions selected in section 3.2 (t ) 30 min and T ) 25 °C). Results obtained are shown in Figure 5. It can be observed that, for all solvents, the extraction yields increase with increasing solvent/oil ratio up to a point at which they stabilize and that the solvent/oil ratios at which stabilization occurs with ketones are smaller than those obtained with alcohols (vertical lines 1, 2, and 3 in this figure indicate the solvent/oil ratios at which stabilization occurs with methyl n-propyl ketone (line 1), methyl ethyl ketone (line 2), and alcohols (line 3)). It can also be seen that, once yields stabilize, those obtained with alcohols and ketones of equal numbers of carbon atoms are similar and that for both families of solvents the extraction yields decrease with decreasing solvent molecular weight. Finally, it should
Figure 5. Effect of solvent:oil ratio on extraction yields.
be noticed that base oil extracted was blackish in all cases and that coloration became darker when either high molecular weight solvents or low solvent/oil ratios were used. The reason extraction yields increase with increasing solvent/oil ratios and then stabilize is probably because at the smaller ratios the solvents saturate and do not dissolve all base oil present. With increasing solvent/ oil ratios the base oil dissolved increases, but only up to a ratio at which the base oil fraction that each solvent may dissolve is exhausted, and consequently, from this moment the extraction yield will not increase anymore. As regards the fact that yield stabilization occurred with ketones at solvent/oil ratios lower than with alcohols, it could be attributed to ketones having lesser viscosities and solubility parameters than alcohols.18 Somewhat surprising is that, once stabilization was reached, extraction yields obtained with alcohols and ketones of equal chain length were similar although their solubility parameters were different. A possible explication of this finding might be that, in the case of the complex system constituted by the solvent and the multicomponent mixture that makes up the waste oil, the effect on base oil solubility of the solvent solubility parameter is less important than the effect of other factors such as the system viscosity and/or the solvent polarity. In relation to the lesser yield and lighter extract coloration attained with the lower molecular weight solvents of a given family, it is a result that can be explained by considering the polarity of the solvents and the difference of the solubility parameters of base oil and solvents, both of which are larger for the lighter solvents. Last, the darker coloration of the base oil extracted at the smallest solvent/oil ratios probably indicates that, in these cases, the solvent has dissolved not only the base oil fraction but also waste oil impurities. It may be imputed to the combined effect of the following factors. First, it could be that at the smaller solvent/oil ratios the dilution of detergent-dispersant additives was not large enough to completely destabilize the waste oil dispersion. Then, impurities did not completely agglomerate and were partly coextracted together with the base oil. Second, agglomeration and further separation of impurities could be hindered because, as a consequence of the lesser viscosity decrease produced in the system at the lower solvent/oil ratios,19 they
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Figure 7. Effect of solvent:oil ratio on metallic concentration of the extracted oil: (A) methyl ethyl ketone; (B) methyl n-propyl ketone.
Figure 6. Effect of solvent:oil ratio on metallic concentration of the extracted oil: (A) 2-propanol; (B) 2-butanol; (C) 2-pentanol.
would remain in solution and would be coextracted with the base oil. Finally, it should be taken into account8 that increasing solvent/oil ratios lead to increasing differences between the average solubility parameter of the base oil solvent mixtures and that of the impurities (recall that they usually have high molecular weights) and, therefore, such impurities would be rejected from the solution more easily when high solvent/oil ratios are used. 3.4. Base Oil Quality. In the last section it has been shown qualitatively that waste oil extraction with any of the solvents under study does not lead to pure base oil. Here, this finding will be quantified and results obtained for several quality parameters (concentrations in the extracted base oil of polymeric additives, metallic compounds, and oxidation products) will be shown. Figures 6 and 7 show the metallic content of the base oil extracted with the different solvents at all the
solvent/oil ratios tested. It can be observed in all cases that the metallic concentration of the oil decreases with increasing solvent/oil ratio. This is due to the same factors that cause a lighter coloration of the base oil extracted at higher solvent/oil ratios, i.e., that increasing ratios produce a larger dilution of the additives that stabilize waste oil impurities, a larger viscosity decrease of the system, and a larger difference between the solubility parameters of the oil/solvent mixture and the waste oil impurities. When comparing results obtained with alcohols and ketones, we find that 2-propanol is the solvent giving the best results, probably because the high molecular weights of metallic compounds make them more difficult to be dissolved by the solvent with the lowest molecular weight. Figure 6 also shows that, unlike with the other solvents, the metallic concentration in the base oil extracted with 2-propanol decreases sharply when varying the solvent/oil ratio from 2 to 5 g/g. This can probably be imputed to the larger oil yield variation observed with this solvent in this range (see Figure 5). On the other hand, when compounds of different families but similar chain lengths are compared, although a small difference exists, similar results are found. Thus, the metallic concentration in base oil extracted with 2-butanol is slightly larger than that obtained with methyl ethyl ketone while with 2-pentanol it is slightly lower than that obtained with methyl n-propyl ketone. These results may be explained by considering both the polarity and the solubility parameter of the solvent. Thus, the higher polarity of alcohols may lead to base oil with a higher level of impurities (it should be recalled that waste oil impurities have
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Table 3. Effect of Solvent on Acid Number
2-propanol 2-butanol 2-pentanol methyl ethyl ketone methyl n-propyl ketone a
solvent/waste oil (g/g)
acid number (mg of KOH/g)
5 10a 5 10a 5 10a 5 7a 5a
1.95 1.08 2.89 2.34 3.15 2.94 2.60 2.52 2.86
Table 4. Effect of Solvent on Extraction Yield for Additived Base Oil and Nonadditived Base Oil
solvent 2-propanol
SAE 20 SAE 30 SAE 50 SAE 20 SAE 30 SAE 50 SAE 20 SAE 30 SAE 50 SAE 20
88 68 45 100 100 95 100 100 100 100
84 63 39 96 95 89 98 96 96 99
4 5 6 4 5 6 2 4 4 1
SAE 30 SAE 50 methyl n-propyl SAE 20 ketone SAE 30 SAE 50
100 100 100
98 97 100
2 3 0
100 100
100 100
0 0
2-butanol 2-pentanol
Solvent/waste oil ratio at which yield stabilization occurs.
polar character20) than that obtained with ketones, as occurred with 2-butanol (solubility parameter 5.3 × 10-3 (J/m3)1/2) and methyl ethyl ketone (solubility parameter 4.5 × 10-3 (J/m3)1/2). However, a larger difference between the solubility parameters of the solvents being compared may disguise the polarity effect, such as in 2-pentanol (solubility parameter 5.2 × 10-3 (J/m3)1/2) and methyl n-propyl ketone (solubility parameter 3.9 × 10-3 (J/m3)1/2). Last, the decrease of the metallic concentration with decreasing solvent chain length found in each solvent family may be explained by bearing in mind that metallic impurities usually have high molecular weight20 and, according to the Hildebrand solubility theory,21 they should be dissolved more easily by the heavier solvents because their solubility parameters are closer to those of the metallic compounds. Other major waste oil impurities that may be coextracted together with the base oil are oxidation products. This is because under normal service conditions an important part of lubricant degradation is due to the oxidation of the lubricant base oil. Their presence in the oil is important because it may lead to both the formation of carbonaceous material and increasing oil viscosity. Besides, these compounds are difficult to separate by vacuum distillation;7 therefore, their elimination by organic solvent extraction could be very important. Obviously, to separate these degradation compounds from the used oil, the solvent should be able to destabilize the dispersion and to dissolve selectively the base oil fraction throwing out the undesirable oxidation products. To find out the effect of the solvent type and solvent/oil ratio on the extent to which separation occurred, the base oils obtained with solvent/oil ratios equal to 5 g/g and those corresponding to the stabilization of the extraction yields (see Figure 5) were analyzed as follows. First, to investigate the presence of oxidation products, the extracts IR spectra were obtained, and second, once a band at 1700 cm-1 confirmed their presence, the concentration of these compounds was determined by measuring the extract acid number.2,14 The values obtained for this parameter are shown in Table 3. It can be observed that the acid numbers obtained with alcohols and ketones of similar chain lengths are very similar and that for both families of compounds the parameter increases with the solvent molecular weight. These results are similar to those obtained when studying the elimination of metallic impurities, and the reasoning stated there is also valid here. Table 3 also shows that the acid numbers are far from the accepted limit for a virgin oil,2 0.05 mg of KOH/ g, probably because of the polar character of the solvents.6,7,10 Last, in Table 3 is also shown that the
extraction yield (%) decrease of nonadditived additived extraction base oil oil oil yield (%)
methyl ethyl ketone
extract acid number increases with decreasing solvent/ oil ratio and that for the heavier solvents this parameter is practically similar to that of the untreated waste oil. This is probably because oxidation compounds are stabilized in the oil by the presence of dispersantdetergent additives and with the introduction of a solvent their concentrations decrease. As a sequel, the removal of oxidation compounds increases with increasing solvent concentration in the system. This hypothesis may be confirmed by measuring the concentration in the extracts of both dispersantdetergent additives and oxidation products. On the other hand, since dispersant-detergent additives usually contain calcium in their molecules, their concentration in the extract may be related to that of the metal and, consequently, also to the concentration of oxidation compounds. In Figures 6 and 7 is shown that, effectively, the calcium concentration in the extract increases as that of oxidation products, a fact that confirms the above suggested hypothesis relating the concentration of dispersant-detergent additives and oxidation products. Besides, as in the case of metallic impurities, agglomeration and further separation of impurities may decrease with decreasing solvent/oil ratios both because of the smaller viscosity decrease produced in the system and because of the smaller differences between the average solubility parameter of the base oil-solvent mixture and that of the impurities. Finally, to analyze the capacity of the solvents to segregate polymeric additives, several experiments were performed using pure base oils (SAE 20, SAE 30, and SAE 50) and mixtures of them with the commercial polyolefinic additive HITEC 5748 EuroDrum, a polymeric additive commonly used as a viscosity improver. The concentration of the additive in the mixtures was 5% w/w, and the weight solvent/oil ratio was that corresponding to the stabilization of the extraction yields (see Figure 5). Table 4 shows the results obtained. It can be observed in Table 4 that for both families of compounds tested, alcohols and ketones, the extraction yields increase with the solvent molecular weight, irrespective of whether experiments were performed with pure base oil or with mixtures of oil and additives. Table 4 also shows that for solvents with similar chain lengths the extraction yields obtained with additived oil are slightly larger with ketones than with alcohols. Both results may be explained on the basis of the Hildebrand
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solubility theory21 that establishes a larger solute solubility in those solvents with solubility parameters closer to that of the solute. On the other hand, Table 4 also reflects that the percentage of oil recovered is larger when the oil to be extracted did not contain additives and that the difference between results obtained with additived and nonadditived oils increases with both the solubility parameter of the solvent and the average molecular weight of the oil. Most probably these results obey the fact that the additive is a high molecular weight compound and, therefore, is more difficult to dissolve than the lighter base oil fractions. Consequently, for those solvents with higher capacity to dissolve heavier base oil (lesser solubility parameter) these differences are smaller, or even nonexistent, as in the case of the methyl ethyl ketone. In short, results obtained indicate that, from the polymeric additive elimination point of view, alcohols are more efficient than ketones and that such efficiency increases when their molecular weights decrease. 4. Conclusions The base oil yields obtained by extraction of waste oil with polar solvents (2-propanol, 2-butanol, 2-pentanol, methyl ethyl ketone, and methyl n-propyl ketone) increase with increasing solvent/oil ratio up to a point at which they stabilize. This is due to the combined effect of factors such as the system viscosity, the detergent-dispersant additive concentration, and the difference between the solubility parameters of the system components. Stabilization with ketones occurs at a lower ratio due to their smaller viscosities and solubility parameters. Once yields stabilize, those obtained with alcohols and ketones of equal numbers of carbon atoms are similar, probably because the base oil solubility is affected not only by the system solubility parameter but also by factors such as the system viscosity and/or the solvent polarity. On the other hand, for both families of solvents the extraction yields increase with increasing molecular weight of the solvent because the difference between the solubility parameters of solute and solvent decreases in this way. As regards the quality of the recovered base oil, it may be stated that with all solvents the removal of impurities increased with increasing solvent/oil ratios. Metallic and oxidation compounds removal was almost the same for alcohols and ketones of equal numbers of carbon atoms, but alcohols were slightly more efficient than ketones when the polymeric additive elimination was considered. As indicated in the case of analyzing the extraction yields, these results may be explained by taking into account the effect of factors such as the dilution of the additives that stabilize waste oil impurities, the decrease of viscosity that produces the addition of the solvent, and the difference between the solubility parameters of the oil/solvent mixtures and the waste oil impurities.
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Received for review October 4, 2004 Revised manuscript received February 23, 2005 Accepted April 6, 2005 IE040254J
